Milestones in Geology (Geological Society Memoir No. 16)

Milestones in Geology Reviews to celebrate 150 volumes of the

Journal of the Geological Society

Geological Society Memoirs

Series Editor A. J. FLEET

, !j

Parliamentary-style meeting room of the Geological Society at Burlington House before 1975.

The meeting room after renovation.

Milestones in Geology Reviews to celebrate 150 volumes of the

Journal of the Geological Society E D I T E D BY

M. J. LE BAS University of Leicester, UK

Memoir No. 16 1995 Published by The Geological Society London

THE GEOLOGICAL SOCIETY The Society was founded in 1807 as the Geological Society of London and is the oldest geological society in the world. It received its Royal Charter in 1825 for the purpose of 'investigating the mineral structure of the Earth'. The Society is Britain's national society for geology with a membership of 7500 (1993). It has countrywide coverage and approximately 1000 members reside overseas. The Society is responsible for all aspects of the geological sciences including professional matters. The Society has its own publishing house, which produces the Society's international journals, books and maps, and which acts as the European distributor for publication of the American Association of Petroleum Geologists and the Geological Society of America. Fellowship is open to those holding a recognized honours degree in geology or cognate subject and who have at least two years' relevant postgraduate experience, or who have not less than six years' relevant experience in geology or a cognate subject. A Fellow who has not less than five years' relevant postgraduate experience in the practice of geology may apply for validation and, subject to approval, may be able to use the designatory letters C Geol (Chartered Geologist). Further information about the Society is available from the Membership Manager, The Geological Society, Burlington House, Piccadilly, London W1V 0JU, UK.

Published by the Geological Society from: The Geological Society Publishing House Unit 7 Brassmill Enterprise Centre Brassmill Lane Bath BA1 3JN UK (Orders: Tel 01225 445046 Fax 01225 442836) First published 1995 The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility for any errors or omission that may be made. © The Geological Society 1995. All rights reserved. No reproduction, copy or transmission of this publication may be made without written permission. No paragraph of this publication my be reproduced, copies or transmitted save with the provisions of the Copyright Licensing Agency, 90 Tottenham Court Road, London W1P 9HE. User registered with the Copyright Clearance Center, 27 Congress Street, Salem, MA 01970, USA: the item fee code for this publication is 0435-4052/95/$7.00.

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Foreword

vii

LE BAS, M. J. Introduction

1

RUDWlCK, M. J. S. Historical origins of the Geological Society's Journal

5

WINDLEY, B. F. Uniformitarianism today: plate tectonics is the key to the past

11

HALL, R. P. & HUGHES, D. J. Early Precambrian crustal development: changing styles of mafic magmatism

25

ROGERS, G. & PANKHURST, R. J. Unravelling dates through the ages: geochronology of the Scotting metamorphic complexes

37

BLUCK, B . J . W . Q .

57

Kennedy, the Great Glen Fault and strike-slip motion

BROWN, M. P - T - t evolution of orogenic belts and the causes of regional metamorphism

67

MCKERROW, W. S. The development of Early Palaeozoic global stratigraphy

83

FORTEY, R. A. Charles Lapworth and the biostratigraphic paradigm

93

RILEY, N. J. Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles

105

CALLOMON,J. H. Time from fossils: S. S. Buckman and Jurassic high-resolution geochronology

127

SAVAGE, R. J. G. Vertebrate fissure faunas with special reference to Bristol Channel Mesozoic faunas

153

COCKS, L. R. M. Triassic pebbles, derived fossils and the Ordovician to Devonian palaeogeography of Europe

165

ALLEN, J. R. L. Sedimentary structures: Sorby and the last decade

175

SELLWOOD,B. W. Structure and origin of limestone

185

WALKER, G. P. L. Flood basalts versus central volcanoes and the British Tertiary Volcanic Province

195

WILSON, M. Magmatic differentiation

205

ATHERTON, M. P. Granite magmatism

221

RANKIN, A. H. Hydrothermal orefields and ore fluids

237

BAILEY, D. K. Carbonate magmas

249

Index

265

Foreword The Geological Society, which is the senior Earth science society in the World, was founded in 1807 for the purpose 'of investigating the mineral structure of the Earth'. In keeping with the place of science in society at the time, it soon received its Royal Charter (1825). The Society's role today is not so different in essence: as a learned society it is primarily concerned with the furtherance of scientific knowledge. This is achieved through debate and, of particular relevance here, through the publication of the results of scientific investigation, analysis and discussion of findings. The Society's principal medium for publication is the Journal. It first appeared in 1845 and has continued, without break, since that time. Hence we arrive at volume 150, and this book celebrates that event. I am sure that readers of this book will not only learn much about how our science has progressed and where the frontiers lie, but will also find interesting the manner in which the Geological Society played the major role in this advance. The Society has grown over the years both in its membership (now over 7000) and in the range of its activities, publications and responsibilities. To its role as the leading UK Earth science society, has been added that of representing professional geologists in the UK and, through the European Federation of Geologists, throughout Europe. I am pleased of this opportunity to recommend this book, edited by the Journal's Chief Editor, Dr Mike Le Bas, to all Earth scientists. His introduction sets the scene. Charles Curtis President 1992-1994

Foreword

vii

LE BAS, M. J. Introduction

1

RUDWlCK, M. J. S. Historical origins of the Geological Society's Journal

5

WINDLEY, B. F. Uniformitarianism today: plate tectonics is the key to the past

11

HALL, R. P. & HUGHES, D. J. Early Precambrian crustal development: changing styles of mafic magmatism

25

ROGERS, G. & PANKHURST, R. J. Unravelling dates through the ages: geochronology of the Scotting metamorphic complexes

37

BLUCK, B . J . W . Q .

57

Kennedy, the Great Glen Fault and strike-slip motion

BROWN, M. P - T - t evolution of orogenic belts and the causes of regional metamorphism

67

MCKERROW, W. S. The development of Early Palaeozoic global stratigraphy

83

FORTEY, R. A. Charles Lapworth and the biostratigraphic paradigm

93

RILEY, N. J. Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles

105

CALLOMON,J. H. Time from fossils: S. S. Buckman and Jurassic high-resolution geochronology

127

SAVAGE, R. J. G. Vertebrate fissure faunas with special reference to Bristol Channel Mesozoic faunas

153

COCKS, L. R. M. Triassic pebbles, derived fossils and the Ordovician to Devonian palaeogeography of Europe

165

ALLEN, J. R. L. Sedimentary structures: Sorby and the last decade

175

SELLWOOD,B. W. Structure and origin of limestone

185

WALKER, G. P. L. Flood basalts versus central volcanoes and the British Tertiary Volcanic Province

195

WILSON, M. Magmatic differentiation

205

ATHERTON, M. P. Granite magmatism

221

RANKIN, A. H. Hydrothermal orefields and ore fluids

237

BAILEY, D. K. Carbonate magmas

249

Index

265

From Le Bas, M. J. (ed.), 1995, Milestonesin Geology, Geological Society, London, Memoir No. 16, 1-4

Introduction M . J. L E

BAS

Department of Geology, University of Leicester, Leicester LE1 7RH, UK

Science advances by taking new and unexpected turnings, pioneers opening up pathways which later workers follow and explore. This book takes the reader along several such geological paths that have followed from observations and theories first printed in The Quarterly Journal of the Geological Society of London (since 1971, the Journal of the Geological Society). Following the paths, one sees the history of geological thought during the nineteenth and twentieth centuries. To achieve this desired structure for the book, leading geologists were invited to present their personal views on significant topics that had been brought to the fore in earlier contributions to the Journal, to evaluate the evidence presented and to give their view of how these seminal papers affected our present understanding of geological processes, and further to hazard where future paths of investigation may lie. Bringing these together under one cover serves two purposes: first to celebrate 150 years of continuous publication of papers by the Geological Society of London: second, it makes a British review of the current 'battle lines' across many fields of geological research. Not only will the serious research investigator discover the several turning points which have governed the paths of his study, but the general reader also will discover the delights, the fortunes and machinations taken by many leading British geologists. Others will use the chapters to answer the question: 'Does historical contingency govern the paths of geological exploration, as it has been said to govern the evolution of living creatures?' Most of the papers have already been published as Celebration Papers during 1993 in Volume 150 of the Journal of the Geological Society, hence the size and format of this book. To emphasize the historical context, all the chapters are preceded by the abstracts or prefaces of the seminal papers reproduced from the Journal. The exception is Rudwick's, which sets the scene on why the Geological Society of London rated so highly the publication of geological observation. He recounts the creation of the Journal, and how it survived growing pains to become a leading international ,journal. One of the several maxims followed by geologists is that of uniformitarianism. It was defined by Hutton over 200 years ago and popularized by Charles Lyell in his Principles of Geology and in his many papers read before and published by the Geological Society. Earlier this century, it was considered applicable only to the Phanerozoic. Nowadays with the great advances made in dating techniques, Windley is able to argue cogently that the plate tectonic paradigm can be successfully applied not only to the Proterozoic but even to the Archaean. He shows how the growth of the North American plate and the Kapvaal craton can be explained by the plate tectonic model, and how

secular changes in heat production changed the course of development of igneous and metamorphic processes at subducting plate boundaries. The application to the Archaean is still not universally accepted, but that working model helps interpret the rocks formed during the first half of the Earth's existence. The magmatism during those early times used to be thought to be no different from present-day ones, on the principle of uniformitarianism. In 1951 came the seminal work of Sutton and Watson on the Lewisian gneiss complexes and the mafic dyke swarms cutting them. This became the cornerstone on which gneiss and greenstone dyke complexes were interpreted, and later contributed to the concept of terranes. Pursuing this, Hall & Hughes narrate the magmatic differences that emerged, mainly as the result of the early high heat flow: the unstable komatiitic volcanic-dominated crust mainly in the Archaean; and then the onset of noritic magmatism and the concomitant crustal accretion super-event, as markers of the transition from the Archaean to the Proterozoic. Their contribution provides greater understanding of the evolution of Precambrian mafic magmatism and the formation of the Earth's early crust. The single great technique that enabled the mysteries of Precambrian metamorphic complexes to be unravelled, was the application in 1961 of R b - S r isotope systematics to age determination, by the Oxford school led by Moorbath. K - A r isotope studies had not been enough; too often they produced only thermally reset ages. Nowadays, any study of high-grade metamorphic rocks automatically includes isotope determinations, and the same now applies to igneous rocks. But the whole process has become very sophisticated, and Rogers & Pankhursl present a thorough analysis of the process as applied to Scotland. They show how the techniques were expanded to include the use of the isotopes of lead, uranium, samarium and neodymium, with great success but not without considerable controversy, much of it still running (e.g. Ben Vuirich). Another break-through in studies of the Earth's crust via the rocks of Scotland was the identification by W.Q. Kennedy in the 1940s of the extent of the Great Glen Fault. Here was a fault of apparent massive strike-slip displacement; hitherto faults had been mainly normal, reversed or thrust phenomena. Bluck analyses the extent of this and other major Scottish faults, and the implication is made that massive displacement along lines of fracture are possible through the Earth's crust. That such displacements could occur was an essential ingredient to the theory of sea-floor spreading and to the analysis of many tectonic basins and oil-bearing structures, even to creating space as sphenochasms for the permissive emplacement of granites. Another process which came to be understood through careful field work in Scotland was the identification by George Barrow 100 years ago of metamorphic zones in the

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M . J . LE BAS

Dalradian schists. It marked the birth of meaningful metamorphic petrology. Barrow's 1893 paper quickly became a classic and was much quoted, even though he mis-identified the heat source for the metamorphism; demonstrating that observation is more important than interpretation. Brown shows how a knowledge of mineral chemistry, textural and field relations which identify stability relations, together with a knowledge of the time relations deduced from age determinations, can produce paths across petrogenetic grids which give information on how pressure and temperature varied with time in different tectonic environments. Even within a single tectonic environment, more than one P - T - t path can be recognized. The next four papers are on stratigraphical classification and correlation, particularly using fossils, one of the main bases of our science and one whose terminology eventually becomes part of everyday language. McKerrow discusses Palaeozoic stratigraphy in general, beginning with the works of Sedgwick and Murchison, and then sets out a logical sequence of steps that might, perhaps should, be followed in developing the stratigraphy of an area, once the structural relations of strata within an area are appreciated. Fortey focuses on the contribution of Lapworth's work on graptolites in the Southern Uplands of Scotland, Riley compares Vaughan's and later work on Early Carboniferous bio- and chronostratigraphy, and Callomon, in reviewing Buckman's pioneering work on the Jurassic of Dorset, goes on to discuss the current limits of resolution attainable in biochronology. It is interesting to reflect on how views on some of the problems addressed and concepts advanced by our predecessors have changed over the years. For instance, Fortey shows how durable Lapworth's biostratigraphy (as we should now call it) has been, while ideas on structure and palaeogeography that he derived concurrently with it have changed almost beyond recognition within the scientific lifetimes of many still active in the field. As Callomon describes, Buckman's practice in collecting and recording fossils in the field, and his ideas on the prevalence of gaps in the rock record, were far in advance of their time, entirely compelling, and relevant to many current concerns in sedimentology and sequence stratigraphy. Yet his contribution has been under-recognized for many years, partly because his views on evolutionary palaeontology were based on theories now superceded, and in his own later practice he departed from his earlier standards. Savage relates the remarkable story of the discovery by Charles Moore of Mesozoic terrestrial vertebrate remains within ,:fissured Carboniferous Limestone, and how the search widened as more species were discovered. This most fortunate means of preservation provided an abundance of material as well as species, and this occurrence and others subsequent have given vertebrate palaeontology a foundation upon which much of our present-day understanding of evolutionary history is based. Even more remarkable is the story unfolded by Cocks of four different faunal assemblages within one set of strata. When fossiliferous quartzite pebbles were found in the Budleigh Salterton Pebble Bed of Triassic age along the Devonshire coast, Salter realized in 1864 that they were different from anything known in Britain but could correlate some of them with Lower Palaeozoic strata in Europe. Then Davidson in 1870 showed there were Devonian fossils as well, and modern work reveals there are four faunas, two

Ordovician and two Devonian, with tectonic reconstruction now explaining all in terms of adjacent palaeocontinents. Present-day geologists sometimes forget the importance of accurate description, going straight to interpretation often based on generalizations of assumed facts. Sorby was a 'quantifier' and had a particularly keen eye, creating meticulous drawings to accompany his precise writings. His President's Address on the study of sedimentary structures, published in 1908 after his death, is taken by Allen as the starting point for marrying description to interpretation. After detailed descriptions, he examines the current understanding of aeolian bedforms, sand-wave bedding, tidal bedding, marine storm bedding, hummocky and swaley cross-stratification, soft sediment deformation and dewatering structures, particularly reviewing the past ten years research into these. Sorby appears again in the next contribution. His 1879 President's Address on the structure and origin of limestone can still be read with pleasure and profit by any modern student. He was far ahead of his time, and virtually invented geological microscopy. From that starting point, Sellwood develops current ideas on limestone classification, their environment of deposition and diagenesis, and their significance in sequence stratigraphy. In the last quarter of the eighteenth century, controversy arose between Geikie and Judd on the interpretation of the Hebridean volcanic complexes of Scotland. Until then, most igneous rocks were regarded in isolation, but Geikie and Judd both realised that there was an association of rock types waiting to be interpreted. This opened the 'school of Hebridean petrology' which was so strongly developed by Harker, Bailey and others, and has flourished ever since. Walker investigates the association of flood basalts with volcanic centres and shows that the former have much more to tell us than most have hitherto supposed: evidence of whether the basalt lavas and dykes were fissure-fed or emanated from point-source vents, and the crustal tension implied; tilting of volcanic fields perhaps related to inflation or deflation of volcanic edifices; and direction of flow of magma. As volcanic hazards become a matter of daily concern, more needs to be learnt about the magmatic plumbing of volcanoes, and the magma flow direction of dykes and sills, now laid bare by erosion within old volcanic structures, could supply the vital data. The study of the anisotropy of magnetic susceptibility would seem to be potentially significant in this respect. In contrast to the structural and mineralogical approach taken by Walker, Wilson takes the geochemical approach, which is equally applicable to volcanic and plutonic rocks. One hundred years ago, Harker began a school of thought on the controls of crystallization applied to the differentiation of basic magma as exemplified by variations seen within individual plutonic masses. Nowadays, finegrained rocks are considered to be better representatives of variation in magma chemistry, and Wilson reviews the many possible processes from fractional crystallization, assimilation and magma mixing to thermogravitational diffusion and liquid immiscibility that can be responsible for magmatic differentiation. These processes concern mainly basaltic magmas which, by and large, are partial melt products of the upper mantle of the Earth. By contrast, most granites have their origin in the partial melting of the continental crust of the Earth. They are the

INTRODUCTION layman's best known coarsely crystalline rock. Most granites are superficially similar, but all are individually distinct when studied geochemically. Fifty years ago, the 'granite controversy' raged, with the Read school upholding the metamorphic and migmatitic association, and the TilleyBowen school maintaining that granites were the product of crystal fractionation from basic magmas. Atherton reviews the pros and cons of granitization, the 'room problem' for plutons, the evidence of melting experiments, the use of rock and mineral geochemical analytical data in determining the production of acid magma by partial melting or by fractional crystallization, and the manner in which isotopes may identify the source rock and the region. He takes the example of Garabal Hill in Scotland, which is one of the few Caledonian complexes including ultrabasic to acid igneous rocks, where crustal contamination might be argued. Also considered are thoughts on the association of basins, granites and thermal highs with high-T/low-P metamorphism. In the penultimate chapter, Sorby's observations on fluid inclusions published almost 140 years ago are shown by Rankin to have led directly to the present state of knowledge about the pressures and temperatures of fluids in rocks, especially igneous-related ones. Fluid inclusions tell us much about mineralization processes, fluids being the carriers of the ore components. Rankin analyses the contribution of current fluid inclusion studies to understanding the many mineralization processes, taking several classical examples from the UK and abroad, some related to igneous bodies and others to tectonically driven crustal circulation of fluids. He also includes some original thermometric data on the main British ore fields. Only recently has undoubted carbonatite been discovered in Britain (and published in the Journal of the Geological Society, 1994, 151, 945). These exotic igneous rocks confounded geologists early this century, who could not believe in igneous 'limestones'--an apparent contradiction. The Journal of the Geological Society has a long history of publishing papers on African geology, a product of the past colonial era. In 1956 Campbell-Smith presented his review of African carbonatites, coinciding with a similar review by Pecora in the USA, and the two changed world opinion. Geologists flocked to the 1960 International Geological Congress in Norway and Sweden and saw the Fen and Alno carbonatite complexes. Calcite carbonatite became an acceptable igneous phenomenon. CampbellSmith's review showed the igneous nature of carbonatites: their occurrence as cross-cutting dykes with fine-grained margins (i.e. chilled) and as small plugs with thermal contacts marked by alkali metasomatic reaction aureoles (fenitization). Their origin remains controversial with three main current theories: they were produced by fractional crystallization of nephelinitic magmas; they were separated by liquid immiscibility from a nephelinitic (or melilititic) magma; they were produced by direct partial melting of the upper mantle. Bailey examines the last of these, on the premise that many carbonatites are found without associated nephelinite/phonolite, and on the experimental evidence that dolomite carbonatite melts can be produced in the mantle under CO2-saturated conditions. This view is contrary to the powerful consensus that now exists: that carbonatites are essentially infracrustal differentiates of alkali silicate melts, to the extent that most modern

3

discussions of petrogenesis begin with this assumption. Bailey's chapter re-dresses this imbalance in the literature. The relation of dolomite carbonatites to calcite carbonatite remains uncertain; perhaps like 'granites and granites' there are 'carbonatites and carbonatites'. Their importance in understanding Earth history is undoubted because, having minimal partial melt compositions, they are potentially the best natural products to give clues to the chemical and thermal evolution of the Earth's mantle. Many more seminal papers could have been selected from the pages of the Journal of the Geological Society for essaying in this book. One which changed the character of the British geologist, is that by Howell Williams on 'The geology of Snowdon (North Wales)' published in 1927 (83, 346). Beginning with clear field descriptions, he explicitly interprets glowing avalanches, mass-flow epiclastic deposits, and proximal and distal water-lain tufts from rocks considered by many to be among the most difficult to interpret. Until this exposition, pyroclastic rocks had been by-passed by geologists in Britain, but this paper fired imaginations. The area described became a training ground, and has spawned several generations of geologists renowned for their pyroclastic expertise. On more traditional grounds is the 1938 President's Address by O.T. Jones 'On the evolution of a geosyncline' (94, lx). For the next 30 years, students pondered on geosynclines which were understood to be crustal downwarped structures filled with sediment, and in so doing opened the subject of conditions of sedimentation and the sources of the sediments. Distinct sedimentary basins were recognized. When 'plate tectonics' burst on the scene, the data accumulated supplied the vital items allowing reinterpretation of geosynclines as oceanic trenches. A paper which was to turn geological interpretation upside-down was Bob Shackleton's 1957 paper in the Journal (113, 361) on 'Downward-facing structures of the Highland Border'. These Scottish schistose rocks had been observed to be largely flat-lying but synformal near Aberfoyle. Shackleton's revelation that they were all upside-down led to the re-interpretation that the 'flat belt' of Loch Tay was the lower half of a nappe with the synform being the inverted anticlinal nose of the nappe, this structure extending across the whole of Scotland. Whereas many geological advances are made on a broad front of carefully documented data, here the break-through depended on a few astute observations of way-up criteria on a bleak mountain side. Some regard geophysics as beyond the bounds of normal geology. It is not. In 1906, Oldham presented 'The constitution of the interior of the Earth, as revealed by earthquakes' (62, 456), which foretold how geophysics would contribute to the fundamentals of geology, i.e. the constitution of the Earth's core, mantle and crust. He pointed out that the seismograph 'enables us to see into the Earth' and that the three wave motions observed allowed interpretation of a shell-structure of the Earth. Having defined the depth to the core-mantle boundary and shown the seismic 'shadow zone', he goes on to discuss the possibility of other discontinuities (now mostly confirmed). This in turn has led on to an explanation of the Earth's geomagnetic field, and to the constitutions of the Moon and planets. This was a truly seminal paper. In bringing all these topics together, it is hoped that

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M.J.

readers will find that the separate topics are not so unrelated; some topics need the others to sustain them, some merge into new topics, but all combined are essential to advancing the frontiers of science. The chapters also give hints on how this frontier may be further advanced. I thank most sincerely each of the authors of the chapters, for writing so assiduously to the briefs given them.

LE BAS I am grateful to the editors of the Journal of the Geological Society for guidance in the early stages of planning this volume and for editing the versions of the chapters published in volume 150 of the journal, and am particularly indebted to John Hudson who provided unstinting assistance at several critical stages.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 5-8 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 3-6

Historical origins of the Geological Society's Journal MARTIN

J. S. R U D W I C K

Science Studies Program, University o f California San Diego, La Jolla, CA 92093-0104, USA Transactions, its earliest periodical (from 1811), published the full texts of a few selected papers, with fine illustrations, but generally long after they had been read at one of the meetings. Conversely, the Proceedings (from 1826) recorded all the papers soon after they had been delivered, but only in abstract and without illustrations. The launching of the Quarterly Journal (1845) was an attempt to combine the advantages and eliminate the disadvantages of the earlier periodicals. After a shaky start, it proved highly successful through the rest of the nineteenth century and much of the twentieth, and was the direct forerunner of today's Journal. Abstract: The Geological Society's

The Geological Society was a publishing body even before it was founded. That paradox is easily explained. One of the reasons for its foundation was the desire of a group of London 'men of science' (the later term 'scientist' would be anachronistic and highly misleading in this context) to give permanent form to meetings that had been concerned with the publication of a specific scientific work. Another reason was the frustration felt by others at the inability of the Royal Society to provide an adequate publication outlet for geological work, and particularly for work that was highly factual in character and localized in content. Those two reasons for the foundation of the Society (there were others too) epitomize the two distinct kinds of publishing activities that have characterized learned societies ever since they proliferated in the eighteenth century. On the one hand there was, and still is, the need to publish the completed results of scientific research and thereby place them permanently on record. On the other hand there was, and still is, the need to inform those with particular interests about the current work of others with the same interests, whether the reasons for seeking such information are those of competition or collaboration or a mixture of the two. Scientific societies have tried to meet both needs through their own publishing activities. By its very nature, the detailed results of scientific research generally appeal to only a relatively small and specialized public, and are therefore often unattractive to ordinary commercial publishers. One solution to this problem that was widely adopted before the twentieth century, and not only for scientific works, was to appeal for subscribers to a particular book before the printing process began; the subscribers' advance payments guaranteed the publisher against loss, and any further sales made after publication could go towards a profit. An alternative solution, however, was for all members of a scientific body, who by definition were a specialized public with common interests, to receive a continuing series of shorter publications in return for a continuing subscription. In effect, members with a great interest in one particular subset of papers received copies of those papers, which might not otherwise have been published at all, in return for subsidizing the papers that were of great interest to other members. This was, and of course remains, part of the rationale behind the publication of any specialized scientific periodical, and the Geological Society's Journal was and is no exception.

At the same time, however, the specialized common interests of the members of any scientific society provide the opportunity for the exchange of opinions and conclusions, and often of course for vehement controversy; indeed the desire for such exchanges has been one of the most common reasons for founding such societies. But unless all the members meet regularly face-to-face, and even more if they are spread widely and unable to meet in that way, they have often felt a need for some kind of newsletter to keep them informed of the current activities of others. Again, this was, and remains, part of the rationale behind the publications of scientific societies, and again the Geological Society was and is no exception. Some of those who founded the Geological Society in 1807 were already subscribers to an important but costly publication. This was a three-volume monograph (1808) on the mineralogy and crystallography of calcium carbonate, by Jacques Louis, Count de Bournon, a French aristocrat who had fled to England from the Revolution in France. The work dated from before the profusion of crystal forms was explained satisfactorily in terms of a small number of types of symmetry and sets of crystal faces. In de Bournon's view, and that of his subscribers, his work required many expensive engraved plates, in order to reproduce a large number of detailed drawings of specific crystal specimens: in terms of illustration, crystallography was in the state in which palaeontology necessarily still remains. So the work was expensive, and could best be published by subscription. The subscribers were of course united by their common interest in research such as de Bournon's, and it was natural for them to regard themselves as a potential core for a permanent society to foster that kind of scientific work. Most of them, however, were already Fellows of the longestablished Royal Society, which had its own Philosophical Transactions for the publication of high-level scientific research (though not of book-length works such as de Bournon's). When, after the Geological Society was founded, its leaders began to talk about starting a periodical of its own, some of the members who were also FRSs were highly critical of that proposal; a few, including the Royal Society's autocratic president, Sir Joseph Banks, even resigned from the Geological Society and for a time put its future in jeopardy. In fact, however, the proposal had been for a periodical that would supplement, and not necessarily compete with, the Philo-

sophical Transactions.

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M A R T I N J. S. R U D W I C K

The concern of the leaders of the Geological Society was that the Royal would not, and perhaps could not be expected to, publish geological papers with highly detailed descriptions of mainly local interest. In other words they argued that there was a need for a more specialized periodical, to supplement the Royal Society's coverage of all the natural and mathematical sciences. But they also had in mind such Continental periodicals as the Parisian Journal des Mines (founded 1795), which published much practical material of interest to mining geologists as well as reports of more fundamental importance. The conflict within the early Geological Society reflected in part a difference of opinion as to whether it should model itself on a learned society such as the Royal Society, or make itself useful in a more practical way to the owners and managers of Britain's mineral resources. In the event, the former opinion triumphed, and the Society was not, after its earliest years, notably congenial to those whose interests were mainly practical or commercial, still less to those (such as the mineral surveyor William Smith) who did not share the wealth and social status of the Society's leaders. Even within the former model, however, there was in fact a clear precedent for the Geological Society's publication plans. This precedent, which had never aroused the hostility of Banks and the Royal Society, lay in the volumes of the Linnaean Society's Transactions (founded 1791), with their detailed and specialized papers on plant taxonomy. In any case, the Geological Society soon launched its own Transactions, modelled on those of both the Royal and the Linnaean. The quarto format was decidedly lavish, and clearly designed to match the gentlemanly tastes and pockets of the Society's members. The first volume (1811) was priced at £1 12s [£1.60], the second (1814) at £3; these were substantial sums. Added to the membership fee, which rapidly rose to £4 a year, an active gentlemanly interest in geology did not come cheaply. (As a rough-and-ready guide to real values, a n inflation factor of at least 50, and perhaps even 100, should be applied to these prices to make them comparable to modern prices.) Since the price of the Transactions was not included in the membership fee, only the more enthusiastic or more wealthy members bought the volumes, and the sales languished accordingly. As with de Bournon's book, a major expense was the illustrations. Engraving on copper was by far the best medium for the pictures, maps and other diagrams that geological papers required; but engraving was a highly skilled and timeconsuming craft, and correspondingly expensive. Furthermore, many geological illustrations required, or at least were greatly enhanced by, the use of colour. This could only be provided by applying watercolour washes by hand to every copy; and although this work was generally done by poorly paid female labour, it added further costs to the final plates. Still, the volumes were impressive, with handsome letterpress and fine illustrations. The Transactions helped to establish the scientific reputation of the Society, and of the selfconsciously new science of geology, both in Britain and abroad. But the periodical remained a medium of record rather than one for reporting work in progress. The intervals between successive volumes narrowed to about two years, as the Society became more established and the quantity of completed research increased; but there was still generally a long delay between the reading of a paper and its eventual publication. This was ill-suited to a science that was burgeoning rapidly into a major area of research internationally. Members of the Society could and did often seek alternative outlets for more rapid publication; but monthlies such as the Philosophical

Magazine, which at this period carried many geological papers, could not provide comparable illustrations, which were so important in geology. After the first decade, the Society took over the management of the Transactions from the commercial publishers who had handled it initially. A 'Second Series' was launched in 1822 to give the work a new look and to boost its sales. At the same time the opportunity was taken to adopt the new and cheaper technique of lithography in place of copper engraving. The price of the volumes was roughly halved, and authors could now be offered more space for their illustrations; an added bonus was that for most geological subjects (except perhaps maps) the more subtle tones of lithography were positively an advantage. Meanwhile, however, the Society had hardly taken any steps to improve the exchange of provisional ideas and ephemeral information, beyond the primary arena of its meeting room. An 'arena' is what its meetings had famously become: in contrast to the other learned societies in London, the Geological permitted discussion of the papers that had just been read. This was at first a cautious experiment, because there were those who feared it would lead to acrimonious argument; but it soon became an established and successful tradition of lively debate. Almost from its foundation, however, the Society had appealed for the collaboration of those living outside London. Its founders recognized that a geographical spread of the membership was even more valuable for geology than for many other sciences, since widely scattered members could report on local areas that they knew thoroughly. Such informants were enticed with the offer of free 'honorary' membership. But these provincial members could not get a first-hand impression of the current state of geological opinions in the Society, unless they were able to attend its meetings in person, on trips to London that for many of them were expensive, uncomfortable and therefore infrequent. The Society's very first publication, mooted almost immediately the Society was founded, and issued three years before the first volume of the Transactions, was in fact directed at these provincial members, and at those of the 'ordinary' or London members who found themselves travelling for any reason. The publication was a small booklet of 'Geological Inquiries' (1808), which listed the kinds of observations that could usefully be made, and the kinds of specimens collected, in more or less remote areas. It was probably inspired by, and partly based on, the famous 'Agenda' published in 1796 by the great Swiss naturalist Horace-B6n6dict de Saussure. Like that model, it was based on the belief that far more empirical information needed to be collected in the field, before it would be appropriate or profitable to indulge in high-level theoretical speculation about the structure or history of the earth. The Society's booklet certainly produced plenty of local information, most of it in the form of letters to the first President, George Bellas Greenough; in due course he incorporated much of it in his great geological map of England and Wales (1820). Together with the provincial members themselves, the 'Inquiries' gave the Society a network of local informants, so that its premises in London quickly became a centre of research material for the whole of Britain and beyond. However, this still did not give those informants much in return. In 1826, just 20 years after the foundation of the Society, a decision was taken to publish summaries of the papers that had been read, without waiting for their possible and eventual appearance in full in the Transactions. This marked the start of

H I S T O R I C A L O R I G I N S OF THE G E O L O G I C A L SOCIETY'S JOURNAL the Society's Proceedings, a publication that in effect complemented the older and grander periodical. The papers had been summarized in writing since soon after the Society was founded, but only in manuscript for its official minute books. From 1827 the summaries began to be printed and distributed to the Fellows (as they had been termed since the Society's formal incorporation in 1825). The Proceedings was published as a small octavo booklet about six times a year, during the Society's 'season' from November to June. Each issue contained summaries of the papers read at the most recent meetings, together with the names of new Fellows elected and other Society business. One issue each year was devoted to the business of the AGM, and also contained the president's 'Anniversary Address'. The latter had grown from a mere review of the Society's domestic affairs into a summary and assessment of all the papers read during the previous year. Some presidents expanded their survey beyond the Society, giving a major evaluation of the state of geological research nationally and even internationally, and often focusing on some particular aspect of the science. The Proceedings immediately became an important medium for the rapid exchange of news and views about geology in Britain. The periodical was not primarily designed to keep provincial Fellows informed, and indeed they were again at a disadvantage: in view of the high costs of postage, the newsletter (as it was in effect) was distributed only within London, and provincials did not receive it unless they could arrange for a friend in London, or their London club, to hold it or forward it for them. But in practice it was distributed and read widely beyond the capital. Furthermore, the summaries of papers could soon be read even by those who were not FGSs, because the general scientific monthlies took to reprinting them from the Proceedings. So any author who had his paper read at a meeting of the Society could be sure of having at least a summary in print, and widely read, within a month or two. By contrast, the authors of papers selected for publication in the Transactions (after a refereeing procedure much like that of the present) often had to wait a couple of years or more, before seeing their work fully in print and with its illustrations attached. As the volume of work presented at the Society's meetings grew, and its average quality improved, so the disadvantages of this two-track system of publication became more and more apparent. The Transactions languished again, as authors became impatient at the long publication delays; sales remained small, and the financial burden on the Society correspondingly great. Conversely, although the Proceedings provided rapid publication, it was at the cost of omitting the details, and particularly the illustrations, that would have given the papers most of their value and persuasive power. The effects of that dilemma can be seen in the successive issues of both periodicals. The number of papers published in the Transactions declined, in proportion to the number read, while the summaries published in the Proceedings became on average progressively longer. Even a few illustrations crept into the latter, as the Society began to adopt the technique of wood engraving. This was less effective for fine detail than copper engraving; but it was adequate for small maps and sections, it was much cheaper, and above all a wood engraving could be printed on the same page as the text to which it referred, rather than having to be bound separately at the end of the volume. In 1842, a substantial issue of the Transactions brought the problem to a head, because although it was a scientific success

7

it finally made the financial burden of the periodical almost intolerable. The following year the trend mentioned above was formally recognized, when the Society resolved to modify the format of the Proceedings to include much fuller summaries of the papers, with small illustrations on a regular basis. Even a few folding lithographed plates, of maps, sections and fossils, were included. But this palliative failed to yield the anticipated increase in sales. So in 1844 the Society tried another tack. The commercial publishers Longmans agreed to produce a new Quarterly Journal in octavo format, at their own risk and profit and for a trial period of one year. This was to incorporate the Proceedings, now extended to full texts of the papers, and fully illustrated with wood engravings and larger lithographed plates. A 'second, or miscellaneous part' would make the new periodical still more attractive, by reporting on recent geological books and other publications in Britain, and by printing abstracts or extracts, in translation, of significant work from abroad. The intention was that the Transactions would meanwhile continue 'when a paper could only be advantageously given in quarto'. The Quarterly Journal started to appear in 1845, but after the first year Longmans reported that they had made a loss on the venture and would not renew the agreement. In retrospect the reason for the failure is clear. The Society had allowed Fellows to continue to receive the Proceedings free, as they had always done, as an alternative to subscribing to the new quarterly (incorporating the Proceedings) at the commercial price. As the Society's centenary historian commented, many Fellows were evidently 'more concerned in appending F.G.S. to their names than in adding the Quarterly Journal to their bookshelves' (Woodward 1907, p. 157). However, the format of the new periodical was so attractive that its publication was continued at the Society's own expense and risk. Significantly, the Proceedings were no longer to be available separately; Fellows were now faced with an all-ornothing choice. Conversely, the Transactions virtually came to an end as soon as the Quarterly Journal began. Three small issues appeared in 1845~,6, printing papers that had been in the pipeline before the change was decided. By the time a final issue appeared a full decade later, the Transactions had clearly become redundant. The Society had thus decided, in effect, to adopt a compromise between the two earlier forms of periodical, between lavish but slow publication on the one hand, and quick but abbreviated publication on the other. As its name implied, the Quarterly Journal was published rather less frequently than the old Proceedings, but much more frequently than the Transactions. Like the former, it ensured reasonably quick publication; like the latter, what it published were the full texts of papers. Its octavo format made it look like the Proceedings; but it provided illustrations virtually as good as those in the Society's original periodical. They ranged from small wood engravings embedded in the pages of text, to substantial folding engraved plates of geological maps, some of them handcoloured, and lithographed plates of fossils and geological sections. Although initially regarded as an uneasy compromise, the Quarterly Journal proved to be a highly successful formula. It combined the advantages of both its predecessors, with just the right balance to satisfy most authors and most of their readers. In particular, it combined in an adequate manner the functions of both newsletter and medium of record. After the first few years its cost was absorbed into the Fellows' annual fee, so that its purchase became in effect a compulsory condition of

8

M A R T I N J. S. R U D W I C K

membership; that ensured a steady and predictable level of sales, which made it financially sustainable. The Quarterly Journal continued to serve as the Society's sole periodical throughout the rest of the nineteenth century and beyond the middle of the twentieth. The volumes became fatter, and the techniques of illustration were improved, or at least enlarged, by the adoption of photography for landscapes, rock exposures and fossils, and of chromolithography and other methods for coloured geological maps and sections. But the format remained almost unchanged until 1971, when the 'Quarterly' was dropped and the present Journal appeared in its place. Significantly, it has reverted to a larger format similar to the original Transactions, allowing for many larger illustrations to be included without the expense of fold-outs. Even before that change, the need for a separate newsletter had reemerged, for the quick publication of relatively ephemeral material; in that respect the modern Circular (Newsletter from 1972-1990), and its recent successor Geoscientist, represent a revival of one of the functions of the old original Proceedings. In conclusion, the Society's periodicals are now once more surprisingly similar, in form and function, to those of its earliest decades and first Golden Age.

Bibliographical note The system of references conventional in scientific papers is ill-suited to a historical article such as this. Readers who want to pursue this topic further will find that the following historical works ('secondary' sources, in historians' jargon) provide some starting points; they also give references to the contemporary ('primary') sources on which all historical research is properly, indeed necessarily, based. It should be noted that although the pace of research in the history of science is quite as intensive as in geology, historical books and articles generally enjoy a much longer useful life than those in the sciences. Woodward's centenary history (1907) of the Society is still a valuable source, since it prints much otherwise unpublished material from the Society's archives; but it is chaotically organized, and scarcely attempts any historical analysis or interpretation. My article on the foundation of the Society (Rudwick 1963) was based particularly on the manuscript papers of the Society's first president; a more recent analysis of

the Society's "prehistory' is by Weindling (1979). kaudan (1977) and Miller (1986) both analyse the micropolitics behind the Society's early emphasis on fact-gathering and its rejection of theorizing; Moore et al. (1991) describe its museum and early collecting activities. The present paper is, as far as I am aware, the only analysis, albeit a very brief one, of its early publications; my earlier review of the origins of what I termed the 'visual language' of geology (Rudwick 1976) discusses the importance of illustrations, and emphasizes the crucial role of the Society's publications in the establishment of a consensual practice that routinely combined maps, sections and other illustrations. Recent detailed analyses of two major geological controversies serve incidentally to demonstrate the role of the Society's publications in the concrete practice of geologists during the period covered in this paper: they are Secord's account (1986) of the famous arguments over the Cambrian and Silurian systems, and my account (Rudwick 1985) of the establishment of the Devonian.

References LAUDAN, R. 1977. Ideas and organizations in British geology: a case study in institutional history. Isis, 68, 527 538. MILLER, D. P. 1986. Method and the 'micropolitics' of science: the early years of the Geological and Astronomical Societies of London. In: SCHUSTER, J. A. & YEO, R. R. (eds) The politics and rhetoric of scientific method. Reidel. Dordrecht, 227-257. MOORE, D. T., THACKRAY, J. C. & MORGAN, D. L. 1991. A short history of the museum of the Geological Society of London, 1807-19l 1, with a catalogue of the British and Irish accessions, and notes on surviving collections. Bulletin of the British Museum (Natural History), Historical series, 19, 51-160. RUDWlCK, M. J. S. 1963. The foundation of the Geological Society of London: its scheme for cooperative research and its struggle for independence. British Journaljor the History of Science, 1, 325 355. -1976. The emergence of a visual language for geological science 1760-1840. Histoo, of science, 14, 149-195. - - - 1985. The great Devonian controvers:v. the shaping of scientific knowledge among gentlemanly specialists. University of Chicago Press, Chicago. SECORD, J. A. 1986. Controversy in Victorian geology: the Cambrian-Silurian dispute. Princeton University Press, Princeton. WHNDLING, P. J. 1979. Geological controversy and its historiography: the prehistory of the Geological Society of London. In: JORDANOVA, L. J. & PORTER, R. S. (eds) Images of the earth. British Society for the History of Science, Chalfont St Giles, 248-271. WOODWARD, HORACE B. 1907. The history of the Geological Society of London. Geological Society, London.

Received 18 August 1992; accepted 21 August 1992.

THE

QUARTERLY JOURNAL OF

THE

GEOLOGICAL SOCIETY OF LONDON,

EDITED

RY

THE VICE-SECRETARY OF THE GEOLOGICALSOCIETY.

VOLUME THE FIRST.

1845.

LONDON: LONGMAN, BROWN, GREEN, AND LONGMANS, PA.TERNOSTER-ROW.

MDCCCXLV.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 11-23 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 7-19

Uniformitarianism today: plate tectonics is the key to the past BRIAN

F.

WINDLEY

Department o f Geology, The University, Leicester LE1 7RH, UK

Abstract: James Hutton published the first two volumes of The Theory of the Earth in 1795 and the third volume was published posthumously by the Geological Society in 1899. Charles Lyell in his four addresses (1836, 1837, 1850, 1851) to the Society put the uniformitarian paradigm of Hutton (the present is the key to the past) into the perspective of his era. Uniformitarianism today can be expressed in the view that plate tectonics is the key to the past. This paper summarizes key data and ideas which confirm that the plate tectonic paradigm can be applied convincingly back to the beginning of the geological record. In spite of the fact that heat production was greater in the early Precambrian than now, tectonophysical and geochemical processes that produced oceanic and continental rocks since the early Archaean have not been fundamentally different from those that operate today.

'The Present is the key to the Past' was the uniformitarian paradigm of James Hutton (1788). He published the first two volumes of his book Theory of the Earth in 1795. In the conclusion of the second volume he said 'In pursuing this object I am next to examine facts with regards to the mineralogical part of the theory etc', but he never published his intended third volume before his death in 1797. The manuscript was passed via Playfair and Webb Seymour to Leonard Horner who gave it to the Geological Society in 1856, where it was re-discovered in 1895 by F.D. Adams (1938, p. 242). In 1899 the Geological Society published volume 3 of Theory of the Earth as a book edited and indexed by Archibald Geikie. The uniformitarian paradigm was promoted and developed by Charles Lyell in his Principles of Geology (1830 and in 10 subsequent editions). Lyell also presented to the Society two anniversary addresses in 1836 and 1837 and two presidential addresses (1850 on tectonics; 1851 on palaeontology), in which he addressed the question of how far the leading contemporary discoveries had confirmed the uniformitarian argument, namely: 'that the ancient changes of the animate and inanimate world, of which we find memorials in the earth's crust, may be similar both in kind and degree to those which are now in progress'. Also having coined the term 'metamorphism' in the third edition of his book, he took the opportunity to follow the development of 'his metamorphic theory' in 1850. The uniformitarian idea of Hutton and Lyell was an important progenitor of the way of thinking of many generations of geologists. Lyell was not concerned with the building of the history of a continent, so he started with the Recent and worked backwards to 'conduct us gradually from the known to the unknown' (Bailey 1962). Adopting a similar time procedure, the aim of this paper is to summarize key ideas and data that suggest that the plate tectonic paradigm can be applied back to the beginnings of the geological record.

constrained, modern analogues of pre-Mesozoic orogens. One or two decades ago there was not much information about mid-early Precambrian island arcs, accretionary prisms, oceanic plateaus, foreland basins, indentation and escape tectonics, ophiolites with sheeted dykes, suture zones, basic dykes intruded in failed arms and passive margins during continental break-up, the seismic pattern of the crust, and terrane accretion in the lower and upper crust. Kerr (1985) and Kr6ner (1984) concluded that modern-style plate tectonics began at 2 Ga and Meissner (1983) at 1 Ga. However, more recent advances in all the above fields now enable us to postulate reasonably that plate tectonics goes back to 4 Ga.

The Phanerozoic Accretionary and collisional orogens can be considered to be two ends of a spectrum of orogens (Murphy & Nance 1991). The former developed largely by the amalgamation of numerous island arcs, accretionary prisms and ophiolites, and they represent almost total crustal growth of juvenile material; Phanerozoic examples include the Kun Lun orogen in Central Asia ($eng6r & Okurogullari 1991), and incomplete, ongoing, accretionary orogens include the Japanese islands and the Cordillera of western North America. Collisional orogens formed largely by the abutment of one continental block against another, and represent little or no crustal growth; modern examples include the Swiss Alps and the central-eastern Himalayas. The important developments in Phanerozoic geology that are relevant to the uniformitarian argument will be considered in the Precambrian sections below where Phanerozoic analogues can be discussed in their appropriate context.

The Late Proterozoic (1.0-0.6 Ga) Current evidence sugggests that in the last 400 Ma of Proterozoic time, widespread terrane accretion and plate collision led to the formation of a supercontinent, which rifted and broke-up into separate continental blocks before the inception

The plate tectonic uniformitarian model Ideas about the origin of orogens and the continental crust are evolving fast, and this information now provides us with better 11

12

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WINDLEY

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SUPERIOR

Penokean Yavapai

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1.9-1.8GaJuvenilcrust e 1.8-1.7GaJuvenilcrust e 1.7-1.6GaJuvenilcrust e

of the Phanerozoic. Most prominent are the many orogens grouped within the terms Pan-African, Cadomian and Avalonian. The Pan-African includes the Arabian-Nubian Shield, the Mozambique belt and the Damaran orogen.

A ccretionary orogens The Arabian-Nubian Shield. This is an assemblage of accreted island arcs, ophiolitic belts, and probable microcontinents and oceanic plateaus, and thus provides good evidence of processes of lateral crustal growth and modern-type obduction-accretion tectonics (Kr6ner 1985; Stoeser & Camp 1985; Windley in press a). Disrupted ophiolites occur in linear belts up to 900 km long defining sutures between island arcs and microplates (Kr6ner 1985; Pallister et al. 1988). Some ophiolites contain a complete (Penrose definition) succession (Shanti & Roobol 1979). In Arabia in addition to the island arcs there are remnants of pre-Pan African (i.e. > 1.0 Ga) microcontinents and possibly oceanic plateaus, whereas in Egypt and Sudan the deformed passive continental margin of the Mozambique belt was partly transformed into an active margin along which there are ophiolites and inter-thrust arc volcanic rocks (Kr6ner 1985). In the Shield, there are three ages of island arcs that are very similar to modern arcs formed at sites of plate convergence (Stoeser & Camp 1985). (1) The earliest are chemically immature bimodal suites of low-K tholeiites and sodic dacites/rhyolites depleted in lithophile elements. After deformation, they were intruded by plutons of diorite and trondhjemite at 910 Ma. The lavas have chemical characteristics similar to immature island arcs such as the Tonga-Kermadec and Lesser Antilles arcs.

Fig. 1. Map of Laurentia showing the distribution of Early Proterozoic collisional orogens in the north of the Baltic Shield and three belts of Early Proterozoic accretionary orogens that extend from W. USA to Finland. Modified after Hoffman (1989). MK, Makkovik; KL, Killarney.

(2) Younger lavas are predominantly calc-alkaline and low-K arc tholeiites, andesites, dacites and tufts which were intruded by granitic batholiths dated at 816 Ma and 743 Ma. These are similar to more mature, partly emergent, intraoceanic island arcs in the western Pacific. (3) The youngest voluminous lavas have calc-alkaline or high-K, calc-alkaline compositions with moderately high lithophile element abundances; they are comparable to volcanic arcs as in Central America and Indonesia which are transitional between island arcs and continental margin volcanic arcs.

Collisional orogens The Mozambique belt. This complicated high-grade and highly deformed orogen in East Africa is still understood only in reconnaissance outline. Shackleton (1986) suggested that widespread thrusts, nappes and high-grade metamorphism imply crustal thickening as a result of continent-continent collision tectonics, and Burke & Seng6r (1986) proposed that the belt was the site of a Tibetan-style continental collision. Berhe (1990) described many ophiolitic remnants in deep crustal gneisses. The most detailed, recent work in the Mozambique belt was by Key et al. (1989) in Kenya who concluded from considerable field and geochronological results that the belt represents a deep crustal section through a Pan-African continent-continent collision zone. Orogens surrounding the West African craton. This Precambrian craton is surrounded by Pan-African sutures, arcs and collisional orogens. In Morocco there is a complete ophiolite at Bou Azzer dated at 788 Ma that is overlain by an island arc

U N I F O R M I T A R I A N I S M : PLATE TECTONICS consisting of calc-alkaline lavas and diorites (Bodinier et al. 1984). Many ophiolites, accretionary m61anges and fore-arcs occur as dismembered slivers on a suture between the craton and the island arc (Saquaque et al. 1989). In the Sahara on the east side of the craton in the central Hoggar, there is a collisional orogen that retains evidence of a complete Wilson Cycle spanning the period 900-550 Ma (Caby et al. 1981).

The Mid-Proterozoic (1.6-1.0 Ga) During the mid-Proterozoic a number of orogens formed, the best-known of which is the Grenville in North America (Davidson 1986) that was preceded by its genetically-related period of so-called anorogenic magmatism (Windley 1993). The Grenvillian Wilson Cycle started with prominent 1.481.43 Ga anorogenic magmatism in Canada, especially anorthosites, and in the central/southern USA, mostly rhyolitic ashfall tufts and peraluminous granites (Van Schmus et al. 1987). This magmatism most likely developed on the continental margin of the Grenvillian Ocean; modern analogues border the Atlantic Ocean (Kay et al. 1989; Windley 1993). Closure of the ocean by subduction is indicated by the 1.28-1.25 Ga island arc of the Central Metasedimentary Belt of Ontario, and by an island arc associated with an incomplete ophiolite in Texas which was thrust northwestwards onto a foreland and shelf (Garrison 1981). Collision of the Belt with adjacent continental blocks gave rise to the 1.25-1.22 Ga Elsevirian orogeny and the 1.12-1.03 Ga Ottawan orogeny. The result of these orogenies was the formation of the collisional Grenville orogen, which consists of several major inter-thrusted terranes (Rivers et al. 1989) bounded by sutures that can be recognized on COCORP deep seismic profiles (Culotta et al. 1990), and which shares some fundamental similarities with the Himalayas (Windley 1986). The northwestdirected deformation caused by the terminal Ottawan orogeny fractured the foreland giving rise to the 1.1 Ga Keweenawan rift, that in origin is comparable to the Rhine graben caused by the Tertiary Alpine deformation in Europe.

The Early Proterozoic (2.5-1.6 Ga) No significant orogens formed from 2.5 Ga to 2.1 Ga (a supercontinent?), but from 2.1 Ga to 1.6 Ga many orogens did form of both accretionary and collisional type. A ccretionary

orogens

Early Proterozoic 'growth' orogens, include: 1.7-1.6 Ga Mazatzal (North America) 1.8-1.7 Ga Killarney, Central Plains and Yavapai (all in North America) 1.9-1.8 Ga Svecofennian (Baltic Shield), Ketilidian (Greenland), Makkovik and Penokean (North America) 2.1 Ga Birimian (West Africa). Except for the Birimian, all the above orogens belong to a mega-orogen that extends across what is now North America and Europe and which youngs southwards (Fig. 1). Just key examples will be discussed. The Svecofennian. Extending from Central Sweden and Finland southwards to the Tornquist Line in Poland, this 1200 km wide orogen developed by the accretion of 1.9 Ga island arcs (Park 1991) and accretionary prisms, and by extensive crustal melting in the period 1.8-1.55 Ga. Extensive isotopic data in-

13

dicate that it contains no Archaean material (Huhma 1987; Patchett et al. 1987; Romer 1991). Extending along its northern margin with the KolaKarelian orogen to the north, the Lule&-Kuopio suture zone contains ophiolitic lenses. The 1.96 Ga (U-Pb) Jormua ophiolite with sheeted dykes was thrust about 30 km onto the northern continental margin (Kontinen 1987). Within the Svecofennian orogen there are several island arcs, whose lavas are chemically comparable with modern calc-alkaline arc lavas (Pharaoh & Brewer 1990). U-Pb zircon data indicate that many of the arc lavas were erupted in the short period of 1.92-1.87 Ga contemporaneously with the intrusion of innumerable 1.91-1.86 Ga, subduction-derived granitic plutons (Nurmi & Haapala 1986). Between many of the Svecofennian arcs there are biotitebearing granitic gneisses and schists which, because of chemical similarities, have been widely regarded as metagreywackes and metapelites, and which were most likely derived from accretionary prisms. Thrusting and folding was associated with high amphibolite facies metamorphism that locally reached granulite grade. Crustal thickening led to the formation of three types of crustal melt granites, the last of which were 1.7-1.55 Ga rapakivi granites and coeval gabbros, anorthosites and basic dykes (Haapala & R~im6 1990). These formed as a result of the internal slow heating of the thickened crust, its final extension and collapse, and thus to decompression melting of the mantle and melting of depleted granulitic lower crust (Windley in press a). The Ketilidian. This orogen in South Greenland (Allaart 1976) is an incomplete segment of an Early Proterozoic accretionary orogen which contains an Andean-type batholith (Fig. 2; Windley 1991 & references therein). A northern foreland of Archaean gneisses is overlain unconformably by a shelf-foredeep succession deposited by turbidity currents into basins on the deepening shelf, a 30 m thick sulphide-facies iron formation (chert-pyrite-shale) similar to that which commonly occurs on the outer ramp of Early Proterozoic foredeeps, and tholeiitic pillow lavas and basicfelsic pyroclastics, like those in the axial zones of other Early Proterozoic foredeeps (Hoffman 1987). The above succession has been thrust northwards over the foreland and back-thrust near the suture, where it and the basement thrusted gneisses are intruded by several 1.775-1.675 Ga granites that contain appreciable crustal-melt components (Kalsbeek & Taylor 1985). These relations are comparable to those that occur in the deformed foreland of modern collisional orogens such as the Himalayas. The Kobberminebugt suture is a 15 km wide vertical shear zone that contains relict greenschist-grade pillow lavas and gabbros, copper and gold mineralization, and late 100 m thick mylonite zones. The Julianehaab batholith is a 80-100 km wide Andean-type tonalitic-granodioritic batholith that contains relicts of pillow lavas, pyroclastic rocks and extensive noritic gabbros (Allaart 1976) that probably belong to an early island arc into which the major calc-alkaline batholith was intruded (Windley 1991). The arc rocks are similar to those in the Kohistan arc in the Himalayas of North Pakistan, the lower part (magma chamber) of which is occupied by the Chilas complex of noritic gabbros (Khan et al. 1989). The southernmost part of the Ketilidian orogen consists largely of metamorphosed, accretionary prism-type, supracrustal rocks that were deformed in three sub-horizontal thrust nappes and metamorphosed at 1.8 Ga. The thrust slab was

14

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KETILIDIAN OROGEN

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intruded by post-tectonic 1.755-1.74 Ga rapakivi granites. The emplacement of such granites within 60 Ma of the peak of regional metamorphism and associated thrusting is consistent with the time lag caused by slow thermal relaxation heating (Sonder et al. 1987; Dewey 1988), between the last thrusting during crustal thickening, and the intrusion of crust-mantle melts in extensional zones in a collapsing crust (Windley 1991). The Birimian. The 2.1 Ga Birimian orogen in West Africa extends for about 1600 km across strike. It consists predominantly of greenschist-grade mafic lavas and tufts, volcanodetrital argillites and turbiditic wackes, and all were intruded by post-orogenic leucogranites. Sm-Nd isotopic data by Abouchami et al. (1990) indicate that the sediments are free of any Archaean or older recycled components, suggesting that they formed in ocean basins far from any continental influence, and they confirm contemporaneity of the Birimian sediments and volcanic rocks. Abouchami et al. (1990) found that the trace element signatures of the volcanic rocks are most comparable to those of basalts in modern oceanic plateaus and thus proposed that this is a very extensive accretionary orogen that formed in a short time around 2.1 Ga from juvenile, mantlederived material. Collisional orogens The Kola-Karelian. This orogen occupies the northern part of the Baltic Shield (Fig.l). It contains five Archaean terranes. The Murmansk and Inari terranes consist of high-grade gneisses, whereas the S6rvaranger, Belomorian and Karelian terranes are composite, consisting of both low-grade greenstone belts and high-grade gneisses. In the period 2.0-1.9 Ga, these Archaean terranes collided and were amalgamated to form the Kola-Karelian orogen (Windley 1991, in press a, b). Early Proterozoic (2.4-1.9 Ga) rocks and structures added to these terranes include island arcs, Andean-type magmatic arcs, sutures and remnant shelf successions. The Early Proterozoic

50 km i

Fig. 2. Map of the Early Proterozoic Ketilidian orogen in South Greenland showing the position of shelf sediments in the foreland, the suture zone, Andean-type batholith, and crustal-melt rapakivi granites in a thrust-thickened nappe stack in the south. After Windley (1991).

structure of this orogen is well constrained by geophysical data (Gafil et al. 1989; Marker 1989). The Kola suture zone is a south-dipping thrust zone up to 40 km wide that has placed the Inari terrane against and over the S6rvaranger terrane (Berthelsen & Marker 1986; Marker 1989). The borders of the suture zone are marked by mylonites and it contains at least two thrust-bound slices made up of the 2.4-2.0 Ga Pechenga Series that contains sediments from the rifted continental margin, shelf-rise transition, and trench and tholeiitic basalts with R E E characteristics resembling those of MORB. On the south side of the suture zone there is a thrustbound, greenschist-grade Early Proterozoic island arc sequence that consists of weakly deformed abundant andesites, basaltic pillow lavas, minor komatiitic lavas, tufts and sulphide-bearing carbonaceous pelites (Berthelsen & Marker 1986; Gafil et al. 1989). A further result of the southward subduction that gave rise to the island arc was emplacement of an Andean-type magmatic arc represented by 1.95-1.9 Ga calcalkaline plutons into the northern border of the Inari terrane (Barbey et al. 1984). The South Kola belt, containing Lapland granulites and gneisses, was metamorphosed at 1.9-2.0 Ga; its turbiditic precursors were possibly deposited in a back-arch basin (Berthelsen & Marker 1986). The Wopmay. Wopmay is a 1.95-1.84 Ga orogen in N W Canada (Hoffman & Bowring 1984) that developed as a result of the collision between the Archaean Slave Province and an unknown Nahanni continental block to the west; a small island arc, the Hottah (that was built offshore on a 2.3-2.1 Ga crust) was trapped between the colliding blocks (Hoffman 1989). The western rifted margin of the Slave Province is overlain by shelf-rise sediments of the westward-facing Coronation Supergroup and succeeded by an eastward-migrating foredeep that formed in a late thin-skinned thrust-fold belt. The shelf began to collapse at 1.97 Ga, and collided at 1.91-1.90 Ga with the 1.95-1.91 Ga Hottah arc as a result of westward subduction below the arc. A new dextral-oblique, east-dipping subduction zone developed on the west side of the accreted arc and led to

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generation of the 1.88-1.86 Ga Great Bear calc-alkaline batholith, partly on top of the Hottah arc and partly on the deformed continental margin to the east. 1.86-1.84 Ga syenogranites were generated from the metasediments of the deformed shelf-rise. Terminal collision at about 1.8 Ga of the Nahanni terrane in the west led to formation of the postulated Johnny Hoe suture, indicated by gravity and magnetic highs.

Proterozoic plate tectonics Current data suggest that the types of orogens that formed throughout the Proterozoic are fundamentally similar to those of the Phanerozoic. In particular, the two end-member types, accretionary and collisional, can be readily recognized back to the early Proterozoic. There may have been a supercontinent in the midProterozoic. The fact that platform carbonates and quartzites were common after, but not immediately before, 1.5 Ga suggests widespread transgressions such as would be expected at a time of continental break-up (Nance et al. 1986). Also Hoffman (1989) pointed out that most orogens in North America formed between 1.98 and 1.65 Ga and that they led to the formation of a supercontinent by 1.5 Ga. The assembly and fragmentation of a mid-Proterozoic supercontinent would provide an ideal framework to explain the long-problematic mid-Proterozoic anorogenic magmatism. Windley (1993) proposed that there were two main periods of formation of such anorogenic rocks that were related to the formation of adjacent orogens. 1.76-1.55 Ga rapakivi granites and rhyolites formed about 60-200 Ma after the last deformation in the Svecofennian, Ketilidian and Penokean orogens, the time-lag being caused by the slow thermal relaxation of thickened lower crust that had been commonly

Fig. 3. Early Proterozoic (2.4 1.85 Ga) mafic dykes and their coeval, related volcanic belts around the perimeter of the Superior Province. Inset shows the suggested relations between the position of ocean openings and the dykes in failed arms and passive margins. After Fahrig (1987).

depleted by the extraction of earlier granitic crustal melts. In contrast, 1.45-1.41 Ga anorthosites in eastern Canada and rhyolitic ash fall tufts and peraluminous granites in central/ southern USA formed in the continental margin of the Grenvillian ocean, modern analogues being found on the borders of the present-day Atlantic Ocean. Thus these anorogenic magmatic rocks formed in the extensional regimes during the formation and the break-up of the 1.5 Ga supercontinent. Mafic dykes are typically intruded into a stable craton in the early stages of continental break-up associated with the formation of an ocean. After the closure of the ocean by plate subduction many of the dyke swarms may be preserved in the foreland of resultant orogens. Fahrig (1987) showed that in North America the 2.4-1.85 Ga Molson, Marathon and Mistassini swarms (Fig. 3) and the 1.2 Ga Mackenzie and Sudbury swarms are all orientated at high angles to, and commonly radiate from, their parent plate boundaries and are related to coeval volcanic belts along those boundaries. This suggests that these swarms occupy failed-arm environments and formed during early spreading. The Payne River dykes of Labrador are orientated parallel to their original passive margin (Fig. 3). The existence of Proterozoic sutures between originally allochthonous continental blocks has long been suggested on the basis of geological and geophysical data (Burke et al. 1977; Fountain & Salisbury 1981). Several sutures in the Canadian Shield (Fig. 4a) show paired negative and positive gravity anomalies (Fig. 4b). G i b b e t al. (1983) and Gibb & Thomas (1976) proposed that the positive anomaly is related to an increased density and thickness of the younger block, and that the negative anomaly is an expression of the increase in crustal thickness of the older block towards the suture. Re-evaluating these data in the light of the more recently established correlation between the age of continental lithosphere at the time of

16

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the loading in mountain belts and its flexural rigidity (Karner & Watts 1983), Pilkington (1990) demonstrated that the negative anomaly may be the result of flexure of the older lithosphere as it is thrust under the younger, more buoyant block, and that the positive anomaly is related to the presence of a subsurface load, indicated by the absence of any visual correlation between topography and observed gravity. Figure 4c shows that the sutures of the Grenville Front (1.1 Ga), Labrador Trough (1.58 Ga), Cape Smith belt (c. 1.8 Ga) and Thelon Front (¢. 2.0 Ga) demonstrate a systematic increase in lithospheric rigidity with age at the time of suturing and loading, and that these relations are in agreement with comparable data for the Alps, Himalayas and Appalachians and oceanic lithosphere. Sutures contain two diagnostic rock groups: blueschists and ophiolites.

Ophiolites. There are many ophiolites in the Pan-African of the Arabian-Nubian shield. Beyond Africa there are three important and well-dated early Proterozoic ophiolites, all of which have sheeted dykes: the 1.96 Ga Jormua ophiolite was thrust onto the shelf of the Svecofennian orogen (Kontinen 1987); the 1.9 Ga Purtuniq ophiolite in Canada obducted onto the foreland of the Cape Smith belt (St Onge et al. 1989); the 1.73 Ga Payson ophiolite in Arizona developed on a 1.76-1.75 Ga magmatic arc (Dann 1991). There are innumerable other geological data and relations which support the idea that modern-style plate processes were in operation throughout the Proterozoic, but the above examples suffice to make the point.

Blueschists. Late Proterozoic examples occur in the Mona Complex on Anglesey (560-550 Ma; Dallmeyer & Gibbons 1987), in the Aksu Group of Xinjiang Province of W. China (698-718 Ma; Nakajima et al. 1990), the Hoggar-Iforas orogen in the central Sahara (undated; Caby et al. 1981), and the Delhi orogen in Rajasthan, NW India (undated; Sinha-Roy & Mohanty 1988).

The Archaean regions of the world contain two types of terrane: low-grade, volcanic-dominated greenstone-granite terranes that formed in the Archaean upper crust, and terranes dominated by high-grade granulites and gneisses that represent the Archaean mid-lower crust. Some regions contain both types and their mutual relationships are particularly important.

The Archaean (4.0-2.5 Ga)

U N I F O R M I T A R I A N I S M : PLATE TECTONICS Greenslone-granite

17

terranes

Archaean greenstone-granite terranes contain the oldest major belts of well-preserved volcano-sedimentary rocks, and so they give us much direct evidence of early crustal conditions. They vary in age from c. 3.6 Ga to 2.5 Ga. The volcanic rocks typically consist of a calc-alkaline basalt-andesite-daciterhyolite association, with basaltic and ultramafic komatiitic lavas as a minor but important component. Andesites make up 30% of Canadian greenstone belts, but form a far smaller proportion of belts in southern Africa and Australia. Rare alkaline to shoshonitic volcanic rocks are like those in modern arcs. Most greenstone volcanic rocks show depletion in Nb, Ta and Ti relative to the rare earth and the large ion lithophile elements, this being a geochemical signature of subductionderived igneous rocks. All the features described above are very similar to those in modern immature to mature island arcs (Condie 1989). Intrusive diorite-granodiorite-granite plutons are coeval with the volcanic rocks; they mostly have arc-type chemical characteristics. There is now general agreement that most greenstone belts formed as a result of seafloor spreading followed by obduction or subduction-accretion processes associated with island arcs (de Wit et al. 1987; Hoffman 1990; Taira et al., in press). Possible modern analogues include obducted slabs of ocean floor (de Wit et al. 1987, 1992), island arcs built on oceanic crust (Sylvester et al. 1987), segments of arcs ranging from forearcs to closed backarc basins (Ludden et al. 1986), intra-arc basins, collages of disparate arcs, arcs thrust onto continental crust (Spray 1985), volcanic arcs and pull-apart basins developed on an active continental margin (Thurston & Chivers 1990), and accretionary prisms that seal amalgamated arcs (Hoffman 1990; Percival & Williams 1989; Kusky 1990). Although not yet widely recognized, there were probably many oceanic plateaus in the Archaean. Kusky & Kidd (1992) suggested that in the Belingwe region of Zimbabwe there are major thrusts between the three groups of greenstone belts. In particular, a major detachment separates underlying gneisses from a 2.7 Ga allochthonous block (the Mberengwa allochthon) which contains 6.5 km of lavas including abundant basaltic and peridotitic komatiites, and which therefore they interpreted as a fragment of an accreted oceanic plateau. They went on to suggest that many of the other stratigraphically comparable greenstone belts in the Zimbabwean craton may be dismembered fragments of this large oceanic plateau. The Superior Province of Canada is composed of several 2.7-2.75 Ga subparallel greenstone belts (Fig. 5) of contrasting lithology, age and metamorphic grade that are very similar to modern arc collision zones. There are two main types (Hoffman 1989; Card 1990): (1) volcanic-plutonic terranes, which appear to be composites of several island arcs; (2) metasedimentary belts which resemble accretionary prisms. Subvolcanic complexes in the Abitibi Belt range from pyroxenite cumulates to gabbros (Raudsepp & Ayres 1982), and hornblende-bearing gabbros to anorthosites (Ashwal et al. 1983; Morrison et al. 1986). These appear to be intrusions derived from the magma chamber in deep parts of island arcs; they are comparable to the Jurassic Border Range complex in Alaska (Burns 1985), and the Chilas complex in the Cretaceous Kohistan Arc in the Himalayas of Pakistan (Khan et al. 1989). Considerable isotopic data indicate the major arc terranes get younger southwards; the main terminal orogeny occurred about 2.725 Ga in the Uchi-Sachigo terranes, about 2.705 Ga in the Wabigoon terrane, and about 2.695 Ga in the Wawa-

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Abitibi terrane (Card 1990). Thus the arc terranes were assembled progressively from north to south before finally colliding with the Minnesota foreland in the south, as illustrated in Fig. 5. The Slave Province of Canada comprises two fundamentally different tectonostratigraphic terranes that collided at c. 2.6 Ga; an older gneissic microcontinent in the west and a composite greenstone-granite terrane containing a paired island arc and accretionary prism in the east (Kusky 1990). Most important is an ophiolitic complex (Helmstaedt et al. 1986). Although it is much thicker (11 km) than a typical equivalent section of Phanerozoic ophiolites, there is an obvious resemblance of the whole sequence, and in particular of the sheeted dyke complex, to a modern ophiolite. The implication is that mid-oceanic accretionary processes were active in the formation of the greenstone belts of the Slave Province, and this in turn implies that subduction and collision processes were also in operation in the Archaean. These greenstone belts are the remnants of a trench accretionary complex of juxtaposed island arcs and other crustal bathymetric highs such as fracture zones, seamount chains and oceanic plateaus, delaminated from subducting oceanic lithosphere and overlain by trench turbidites. Subsequently the foreshortened accretion-

18

B . F . WINDLEY

ary complex was extensively intruded by crust- and mantlederived plutons of the prograding magmatic arc (Hoffman 1990). Storey et al. (1991) pointed out that Archean komatiites are chemically comparable to the Tertiary komatiites of Gorgona Island off`the coast of Columbia. Peridotitic komatiites ( > 18% MgO), which have an eruption temperature greater than 1650 °C, require a very high degree of melting (50-80%) of the mantle. This fact may be explained by shallow depths of melting, which may be consistent with expected high rates of heat flow in the Archaean that were concentrated in mantle plumes that may have facilitated the formation of many oceanic plateaus, like the Kaapvaal craton. Similarities between Archaean arc volcanoes in Canada and modern arc volcanoes include (Ayres & Thurston 1985): (a) an upward change from basaltic to calc-alkaline volcanism, and an accompanying increase in the proportion of tufts and volcaniclastic rocks, reflecting a progressive upward chemical trend in the evolution of the volcanoes; (b) subduction signature of trace elements such as a negative Nb anomaly; (c) a gradual emergence of volcanic islands from submarine to subaerial, and the tectonic alignment of these islands. The differences include: (i) very magnesian peridotitic komatiites do not exist in modern arcs; (ii) there are less andesites, more rhyolites and more bimodal volcanic suites in Archaean volcanics; (iii) alkaline shoshonitic volcanics are uncommon in the Archaean; (iv) more rapid eruption rates of Archaean volcanoes during their subaqueous, komatiitic and tholeiitic basaltic phase resulted in a higher incidence of non-pillowed sheet flows, thicker flows, and lava plains; (v) development of larger, longer-lived, zoned magma chambers during the later felsic stages of Archaean volcanism. Granulite-gneiss terranes

These terranes have undergone deep crustal metamorphism that ranged from amphibolite to granulite facies. Harley (1989) pointed out that Archaean granulites formed within a wide range of pressures (6-12 kbar) and temperatures (750980 °C) and that some have near-isothermal decompression P - T paths (S India; Aldan Shield, Scourian, NW Scotland; Limpopo belt), and others have near-isobaric cooling paths (W Greenland; Napier Complex, Antarctica; Pikwitonei, Canada). He suggested that the decompression granulites formed in crust thickened by collision, with magmatic additions that were commonly calc-alkaline tonalites, and that the isothermal paths were generated during rapid thinning (1-2ram a -1 exposure) related to tectonic exhumation during moderate or waning extension. In contrast, the deeplevel isobaric cooling granulites formed in thickened crust which underwent very rapid (5mm a-l) extensional thinning subsequent to collision. These conclusions confirm structural relations that indicate substantial tectonic intercalation of rock units by thrusting and of massive injection of tonalites in many regions. For example in West Greenland the long history of thrusting since c. 3.8 Ga culminated in the juxtaposition of four distinctive thrust-slabs or gneissic terranes at 2.75-2.55 Ga (Friend et al. 1987; Nutman et al. 1989). Because the granulites today are still underlain by some 30-35 km of continental crust, it can be reasoned that the orogenic belts must have reached by the end of the Archaean a crustal thickness of some 60-75 km, comparable to that of the modern Himalayas and Tibet.

The deep crustal levels of Archaean terranes are obviously more difficult to unravel in terms of modern tectonic environments, but Wedepohl et al. (1991) found no systematic changes in chemical composition with age of early to late Archaean granitoid rocks from West Greenland, and concluded that typical modern processes of crust formation started to work early in the Archean. In terranes of the Superior Province of North America, granulite metamorphism formed in three distinct tectonic environments (Percival 1989): (1) the Minnesota River Valley terrane was metamorphosed at modest pressure (4.5-6.5 kbar) in a continental collision zone--it was the continental foreland that collided with the collage of accreted arcs (greenstone belts) to the north; (2) the Kapuskasing and Pikwitonei terranes formed at 7.5-9 kbar in the roots of the island arcs of the greenstone belts; (3) metasedimentary belts between composite island arcs which represent accretionary prisms that were tectonically thickened and metamorphosed during uplift at 4.5-6.5 kbar.

Archaean plate tectonics The late Archaean (2.9-2.7 Ga) greenstone belts of the Superior and Slave Provinces of Canada that formed largely by the amalgamation of island arcs and accretionary prisms are comparable to Proterozoic arc-accretionary orogens like the Pan-African of the ArabianNubian Shield and the Japanese islands today (Taira et al. in press). The late Archaean (3.1-2.6 Ga) Kaapvaal craton evolved by formation of Pacific-type continental margin orogens and Himalayan-type collisional tectonics (Table 1; de Wit et al. 1992). If the late Archaean (3.1-2.55 Ga) craton of W Greenland is the exposed deep level of a Tibetantype plateau, then there are few fundamental tectonic differences between these late Archaean arc-accretionary and collisional orogens and modern orogens. De Wit et al. (1992) described the evolution of the Kaapvaal craton of South Africa in terms of a two-stage formation of an Archaean continent from 3.7 Ga to 2.6 Ga. During the first stage (3.7-3.1 Ga), dominant intra-oceanic processes similar to those operating along mid-oceanic ridges caused separation of continental lithosphere from the mantle and formation of an oceanic plateau comparable to the Ontong-Java plateau. During this stage the 3.5 Ga mafic-ultramafic Barberton greenstone belt was obducted onto volcanic arc-like rocks. Amalgamation of oceanic plateaus/terranes by subduction/accretion processes like those occurring today along oceanic convergent margins gave rise to the Kaapvaal shield by 3.1 Ga (Table 1). According to Matthews (1990) the 2.94 Ga Pongola Supergroup was deposited partly on the passive continental margin of the newly created Kaapvaal shield and partly in an aulacogen extending into it. The second stage (3.1-2.6 Ga) of de Wit et al. (1992) records the accretion of crustal fragments by Cordilleran-type subduction/accretion processes, the formation of intermontane riffs and foreland basins, and finally Himalayan- and Tibetan-type continent-continent collision between the Kaapvaal and Zimbabwe cratons and consequent formation of the Limpopo orogen at 2.68 Ga (Treloar et al. 1992). Figure 6 shows that the Limpopo belt has a symmetrical thrust structure, which is similar to that of many Phanerozoic collisional orogens. Burke et al. (1985) postulated that the Limpopo collision was responsible for the deposition of the Witwatersrand Supergroup in

U N I F O R M I T A R I A N I S M : PLATE TECTONICS Table 1. The first 1 Ga o f formation of the Kaapvaal continent (after de Wit et al. 1992)

Age (Ga) Kaapvaal shield formation: 500 Ma of intra-oceanic tectonics

Regional emergence of Kaapvaal continental lithosphere by intra-oceanic obduction processes Western Pacific-style intra-oceanic amalgamation of oceanic terranes Widespread within-shield melting, granite formation and chemical differentiation in the upper lithosphere Kaapvaal shield stabilized by

3.7-3.2 3.3-3.2 3.2-3.1 3.1

Kaapvaal craton jormation: 500 Ma of inter- and intra-continental tectonics

Regional extension of the Kaapvaal shield; passive continental margin and rift basins Cordilleran-type accretion of composite terranes along north and west margin of Kaapvaal shield; intermontane and foreland basins Alpine or Himalayan-type continentcontinent collision between the Kaapvaal and Zimbabwean cratons Foreland extensional and impact-generated rifts Kaapvaal craton stabilized by

3.1-2.9

2.9-2.7

2.68 2.7-2.6 2.6

a foreland basin, and that the 2.64 Ga Ventersdorp rift system formed by post-collisional extension in the Kaapvaal craton. Although de Wit et al. (1992) concluded that the Archaean thermal and tectonic processes resemble plate tectonic processes occurring today, they emphasize there were some differences. For example, during the first-stage the Kaapvaal oceanic-type plateau formed by intra-oceanic obductiondominated tectonics that gave rise to stacking, tectonic loading and subsidence, and this resulted in melting of the lower parts of the thrust stack to yield extensive trondhjemite-tonalite melts. MacGregor & Manton (1986) found that the variation of major elements with oxygen isotopes of Archaean eclogites from Cretaceous kimberlites in South Africa matches that calculated for modern oceanic volcanic rocks altered by circulating seawater in ridge crest hydrothermal systems, and thus they proposed that the eclogites were derived from subducted, buoyant Archaean oceanic lithosphere (Bickle 1986). Seismic shear velocities indicate that Precambrian shields are underlain by chemically depleted mantle roots of Archaean age (Jordan 1988), and Helmstaedt & Schulze (1989) suggested that these roots were formed of imbricated slabs of partly subducted Archaean oceanic lithosphere.

Discussion

A review of current and recent data and ideas on crustal evolution indicates that Cenozoic-style plate tectonic processes have been in operation since the beginning of the geological record, but that there are some differences which we must consider.

19

According to Murphy & Nance (1991) the Pan-African and Cadomian-Avalonian belts developed by subduction and accretion on the outer margins of a late Proterozoic supercontinent (peripheral orogens), whereas the Mozambique belt formed by continent-continent collision and thus is situated within the supercontinent (internal orogen). Similar earlier Proterozoic accretionary and collisional orogens may have developed in relation to large continental blocks or supercontinents. However, in the early Archaean, when there was a more widespread primordial ocean, accretion was predictably the more important process in the generation of orogens (de Wit et al. 1992). Secular thermal changes have important implications for plate tectonic processes. Total heat production in the Earth in the late Archaean was around three times that of the present and this would have given rise to a higher temperature Archaean mantle, which in turn would have led to increased depth and volume of melting, a thicker continental lithosphere (150-200 km), a thicker oceanic crust (20-50 km) and sea-floor spreading rates 2-3 times than at present (Sleep & Windley 1982; Bickle 1986). Archaean oceanic lithosphere was highly chemically depleted, and buoyant subduction was more common than today (Burke et al. 1976; Hoffman 1990). Plate tectonics could have operated very efficiently in early Earth history, the plates moving over an asthenosphere with a greater heat flux and lower viscosity than now (Nisbet & Fowler 1983). The relatively light, low-viscosity asthenosphere would have facilitated easy movement and rapid subduction of the oceanic plates. Simple models of thick relatively buoyant plates above a hot (c. 1700 °C) upper asthenosphere suggest that the resistive forces to plate motion may have been considerably less in the Archean than today, and that the mean age of subducted oceanic crust would have been around 20 Ma compared with 60 Ma today (Bickle 1986; Nisbet & Fowler 1983). The younger net age of subducting slabs would favour widespread shallowdipping subduction in the Archean (Abbott & Hoffman 1984) and the buoyancy problem during subduction could be overcome by delamination of upper oceanic crust (Hoffman & Ranalli 1988); these relations would be consistent with the widespread evidence of thin-skinned thrust tectonics (Taira et al. in press). However the secular changes in heat production and loss were able to change the course of development of some igneous and metamorphic processes at subducting plate boundaries. (1) There has been a long-term change in the composition of granitic rocks over Earth history from more sodic to more potassic (Dewey & Windley 1981), the main cause of which may have been a change in subduction zone geometry. In Phanerozoic and Proterozoic magmatic arcs high-K magmatism was derived from dehydration-driven melting of the volatile-fluxed mantle prism, whereas in the Archaean when subduction zones were predictably shallower, dehydration melting took place of the hydrated amphibolites of the downgoing slabs, leading to more sodic magmas (Arculus & Ruff 1990). (2) There has probably been a long-term decrease in subduction geotherms (Ellis 1992, and references therein). Newton (1986) suggested that most Precambrian eclogites equilibrated at higher temperatures than younger eclogites (Fig. 7). Also the fact that they equilibrated beyond the stability of glaucophane may explain the absence of early Precambrian glaucophane schists: Potassic granites of crustal melt origin are rare in the Archaean, but common in later orogens. This may be related to the paucity Of Archaean clastic sediments as a result of a

20

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predominant intra-oceanic stage of crustal development (de Wit et al. 1992), compared with later times when more cannibalistic recycling of both crust and sediments took place (Veizer & Jansen 1979). Experimental data show that a high proportion (up to 40%) of S-type granitic liquid can be produced by melting at about 850 °C of pelites (Vielzeuf & Holloway 1988) and greywackes (Patifio Douce & Johnstone 1991). From the late Archaean/early Proterozoic an increasing number of continental blocks were available for erosion, and thus more pelites and greywackes were deposited in accretionary prisms in trenches. It is speculated here that most crustal melt granites in Earth history formed in accretionary prisms, many of which were transported from subduction zones into the deep continental crust (Klemperer 1989) where they underwent adiabatic decompression melting during uplift in extensional collapse orogens. Kr6ner (1984) considered that the formation of most Proterozoic orogens did not involve significant oceanic opening or oceanic subduction tectonics, and instead invoked a model of orogenesis based on intracontinental A-type continental subduction driven entirely by gravitational instability of lower crust and upper mantle. This model was adopted by Etheridge et al. (1987) to account for all the Proterozoic orogens in Australia. However, Ellis (1992) cogently demonstrated that such

Fig, 6. (A) Map showing the position of the Limpopo belt between the Zimbabwe and Kaapvaal cratons and the south- and north-dipping thrusts on the north and south sides of the belt respectively. (B) North-south section across the Limpopo belt (for line of section see A) based on geological and geoelectric, magnetic, seismic (refraction and vibroseis reflection) and gravity data. After de Wit et al. (1992).

a speculative type of orogenesis does not happen. Indeed Myers (1990) showed that four Proterozoic orogens in Australia do have the characteristic signatures of modern collisional orogens. So, as Ellis (1992) pointed out, the explanation for Australian or other Proterozoic intracontinental orogens that have no sutures or magmatic arcs is not A-type subduction but rather the late Cenozoic, post-collisional Tien Shan orogen in central Asia, which has no suture or arc, and which is 2000 km from its deformation front, the India/Asia suture zone (Windley et al. 1990). Uniformitarianism today means that plate tectonics provides a paradigm for understanding the past, but that does not mean that the present and the past are identical. Many features of the earliest Precambrian are predictably the result of the greater heat production at that time. With this thermal caveat it is possible to say 'It is unlikely that any of the continental material preserved on Earth today was produced by processes significantly different from those that operate now' (Burke & Seng6r 1986). With this Hutton and Lyell would have agreed.

I wish to thank the Geological Society for the invitation to present a conceptual synthesis of what uniformitarianism means today for the Earth Sciences. M. Allen made valuable comments on the manuscript.

UNIFORMITARIANISM:

25

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Fig. 7. Calculated P - T equilibration c o n d i t i o n s o f eclogites a n d transitional eclogites o f different ages, suggesting a possible decrease in s u b d u c t i o n g e o t h e r m s with time. D a s h e d lines show core-rim P - T conditions. R e a c t i o n b o u n d a r y o f g l a u c o p h a n e - q u a r t z to albite-talc f r o m K o o n s (1982). After N e w t o n (1986) and Ellis (1992).

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PLATE

TECTONICS

21

BICKLE, M. J. 1986. Implications of melting for stabilization of the lithosphere and heat loss in the Archean. Earth and Planetary Science Letters, 80, 314-324. BODINIER, J. L., DUPUY, C. & DOSTAL, J. 1984. Geochemistry of Precambrian ophiolites from Bou Azzer, Morocco. Contributions to Mineralogy and Petrology, 87, 43-50. BURKE, K. & SENGOR, A. M. 1986. Tectonic escape in the evolution of the continental crust. American Geophysical Union, Geodynamic Series 14, 41 53. --, DEWEY, J. F. & KIDD, W. S. F. 1976. Dominance of horizontal movements, arc and rnicrocontinental collisions during the later permobile regime, In: WINDLEY, B. F. (ed.) The Early History of the Earth. Wiley, New York, 113-130. - - - - - - & - - 1977. World distribution of sutures--the sites of former oceans. Tectonophysics, 40, 69-99. --, KIDD,W. S. F. & KUSKY,T. 1985 Is the Ventersdorp rift system of southern Africa related to a continental collision between the Kaapvaal and Zimbabwe cratons at 2.64 Ga? Tectonophysics, 115, 1-24. BURNS, K. E. 1985. The Border Ranges ultramafic and mafic complex, southcentral Alaska; cumulate fractionates of island-arc volcanics, Canadian Journal of Earth Sciences, 22, 1020-1038. CABV, R., BERTRAND,J. N. L. & BLACK,R. 1981. Pan-African ocean closure and collision in the Hoggar-Iforas segment, central Sahara. In."KR6NER, A. (ed.) Precambrian Plate Tectonics. Elsevier, Holland, 407-434. CARD, K. D. 1990. A review of the Superior Province of the Canadian Shield: a product of Archean accretion. Precambrian Research, 48, 99-156. CONDIE,K. C. 1989. Plate Tectonics and Crustal Evolution, 3rd edition. Pergamon, Oxford. CULOTTA, R. C., PRATT, Y. & OLIVER, J. 1900 A tale of two sutures: COCORP's deep seismic surveys of the Grenville Province in the East US midcontinent. Geology, 18, 646 649. DANN, J. C. 1991. Early Proterozoic ophiolite, central Arizona. Geology, 19, 594-597. DALLMEYER,R. D. & GIBBONS,W. 1987. The age of blueschist metamorphism in Anglesey, North Wales: evidence from 4°Ar/39Ar mineral dates of the Penmynydd schists. Journal of Geological Society, London, 144, 843-850. DAVIDSON,A. 1986. New interpretations in the southwestern Grenville Province. In: MOORE, J. M., DAVlDSON,A. & BAER, A. J. (eds) The Grenville Province. Geological Association of Canada, Special Paper, 31, 61-74. DE W~T, M. J., HART,R. A. & HART, R. 1987. The Jamestown ophiolite complex, Barberton mountain belt: a section through 3.5 Ga oceanic crust. Journal of Afi'ican Earth Sciences, 6, 681-730. --, ROERING, C., HART, R. J., ARMSTRONG,R. A., DE RONDE, C. E. J., GREEN, R. W. E., TREDOUX, M., PEBERDY, E. & HART, R. A. 1992. Formation of an Archaean continent. Nature, 357, 553-562. DEWEY, J. F. 1988. Extensional collapse of orogens. Tectonics, 7, 1123-1139. -& WINDLEY, B. F. 1981. Growth and differentiation of the continental crust. Philosophical Transactions qf Royal Society of London, A301, 189-206. ELLIS, D. J. 1992. Precambrian tectonics and the physicochemical evolution of continental crust. II. Lithospheric delamination and ensialic orogeny. Precambrian Research, 55, 507-524. ETHERIDGE,M. A., RUTLAND,R. W. R. & WYBORN,L. A. L. 1987. Orogenesis and tectonic process in the Early to Middle Proterozoic of northern Australia. In: KR6NER, A. (ed.) Proterozoic Lithospheric Evolution. American Geophysical Union, Geodynamic Series 17, 131-147. FAHRIG,W. F. 1987. The tectonic setting of continental mafic dyke swarms: failed arm and early passive margin. In: HALLS,H. C. & FAHRIG,W. F. (eds) Mafic Dyke Swarms. Geological Association of Canada, Special Paper, 34, 331-348. FOUNTAIN,D. M. & SALISBURY,M. H. 1981. Exposed cross-sections through the continental crust: implications for crustal structure, petrology and evolution. Earth and Planetary Science Letters, 56, 263--277. FRIEND, C. R. L., NUTMAN, A. P. & MCGREGOR, V. R. 1987. Late Archaean tectonics in the Faeringehavn-Tre Brodre area, south of BukseI]orden, southern West Greenland. Journal of Geological Society, London, 144, 369-376. GA~,L, G., BERTHELSEN,A., GORBATSCHEV,R., KESOLA, R., LEHTONEN, M. I., MARKER, M. & RAASE, P. 1989. Structure and composition of the Precambrian crust along the POLAR Profile in the northern Baltic Shield. Tectonophysics, 162, 1-25. GARRISON, J. R. 1981. Coal Creek serpentinite, Llano Uplift, Texas: a fragment of an incomplete Precambrian ophiolite. Geology, 9, 225 230. GIBB, R. A. & THOMAS,M. D. 1976. Gravity signature of fossil plate boundaries in the Canadian shield. Nature, 262, 199-200. --,--, LAPOINTE, P. L. & MUKHOPADHAY,M. 1983. Geophysics of proposed Proterozoic sutures in Canada. Precambrian Research, 19, 349-384. HAAPALA,I. & R~,M£),T. 1990. Petrogenesis of the Proterozoic rapakivi granites of Finland. Geological Society of America, Special Paper, 246, 275 286. HARLEY, S. L. 1989. The origins of granulites: a metamorphic perspective. Geological Magazine, 126, 215-247.

22

B.F.

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SAQUAQUE,A., ADMOU,H., KARSON, J., HEFFERAN,K. & REUBER, I. 1989. PreTRELOAR, P. J., COWARD,M. P. & HARRIS, N. B. W. 1992. Himalayan-Tibetan cambrian accretionary tectonics in the Bou Azzer-El Graara region, Antianalogies for the evolution of the Zimbabwe craton and Limpopo belt. Atlas, Morocco. Geology, 17, 1107-1110. Precambrian Research, 55, 571-587. ~ENGOR, A. M. C. & OKUROGULLARI,A. H. 1991. The role of accretionary prisms VAN SCHMUS,W. R., BICKFORD,M. E. • ZIETZ, I. 1987. Early and middle Proin the growth of continents: Asiatic examples from Argand to plate tecterozoic provinces in the central United States. In: KRONER, A. (ed.) Protonics. Eclogae Geologicae Helvetiae, 84, 535-597. terozoic Lithospheric Evolution, American Geophysical Union, Washington, SHACKLETON,R. M. 1986. Precambrian collision tectonics in Africa. In: COWARD D.C. Geodynamic Series, 17, 43-68. M. P. & RIES A. C. (eds) Collision Tectonics. Geological Society, London, VEIZER, J. & JANSEN, S. L. 1979. Basement and sedimentary recycling and conSpecial Publication, 19, 329-349. tinental evolution. Journal of Geology, 87, 341-370. SHANTI,M. & ROOBOL,M. J. 1979. A late Proterozoic ophiolite complex at Jabal V1ELZEUF, D. & HOLLOWAY, J. R. 1988. Experimental determination of the Ess in northern Saudi Arabia. Nature, 279, 488-491. fluid-absent melting relations in the pelitic system: consequences for SINHA-RoY, S. t~g MOHANTY, M. 1988. Blueschist facies metamorphism in the crustal differentiation. Contributions to Mineralogy and Petrology, 98, ophiolitic m61ange of the late Proterozoic Delhi fold belt, Rajasthan, India. 257-276. Precambrian Research, 42, 97-105. WATTS, A. B., KARNER,G. D. & STECKLER,M. S. 1982. Lithospheric flexure and SLEEP, N. H. & WINDLEY,B. F. 1982. Archaean plate tectonics; constraints and the evolution of sedimentary basins. Philosophical Transactions of the Royal inferences. Journal of Geology, 90, 363-379. Society of London, A305, 249-281. SONDER, L. J., ENOLAND,P. C., WERNICKE,B. P. & CHmSTIANSEN,R. L. 1987. A WEDEPOHL,K. H., HEINRICHS,H. & BRIDGWATER,D. 1991. Chemical characterisphysical model for Cenozoic extension of western North America. In: tics and genesis of the quartz-feldspathic rocks in the Archean crust of COWARD, M. P., DEWEY, J. F. & HANCOCk, P. L. (eds) Continental ExtenGreenland. Contributions to Mineralogy and Petrology, 107, 163-179. sional Tectonics. Geological Society, London, Special Publication, 28, WINDLEY, B. F. 1986. Comparative tectonics of the western Grenville and the 187-201. western Himalaya. In: MOORE, J. M., DAVIDSON,A. & BAER, A. J. (eds) The SPRAY, J. G. 1985. Dynamothermal transition zone between Archean greenstone Grenville Province. Geological Association of Canada, Special Paper, 31, and granitoid gneiss at Lake Dundas, western Australia. Journal of Struc341-348. tural Geology, 7, 187-203. -, ALLEN, M. B., Zr~ANG, C., ZHAO, Z-H. & WANG, G-R. 1990. Paleozoic STOESER, D. B. & CAMP, V. E. 1985. Pan-African micro-plate accretion of the accretion and Cenozoic redeformation of the Chinese Tien Shan range, CenArabian shield. Geological Society of America Bulletin, 96, 8 l 7-826. tral Asia. Geology, 18, 128-131. STOREY, M., MAHONEY, J. J., KROENKE, L. W. & SAUNDERS,A. J. 1991. Are 1991. Early Proterozoic collision tectonics, and rapakivi granites as inoceanic plateaus sites of komatiite formation? Geology, 19, 376-379. trusions in an extensional thrust-thickened crust: the Ketilidian orogen, SYLVESTER, P. J., ATTOrt, K. & SCHULZ, K. J. 1987. Tectonic setting of late South Greenland. Tectonophysics, 195, 1-10. Archean bimodal volcanism in the Michipicoten (Wawa) greenstone belt, 1993. Proterozoic anDrogenic magmatism and its orogenic connections. Ontario. Canadian Journal of Earth Sciences, 24, 1120-1134. Journal of Geological Society, London 150, 39-50. TAIRA, A., PICKEPdNG,K. T., WmDLEY, B. F. &SOH, W. In press. Accretion of In press a. Proterozoic collisional and accretionary orogens. In: CONDIE,K. Japanese island arcs and implications for the origin of Archean greenstone C. (ed.) Proterozoic Crustal Evolution. Elsevier, Amsterdam. belts. Tectonics. In press b. Precambrian Europe. In: BLtJNDELL,D. FREEMAN,R. & MUELLER, TmJRSTON, P. C. & CraVERS, K. M. 1990. Secular variation in greenstone ST. (eds) Tectonic Evolution of a Continent: The European Geotraverse. Camsequence development emphasizing Superior Province, Canada. Precambridge University Press, Cambridge. brian Research, 46, 21-58. Received 30 July' 1992; accepted 25 August 1992. -

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Extract from the Anniversary address of the President, Sir Charles Lyell, QJGS,6, xxxii. GENTLEMEN,witis n0W my duty, in accordance with the usual custom of my predecessors in office, to say something of the scientific labours of geologists during the past session. It is nearly twenty years since I announced, in the first edition of my ' Principles of Geology,' the conviction at which I had then arrived, after devoting some time to observation in the field, and to the study of the works of earlier writers, that the existing causes of change in the animate and inanimate world might be similar, not only in kind, but in degree, to those which have prevailed during many successive modifications of the earth's crust. I attempted to adapt the views which Hutton and Playfair had first promulgated, to a more advanced state of our science, and to extend their application, by showing, that should the same causes continue to act with unabated energy, for indefinite periods of the future, they must bring about revolutions not inferior in magnitude to those recorded in the monuments of past ages. After an interval of twenty years, during which Geology has been enriched by a vast accession of new facts, and when so many powerful minds, in every civilized country, have brought their intellectual energies to bear on the philosophy of our science, I may I think affirm that the idea of comparing the modem agents of change with those of remote epochs, as not inferior in power and intensity, appears even to the most sceptical a far less visionary and extravagant hypothesis than when I first declared my belief in its truth. As, however, tt~ere are not a few original observers, whose opinion I respect, who are still opposed to this doctrine, I cannot I believe do better on the present occasion than take a brief view of the bearing of some leading discoveries of modem date on this much-controverted question. I adopt this course the more

Extract from the Anniversary address of the President, Sir Charles Lyell, QJGS,7, xxxii-xxxiii. GENTLEMEN,--In my Anniversary Address of last year, I entered into an examination of the question, how far the leading discoveries of modem date tend to confirm or invalidate a doctrine which I had advocated twenty years before, in the first edition of my ' Principles of Geology,'--that the ancient changes of the animate and inanimate world, of which we find memorials in the earth's crust, may be similar both in kind and degree to those which are now in progress. But in order to keep myself within due bounds, I confined my remarks on that occasion to the revolutions of the inorganic world, reserving for the present opportunity a comparison of the organic creation in ancient and modem times, and a consideration of the light thrown by Pal~eontology on the laws which govern the fluctuations of the living inhabitants of the globe.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 25-35 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 625-635

Early Precambrian crustal development: changing styles of mafic magmatism R.

P.

HALL

& D.

J.

HUGHES

Department of Geology, University of Portsmouth, Burnaby Road, Portsmouth P 0 1 3QL, UK Abstract: The work of Sutton & Watson (1951) was one of the cornerstones in the interpretation of the evolution of early Precambrian (Archaean and early Proterozoic) gneiss complexes and in the documentation of mafic dyke swarms that intruded recently accreted Archaean cratons. Since then, the concept of geological terranes and terrane accretion has been extended back to the Archaean, and different events can be distinguished by increasingly precise isotopic dating techniques. There is growing evidence that, with certain clear provisos, the principle of uniformitarianism can be applied as equally to the early Precambrian as it can through the rest of the geological record. In terms of mafic magmatism, there are similarities between some ancient and modern suites, both intrusive and extrusive, but there are also some major differences, such as the presence of komatiites in the Archaean and the formation of large-scale layered intrusions and extensive dyke swarms at the end of the late Archaean and in the early Proterozoic. The mantle has evolved continuously with the progressive extraction of mafic magmas but, allowing for this, continental tholeiites appear to have changed little with time. Ancient mid-ocean ridge basalts are elusive because of their poor preservation record. The geochemistry of mafic volcanics occurring in many greenstone belts commonly suggests a continental contamination component and that these volcanic rocks are most closely akin to those from present-day arc systems. The relative abundance of early Precambrian intrusive mafic suites parallels the crustal growth curve, reflecting the global crustal accretion-differentiation superevent towards the close of the Archaean. Bimodal tholeiitic-noritic mafic magmatism associated with many of these intrusive suites stems from this rapid turnover in the mantle-mafic crustcontinental crust system. This, in turn, gives a possible link between the respective evolutions of Archaean to Recent komatiitic, noritic and boninitic suites.

One of geologists' most precious tools in deciphering the geological record is the principle of uniformitarianism--that the present is the key to the past. However, the reliance on this principle becomes proportionally weaker with the increasing age of the rocks under investigation. Clearly, the earliest formed rocks cannot have developed in precisely the same way, or at least in response to the same tectono-magmatic constraints, as those formed most recently. The Earth's heat production, mantle composition, tectonic processes and crustal accretion have evolved with time and so too have the concomitant sedimentary, metamorphic and magmatic provinces. Over the past one or two decades, there have been considerable advances in our understanding of the evolution of the Earth's crust during the Precambrian. In particular, our knowledge of major Archaean terranes in, for example, Greenland, southern Africa and Australia was expanded very significantly during the late 1960s and early 1970s (e.g. Viljoen & Viljoen 1969a, b; Anhaeusser 1973; Arriens 1971; McGregor 1973). There have been numerous reviews since then monitoring the progress in our comprehension of Archaean tectonics and crustal evolution and the development of Precambrian granite-greenstone terranes (e.g. Windley 1976; McCall 1977; Condie 1981; Glover & Groves 1981; KrOner 1981; Park & Tarney 1987a; Pharaoh et al. 1987; Hall & Hughes 1990a). It is generally accepted that the end of the Archaean saw the onset of the development of supracrustal and intrusive complexes which can be recognized as having been related to modern-style plate

tectonic processes, and the accretion of Proterozoic tectonic terranes have been proposed accordingly. By the midProterozoic, geological processes had certainly come to a state whereby Phanerozoic-type igneous and sedimentary assemblages were being formed. However, controversy remains concerning the application of a uniformitarian approach to the interpretation of Archaean suites (Windley 1993; Rapp 1993). Some of the earliest and most formative work unravelling early Precambrian rocks was done in Britain, on the Lewisian Complex of N W Scotland, and few people would deny the work of Janet Watson and John Sutton as being regarded a cornerstone to countless students of Precambrian geology. Their seminal paper published in the Quarterly Journal of the Geological Society (Sutton & Watson 1951) is one of the foundation stones of our understanding of Precambrian terranes worldwide, and these authors were still asking questions of Precambrian complexes (Sutton & Watson 1987) at one of the more recent conferences on the evolution of the Lewisian and comparable high-grade tarranes, papers from which were compiled by Park & Tarney (1987a). Our legacy from the publications by Sutton and Watson is indeed a considerable one (e.g. Bowes 1987)

Crustal accretion in the Precambrian Our understanding of the origin and evolution of large tracts of what were once referred to as 'undifferentiated gneisses' 25

26

R.P.

HALL & D. J. HUGHES

has, over the past few decades, become quite refined. From detailed mapping of the various components of the relatively undeformed parts of gneiss complexes preserved in low strain zones, and the tracing of these units into more highly deformed gneissic equivalents, even high-grade 'mobile belts' can be recognized to have formed from decipherable and manageable sequences of magmatism, sedimentation, tectonism and metamorphism. One of the most fundamental questions in interpreting these sequences is whether or not equivalents to more recent plate-tectonic related episodes and products can be identified progressively further back into the Precambrian record. Some would argue that modern-style plate tectonics cannot be postulated earlier than the Archaean-Proterozoic transition. Proterozoic supracrustal and associated intrusive suites often bear a close resemblance to sequences developed in different modern tectono-magmatic provinces. This is perhaps less often the case in Archaean greenstone and high-grade belts. The assembly of Archaean volcanosedimentary successions and their apparently inevitable ultimate demise to produce the almost ubiquitous syn-tectonic tonalite-trondhjemite-granodiorite suite (Jahn et al. 1988; Rapp et al. 1991), which forms a major component of most gneiss terranes, has distinct modern analogues (e.g. Tarney et al. 1976). However, certain sediment types (e.g. banded iron-formation) and volcanic rocks (e.g. komatiites) are far more prominent in the Archaean. The progressively dubious value of applying discriminatory geochemical parameters derived from modern volcanic suites to increasingly ancient suites also casts some doubt on the correlation of Archaean magmatic, sedimentary and tectonic provinces with those associated with modern plate tectonics. The end of the Archaean saw the accretion of extremely large volumes of continental crust. Classic Archaean granite-greenstone terranes comprise curvilinear supracrustal belts swamped by dioritic to granitic 'plutons'. The shape of these belts is variously controlled by unconformable contacts, intrusive contacts, folding and shear zones. In other granite-greenstone terranes and higher grade 'mobile belts', the supracrustal belts are more uniformly linear and more strongly controlled by ductile shear zones. Isotopic evidence from some regions supports lithostratigraphic and structural interpretations that many granitic (s.l.) gneiss suites are only slightly younger than the relatively thin supracrustal belts that they envelop. The formation of mantle-derived basic volcanics and deposition of accompanying sediments was rapidly followed by their subduction and related metamorphism and by partial melting to give rise to syn-tectonically emplaced granitoids. This immense magmatic episode or 'crustal accretion-differentiation superevent' was commonly accompanied by tectonic crustal thickening (Moorbath 1977; Moorbath & Taylor 1981). The oldest rocks of the Lewisian complex of NW Scotland comprise Scourian (c. 2900Ma) high-grade gneisses which were variably metamorphosed, some up to granulite facies, during the deepest Badcallian metamorphism (c. 2660Ma; Pidgeon & Bowes 1972). Soon after, gneisses from different crustal levels were segmented and juxtaposed along major retrograde shear zones (Park & Tarney 1987b). R e f n e d isotopic geochemistry techniques reveal that, as might be expected, the assembly of Archaean cratons does not occur as a simple, single event, but that different components of a craton may differ somewhat in

age, signifying their sequential accretion and long geological history. Recent Sm-Nd and UoPb isotopic studies have indicated the possibility of source heterogeneities, and that the various gneisses from different parts of NW Scotland represent a diachronous Archaean accretion-differentiation sequence (Whitehouse 1989, 1990). Interpretations of the assembly of the somewhat more extensive portion of the North Atlantic Archaean craton in SW Greenland have been taken a stage further. Here, McGregor (1973) demonstrated that the Archaean quartzofeldspathic gneisses could be broadly separated into two suites, the Amitsoq and Nfik gneisses distinguished by the presence of abundant amphibolitic relicts of an intervening mafic dyke swarm (the Ameralik dykes) in the older of the two. The Amltsoq gneisses are around 3.7 Ga old, and the Nfik gneisses about 2.9 Ga. Both suites form part of major accretion-differentiation events (Moorbath & Taylor 1981) and, at first approximation, possibly represent components of Wilson-cycle crustal development in that both were preceded by the deposition of supracrustal volcanicsedimentary associations and followed by the intrusion of marie dyke swarms. However, it is now clear that the 'younger' Archaean gneisses of this broad region comprise varied suites ranging from 3.0 to 2.6Ga and, based on lithostratigraphic, structural, metamorphic and further detailed isotopic evidence, it has been shown that different portions of the craton developed at significantly different times and underwent different tectono-metamorphic events (Friend et al. 1988; Nutman et al. 1989; and references therein). The SW Greenland Archaean 'craton' has now been interpreted to comprise at least five different terranes assembled during the late Archaean (Friend & Nutman 1994)

Mafic magmatic suites The importance of understanding in detail not only the tectonic and metamorphic evolution but also the timing of emplacement and subsequent paragenetic histories of quartzo-feldspathic gneiss suites is paramount in interpreting the way in which they were assembled. However, the honing of analytical techniques on which these interpretations rest means that large-scale crustal models rely increasingly on finer pin-points of information. The ion-microprobe dating of the cores and the margins of individual zircons can be the determining factor in the recognition of a 'terrane' (e.g. Friend & Nutman 1994), although even such highly refined data as this may also lead only to the question of these zircons having been derived from yet older rocks. Compared to granitic intrusives, the style of the mafic magmatic rocks more readily reflects the environment in which they formed. Gabbros, pillowed basalts and mafic dykes commonly retain their distinctive features even in areas of high deformation and metamorphism. One of the fundamental questions concerning Precambrian gneiss terrane accretion models is whether, or to what extent, modern plate tectonic styles are indicated. In this regard the character and chemistry of the mafic magmatic rocks (or their metamorphic equivalents) are clearly very important. The geochemical comparison of progressively older magmatic suites with those of different modern tectonic regimes becomes increasingly tenuous. However, coupled together with comparisons of different types of Precambrian suites of the same age and sequentially younger suites, this

E A R L Y P R E C A M B R I A N MAFIC MAGMATISM gives us some of the only clues as to the similarities and changes in mafic magmatism with time and, thus, with the evolution of the Earth's crust. Do styles of mafic magmatism therefore evolve with time? Is there some logical sequence of changing mafic chemical characteristics that can be related to the formation and development of the crust? In some cases these questions are easily addressed by a straightforward examination of the stratigraphic relationships of the mafic components and their petrological characteristics. Marie magmas are derived overwhelmingly from the mantle and the geochemistry of marie rock suites thus tells us about the composition of the mantle source, the degree of partial melting of that source, the composition of the parental magma and the nature of its evolution by fractional crystallization, magma replenishment or mixing, and contamination at high or low crustal levels. These petrogenetic features have to be deduced paying due regard to the effects of crystal accumulation, alteration and metamorphism. It may be argued that even fresh modern basalts do not exactly correspond to the composition of the liquid from which they are derived. Analysing metamorphosed Archaean mafic rocks is clearly problematical.

Preservation and characteristics of ArchaeanProterozoic suites How well do early Precambrian mafic suites represent the range of mafic rocks actually formed at that time, or do they better reflect the differential processes of preservation due to their association with certain tectonic regimes? Two thirds of the Earth's present-day crust is oceanic and less than 200 Ma old. Most of this oceanic crust will presumably no longer exist at the surface in another 200 Ma years time. Marie volcanic rocks with mid-ocean ridge basalt (MORB) geochemical characteristics are extremely rare in both Archaean and Proterozoic metavolcanic sequences, but it is highly unlikely that early Precambrian assemblages are genuinely volumetrically representative of the original distribution of marie volcanic suites, because of selective preservation (Condie 1990). Simplistically, the three principle styles of mafic magmatic suites, lavas, dykes and gabbroic bodies indicate their depth of formation. This sequence has been logged recently, for example, in a 2 k m drill hole through the eastern equatorial Pacific oceanic crust (Dick et al. 1992). Ophiolitic fragments comprise similar sequences but are representative mostly of fore-arc or back-arc basin, atypical oceanic crust, formed over subduction zones and tectonically emplaced into mountain belts. The form of marie intrusive complexes within continental crust says more about the rigidity and breakup of that crust, giving rise to either layered lopolithic, sub-horizontal intrusions or (sub-)vertical dyke swarms. Because of their intrusive character into established bouyant crust, these suites are commonly well preserved. Thus, despite the fact that they are grossly volumetrically subordinate to oceanic basaltic crust, we have a far greater abundance of material and data for these continental suites. One of the premises that holds true, even in examining Archaean suites, is that the present is the key to the past (Windley 1993). Some types of marie magmatism appear to have changed very little through geological time. Continental tholeiites, for example, are virtually ubiquitous as mafic dyke swarms in Precambrian cratons throughout the world

27

(Halls & Fahrig 1987; Parker et al. 1990) and the similar chemistry of young and old continental mafic rocks reflects simply the similar petrogenetic processes involved in their formation (e.g. Weaver & Tarney 1983; Tarney 1993). This shows that not only do these magmas continue to form in the same way, but that they formed worldwide at certain broad periods in the geological record. Mafic dykes intruded into continental crust are a relatively easy case; their tectonic provenance is seldom in dispute. Comparisons of their trace element ratios with those used to discriminate modern basalt types are normally made only to illustrate and formalize their geochemical characteristics. The reliance on geochemical discriminant diagrams to determine the tectonic setting of ancient 'greenstone' belts is perhaps progressively more tenuous with increasing age, but the combined evidence of their lithostratigraphy, structural evolution and geochemical characteristics strongly suggests that many early Precambrian greenstone belts are analogous to modern arc-related volcanic belts. However, the analogies are not absolute. There are some significant differences between these belts and those forming at the present day. The relative abundance of komatiites clearly reflects the higher heat production and mantle temperatures, and increased partial melting giving rise to mantle plume-related magmatism producing thickened oceanic crust during the Archaean (O'Nions et al. 1978; White & McKenzie 1989; Bickle 1990; Campbell & Griffiths 1990). This does not negate uniformitarianism, it simply means that there were additional processes and concomitant magma types in the early Precambrian compared with the present.

Metavolcanic rocks of SW Greenland: Archaean ophiolites? Examples of supracrustal suites representing early Precambrian oceanic crust are provided by some of the extensive Archaean mafic metavolcanic-dominated supracrustal belts of SW Greenland. Despite their amphibolite facies metamorphism, this interpretation is clearly indicated by the stratigraphic and geochemical evidence (e.g. Chadwick 1990; Hall et al. 1990). Metavolcanic rocks in neighbouring belts, on the other hand, appear to have closer affinities with island-arc assemblages. These are not dissimilar in composition to the Ameralik marie dykes which intruded the ¢. 3.7 Ga Am]tsoq gneisses. These suites may have been deposited onto a continental marginal gneissic basement (Chadwick 1981; Nutman & Bridgwater 1983). Just as Sutton & Watson (1951) recognized in NW Scotland that the Scourian (c. 2.9-2.7 Ga) and Laxfordian (c. 1.9 Ga) metamorphic events were separated by a long gap during which there was a major mafic magmatic episode--the intrusion of the Scourie mafic dyke swarms-so the early Archaean Am]tsoq gneisses in Greenland were differentiated from the mid- to late Archaean Nfik gneisses by the presence of the intervening Ameralik tholeiitic mafic dykes (McGregor 1973) (various late Archaean gneisses have since been lithologically and chronologically distinguished from the 'Nfik' gneisses, see Friend et al. 1987, 1988). However, in SW Greenland the large metavolcanic belts and Fiskenaesset-type layered gabbro-anorthosite complexes also fall stratigraphically between the early and mid- to late Archaean gneisses. Geochemical evidence suggests that it was probably the subduction and remelting of these types of marie rocks worldwide, soon after their separation from the

28

R.P.

HALL & D. J. H U G H E S

mantle, that gave rise to the voluminous meta-tonalitic and trondhjemitic gneisses (Moorbath 1977; Barker 1979; Martin 1987). The simple question in West Greenland is whether or not the layered intrusions, the dykes and the volcanic rocks simply represent plutonic, hypabyssal and extrusive equivalents derived from the same mafic magmas, and whether the Ameralik dykes could thus have been feeders to some at least of the abundant mafic volcanics. If they were all petrogenetically related, then they should share common geochemical characteristics, allowing for the contrasts in crystallization conditions. The Archaean stratigraphy of West Greenland shows that, away from the central early Archaean, Arnltsoq gneisses (the Faeringhavn terrane, Friend et al. 1987; Friend & Nutman 1991), the various mafic belts clearly represent the oldest crust. The metavolcanic rocks commonly display deformed pillow structures and are typically associated with thick slices of harzburgitic and lherzolitic peridotitic material, which are here considered to represent tectonically intercalated, thrusted mantle remnants rather than intrusive sills (Chadwick 1990). Coarse layering is well preserved within the Fiskenaesset-type metagabbros and anorthosites (Myers 1985). The geochemistry of the mafic volcanic rocks varies from komatiitic to tholeiitic, the closest modern analogies of the tholeiitic members being MORB-Iike in character. For example, they have fiat, unfractionated chondrite-normalized rare-earth element (REE) distribution patterns, indicative of their primitive nature and of their having been derived from an undepleted mantle source. Their petrogenesis was strongly controlled by combined olivine and clinopyroxene fractionation from a komatiitic parental magma (Hall et al. 1987). The abundant coarse plagioclase cumulate gabbros and anorthosites, on the other hand, reflect the dominance of plagioclase as a controlling fractionating phase and the relatively aluminous nature of the parental tholeiitic magma of the layered complexes (Weaver et al. 1981). While komatiites to a large extent probably reflect primitive oceanic crust (Arndt 1983; Nisbet 1987), the primordial geochemical nature of these Greenlandic mafic metavolcanic rocks appears, in fact, not to be a common feature (Cattell & Taylor 1990; Hall & Hughes 1990a). Although some examples have primary chemical characteristics, many komatiitic-tholeiitic sequences in greenst~ne belts show evidence of a crustal contamination component, and the nature of the original magma and mantle source is, thus, frequently somewhat obscured (Arndt 1986; Arndt et al. 1986; Jochum et al. 1992). The relatively high metamorphic grade cannot realistically be held responsible for differences in the geochemistry between the different mafic units in individual component Archaean terranes in West Greenland, since these units have undergone the same metamorphism. Thus, the minor, though significant geochemical contrasts between the Malene metavolcanic rocks and Ameralik mafic dykes within the Arn~tsoq gneisses of the Faeringhavn terrane cannot be attributed to secondary processes. Although the compositions of mafic lavas and their feeder dykes can be slightly different, the higher Mg, Ni, and Cr contents, and lower Ti, Zr, AI and Fe/Mg in the komatiitic metavolcanic rocks compared to the Ameralik tholeiitic dykes (Hall et al. 1987) reflect significant geochemical differences between their respective parental magmas. Only some, if any, of the

Malene-type metavolcanic rocks (McGregor 1973) can have been fed via the Ameralik dykes through the Arn~tsoq granitic crust. The age of the widespread mafic metavolcanicdominated belts is not known, and in many cases cannot be narrowed to within one or two hundred million years. It is doubtful whether the belts throughout the Archaean craton are all of the same age. Nonetheless, their geochemistry and lithostratigraphy suggest that many of these primitive, pillow-structured, komatiitic-tholeiitic volcanic belts are similar in origin, and analagous to oceanic suites. The structural evidence (Hall & Friend 1979; Friend & Nutman 1991), and the close geochemical similarities between metavolcanic rocks remote from the older Arn~tsoq 'continental' segment in the centre of the craton, with those at present intercalated with these gneisses, together suggest that, the metavolcanic rocks represent oceanic crust, tectonically juxtaposed with the remnants of the earlier Archaean continental crustal fragments even within the heart of the the Archaean craton (Hall et al. 1987). Many of these belts carry large horizons of coarse lherzolitic, dunitic and commonly harzburgitic peridotite, which often show weak relict layering. Some are continuous for several kilometres and have been used as markers in defining the structural configuration of the region (e.g. Hall & Friend 1979; Chadwick 1990). While some of these ultrabasic lenses may have been intrusive (Chadwick 1990), most are considered to be mantle residua, their harzburgitic compositions reflecting the extraction of the voluminous komatiitic-tholeiitic mafic suites. Their present intercalation with the metavolcanic rocks probably resulted from oceanic and subsequent tectonic interleaving (Friend & Hughes 1978; Crewe 1984). Mafic dykes do occur within some of the metavolcanic units, although true sheeted dyke swarms have not been recognized. The overall package of pillow-structured komatiitic and low-K tholeiitic lavas with mafic dykes, large-scale layered gabbroic bodies and abundant tectonic slices of peridotite are considered to constitute the Archaean equivalent of an ophiolite complex. Because of their disruption by the voluminous tonalitic, trondhjemitic and granitic intrusives and subsequent severe deformation, not all of the belts contain all the ophiolitic components. However, collectively they constitute a considerable segment of Archaean oceanic crust and, apparently, are among the few to preserve so high a proportion of such material together with depleted oceanic mantle. Perhaps the most extensive relict oceanic crust represented by a lower grade, greenstone terrane is in the Belingwe belt of Zimbabwe (Kusky & Kidd 1992). The tectonic setting of many komatiite suites remains uncertain. The Archaean Earth was presumably dominated by komatiitic oceanic crust (Nisbet 1987). However, many komatiite-dominated belts show clear geochemical evidence, for example rare-earth element and isotopic, of a subduction zone related crustal contamination component (Cattell & Taylor 1990). High-Mg komatiitic lava would have been very capable of digesting crustal material (Huppert & Sparks 1985a, b) and contaminated komatiitic basalts are commonly encountered (Arndt & Nesbitt 1984; Nisbet 1984; Arndt & Jenner 1986). Our view of parental komatiitic magma and its source is, therefore, generally through a 'dirty window' (Arndt 1986), although paradoxically, some

E A R L Y P R E C A M B R I A N MAFIC M A G M A T I S M komatiites associated with continental crust appear to show no such contamination (Claou6-Long & Nesbitt 1985; Arndt et al. 1986). The question of how confidently the volcanic belts of Greenland can be correlated chronologically is, of course, a serious one in terms of establishing a crustal evolution model. Recent detailed isotopic studies, particularly of Pb isotopes in single zircons, of various Archaean greenstone belts in the Superior Province of the Canadian Shield have shown that what were once regarded as single composite belts in fact comprise discrete metavolcanic and sedimentary 'panels' which may differ in age by more than 100Ma (Thurston et al. 1987). These different belts form major parts of the dominantly E-W-trending tectonic subprovinces (Card & Ciesielski 1986) accreted together in the mid- to late Archaean (Thurston 1990). Stratigraphic and geochemical interpretations suggest that individual composite belts, such as those comprising belts in the Geraldton-Beardmore and Onaman-Tashota terranes in the Wabigoon sub-province (Williams & Stott 1991) contain volcanic and sedimentary assemblages indicative of .(i) shallow platformal, (ii) deep mafic plain, (iii) arc and (iv) pull-apart basin environments (Thurston 1990). Some of t h e s e may reflect lateral equivalents formed penecontemporaneously, but there is also isotopic evidence for the evolution from one type to another with time (Thurston & Chivers 1990). Thus, while greenstone belts are sometimes regarded as structural marker horizons, some may themselves comprise completely different components (terranes) welded together during subsequent tectonism and acid intrusive activity associated with the accretion of one subprovince onto another (Hoffman 1989; Card 1990). Despite the relatively short time span of crustal accretion-differentiation super-events, all of the Greenlandic belts and layered intrusions could not have formed synchronously. The recognition on structural and metamorphic grounds of the different terranes which make up this craton (Friend et al. 1988) emphasizes the point that, of course, the supracrustal belts may have formed in different environments and at widely different times.

Late Archaean to Early Proterozoic noritic magmatism The end of the Archaean is not marked by a common, contemporaneous event. The boundary between the Archaean and Proterozoic is normally considered to be 2.5 Ga, but there is no complete uniformity in the nature of igneous activity at that time. In many parts of the world, and SW Greenland is no exception, syntectonic tonalitetrondhjemite-granodiorite suite gneisses were derived from young subducted volcanic and associated intrusive basic rocks. The final part of the Archaean saw the emplacement of post-tectonic potassic granites, marking the waning stages of cratonization. This sequence crudely defines the 'Wilson cycle'. It did not, of course, happen synchronously throughout the world. Just as there occurred a diachronous change in granitic magmatism, so there was a gradual change in the style of mafic magmatic activity. The birth of the cratons, and the marked increases in crustal thickening by the immense mafic to granitic magmatic and associated tectonic episodes, gave way subsequently to the intrusion of mafic sills or dykes, as the continents either assembled or

29

rifled (Hatton & Von Gruenewaldt 1990; Parker et al. 1990). There is an apparent hiatus in oceanic magmatism during this period of Earth history, from 2.5 to 2.1 Ga (Windley 1984), while the array of major and minor intracratonic layered marie intrusions and dyke swarms during the same period is considerable (Halls & Fahrig 1987; Hatton & Von Gruenewaldt 1990; Parker et al. 1990). Few Archaean cratons are without early Proterozoic swarms of marie dykes. This phase of marie magmatism clearly marks the beginning of a new marie magmatic cycle. The intrusion of the Great Dyke has, indeed, been used to define the Archaean-Proterozoie transition in Zimbabwe (Nisbet 1982; Wilson 1990). Some of the intrusions, and many of the extensive early Proterozoic dyke swarms, comprise normal continental tholeiites and have recent analogues (e.g. Tarney 1993). Others are more complex and their characteristics are effected by the varying influences of source composition, lower crustal contamination of the magma chamber, high-level intrusion contamination, fractionation, crystal accumulation, magma replenishment, and the mixing of magmas of different compositions. Intrusion forms fall into three main categories, namely sub-horizontal sills, sub-vertical dykes, and 'canoe'-shaped intrusions that have mixed sill and dyke features. These contrasting forms reflect the different tectonic environments with which they are associated (e.g. Hatton & Von Gruenewaldt 1990). The intrusion and mixing of two (or more) different magmas has been postulated to explain the formation of contrasting noritie and gabbroie layers within stratiform intrusions such as the 2.05Ga Bushveld and 2.7Ga Stillwater complexes (Todd et al. 1982; Irvine & Sharpe 1982; Barnes & Naldrett 1986). Noritic layers are also encountered in some 'canoe'-shaped intrusions such as the 2.46Ga Great Dyke of Zimbabwe (Wilson 1982; Podmore & Wilson 1987), the 2.44 Ga belt of intrusions between Tornio and Koillismaa in northern Finland (Ga~il 1985; Alapieti & Lahtinen 1986) and the 2.37 Ga Jimberlana and Binneringie intrusions of Western Australia (McCall & Peers 1971; McClay & Campbell 1976). The parental magma compositions of such intrusions are difficult to decipher, and attention has to be turned to minor satellite dykes that could represent feeders to, or off-shoots from, these larger intrusions. That highly variable suites of dykes and sills are associated with both the Stillwater and Bushveld complexes (Helz 1985; Sharpe & Hulbert 1985; Premo et al. 1990), lends support for their heterogeneous parentage. Evidence for contemporaneous bimodal marie magmatism comes also from dyke swarms which are not closely associated with layered intrusions. One of the most clear-cut examples is given by the various dyke swarms of SW Greenland, and the probable eastward extension of these swarms to SE Greenland and the Scourie dykes of NW Scotland. Six different sets of dykes have been distinguished in SW Greenland, based on their geographic distribution, crosscutting relationships, mineralogy and geochemical characteristics (Hall & Hughes 1990b). Three generations of progressively evolved tholeiitic dolerite dykes (the MD1, 2 & 3 dykes) fan northwards from the southern margin of the eraton. The oldest ones are chemically primitive and have, for example, flat, unfractionated chondrite-normalized REE

30

R. P. H A L L & D. J. H U G H E S

distributions and MORB-like incompatible trace element proportions. The later ones show progressive enrichment in the incompatible elements and higher and slightly fractionated R E E distributions (LaN = 15-50; LaN/LUN = 2-4). These traits are fairly typical of continental tholeiites (Norry & Fitton 1983; Weaver & Tarney 1983). These three dyke generations appear to represent sequential, progressively evolved batches of the same parental magma. A younger set of less common plagioclase-phyric dykes in the southern part of the craton are richer in large ion lithophile elements (LILE: K, Ba, Rb) and high field strength elements (Ti, Zr, Nb, P) at a given Mg number and do not fall on the same petrogenetic trends as the earlier tholeiitic MD dykes (Hall & Hughes 1990b). They have characteristically high LaN and LaN/LUN values (up to 150 and 8 respectively), and appear to have been derived from a separate magma, one possibly more strongly contaminated by continental crust. A second, very dense swarm of tholeiitic dolerites (the Kangamiut dykes) coincides with the Proterozoic (Nagssugtoqidian) mobile belt which marks the northern boundary of the Archaean craton (Kalsbeek et al. 1978; Zeck & Kalsbeek 1981). In contrast to the earlier tholeiitic MD dykes which were intruded into stable continental crust, this swarm was injected between the two stages of the Nagssugtoqidian deformation, the late Archaean transcurrent shearing and the early Proterozoic ductile overthrusting, i.e. into a tectonically active zone at the margin of the craton (Korstg~rd et al. 1987). Despite this major difference in their tectonic setting, preliminary analysis reveals no equivalent significant contrast in their respective geochemical characteristics (Hall & Hughes 1990b), although further work is required to test this more rigorously. Another set of dykes is also abundant in the north of the craton, and it fans out southwards away from the Nagssugtoqidian mobile belt front and parallel to it (Hall & Hughes 1987). Noritic dykes also occur to the north of the boundary (Zeck & Kalsbeek 1981), but the northerly extent of these dykes, into the Nagssugtoqidian belt, is not known. Some also trend E - W , roughly parallel to the mobile belt. These dykes (the so-called 'BN' dykes) contrast markedly with the more normal ophitic and sub-ophitic dolerites in that they comprise olivine (gabbro)norites in which forsterite (Fo9o-7o) and weakly zoned bronzite (Eng~6o) primocrysts are poikilitically enclosed by plagioclase (An6s_4o) (Hall & Hughes 1987). Rare dykes have preserved partial quench textures. The geochemistry of the quenched dykes is not significantly different from that of the coarser grained ones and the compositions of these noritic dykes is considered to reflect closely that of their parental magma. These noritic dykes are geochemically distinctive in that while they are magnesian (up to 22% MgO), they are also relatively siliceous and rich in the light rare-earth and LIL elements, compared to the tholeiitic MD dolerites (Fig. 1). The immediate question regarding the petrogenesis of these noritic dykes is whether they could have been related to the tholeiitic ones by varying degrees of partial melting, fractional crystallization or crystal accumulation within the same parental magma, or perhaps the contamination of a MD-type tholeiitic liquid. None of these models appears to be tenable; the noritic BN and tholeiitic MD dykes were derived from different mafic magmas. While increased partial melting would produce the higher Mg, Cr and Ni and lower Zr and Ti

i-::.:.:.::%"i!:.

I

I

Rb Ba

I

K

I

I

1

I

1

Nb La Ce Sr Nd

I

1

I

I

I

P Z r S m Eu Ti

1

I

Y Yb

Fig. 1. MORB-normalized multi-element diagram ('spidergram')

showing the geochemical range of the BN noritic dykes of SW Greenland (stippled field) compared to an early MD tholeiitic dolerite dyke from the same region (filled circles), a contaminated komatiite (asterisks; MgO = 15.6%; anal. 3 of Arndt & Jenner 1986), and estimated bulk crust (C; Taylor & McLennan 1985). Diagram adapted from Hall & Hughes (1990c).

contents of the noritic BN dykes, it could not at the same time account for their relatively high Rb, Ba, K, and La contents. Equally, neither the accumulation within a tholeiitic magma of the norite dyke primocryst phases, magnesian olivine and orthopyroxene, nor the separation of these phases from a BN dyke-type liquid could have given rise to a tholeiitic liquid with the composition represented by that of the MD dykes (Fig. 1). To produce the characteristically high LILE and L R E E geochemistry of the noritic dykes, a high-level upper crustal contamination component would have had either to enrich a tholeiitic basaltic magma also in elements such as Mg, Cr and Ni or, more reasonably, to have contaminated a magma originally even more rich in these elements, and to have reduced their levels by a factor of at least 2. As many of the norite dyke have MgO values of c. 16 wt %, and Cr and Ni contents of 2000 and 1000 ppm respectively, this model would require their parental magma to have been ultrabasic (Liang & Elthon 1990). There would seem to be no justification whatever for this hypothesis (Hall & Hughes 1987). The conclusion is that the BN noritic dykes represent a considerable volume of mafic magma whose composition was very similar to that of the dykes themselves; magnesian, siliceous and rich in LIL and light rare-earth elements. These SW Greenlandic noritic dyke are by no means unique. A few similar ones outcrop in SE Greenland. In the central part of the Ammassalik Proterozoic mobile belt they trend E - W (Hall et al. 1989), and to the south of it rare norite dykes trend SE (Walton 1987). Norites and picrites

E A R L Y P R E C A M B R I A N MAFIC M A G M A T I S M

GP

Fig. 2. Schematic map showing the distribution of noritic dykes possibly fanning from the north of the North Atlantic craton in SW Greenland, through SE Greenland to NW Scotland. Exactly how the Nagssugtoqidian mobile belt (NMB), the Amassalik mobile belt (AMB), and the Archaean cratons to the north and south of the AMB correlate with the Lewisian of NW Scotland is not certain. Also shown are Ketilidian mobile belt (KMB) to the south of the Archaean craton; the Churchill province (ChP) and Nain province (NP) of Labrador, and the Grenville province (GP) and Superior craton (SC). Adapted from Korstg~rd et al. (1987) and Hall et al. (1990). also constitute a part of the Scourie dykes of NW Scotland (Tarney & Weaver 1987). The noritic BN swarm of SW Greenland thus appears to extend 500km southwards, normal to the Nagssugtoqidian mobile belt front and curving from NNE to N - S and NNW trending roughly parallel to the coast, and to E - W - and SE-trending dykes in SE Greenland and NW Scotland, normal to the coast and roughly parallel to the Proterozoic mobile belt deformation (Fig. 2). No coast-parallel noritic dykes have as yet been reported from the Nain Province of Labrador, the N - S boundary of the Churchill Province having been postulated as the continuation of the Nagssugtoqidian belt (Korstg~rd et al. 1987). It is perhaps significant that these noritic dykes appear to fan from the northern 'nose' of the Archaean craton in SW Greenland, but it is still not proven whether these bounding major shear zones are all part of the same single, composite mobile belt (Chadwick et al. 1989; Hall et al. 1990). Similar noritic dykes and sills also occur in several other parts of the world. Those associated with the Bushveld and Stillwater complexes have already been mentioned, and these too share the decoupled geochemical characteristics, the high Mg, Cr, and Ni in association with high LILE and LREE, displayed by the dykes of the North Atlantic craton. They are also to be found in the Laramie and Bighorn mountains and Hartville uplift of the Wyoming craton (Snyder et al. 1990; Hall & Hughes 1990c), Western and South Australia (Hallberg 1986; Fletcher et al. 1987; Mortimer et al. 1988), South America (Sial et al. 1987) and Antarctica (Sheraton et al. 1987; Kuehner 1989). The dykes from these different cratons are, of course, not identical but their distinctive compositions do show considerable similarities (Fig. 3).

31

The occurrence of all of these magnesian noritic intrusive suites at the end of the Archaean and into the early Proterozoic is not coincidental. Komatiites represent a characteristic oceanic mafic magma type predominantly of the Archaean (Arndt & Nisbet 1982; Nesbitt et al. 1982), reflecting the relatively high heat flow and mantle plume activity during the early stages of crust formation. Mesozoic komatiites such as those of Gorgona (Aitken & Echeverria 1984) and the vast increase in oceanic crust formation and its obduction equally reflect an immense episode of fertile mantle plume-generated mafic magmatism at this time (White & McKenzie 1989; Larsen 1991; Brooks et al. 1991; Storey et al. 1991; Tarney 1993). The norites must represent either the continuation and the waning stages of this Archaean komatiitic magmatism, or the start of a new style of high-Mg magmatism (Fig. 4). The environments in which the intrusive norite dykes, sills and lopoliths are preserved are clearly very different to those in which komatiitic lavas formed. It has already been argued that the crustal contamination of a komatiitic liquid would not produce the compositions of the norites (Figs 1 and 3). Contaminated komatiites and siliceous high-Mg basalts are slightly less strongly LILE-enriched and have less steeply fractionated REE distributions (Arndt & Jenner 1986; Barnes 1989; Hall & Hughes 1990c). Our contention is that it was the sub-continental lithospheric mantle, previously depleted from the production of immense volumes of Archaean mafic komatiitic-tholeiitic volcanic suites, that was contaminated and replenished by hydrous partial melts from subducted crust at the end of the Archaean and during early Proterozoic tectonism, and which partially melted to give rise to these noritic intrusions (cf. Weaver & Tarney 1981; Hatton & Von Gruenewaldt 1990). In this respect, there is an analogy between the formation of these magnesian, siliceous, LILE- and LREE-rich mafic magmas, and those which form boninitic suites more recently (Crawford 1989). A most significant feature of the various mafic-ultramafic suites of that part of the North Atlantic Precambrian craton of SW Greenland is that the Archaean komatiites, the residual depleted harzburgites, and the subsequent early Proterozoic norites are all present.

Crustal evolution and changing mafic magmatism There is an apparent anomaly in the nature of mafic magmatic assemblages formed during the Archaean and early Proterozoic. There are no obvious extrusive equivalents to the late Archaean-early Proterozoic noritic intrusions, and no common intrusive equivalents to Archaean komatiites. Perhaps this simply signifies a very considerable change in mafic magmatic styles during this Archaean-Proterozoic transition (Fig. 4b). There is also no agreement about the timing of maximum continental crustal growth (Taylor & McLennan 1985). There is, however, abundant geological and isotope geochemical evidence for considerable crustal accretion towards the end of the Archaean, and the growth and stabilization of cratons at this time and, therefore, perhaps the most geologically reasonable crustal growth model should reflect this (Fig. 4; McLennan & Taylor 1982; Veizer & Jansen 1979), taking into cognizance several indirect lines of evidence for the existence of much older crust (e.g. Compston et al. 1986; Edwards & Nisbet 1986; Harper & Jacobsen 1992). There are subtle changes in the geochemistry of

32

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metabasaltic suites from the early Archaean to the late Proterozoic (Condie 1989, 1990), reflecting in part the contribution played by changing tectonic conditions under which these belts formed and were preserved, the evolution of the mantle from which they were derived (Bickle 1990; McKenzie & Bickle 1988) and the Earth's evolving thermal gradient and heat loss (O'Nions et al. 1978; Bickle 1986). The poor preservation record of oceanic crust, both old and young, is clear. The change in chemistry and style of formation from komatiitic to noritic magmatism seems to represent something even more fundamental than a simple temporal modification and evolution of the chemical characteristics of the same type of mafic magmatism with time. It signifies a major link between changing crustal states, from Archaean unstable mafic-dominated crust prone to rapid recycling, to "early Proterozoic stable continental crust, subject to collision or rifting. It was probably due to the significant thickening of the crust at this time that there was melting of the sub-continental lithosphere, which produced the abundant noritic intrusions that a thickened crust could also now support. The supercontinents were being assembled at this time. In the case of the formation of layered intrusions (Hatton & Von Gruenewaldt 1990), marginal sediments were subducted and contaminated the the underlying lithosphere, and further subduction of the ocean basin could have led to eventual upwelling of the asthenosphere (Damon 1983), resulting in the melting of the overlying, newly contamin-

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Fig. 3. MORB-normalized multielement diagrams for norite dykes and sills from: (a) average of 9 sills associated with the Bushveld complex; (b) NW Scotland; (¢) South Australia; (d) Antarctica; (e), (f) and (g) Beartooth, Bighorn and Laramie mountains, Wyoming craton, USA (open symbols represent tholeiitic dolerites with less fractionated element distribution patterns. Data from references quoted in the text; diagram reproduced from Hall & Hughes (1990c).

ated sub-continental lithosphere. The rifting of the newly formed continents gave rise to abundant dyke swarms worldwide. During this rifting, the ascending asthenosphere would equally have caused the melting of the sedimentcontaminated sub-continental lithosphere. It is the intrusion form which perhaps signifies the overall tectonic environment in which these boninite-like magmas intruded. In this scenario, norites more closely form part of a lineage with boninitic suites than they do with komatiites (Fig. 4). Since the seminal work of Sutton & Watson (1951), we continue to learn a great deal about the accretion of the Precambrian cratons, in terms of their structural assembly and their magmatic make-up and evolution. It is now recognized that cratons comprise discrete components with different geological histories, juxtaposed by tectonism. In this respect, they are analagous to composite terranes. The major crustal accretion event, involving magmatic and tectonic thickening towards the end of the Archaean saw the end of significant komatiitic production, and subsequently the birth of a new style of magnesian but relatively siliceous magmatism which formed extensive noritic sills and dyke swarms. Despite the continuance of some magmatic styles from the early Precambrian to the present day, the onset of noritic magmatism is another feature which helps define a petrogenetic break between the Archaean and the Proterozoic. It reflects and results from the crustal thickening event, and marks the beginning of the genesis of boninitic suites.

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We wish to thank J. Tarney and an anonymous reviewer for their helpful comments on the original manuscript.

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MAFIC

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33

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-

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1 5 0 .

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-

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EARLY

PRECAMBRIAN

PARKER, A.J., RICKWOOD, P.C. & TUCKER, D.H. (eds) 1990. Mafic Dykes and Emplacement Mechanisms. Balkema, Rotterdam. PHARAOH, T.C., BECKINSALE, R.D. & RICKARD, D.T. (eds) 1987. Geochemistry and Mineralization of Proterozoic Volcanic Suites. Geological Society of London Special Publications, 33. PIDGEON, R.T. & BOWLS, D.R. 1972. Zircon U-Pb ages of granulites from the central region of the Lewisian, north-west Scotland. Geological Magazine, 109, 247-258. PODMORE, F. & WILSON, A.H. 1987. A reappraisal of the structure, geology and emplacement of the Great Dyke, Zimbabwe. In: HALLS, H.C. & FAHRIG, W,F. (eds) Mafic Dyke Swarms. Geological Association of Canada Special Papers, 34, 317-330. PREMO, W.R., HELZ, R.T., ZIENTEK, M.L. & LANGSTON, R.B. 1990. U-Pb and S-Nd ages for the Stillwater Complex and its associated sills and dikes, Beartooth Mountains, Montana: identification of a parent magma? Geology, 18, 1065-1068. RAPP, R.P. 1993. Origin of Archean granitoids and continental evolution. Los, 72, 225-229. , WA'rSON, E.B. & MILLER, C.F. 1991. Partial melting of amphibolite/ eclogite and the origin of trondhjemites and tonalites. Precambrian Research, 51, 1-25. SHARPE, M.R. & HULBERT, L.J. 1985. Ultramafic sills beneath the eastern Bushveld Complex: mobilized suspensions of early lower zone cumulates in a parental magma with boninitic affinities. Economic Geology, 80, 849-871. SHERATON, J.W., THOMSON, J.W. & COLLERSON, K.D. 1987. Marie dyke swarms of Antarctica. In: HALLS, H.C. & FAHRIG, W.F. (eds) Mafic Dyke Swarms. Geological Association of Canada Special Papers 134 419-432. SIAL, A.N., OLIVIERA, E.P. & CHOUDHURI, A. 1987. Mafic dyke swarms of Brazil. In: HALLS, H.C. & FAHRm, W.F. (eds) Mafic Dyke Swarms. Geological Association of Canada Special Papers, 34, 467-481. SNYDER, G,L., HALL, R.P., HUGHES, D.J. & LUDWIG, K. 1990. Early Precambrian basic rocks of the USA. In: HALL, R.P. & HUGHES, D.J. (eds) Early Precambrian Basic Magmatism. Blackie, Glasgow, 191-220. STOREY, M., MAItONEY, J.J., KROENKE, L.W. & SAUNDERS, A.D. 1991. Are ocean plateaus sites of komatiite formation? Geology, 19, 376-379. SUTTON, J. & WATSON, J. 1951. The pre-Torridonian metamorphic history of the Loch Torridon and Scourie areas in the north-west highlands, and its bearing on the chronological classification of the Lewisian. Quarterly Journal of the Geological Society of London, 106, 214-297. -& 1987. The Lewisian complex: questions for the future. In: PARK, R.G. & TALLY, J. (eds) Evolution of the Lewisian and Comparable High Grade Terrains. Geological Society, London, Special Publications, 7-11. TARNEY, J. 1993. Geochemistry and significance of marie dyke swarms in the Proterozoic. In: COND1E, K.C. (ed.) Proterozoic Crustal Evolution. Elsevier, Amsterdam (in press). • WEAVER, B.L. 1987, Mineralogy, petrology and geochemistry of the Scourie dykes: petrogenesis and crystallization processes in dykes intruded at depth. In: PARK, R.G. & TARNEY, J. (eds) Evolution of the Lewisian and Comparable High Grade Terrains. Geological Society, London, Special Publications, 27, 217-233. --, DALZIEL, I.W.D. & DE WIT, M.J. 1976. Marginal basin 'Rocas Verdes' complex from S. Chile: a model for Archaean greenstone belt formation. In: WINDLEY, B.F. (ed.) Early History of the Earth. Wiley, London, 131-146. TAYLOR, S.R. & MCLENNAN, S.M. 1985. The Continental Crust: its Composition and Evolution. Blackwell, Oxford. THURSTON, P.C. 1990. Early Precambrian basic rocks of the Canadian shield. 27,

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MAFIC

MAGMATISM

35

In: HALL, R.P. & HUGHES, D.J. (eds) Early Precambrian Basic Magmatism. Blackie, Glasgow, 221-247. d~ CHIVERS, K.M. 1990. Secular variation in greenstone sequence development emphasizing Superior Province, Canada. Precambrian Research, 46, 21-58. --, CORTIS, A.L. & CHIVERS, K.M. 1987. A reconnaissance re-evaluation of a number of northwestern greenstone belts: evidence for an early Archean sialic crust. Ontario Geological Survey Miscellaneous Papers, 137, 4-24. TODD, S.G., KEITH, D.W., LERoY, L.W., SCHISSEL, D.J., MANN, E.L. & IRVlNE, T.N. 1982. The J-M platinum-palladium reef of the Stillwater Complex, Montana: I. Stratigraphy and petrology. Economic Geology, 77, 1454-1480. VEIZER, J. & JANSEN, S.L. 1979. Basement and sedimentary recycling and continental evolution. Journal of Geology, 87, 341-370. VILJOEN, M.J. & VmJOEN, R.P. 1969a. An introduction to the geology of the Barberton granite-greenstone terrain. Special Publications of the Geological Society of South Africa, 2, 9-28. -& -1969b. The geology and geochemistry of the Lower Ultramafic Unit of the Onverwacht Group and a proposed new class of igneous rocks. Special Publications of the Geological Society of South Africa, 2, 55-86. WALTON, B.J. 1987. Field diary (unpublished). Geological Survey of Greenland. WEAVER, B.L. & TARNEY, J. 1981. The Scourie dyke suite: petrogenesis and geochemical nature of the Proterozoic sub-continental mantle. Contributions to Mineralogy and Petrology, 78, 175-188. & -1983. Chemistry of the subcontinental mantle: inferences from Archaean and Proterozoic dykes and continental flood basalts. In: HAWKESWORTH, C.L. & NORRY, M.J. (eds) Continental Basalts and Mantle Xenoliths. Shiva, Nantwich, 209-229. --, -& WINDLEY, B.F. 1981. Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland; nature of the parent magma. Geochimica et Cosmochimica Acta, 45, 711-725. WHITE, R.S. & MCKENZIE, D.P. 1989. Magmatism at rift zones: the generation of volcanigenic continental margins and flood basalts. Journal of Geophysical Research, 94, 7685-7730. WHITEHOUSE,M.J. 1989. Sm-Nd evidence for diachronous crustal accretion in the Lewisian complex of northwest Scotland. Tectonophysics, 161, 245-256. -1990. Isotopic evolution of the southern Outer Hebridean Lewisian gneiss complex: constraints on Late Archaean source regions and the generation of transposed Pb-Pb palaeoisochrons. Chemical Geology (Isotope Geoscience Section), 86, 1-20. WILLIAMS,H.R. & STOW, G.M. 1991. Subprovince accretion in the southern Superior Province. Toronto '91 Field Trip B6 Guidebook. Geological Association of Canada. WILSON, A.H. 1982. The Geology of the Great 'Dyke', Zimbabwe: The ultramafic rocks. Journal of Petrology, 23, 240-292. WILSON, J.F. 1990. A craton and its cracks: some of the behaviour of the Zimbabwe block from the Late Archaean to the Mesozoic in response to horizontal movements, and the significance of some of its mafic dyke fracture patterns. Journal of African Earth Sciences, 10, 483-501. WINDLEY, B.F. (ED.) 1976. The Early History of the Earth. Wiley, New York. -1984. The Evolving Continents (2nd edition). Wiley, Chichester. 1993. Uniformitarianism today: plate tectonics is the key to the past. Journal of the Geological Society of London, 150, 7-19. ZECK, H.P. & KALSBEEK, F. 1981. Geochemistry of amphibolite facies metamorphism of a suite of basic dykes, Precambrian basement, Greenland. Chemie der Erde, 40, 1-22. -

-

-

-

Received 26 February 1993; revised typescript accepted 12 March 1993.

Addendum

Additional references

C o n s i d e r a b l e c o n t r o v e r s y still s u r r o u n d s the i n t e r p r e t a t i o n of K u s k y & K i d d (1992) that the Belingwe g r e e n s t o n e belt of Z i m b a b w e comprises a mafic o c e a n i c c o m p o n e n t tectonically j u x t a p o s e d with an ensialic s e q u e n c e ( B l e n k i n s o p p et al. 1993). T h e r e c o g n i t i o n o f A r c h a e a n oceanic crust thus r e m a i n s e v e n m o r e elusive t h a n is c o n c l u d e d in this r e v i e w (e.g. Bickle et al. 1994).

B1CKLE, M.J., N1SBET, E.G. & MARTIN, A. 1994. Archaean greenstone belts are not oceanic crust. Journal of Geology, 102, 121-137. BLENKINSOPP,T.G., FEDO, C.M., BICKLE, M.J., ERIKSSON, K.A., MARTIN, A., NISBET, E.G. & WILSON, J.F. 1993. Ensialic origin for the Ngezi Group, Belingwe greenstone belt, Zimbabwe. Geology, 21, 1135-1138.

Added April 1994.

From QJGS, "10&, 241-242. THE PRE-TORRIDONIAN METAMORPHIC LOCH TORRIDON AND SCOURIE AREAS HIGHLANDS, AND ITS BEARING ON CLASSIFICATION OF THE LEWISIAN BY

JOB-N S U T T O N ~

PH.D., F.G.S., A N D

HISTORY OF THE IN THE NORTH-WEST THE CHRONOLOGICAL

JANET

WATSON,

PH.D., F.G.S.

Read 1 February, 1950 [PLATES X V I I - X X ] CONTENTS Page Introduction and previous work ................................................ 241 P A R T l.--The area around Loch Torridon, Ross-shire (J. S.)

......... 243 I. The complex produced in the first metamorphism ............... 243 II. The dolerite dykes ......................................................251 III. The complex produced in the second metamorphism ............ 254 IV. S u m m a r y and conclusions .............................................262 P A R T 2 . - - T h e a r e a a r o u n d Scourie, S u t h e r l a n d (J. W.) . . . . . . . . . . . . . . . . . . 263 ][. The Seourian c o m p l e x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 I I . The dolerite d y k e s ...................................................... 273 I I I . The L a x f o r d i a n complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276 P A R T 3 . - - S u m m a r y a n d conclusions (J. S. and J. W.) . . . . . . . . . . . . . . . . . . 291

List of works referred to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

295

SUMMARY The p a p e r describes the m e t a m o r p h i c h i s t o r y of two areas of Lewisian gneiss in the N o r t h - W e s t H i g h l a n d s of Scotland. M e t a m o r p h o s e d s e d i m e n t a r y rocks are described for the first time from the Loch Torridon area. I t is shown t h a t b o t h in the L o e h Torridon a n d t h e Scourie districts the gneisses have been p r o d u c e d in two s e p a r a t e periods o f m e t a m o r p h i s m , m i g m a t i z a t i o n and deformation. These two m e t a m o r p h i c episodes, which are named the Scourian and L a x f o r d i a n episodes, are s e p a r a t e d in time b y a period of uplift and tension during which a series of uniform dolerite d y k e s was i n t r u d e d . Since a v e r y g r e a t interval of time appears to have elapsed b e t w e e n the two m e t a m o r p h i c episodes, it is suggested t h a t the rocks p r o d u c e d during these episodes should be regarded as members, n o t of a single f o r m a t i o n as heretofore, b u t of two distinct chronological units. INTRODUCTION AND PREVIOUS W O R K

In the Northern Highlands of Scotland, between the Moine Thrust and the west coast, lies an area of crystalline Archaean or Lewisian rocks unconformably covered by unmetamorphosed Torridonian and Cambrian strata. The Archaean rocks consist mostly of banded gneisses varying in composition from ultrabasic to very acid types. During the course of work carried out in the regions of Loch Torridon in Ross-shire and Scourie in Sutherland, the writers reached certain general conclusions as to the nature and mutual relations of the Lewisian rocks. Confirmatory evidence was sought in the region of Gruinard Bay, arid the results of the whole investigation are given in the present paper. The two areas studied in detail are described first, while in the final section the results obtained are correlated and the conclusions reached are extended to cover much of the Lewisian of the mainland north of Loch Torridon. It has long been realized that the Lewisian complex, as it stands, is the product of a prolonged series of events in which sedimentation, metamorphism and intrusion have all played a part. In 1907 Teall wrote: " the Lewisian gneiss is not a geological formation in the ordinary sense of the word. Even if we exclude from it the later dykes and sills, there still remains a petrographical complex which future research will probably separate into its component parts."

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 37-54 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 447-464

Unravelling dates through the ages: geochronology of the Scottish metamorphic complexes G.

ROGERS

1 & R.J.

PANKHURST

2

l lsotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, UK 2British Antarctic Survey, c/o N E R C Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham N G 1 2 5GG, UK Abstract: The paper by Giletti et al. (1961) is seen as a major landmark in the evolution of dating techniques in polymetamorphic terrains. We consider certain critical issues from each of the main complexes of the Scottish Highlands studied by Giletti et al. to illustrate how subsequent developments in geochronological methodology have influenced our understanding of metamorphic belts. Lewisian examples focus on the formation of Archaean crust, and the age of the main high-grade metamorphism and the Scourie dyke swarm. The antiquity of Moinian sedimentation, its relationship to the Torridonian sandstones, and the timing of Precambrian metamorphism have been controversial issues. The timing and nature of Caledonian orogenesis, most clearly expressed in the Dalradian complex, have been the focal points for the refinement of radiometric investigation. These complexes have been subject to successive developments in methodology, with ever-tighter constraints from Rb-Sr and K-Ar mineral dating, through Rb-Sr and Pb-Pb whole-rock studies, U-Pb dating of bulk zircon fractions, and Sm-Nd whole-rock and mineral investigation, up to the latest technologies of single-grain zircon and ion microprobe analysis. The rocks have released their secrets reluctantly, and many of the questions posed in 1961 have still not been definitively answered. However, the hope of unambiguous solution leads towards greater efforts, ever more reliable data, and a clearer evolutionary picture.

Rb-Sr data and conclusions that, in general terms, have proved correct. The initial impact on the geological community may be judged by the six pages of written discussion following the paper. More importantly, they set the goals and standards for future work. Only the most enlightened of observers can have foreseen the growth of the subject that has followed during the past 30 years, and the degree to which modern geological theories now hinge on a reliable geochronological background (or, occasionally, its absence!). This paper was so influential in both establishing the geochronological groundwork, and dictating the course of subsequent research, that it is all too easy to forget the relatively primitive technical facilities of the early days. This is especially true of the mass-spectrometry where the subsequent advent of digital measurement (instead of the use of chart-recorders for measuring isotope ratios) was accompanied by better methodology: Giletti et al. made empirical corrections for electron-multiplier discrimination, but not for mass fractionation during ionization,, and were restricted to Rb-enriched micas for dating, mostly without control of the initial 875r/86Sr ratio. They were content to obtain errors of + 5 % on calculated ages, whereas today errors of +0.5% are possible even on whole-rock isochrons (with statistical tests for significance), and U-Pb zircon ages are often quoted to c. 2-3 Ma over the whole geological time-scale. Nevertheless, they demonstrated the need for sound practices such as routine measurement of chemical blanks and inter-laboratory standards. Even after many of these advances, the geological application of Rb-Sr

The value of radioactive decay as a principle in determining geological time was recognized almost as soon as radioactivity was discovered: a brief summary of this fascinating history is given by Faure (1986), who points out that Rutherford, about 1905, was the first to make meaningful measurement of mineral ages (using the U-He method). Naturally it took further discoveries and much work before theory and techniques (the work of Nier in the development of mass-spectrometers was particularly critical (e.g. Nier 1940)) were developed to the point where geochronology could be investigated in a practical way. This occurred during the 1950s, when laboratories were set up world-wide and the first data were produced, mostly using K-Ar and Rb-Sr mineral methods. Thus, by 1960, the foundations of modern radiogenic geochronology had been laid, although theory was some way ahead of analytical capability. The landmark represented by the publication of Giletti, Moorbath & Lambert (1961) was that of a regional geochronological study, based upon well-constrained field relationships--the first in Britain and among the first anywhere. The metamorphic rocks of the Scottish Highlands had always been of great importance in the development of geological interpretation of both Archaean gneiss terrains and orogenic mobile belts, at times generating heated controversy. The lack of stratigraphical control, and the wide span of geological history represented in a relatively small area, made this a prime target for testing methods that could provide an absolute time frame. This was addressed by Giletti et al. (1961) with a significant body of reliable 37

38

G. ROGERS & R. J. P A N K H U R S T

geochronology was hampered by uncertainty over the half-life of 87Rb (not resolved until the work of Davis et al. 1977 and the subcommission report of Steiger & J~iger 1977), and poorly constrained calibration of the stratigraphical time-scale (that of Holmes 1960 being the most recent available to Giletti et al.). All the progress made in these aspects of analysis was stimulated by the demonstration, in studies such as that of Giletti et al., that radiometric geochronology indeed had the potential to solve geological problems. Our objective in this paper is to use the progress that has been made in our understanding of the Scottish Highlands since the pioneering work of Giletti et al. (1961) as a means of illustrating the evolution of methods of dating metamorphic events in general. This evolving research has seen publications which themselves have been amongst the first of their kind and represent landmark papers in their own right (e.g. Long 1964; Moorbath et al. 1969; Dewey & Pankhurst 1970; Pidgeon & Aftalion 1978; Hamilton et al. 1979). We do not intend to provide a comprehensive review of all the geochronological data or the geology; thorough and regularly up-dated reviews of Scottish geology have been provided by Craig (1965, 1983, 1991)--the first of these already showing the impact of Giletti et al.'s work. Instead we highlight certain critical issues where the use of new analytical techniques has helped to elucidate the complexities of polymetamorphic terrains and crustal development in orogenic belts (or, in some cases, to present the challenge of a more confusing story!). All ages referred to in this paper have been recalculated using the decay constants recommended by Steiger & J/iger (1977); we have retained the error estimates quoted by the original authors in all cases.

Lewisian Following the basic chronological subdivision of the Lewisian complex by Peach et al. (1907) the classic paper of Sutton & Watson (1951) placed Lewisian evolution in an orogenic context, defining the following episodes. (1) Scourian: consisting of granulite facies gneisses and late granitic pegmatites. (2) Intrusion of a suite of basic dykes--the Scourie dyke suite. (3) Laxfordian: reconstitution of the Scourian gneisses and dykes, mainly to the north of Laxford Bridge (the northern region) and south of Gruinard Bay, under amphibolite facies conditions; granite emplacement. Although a relative chronology had been erected there was no information regarding the absolute ages of the orogenic events, and hence it was impossible to assess the rates of geological processes or the genetic links between them. One of the major achievements of the paper by Giletti et al. was to place firm constraints on the timings of the main Scourian and Laxfordian events. They used Rb-Sr dating of muscovites and K-feldspars from late Scourian pegmatites to assess the age of the Scourian complex, as these geochronological systems were considered to be more robust than K-Ar ages or Rb-Sr biotite ages, observations later encapsulated in the concept of blocking/closure temperatures (e.g. Macintyre et al. 1967; Dodson 1973). The oldest K-feldspar age obtained was 2460Ma (mean of three determinations) with others ranging down to 2140Ma. Biotite dates varied from 2090 to 1480 Ma, in each case giving a younger age than coexisting K-feldspar. Rb-Sr

K-feldspar and muscovite ages from a Laxfordian pegmatite were 1600 (mean) and 1500 Ma respectively, with mean biotite ages from other pegmatites giving 1160-1510Ma. Giletti et al. concluded that the main Laxfordian metamorphism occurred between 1500 and 1600 Ma with the spread to lower ages reflecting mild reheating later than 1100 Ma. The Scourian complex was considered to be older than 2460 Ma with the lower Scourian pegmatite ages being due to variable resetting during the Laxfordian. Giletti et al. therefore established that there was c. 800 Ma between the two metamorphic events then recognized within the Lewisian complex. The subsequent history of geochronology in the Lewisian has been directed in the main at trying to establish the ages of: (1) the protoliths to the Scourian and Laxfordian complexes, (2) the granulite and amphibolite facies metamorphic events, (3) the intrusion of the Scourie dyke suite, (4) any regional variations in these events. The following sections will discuss certain aspects of these problems in the light of evolving geochronological techniques and methodology. Detailed reviews of the chronology of the Lewisian have been presented by Park (1970), Bowes (1978) and Park & Tarney (1987). Protolith formation and Badcallian metamorphism

Following Giletti et al.'s study, Evans conducted an extensive K-Ar investigation in the Lochinver area and, using hornblende data from ultrabasic gneisses, concluded that the Scourian granulite facies metamorphism (termed 'Badcallian' by Park 1970) occurred prior to 2600 Ma (Evans 1965). He also identified an amphibolite facies event which took place penecontemporaneously with the intrusion of the Scourie dykes, which he called the Inverian (see later). A major advance was made by Moorbath et al. (1969) who showed that acid and basic gneisses from the Scourian and Laxfordian complexes gave a single Pb-Pb whole-rock isochron age of 2860 + 100 Ma (Fig. 1), interpreted as the time of major U-depletion during the granulite and amphibolite facies metamorphism. The data also showed that both the gneiss complexes had been in existence c. 2900Ma ago, and therefore that evolutionary models proposing that the gneisses of the Laxfordian complex were formed of completely reworked Scourian rocks (e.g. Sutton & Watson 1951; Bowes 1968; Holland & Lambert 1973) were untenable. Moorbath et al. favoured an interpretation in which the precursors to the gneisses had separated from the mantle shortly before 2900 Ma, and consequently that crustal extraction and metamorphism were part of one 'superevent'. Such ideas led, through further work in Greenland, to the 'Crustal Accretion Differentiation Superevent' model of crustal growth (Moorbath 1977). Evidence for a younger age of Badcallian metamorphism was provided by a series of U-Pb zircon studies, and subsequently by Rb-Sr and Pb-Pb whole-rock investigations. Pidgeon & Bowes (1972) obtained zircon ages of 2663 + 20 and 2648 + 20 Ma for granulite facies gneisses from Kylesku and Badcall, which they interpreted as dating the Badcallian granulite facies metamorphism. A zircon age of 2675 ± 20Ma from Rona (Lyon et al. 1973) indicated the widespread nature of this event, although in this case the metamorphic grade only reached amphibolite facies. A zircon upper intercept age of 2 7 0 2 ± 1 0 M a from an amphibolite gneiss from Harris (Pidgeon & Aftalion 1972)

U N R A V E L L I N G DATES

16.0[-

207pb

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Fig. 1. Pb-Pb whole-rock isochron for granulite and amphibolite facies gneisses from the Lewisian complex showing that both types of gneisses were in existence 2900 Ma ago. (Figure reproduced from Moorbath et al. 1969 with permission from Elsevier Science Publishers BV and S. Moorbath).

39

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3000

500

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was considered to reflect the timing of Scourian granulite facies metamorphism, with the lower intercept age of 1625 + 60 Ma representing isotopic disturbance during Laxfordian retrogressive metamorphism. The presence of euhedral cores in some grains suggested an even older zircon component, possibly related to igneous crystallization, although Pidgeon & Aftalion (1972) argued that, if this were the case, the time interval between crystallization and the early metamorphism was short. However, as the only granulite facies assemblages on Harris occur in the South Harris igneous complex and are Laxfordian in age (Cliff et al. 1983), it is possible that the 2700 Ma age may not be dating a granulite facies event, but rather either igneous crystallization or an early amphibolite facies event. Lyon & Bowes (1977) reported a zircon age of 2826 + 50 Ma from an amphibolite facies gneiss from the Laxfordian complex near Durness, and, whereas the date again indicated an Archaean age for the Laxfordian gneiss complex, the poorer precision hindered detailed interpretation. Rb-Sr wholerock ages of 2633 + 120 to 2731 + 210 Ma (Lyon et al. 1973, 1975; Moorbath et al. 1975), determined from various areas, and a Pb-Pb whole-rock age of 2665 + 26 Ma on samples from a small area near Scourie (Cohen et al. 1991) were again interpreted as dating widespread Rb and U depletion during Badcallian metamorphism. A Pb-Pb whole-rock age of 2680 + 60 Ma was reported by Chapman & Moorbath (1977) on rocks considered on structural grounds to represent the oldest components of the Lewisian; the data did not support a substantial crustal prehistory for these rocks. Recent work by Whitehouse has raised important questions over the interpretation of Pb-Pb whole-rock ages from metamorphic terrains, and whether they have any geological significance. On the basis of assumed peak Scourian metamorphism at c. 2680 Ma a Pb-Pb whole-rock age of 2950 + 70 Ma from the amphibolite facies gneisses north of Laxford Bridge was interpreted as seeing through the effects of the metamorphism to give the date of crustal accretion (Whitehouse & Moorbath 1986). In the light of his other Pb-Pb whole-rock ages (Whitehouse 1989a, 1990) he argued that interpretation of Pb-Pb whole-rock ages from amphibolite facies and lower grade suites must be treated with caution due to the effects of incomplete homogenization of Pb isotopes during metamorphism. Consequently the 'age' of 2950 Ma (Whitehouse & Moorbath 1986), although well-constrained by a low MSWD, was considered spurious. Whitehouse further argued that the only 'reliable' data could be generated from pyroxene-granulite facies assemblages. However, in granulite terrains where the reduction in U / P b ratios has been severe (such as in the

13"5

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Lewisian), the Pb-Pb ages would have poor precision owing to the reduction and homogenization of U / P b ratios during the metamorphism. Whereas the Rb-Sr, Pb-Pb and U-Pb work had given ages that were thought to date the Badcallian metamorphism, the advent of Sm-Nd dating in the late 1970s, coupled with the observation that Sm/Nd does not change during metamorphism (Green et al. 1969), led Hamilton et al. (1979) to study the Lewisian complex using Sm-Nd on whole rocks in an attempt to determine the time of separation of the gneissic precursors from the mantle. Using samples from both granulite and amphibolite facies areas they obtained an age of 2920 + 50 Ma (Fig. 2) which was interpreted as dating the crustal generation of the complex. As this study used such widely separated samples and different rock types, Whitehouse (1988) analysed tonalitic gneisses from the central region and obtained a Sm-Nd whole-rock age of i 2600 + 155 Ma (MSWD = 5.7) with an end of -- 2.4 + 1.9. Sm-Nd whole-rock isochrons from ultramafic intrusions at Drumbeg, Achiltibuie and Scourie gave ages of 2910 + 55, 2810 + 95 and 2670 + 110 Ma (MSWD's = 0.2 - 5.0), which, i coupled with their end values of between + 1 + 0.7 and + 1 . 9 + 0 . 5 (Whitehouse 1989b), were interpreted as indicating the dates of crystallization from mantle-derived I

I

,

/

0.5135 0-5130 Z

/

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Fig. 2. Sm-Nd whole-rock isochron for Lewisian gneisses giving an age of 2920 + 50 Ma, interpreted as dating Lewisian crustal accretion. (Figure reprinted with permission from Nature (Hamilton et al. 1979): copyright (1979) Macmillan Magazines Limited, and from P.J. Hamilton).

40

G. R O G E R S & R. J. P A N K H U R S T

magmas. These results implied that the Nd isotopic composition of the sub-Lewisian mantle was close to that of depleted mantle (DePaolo 1981). The data for the Scourie i tonalitic gneisses which had low end were therefore considered to be due to Nd isotopic homogenization of older gneisses at 2660 Ma during the Badcallian metamorphism. Whitehouse (1989b) also dated hornblende-granulite gneisses from Gruinard Bay and amphibolite-facies tonalitic gneisses from the northern region which gave Sm-Nd i __ whole-rock ages of 2965 + 130Ma (MSWD = 3.8; eNd+3.7 + 2.2) and 2970 + 190Ma (MSWD = 7.9; ~ Ni d = + 4.7 + 2.6), but questioned the significance of these ages on the basis that their eNo i values were higher than DePaolo's (1981) depleted mantle "terqa 2970~ +1.8). He argued that as the mafic bodies studied all appeared to have been derived from a mantle source similar to DePaolo's depleted mantle, and that if the gneisses were derived from mafic/ultramafic precursors, then, given that intracrustal melting and metamorphism have only limited affect on Sm/Nd ratios, the TDM Na ages of the felsic gneisses should give the approximate timing of crustal accretion of each region. Thus, the Scourie region was deemed to have accreted at c. 2930 Ma, the Gruinard Bay region at c. 2860 Ma, and the northern region at c. 2780 Ma. However, if one chooses a different model for the evolution of the depleted mantle _2970 (e.g. Goldstein et al. 1984) then the eNa values for the tonalitic gneisses of Gruinard Bay and the northern region are broadly consistent with derivation from such a source, and both areas could have accreted at the same time. Therefore, if one defines different boundary parameters-even within the same genre of model--significantly different conclusions may be drawn. Whereas Nd model ages may give some insight into the approximate average ages of components of Nd in rock units relative to a specific starting composition, great care must be taken in their interpretation. (See Arndt & Goldstein 1987 for a review of Nd model age applications). Cohen et al. (1991) have also recently addressed the problems of the physical extent and timing of depletion by studying a small area at Scourie. Sm-Nd whole-rock data for mafic and ultramafic samples gave an age of 2702 + 52 Ma (eiNd = +1.8) which was interpreted as the time of Sm/Nd fractionation between these lithologies. This age is identical, within error, to the best estimates of the time of Badcallian 27(X) metamorphism. Lower eNa values for tonalites and metasediments were taken to indicate that these had had a previous crustal history, and that there had been minimal exchange of Nd between the tonalites and more mafic samples during Badcallian metamorphism at c. 2700Ma, thus limiting Nd movement to no more than a metre scale. Consequently, it was concluded that the 2702Ma age represented the time of igneous differentiation. Cohen et al. also argued that Whitehouse's apparently older ages of 2850 and 2910Ma for the ultramafic bodies at Achiltibuie and Drumbeg could be artefacts produced through mixing mafic magma emplaced at 2700 Ma with older tonalitic material: this interpretation depends on these ultramafic intrusions also being c. 2700 Ma old, for which there is no published geochronological evidence. Sm-Nd mineral data for basic gneisses around Scourie were presented by Humphries & Cliff (1982). They obtained uniform errorchron ages of 2490 Ma, largely controlled by garnet, which, they argued, implied that the Scourian complex cooled slowly from the peak of Badcallian

metamorphism at 2660Ma to c. 600°C at 2490Ma. Although Cohen et al. (1988a, 1991) also stated that Sm-Nd and Pb-Pb mineral isochrons from mafic/ultramafic gneisses gave ages of c. 2420Ma (supporting data were not presented) they interpreted the results as dating the final mineral crystallization of the granulite-facies assemblage rather than simply the slow cooling of the complex, on the basis of similar diffusion rates for Sm and Nd to the major cations in garnet and clinopyroxene. Consequently, whereas the Sm-Nd whole-rock data give the age of emplacement of these ultramafic bodies at c. 2700 Ma, Sm-Nd equilibration at the mineral level was not achieved until about 250 Ma later. Major regional U, Th and Rb depletion was accomplished during the peak of Badcallian metamorphism at 2660 - 2700Ma. This interpretation has a number of implications and associated problems. Most significantly, the 2420 Ma age of Cohen et al. is identical to the 2418 Ma U-Pb baddeleyite age of the oldest reliably dated Scourie dyke (Heaman & Tarney 1989; see below). As the dykes cross-cut the Badcallian fabrics (see Park 1991) and were intruded penecontemporaneously with the Inverian amphibolite facies retrogression, it seems highly improbable that the granulite facies fabrics could have been produced at 2420 Ma. Burton & O'Nions (1992) have recently obtained Sm-Nd and Pb-Pb mineral ages from an amphibolite facies agmatite complex at Gruinard Bay. The host amphibolites give ages of c. 3300 Ma whereas the later trondhjemite gneisses are c. 2400 Ma. The date for the amphibolites is the oldest Sm-Nd age yet reported from the Lewisian complex. As the amphibolite date is about 650 Ma older than the age of the Badcallian metamorphism further north, Burton & O'Nions (1992) concluded that information relating to the timing of crustal differentiation or of prograde metamorphism would be unlikely to be preserved in rocks which had suffered granulite facies metamorphism. Such a conclusion presupposes that the rocks further north in the central region were formed significantly before the Badcallian metamorphism, and that Sm-Nd evidence for their original age has been overprinted. The geochronological history of Laxfordian activity does not uniquely serve to illustrate any points we wish to make, and so is not considered in detail. However, for the sake of completeness the granite sheets along the Laxford Front have yielded Rb-Sr and Pb-Pb whole-rock ages of 1650 + 55 Ma to 1754 + 18 Ma (Taylor et al. 1984). Other intrusive granites have given Rb-Sr whole-rock ages of 1429 + 350, 1645 + 170 and 1713 + 34 Ma (van Breemen et al. 1971; Lyon et al. 1973; Lyon & Bowes 1977), and a U-Pb zircon age of l~7~+2°Ma,,-H) (van Breemen et al. 1971). Lambert & Holland (1972) and Lyon & Bowes (1972) reported Rb-Sr whole-rock ages for quartzofeldspathic gneisses from the northern region of 1862+50 and 1713 d:135 Ma, which they interpreted as the time of Laxfordian metamorphism. Consequently, the timing of the Laxfordian metamorphism and granite emplacement is thought to have occurred c. 1700 Ma. Despite 30 years of effort there are still many major unresolved problems regarding the chronology of the Lewisian gneiss complex. This discourse makes it apparent that unravelling the geochronological evolution of such a polymetamorphic, high-grade terrain is a complex business. Not only can the structural geologists not always agree about which structures are being dated (e.g. Inverian or

U N R A V E L L I N G DATES Laxfordian), but many of the ages so far produced are by and large of dubious chronological validity. There are often major difficulties encountered in the interpretation of whole-rock ages, and in certain instances the ages may even be meaningless. K-Ar ages may be largely a function of thermal resetting or excess Ar; Rb-Sr mineral ages may reflect post-crystallization effects and are subject, in some cases, to the choice of initial S7Sr/~'Sr ratio leading to potentially spurious ages; earlier published U-Pb ages based upon highly discordant data may be inaccurate; and Sm-Nd mineral data require a detailed knowledge of the textural relations in order to interpret correctly the precise 'ages' that may be obtained. It is likely that only through detailed mineral geochronology, coupled with perhaps whole-rock data to obtain information on process length scales, will the temporal complexities of the processes involved be further unravelled. The Scourie dykes

The emplacement of the Scourie dyke swarm presents one of the most critical and controversial events in the development of the Lewisian complex. Their generally fine-grained texture and basic composition and the effects of post-intrusive tectono-thermal events all militate against straightforward radiometric dating. The history of study of the Scourie dykes thus provides an interesting example of how evolving analytical methodology may be progressively used to tackle more difficult geochronological problems with increased precision and accuracy, and hence may be able to address regional geological evolution. Sutton & Watson (1951) considered structures that were cut by the dykes to be correlated with the Scourian orogeny and those that affected the dykes to be Laxfordian. Such a model assumed that there was only one phase of dyke injection, and therefore that it represented a unique time marker. This interpretation was challenged by Bowes and co-workers (e.g. Bowes & Khoury 1965; Bowes 1968; Dash et al. 1974) who argued on the basis of structural evidence that dyke injection must have occurred over a considerable timespan. In contrast to the view of Sutton & Watson (1951) that the dykes were anorogenic and emplaced into a cold gneiss complex, O'Hara (1961) and Tarney (1963) proposed that the dykes were intruded at depth into hot country rocks. It was also recognized that the Inverian deformation and amphibolite facies metamorphism occurred after the Badcallian granulite facies event, but prior to and penecontemporaneous with the intrusion of at least some of the dykes (Tarney 1963; Evans 1965). It was argued that the foliated, more deformed, Scourie dykes represented the earliest members of the suite intruded at higher temperatures, whereas the unfoliated variants were emplaced later into cooler crust (Tarney 1963, 1973; Park 1970). The objections highlighted by Bowes were attributed by Park (11970) to both Inverian and Laxfordian structures having similar orientations coupled with ambiguities in the interpretation of early geochronological data. Although Giletti et al. (1961) did not date any of the Scourie dykes, their importance in the chronological framework of the Lewisian was clearly recognized. Shortly afterwards Evans & Tarney (1964) presented the first K-Ar whole-rock data for the dykes covering a range of compositions, which gave ages between 1390-3860 Ma. Due to an overall cluster of ages Evans & Tarney preferred an

41

age of c. 2200 Ma for the intrusion of the dykes as a whole, although one fresh tholeiite gave an age of 1950Ma, suggesting that dyke emplacement may have been a protracted or pulsed affair. The oldest ages (>2200Ma) were attributed to the presence of excess At, whereas younger ages (with the exception of the sample cited above) were considered to be due to variable Ar loss during Laxfordian metamorphism. In contrast to Rb-Sr mineral and K-At studies, which may be reset by later thermal events, Rb-Sr whole-rock dating may in certain circumstances see through these later events to yield information regarding the igneous crystallization of protoliths. Using this philosophy, Chapman (1979) analysed three dolerite dykes--two from Scourie and one from Kylesku--using Rb-Sr whole-rock techniques. Owing to the low Rb/Sr ratios and to the lack of fractionation of Rb/Sr within each individual dyke the errors on the ages were high, although it was stated that the ages and the initial ~7Sr/~'Sr ratios for the individual dykes were not significantly different. However, each dyke had a distinct (if restricted) range of Rb/Sr, and so, by combining the data for the three dykes, a greater spread of Rb/Sr was achieved and hence a reduction in the calculated error. The combined age of 2390 + 20 Ma was thought to represent the emplacement of the Scourie dykes; this date also placed a minimum age on the Inverian metamorphism. No evidence was presented to substantiate the c. 1950 Ma age of Evans & Tarney (1964). In order to obtain more precise ages from individual dykes Cohen et al. (1988c) undertook a Sm-Nd mineral study of Scourie dykes in which an apparently primary igneous mineralogy was still preserved: the detailed results of this study were given in Waters et al. (1990). Olivine gabbro and quartz dolerite dykes gave Sm-Nd mineralwhole-rock ages of 2015 + 42 to 2102 + 77 Ma and 1758 + 7 and 1982 + 44 Ma respectively (Table 1). In the case of the Graveyard dyke a feldspar datum lay below the regression line and was therefore excluded from the calculation. A Rb-Sr mineral-whole-rock age of 1978 + 13 Ma was also obtained from the Rhegreanoch dyke, within error of the Sm-Nd age. Rb-Sr mineral-whole-rock ages for the other dykes generally gave younger ages than the Sm-Nd data. An exception was the Graveyard dyke which gave a Rb-Sr amphibole-feldspar age of 2027-1-11 Ma compared to the Sm-Nd age of 1758-t-7 Ma. This discrepancy was explained by invoking recrystallization and growth of garnet and ilmenite during Laxfordian metamorphism; amphibole and feldspar were deemed to have formed shortly after dyke emplacement, and to have remained closed systems throughout the later event. Clearly a primary igneous mineralogy had not remained undisturbed in this case. No evidence was found for dyke emplacement at 2400Ma, which led Cohen et al. (1988c) to state that the Rb-Sr age of Chapman (1979) was 'erroneous', and probably reflected contamination with continental crust. Interestingly, the Graveyard dyke at Scourie was also one of those used by Chapman in the Rb-Sr study. Whereas the studies considered so far have either been prone to post-emplacement thermal disturbance of isotope systematics, or have had to make assumptions about the geochronological significance of isotopic equilibration between minerals and/or whole-rocks, U-Pb geochronology on phases such as zircon, baddeleyite and titanite, potentially represent self-contained systems with high

42

G. R O G E R S & R. J. P A N K H U R S T Table 1. Geochronological data for Scourie dykes (Waters et al. 1990)

Dyke

Rock type

Phases

Badnaban

Olivine gabbro

Rhegreanoch

Olivine gabbro

Loch Torr an Lochain Graveyard

Olivine gabbro

Poll Eorna

Quartz dolerite

Quartz dolerite

Isotope System Age (Ma)

Fsp-cpx-amph-WR Bi-WR Fsp-cpx-amph-WR Fsp-cpx-amph-WR Fsp-WR

Sm-Nd Rb-Sr Sm-Nd Rb-Sr Sm-Nd

2031 + 62 1714 + 8 2015 + 42 1978 + 13 2102 + 77

Amph-gt-ilm-WR Amph-fsp IIm-WR Fsp-cpx-ilm-WR IIm-WR

Sm-Nd Rb-Sr Rb-Sr Sm-Nd Rb-Sr

1758 + 7 2027 + 11 1738 + 11 1982 + 44 1733 + 7

Fsp, feldspar; Cpx, clinopyroxene; Amph, amphibole; Bi, biotite; Gt, garnet; Ilm, ilmenite; WR, whole-rock

data of Waters et al. (1990) discussed above. The c. 2200 Ma K-Ar whole-rock ages of Evans & Tarney (1964) may therefore represent prolonged cooling following dyke intrusion at deep crustal levels, whereas the younger K-Ar whole-rock age of 1950 Ma may indicate more rapid cooling of later dykes intruded at higher crustal levels. In conclusion, if the Scourie dykes do, in fact, represent a unique structural time marker, then the time interval over which there has to be structural quiescence is about 400 Ma. The final recognition of such a long interval led Park (1991) to state that 'in view of the geochronology, it is likely that such tectonic activity did take place [during the 400 Ma] and may ultimately be proved by more adequate dating.' The studies of Cohen et al. (1988c), Waters et al. (1990) and Chapman (1979) indicate some of the problems of geochronological techniques which require an assumption of isotopic equilibration between coexisting phases or wholerocks, and highlight the great care required in determining which phases are primary and which may be either secondary or have recrystallized. If incorrect textural observations are made then isolated results may yield erroneous interpretations. It is clear, however, that if suitable material can be found and careful petrographic analysis undertaken, then both detailed Sm-Nd and U-Pb mineral studies may yield meaningful emplacement ages for mafic dyke suites. The significance of such ages in the structural evolution of the Lewisian complex requires a

closure temperatures to parent-daughter migration. Although material for analysis has to involve the selection of high-integrity grains in order to avoid the effects of low-temperature Pb loss, and there is always the possibility of analysing grains which have experienced multiple episodes of growth (e.g. Pidgeon & Aftalion 1978), the presence of two internal U-Pb radiometric clocks enables departure from simple closed-system behaviour to be generally readily identified. The development of improved techniques for the production of accurate and precise U-Pb data on small samples (e.g. Krogh 1982b; Parrish & Krogh 1987), and the discovery that mafic dykes may contain trace amounts of zircon and/or baddeleyite (Heaman et al. 1986; Krogh et al. 1987) enabled Heaman & Tarney (1989) to obtain ages on three individual Scourie dykes. A bronzite picrite from Beannach and an olivine gabbro from Strathan +3 gave baddeleyite ages of 2418_+TMa and 1992_~Ma respectively (Fig. 3). The latter was interpreted as the time of dyke emplacement whereas the former, owing to the slight discordancy of the data, was thought to be the minimum age of intrusion with the true age being a little older. Zircons from a norite from Badcall Bay yielded discordant data but their 2°7pb/2~pb ages of 2166-2179 Ma were considered to represent minimum estimates for the age of the dyke. The U-Pb data provide clear evidence for two phases of dyke emplacement, the first at c. 2418 Ma and the second at 1992 Ma, this latter date being consistent with the

a

0.37

2,420

b 2,000

0.45

j,

co

¢o o~

/

/

:~ 0 " 3 6 co

Fig. 3. U-Pb concordia diagrams for

tn IX

J~ 0 . 4 3 IX co 0

1,992+3/-2Myr I

0"35

-~1,071Myr 0"41 8"6

818

I 9-0

91"2

I 9"4

207pb[235 u

91"6

91-8

034 58

5.9

6.0

2o7pb/235

6 1

6.2'

two Scourie dykes, confirming the episodic emplacement of the suite. (a) Beannach dyke. (b) Strathan dyke. Error ellipses are drawn at the 2a level. (Figures reprinted with permission from Nature (Heaman & Tarney 1989): copyright (1989) Macmillan Magazines Limited, and from L.M. Heaman).

U N R A V E L L I N G DATES more reliable knowledge of the timing of the igneous and metamorphic events within the region, without which the structural debate may continue in sterile argument.

The Moinian Supergroup The age, metamorphic history and wider affinities of the Moinian Supergroup have long been contentious issues. Following the great debates of the last century (see Oldroyd 1990 for a review) two main hypotheses emerged regarding the evolution of the area: (1) that the Moinian and Torridonian sediments were of the same age and that orogenesis was entirely Caledonian (Peach in Peach & Horne 1930); (2) that the metamorphism was, at least in part, pre-Cambrian in age (Horne in Peach & H o r n e 1930). With the advent of radiometric dating techniques Giletti et al. (1961) set out to address this problem through Rb-Sr and K-Ar dating of micas from Moinian metasediments and pegmatites. Biotites from schists covering much of the strike length of the Moine gave an average age of 420 + 15 Ma which Giletti et al. interpreted as dating widespread metamorphism of the region. The age also placed a maximum age on the movement of the post-metamorphic Moine thrust. More interesting, however, w e r e Rb-Sr muscovite ages of 690-750Ma from pegmatites from Knoydart and Sgurr Breac. Giletti et al. put forward several hypotheses in which the dates might be a function of later isotopic disturbance, yet all reasonable solutions required the pegmatites to be Precambrian in age. This led Giletti et al. to conclude 'that the Moine sediments, at least in the Knoydart-Morar area are older than 740 m.y.' and that the pegmatites were formed 'possibly at the time of the first, or an early, metamorphism of the Moine sediments', thus supporting Horne's view of a Precambrian metamorphism. The Moinian Supergroup has been shown to have experienced polyphase deformation and two main metamorphic events (e.g. Ramsay 1963; Powell 1974). The timing of the metamorphism and associated deformational episodes are largely constrained by dating intrusive bodies which both pre- and post-date the events. The initial results of Giletti et al. stimulated further research into the geographical extent of the Precambrian event and the timing of the early and late metamorphisms. The results of this work, inextricably linked with the highly contentious issue of the structural interpretation of the Moine (see Harris & Johnson (1991) and references therein), have prompted much heated debate and several major issues are still unresolved. Various studies have shown that the late event in the Moine is dated by: (1) a bulk fraction U-Pb zircon age of 456 + 5 Ma for the Glen Dessarry syenite (van Breemen et al. 1979) which was deformed during this event (Roberts et al. 1984); (2) a concordant U-Pb monazite age of 450 + 10 Ma and Rb-Sr muscovite ages of 438-450 Ma for late pegmatites (van Breemen et al. 1974); (3) concordant U-Pb monazite ages of 455 + 3 Ma from the Glenfinnan area (Aftalion & van Breemen 1980); (4) Rb-Sr muscovite ages from semi-pelitic units of 427 + 8 to 462 + 10Ma (van Breemen et al. 1978). The high closure temperature of the U-Pb monazite system (c. 725 °C; Parrish 1990) suggests that the peak of Caledonian metamorphism was c. 455 Ma, with pegmatite injection and cooling below 500 °C following shortly thereafter. In a classic piece of work Long (1964) showed that the Carn Chuinneag granite, which was intruded between the

l

~k 'Sr

43

CARN CHUlNNEAG GRANITE

/~

°Sr

1 10 I . . . . . . . . . . . . .

/

o,t

0-8J ~

07

/

/ f \ ~ ~ M

/

1/

~ Mine~l,Isochron 403±5Ma

~Y /

WR~qbm="Isochron

( ./ "WR 10

20

30

750

87Rbl~ 86Sr I~ 1000

Fig. 4. Rb-Sr isochron diagram for whole-rocks and minerals from the Carn Chuinneag granite (Long 1964) illustrating isotopic homogenization of the minerals during regional metamorphism while the whole-rocks remained closed systems. two metamorphic episodes (Shepherd 1973; Wilson & Shepherd 1979), gave a Rb-Sr whole-rock age of 5 4 8 + 1 0 M a , but that the minerals from one of these whole-rocks yielded an age of 403 + 5 Ma (Fig. 4). These results were taken to indicate that the granite was intruded at 548 Ma, but that during the later metamorphism, whereas the whole-rocks remained closed systems, the minerals within the whole-rocks isotopically equilibrated to give the age of the reheating event. The whole-rock data clearly indicated that the earlier metamorphism had to be pre-550Ma. Pidgeon & Johnson (1974) performed bulk fraction U-Pb zircon analyses on three facies of the pluton. Data from the Inchbae and Lochan a' Chairn phases defined a reverse discordia with lower intercept ages of 563 + 10 and 502 Ma respectively, whereas the riebeckite gneiss gave a simple discordia line but also with an upper intercept of 522 -1-20 Ma. All these phases were considered to have been emplaced at c. 560 Ma, broadly consistent with the Rb-Sr date. A Rb-Sr whole-rock age of 416 + 15 Ma from the riebeckite gneiss showed that although the Inchbae and Lochan a' Chairn facies had remained closed to Rb-Sr migration on the scale of the whole-rocks during the Caledonian, the riebeckite gneiss had been open. Consequently, whereas the studies at Carn Chuinneag indicated the potential benefits of combined mineral and whole-rock studies, they also sounded a severe note of caution regarding whole-rock methods in polymetamorphic terrains. Further work on the older pegmatites of Giletti et al. has produced Rb-Sr muscovite ages from 647 + 20 to 776 + 15 Ma (Long & Lambert 1963; van Breemen et al. 1974, 1978; Powell et al. 1983; Piasecki & van Breemen 1983). In a detailed study of the C a m Gorm locality van Breemen et al. (1974) found that there was probably only minor disturbance of the Rb-Sr muscovite systematics during the Caledonian metamorphism. In contrast K-Ar muscovite dates from the Ardnish pegmatite (Powell et al. 1983) yield younger ages of 498-410 Ma, indicating partial or total Ar loss during the Caledonian, in accord with other KoAr determinations from the NW Highlands (Giletti et al. 1961; Miller & Brown 1965; Brown et al. 1965a, 1965b). In an attempt to address the possibility that the Rb-Sr muscovite ages might reflect partial Caledonian overprinting of Grenvillian pegmatites van Breemen et al. (1974, 1978)

44

G. R O G E R S & R. J. P A N K H U R S T analysed bulk fraction zircons from an MP1 pegmatite lit from the gneiss which gave a lower intercept age of 556 -t- 8 Ma. Yet again there was no hint of a Grenvillian age in the data (Fig. 5). Nonetheless, Aftalion & van Breemen, largely on the basis of the 1028Ma age, constructed elaborate models of multi-stage Pb loss to account for the observed zircon discordance in terms of a Grenvillian crystallization age for the gneiss. Sm-Nd mineral dating has also thrown a spanner into the works regarding the presence of a Grenvillian event in the Moine. Sanders et al. (1984) obtained Sm-Nd g a r n e t clinopyroxene-whole-rock ages of 1082-1-24 and 1010 + 13Ma for eclogites from the Glenelg inlier which are considered to date the eclogite facies metamorphism. As the Morar group sits unconformably on the Glenelg inlier (Clough in Peach et al. 1910; Ramsay 1958) and is only at low metamorphic grade (e.g. Fettes et al. 1985) it follows that the Morar group must have been deposited after c. 1000Ma. Given that there is stratigraphic continuity throughout the Moinian succession, and that the early Moinian metamorphism is considered to have occurred at pressures of about 6.5 kbar (Fettes et al. 1985), there is considerable difficulty in reconciling the Sm-Nd data with the Rb-Sr data from the Ardgour gneiss (1028 + 46 Ma). In summary, whereas the ages of the Caledonian metamorphism and the Knoydartian pegmatite emplacement are now fairly well constrained, the significance of the Knoydartian event and the timing of the Precambrian metamorphism are still unclear.

performed U-Pb analyses on monazite and zircon - which have higher closure temperatures - from two localities. Monazite analyses were concordant at 780 + 10 Ma whereas discordant zircon data gave ages of 7 4 0 + 3 0 and 815 + 30Ma. Given that these pegmatites were emplaced into rocks of low metamorphic grade, van Breemen et al. (1978) concluded that the ages must represent the time of pegmatite intrusion rather than reflecting slow cooling. Furthermore, the field evidence of pegmatite concordance with the main foliation in the host lithologies suggested that this was also the time of peak metamorphism (termed the 'Knoydartian' by Bowes (1968) and the 'Morarian' by Lambert (1969)). In this interpretation they agreed with the views of Giletti et al. (1961), Long & Lambert (1963), Bowes (1968) and Lambert et al. (1979) in ascribing the pegmatites to orogenic activity. Evidence against this interpretation was provided by Powell et al. (1983) who showed that the Ardnish pegmatite, which they had dated using Rb-Sr on muscovites at 776 + 15 Ma, post-dated the early folding and metamorphism of the Moinian metasediments and was deformed during a later event. The timing of the early metamorphism, however, remained unclear, although a Grenvillian age was plausible in the light of other data (see below). Perhaps the most controversial aspect of Moinian geochronology has centred on the age of the Ardgour gneiss. This was originally held to have been produced by in situ metasomatism during the peak of metamorphism (e.g. Dalziel 1966), but has since been shown to have been intruded during the early metamorphism (Barr et al. 1985). Brook et al. (1976), using large samples, produced a Rb-Sr whole-rock age of 1028 + 46 Ma which they interpreted as indicating that the early metamorphism was Grenvillian in age. Pidgeon & Aftalion (1978) investigated the gneiss using bulk fraction U-Pb zircon techniques. The data defined a reverse discordia giving a lower intercept age of 574 + 30 Ma and an upper intercept age of 1556 ' ~ Ma, which they had great difficulty in reconciling with the 1028 Ma Rb-Sr age of Brook et al. (1976). Aftalion & van Breemen (1980) also --

oso

I

I

Torridonian The dating of unfossiliferous sedimentary successions which do not contain volcanic horizons or igneous intrusions presents a considerable geochronological challenge as such combinations militate against dating strategies such as have been used in the Moine and Dalradian. The Torridonian sandstones are one such succession. Although samples from the Torridonian sandstones

~~

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li re 206pb' 238 0

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P ~tlgl~ 1 lOOMax / ,~55.53uNM R __ _ _ >d" / ~ "106+841JNM GRANITI,,. b Sr AGE,o,~,..,oo l OOO~ ~ ~ / / / .•. .• . .' t~ . . . .

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r 2 ~ / - GLEN DESSARY SYENITE

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~/~ ....

R C 9 0 9 and RC 1 5 2 4

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Z i r c o n s i z e fractions: RC1524 Paragneiss Glenfinnan [J R C 7 0 5 G r a n i t i c G n e i s s :\ R C 9 0 9 Lit in Granitic G n e i s s

30

/

40 3'0

235 U

r5

50

6 ~ 40 - - - -

Fig. 5. U-Pb concordia diagram for samples from Glenfinnan. Inset shows modelled zircon evolution assuming Pb loss during Caledonian and Grenvillian times. See Aftalion & van Breemen (1980) for detailed discussion. (Figure reproduced from Aftalion & van Breemen 1980 with permission from Springer-Verlag and M. Aftalion).

UNRAVELLING DATES which unconformably overlie the Lewisian complex were not analysed by Giletti et al. (1961), the data they obtained regarding the age of the Laxfordian led them to conclude that the Torridonian must be younger than c. 1600 Ma, or possibly 1160 Ma based on an Rb-Sr biotite age from a gneiss from Loch Torridon whose interpretation was problematical. If the Moine and Torridonian sediments were lateral equivalents (e.g. Sutton 1963) then the latter must be older than the c. 740 Ma pegmatites in the Moine. However, given that the Torridonian is unconformably overlain by Cambrian sediments, Giletti et a l . ' s data permitted the Torridonian to be younger than the Moine and to have been derived from it. In order to assess the age and provenance of the Torridonian, Moorbath et al. (1967) analysed both detrital grains and individual pebbles. In the Applecross Formation muscovites from schistose pebbles gave K-Ar ages of 1659-1802Ma whereas Rb-Sr K-feldspar dates from microcline and quartz porphyry pebbles ranged from 1320-1637 Ma. Detrital muscovites from the Diabaig Formation gave a restricted range of Rb-Sr and K-Ar ages from l160-1190Ma. From these results Moorbath et al. concluded that the schistose pebbles were not derived from the Moine metasediments as these had been metamorphosed at least 740 Ma ago, and that some of the pebbles had a provenance in rocks which had been last metamorphosed c. 1700Ma, which they equated with the Laxfordian complex. The ages from the detrital micas indicated that the Torridonian must be younger than about 1190 Ma. In an attempt to define the age of the Torridonian more closely Moorbath (1969) used Rb-Sr whole-rock analyses of shales to construct isochrons for the Stoer and Applecross Formations, which gave ages of 968 + 24 and 788 + 17 Ma respectively (Fig. 6). Moorbath argued that these reflected isotopic homogenization during diagenesis which would have closely followed deposition. This interpretation was questioned by Smith et al. (1983) who suggested, on the basis of palaeomagnetic results which indicated that the Stoer and Applecross Formations were c. 1100 and 1040 Ma respectively, that diagenesis could have occurred significantly later than deposition. Allen et al. (1974) presented 4°Ar-39Ar data on exotic quartz tourmaline pebbles from the Applecross Formation

0-85

I

•

45

which gave ages from 802-2926 Ma. Taking the results at face value and using known palaeocurrent indicators they concluded that the data were consistent with a source in the Preketilidian and Ketilidian belts of Greenland and the Grenville of Labrador. However, it is now recognized that tourmalines are prone to excess Ar and give anomalously old ages (Damon & Kulp 1958); consequently no reliable information can be gleaned from the data. Rb-Sr dating of biotites has provided evidence of uplift and cooling of the Lewisian complex of northern Harris and Lewis at c. 1100 Ma (Cliff & Rex 1989). By comparing these data with Rb-Sr and K-Ar ages for detrital muscovites from the Torridonian and a K-Ar biotite age from a gneiss boulder in a Lower Torridonian tilloid (Moorbath et al. 1967), Cliff & Rex suggested that the northern part of the Outer Hebrides could have acted as a source for some of the Torridonian sediment. As zircon has a high closure temperature for U-Pb compared with either K-Ar or Rb-Sr in micas, U-Pb dating of individual zircon grains can potentially yield information regarding the ages of formation of the sources of sedimentary piles rather than later uplift and/or metamorphic ages. Moreover, because they are generally mechanically and chemically resistant they are most likely to survive sedimentary processes and so provide a full coverage of the provenance spectrum, provided, of course, that all the source rocks contain zircon. Rogers et al. (1990) analysed single detrital zircon grains from the Applecross Formation. The analyses were generally less than 1% discordant and fell into three groups: 1088-1193Ma; 1625-1662Ma; 26282857 Ma. These ages provide a maximum age for the the Applecross Formation of 1088 Ma consistent with both the palaeomagnetic and earlier geochronological results. Rogers et al. concluded that the data were more consistent with a provenance in Labrador than E Greenland. Given the lack of published evidence for zircons of c. 1150 and c. i650 Ma from the Outer Hebrides, a source in such an area also seems unlikely. Thirty years on, and despite the increasing sophistication of geochronological approaches, the problems of the age and affinities of the Torridonian are still unresolved. Stratigraphical correlation with the Moine is still equivocal and would appear to require more detailed knowledge about the age of the Torridonian sediments and the tectono-

a

26-5

Sr87 Sr 88

/

0.85

~ . , 26- 7 /

2~6_4 c 26-3

0-80

• Sr 87 _ Sr86

12a~/ /-i A

b /

/

0-80

26_//~8'26-6

Fig. 6.

Rb-Sr whole-rock isochrons for siltstones and shales from (a) the Stoer and (b) the Applecross Formations of the Torridonian sandstone. (Figure adapted from Moorbath 1969 with permission from Scottish Journal of Geology).

0"75

6_1o ~/~26_12 968-+24Ma ~ 26-11 26-9

0 -7(2 0

-~ 0 . 7 0 8 6 + 0 . 0 0 1 6

Rb87 I

12e~'~12d 12c~12h 0-75 - 12fQ, ,.J U'i29 12b /3A '~4A -,~0.7215+0,0014 0.7C I 0 5

788+_1 7Ma Rb87 Srr "~-6II~ I 10

46

G. R O G E R S & R. J. P A N K H U R S T

thermal history of the Moine. Nonetheless, significant progress has been made regarding the provenance of the sediments which points to areas of fruitful research in the future.

The Dalradian Supergroup Nowadays we look to geochronology to determine both the time of metamorphic recrystallization within a complex belt and its post-orogenic uplift history. These two aspects were already inherent in the treatment of the Dalradian metamorphic complex presented by Giletti et al.

following a brief early Ordovician climactic episode of metamorphism. They drew age contours ('chrontours' or 'thermochrons'), taken to represent lines of synchronous cooling through the respective closure temperatures, and proposed early uplift of the low-grade rocks against the bounding faults and later uplift of the hotter, high-grade rocks in the central areas (Fig. 7). Their model also related the cycle of deposition, deformation, metamorphism, uplift and erosion to the still new ideas for subduction and closure of the Iapetus Ocean to the south, and they further suggested that the post-orogenic Newer Granites (which had mostly given ages of 420-370Ma) were triggered by pressure-release during the uplift phase.

M i n e r a l ages: cooling histories Seven Rb-Sr mica model ages from the Grampian Highlands and three from the Connemara schists of western Ireland were reported by Giletti et al. By subsequent standards this is a small body of data, but they were enough to reveal an unexpected problem. Whereas their data for the Moinian Supergroup (see above) had suggested a Late Silurian/Early Devonian metamorphism at about 420 Ma, consistent with the stratigraphically-controlled age of Caledonian deformation in the Southern Uplands, England and Wales, most of the Dalradian and Connemara schist ages were significantly older at about 475Ma. Although this supported a correlation of the Dalradian and Connemara schists, it also indicated that both had experienced an Early-MidOrdovician metamorphism. Additionally, three of the Dalradian biotite ages were younger and within error of the Moine schist results, suggesting that the later Moinian event had also affected the Dalradian rocks; consequently, 475 Ma could be regarded only as a minimum age for the early metamorphism of the Dalradian. The idea of mineral phases having different 'blocking temperatures', with muscovite being more resistant to overprinting than biotite, was already emerging at the time of Giletti et al.'s paper, and in their Discussion reply to R. Rutland, the authors cautioned that the younger age of 420Ma might not represent the real time of prograde metamorphism but merely that of post-metamorphic cooling. In this they admitted a fundamental property of mineral geochronology, but also predicted how this might be turned to advantage in the interpretation of the thermal history of metamorphic belts. The following years saw the acquisition of a great deal more mineral data in an attempt to resolve this issue (K-Ar: Miller & Brown 1965; Brown et al. 1965a; Harper 1967: Rb-Sr: Bell 1968). These showed a wide spread of ages, from 500 to less than 400Ma in both the Moine and Dalradian. This was still generally taken to represent the effects of overprinting of a late metamorphism on an early one, although authors such as Brown et al. (1965a) carefully discussed the alternative possibility of slow cooling. Particularly influential was the idea of Harper (1967) that low-grade rocks that had only reached their maximum temperature during a short period prior to uplift, should give a better estimate for the metamorphic age than high-grade rocks; he obtained K-Ar whole-rock ages of c. 490-520 Ma from the southern margin of the belt, close to the Highland Boundary fault. The case for the cooling hypothesis was advanced by Dewey & Pankhurst (1970), who interpreted the entire body of data for both the Moine and Dalradian rocks as representing uplift and cooling

"~

IIW,'

• ) .'J / o / ~,~ f ~ ' // . ,'~'" / " \~_.~" ~// L//'/ ~ / / //-~--~z~

/

.} /;11 zx/ ,"),11/~1 /;1~

.)//,'I,'/J" Ii ~

I//

IIi

j / / t " ~ X-~',\ \ ~7. " ~-~'o.. ~. fl}" ~Oo~ - ~ . \ \ ,

/

>soo 490-499 480-489 470-479 460-469 450-459 440-449

° Q 0 • [3 .

4. 2o0-- .49 29 410-419 400-409 > 400

A • v v

%

2 ff

./J~

K-,,.o.m.,.

AA

J

I

40 miles

I

Fig. 7. K-Ar muscovite age-contours ('chrontours' or 'thermochrons') for the Scottish Highlands, as presented by Dewey & Pankhurst (1970). These were interpreted as representing lines of synchronous uplift and cooling through the blocking temperature (c. 350 °C) following a relatively brief climactic episode of deformation and metamorphism, 480-500 Ma ago. Early uplift occurred along the bounding faults in the marginal parts of the orogen (Highland Boundary fault and Moine thrust), whereas the central high-grade areas that had been most deeply buried did not finally cool to 350 °C until 80-100 Ma later. Subsequent work has suggested a more complex pattern of local uplift events, and attainment of peak metamorphism as late as 455 Ma in the NW Highlands (Figure adaptation reproduced by permission of the Royal Society of Edinburgh from Dewey & Pankhurst 1970).

UNRAVELLING DATES This was a very 'broad-brush' approach to the problem. Proper control of time-temperature trajectories for metamorphism requires a great deal of high-quality data. The principles usually applied have been developed from the pioneering work in the Swiss Alps (e.g. J/iger 1979). K-Ar hornblende and Rb-Sr muscovite are generally thought to have relatively high closure temperatures of about 550 and 500°C respectively, K-Ar muscovite about 350°C and biotite (Rb-Sr and K-Ar) about 300°C. The highest temperatures of metamorphism require independent control, possibly using Rb-Sr whole-rock or U-Pb zircon geochronology, whereas fission track data are necessary to date cooling down to 100°C. These methods must be applied to a volume of rock small enough to have had a uniform cooling path, and with sufficient precision to distinguish the separate stages: even then, no direct time-temperature information can be obtained for the prograde heating path prior to the maximum temperature. It is rare that the full set of such measurements is available; the only case in Scotland is that of the Glen Dessarry syenite intruded into the Moinian Supergroup (see above). The data for Glen Dessarry, collated and interpreted by Cliff (1985), are consistent with a fairly simple pattern, showing an initial cooling rate of 30 °C/Ma, falling to 10°C/Ma over the first 40 Ma, and followed by dramatically slower cooling from about 300 °C (Fig. 8). This last stage is, however, governed by an apatite fission track age which may reflect the effect of later re-heating rather than regional post-Caledonian cooling. In general, closure temperatures must depend on a variety of factors, such as mineral composition, grain-size, cooling rate and fluid interactions. Furthermore, Giletti (1991) has claimed that the variations in diffusive exchange rates for Sr are such as to cause major errors in the estimated closure temperature for Rb-Sr in biotite. Various closure temperatures have been proposed for Sm-Nd garnet systems (e.g.c. 500-700 °C: Humphries & Cliff 1982; 900 °C: Cohen et al. 1988b). Mezger et al. (1992)

I

I

I

Zircon

1000

0

0

800 Ik,.

=1

600 E

400

1--

l~

hene

hHornblende ~ Muscovite \ Muscovite Apatite Biotite - - ~

20O

Ap-f-t

(,)

i

~

400

I

I 300

I

Age (Ma) Fig. 8. Cooling pattern for the Glen Dessarry syenite showing the relationship between blocking temperature for each phase and the age determined for that phase. U-Pb ages shown by circles; Rb-Sr ages, filled stars; K-Ar ages, squares; Fission track age, open star. Fission track datum represents mean fission track age north of the Great Glen fault (Hurford 1977). All other data from van Breemen et al. (1979). (Figure reproduced from Cliff 1985 with permission from R.A. Cliff).

47

have recently argued for a closure temperature of c. 600 °C in metamorphic garnets <5 cm in diameter. This implies that for rocks above middle amphibolite facies the Sm-Nd ages obtained using garnet will generally record cooling rather than prograde mineral growth. The most detailed studies that have been made on the Dalradian metamorphic complex have suggested that the cooling history was more complicated than that considered by Dewey & Pankhurst (1970). Dempster (1985) presented Rb-Sr and K-Ar data for two transects from the Highland Border into the Central Highlands, which he interpreted in terms of thermal modelling and tectonics. He found the expected decrease in ages northwards with increasing grade, but with a reversal in Rb-Sr muscovite ages in the area of the Tarfside Culmination, which he explained by early uptilting. From the comparison of the various isotope systems he deduced large fluctuations in cooling rates (1-25°C/Ma) in different sectors, and at different times within individual sectors (Fig. 9). Dempster also favoured the idea that the metamorphic climax might have been reached at progressively later times at greater depths, which could be seen as a reconciliation of the two hypotheses (one metamorphism or two?) first proposed by Giletti et al. (1961). Consequently, despite uncertainties in absolute closure temperatures, the mineral dating approach can be used to explore and elucidate the main elements of orogenic evolution. It must be emphasized that a painstaking collection of adequate geochronological data must be combined with good structural control in order to achieve this. Crystallization ages: dating o f specific e v e n t s

The indirect approach to dating metamorphic and structural events is based upon the accurate and precise dating of igneous rocks with structurally-defined relationships, such as was discussed above in relation to the Carn Chuinneag and Glen Dessarry intrusions. Giletti et al. (1961) presented data for two such intrusions within the Dalradian tract, the pre-metamorphic Ben Vuirich granite (600 + 100 Ma) and the post-tectonic Galway granite (365 + 10 Ma). Although these ages gave only a very loose constraint for a broad span of Caledonian activity, the technique was seen to be a very promising one, and it was applied extensively thereafter in Dalradian geochronology, as can be seen from the results listed in Table 2. The age of the Ben Vuirich granite in particular became a fascinating problem for subsequent study, and our erratic approach to the truth--as we now perceive it--is very instructive. The Ben Vuirich granite is regarded as having been intruded after nappe formation (regional D1/D2) but prior to later, more ductile deformation (regional D3) and the local peak of kyanite-grade metamorphism (Bradbury et al. 1976; Rogers et al. 1989). Giletti et al. (1961) were able to show that, although Rb-Sr mica systems in the granite were obviously reset or strongly affected by Caledonian metamorphism (muscovite and biotite gave ages of c. 500Ma and 420Ma respectively), whole-rock samples preserved evidence of a pre-metamorphic history. They argued that the closed-system model age of these samples of c. 800 Ma, calculated assuming a meteoritic initial 878r/86Sr ratio, could be regarded as a maximum possible age for intrusion. Assuming a more typical crustal value of 0.710 for

48

G. R O G E R S

& R. J. P A N K H U R S T

520

"520 V. RAPID COOLING 15-25"C/Ma

500

500

SLOW COOLING

5 "c/Ma

3

/

480

480

\R~-SF Mo,~ I ~ - s oo'c ~

"\ 460

440

.......

',

\

~'

...-~"

SL% COOL,N~

,'

/

f-..

~'~V SLOW COOLING K-Ar Musc 2-3"C,/~a

""-~O0~-AF C

420

J /"

-440

D

400

460

420

400

Fig. 9. Summary of the mineral age data from a N W - S E transect across the Dalradian of Angus, showing approximatc cooling rates and assumed closure temperatures. A, B, C and D are different structural units. (Figure reproduced from Dempster 1985 with permission from T.J. Dempster).

Table 2. Geochronology of intrusions with known structural ages Rocks

Age (Ma)

Method

Reference

600 + 1(~1" 497 ± 37

(1) (2)

590 ± 2 597 ± 11

Rb-Sr W R model ages Rb-Sr WR, with other Older granites Rb-Sr errorchron U-Pb zircon population lower intercept U-Pb zircon abraded needles U-Pb zircon, ion microprobe

655 ± 519 + 481 ± 492 + 487 ± 482 ± 49() ± 477 + 489 ± 456 + 444 ±

Rb-Sr W R Rb-Sr WR Rb-Sr WR Rb-Sr WR Rb-Sr WR Rb-Sr WR U-Pb zircon Rb-Sr W R Rb-Sr W R U-Pb zircon Rb-Sr muscovite

(6) (7) (3) (8) (8) (8) (9) (10) (11) (12) (13)

Rb-Sr W R Rb-Sr muscovite U-Pb monazite

(6) (14) (15)

I. Ben Vuirich granite

552 + 24 51A~ •~ 7

(3) (3) (4) (5)

!I. Other key intrusions a. Pre-/syn-tectonic intrusions

Portsoy granite gneiss Shetland migmatites Dunfallandy Hill granitc lnsch Upper Zone gabbros Haddo Housc aureole Minor foliated intrusions CasheI-Loch Wheelaun Slieve G a m p h granitc Ox Mountains granodioritc Glen Dessarry syenite Glen Kyllachy pegmatites

17 25 15 26 23 12 1 6 18 5 4

b. Post-tectonic intrusions ?

K e n n e t h m o n t granite Belhelvie pegmatite Strichen granite

453 + 4 463 ± 5 475 + 5

* As reported: all other Rb-Sr ages recalculated with Z8TRb = 1.42 × 10 - t l a 1. ~-Oldest intrusions only; the majority of Newer Granites have given ages of 390-430 Ma by a variety of methods. WR, Whole rock. References: (1) Giletti et al. (1961); (2) Bell (1968); (3) Pankhurst & Pidgeon (1976); (4) Rogers et al. (1989); (5) Pidgeon & C o m p s t o n (1992); (6) Pankhurst (1974); (7) Flinn & Pringle (1976); (8) Pankhurst (1970); Jagger et al. (1988); (10) Pankhurst et al. (1976); (11) Max et al. (1976); (12) van Breemen et al. 1979; (13) van Breemen & Piasecki (1983); (14) van Breemen & Boyd (1972); (15) Pidgeon & Aftalion (1978).

UNRAVELLING DATES

peak of metamorphism a n d / o r due to variation in initial STSr/S6Sr ratios during emplacement. The best age that could be obtained from the data was 552 ± 24 Ma. The geochronological method now most widely used to obtain pre-metamorphic ages is U-Pb dating of zircon. This mineral is highly resistant to open-system behaviour, and in theory even a single, brief episode involving U mobility or Pb loss could be corrected for by extrapolation of linear discordance patterns in the concordia diagram. Zircon fractions were often separated according to their grain size and magnetic susceptibility to obtain a spread of data in this diagram. The Ben Vuirich zircons analysed in this way by Pankhurst & Pidgeon (1976) were unusual in that they defined a statistically excellent discordia line with the lower intercept (514+6Ma) being interpreted as the age of emplacement and the upper intercept of 1316 +2~ 2.~Ma apparently reflecting the age of the source material (Fig. 10). This type of reverse discordia, due to inheritance of pre-magmatic zircons, had already been found in the Carn Chuinneag granite cutting the Moinian Supergroup (see above). It was subsequently demonstrated to be a general feature of Caledonian granites north of the Highland Boundary fault by Pidgeon & Aftalion (1978), who ascribed this to underlying Proterozoic crystalline basement in contrast to Palaeozoic basement further south. Thus, at this time, it was believed that the Ben Vuirich granite was emplaced only shortly before the climax of metamorphism, probably in latest Cambrian-early Ordovician times.

the initial ~TSr/S°Sr ratio, the growth period of radiogenic Sr in the granite would have been about 500 Ma, and so they proposed emplacement at 600-t-100Ma. This, therefore, must also be a minimum age for Dalradian sedimentation. The procedure that they used is closely analogous to the calculation of TDM Nd and Nd model ages in the Sm-Nd system discussed earlier, although the assumption of insignificant parent-daughter fractionation during metamorphism or crustal anatexis is less obviously justified in the case of Rb-Sr. The proper treatment of this problem is, of course, the establishment of a whole-rock isochron for several whole-rock samples with different Rb/Sr ratios, which potentially defines both the age of crystallization and the true initial ~7Sr/g°Sr ratio. The isochron method was still being developed at the time of Giletti et al. (Nicolaysen 1961), and its application in the Scottish Highlands followed within a year or two (Long & Lambert 1963; Long 1964; see above). In the case of the Ben Vuirich granite this approach was complicated by a lack of variation in Rb/Sr (as well as the relative difficulty of collecting a good suite of whole-rock samples from a rather remote locality!). High-precision mass-spectrometry and strict statistical treatment are also essential to the technique, and both were increasingly developed through the late 1960s (e.g. York 1969). Even when this was applied to the Ben Vuirich granite (Pankhurst & Pidgeon 1976), it was found that the isochron model broke down, either due to open system behaviour during the

TCHUR

0"20 B E N VUIRICH Euhedral grains

a

0'15 _

206p b

0-30 ~ , ~ ' ' 0 26 / b B E N V U I R I C H " ~Subhedralgrains 0 22~- 2 0 6 p b 1250~ ' 1 19;i/--"

IO00/A//*~

~ -

28-1

0"10

400/"~ 25"1 0"05 0"00

200/ /

597-+1 1Ma

, u

0"50

0"00

BEN

0"18

0 14[ -16"1

Aqe_+2o

/ ....

°°61-,

0"6

1-50

I

~ 1 5 0 0 ~ ~~-1.~ 1-1

,ooo,. 6.1 / ~ . ~ 15"1

0'10~"

,

1.00

49

207pb/235U ,

,

1"2

1"8

.

2-4

,

,

,

3"0

3-6

4-2

VUIRICH

1000~/'/'//

C 0"16

014

800 , /

/

.

.

0'12

// oooJ,

0 '10

____

///~2 0 08

207 p b / 2 3 5 0 ..... I

0.60

0"85

I 1"10

I

1-35

I

1-60

I

1"85

Fig. 10. Concordia diagrams for zircons from the Ben Vuirich granite. SHRIMP analyses showing (a) the cluster of data points around 600 Ma for euhedral grains and (b) the spread of analyses for subhedral grains to higher ages. Error boxes on (a) are drawn at the l o level whereas on (b) they are 20. Also shown on (b) as filled circles are the bulk fraction analyses of Pankhurst & Pidgeon (1976). (Figure adaptation reproduced by permission of the Royal Society of Edinburgh and R.T. Pidgeon from Pidgeon & Compston 1992). (¢) High precision, selected grain analyses with the inset showing the points for abraded, high-integrity needles giving an age of 590 + 2 Ma. Error ellipses are drawn at the 20 level. (Figure reproduced from Rogers et al. 1989).

50

G. R O G E R S & R. J. P A N K H U R S T

Following the work of Krogh (1982a, b), more emphasis was placed on the selection, cleaning and abrading of only the most perfect, uncracked, euhedral zircons that had clearly formed during igneous crystallization. Selection of just one or a few such high-integrity grains, although requiring demanding low-blank chemistry and very sensitive mass-spectrometry, was usually found to yield concordant data points, with a consequent minimization of ambiguity in interpretation. Even though the discordia extrapolation involved in the Ben Vuirich granite data of Pankhurst & Pidgeon (1976) was relatively short, and apparently well-controlled, the zircons which they had analysed consisted of a mixture of at least two populations: rounded or stubby subhedral crystals (inherited) and a few clear needles (melt-precipitated). In order to eliminate the effects of inheritance and obtain an accurate and precise date for igneous crystallization, Rogers et al. (1989) carried out U-Pb analyses of only high-integrity zircons, especially the clearest, needle-shaped, meltprecipitated crystals. The purest, most strongly abraded fraction analysed was indeed concordant, with two others defining a small degree of normal Pb-loss discordance (Fig. 10). No evidence of inherited U-Pb systematics was detected in these fractions, and so the age was determined accurately and precisely at 5 9 0 ± 2 M a . This result is extremely significant, both for the timing of Dalradian events and for the methodology of determining such events in other metamorphic belts. Firstly, it requires that all preceding Dalradian events (i.e. sedimentation and nappe formation) must have been Precambrian, and that the fossiliferous rocks of the Leny Limestone, previously considered to be part of the Dalradian Supergroup, and as such to link the Dalradian to the Laurentian margin, could no longer be regarded as part of the Dalradian (see also Harris, in discussion of Rogers et al. 1989). Secondly, it cast even more doubt on the general ability of other methods to determine pre-metamorphic ages with any reliability. The validity of the age determined by Rogers et al. (1989) has received support from the work of Pidgeon & Compston (1992), which has also gone some way towards quantitatively explaining the failure of the earlier attempt at zircon dating. These authors have used a highly specialized and still newly-developed method of dating based on ion-microprobe analysis--direct sputtering of ions from the sample, followed by high-resolution mass-spectrometry (Compston et al. 1984). The sensitivity and spatial resolution of this instrument (SHRIMP), which can measure the Pb-isotopic composition of material excavated from a volume of about 10-6mm 3, allows the discrimination of U-Pb ages between the growth zones of individual zircons. It is thus a unique tool for potentially obtaining concordant (crystallization) ages for both the younger igneous and inherited components in a zircon population. The data produced by analysis of zircon grains from the concentrates originally used by Pankhurst & Pidgeon (1976) results in two major observations (Fig. 10). Firstly, euhedral grains cluster around concordia at a weighted mean age of 597 ± 11 Ma, within error of the more precise value of 5 9 0 + 2 M a obtained by selected-grain zircon analysis. Secondly, the subhedral grains exhibit two separate, older groups of concordant or near-concordant ages, interpreted as the primary crystallization ages of zircons inherited by the younger magma; three grains form a group at 950-1100 Ma and two analyses of one grain are close to 1700 Ma. These

dates could potentially correspond to metamorphic recrystallization events in the underlying crust at Grenvillian and Laxfordian times respectively, regardless of whether the Ben Vuirich magma were derived by anatexis of such crust or had merely assimilated it. Alternatively, grains of these ages may have been eroded into sediments (e.g. Dalradian Supergroup) and then entrained by the Ben Vuirich magma either at source or en route through the crust. The data do not provide unequivocal evidence for the presence of crust of these ages beneath the Central Highlands. The upper intercept of the discordia line of Pankhurst & Pidgeon (1976) may be envisaged as the 'weighted mean' of such a mixed population, modified by the effects of Pb loss. The erroneous lower intercept may similarly represent a compromise between the times of igneous crystallization, peak metamorphism, multi-aged inheritance and Pb loss (see also Rogers et al. 1989 for a discussion of this). The perfect linear alignment of different fractions along the discordia line is still hard to explain, but must be related to the way in which this complex mixture of zircons fractionated during the mineral concentration procedures. Nevertheless, the warning is very clear: only genuinely concordant U-Pb zircon data may be used to define crystallization ages in disturbed or polygenetic systems. This experience should lead us to caution in interpreting the data listed in the second part of Table 2, almost all of which are from pre-1980 studies, even though they support a consistent story. Many of these rocks are key eventmarkers, some of which are the subjects of on-going precise modern geochronological analysis. The 655 + 17 Ma Rb-Sr whole-rock isochron for the Portsoy granite gneiss (Pankhurst 1974), another pre-metamorphic 'Older Granite' that gave a 440 Ma Rb-Sr biotite age in Giletti et al. (1961), seems to have survived the later metamorphic overprinting, and, at face value, is fully compatible with late Precambrian Dalradian sedimentation and deformation. However, Hall±day et al. (1989) reported a bulk fraction U-Pb zircon age of 595 -t- 5 Ma from the Tayvallich volcanic sequence in the SW Highlands, at the top of the Middle Dalradian and supposedly correlative with the Portsoy beds. This is thus inconsistent with the Portsoy Rb-Sr age, and only consistent with the new age of the Ben Vuirich granite if deposition of the Upper Dalradian and nappe formation were all accomplished within less than 12Ma prior to granite emplacement. The data of Pankhurst (1970) for the basic intrusions in the NE Grampians have been very influential in ascribing an age of about 490 Ma to the peak of metamorphism in the Buchan area of high-T, low-P metamorphism. This date is supported by an age of 490 Ma--being the mean 2°7pb/Z°6pb age of six magnetic fractions of discordant zircon--from the Cashel-Lough Wheelaun intrusion in Connemara which occupies a similar structural position (Jagger et al. 1988). The age for these intrusions compared with that for the Glen Dessarry syenite ( 4 5 6 + 5 M a ) has been taken as indicating diachronism of Caledonian metamorphism and deformation across the orogen with younger times in the west (e.g. van Breemen et al. 1979; Powell & Phillips 1985). Van Breemen & Piasecki (1983) reported an average Rb-Sr muscovite age of 444 ± 4 Ma from late pegmatites associated with the Glen Kyllachy granite in the NW Grampian Highlands which they said was intruded late in the local F3 stress field. They argued that this indicated that there was no significant difference between the timing of deformation

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in the NW Grampian Highlands and the Northern Highlands, and that any major diachronism occurred between the NW Grampians and the other parts of the Dalradian block. However, a number of the late- F3 pegmatites analysed by van Breemen & Piasecki (1983) gave ages down to 422 Ma which they interpreted as being due to resetting by the later Findhorn granite: this raises doubts as to whether the other pegmatite dates have also been partially reset. Moreover, as there is no evidence in the Northern Highlands for the c. 600Ma deformation and metamorphism (Grampian orogeny) which affected the Dalradian block (Rogers et al. 1989), correlation across the Great Glen must still be considered doubtful. If the Glen Kyllachy granite really is 444 Ma old, then the fact that the post-tectonic Strichen granite from the NE Grampians has given a concordant U-Pb monazite age of 475 + 5 M a (Pidgeon & Aftalion 1978) suggests that there may be lateral diachroneity within the Dalradian block. As pointed out by Dempster (1985), however, such observations could also be due vertical diachronism exposed by differential uplift. Consequently, in order to assess the relationships between the timing of deformation and metamorphism in differing areas a body of reliable geochronological data is required based on sound field relationships. Defining the full chronicle of events during orogeny is still not a simple matter, more than 30 years after the work of Giletti et al.

Future d e v e l o p m e n t s The advent of high-precision mass-spectrometry, coupled with careful mineral geochronology is clearly reaping significant rewards in our understanding of Highland evolution, and of orogenic belts elsewhere (e.g. Corfu 1988; Mezger et al. 1992). Of particular importance is the advent of multi-isotopic techniques applied to garnet (e.g. Mezger et al. 1989, 1992) as these may potentially be able to relate geochronological information to P-T conditions on the same sample (but see the note of caution in Mezger et al. 1992). The use of a laser micro-probe for K-Ar and 4°mr-39Ar studies of individual detrital micas (e.g. Kelley & Bluck 1989, 1992) is providing exciting data relating to sedimentary provenance and regional tectonics. As mentioned earlier, the ability of the SHRIMP instrument to date multiple stages of growth within zircon grains provides the potential for determining the age spectrum of inherited zircon cores within granitoid magmas; moreover, it may prove a relatively rapid technique for assessing the overall spread of ages in detrital zircons from a given rock, where high precision data are not necessarily required. All these applications should make important contributions to future research. Conclusions The seminal paper by Giletti et al. (1961) did indeed, to quote J. Sutton's words in the written Discussion of the paper, 'prove a landmark in Highland investigation'. The Highlands have been at the forefront of many subsequent developments in geochronOlogical approach. Using more precise analytical methodology and diverse isotope systems, the chronology has been refined somewhat, though as indicated throughout the text, many fundamental questions still remain unanswered, though not necessarily unanswerable. The m o d u s operandi for major future progress

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51

lies in detailed mineral studies, allied, of course, to sound field and textural control of samples. The manuscript benefitted from c o m m e n t s b y S. M o o r b a t h , R . A . Cliff and M.J. W h i t e h o u s e

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timing of orogenic activity in the Caledonian rocks of the British Isles.

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1976. Age and structural setting of the Slieve Gamph igneous complex, Co. Mayo, Eirc. Journal of the Geological Society, London, 132, 327-336. PARK, R.G. 1970. Obscrvations on Lewisian chronology. Scottish Journal of Geology, 6, 379-399. 1991. The Lcwisian complex, ln: CRAIG, G.Y. (ed.) Geology of Scotland, 3rd edition. The Geological Socicty, London, 25-64. & TARNEY, J. 1987. The Lcwisian complcx: a typical high-grade gneiss terrain? In: PARK, R.G. & TARNEY, J. (eds) Evolution of the Lewisian and comparable Precambrian high-grade terrains. Geological Society, London, Spccial Publications, 27, 13-25. PARRISH, R.R. 1990. U-Pb dating of monazite and its application to geological problems. Canadian Journal of Earth Sciences, 27, 1431-1450. & KROGH, T.E. 1987. Synthesis and purification of 2°sPb for U-Pb geochronology. Chemical Geology (Isotope Geoscience), 66, 103-110. PEACH, B.N. & HORNE, J. 1930. Chapters on the geology of Scotland. Oxford University Press, London. ---, GUNN, W., CLOUGH, C. T. , H1NXMAN, L. & TEALL, J . J . H . 1907.

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The geological structure of the North-West Highlands of Scotland.

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Geological Society, London, Memoirs, 9, 17-39. BROOK, M. & BAIRD, A.W. 1983. Structural dating of a Precambrian pegmatite in Moine rocks of northern Scotland and its bcaring on the status of the 'Morarian Orogeny'. Journal of the Geological Society, London, 140, 813-823. RAMSAY, J.G. 1958. Moine-Lewisian relations at Glenelg, Inverness-shire. Quarterly Journal of the Geological Society of London, 113, 487-520. 1963. Structure and metamorphism of the Moinc and Lcwisian rocks of the north-west Calcdonides. In: JOtINSON, M.R.W. & STEWARt', F.H. (cds) The British Caledonides. Oliver & Boyd, Edinburgh, 143-175. ROBERTS, A.M., SMrrlt, D.I. & HARRIS, A.L. 1984. The structural setting and tectonic significance of the Glen Dessary syenite, Inverness-shire. Journal of the Geological Society, London. 141, 1033-1042. ROGERS, G., DEMPSTER, T.J., BLUCK, B.J. & TANNER, P.W.G. 1989. A high-precision U-Pb age for the Ben Vuirich granite: implications for the evolution of the Scottish Dalradian Supergroup. Journal of the Geological Society. London, 146, 789-798. --, KROGH, T.E., BLUCK, B.J. & KWOK, Y.Y. 1990. Provenance ages of the Torridonian sandstone of NW Scotland using single grain U-Pb zircon analysis. Geological Society of Australia Abstracts, 27, 84. SANDERS, I.S., VAN CALSTEREN, P.W.C. & HAWKESWORTrf, C.J. 1984. A Grcnville Sm-Nd age for the Glenelg eclogite in north-west Scotland. Nature, 312, 439-440. SHEPHERD, J. 1973. The structure and structural dating of the Carn Chuinneag intrusion, Ross-shire. Scottish Journal of Geology. 9, 63-88. SMrrH, R,L., STEARN, J.E.F. & PIPER, J.D.A. 1983. Palaeomagnetic studics of the Torridonian sediments, NW Scotland. Scottish Journal of Geology, 19, 29-45. STEIGER, R.H. & JXGER, E. 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359-362. Su'vrON, J. 1963. Some events in the evolution of the Caledonides. In: JOHNSON, M.R.W. & STEWART, F.tt. (eds) The British Caledonides. Oliver & Boyd, Edinburgh, 249-269. & WATSON,J.V. 1951. The prc-Torridonian metamorphic history of thc Loch Torridon and Scourie areas in the north-wcst ttighlands, and its bearing on the chronological classification of the Lewisian. Quarterly Journal of the Geological Society of London, 106, 241-307. TARNEY, J. 1963. Assynt dykes and their metamorphism. Nature, 199, 672-674. 1973. The Scourie dyke suite and the nature of thc lnverian event in Assynt. In: PARK, R.G. & TARNEY, J. (eds) The early Precambrian of Scotland and related rocl~ of Greenland. University of Keele, 105-118. TAYLOR, P.N., JONES, N.W. & MOORBATH, S. 1984. Isotopic assessment of rclative contributions from crust and mantle sources to the magma genesis of Precambrian granitoid rocks. Philosophical Transactions of the Royal Society of London, A310, 61)5-625. VAN BREEMEN, O. • BOYD, R. 1972. A radiometric agc for pegmatitc cutting the Belhelvie mafic intrusion, Aberdeenshire. Scottish Journal of Geology, 8, 115-120. & PIASECKI,M.A.J. 1983. The Glen Kyllachy granite and its bearing on the nature of the Calcdonian Orogcny in Scotland. Journal of the Geological Society, London, 140, 47-62. --, AVrALION,M. & PIDGEON, R.T. 1971. Thc agc of the granitic injection complex of Harris, Outer Hebrides. Scottish Journal of Geology, 7, 139-152. --, PIDGEON, R.T. & JOHNSON, M.R.W. 1974. Precambrian and Palaeozoic pegmatites in the Moincs of northern Scotland. Journal of the Geological Socie~, London, 130, 493-507. --, HALLIDAY, A.N., JOHNSON, M.R.W. & BOWLS, D.R. 1978. Crustal additions in late Precambrian times. In: BowLs, D.R. & LEAKE, B.E. (cds) Crustal evolution in northwestern Britain and adjacent regions. Geological Journal Special Issue, 10, 81-106. --, AFrALION,M., PANKHURST,R.J. & RICHARDSON,S.W. 1979. Agc of the Glen Dessary syenite, Inverness-shire: diachronous Palaeozoic metamorphism across thc Great Glcn. Scottish Journal of Geology, 15, 49-62. WATERS, F.G., COHEN, A.S., O'NtoNS, R.K. & O'HARA, M.J. 1990. Development of Archaean lithosphere deduced from chronology and isotopc chemistry of Scourie dykes. Earth and Planetary Science Letters, 97, 241-255. WHITEtIOUSE, M.J. 1988. Granulite facies Nd-isotopic homogenization in the Lewisian complex of northwest Scotland. Nature, 331, 705-707. 1989a. Pb-isotopic evidence for U-Th-Pb behaviour in a progradc amphibolite to granulitc facies transition from the Lewisian complex of north-wcst Scotland: implications for Pb-Pb dating. Geochimica et Cosmochimica Acta, 53,717-724. 1989b. Sm-Nd cvidencc for diachronous crustal accrction in the Lcwisian complex of northwest Scotland. Tectonophysics, 161, 245-256. --,

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WILSON, D. & SHEPHERD, J. 1979. The Cam Chuinneag granite and its aureole. In: HARRIS, A.L., HOLLAND,C.H. & LEAKE, B.E. (eds) The Caledonides of the British Isles--reviewed. Geological Society, London, Special Publications, 8, 669-675. YORK, O. 1969. Least squares fitting of a straight line with correlated errors. Earth and Planetary Science Letters, 5, 320-324.

Received 28 November 1992; revised typescript accepted 21 December 1992.

Addendum Aspects of the geochronological evolution of the Lewisian complex have recently been studied using detailed U-Pb techniques (such as those described in the Ben Vuirich section of this paper) by Corfu et al. (in press) in an attempt to unravel some of the complexities outlined earlier in the Lewisian section of this paper. By selecting high-integrity zircons and zircon fragments of specific morphologies from granulite facies gneisses and pegmatites Corfu et al. (in press) documented the importance of a major event at c. 2480 Ma (early Inverian?) within the Scourian complex. This event included: (1) pegmatite emplacement (the suite of potash pegmatites dated at 2220-2550 Ma by Giletti et al. (1961) and regionally considered to be between 2325 and 2555 Ma by Evans & Lambert (1974) on the basis of Rb-Sr whole-rock data); (2) strong resetting of zircons within the gneisses; (3) the development of zircon overgrowths in some of the gneisses. Similar U concentrations and T h / U in the overgrowths and cores of zircons from both mafic and felsic gneisses implied an isochemical process which Corfu et al. (in press) attributed to high-grade granulite facies metamorphism. Because of the intensity of this 2480Ma event, the exact timing of protolith formation and early metamorphism remain uncertain, though the data suggested that these occurred prior to 2710MR. The previous age of c. 2660 Ma for Badcallian metamorphism (Pidgeon & Bowes 1972) is probably too young due to rotation of the discordia line as a result of later Pb loss, and to the averaging of the complex age distribution pattern of the zircons in the gneisses by using large sample sizes. Corfu et al. (in press) also showed that a banded gneiss from the Scourian complex contained zircon growth at >2716Ma and <2482 MR. In contrast to the other gneisses studied, however, Corfu et al. (in press) argued that, on the basis of zircon morphology and the T h / U of the grains, the younger zircon growth was due to melt infiltration and not to isochemical metamorphism; consequently some of the leucocratic bands in this gneiss were formed during the Inverian, coeval with the pegmatites which intrude the gneiss complex. It thus appears that there were two periods of granulite facies metamorphism within the central region of the Lewisian complex: one at ->2710MR and the other at 2480-2490 MR. Such a late high-grade event would help to explain

the Sm-Nd mineral ages of 2490Ma of Humphries & Cliff (1982) which would thus reflect more rapid post-metamorphic cooling rather than slow cooling over 150-200 MR. By using U-Pb techniques on other minerals Corfu et al. (in press) were able to more fully document the thermal history of the area. Titanites from a metasedimentary layer at Scourie More were shown to have grown during the early Inverian (2480Ma) and to have uffered Pb loss and probable regrowth during Laxfordian events (1750Ma). Titanites from other gneisses indicated Laxfordian growth, but also isotopic disturbance and new growth at c. 1670MR. This late event was also reflected in rutile growth (HeRman & Tarney 1989; Corfu et al. in press). Within the Laxford Front zone, but within gneisses of the Laxfordian complex, Corfu et al. (in press) found a completely different pattern of zircon discordance to that of the Scourian complex. One near-concordant zircon fragment gave a 2°7pbF°7pb age of 2882 MR, but despite detailed picking and air abrasion, the other data were all strongly discordant. The intense early Inverian event of the central region was not evident in this sample. These new detailed U-Pb studies have revealed the importance and varied nature of early Inverian events (c. 2480Ma), have highlighted the occurrence of a late event (c. 1670MR), and have indicated contrasting thermal histories between the Scourian and Laxfordiancomplexes. However, the complexity and intensity of processes occurring repeatedly in such a high-grade terrain have obscured the timing of the earlier events, such that definitive ages for protolith formation and early Badcallian metamorphism are still uncertain. Despite the application of increasingly sophisticated techniques many fundamental questions are still unanswered.

Additional references CORFU,F., HEAMAN,L.M. & ROGERS,G. 1994. Polymetamorphic evolution of the Lewisian complex, NW Scotland, as recorded by U-Pb isotopic compositions of zircon, titanite and rutile. Contributions to Mineralogy and Petrology, 117, 215-228. EVANS, C.R. & LAMBERT, R.ST.J. 1974. The Lcwisian of Lochinver, Sutherland; the type area for the lnverian metamorphism. Journal of the Geological Society, London, 130, 125-150.

Added November 1994.

From QJGS, 1 17, 233-234. A GEOCHRONOLOGICAL STUDY OF THE METAMORPHIC COMPLEXES OF THE SCOTTISH HIGHLANDS BY BRUNO J. GILETTI~ PH.D.~ STEPHEN MOORBATH~ M . A . D . F t t I L . F.G.S. AND RICHARD ST. JOHN LAM'BERT~ M.A. P H . D . F . G . S . S u b m i t t e d 19 October 1960 ; revised m a n u s c r i p t received 13 F e b r u a r y 1961 ; read 4 J a n u a r y 1961

[PLA~ I X ] Co~m~rs I. I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I . SnmrniEry of geological d a t a on t h e age of t h e m e t a m o r phic complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) The Lewisian complex . . . . . . . . . . . . . . . . . . . . . . . ,. (b) The Moine a n d D a l r a d i a n Series . . . . . . . . . . . . . . . . III. Analytical methods ................................. IV. Geochronological d a t a a n d discussion . . . . . . . . . . . . . . . . . (a) T h e Scourie area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) L a x f o r d i a n m e t a m o r p h i s m . . . . . . . . . . . . . . . . . . . . . . (c) The Moine Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (d) T h e D a l r a d i a n Series . . . . . . . . . . . . . . . . . . . . . . . . . . (e) R o c k s f r o m C o n n e m a r a , I r e l a n d . . . . . . . . . . . . . . . . . V. Correlations w i t h o t h e r areas . . . . . . . . . . . . . . . . . . . . . . . . . VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V I I . A p p e n d i x . Localities a n d descriptions of a n a l y s e d samples V I I I . L i s t of references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PAGE 234 234 234 235 238 240 241 243 245 249 253 253 254 255 262

SUMM~RY R u b i d i u m - s t r o n t i u m a g e - d e t e r m i n a t i o n s are p r e s e n t e d for m i n e r a l s a n d whole rocks from the Lewisian, Moinian a n d D a l r a d i a n m e t a m o r p h i c complexes o f Scotland a n d from the C o n n e m a r a schists of western I r e l a n d . Ago d a t a from the Lcwisian eoml)lex confirm t h a t it was affected b y two m a j o r periods of m e t a m o r p h i s m . P e g m a t i t e s associated w i t h the Scourian p a r t of t h e Lowisian complex are s h o w n to be a t least 2460 m . y . old, whereas t h e L a x f o r d i a n m e t a m o r p h i s m occured a b o u t 1600 m.y. ago. T h e effect of t h e L a x f o r d i a n m e t a m o r p h i s m on t h e Scourian p e g m a t i t e s is to p r o d u c e a s c a t t e r of ages in which coexisting p o t a s s i u m feldspars a n d biotites show the p a t t e r n potassium-feldspar age > biotite age. Six biotites, a microcline a n d a m u s c o v i t e from t h e Moine Series h a v e ages in t h e range 435 to 405 m.y., showing t h a t a widespread Caledonian (sen~u atr/c~) metam o r p h i s m affected the Moine Series 420 4- 15 m . y . ago. Two p e g m a t i t e s f r o m t h e K n o y d a r t - M o r a r a r e a yielded m u s c o v i t e s w i t h ages of 740 m . y . a n d 665 m . y . ; a s u r v e y of the geochemical possibilities a n d consideration of the geological s e t t i n g of the p e g m a t i t e s suggest t h a t the Moine s e d i m e n t s in this area are older t h a n 740 m . y . a n d m a y h a v e u n d e r g o n e an e a r l y m e t a m o r p h i s m before this date. Specimens from t h e D a l r a d i a n Series of P e r t h s h i r e suggest a m a j o r m e t a m o r p h i s m a t 475 -t- 15 m . y . ago, i n t e r p r e t e d as L o w e r or Middle Ordovician in age. Two whole-rock a n d t h r e e mineral a n a l y s e s f r o m the p r o - m e t a m o r p h i c B e n Vuroch granite-gneiss suggest t h a t the intrusion was f o r m e d 600 4- 100 m . y . ago a n d t h a t a partial r e c o n s t i t u t i o n occurred 415 -[- 10 m . y . ago. T h e Ben V u r o c h g r a n i t e complex as a whole appears to h a v e b e h a v e d as a closed s y s t e m w i t h respect to r u b i d i u m a n d s t r o n t i u m d u r i n g later m e t a m o r p h i s m . Three specimens of m u s c o v i t e a n d biotite from the Cormomara schists of western I r e l a n d have a m e a n age of 475 m . y . ; this finding t e n d s to s u p p o r t t h e generally supposed e o n t e m p o r a n e i t y of t h e I ) a l r a d i a n a n d C o n n e m a r a m e t a m o r p h i s m s . B i o t i t e f r o m t h e G a l w a y g r a n i t e h a s a n age of 365 ± 10 m.y., which suggests t h a t this granite m a y be c o n t e m p o r a n e o u s with o t h e r d a t e d Caledonian granites of the British Isles. F o u r p o t a s s i u m - a r g o n ages s u p p o r t the conclusions on the age of t h e L a x f o r d i a n a n d Caledonian-Moinian m e t a m o r p h i s m s .

From Le

Bas, M. J. (ed.), 1995, Milestonesin Geology, Geological Society, London, Memoir No. 16, 57-65

W. Q. Kennedy, the Great Glen Fault and strike-slip motion B.

J.

BLUCK

Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK Abstract: At the time it was written, Kennedy's paper on the Great Glen Fault had clear evidence for

a known lateral displacement, and the evidence was so well presented that it convinced a sceptical geological world that such movements were possible. The acceptance of large scaled lateral movements led to the concept of great fundamental fractures and, with the advent of plate tectonics and a climate of mobilistic thinking, many of these great fractures were later recognized as plate or terrane boundaries. Along with this thinking, new criteria evolved for recognizing those fractures that had been involved in major displacements--in fact the concept of throw became replaced by the concept of role. Role was identified from the history of the blocks on either side of the fracture, and where that history was incompatible with them being together, then a large role was possible for the fault itself. Taking a new look at the Great Glen Fault in these terms, it becomes clear that there are insufficient data on the rocks on either side to allow any conclusions about the nature and timing of its role to be deduced. If the later, displacements, which are at present the main concern of researchers, are the sum of the movements on the fracture then it is ironically the least significant of the four NE-SW fractures in Scotland.

There can hardly be a student who graduated in the 1950s and 1960s who either did not read or was not aware of the paper by W. Q. Kennedy (1946) on the Great Glen Fault. Its impact lay in two areas: he raised the possibility of large scale lateral movements within continental crust, a concept not highly regarded in the fixist days before plate tectonics; the other in the masterly way in which evidence from a range of disciplines was mustered to focus on a single solution. In both these areas lay the future of a large segment of geological thinking. Lateral displacements along faults had been described in New Zealand (McKay 1890) and subsequently in Japan and California. But, as Sylvester (1988) pointed out, there was a reluctance on the part of geologists to apply the clear evidence for lateral movements in recent earthquake zones to the geological record. This reticence was partly lodged in the difficulty in understanding how the crust at either end of the fault accommodated the movement. Kennedy grew up geologically in an environment which was making much of the concepts of faulting. H e was born in Scotland, where, at that time, there was not only the greatest concentrations of faults on the ground, but where obvious large scaled structures, with clear geomorphological and structural signature cut the country from shore to shore (Fig. 1). It was the country where controversy over major structural features had been in existence for, and occasionally raged for, some 80 years. During his undergraduate days with Gregory at Glasgow he would have no doubt been introduced to these controversies and also to the concept of rifting (and the excitement aroused by Africa). Subsequently at the Geological Survey, he was introduced to the concept of wrench faulting by such fellow officers as Anderson who he readily acknowledges as a stimulus to his own paper of 1946. More apposite to the culmination of these ideas in the geographical region of the Great Glen, he would almost certainly have read Cunningham Craig (in H o m e & H i n x m a n 1914) who had already discussed the existence of strike slip movements in the vicinity of the Great Glen Fault.

The 1946 paper (originally given in 1939) was sufficiently rigorous and convincing to justify its publication in geological climate unsympathetic to strike-slip faulting on anything but the smallest scale. Although it may not have overcome the prejudice of the larger geological community against strike-slip movements of some magnitude, it released a number of important papers describing large scaled lateral movements on fault systems throughout the world. Kennedy's paper had, as daring papers so often do, crystallized a whole undertow of feeling that there was more to seen in the world of strike-slip faulting than the prevailing climate of prejudice would allow to be seen. In these ensuing papers, and to some extent in his own paper of 1946, there were faults described which had a long and variable geological history of movement: they had greatly different displacements on them at different times (e.g. the San Andreas Fault by Taliaferro 1941). The concept of the great fundamental fault, as De Sitter (1956) was later to call them, was born; and along with its birth came a change in geological thinking towards a more mobilistic view of geology. There was much discussion on the nature of fundamental fractures, and Bailey & McCallien (1953) pointed out that there were a group of major fractures which were associated with serpentinites and may, for that reason, be tapping magmas from great depths. As more of these large fracture zones were described throughout the world, so grew the numbers of papers which recorded or estimated that blocks of crust could move hundreds of kilometres instead of a few. The 100km lateral displacement estimated for the Great Glen and Dead Sea Rift faults was followed by 450 km for the Alpine fault in New Zealand (Wellman in Benson 1952); > 5 6 0 k m for the San Andreas (Hill & Dibblee 1953) amongst many others. Yet there still remained a whole gamut of faults being described having all the characteristics of fundamental fractures but for which no lateral movement could be determined. These faults were marked by a lack of correlation on either side of them: the Highland Bounday 57

58

B . J . BLUCK

~

I

ol)hioliteaml)hibolite sole 490Ma

thrusting & orl)hism c . 4 6 0 M a ges 4 6 0 - 3 9 0

ROCKS Cover

t Dalradian ] Grampian t Central Highland Granulites

t Crust

c.2.0 S. of

Moine Foreland

0

Km

100 J

Fig. 1. Outline of the major fractures in Scotland, the distribution of the basement units and some important features along the Great Glen Fault. MT, Moine Thrust; GGF, Great Glen Fault; HBF, Highland Boundary Fault; SUF, Southern Upland Fault; SL, Solway line; BV, Ben Vuirich; CC, Carn Chuinneag.

Fault, as described by Anderson (1946) was such an example.

A n e w role for f u n d a m e n t a l fractures At the same time as there was a change in attitude towards a more mobile earth, triggered by Kennedy's 1946 paper, so another Celt working in another Celtic realm had challenged another geological dogma. Although Dana (1873) and Bertrand (1897) had impicitly or explicitly invoked a tectonic control on sedimentation, it was O. T. Jones (1938) who revived and redefined the notion at a critical time, pointing out that in the Lower Palaeozoic rocks of North and Central Wales, tectonics had an influence on sedimentation. Accepting this was the first step towards rejecting a view of geological history which saw it as comprising long periods of quiescence when sedimentary rocks were laid down punctuated by short periods of world-wide change during which time these rocks were deformed and thrown into mountain chains. The mountains were then slowly eroded down to yield sediment for the next cycle. Even Krynine (1945) who was a vigorous proponent of the tectonic control on sedimentation still produced a cycle in which shelves were converted to orogens. The mobilistic view, as presented by Gilluly (1949), effectively brought to an end the concept of punctuated orogeny. Following on the recognition of the nature and diversity

of fundamental fractures and the close relationship between tectonics and sedimentation, came the recognition that faults had an essential role in controlling sedimentation and stratigraphy. Their movements were not accomplished in an instant nor always associated with a stratigraphic void, but it became clear that they exerted a strong control on the nature of the sedimentary record, they often defined source-basin margins and some of the history of their activity could be read from the rocks they helped to generate. It was in this atmosphere of tectonic control on sedimentation, and with the newly described models for the genesis of the Swiss molasse where there was a direct influence of faulting on sedimentation, that the role of great fundamental faults was extended in this country, again by Kennedy (1958) when he turned his attention to the big fractures to the south of the Great Glen. Seemingly detached from his view of the Great Glen Fault, Kennedy again entered the scene of fundamental fractures with postulating that the evolution of Midland Valley of Scotland was largely controlled by the Highland Boundary Fault to the north and Southern Uplands fault to the south. As with the Great Glen Fault, Kennedy had a dynamic view of Midland Scottish geology. Here he traced the long history of the Faults, particularly the Highland Boundary Fault, recognizing activity stretching back to the Arenig when he saw its earliest inception. During Early Devonian times he envisaged that a developing graben had been bounded on the north by the Highland Boundary Fault and to the south by the Southern Uplands Fault, and clearly saw the Highlands and Southern Uplands as the sources of the sediments which filled the rift. The mid-Devonian folding he read as the closing of the 'jaws' of the rift as the Southern Uplands and Highlands closed on each other and folded the rocks of the Midland Valley between. In this account of Kennedy, we see that him interpreting fundamental fractures, such as the Highland Boundary fault, as having differing roles at different times, but he never suggested that either the Southern Uplands or the Highland Boundary faults as having a history of substantial strike-slip. It is instructive to view now the repost by George (1960) who, in strong contrast to Kennedy came from a background of stratigraphy worked out on horizontal or gently folded rocks in South Wales and the Borders. It was the stratigraphy rooted in sequences of events through time rather than sequences through space and time. He saw the stratigraphy of the Midland Valley much more in a layer-cake form (although Kennedy himself was not free of such a view) with no evidence for the bounding faults having anything like the history of control on sedimentation as envisaged by Kennedy, but recognized and extended the concept of multiple movement along the fractures. In addition he pointed out that there were faults within and bounding the Midland Valley which had a control on sedimentation during the Carboniferous.

A n e w e r still role for f u n d a m e n t a l fractures The advances in understanding the nature of fundamental fractures in the interval from 1946 to 1966 were minimal compared with the advances which followed in the next 15 years. An acceptance of plate tectonics had the effect of releasing geologists from the constrains of limited movement of crustal blocks. Movements of continental masses were

W. Q. K E N N E D Y & THE G R E A T GLEN F A U L T proposed which were so large scaled as to make the propositions of Kennedy over 20 years earlier seem trivial. However, of more direct relevance to the interpretation to fundamentral fractures such as the Great Glen was the discovery in 1970s of exotic terranes which had accreted to the west coast of North America but which had a provenance in the east coast of Asia. These blocks had accreted during subduction at various points along the western edge of N America and had migrated northwards along the coast towards Alaska, their probable final home (Jones et al. 1972; Howells 1989). In order to achieve these large scaled movements they had to be bounded by large scaled faults, and one of the identified Earth's fundamental fractures of earlier times, the San Andreas Fault, was identified as such a terrane-bounding fracture. Workers in other regions were quick to see the importance of the terrane concept and, freed from the old constraints of limited lateral movement, began identifying many terranes and boundaries with varying degrees of lateral movement. But in any case the scales of movement now conceived were greater than the lateral persistence of a recognizably similar geology on either side of the fault. The old criteria used to identify major strike-slip displacement (i.e. the distance between correlatable elements across the fault) changed to one where there was no expected correlations across faults. A new set of criteria evolved by which large scaled faulting was determined and t h e y had very little to do with those used by Kennedy on the Great Glen Fault. After Dewey (1969) had demonstrated that the Lower Palaeozoic rocks of northern Britain had formed on a destructive plate margin, there was a significant change in the climate of thinking amongst those working in Palaeozoic geology. Discoveries in Cyprus and Newfoundland of oceanic crust sited on continental blocks prompted much discussion about the mechanics and scales of movement at destructive margins of this kind. When, later, the nature of the Ballantrae Ophiolite was clearly determined (Church & Gayer 1973; Dewey 1974), the problem came to Scotland and it became clear to those working in the Caledonides, as it had to those in the Mediteranean and Appalchians, that there was considerable tectonic significance in having oceanic crust lying on the continent: it implied that there was an enormous displacement of crust, and the faults which bounded the ophiolite were of unimaginable throws. Even the most conservative thinkers had eventually to concede that if the origin of the ophiolite was indeed in an oceanic setting, then from the growing background of what we understood from the oceans and their continental boundaries, its emplacement implied substantial structural activity. In this we see the development of a new argument: the geological nature of two blocks; their origin, associati~on and history of genesis have far more significance to nature of their boundaries than the visible structure of the boundary itself. And it left structural geologists who look closely at the fabrics of fault zones with a new challenge and wider boundaries within which to work. In the Ballantrae Complex this logical demand for substantial displacement was further compounded when, following Williams & Smyth (1973), Spray & Williams (1980) and Treloar et al. (1980) were able to determine that there was a depth of provenance for lithological elements of the metamorphic aureole which exceeded 12 kbar (36 km).. In this, as in many other examples along major tectonic

59

boundaries where many kilometres of displacement can be demonstated, the preservation of the history of movement is recorded only in fragmentary evidence along the lateral extent of the fault zone. Subsequent movement along this critical thrust at Ballantae has, in places, obscured these metamorphic rocks and placed unmetamorphosed spilite in contact with serpentinite with only a minor shear between. The lessons are clear: faults and the rocks in fault zones may only partly record the history of movement between blocks; subsequent movement may cut-out a critical earlier history so it is dangerous to read the history of a fracture from any one point or sector of its length. As already stated, the magnitude of movement must be seen in the context of the history of the blocks on either side of the fault as well as the fault zone itself. The acceptance of this type of evidence for large scale movement, paved the way for the more mobilistic views that followed in the immediately succeeding years. Barber (1985) and Bluck (1985) followed with the view that the major fractures of Scotland, including the Great Glen Fault, bound allochthonous blocks which were suspect terranes i.e. they were blocks which had no direct evidence to indicate that they were adjacent to each other for their entire history. The presence of large faults which bounded deformed blocks with apparently different histories was sufficient proof of suspect terranes, and the analogy with Mesozoic terranes of the American west set a new thinking going in the British Palaeozoic.

Blocks and boundaries: the way forward There are now clear indications in the Tertiary and Recent history of areas like SE Asia and western North America that continents are growing by the continual addition of discrete terranes with a history quite different from that of the block to which they are accreting. Taking this concept a stage further, Dalziel (1991, 1994) has attempted reconstructions of the 1.0 Ga continent and its subsequent break-up and re-assembly into Pangaea. In this reconstruction the re-arrangement of continents requires them to move great distances so that regions now quite remote from each other (western South America and UK for example) may have been juxtaposed in the Neoproterozoic. In the Wilsonian cycle of megacontinent growth and dispersal, there is therefore great potential for bringing together blocks with totally different histories and provenances to amalgamate onto continents with a history quite different from any of the blocks accreted to it. This dispersal of continents is achieved by ocean spreading, but it is the repeated change in the direction of spreading (as can be ciearly demonstrated on most destructive margins today), which makes it probable that continental blocks of all sizes are spread widely before they are re-assembled. But within this regime of widespread dispersal, there is a lower order of lateral movement which is of considerable significance. Tectonic elements, such as arcs, fore-arcs etc. on a single destructive margin, may be broken up and move laterally to re-assemble along the same continental edge as has been demonstrated for areas of the western Pacific such as the Phillipines (Karig 1983). The boundaries between these major tectonic elements on destructive marginsare often zones of weakness, so that fore-arcs can be detached from arcs and back-arcs and

60

B.J.

independenly move along continental margins far from their position of genesis. C a l e d o n i a n terranes a n d their b o u n d a r i e s

It follows from this discussion that a most important step to take in the Caledonides was to define the tectonic elements and terranes and locate their limits. By 1977, following closely on the work of Dewey (1974) and Church & Gayer (1973), McKerrow et al. (1977) made a major step forward in interpreting the Southern Uplands as an accretionary prism. England and Wales was now firmly regarded as a fragment alien to Laurentia, and in looking for the southern limit to the Southrn Uplands, there grew mounting evidence for the presence of another fracture in the Caledonides which had not been recognized by either Kennedy or George: the Solway line. The Solway line is a major fracture sitting alongside the others in Scotland. It owes its birth in the geological literature to the same sort of reasoning which was applied to terranes in the Mesozoic of the American west and to the Ballantrae Complex--it had to be there to satisfy the juxtaposition of an Ordovician arc in the Lake district with a fauna in its sediments quite different from the accretionary prism to the north. In terms of what can be seen in present-day plate regimes, the Lake District Borrowdale arc would have had a fore-arc in front of it and a southerly dipping plate beneath it: the Southern Uplands was a fore-arc, but its structure and stratigraphy supported the palaeontology in suggesting it to be a fore-arc above a plate that was subducting towards the north. There was a lot of ground clearly missing between the two tectonic elements. There was yet another turn in the thinking about major faults: throw was not a meaningful concept any more. An ocean plate of uncertain width had been consumed along the zone where now lies the Solway line. In addition, the presence of a fault that had yet to be seen, was accepted as a necessity because without it a section drawn from southern Scotland to northern England did not make sense in terms of the present-day distribution of tectonic elements. In this way a new form of critical thinking was used in evaluating major tectonic boundaries; a form of thinking used so effectively along the west coast of North America. So great was the role of this previously undiscoverd fault that the meaning of the known and fully exposed fractures had to be questioned in this new light. As with terrane workers elsewhere in the world, the stratigraphy, sedimentation, palaeontology, igneous and metamorphic history and tectonic history all had to be evaluated and rigorously examined for any mismatches or evidence of missing ground across the known major fractures. The Southern Uplands Fault. As an example of the new approach to the evaluation of blocks and boundaries, the case of the Southern Upland Fault is taken. The fault terminates the accretionary prism of the Southern Upland on its northern margin. The Southern Uplands, through the meticulous work of Walton and his many students over the years (Kelling 1962; Walton & Oliver 1991), was demonstrated to have a provenance in a metamorphic block which was associated with volcanic and plutonic rocks and an ophiolite. A fore-arc (accretionary prism) should have an arc to the north of it and it seemed safe to assume that the

BLUCK volcanic rock fragments in the greywackes of the Southern Uplands came from such a source. In the area around Girvan there is a proximal, fault-controlled sequence which overlaps in age with the finer grained turbidites of the Southern Uplands. Age determinations from boulders of granite in the conglomerates at Girvan showed them to have come from a contemporary igneous province which was clearly only a little distance to the north (Longman et al. 1979). This implied that the source of the Southern Uplands sediment was within the region of the Midland Valley or its lateral equivalent. It then became clear that the nature of that igneous source" was almost certainly a dissected arc and the Girvan sequence was its fore-arc. On the assumption that the Southern Uplands was a trench sequence, and there were many doubters (Murphy & Hutton 1986; Stone et al. 1987), there was now a problem that the gap beween the trench and the arc was only a few kilometres wide, so it was proposed that the Southern Uplands block was allochthonous, having been thrust over a continental basement and the gap thus reduced (Bluck 1985). Geophysical investigations have shown shallow continental basement beneath the Southern Uplands (Hall et al. 1983) but this can be regarded only as supportive of the view that they have been displaced northwards if the hypothesis of them being an accretionary prism is correct: a back or fore-arc for instance can be founded on continental crust. As with the Solway line, the history of the Southern Upland Fault is determined from close reasoning over the history of the blocks on either side: the fracture is only poorly exposed and very little of its history is likely to have been preserved in the fault itself. It is easy to imagine that after their initial suturing, movement continued between the receiving continent and the donated terrane and that later movement was likely to overprint or somehow obscure the earlier record of initial suturing. Interpretation of the geological history of terrane boundaries which has undergone this type of accretion is therefore likely to be thwart with potential problems of an incomplete structural record of the amalgamation. The Highland Boundary Fault zone. This boundary differs from the two previously discussed fractures in that there is comparatively good exposure of the margins of the blocks on either side of it. In addition, there is a range of rock types and ages (from Cambrian to Carboniferous) which are available to record the history of movement and for these reasons it is discussed in a little detail (Fig. 2). The Highland Boundary Fault has a sinuous trace across Midland Scotland, bifurcates at its southern end and variably dips to the northwest (Dentith et al. 1992), southeast, or is vertical. To the north lies the metamorphic basement of the Dalradian, and to the south it bounds rocks of Cambrian, Ordovician, Silurian, Devonian and Carboniferous age. The Dalradian block is a polyphase folded, late Proterozoic (Halliday et al. 1989) metamorphic sequence of passive margin style rocks which have been metamorphosed at least once. A n early phase of folding (Tanner & Leslie 1994) and possibly a phase of metamorphism is cut by the Ben Vuirich granite which is 590 ± 2 Ma (Rogers et al. 1989) or 597 = +11 Ma (Pidgeon & Compston 1992), and a later phase of folding, metamorphism and uplift occurred in the interval 515 to c. 430 Ma. (Dempster 1985).

W. Q. KENNEDY & THE G R E A T GLEN FAULT DALRADIAN

MIDLAND

TERRANE

Ma

Carboniferous

-~c-~- ~

I

STATE OF TERRANES

VALLEY

TERRANE

HBFZ

AMALGAMATED

Carboniferous overstep

\ c=,o,.

J

Peneplain

2

Devonian Igneous activity

Silurian

~+ * \

-

Peneplain 450

1

,° . , 0 , o o 0

]~,~... ~ ~ / ' ~ ~ ~ -~ ° ° ' ~ ~

Valley (UORS) Thrust convergence:

~

Strathmore syncline

AMALGAMATION

Strike-slip basins

Ordovi I cia~500 rapid uplift

61

\ \

\

APART

arc, back arc basin ~,~/~.-,~,.,~~/~ ~,~ • a.~, • -

The history of the south side is particularly revealing and is traced from Cambrian times. (1) C a m b r i a n is represented on the Island of Bute by a sliver of metamorphic rock associated with serpentinite which resembles the sole to an ophiolite (Henderson & Robertson 1982). A cooling age of 540 Ma was reported from the amphibolite suggesting an obduction at about that time (Dempster & Bluck 1991). Near Callander, a sliver of distal, shelf-type Lower Cambrian limestones, overlaps in age with this cooling age. The association and relationship to the fault history of both these slivers is uncertain but is likely to be related to the convergence of the Midland Valley and Dalradian terranes (see Fig. 2). (2) Ordovician. Undoubted rocks of Ordovician age have been described (Curry et al. 1982) and these are overlain by sedimentary rocks which are themselves overlain by Old Red Sandstone rocks. The latter, formerly regarded as approximately Devonian are now thought to be well into the Silurian (Thirlwall 1988; Richardson et al. 1984). This part of the Highland Border Complex is therefore likely to be essentially Ordovician in age and comprises limestones, black shales, metamorphic rocks and basic-ultrabasic igneous bodies. The juxtaposition of the Highland Border Complex with the Dalradian block is incongruous in that the Dalradian block which was being uplifted in Ordovician time should have been yielding sediment to a coeval basin now preserved in the Highland Border Complex (Bluck 1985). The fact that it did not, suggested that the two blocks were not near each other in Ordovician times. This in turn suggested that there was a substantial displacement on the Highland Boundary Fault and one or both were displaced terranes in the sense used by workers on the W USA. Tanner (1994) has recently suggested that the earlier work of Johnson & Harris (1967); Harris & Fettes (1972) and others in having the Highland Border Complex or part of it belonging to the Dalradian, may be correct. This would therefore remove the evidence for the Highland Boundary Fault being a terrane boundary of any significance. This would appear to be the classical case of weighing the evidence from the fault zone itself against the evidence from

Fig. 2. Diagram illustrating the history of the blocks on either side of the Highland Boundary Fault zone (HBFZ) and how this is recording the interaction of the blocks north and south of the fault. Peneplain 1, refers to the Late Silurian peneplain as recorded for example in the Lorne area where Late Silurian lavas rest on a flat Dalradian surface; Peneplain 2 is the Late Devonian-Carboniferous peneplanation.

the history of the blocks on either side. However, if it is agreed that there is a major break between the Dalradian and Highland Border Complex then it is instructive to see if younger rocks at the Highland Border have recorded the amalgamation of the two disparate blocks. (3) S i l u r i a n - D e v o n i a n rocks of this age occur in the asymmetrical Strathmore syncline which runs parallel with the outcrop of the Highland Boundary Fault. On the steep northern limb of the syncline, the Highland Border Complex provided the basin floor for the Old Red Sandstone south of the fault as the Dalradian does to a few outliers of Old Red Sandstone to the north. The Highland Border Complex was therefore flat lying or gently dipping at the begining of Old Red Sandstone time. The provenance of the sediment of this age in the basins north and south of the faults is very different despite being of roughly the same age: Dalradian clasts dominate the sequences to the north and polycyclic quartzites and volcanic rocks dominate the very much thicker sequences to the south. These basins south of the fault have no unequivocal Dalradian clasts in them, but the Dalradian block could not have been far away from the Strathmore basin on its southern side as an ignimbritic flow overlies both Dalradian and Strathmore basin sediment. Clearly this was a time of terrane amalgamation, when the Midland Valley and Dalradian blocks were joining together but not clearly exchanging sediment. The two basins north and south of the fault are clearly not displaced margins of one single basin, but are distinctive basins which have been brought together by subsequent fault movement and intervening ground has been lost in this convergence. One stage of that convergence history occurred before the deposition of the Upper Old Red Sandstone when the Lower ORS was folded into the asymmetrical Strathmore syncline. The axis of this syncline runs parallel to the Highland Boundary Fault and indicated a thrust convergence in this zone (Fig. 2) By Upper Old Red Sandstone times, in a coarse fanglomerate near the Highland Boundary Fault in the SW Midland Valley, the first clasts of unequivocal Dalradian provenance entered the Midland Valley sequence (Fig. 2). A

62

B.J. Ma

450

--

NW HIGHLANDS

500

-

DALRADIAN BLOCK Reburial

Cambro-Ordovician

Erosion of mountain b e l t

550

Torridonian

Collapse of Dalradian basin

?

600

650

Dalradian basin

700

-

750

--

800

L

Torridonian basin

Gondwanan b a s e m e n t

Fig. 3. Diagram showing the nature of the late Proterozoic history of the foreland (west of Moine thrust) and Dalradian blocks. When there was a passive margin on the west of the Moine Thrust, there was the large expanse of ductile folded and at least partly metamorphosed rock lying to the west. Bluck & Dempster (1991) point out this paradox. slight relative uplift of the Dalradian is probable at this time, possibly as a result of further convergence of the Midland Valley and Dalradian terranes. (4) Carboniferous. Carboniferous volcanic and sedimentary rocks transgressed the fault to rest on Dalradian, but subsequent movement in the fault zone alternately threw Carboniferous rocks down to the northwest and to the southeast. The Highland Boundary Fault has a fairly complete record of the movements along it and from reading these in terms of the history of the blocks on either side a convergence history can be reconstructed. But this has to be read from the a study along the fault: any one locality may record only its latest (post-Carboniferous) movement. In this respect it is similar to the many terrane boundary faults seen elsewhere in the world.

Back to the Great Glen Armed with the degrees of freedom which the concepts of plates and terranes have given and in the light of the more intensive examination of faults which these concepts have stimulated, the Great Glen Fault can be now approached with a very different mind from the one which took Kennedy there some 50 years ago. But a bigger framework of knowledge brings with it a wider range of prejudice. Examination of terrane boundaries, such as the Highland

BLUCK Boundary Fault, has taught the geological community to be suspicious of reading the history of fundamental fractures in terms only of the displacements which we can now observe: these may be young modifications produced on reactivation of the older fractures. The magnitude and role of the fault is often demonstated by critically examining the whole history of the rocks which lie on either side of it. Moreover some of that history is often written in the slivers which occur along its entire length. The Great Glen Fault is thought to extend into the Shetland Isles where it is identified as the Walls Boundary Fault (Flinn 1992). Its extension to the southwest is far less certain but it is clear that its great length through Scotland can be matched only by its poor exposure compared with the other large fractures. A critical line of evidence, the composition and history of the blocks within the fault zone as used in the Highland Boundary Fault, is not widely available on this fracture. However, there is ample exposure of the rocks on either side over a considerable lateral distance, and Kennedy made dramatic use of them in his original work. Since Kennedy's time there has been much work done on attempting to establish the nature, timing and magnitude of the movements on the Great Glen Fault. Palaeomagnetic measurements have led to a view of unusually large displacements (Van der Voo & Scotese 1981; Storetvedt 1987) which are usually unacceptable because of the constraints of the existing geology on either side of the fault and more rigorous palaeomagnetic studies (Torsvik 1984). But others have estimates which vary greatly amongst themselves in timing, amount and nature of displacement. Rogers & Dunning (1991) and Hutton & McErlean (1991) show clear evidence for sinistral shear in the region of the Great Glen at c. 425 Ma. Hutton & McErlean (1991) see further evidence for sinistral movements contemporaneously with the intrusion of dykes at 410-395 Ma. The main source of this evidence comes not from the fault itself but from shears thought to be related to it. Although movement along the fault itself is difficult to establish, Donovan & Mayerhoff (1982), Parnell (1982) and Rogers et al. (1989) have appealed to displacements of the Old Red Sandstone outcrops in the Moray Firth region. Rogers et al. (1989) suggest post-Frasnian to pre-Permain dextral movements of 2 5 - 1 2 0 k m movement, but Flinn (1992) deduces a pre-Carboniferous sinistral net movement of c. 100 km and a dextral 65 km movement in Jurassic times. Most of the discussion of the Great Glen Fault, both recently and at the time of Kennedy, has concentrated on establishing the magnitude of its throw. In the light of terrace accretion tectonics the whole emphasis with respect to major faults has changed: the throw of the fault is now subordinate to its role. From the more recent work on the three major Scottish fractures to the south, and particularly illustrated by the Highland Boundary Fault (Fig. 2), major fractures are evaluated on the following criteria. (1) Degree of separation of the two blocks on either side of the fracture: were they great distances apart? At this stage oceanic crust normally separates the blocks. (2) The history of amalgamation of the blocks. (3) Their post-amalgamation history. Magnitudes or throws of displacement have relevance only to the third and possibly to part of the second of these criteria. The work on the Great Glen Fault so far, including Kennedy's, has addressed only the third and possibly second

W. Q. K E N N E D Y & THE G R E A T GLEN F A U L T of these criteria: it is clear that the fault, in Silurian and post-Silurian, times can be regarded as undergoing relatively minor adjustments. In order to establish if it is a terrane-scaled fracture satisfying the first of the above criteria, it is necessary to examine the pre-Silurian history of the blocks on either side and evaluate the compatiblity of their histories.

The pre-Silurian Great Glen fault The pre-Silurian rocks on either side of the Great Glen fault are basements of various ages and associations (Fig. 1). The ways of reading basement history are very different from ways of reading the Palaeozoic history as it has been applied to the ground further south. Although the basement history may not be subject to the same degree of rigour, its evaluation should not be based on no rigour at all, and if one considers that the Scottish Highlands are amongst the most intensively worked basement areas in the world, there should emerge some substantial criteria by which to evaluate the role of the Great Glen fault in terms of the basements on either side of it. As with the analysis of the Scottish terranes to the south, it is important first to establish the blocks and then their boundaries. There are four recognized basement blocks to the south of the Great Glen Fault. (1) The furthest south is the Dalradian block which comprises a passive margin type sequence (Anderton 1985) which has been repeatedly folded up to four times. An early phase of folding (Tanner & Leslie 1994) and possibly a phase of metamorphism is cut by the Ben Vuirich granite which is 590 + 2 Ma (Rogers et al. (1989) or 597 = +11 Ma (Pidgeon & Compston 1992), and some of the subsequent fold phases are seen to post-date the intrusion. After a substantial gap of c. 75Ma, a major cooling event was recorded so the Dalradian block, if the data relating to Ben Vuirich are accepted, was probably subject to two periods of metamorphism. (2) To the northwest of the Dalradian lies the Grampian Group that comprises, as far as can be read, a shelf-like sequence which is apparently cut by the c. 750 Ma suite of pegmatite sheets. Many workers (e.g. Harris et al. 1978) include the Grampian Group within the Dalradian sequence and point out, along with Treagus (1987) and Winchester (1992), that there is a transition between them. These rocks are, in turn, at least partly in shear contact with rocks of the Central Highland Division (3) The Central Highland Division comprises migmatitic gneisses the age of which is uncertain but which have been correlated with Moine rocks on the north side of the Great Glen fault (Piasecki & Van Breemen 1982). Southwest, along the Great Glen, the Central Highland Division is cut out of the sequence, and rocks of the Grampian Group lie in contact with faults near to and parallel with the Great Glen Fault (Phillips et al. 1993). (4) Further to the southwest, on the islands of Colonsay and Islay rocks previously thought to be Lewisian have now been shown to be Ketilidian (Marcantonio et al. 1988). The Central Highland Division has many similarities with the Moine rocks to the north and most workers accept them to belong to the same group. This view of continuity across the Great Glen is supported by the presence of the c. 750 Ma group of pegmatites along some of the notable shears within both the Moine on the northern side and the

63

Central Highland Division to the south. There is therefore no evidence for post-750 Ma displacement of terrane scale along this zone. There are however a number of important points to bear in mind before assigning the Great Glen fault to the category of a minor fracture. (1) There is a considerable similarity in late Proterozoic sediments throughout areas of the world, and to some extent the distinctiveness of sedimentary sequences is often variably obscured when they are metamorphosed. In addition dykes and sheets can be intruded over a wide area in extensional regimes, although this applies to basic more than to acid ones. These considerations may still leave room for considerable displacement along the fault. (2) There is an ophiolitic assemblage in the Feltar mass in the Shetland Isles, and at the SW Scottish end of the fault, is Ketilidian crust which does not have clear associations with much else in Scotland. Terrane-scaled movement most commonly take place on oceanic crust which often gets preserved somewhere along the fracture. (3) The history of the rocks on the northern side of the Great Glen is also puzzling and not easily related to those on the south. They are thought to rest on gneisses considered to be Lewisian and are now recognized to be polymetamorphic. The earliest metamorphic episode was thought to be pre-750 Ma, the age of a suite of pegmatites which are found in some of the slides (Piasecki & van Breemen 1982). The later, c. 460 Ma metamorphic event was accompanied by at least three folded thrust sheets (Barr et al. 1986) which involved ductile translation towards the WNW at temperatures estimated to be c. 600 °C (Barr et al. 1986). Retrogression began at c. 440 Ma with cooling ages down to c. 400 Ma. There are three problems raised by these data. (1) The Moine at 460 Ma (Caradoc) and at its present level would need to have a cover of c. 20 km to maintain the temperatures prevailing at that time (Soper & Barber 1982), and even at 420 Ma (Ludlovian) the rocks were at c. 350 °C implying, at reasonable geothermal gradients, a cover of at least 10km. The nature, source and the means of post-Caradoc disposal of this cover are uncertain. (2) This type of deformation and metamorphism implies plate-scale interactions, but the Dalradian block lies beween the Moine and the paratectonic zone. Along with the cooling of the Moine the Dalradian also cooled rapidly at 460 Ma--as did many metamorphic blocks southwest along strike in the Appalachians. The provenance of the cover to the Moine can only be speculated, but there are a number of possibilities (see Bluck & Dempster 1991). It could either: (a) have been rooted in the area now the Great Glen fault; (b) have come over both the Dalradian and the Moine and be rooted to the south of the Dalradian outcrop; (c) the northern margin of the Dalradian (no longer visible) covered the Moine. Only in (1) above needs there have been any large scaled movement on the Great Glen Fault. (3) Whilst this ductile folding and metamorphic history was taking place east of the Moine thrust, there was a passive margin sequence being deposited to the west. Bluck & Dempster (1991) has pointed out the paradox of this situation (Fig. 3), and there may therefore be good reason to examine in detail the role of the Great Glen Fault and its subsurface relationship with the Moine Thrust.

B.J.

Conclusions There exists no compelling evidence for the the Great Glen Fault to have been a terrane boundary: the later history shows throws which are not in the mega-shear class. H o w e v e r there are anomalies in the history of the basements on either side of the fracture, but a great deal of work needs to be done on refining the significance of these, the boundaries to a n o m a l o u s ground as well as on the details of the timing of events before anything can be resolved from them. If ground so well k n o w n as this in Scotland can throw up anomalies of this kind which are not resolvable with the current state of information, then large b a s e m e n t areas the world over must have m a n y problems yet to be discovered. I wish to thank B. E. Leake for pointing out some details of the life of W. Q. Kennedy, and T. Dempster, G. Rogers and N. J. Super for valuable discussion.

References ANDERTON, R. 1985. Sedimentation and tectonics in the Scottish Dalradian. Scottish Journal of Geology, 21,389-406. ANDERSON, J.G.C. 1946. The geology of the Highland Border: Stonehaven to Arran. Transactions of the Royal Society of Edinburgh, 41, 479-515. BAILEY, E.B. & McCALLIEN, W.J. 1953. Serpentinite lavas, the Ankara melange, and the Analolian thrust. Transactions of the Royal Society of Edinburgh, 62, 402-442. BARBER, A.J. 1985. A new concept of mountain building. Geology Today, 1, 116-121. BARR, D., HOLDSWORTH, R.E. & ROBERTS, A.M. 1986. Caledonian ductle thrusting in a Precambrian metamorphic complex: the Moine of Northwestern Scotland. Geological Society of America Bulletin, 97, 754-764. BENSON, W.N. 1952. Meeting of the geologic division of the Pacific Science Congress in New Zealand, February 1949. International Proceedings Geological Society of America Bulletin, 63, 11 - 13. BERTRAND, M. 1897. Structure des Alpes Francaises et recurrence de certain facies sedimentaires. Comptes Rendus, Congress International 6th Session, 1894, 164-177. BLUCK, B.J. 1985. The Scottish paratectonic Caledonides. Scottish Journal of Geology, 21, 437-464. & DEMPSTER, T.J. 1991. Exotic metamorphic terranes in the Caledonides: tectonic history of the Dalradian block, Scotland. Geology, 19, 1133-1136. CHURCH, W.R. & GAYER, R.A. 1973. The Ballantrae ophiolite. Geological Magazine, 110, 497-510. CURRY, G.B., INGHAM, J.K., BI.UCK, B.J. & WILLIAMS, A. 1982. The significance of a reliable Ordovican age for some Highland Border rocks in Central Scotland. Journal of the Geological Society, London, 139, 451-454. DALZIEL, I.W.D. 1991. Pacific margins of Laurentia and East AntarcticaAustralia as a conjugate rift pair: Evidence and implications for an Eocambrian supercontinent. Geology, 19, 598-601. -1994. Precambrian Scotland as a Laurentia-Gondwana link: Origin and significance of craton promontories. Geology, 22, 589-592 DANA, J.D. 1873. On some results from the earth's contraction from cooling, including a discussion of the origin of mountains and the nature of the earth's interior. American Journal of Science, 5, 423-443; 6, 6-14; 1114-115; 161-171. DEMPSTER, T.J. 1985. Uplift patterns and orogenic evolution in the Scottsh Dalradian. Journal of the Geological Society, London, 142, 111-128. -& BLUCK, B.J. 1991. The age and tectonic significance of the Bute amphbolite, Highland Border Complex, Scotland. Geological Magazine, 128, 77-80. DENTITH, M.C., TRENCH, A., & BLUCK, B.J. 1992. Geophysical constraints on the nature of the Highland Boundary Fault zone in western Scotland. Geological Magazine, 129, 411-419 DE S~YrER, L.U. 1956. Structural Geology. Mc Graw-Hill. London. DEWEY, J. F. 1969. Evolution of the Caledonian/Appalachian orogen. Nature, 222, 124-129. 1974. Continental margins and ophiolite obductions: Appalachian/ Caledonian system. In: BURKE, C.A. 8¢~ DRAKE, C.L. (eds) Geology of continental margins. Springer-Verlag, New York, 933-950.

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DONOVAN, R.N. & MEYERHOff, A. A. 1982. Comments on 'paleomagnetic evidence for a large (2000 km) sinistral offset along the Great Glen fault during Carboniferous time' Geology, 10, 604-605. FLINN, D. 1992. The history of the Walls Boundary fault, Shetland: the northward continuation of the Great Glen fault from Scotland. Journal of the Geological Society, London, 149, 721-726. GEORGE, T.N. 1960. The stratigraphical evolution of the Midland Valley. Transactions of the Geological Society of Glasgow, 24, 32-107. GILLULY, J. 1949. Distribution of mountain building in geologic time. Geological Society of America Bulletin, 60, 561-590. HALL,J., POWELL, D.W., WARNER, M.R., EL-ISA, Z.M.H., ADESANYA, O. BLUCK, B.J. 1983. Seismological evidence for shallow crystalline basement in the Southern Uplands of Scotland. Nature, 305, 418-420. HALLIDAY, A.N., GRAHAM, C. M., AFTALION, M. & DYMOKE, P. 1989. The depositional age of the Dalradian Supergroup: U-Pb and Sm-Nd isotopic studies of the Tayvallich volcanics, Scotland. Journal of the Geological Society, London, 146, 3-6 HARRIS, A.L. & FETrES, D.J. 1972. Stratigraphy and structure of the Upper Dalradian rocks at the Highland Border. Scottish Journal of Geology, 8, 253-264. --, BRADBURY, H. J., JOHNSON, H.D. &SMITH, R.A. 1978. Ensialic basin sedimentation: the Dalradian Supergroup. In: BowLs, D.R. & LEAKE, B.E. (eds) Crustal evolution in Northwestern Britain and adjacent regions. Seel House Press, Liverpool, 115-138 HENDERSON, W.G. & ROBERTSON, A.H.F. 1982. The Highland Border Rocks and their relation to marginal basin development in the Scottish Caledonides. Journal of the Geological Society, London, 139, 433-450. HILL, M.L. & DIBBLEE, T.W. JR. 1953. San Andreas, Garlock and Big Pine faults, California--a study the character, history and tectonic significance of thier displacements. Geological Society of America Bulletin, 64, 443-458. HORNE, J. 8¢ HINXMAN, L.W. 1914. The Geology of the country round Beauly and Inverness: Scotland. Geological Survey Memoir sheet 83. HOWELLS, D.G. 1989. Tectonics of suspect terranes. Chapman Hall, London. HUTI'ON, D.W.H. & McERLEAN, M. 1991. Silurian and Early Devonian sinistral deformation of the Ratagain granite, Scotland: constraints on the age of the Great Glen fault system. Journal of the Geological Society, London, 148, 1-4. JOHNSON, M.R.W. & HARRIS, A.L. 1967. Dalradian-?Arenig relations in parts of the Highland Border, Soctland and their significance to the chronology of the Caledonian Orogeny. Scottish Journal of Geology, 3, 1-6. JONES, D.L., SILBERLING, N.J. & NELSON, W.H. 1972. Southerastern Alaska--a displaced continental fragment? US Geological Survey professional Paper, 800B, B211-B217 JONES, O.T. 1938. On the evolution of a geosyncline. Quarterly Journal of the Geological Society of London, 94, lx-cx. KARIG, D.E. 1983. Accreted terranes in the northern part of the Philippine archipealigo. Tectonics, 2, 211-236 KEELING, G. 1962. The petrology and sedimentation of Upper Ordovician rocks in the Rhinns of Galloway, southwest Scotland. Transactions of the Royal Society of Edinburgh, 65, 107-137. KENNEDY. W.Q. 1946. The Great Glen Fault. Quarterly Journal of the Geological Society of London, 102, 41-76. 1958. The tectonic evolution of the Midland Valley of Scotland. Transactions of the Geological Society of Glasgow, 23, 107-133. KRYNINE, P.D. 1945. Sediments and the search for oil. Producers Monthly, 9, 17-22. LONGMAN, C.D. BLUCK, B.J. & VAN BREEMEN, O. 1979. Ordovician conglomerates and the evolution of the Midland Valley. Nature, 280, 578-581. MARCANTONIO, F., DICKIN, A.P., McNurr, R.H. & HEAMAN, L.M., 1988. A 1800 Ma Proterozoic gneiss terrane in Islay with implications for the crustal struture and evolution of Britain. Nature, 335, 62-64. MURPHY, F.C. & Hurroh, D.H.W. 1986. Is the Southern Uplands of Scotland really and accretionary prism? Geology, 14, 354-357. McKAY, A. 1890. On the earthquake of September 1888 in the Amuri and Marlborough Districts of the South Island. New Zealand Geological Survey Report of Geological Explorations 1888-1889, 20, 1-16. MCKERROW, W.S. LEGGETT, J. K. & EALES, U. H. 1977. Imbricate thrust model of the Southern Uplands of Scotland. Nature, 267, 237-239. PARNELL, J. T. 1982. Comment on 'paleomagnetic evidence for a large (2000 km) smistral offset along the Great Glen fault during Carboniferous time' Geology, 10, 605. PHILLIPS, E.R., CLARK, G.C. & SMITH, D.I. 1993. Mineralogy, petrology and microfabric analysis of the Eilrig Shear Zone, Fort Augustus. Scottish Journal of Geology, 29, 143-158. PIASECKI, M.A.J. & VAN BREEMEN, O. 1982. Field and isotopic evidence for a c. 750 Ma tectonothermal event in Moine rocks in the Central Highland

W.

Q.

KENNEDY

& THE

region of the Scottish Caledonides. Transactions of the Royal Society of Edinburgh, Earth Sciences, 73, 119-134. PIGEON, R.T. & COMPSTON, W. 1992. A SHRIMP ion microprobe study of inherited and magmatic zircons from four Scottish Caledonian granites. Transactions of the Royal Society of Edinburgh, Earth Sciences', 83, 473-483. RICHARDSON, J.B., FORD, J.H. & PARKER, F. 1984. Miospores, correlation and age of some Scottish Lower Old Red Sandstone sediments from the Strathmore region (Fife and Angus). Journal of Micropalaeontology, 3, 109-124. ROGERS, D.A. MARSHALL, J.E.A. & ASTIN, T.R. 1989. Devonian and later fault movements long the Great Glen fault system, Scotland. Journal of the Geological Society, London, 146, 369-372. ROGERS, G. & DUNNING, G.R. 1991. Geochronology of appinite and related granitic magmatism in the W Highland of Scotland: constraints on the timing of transcurrent fault movement. Journal of the Geological Society, London, 148, 17-27. - - , DEMPSTER, T.J., BLUCK, B.J. & TANNER, P.W.G. 1989, A high precision U-Pb age for the Ben Vuirich Granite: implications for the evolution of the Scottish Dalradian Group. Journal of the Geological Society, London, 146, 789-798. SOPER, N,J. & BABER, A.J., 1982. A model for the deep structure of the Moine thrust zone. Journal of the Geological Society, London, 139, 127-138. SPRAY, J.G. & WILLIAMS,G.D. 1980. The sub-ophiolite metamorphic rocks of the Ballantrae Igneous Complex. Journal of the Geological Society, London, 137, 359-368. STONE, P., FLOYD, J.D. BARNES, R.P. & L1NTERN, B.C. 1987. A sequental back-arc and foreland basin thrust duplex model for the Southern Uplands, Journal of the Geological Society, London, 144, 753-764. STORETVEDT, K.M. 1987. Major late Caledonian and Hercynian shear movements on the Great Glen fault. Tectonophysics, 143, 252-267. SYLVESTER, A.G. 1988. Strike-slip faults. Geological Society of America Bulletin, 100, 1666-1703. TALIAFERRO, N.L. 1941. Geological history and structure of the central Coast

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Ranges of Calilifornia. In: JENKINS, O.P. (ed.) Geologic" formation and economic development of the oil and gas fields of California, Part 2. Geology of California and the occurrence of gas. California Division of Mines Bulletin, 118, 119-163. TANNER, P.W.G. 1994. Caledonian Terrane relationships in Britain: Programme with Abstracts. B.G.S. Keyworth, 10. & LESLIE, A.G. 1994. A pre-D2 for the 590 Ma Ben Vuirich Granite in the Dalradian of Scotland. Journal of the Geological Society, London, 151, 209-212. THIRLWALL, M. 1988. Geochronology af Late Caledonian magmatism in Northern Britain. Journal of the Geological Society, London, 145, 951-968. TORSVIK, T. 1984. Palaeomagnetism of the Foyers and Strontian granites, Scotland. Physics of the earth and Planetary Interiors, 36, 163-177. TREAGUS, J. E. 1987. The structural evolution of the Dalradian of the Central Highlands of Scotland. Transactions of the Royal Society of Edinburgh, Earth Science, 78, 1-15. TRELOAR, P.J. BLUCK, B.J., BOWES, D.R. & DL'DEK, A. 1980. Hornblendegarnet metapyroxenite beneath serpentinite in the Ballantrae complex of SW Scotland and its bearing on the depth of provenance of obucted ocean lithosphere. Transactions of the Royal society of Edinburgh, Earth Science, 71, 201-212 VAN DE VOO, R. & S¢OTESE, C. 1981. Paleomagnetic evidence for a large (sinistral) offset along the great Glen fault during the Carboniferous time. Geology, 9, 583-589. WALTON, E.K. & OLIVER, G.J.H. 1991. Lower Palaeozoic-stratigraphy, In: CRAIG, G.Y. (ed.) Geology of Scotland 3rd Edition. Geological Society, London. 161-193. WILLIAMS, H. & SMYTH, W.R. 1973. Metamorphic aureoles beneath ophiolite suites and alpine peridotites: tectonic implications with West Newfoundland examples. American Journal of Science, 273, 594-621. WINCHESTER, J. A. 1992. Comment and Reply on 'Exotic metamorphic terranes in the Caledonides: Tectonic history of the Dalradian block, Scotland' Geology, 20, 764-765. -

-

From

QJGS, ] 0 2 , 41-42. THE GREAT GLEN FAULT BY WILY.TAM QUARRIER KENNEDY, D.SC. F.G.S.

Read 8 February, 1939 [PT,AWm I I I ] CONTENTS I. I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I . GeneraI features of t h e dislocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) S t r u c t u r a l c h a r a c t e r s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) Topographic effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) Seismic a c t i v i t y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I I . S u m m a r y of fault-line geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. The n a t u r e of t h e displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) General discussion . . . . . . . . . . . . . . . . . . ~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) E v i d e n c e of lateral displacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. I n t e r p r e t a t i o n of m o v e m e n t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) Age of t h e m a i n (lateral) displacement . . . . . . . . . . . . . . . . . . . . . . . . (b) D y n a m i c s of m o v e m e n t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) Tectonic significance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. List of works to which reference is made . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page 42 43 43 46 46 47 52 52 54 64 64 67 70 71

S~r~ARr The powerful dislocation which intersects Scotland along the line of the Great Glen has, in the past, been r e g a r d e d b y most geologists as a n o r m a l or dip-slip fault with a p r e d o m i n a n t vertical d o w n t h r o w to the south-east• A reconsideration of the ~atire problem n o w suggests t h a t this view is no longer tenable and t h a t the dislocation is, in reality, a lateral-slip or wrench fault with a horizontal displacement of approximately 65 miles. Such an interpretation is s u p p o r t e d b y several independent lines of evidence, as follows : - • (1) The dislocation possesses physical characters unlike those of most n o r m a l faults b u t similar to the g r e a t strike-slip shears of the California Coast Range. (2) I t belongs to t h e same s y s t e m as the S t r a t h c o n o n , E r i c h t - L a i d o n a n d Loch Tay faults, all of which h a v e p r o v e d lateral displacements of up to 5 miles. (3) I t displaces t h e g r e a t belt of regional injection which affects the Moine Schists of the n o r t h e r n a n d G r a m p i a n Highlands, the n a t u r e a n d a m o u n t of the displacement being consistent w i t h lateral shift b u t not with vertical d o w n t h r o w . (4) I t similarly displaces t h e m e t a m o r p h i c zones of t h e H i g h l a n d s in an equally significant m a n n e r . (5) I t t r u n c a t e s t h e S t r o n t i a n Granite, the southern p o r t i o n of which, according to the detailed s t r u c t u r a l evidence, is missing. The missing portion, moreover, can be identified in t h e F o y e r s mass which outcrops on the o t h e r side of the fault-line some 65 miles to t h e n o r t h - e a s t a n d is similarly t r u n c a t e d b y the fault. These two m a j o r Caledonian intrusions consist of identical rock t y p e s a n d are s t r u c t u r a l l y homologous. (6) Finally, t h e occurrence of Lewisian a n d Torridonian rocks in I s l a y and Colonsay and t h e presence of t h e Moine Thrust-plane in the former island are more readily explained on the a s s u m p t i o n of a lateral r a t h e r t h a n a vertical displacement along the fault. A l t h o u g h the dislocation is still active, the available evidence indicates t h a t the mare lateral m o v e m e n t was accomplished prior to t h e deposition of the Upper Carboniferous s e d i m e n t s of LochaHne a n d subsequent to t h e intrusion of the S t r o n t i a n and F o y e r s (Lower Old R e d Sandstone) granites. Middle Old R e d Sandstone s t r a t a along t h e G r e a t Glen h a v e , moreover, suffered intense crushing and d e f o r m a t i o n during t h e faulting, w h i c h m u s t , therefore, be referred p a r t l y if n o t wholly to a postMiddle Old R e d S a n d s t o n e epoch. The sinistral n a t u r e of t h e displacement, i.e. t o w a r d s the south-west on the northwest side of t h e f r a c t u r e a n d t o w a r d s the north-east on its south-east side, implies t h e o p e r a t i o n of a stress s y s t e m involving regional compression a c t i n g in a general n o r t h - a n d - s o u t h direction a c c o m p a n i e d by an east-and-west relief of pressure. This is r e g a r d e d as evidence of the fact t h a t the Herc).~ian forces, to which the f o r m a t i o n of t h e G r e a t Glen F a u l t is ascribed, were a l r e a d y m o p e r a t i o n during Upper Old R e d S a n d s t o n e or L o w e r Carboniferous times.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 67-81 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 227-241

P-T-t

evolution of orogenic belts and the causes of regional metamorphism MICHAEL

BROWN

Department of Geology, University o f Maryland at College Park, College Park, M D 20742, USA Abstract: Barrow (1893) introduced three important ideas that furthered understanding of metamorphic processes: (i) the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism; (ii) the concept of progressive metamorphism; and (iii) the concept of magmatic advection of heat as a possible cause of regional metamorphism. This article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the P-T-t paths of metamorphic rocks, and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts. Understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum T and maximum P. Orogenic belts characterized by clockwise P - T paths achieved maximum P before maximum T, the metamorphic peak normally post-dated early deformation within the belt and additional heating above the 'normal' conductive flux has been related to the amount of overthickening. By contrast, orogenic belts characterized by counterclockwise P-T paths achieved maximum T before maximum P, the metamorphic peak normally pre-dated or was synchronous with early deformation within the belt and additional heating above the 'normal' conductive flux has been related to the emplacement of plutons. Techniques used to constrain portions of P-T-t paths include: the use of mineral inclusion suites in porphyroblasts and reaction textures; thermobarometry; the use of fluid inclusions; thermodynamic approaches such as the Gibbs method; radiogenic isotope dating; fission track studies; and numerical modelling. We can utilize specific mineral parageneses in suitable rocks to determine individual P-T-t paths, and a set of P-T-t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism. Recent advances are reviewed with reference to collision metamorphism, high-temperature-low-pressure metamorphism, granulite metamorphism, and subduction zone metamorphism, and some important directions for future work are indicated.

of zones to include, in order of decreasing metamorphic grade below the staurolite zone, a garnet zone, a biotite zone and a zone of digested clastic mica, bounded at lower grade by a local margin with clastic mica, and to formalize the sillimanite zone on the map. In the discussion of Barrow's 1893 paper, Teall commented that 'It appeared p r o b a b l e . . , that the line separating the sillimanite zone from the cyanite zone was an isothermal'. This point was picked up by Barrow in his 1912 paper which included a description of how a metamorphic zone is defined, as follows: 'Proceeding across Zone i we reach rocks in which biotite is developed; this marks the commencement of Zone ii. But no matter how far to the north-west we go, brown mica is usually present in many of the rocks; so that the distance to which it extends has no zonal value; it is the line marking the first oncoming or the 'outer limit' of the mineral that gives the zonal line. Farther north-west the common (non-calcareous) garnet is met with; this, too, extends into the highest zones, so that again the line showing its first oncoming or 'outer limit' is the zonal line . . . . It gradually becomes clear that these 'outer limit' lines correspond with isothermals; i.e., they indicate the point where the rocks have been raised to a sufficiently high temperature to develop the index-mineral of the zone'. In the Quarterly Journal of the Geological Society for 1924, Tilley proposed the replacement of Barrow's 'local margin with clastic mica' and 'zone of digested clastic mica' by the zone of chlorite. Tilley argued that chlorite is a typical mineral of the lowest zone of metamorphism and

This review takes as its starting point the classic article published in the Quarterly Journal of the Geological Society in 1893 by George Barrow 'On an I N T R U S I O N of MUSCOVITE-BIOTITE GNEISS in the S O U T H E A S T E R N H I G H L A N D S of S C O T L A N D , And Its A C C O M P A N Y I N G M E T A M O R P H I S M ' . It is instructive to note, in the centennial year of this paper, that one of the questions still debated in metamorphic geology was its underlying theme, namely what is the principal cause of regional metamorphism, as reflected in Barrow's introductory statement 'It is proposed to show in the present communication that this area contains several masses of intrusive rock which are probably connected underground, and that the highly crystalline character of the surrounding schists is mainly the result of thermometamorphism'. That paper represents the first attempt to bring precision to a study of regional metamorphism by publishing a 'map of a portion of North-East Forfarshire embracing the area of outcrop of the muscovite-biotite-gneiss, and showing the zones of occurrence of the silicates of alumina which are connected with the intrusion'. The map (Barrow 1893, plate XV) illustrated a staurolite zone, a cyanite [sic] zone and indicated an area where sillimanite occurs, referred to as a sillimanite zone in the text. In the explanation of plate X V Barrow wrote 'The zones of staurolite- and cyanite-gneiss or schist really represent the variation in height above the upper limit of the underlying gneiss'. Nineteen years later Barrow extended the area of his zonal metamorphic map to the Highland fault, at the same time increasing the number 67

68

M. BROWN

that it generally decreases in amount as successive higher-grade zones are entered, interpreted to reflect its consumption during synthesis of higher-grade index minerals. Furthermore, as Tilley pointed out, white mica persists throughout, and is often very abundant in the higher-grade zones. Finally, it was Tilley who explicitly made the point that rocks of restricted chemical composition must be utilized for such zonal mapping, as follows 'The widespread distribution of sediments of argillaceous t y p e . . . at once marks o u t . . , these rocks as a suitable group in which to study successive and progressive mineral changes. Such are the rocks which Barrow used in his zonal m a p p i n g . . . ' . Thus was established the classic sequence of index minerals that commonly develop in pelites during 'normal' regional metamorphism, widely referred to as 'Barrow's zones of progressive regional metamorphism'; this particular sequence of index minerals is diagnostic of medium-P regional metamorphism which traditionally has been referred to as 'Barrovian metamorphism'. In addition to producing a map of metamorphic zones characierized by specific index minerals, Barrow (1893) considered the reasons for the great extent of the area affected and the intensity of the alterations produced by what he called the 'thermometamorphism of the Southeastern Highlands.' Barrow wrote '... we are led to the conclusion that these gneisses occur in huge sills or laccolites [sic] having approximately horizontal upper s u r f a c e s . . , it follows that one of the chief factors in increasing or decreasing metamorphism of rocks affected must be the variation in depth of the sills below the surface.., the intensity of the metamorphism is doubtless largely due to the great depth below the surface of the rocks affected by the intrusion.' Finally, Barrow commented that ' . . . these special features m a y . . , be due to the depth in the earth's crust at which the metamorphism took place, rather than to any physical conditions peculiar to early geological t i m e . . . and strengthens Dr. Barrois's conclusion that 'regional metamorphism and contact-metamorphism are much the same thing". Barrow's depth of geological understanding is reflected in his interpretation, in his 1912 paper, of his regional metamorphic map (Barrow 1912, folding map), as follows 'A key to the form of the zones of this area of regional metamorphism is given by the 'outer limit' lines of the sillimanite zone (6) and of the biotite zone (2). • . . sillimanite-bearing rocks occur over an area of, roughly, 200 square m i l e s . . , roughly t r i a n g u l a r . . , but the line marking the outer limit of biotite is almost s t r a i g h t . . . (3) defining the limit of garnet, slowly diverges from the (2) just described. The divergence is more marked in the case of staurolite (4); its boundary begins to assume a rude parallelism with (6), defining the outer limit of sillimanite. The parallelism is still closer in the case of cyanite (5) . . . . It thus gradually becomes apparent that while the line marking the lower zones is almost straight, the intermediate zones surround lenticles of the most highly altered rocks containing sillimanite; further, the whole masses of crystalline rocks are somewhat lenticular in f o r m . . . It thus appears that the Highland a r e a . . , is essentially built up on the lines of an aureole of metamorphism around a granite intrusion; but instead of aureoles we have zones or belts which diverge more and more from a lenticular, highly crystalline nucleus, until the lines bordering the lower temperature zones are nearly straight.' Thus, Barrow was firmly convinced that this regional metamorphic terrane is

nothing more than a large-scale contact aureole around numerous small granitic intrusions; he saw no essential difference between regional and contact metamorphism• Although Barrow's work is widely known today, at least through the naming of medium-pressure regional metamorphism and associated metamorphic field gradients as 'Barrovian', it was not well known early during this century in Europe, possibly because of the rather non-informative title of his 1893 paper (above). Indeed, Goldschmidt (1916, summarized in Mason 1992), was able to map zones of progressive metamorphism in the Trondhjem district of the Norwegian Caledonides marked by the index minerals chlorite, biotite and garnet in argillaceous sediments, unaware of the earlier work of Barrow in the southeast Highland of Scotland. In a clear example of the parallel development of ideas in science, Goldschmidt had summarized the aim of his research in his inaugural lecture as Professor of Mineralogy at the University of Stockholm on 28 September 1914 (quoted in Mason 1992, from a translation by G. Kullerud) with the following 'It is . . . of great i n t e r e s t . . , to determine the physical conditions under which an individual mineral has been formed• It is of much greater i m p o r t a n c e . . , to study thoroughly a sizeable area in order to investigate the temperature-pressure distribution during a certain geological era. Such an investigation, no doubt the first of its kind, is being performed by myself in the Norwegian mountain areas, from Ryfylke (near Stavanger) to Trondhjemsfjorden, in order to determine the temperature and pressure conditions in this part of the earth's crust during the formation of the Norwegian Caledonides at the beginning of the Devonian . . . the sum of all observations gives us a picture of the temperaturepressure distribution during the formation of a mountain chain.' Indeed, Goldschmidt could not have described better the aim of modern metamorphic petrology. During the 100 years since publication of Barrow's seminal paper, Scotland has proven to be a fertile ground for the development of ideas in metamorphism. With respect to the zonal distribution of metamorphism and the quantitative estimation of P and T, the reader is referred to Chinner (1966), Atherton (1977) and Harte & Hudson (1979). Metamorphic reactions in the higher grade zones of the Barrovian type area have been investigated by McLellan (1985), who has emphasized also the importance of both sub-solidus and anatectic processes in the generation of migmatite leucosomes (McLellan 1983, 1989) interpreted by Barrow (1893) to be related to the granites by fractionation.

Orogeny and regional metamorphism Orogeny is characterized by a distinctive relationship between sedimentation, tectonic deformation, regional metamorphism and magmatism. It leads to structural inversion of sedimentary basins, mountain building and eventual exhumation of metamorphic belts, either during the same cycle or subsequently. Modern orogenic belts are located at convergent plate boundaries and along lines of continental and/or arc collisions. Orogenesis is responsible for crustal differentiation through anatexis and transfer of granitic magma from the lower crust to the upper crust, leaving behind a depleted residuum. Large tracts of the continents of the Earth are formed by rocks that have been metamorphosed on a regional scale, that is, their secondary mineral assemblages indicate that

P-T-t

E V O L U T I O N OF O R O G E N I C BELTS

these regions have been subjected to elevated temperatures and pressures at some time in their past. Generally accepted tectonic settings for regional metamorphism are continental margins associated with subduction, such as the west coast of South America; island arcs, such as those of Southeast Asia; and zones of continental collision, such as the Alps and Himalayas. Traditionally, such regional metamorphism has been separated from metamorphism that occurs in aureoles surrounding intrusives, so-called contact metamorphism, because of the scale of the area affected on the one hand and because of the spatial relationship to the intrusive heat source on the other hand. However, to maintain that metamorphism of regional extent shows no apparent relation to intrusive rocks as heat sources is to deny one of the main conclusions of Barrow's 1893 paper. Indeed, magmatic arcs at convergent plate boundaries are a locus for plutonic magmatism and, as a consequence, contact metamorphism, and such a relationship occurs on a regional scale (e.g. Barton & Hanson i989). The observation that belts of regional metamorphism typically contain abundant intrusive rocks leads to the postulate implied in Barrow's paper that intrusive rocks collectively increase the regional thermal gradient and might be a primary cause of some regional metamorphism, even though in the particular case discussed by Barrow the 'intrusive rocks' are sub-solidus and anatectic migmatites (McLellan 1983, 1989). On the other hand, Harker (1932) emphasized that regional metamorphism is characterized by both a wide areal extent and a regional-scale temperature distribution, as shown by mineral zones, independent of the distribution of individual plutonic masses. It is plausible that medium-P regional metamorphism may grade with decreasing crustal depth into regional-scale contact metamorphism; an example, likely representing such an oblique crustal section, may be the New England Appalachians from Connecticut to Maine (Tracy & Robinson 1980; Armstrong et al. 1992; De Yoreo et al. 1989a, b).

69

values of mantle heat flux, thermal conductivity and heat production.

Progressive regional m e t a m o r p h i s m

In detail, regional metamorphic terranes have been classified by their mineral assemblages into facies series types such as andalusite-sillimanite, kyanite-sillimanite and jadeite-glaucophane (Miyashiro 1961). It is implicit in this classification that an entire metamorphic terrane can be described by a single geothermal gradient and that higher-grade mineral assemblages develop from lower-grade ones similar to those now found along the present erosion surface. This view derives directly from Barrow (1893) who argued that the sequential development of a coarse-grained sillimanite-gneiss from a kyanite-gneiss, and the kyanitegneiss from a staurolite-schist within one continuous stratigraphic horizon, presented a conclusive proof of progressive metamorphism. However, metamorphism is not a static process but an evolutionary one. With our acceptance of a dynamic tectonic environment for regional metamorphism, we realize now that rocks follow more complex routes in P - T space, reflecting burial, heating and exhumation. This does not negate the concept of progressive metamorphism, and it is likely that, with the exception of circumstances in which heating rates are relatively rapid, the progressive model for medium- to high-grade segments of P - T paths is essentially correct, but the sequential change is not simply the one observed by following the sequence of assemblages along the metamorphic field gradient. The traditional view has been replaced by one in which individual rocks (e.g. Thompson & England 1984) and minerals (e.g. Spear & Selverstone 1983) can be used to derive paths in P - T space, that can be related to the tectonic setting (e.g. England & Thompson 1984). The derivation of such paths along the length and breadth of an orogenic belt enables us to unravel the three-dimensional reality of orogenic processes.

Paired m e t a m o r p h i c belts

Some metamorphic terranes have developed as paired belts, in which one member is characterized by high-temperature metamorphism and the development of migmatites and anatectic granites, and is interpreted as having been developed at a site of high heat flow such as that beneath an associated volcanic arc; the other member is characterized by blueschists and eclogites that indicate relatively low geothermal gradients and relatively high-pressure conditions, interpreted as having been developed at a site of low heat flow such as a subduction zone. Commonly, the first member is referred to as a low-pressure-high-temperature metamorphic belt, although many such belts exhibit a high-pressure history prior to the development of the final low-pressure mineral assemblage as in the Abukuma Plateau (Kano & Kuroda 1968; Hiroi & Kishi 1989) and in Southern Brittany (Cogn6 1960; Jones & Brown 1990). Intrusions advecting heat are commonly proposed as the cause of the thermal anomaly for these high-temperature metamorphic belts because of the abnormally high geothermal gradients implied (De Yoreo et al. 1991; Furlong et al. 1991). However, Treloar & Brown (1990) have shown that moderate overthickening of sedimentary basins during structural inversion may lead to high-temperature metamorphism at mid-to-lower crustal depths for reasonable

P - T - t paths of metamorphism Two fundamentally different types of orogenic belt are distinguished by relative timing of maximum T and maximum P, as revealed by metamorphic rocks within the belt. One type of orogenic belt is characterized by an evolutionary path in P - T space that is clockwise (Fig. 1; CW paths). Orogenic belts of this type are generated by basin inversion or crustal thickening followed by erosional exhumation and/or extensional thinning and/or lithospheric delamination and orogenic collapse (Oxburgh & Turcotte 1974; Bird et al. 1975; Houseman et al. 1981; Thompson 1981; Thompson & England 1984; Thompson & Ridley 1987). Orogenic belts characterized by clockwise P - T paths achieved maximum P before maximum T, and the metamorphic peak normally post-dated early deformation within the belt. Such an evolutionary path will lead to decompression dehydration-melting of common crustal rock types (Thompson 1982, 1990; Jones & Brown 1990), and may lead to granite magmatism which is a consequence of the regional metamorphism (e.g. Patin6-Douce et al. 1990). Experiments on natural rock compositions (Le Breton & Thompson 1988; Rushmer 1991) and the results of thermal models (De Yoreo et al. 1989a) indicate significant volumes of crustal melt can be generated through crustal thickening.

70

M. BROWN

500

700

900

1100 70

CW 17.5

15.0

CWa

50

12.5 N

E

.{3

10.0 ~[_

a

09

30

7.5

5.0

2.5

Ms(ss) Ab Q t z / \ a5 ~1 ~3 ~ CCW

for

500

/ BA

10

TC

AIs V

700

900

Indeed, significant amounts of melt are generated for overthickening as small as 10-15 km (see De Yoreo et al. 1989a) and De Yoreo (1988) has shown that partially molten (>0.3 melt fraction) sections of crust substantially thicker than 1 km may be generated in less than 40 Ma. Crustal anatexis may be a normal consequence of some types of high-temperature metamorphism, particular those which involve decompression at high temperature (e.g. Jones & Brown 1990). The relationship between anatectic migmatites and higher-level granites has been considered by D'Lemos et al. (1992), and the thermal consequences of crustal melting have been considered by Haugerud & Zen (1991) who make the point that the complement of a high-level zone of heating by magmatic injection is a zone of retarded heating in the middle-to-lower crust because melting buffers the geothermal gradient. The issue of magmatic advection driving high-temperature metamorphism v. crustal melting to derive granites subsequently emplaced into the high-temperature metamorphic belt needs to be resolved on an individual basis. -1"he other type 0i~ orogenic belt is characterized by an evolutionary path in P - T space which has a counterclockwise direction (Fig. 1; CCW path). In such an evolution, heating preceded crustal thickening or the two may have

1100

Fig. 1. Pressure-temperature diagram to show: (1) Some of the initial melting reactions in metapelites (from Thompson 1990, and primary references therein). (2) Initial melting reactions for amphibolites, including the H20saturated solidus for olivine tholeiite and an approximate H20-saturated solidus for quartz tholeiite based upon albite + quartz + H20 (after Thompson 1990 and primary references therein). (3) P - T - t paths (CW; 50 km and 70 km depth after thickening) for thickening of continental crust from 35 km to 70 km, followed by erosional thinning in 100 million years, after a post-thickening isobaric metamorphism of 20 million years for an initially 'hot' geotherm that certainly reaches granulite facies conditions (after Thompson 1990 and primary references therein). (4) P - T - t path (CCW) for heating followed by crustal thickening and near isobaric cooling (after Thompson 1990 and primary references therein). CW, clockwise path in P - T space; CCW, counterclockwise path in P-T space; GWS, granite wet solidus; BWS, basalt wet solidus; IAT, island arc tholeiite; BA, alkali basalt. Reactions and processes that occur as a consequence of evolution along these paths (a-j and p-y) are discussed in Brown (1993).

gone hand-in-hand. Models to generate such counterclockwise paths include intraplating of mantle-derived magmas (Bohlen 1987, 1991; Bohlen & Mezger 1989) and crustal thickening with concomitant mantle lithosphere thinning (Loosveld & Etheridge 1990; Sandiford & Powell 1991). Orogenic belts characterized by counterclockwise P - T paths achieved maximum T before maximum P, and the metamorphic peak normally pre-dated or was synchronous with early deformation within the belt. Once again, such a process will generate dehydration melting (Thompson 1990) and may lead to granite production as a consequence of regional metamorphism (Collins & Vernon 1991). Clockwise and counterclockwise paths may occur in adjacent parts of the same orogenic belt, as exemplified by the Acadian orogenic events in the northern Appalachians (Tracy & Robinson 1980; Schumacher et al. 1989; Armstrong et al. 1992), and illustrated in Fig. 2. In order to unravel the history of an orogenic belt we n e e d knowledge of the change of pressure and temperature with time, and on the relationship of these to deformation. Information that will enable us to address this issue potentially includes the following: data on the type and age of protolith lithologies and if possible the tectonic environment of their formation; data on the P - T evolution

P-T-t

E V O L U T I O N OF O R O G E N I C BELTS

71

14 12

Western Acadian

10

L_

n

0 0

200

400

600

800

T (°C) Fig. 2. P - T diagram to show postulated typical P - T trajectories for each of the metamorphic realms discussed by Armstrong et al. (1992) from central and western New England, USA. The patterned ovals indicate the approximate part of each trajectory at which the peak P - T conditions were recorded. Note: for broad metamorphic realms such as the Western Acadian and Taconian, what Armstrong et al. have shown is only one typical path from a nested family of similar paths.

of the metamorphic rocks; data on the relationship between metamorphic mineral growth and deformation; age data to constrain the timing of prograde and peak metamorphic conditions; age data from pre-, syn- and post-orogenic plutons; data on cooling and exhumation using various mineral geochronometers; and the unroofing history of the belt as reflected in the erosional debris deposited in its foreland. Although a complete understanding of the evolutionary history of an orogenic belt requires much or all of this information, studies to date are rarely so complete. Nevertheless, our understanding of the relationship between metamorphism and tectonics has increased dramatically during the past few years through the combination of several of these types of field, analytical, and numerical investigations of metamorphic P - T - t paths. M i c r o s t r u c t u r a l studies a n d the use o f textures

The identification of clockwise v. counterclockwise paths requires the relationship between mineral growth and deformation to be established from textural relationships; an example is given in Fig. 3. In effect, we need to recognize both a sequence of overprinting metamorphic events and the relationship between a particular mineral assemblage and the deformation phases that have affected the rock during thickening and exhumation. Textural analysis enables us to establish relative timing of metamorphic and deformational events (see Fig. 3), which may then be quantified using the increasingly sophisticated isotopic techniques that can be applied to individual minerals. Pioneering work on microstructural studies, in particular the relationship between porphyroblast growth and matrix development, was undertaken by Zwart in the Pyrenees (1962) and Johnson in the Scottish Highlands (1963). Careful petrographic analysis

Fig. 3. Relatively straight inclusion trails in the centre of kyanite (Ky) porphyroblast curve through the rim (lines emphasize trail orientation) and are consistent with early syn-foliation growth, a conclusion supported by the overall shape of the porphyroblast which exhibits small 'tails' that have grown into the foliation at the top and bottom. The dominant schistosity which encloses the kyanite porphyroblast is $2 in this particular rock, which is from the Port aux Basques Complex in Southwest Newfoundland, Canada. Long dimension of field of view is 4.5 mm, crossed polars.

is critical, yet many metamorphic textures are ambiguous, and interpretations consequently may be subjective; for example, we cannot yet agree on whether or not porphyroblasts rotate during deformation or, more likely, accept that in some cases porphyroblasts have rotated, but in other cases they have not (see the debate between Passchier et al. (1992) and Bell et al. (1992), and primary references therein). Suitable textures to elucidate P - T - t paths include relict and replacement features, porphyroblast-inclusion-matrix relationships and high strain zones cutting through metamorphic belts that may have developed during the exhumation part of the P - T - t path. Further, it is important to decide which minerals, if any, might represent equilibrium assemblages that can be used in quantitative calculation of P - T conditions at points on the P - T - t path. Minerals of particular growth stages can be used to elucidate t at points on the P - T - t path. Thus, a combination of microstructural, thermobarometric and geochronological studies will allow the identification of a well-constrained P - T - t - d e f o r m a t i o n path. Particularly important, but largely unknown, is information on the rates of processes such as heating, mineral reactions, partial melting and tectonic

72

M. BROWN

deformation; one example of such information is given by Mezger (1990).

Equilibrium v. disequilibrium The main development that has occurred in metamorphic petrology during the past twenty years is the realization that our previous obsession with 'equilibrium' ignores the evidence of a dynamic evolution represented by mineralogical and chemical 'disequilibrium'. Equilibrium is the basis of the metamorphic facies concept, proposed by Eskola in 1914 and developed by Goldschmidt and Eskola, in particular during a visit by Eskola to work with Goldschmidt in Oslo during 1919-1920 (Eskola 1920); and it is the sequence of metamorphic facies exposed along the erosion surface through a metamorphic belt that represents the metamorphic facies series of Miyashiro (1961). The thermodynamic basis for the metamorphic facies concept was provided by Thompson (1955) which set the ground for quantitative geothermobarometrical work that has proven so profitable in the quantification of metamorphic P and T over the last three decades. One basic aim of modern metamorphic petrology is to relate observed mineral assemblages to

P - T - t history and to utilize this information to distinguish between various possible tectonic processes that may operate in orogenic belts. To assess 'peak' P - T history, we have relied on thermodynamics and phase equilibria, and we have made t h e assumption that either the mineral assemblages, through use of a petrogenetic grid, or the mineral chemistries, utilizing thermobarometry, will reveal a P and T that are geologically significant. If equilibrium is achieved and preserved over significant portions of the mineral assemblage, as reflected by completely homogeneous mineral chemistry and straight boundaries between adjacent grains, then this procedure will be valid; but by the very nature of equilibrium, the effects of any previous or subsequent processes are lost completely. Thus, the dynamics or history of a rock are relegated to a secondary role. It follows from this discussion that a study of a whole series of samples from a single metamorphic belt that are thought to reflect equilibrium can only yield individual points on each of a set of P - T - t paths and can yield no information about the dynamic aspects of the tectonic processes involved. Disequilibrium features in rocks, however, reveal dynamic history because features such as inclusions in porphyroblasts, replacement textures and

!

0

12 Fig. 4. (A) Partially resorbed garnet (Grt) from granulite facies metapelite, Sharyzhalgay complex, Lake Baikal, Russia, exhibits a partial orthopyroxene necklace (Opx) that outlines the original garnet porphyroblast (dashed line). Inside the orthopyroxene necklace, a symplectite (Sym) cdmposed of cordierite, orthopyroxene and biotite has partially resorbed the garnet. This delicate texture often is interpreted to represent decompression, and indicates further that any deformation associated with decompression was concentrated in rocks other than this one since the texture would not have survived significant ductile strain. Long dimension of field of view is 13.5 mm, plane light. (B) Detail of rock shown in (A) to illustrate two reactions preserved by the textures. First, garnet (Grt) and quartz (Qtz) reacted to give granular orthopyroxene (Opx) and plagioclase (PI), orthopyroxene nucleated against quartz and plagioclase nucleated against garnet (now replaced by subsequent symplectite development). Second, during decompression garnet and quartz, probably in the presence of melt, have reacted to cordierite (Crd), orthopyroxene and biotite (Bt) intergrown as a symplectite. This reaction has not gone to completion which suggests that decompression was relatively rapid. Long dimension of field of view is 2.0 mm, plane light.

P-T-t

EVOLUTION OF O R O G E N I C BELTS

mineral zoning reflect a succession of changing conditions along a P - T - t path; an example is given in Fig. 4. These features have been utilized in an increasing number of studies during the past ten years; see, for example, information in Tracy (1987) and Peacock (1991b) and the many examples of P - T - t paths collected together in the book 'Evolution of metamorphic belts' (Daly et al. 1989). M o d e l l i n g studies

Possible causes of temperature change in the crust include variation in the conductive heat flux from the mantle, advection of heat through emplacement of mantle-derived magma into the crust, variation in the heat productivity from radioactive decay, physico-chemical processes that involve thermal energy such as metamorphic reactions and melting, including magmatic transfer of heat from the lower crust to the upper crust, depression of the crust through thickening and elevation of the crust through thinning. The dependence of the temperature field upon dynamic processes and the consequent pressure-temperature-time evolution of metamorphic rocks have been investigated through thermal modelling of orogenic belts (for a review of early work see Thompson 1981). This modelling has shown that the P - T - t path a rock follows is the result of a complex interaction between tectonic processes and heat flow/heat generation/heat transfer mechanisms. The regional scale complexity of metamorphic belts reflects a variety of factors that include lateral cooling at depth along major tectonic boundaries (Harte & Dempster 1987) and spatial variation in thermal conditions and structural history (e.g. Jaupart & Provost 1985; Allen & Chamberlain 1989; Sonder & Chamberlain 1992). These variations were recognized by Turner (1981) and Richardson (1970) and are acknowledged in most subsequent modelling studies, which by their nature are inherently simplified, either because of our incomplete knowledge of the physical properties of rocks, or by reduction of the number of dimensions from three to two, or even to one, or by regarding certain kinds of geological processes as instantaneous, such as thrusting during crustal thickening (England & Thompson 1984; Shi & Wang 1987; Haugerud 1989; Chamberlain & Sonder 1990; Peacock 1990, 1991a). Nonetheless, we are approaching a first-order understanding of those heat and mass transfer processes that drive metamorphism, exemplified by the saw-tooth geothermal profile that results from models of crustal thickening by instantaneous thrusting and its subsequent evolution with time to a new stable geothermal gradient (e.g. Thompson 1981; England & Thompson 1984). The tectonic history of a region, the thermal history of rocks within that region, and the P - T - t evolution of those rocks are related. In a one-dimensional analysis, by which we mean that the crust is regarded as composed of columns of rock with equal physical properties and between which there is no lateral heat transfer, this relationship can be represented as a surface in P - T - t space. The tectonometamorphic histories of rocks are represented by lines on this surface (Haugerud 1989, fig. 1). The projections of such rock histories onto the three two-dimensional planes of the P - T - t box represent the T-t paths described by the geochronologist, the P - T paths studied by the metamorphic petrologist and the P - t paths inferred by the tectonicist. Of course, the strict correlation of T and t refers to the cooling paths of a metamorphic belt; minerals that grow below their

73

blocking temperatures for which an estimate of pressure of formation can be made may be used to relate P and t. One approach to understanding the P - T - t evolution of rocks in particular tectonic settings and for particular heat-transfer mechanisms is that of forward modelling of the thermal response to tectonism, as demonstrated, for example, by England & Thompson (1984), Haugerud (1989) and Chamberlain & Sonder (1990). However, the metamorphic petrologist investigates the inverse problem, that is the determination of P, T and t from rocks, and from these data infers a tectonic history. A combination of the forward approach used by the modeller and the inverse approach used by the petrologist in understanding the P - T - t evolution of rocks will lead to a better understanding of the history of orogenic belts than either alone. Further, thermal modelling allows us to determine the potential for reaction by giving us rates and duration of overstep of particular equilibrium boundaries, it allows us to predict when reactions should be frozen-in and to determine the amount of bulk difusional resetting of phases such as garnet that are commonly compositionally zoned. Outcomes

A variety of different techniques has been used to constrain portions of P - T - t paths. These techniques include: the use of mineral inclusion suites and reaction textures, for example, in the Wopmay orogenic belt of northwest Canada (St-Onge 1987), in Southern Brittany (Jones & Brown 1990) and in British Columbia and New Hampshire (Selverstone & Chamberlain 1990); thermobarometry, for example, in Antarctica (Harley et al. 1990; Harley & Fitzsimons 1991) and in the Arunta Complex in central Australia (Goscombe 1992); thermodynamic approaches such as the Gibbs method, for example utilizing garnet (Spear et al. 1984; Spear 1988, 1989); radiogenic isotope dating, for example in the Himalayas (Zeitler 1989), the Pikwitonei granulite domain in the Canadian Shield (Mezger 1989) and in the New England Appalachians (Wintsch et al. 1992); the use of fluid inclusions to constrain physical conditions during exhumation (for example, Hollister et al. 1979); and numerical modelling (for example, Thompson & England 1984; Chamberlain & Sonder 1990; Peacock 1990). A number of recent reviews address methods of obtaining P - T - t path information from metamorphic terranes, with many examples, to which the reader is referred for more detailed information (.Ghent et al. 1988; Daly et al. 1989; Harley 1989; Spear & Peacock 1989; Haugerud & Zen 1991; Hodges 1991; Jamieson 1991). Our ability to separate partially overprinted 'equilibrium' mineral assemblages in 'disequilibrium' rocks has enabled the use of thermobarometry to determine different sets of P - T data for individual rocks, although one must proceed with due caution (Selverstone & Chamberlain 1990). The locus of peak or slightly post-peak P - T conditions as preserved by the mineral assemblages--the metamorphic facies series (Miyashiro 1961), metamorphic geotherm (England & Richardson 1977), piezothermic array (Richardson & England 1979), P - T array (Thompson & England 1984), metamorphic field gradient (Spear et al. 1984) or set of metamorphic zones (Harte & Dempster 1987)--results from the intersection of P - T paths for individual rocks with the erosion surface (England & Richardson 1977). Thus, each location along the metamorphic field gradient

74

M. B R O W N

represents a unique point of pressure, temperature and time. Since these points generally will not be contemporaneous, the metamorphic field gradient is necessarily the locus of diachronous P - T conditions. P - T - t paths deduced from petrographic, thermobarometric, fluid inclusion and geochronometric data can be the result of a single orogenic cycle or may be the cumulative effect of several orogenic cycles, in which case the apparent P - T - t path determined from evidence in the rocks may have no real meaning, being a composite of information partially preserved from more than one path. Additionally, single-cycle P - T - t paths are commonly represented as simple smooth curves, whereas in reality they are likely to have a more complex form because rates of burial and uplift, with or without magmatic heating, vary during the orogenic cycle or because of the effects of progressive deformation (e.g. Hames et al. 1990; Dempster 1985; Dempster & Harte 1986). An example of the complexity that occurs in nature and of our increasing ability to resolve such complexity is provided by Goscombe (1992) who has separated an earlier counterclockwise P - T - t evolution from a subsequent clockwise P - T - t evolution within the polymetamorphic Arunta Complex in central Australia. One rapidly advancing area is in the development and application of a wider range of mineral geochronometers; this has proven particularly important in the elucidation of prograde parts of P - T - t paths and exhumation processes. For example, Mezger et al. (1989) have utilized the U-Pb system on garnet to date points during the prograde evolution of parts of the Pikwitonei granulite domain in the Canadian Shield. With respect to the exhumation process, the Adirondack Mountains of New York cooled at time-integrated rates of c. 1.5 °C/Ma for at least 150 million years following the last phase of high-grade metamorphism (Fig. 5), suggesting only limited vertical tectonic displacement and approximate isostatic equilibrium (Mezger et al. 1991). By contrast, the Southern Brittany Migmatite Belt T (°C) 700

Central x \ \Highlands.,,.

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Cooling History Adirondack Highlands

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I

850 800 time (Ma)

Fig. 5. A possible temperature-time cooling path for the Adirondack Highlands based on mineral ages of garnet, monazite, titanite, hornblende, rutile and biotite. The solid vertical bars and shaded areas indicate the mineral ages or range in mineral ages and the range in estimated closure temperatures. The closure temperatures used for the different minerals are as follows: garnet, U-Pb, >800 °C; monazite, U-Pb, 700-650 °C; titanite, U-Pb, 670-500 °C (depending on grain size); hornblende, 4°Ar-39Ar, 500-400°C; rutile, U-Pb, 430-380 °C; and, biotite, 4°Ar-39Ar, c. 300 °C. Data from Mezger et al. (1991) to which the reader is referred for further discussion.

900 800

I

I

I

700

' ~ 1

600

Cooling history Southern Brittany

500

Monazite

~-]

Hornblende

Muscovite

400 300

I

Zircon

--]

Toc

Biotite

200 100

time (Ma)

I

i Apatite

I I I I 400 350 300 250 Fig. 6. A possible temperature-time cooling path for the Southern Brittany Migmatite Belt based on mineral ages of garnet, monazite, hornblende, muscovite, biotite, and apatite. The boxes indicate the range of mineral ages and the range in estimated closure temperatures, based upon likely peak metamorphic temperatures for zircon and fast cooling for hornblende, muscovite, biotite and apatite. The closure temperatures used for the different minerals are as follows: zircon, U-Pb, c. 775 °C; monazite, U-Pb, 730-640 °C; hornblende, 4°Ar-a9Ar, c. 500 °C; muscovite, 4°Ar39Ar, c. 400 °C; biotite, Rb-Sr, c. 325 °C; and, apatite fission track, c. 125 °C. Data are from Peucat (1983) and Dallmeyer & Brown (1992). 0

(Jones & Brown 1990) exhibits extremely rapid cooling with time-integrated rates of c. 50°C/Ma. during 10 million years. (Fig. 6), suggesting tectonic exhumation (Dallmeyer & Brown 1992). Barrow's 1893 paper was concerned with spatial variations in metamorphic grade across an orogenic belt and the underlying causes of regional metamorphism. Much of modern work, however, deals with evolving P-T conditions within individual rock samples or within small parts of orogenic belts. This contrast reflects a change in emphasis, but we are beginning to return more to the spatial variation in P - T - t evolution as the methodology becomes more routine and its application more widespread across orogenic belts. As examples, regional variations across the Scottish Highlands have been summarized succinctly by Harte & Hudson (1979; see also Dempster 1985 and Dempster & Hafte 1986), and regional variations in depth of burial and the implications for denudation history of the southern New England Appalachians have been discussed by Zen (1991; see also Hames et al. 1990). A major feature of European orogenic belts is the great variety of facies types side-by-side or superimposed. For example, the Variscan belt contains very-high-pressure rocks over a wide area, from Poland to Portugal, some of which have been overprinted by a low-pressure-high-temperature metamorphism, reflected in orthopyroxene-bearing assemblages that overprint eclogitefacies mineralogies. The spatial relationships may reflect tectonic dismemberment of a paired metamorphic belt, and even though the structural history may be locally complex, the overall thermal pattern remains deeply ingrained.

R e c e n t a d v a n c e s in r e g i o n a l m e t a m o r p h i s m Any attempt to 'cubbyhole' regional metamorphism into types will produce some overlap between tectonic setting,

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metamorphic processes and metamorphic rock-types. Nonetheless, it is convenient to highlight recent advances in our understanding of different aspects of regional metamorphism by considering four particular types of metamorphism, acknowledging that the distinction between them is not perfect and that my selection of both types and highlights is a personal one. Collision metamorphism

Metamorphism that is a result of substantial crustal thickening due to collisions between continental elements and/or arcs dominates the literature on regional metamorphism. P - T - t paths that result from collision metamorphism are clockwise in P - T - t space and their general characteristics are well understood, both from the standpoint of actual examples, such as the Appalachians (Armstrong et al. 1992, and references therein), the Caledonides (Anderson et al. 1992, and references therein) and the Himalayas (Searle et al. 1992, and references therein) and from a theoretical standpoint in terms of the heat transfer processes involved (England & Thompson 1984, and references therein). Recently, there has been a dramatic increase in the use of 4°Ar/39Ar methods to determine the polymetamorphic and exhumation histories of these metamorphic terranes (e.g. McDougall & Harrison 1988). Also, during the past decade the emphasis has shifted from the Appalachians and Caledonides to the Himalayas; although the former remain important both historically in the development of ideas and in terms of ongoing research, some of the most exciting advances have resulted from detailed work in the Himalayas. The Himalayan mountain chain developed as a result of the Eocene to Recent collision between India and Eurasia; the ongoing convergence has led to the exhumation and exposure of the high-grade metamorphic core of the Himalayan orogen. These high-grade metamorphic rocks of the Greater Himalaya lie above the north-dipping Main Central Thrust system, a major intracontinental thrust system that accommodated a significant proportion of the total shortening across the orogen (Le Fort 1975). The thrust system separates the high-grade metamorphic rocks from generally lower grade metasedimentary rocks of the Lesser Himalaya, although the metamorphic grade of this unit increases to the east (e.g. Swapp & Hollister 1991). Further, metamorphic studies along the Himalayan mountain chain show that high-grade rocks occur at structurally shallower levels than lower-grade rocks, a phenomenon referred to as 'inverted metamorphism' (Gansser 1964). One of the issues that remains unclear is the extent to which polyphase metamorphism is reflected in the observed mineral assemblages in the high-grade metamorphic core of the Himalayan orogen (Hodges et al. 1988; Inger & Harris 1992), and the degree to which the metamorphism is diachronous, propagating to the south (Searle et al. 1992). Syn-metamorphic displacements on fault systems result in thermal decoupling across such systems and the Main Central Thrust system probably was active during high-grade metamorphism and anatexis of the Greater Himalaya rocks (Hodges et al. 1988; Hubbard & Harrison 1989; Searle et al. 1992). This high-grade metamorphic core is .truncated by north-dipping, low-angle normal faults and shear zones of the South Tibetan detachment system (Burg et al. 1984; Burchfiel et al. 1992), structures that developed

75

under the influence of gravity to moderate the extreme topographic and crustal thickness gradients produced by displacement on contractional structures (Burchfiel & Royden 1985). Recent work suggests that displacement on the South Tibetan detachment system was penecontemporaneous with contractional deformation on the Main Central thrust zone (Burchfiel et al. 1992; Hodges et al. 1992; Searle et al. 1992); the thermal consequences of this complex tectonic activity are described by Inger & Harris (1992) and Hodges et al. (1993). Further, Hodges et al. (1993) emphasize that radically different P - T - t paths can be found at different structural levels beneath such extensional structures; this leads them to suggest that P - T - t paths in collision belts may be complex and thus not uniquely diagnostic of the unroofing mechanism, particularly in regions characterized by penecontemporaneous extension and shortening. To the north of the South Tibetan detachment system, in the Tibetan zone, Neogene metamorphic core complexes occur within the Tibetan sedimentary sequence. One example, the Kangmar dome, has been studied in some detail by Chen et al. (1990). On the basis of structural analysis, they infer that the domal structure formed as a consequence of extensional deformation; this draws inevitable comparisons with the Tertiary metamorphic core complexes of the northwestern American Cordillera. High-temperature-low-pressure

metamorphism

High-temperature-low-pressure metamorphic terranes have been explained in a number of different ways; however, a common cause for this type of regional metamorphism is not going to be established because fundamentally different tectonic processes result in similar P - T conditions. High-temperature-low-pressure metamorphism is perceived as a problem because of the extreme thermal anomaly implied by calculated geothermal gradients, commonly in the range 60-150 °C/km (De Yoreo et al. 1991), hence this type of metamorphism is a 'freak of nature'. As a direct consequence of this, the role of advective heat transfer in the formation of high-temperature-low-pressure metamorphic belts has been over emphasized and tectonic transport of heat to change the thermal gradient with time, quite plausible given deformation at reasonable rates and the poor thermal conductivity of rocks, has been underestimated. There are three endmember processes that result in high-temperature-low-pressure metamorphism: (i) contractional deformation and crustal thickening, for example structural inversion of a sedimentary basin developed on thin lithosphere, which results in a clockwise path in P - T space and subsequent tectonic exhumation that produces high-temperature-low-pressure metamorphism as a consequence of high-temperature decompression (e.g. Jones & Brown 1989; Jones & Brown 1990; Treloar & Brown 1990; Dallmeyer & Brown 1992; Thompson 1989); (ii) regionalscale contact metamorphism which results in isobaric heating and cooling (e.g. Lux et al. 1986; Barton & Hanson 1989; De Yoreo et al. 1989a, b); and, (iii) magmatic advection of heat during crustal thickening, which results in a counterclockwise P - T path and either isobaric cooling from the high-temperature-low-pressure peak or even increasing P during cooling (e.g. Wells 1980; Bohlen 1987; Vernon et al. 1990; Collins & Vernon 1991). Metamorphism associated with crustal extension has

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M. BROWN

become widely recognized in the past decade, both in metamorphic core complexes and in collision belts, and the role of extension in the generation of high-temperaturelow-pressure metamorphism should not be underestimated; Peacock (1991b) gives a good summary. As we have seen in the section on collisional metamorphism, a hightemperature-low-pressure metamorphic event is commonly superimposed on an earlier higher pressure metamorphism as a consequence of extensional collapse of an overthickened orogenic belt (Dewey 1988; Inger & Harris 1992; Hodges et al. 1993). Variscan massifs of the Pyrenees are characterized by high-temperature-low-pressure metamorphism that provoked much discussion in the 1980s (e.g. Wickham & Oxburgh 1985; Lux et al. 1986; Wickham 1987; Wickham & Oxburgh 1987). Ironically, many of these massifs have been shown to have followed P - T - t paths that involved decompression under prograde and retrograde conditions, that is to say they are clockwise, such as the Bosost and Lys-Caillaouas massifs of the central Pyrenees (Pouget 1991; Kriegsman et al. 1989), the Canigou massif in the eastern Pyrenees (Gibson 1991), and the Trois Seigneurs and Saint Barth616my massifs of the North Pyrenean Zone (Kriegsman 1989; de Saint Blanquat et al. 1990). Further all of these authors have shown that the Variscan hightemperature-low-pressure metamorphism of the Pyrenees occurred in an extensional tectonic environment that produced the subhorizontal regional foliation; this followed only moderate overthickening evidenced by earlier tectonic structures. Granulite m e t a m o r p h i s m

This topic of regional metamorphism has proven intellectually productive during the past few years, and much information can be found in two recent books (Vielzeuf & Vidal 1990; Ashworth & Brown 1990). Some granulite facies terranes clearly are the result of collisional metamorphism, such as the Grenville Province in North America (Anovitz & Chase 1990); these terranes exhibit little variation in metamorphic conditions over large areas, and apparent disequilibrium textures, such as symplectites of orthopyroxene and plagioclase after garnet and quartz, may be preserved in rocks of the same bulk composition over thousands of square kilometres. Such metamorphic terranes followed clockwise paths in P - T space but have remained incubated as the post-orogenic lower crust to generate a long, nearly isobaric cooling history from high temperatures. Other granulite facies terranes appear to be related to extensional tectonics, and such an example has been described by Armstrong et al. (1992) from central Massachusetts, where a component of advective heat transfer from the mantle also is thought to be important. Finally, granulite facies metamorphism may be driven substantially by advective heat transfer, exemplified by the Proterozoic low-pressure granulites of southwest Finland (e.g. Schreurs & Westra 1986). The extreme conditions characteristic of granulite-facies terranes are of two kinds: high-temperature, such as found in the Enderby Land granulite terrane (e.g. Ellis 1980; Sheraton et al. 1987); and high-pressure, such as found in the European Variscides (e.g. Carswell & O'Brien 1992). With respect to the high-pressure granulites, such rocks may well represent the exposed roots of collisional mountain belts, metamorphism being the result of burial during crustal

overthickening. However, high-temperature granulites require a gross perturbation of the normal continental geothermal gradient, and many of these terranes preserve evidence of prolonged residence in the middle-to-lower crust after deformation and metamorphism. In spite of the debate in the literature during the 1980s concerning the origin of granulite-facies terranes (e.g. Bohlen 1987; Ellis 1987), the general cause of granulite-facies metamorphism in many high-temperature terranes may be external to the rocks that we observe, and possibly external to the crust (Vernon et al. 1990). Much of the argument during the 1980s about the origin of granulite-facies terranes stemmed from two particular kinds of incomplete P - T - t paths, derived largely from evidence preserved from the retrograde rather than the prograde metamorphic history, which has allowed the division of many high-grade metamorphic terranes into two types (Harley 1989): those which show near-isobaric cooling and those which show near-isothermal decompression. Isobaric cooling paths have been identified from many granulite terranes (see reviews by Bohlen 1987 and Harley 1989) but the tectonic setting in which they are generated, either crustal thickening or magmatic accretion (Ellis 1987; Bohlen 1987; Bohlen & Mezger 1989; Harley 1989; Bohlen 1991) or, possibly, lithopheric extension (Sandiford & Powell 1986), remains a matter of debate. Furthermore, since evidence for the prograde path is generally lacking, rocks which exhibit isobaric cooling paths can have followed either a clockwise or a counterclockwise path in P - T space. As an example, the granulite facies rocks of the Grenville province are best modelled by early high-pressure conditions followed by exhumation to lower-middle crustal levels and then slow cooling (Anovitz & Chase 1990). Isothermal decompression paths occur as part of clockwise P - T evolution but have steeper d P / d T s l o p e s than those generated by the erosion-controlled exhumation of the kind discussed by England & Richardson (1977) and England & Thompson (1984). The model of Albar~de (1976) corresponds closely with many of the petrologically determined P - T - t paths, as by Hollister (1982) in the Coastal Range of British Columbia, Canada, Brown & Earle (1983) in Timor, Droop & Bucher-Nurminen (1984) in the Alps, and Harris & Holland (1984) in the Limpopo mobile belt of Southern Africa. Such 'fast exposure paths' (Harley 1989) can be generated by rapid exhumation and probably reflect tectonic thinning by extension of previously thickened crust (e.g. Sonder et al. 1987; Ruppel et al. 1988). Subduction zone metamorphism

Advances in our understanding of subduction zone metamorphism have been achieved through field and petrologic investigations of ancient and modern subduction zones and through numerical modelling of heat and mass transfer at convergent plate margins. Again, this type of metamorphism grades into collisional metamorphism because arcs and continents will be transported to subduction zones where they will slow subduction and generate intracontinental deformation. Recently, an increasing number of localities with evidence of very high pressure, reflected in coesite or coesite pseudomorphs and other exotic minerals, has been identified; this demonstrates that crustal material can be subducted to, and exhumed from, depths greater than 100 km in collision zones, for example in the Alps (Chopin 1984, 1987; Schreyer 1988), in the

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E V O L U T I O N OF O R O G E N I C BELTS

Caledonides of Norway (Smith 1984), and in the Dabie Mountains in central China (Wang et al. 1989). Further evidence of such extreme pressures is the occurrence of diamond-bearing metamorphic rocks derived from crustal protoliths (Sobolev & Shatsky 1990; Xu et al. 1992). The structural and metamorphic features of such very highpressure metamorphic rocks require rapid exhumation that is likely tectonic, effectively involving transport as fault-bounded tectonic slices. Without rapid tectonic exhumation the very high-pressure mineral assemblages are thermally consumed, hence slow erosion-controlled exhumation allows this type of metamorphism to be overprinted under lower-pressure/higher-temperature conditions because it is 'time, that devours all things!'. In terms of P - T - t paths, Ernst (1988) has divided subduction zones into two different types according to the retrograde legs of the P - T - t paths. One type comprises the collisional blueschist belts, such as the Western Alps, which undergo widespread retrogression in the greenschist and/or epidote amphibolite facies during near isothermal decompression associated with the collision of a continental fragment, oceanic plateau, or arc with the subduction complex. Other high-P belts, such as the Franciscan Complex of California in the USA, in which continued subduction results in exhumation P - T - t paths that approximately retrace prograde paths, exhibit minimal retrogression and the preservation of metamorphic aragonite. A correlation evidently exists between the nature of the retrograde metamorphic trajectory and continued underflow that inhibits back-reaction, in comparison with collision and abrupt deceleration of convergence that allows pervasive back-reaction. Thus, retrograde blueschist parageneses can help to constrain the tectonic history of the subduction zone.

Quo vadimus? Further resolution of problems in metamorphism requires advances in a number of different areas. These include, but are not limited to, the following. (1) The continued development of an accurate thermodynamic data base for minerals that occur in metamorphic equilibria through an improvement of our thermodynamic knowledge of individual minerals, and in particular the activity-composition relations in the P - T range of interest. The data on thermodynamic properties of minerals are becoming more accurate and precise with time and the importance of using internally consistent data sets has been realized; however, thermodynamically calibrated thermobarometers must continue to be evaluated against experimentally based equilibria and natural occurrences. More accurate and precise determination of metamorphic P - T conditions will place tighter constraints on modelling studies and advance our ability both to understand metamorphic processes and to identify different tectonic settings through their characteristic metamorphism. (2) The development of a better understanding of the kinetic response of minerals to changes in P and T, and in particular improvement of knowledge of diffusion rates and closure temperatures in minerals that are useful geochronometers. The past ten years have seen significant improvements in our understanding of processes such as intracrystalline diffusion that can modify significantly mineral compositions from their peak metamorphic values and thus obscure the peak conditions (e.g. Spear & Florence

77

1992, and references therein). However, the recent discovery of oxygen isotope zoning within garnet (Chamberlain & Conrad 1991) reminds us how poorly we understand the kinetics of diffusion, although it should be noted that this provides an additional record, or 'tape recording' of events in the evolution of the rock that are likely to have been completely obliterated in the matrix. (3) The further development of radiogenic isotope dating methods for a wider range of minerals to improve our ability to measure time at different points on the prograde and retrograde segments of P - T - t paths, and the increasingly widespread use of the laser 4°mr/39Ar method and the ion microprobe as geochronological tools. Uranium-lead mineral dating is the best source of high-precision ages. The relatively high U/Pb ratios of widely occurring accessory minerals such as zircon, titanite, monazite, rutile and ilmenite, the relatively rapid change in the 2°7pb*/2°6pb* with time, and the relatively short half-lives of 235U and 238U all contribute to the potential of small age uncertainties; uncertainties of less than + 0.1% relative are possible for concordant U-Pb ages. Increased use of high-precision U-Pb ages will improve our understanding of the time involved in the high-temperature parts of P - T - t paths. The usefulness of garnet to the future quantification of rates of tectonometamorphic processes in metamorphism is exemplified in a number of recent papers. Christensen et al. (1989) have utilized the Rb-Sr method in single garnet crystals from schists in southeast Vermont to study the rates of petrological processes, such as growth rate, estimated at 1.4 +0.92/-0.45mm per million years, and average time interval of growth, c. 10.5 + 4.2 Ma. Garnet and its mineral inclusions provide a sequential record of P - T change, strain and chemical reactions during metamorphism; therefore, the technique offers the potential for determination of the rates of those processes as well. In the Vermont example, the growth interval and the observed amount of rotation recorded by inclusion trails, assuming that the porphyroblasts have rotated with respect to the matrix, indicate that the average shear strain rate during garnet growth was 2.4 + 1 . 6 - 0 . 7 x 10 -14 per second. By contrast, Burton & O'Nions (1990) have used a combination of the Sm-Nd, U-Pb and Rb-Sr systems to give whole rock isochron and mineral isochron ages that have revealed in detail the chronology of processes in small-scale granulite formation from Kurunegala in Sri Lanka. Finally, the Sm-Nd system offers the possibility of dating different zones within minerals such as metamorphic garnet (Burton & O'Nions 1991).

Epilogue The past 100 years have produced significant and dramatic progress in our understanding of metamorphic processes, even though the relative contribution of some processes remains unresolved. Recently, we have begun to recognize and appreciate the importance of extension in collisional mountain belts, interpreted previously largely in terms of contraction. Most of us now accept that clockwise and counterclockwise P - T - t paths occur in nature and reflect substantially different tectonic and magmatic processes. It is apparent that high-temperature-low-pressure metamorphism can be generated by different tectonic processes. Granulites not only represent extreme conditions of both pressure and temperature but also are produced by tectonic

78

M. B R O W N

processes that result in both clockwise and counterclockwise P - T - t paths. The causes of regional metamorphism are multiple and may occur individually or in unison, in addition to the 'normal' conductive heat flux from the earth's interior; they include internal radiogenic heat production in thickened continental crust and structurally inverted sedimentary basins, magmatic advective heat transfer both from the mantle into t h e crust and from the lower crust into the upper crust, and lithospheric extension resulting in an enhanced conductive heat flux from the underlying asthenosphere. It is often alleged that serendipity plays a large part in research. Perhaps George Barrow was lucky to be born in the UK during the period when the Geological Survey was at its peak mapping the Scottish Highlands, but without his ability to make careful observations and his insight in their interpretation he could not have written the perspicacious paper that has provided much of the foundation for metamorphic geology today. It is an unfortunate measure of progress in science that some of what Barrow wrote about in detail in the Scottish Highlands has been reinterpreted. The pegmatites, critical to the model of thermal metamorphism preferred by Barrow, represents sub-solidus and anatectic migmatites and the tectonic setting of the metamorphism was one of plate collision. Nonetheless, his work will be remembered in perpetuity as the type example of medium-pressure or 'Barrovian' metamorphism.

States and thermal modelling. Geological Society of America, Bulletin, 101, 1051-1065. BELL, T. H., JOHNSON, S. E., DAVIS, B., FORDE, A., HAYWARD, N. and WILKINS, C. 1992. Porphyroblast inclusion-trail orientation data: eppure non son girate! Journal of Metamorphic Geology, 10, 295-307• BIRD, P., TOKSOZ, M. N. & SLEEP, N. H. 1975. Thermal and mechanical models of continent-continent convergence zones. Journal of Geophysical Research, 80, 4405-1406. BOHLEN, S. R. 1987. Pressure-temperature-time paths and a tectonic model for the evolution of granulites. Journal of Geology, 95, 617-632. --, 1991. On the formation of granulites. Journal of Metamorphic Geology, 9, 223-230. -& LINDSLEY, D. H. 1987. Thermometry and barometry of igneous and metamorphic rocks. Annual Reviews of Earth and Planetary Science, 15, 397-420. & MEZGER, K. 1989. Origin of granulite terranes and the formation of the lowermost continental crust. Science, 244, 326-329. BROWN, M. 1993. The generation, segregation, ascent and emplacement of granite magma: Insights from migmatites. Earth Science Reviews. & EARLE, M. M. 1983. Cordierite-bearing schists and gneisscs from Timor, eastern Indonesia: P - T conditions of metamorphism and tectonic implications. Journal of Metamorphic Geology, 1, 183-203. BURCHFIEL, B. C. & ROYDEN, L. H. 1985. North-south extension within the convergent Himalayan region. Geology, 13, 679-682. , CHEN, Z., HODGES, K. V., LIu, Y., ROYDEN, L. H., DENG, C. & Xu, J. 1992. The South Tibetan detachment system, Himalayan Orogen, -

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extension contemporaneous with and parallel to shortening in a collisional mountain belt. Geological Society of America, Special Paper 269. BURG, J. P., BRUNEL, M., GAPAIS, D., CHEN, G. M. & LIU, G. H. 1984• Deformation of leucogranites of the crystalline Main Central Sheet in Southern Tibet (China). Journal of Structural Geology, 6, 535-542. BURTON, K. W. & O'NIONS, R. K. 1990. The time scale and mechanism of granulite formation at Kurunegala, Sri Lanka. Contributions to Mineralogy and Petrology, 106, 66-89. & --, 1991. High resolution garnet chronometry and the rates of metamorphic processes. Earth and Planetary Science Letters, 107, 649-671. CARSWELL, O. A. & O'BRIEN, P. J. 1992. Spatial and temporal relationships between HP and LP metamorphic assemblages in the Central European Variscides. 29th International Geological Congress, Kyoto, Japan, Abstracts, 2, 581. CHAMBERLAIN, C• P. & CONRAD, M. E. 1991. Oxygcn isotope zoning in garnet. Science, 254, 403-406. & SONDER, L. J. 1990. Heat-producing elements and the thermal and baric patterns of metamorphic belts. Science, 250, 763-769. CltEN, Z., LIU, Y., HODGES, K. V., BURCHFIEL, B. C., ROYDEN, L. H. & DENG, C. 1990. The Kangmar dome: A metamorphic core complex in southern Xizang (Tibet). Science, 250, 1552-1556. CHINNER, G. A. 1966. The distribution of pressure and temperature during Dalradian metamorphism. Quarterly Journal of the Geological Society of London, 122, 159-186. CtlOPIN, C. 1984. Coesite and pure pyrope in high-grade blueschists of the Western Alps: A first record and some consequences. Contributions to Mineralogy and Petrology, 86, 107-118. , 1987. Very high-pressure metamorphism in the Western Alps: Implications for subduction of continental crust. Philosophical Transactions of the Royal Society, London, A321, 183-197. CHRISTENSEN, J. N., ROSENEELD, J. L. & DE PAOLO, O. J. 1989. Rates of tectonometamorphic processes from rubidium and strontium isotopes in garnet. Science, 244, 1465-1469. COGN~, J. 1960. Schistes crystallins et granites en Bretagne mridionale. Le domaine de I'anticlinal de Cornouaille. M6moires pour servir l'explication de la Carte G6ologique d6taill6e de la France• COLLINS, W. J. & VERNON, R. H. 1991. Orogeny associated with anticlockwise P - T - t paths: Evidence from low-P, high-T metamorphic terranes in the Arunta inlier, central Australia• Geology, 19, 835-838. DALLMEYER, R. D. & BROWN, M. 1992. Rapid Variscan (c. 300 MR) exhumation of Eo-Variscan (c. 400 MR) metamorphic rocks from South 40 39 Brittany, France: New A r / Ar age data and tectonic implications•

I acknowledge the contribution made to my metamorphic education by all the participants in IGCP Project 235 (1984-1990). Rapid, critical and constructive reviews that substantially improved this article were provided by T.R. Armstrong, G.T.R. Droop, E.J. Krogstad, E.L. McLellan, K. Mezger, P.J. O'Brien, J.C. Schumacher, R.J. Tracy and two anonymous reviewers. I thank K. Mezger for the provision of Fig. 5 and R.J. Tracy for the provision of Fig. 2, and J. Martin for proficient word processing; however, I take responsibility for those misperceptions and infelicities that remain.

References ALBAREDE, F. 1976. Thermal models of post-tectonic decomprcssion as cxcmplified by the Haut-Allier granulites (Massif Ccntral, France)• Bulletin de la Soci~t( Gdologique de France, 18, 1023-1032. ALLEN, T. & CIIAMBERLAIN,C. P. 1989. Thermal consequences of mantled gneiss dome emplacement. Earth and Planetary Science Letters, 93, 392-404. ANDERSON, M. W., BARKER, A. J., BENNETT, D. G. & DALLMEYER, R. D. 1992. A tectonic model for Scandian terrane accretion in the northern Scandinavian Caledonides. Journal of the Geological Society, London, 149, 727-741. ANOWTZ, L. M. & CHASE, C. G. 1990. Implications of post-thrusting extension and underplating for P - T - t paths in granulite terrancs: A Grenvillc cxamplc. Geology, 18, 466-469. ARMSTRONG, T. R., TRACY, R. J. & HAMES, W. E• 1992. Contrasting styles of Taconian, Eastern Acadian and Western Acadian metamorphism, Central and Western New England. Journal of Metamorphic Geology, 10, 415-426. ASHWORTII, J. R. & BROWN, M. 1990. High-temperature metamorphism and crustal anatexis. Unwin Hyman, London, UK. ATHERTON, M. P. 1977. Carncgie Review Article: The metamorphism of the Dalradian rocks of Scotland. Scottish Journal of Geology, 13, 331-370. BARROW, G. 1893. On an intrusion of muscovite-biotite gneiss in the south-eastern Highlands of Scotland, and its accompanying metamorphism. Quarterly Journal of the Geological Society of London, 49, 330-358. --, 1912. On the geology of lower Dee-side and the Southern Highland Border. Proceedings of the Geologists' Association, 23, 274-290. BARTON, M. D. & HANSON, R. B. 1989. Magmatism and the development of low-pressure metamorphic belts: Implications from the western United

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Geological Society of America Annual Meeting, Cincinnati, Ohio, Abstracts with Program, 24, A236. DALY, J. S., CLIFF, R. A. & YARDLEY, B• W. D. (eds) 1989. Evolution of Metamorphic Belts. Geological Society, London, Special Publication 43.

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DEMPSTER, Z. J. 1985. Uplift patterns and orogenic evolution in the Scottish Dalradian. Journal of the Geological Society, London, 142, 111-128. & HARTE, B. 1986. Polymetamorphism in the Dalradian of the central Scottish Highlands. Geological Magazine, 123, 95-104. DEWEY, J. F. 1988. Extensional collapse of orogens. Tectonics, 7, 1123-1139. DE SAINT BLANQUAT, M., LARDEAUX, J. M. & BRUNEL, M. 1990. Petrological -

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Received 22 October 1992; revised typescript accepted 11 November 1992

Addendum Our understanding of high-temperature-low-pressure metamorphism has been advanced recently by new work from the Ryoke Belt in Japan and the Chugach Metamorphic Complex in Alaska, USA. In the case of the Chugach Metamorphic Complex, observation of the relative timing of deformation, metamorphism and plutonism leads to a model of ridge subduction followed by plate reorganization to account for the abnormally high geothermal gradients in the forearc at the subduction zone separating North America from the Pacific Ocean Basin during the Eocene (Sisson & Pavlis 1993). Perhaps more significant, is the recognition that the Ryoke Belt in Japan also might be a consequence of ridge subduction (Nakajima 1994). The metamorphism of ~he Ryoke Belt has been considered typical of Miyashiro's (1961) low-pressure facies series (andalusitesillimanite type). It is the type example of the high-temperature component of a paired metamorphic belt, with the Sanbagawa Belt to the oceanward side being the high-pressure m e m b e r of the pair. The main part of the Ryoke Belt extends for a length of c. 1000 km, but has a width of only 30-50 km. Metamorphic rocks occupy about one-third of the total area of the Ryoke Belt, because of the large amount of granitic rocks that also occur and which are characteristic of this belt. Higher grade metamorphic zones within the Ryoke Belt exhibit evidence for both fluid-conserving melt-producing reactions and fluid-absent-melting reactions, in particular reactions that involve biotite with aluminosilicate±quartz. A t the highest metamorphic grade exposed, biotite-K-feldspar-cordierite-garnetbearing assemblages are characteristic in rocks with a migmatitic layering. A n upper limit on temperature is provided by the absence of hypersthene-bearing assemblages, which indicates that the stability of biotite + quartz was not exceeded at the crustal level now exposed. Peak metamorphic conditions in the highest grade zones of the Ryoke Belt metamorphism likely correspond to c. 4 kbar and c. 750 °C (Brown & Nakajima 1994). The sequence of mineral assemblages developed in pelites that cover a range in Mg/(Mg + Fe) suggests that the prograde P - T path may be close to isobaric, at least in the higher grade zones, and that P may not vary significantly along the belt (Brown & Nakajima 1994). A t present, there are insufficient data to assess the exhumation P - T path, but the fine-grained nature of the rocks suggests rapid cooling, and

some replacement of garnet by biotite may indicate that the retrograde P - T path also may be close to isobaric. K-Ar and Rb-Sr ages of Ryoke granitoids and metamorphic rocks indicate that the metamorphism is older in the west and younger in the east, following the same systematic eastward younging identified in the San-yo granitoids immediately to the north of the Ryoke Belt (Nakajima et al. 1990). The San-yo and Ryoke granitoids and the Ryoke metamorphic rocks were formed during approximately the same interval of time, and the magmatism and metamorphism shifted eastward between 105 Ma to 65 Ma from Southwestern to Central Japan. Cooling rates are high at 40-80 °C Ma -1. The along-arc age variation is incompatible with a tectonic model based on steady-state subduction, and the metamorphic and granitic rocks are interpreted to have formed by subduction of a single ridge segment that migrated along the Eurasian trench margin with time. Observations consistent with this interpretation include: the narrowness of the belt and the diachronous nature of the metamorphism; the approximately isobaric prograde P - T path; the fine-grained nature of even the highest grade metamorphic rocks; high temperatures at middle crustal depths; and the rapid cooling rates. This model requires the juxtaposition of the Sanbagawa Metamorphic Belt against the Ryoke Metamorphic Belt to be a younger event as a consequence of sinistral strike-slip displacement on the Median Tectonic Line, which raises questions about the usefulness of the concept of 'paired' metamorphic belts (Brown & Nakajima 1994).

Additional references BROWN, M. & NAKAJIMA, T. 1994. High-T-low-P metamorphism in the Ryoke Belt of Japan: consequences of ridge subduction. Geological Society of America, 1994 Annual Meeting, Abstracts with Programs, 26, 7, A-214. NAKAJIMA,T. 1994. The Ryoke plutono-metamorphic belt: Crustal section of the Cretaceous Eurasian continental margin. Lithos, in press. - - , SHIRAHASE,T. • SHIBATA,K. 1990. Along-arc variation of Rb-Sr ages of Cretaceous granitic rocks in southwest Japan. Contributions to Mineralogy and Petrology, 104, 381-389. SISSON, V.B. & PAVL1S, T.U 1993. Geologic consequences of plate reorganization: An example from the Eocene southern Alaska fore arc. Geology, 21, 913-916.

Added November 1994.

From QJGS,47, 330, 343. 29. On an IN~RUSI01~ of MUSC0VITE-BIOTITE GlWglSS ~n the SOUTHEA.STERlffHIOHLA'NDSof SCOTLAND,and it$ ACCO~tPAI~IINO METAmORPHISm. By G~OSa~ BARROW,Esq., :F.G.S. (Communicated by permission of the Director-General of the Geological Survey. Read March 22nd, 1893.) [PLATES X~r. & XVI.] CONTENTS.

I. II. III. IV. V.

VI. VII. ¥III. IX.

Introduction ..................................................................... Distribution and Mode of Occurrence of the Igneous Rocks ......... Petrological Characters oF the Igneous Rocks ........................... Minerals of the Metamorphic Rocks ....................................... Rocks of the Metamorphic Area ............................................. (a) The Sillimanite-zone. (b) The Cyanite-zone. (c) The Staurolite-zonc. Sedimentary Origin of the Metamorphic Rocks ........................ Evidence of Progressive Metamorphism .................................... General Conclusions, and Summary of Results ........................... Analyses of the Rocks .. .......................................................

Page 330 330 339. 337 343

351 352 352 354

L INTRODUCTION.

Ta~ area to which attention is directed in the following pages lies in the north-eastern corner of Forfarshire, and forms part of the singularly flat table-land of the South-eastern Highlands. I t is essentially a moorland district, much covered with peat and heather, and is drained by two rivers, the North Esk and the South Esk. The rocks of which the area is composed consist principally of gneisses and schists ; these are clearly seen in the craggy sides of the valleys through which the two Esks and their tributaries flow. Boulders of these rocks may be noticed in the rough walls by the roadside as one drives up the gleus, and their intensely crystalline aspect is a most striking feature. A brief visit to the crags and the flat-topped moorland speedily convinces the observer t h a t this crystalline aspect is one of the chief characteristics of the district. I t is proposed to show in the present communication that this area contains several masses of intrusive rock which are probably conneeted underground, and that the highly crystalline character of the surrounding schists is mainly the result of thermometamorphism.

V. I~0CKS OF TIIE ~ETAMORPHIC 2~R:EAo

A brief description may ~now be given of the principal types of rock in which the minerals above described occur. They may be divided into four groups: firstly,, those of the silhmanite-zone ; secondly, those of the cyanite-zone ; thirdly, those of the staurolitezone : and lastly, those lying between the third zone and the Great Highland Fault, as seen on the banks of the North :Esk.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 83-90 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 21-28

The development of Early Palaeozoic global stratigraphy W. S. M c K E R R O W Department o f Earth Sciences, Parks Road, Oxford OX1 3PR, UK Abstract: The major steps in the development of Early Palaeozoic stratigraphy are examined, with special emphasis on early Journal papers by Murchison and Sedgwick, and on their conception of systems and series, which permitted long-distance correlation. Unlike other periods, the Ordovician and Silurian were originally split into series; most stages have only been defined in the past 60 years. From 1880, Lapworth's graptolite zones have allowed much greater chronological precision. More recently, other methods have been developed for recognizing small time divisions, including studies in gradational evolution. A significant new advance is the correlation (by biostratigraphy) of short-lived physical events such as magnetic reversals and sea-level and climatic changes.

Sedgwick (1845), in the first paper in Volume 1 of the Journal, presented transverse sections showing the stratigraphic and structural relations of Lower Palaeozoic rocks in North Wales. He acknowledged, on page 6, the palaeontological help of J. C. Sowerby and his assistant, J. W. Salter, but, as we shall see, fossils were scarce in many of these formations, and Sedgwick was clearly more interested in unravelling the structure. In contrast, Murchison applied the methods of William Smith by describing (also with the expert help of Sowerby and Salter) the fossils present in each formation of his Silurian System. By 1845, Murchison had traced the Silurian across much of northern Europe, and was collaborating with several European geologists (e.g. Murchison & Verneuil 1845) to define younger parts of the Palaeozoic. While many of the objectives in modern stratigraphy are similar to those of Murchison and Sedgwick, we have now more techniques available. Just as important, we also have a different approach to biostratigraphy, mainly because we know more about how the Earth works and how animals live than did the geologists of 150 years ago. After a brief discussion of developments prior to 1845, this review gives an account of the evolution of Early Palaeozoic stratigraphy, followed by a more personal view of where biostratigraphy is heading. The principal steps in the development of stratigraphy are set out in a logical sequence which means that they are not all presented in chronological order.

and sections showing beds from the Coal Measures up to the Muschelkalk of Thuringia (Dunbar & Rodgers 1958, p. 290; Geikie 1897, p. 100). The much more celebrated A. G. Werner, working in the same region of Germany as Lehmann and Ffichsel, is famous for his all-embracing doctrine on the origin of rocks; he thought that they were mostly precipitates from sea water: the Neptunist theory. But Werner also proposed, in 1787, five broad stratigraphical units (based, in part, on the work of his associates, in the regions of Erzgebirge, Saxony and Bohemia) which he claimed were world-wide in scope (Geikie 1897, pp. 102-115; Rupke 1983, pp. 112-14). Werner's stratigraphical units were: 5. 4. 3. 2. 1.

Volcanics Unconsolidated deposits Floetz [now classed as Permian to Tertiary] Transitional strata, including greywacke Primitive strata, including granite, gneiss, etc.

Werner is more widely known than Lehmann or Ffichsel. The new theory (together with the ability to give a good lecture and have notable students) provided Werner with more publicity, even although he did not publish very much. Thanks to Hutton and others, Werner's ideas on Neptunism were later discounted, but he retains a position in the development of stratigraphy. In 1831, when Murchison and Sedgwick started field work in Wales and Shropshire, their aim was to examine the beds below the Old Red Sandstone, which were still known by Werner's term: 'Transition rocks or greywacke' (Fig. 1).

Step 1: defining a local succession of formations and mapping them in the field Stratigraphy did not begin with William Smith. In 1751, the French geologist, J. E. Guettard, appears to have created the first geological map (Geikie 1897, pp. 21-2), when he portrayed the geographical distribution of rocks (with fossil localities) in France and western Europe. But Guettard gave no indication that he had any ideas on the chronological or structural relations of the strata he mapped (Geikie 1897, p. 96). A few years later (in 1756), the German geologist J. G. Lehmann, working in the Harz and the Erzgebirge mountains, was the first to publish sections showing the sequence and structure of rocks (Geikie 1897, p. 96-7). Then, in 1762, G. C. Ffichsel, a contemporary of Lehmann, published both maps

Step 2: recognizing characteristic fossils in each formation The British must share the credit with other Europeans for recognizing the importance of fossils to stratigraphy. In the seventeenth century, the Danish physician and mineralogist, Nicholaus Steno recognized that different strata had different fossils. Subsequently, in 1703, the British scientist Robert Hooke clearly envisaged the potential of fossil shells to provide the 'criteria of chronology' in sedimentary rocks, but he does not appear to have put his suggestion into practice (Dott & Batten 1988, pp. 19-24). By 1799, William Smith had drawn up a table of strata in 83

84

W . S . MCKERROW

WERNER 1787

SEDGWICK 1855

MURCHISON 1859

LYELL 1865

LAPWORTH 1879

1993 Pridoli

Upper Upper Silurian

Ludlow

Silurian

Silurian

Wenlock Upper Llandovery

Middle

Transition

t"

Silurian

~

.....

Silurian

Lower Llandovery

Upper strata

Ashgill Cambrian Lower

including

Caradoc 0

Lower Silurian

greywacke

Silurian

Llandeilo Ordovician Llanvim

Middle Arenig Cambrian

O < 0" > Z

Tremadoc Upper Cambrian Cambrian

Cambrian

0

Middle Cambrian

~

Lower Cambrian

~

Lower Cambrian Cambrian Longmynd

Fig. I. Some classifications of the Lower Palaeozoic (modified from Secord 1986, fig. 9.4). England (from the Coal Measures to the Chalk) in which he listed some characteristic fossils in each formation, but this was not published until 1813 (Townsend 1813; Geikie 1897, pp. 230-1). At the same time, following the lead of GiraudSoulavie (Geikie 1897, pp. 204-8), Cuvier & Brongniart (1808) were compiling faunal successions in Tertiary beds in the Paris Basin, although they were at first more interested in the history of life than in the correlation of strata (Hancock 1977, p. 6). Subsequently, Smith (1816-1819) showed how the use of fossils permitted formations to be mapped across much of England. Smith may never have heard of Guettard, Lehmann or F(ichsel, and he had little comment to make on the theories of Werner and Hutton. In 1817 Smith stated: 'My observations ... are entirely original, and unencumbered by theories, for I have none to support.' (Hancock 1977, p. 4). The only theory essential to construct a geological map is the belief that formations are entities. Smith certainly had most influence on subsequent developments in biostratigraphy. Even as late as 1822, Cuvier and Brongniart were still attempting to fit the French sequences into Werner's scheme, but when confronted by [Cretaceous] beds in Poland, with different lithologies but the same fossils

as in France, Brongniart started to argue in favour of the prime position of fossils as a means of correlation (Hancock 1977, p. 7). These new observations also brought new theories. While Lyell preached uniformitarianism, Cuvier thought that the faunal changes which occurred in his successions were the results of a series of extinction events (Hallam 1989, pp. 3740). The catastrophic theories of Cuvier show some signs of revival in recent years but, as Smith observed, theories are not essential for the application of fossils to correlation.

Step 3: facies and type localities One potentially confusing issue in biostratigraphy is the presence of different facies (with different fossils) occurring in rocks of the same age. The problem of facies was recognized early on by Brongniart, Fitton and Phillips and discussed in detail by Gressly (1838), who coined the term. A year after Gressly's paper was published, De la Beche had no great difficulty in regarding the Old Red Sandstone as a local facies of the marine Devonian (Rudwick 1985, pp. 267-8). In 1842, after his visit to New York, Lyell recorded Old Red Sandstone fish and marine

GLOBAL S T R A T I G R A P H Y Devonian rocks sandwiched between the Silurian and Carboniferous (Rudwick 1985, p. 381). Because of facies changes, the beds represented in different regions had different aspects, although the stratigraphical sequences in which they occurred often contained enough fossils for their approximate age to be determined. This was usually done by reference to previously described sequences. The concept of type localities (invented by d'Orbigny) has thus proved a useful tool when correlating over large distances (Hancock 1977, p. 11). Step 4: systems, periods and eras In the 50 years after the Geological Society was founded (in 1807), several of its fellows played a crucial role in the development of global stratigraphy. The long-distance correlation of formations was closely linked with their reclassification into larger groups. At first, the word 'system' had a variety of meanings, but when Murchison (1835, 1839) defined the Silurian System both by its rock sequences and its fossils, it soon became recognizable across Europe and in America, and the term 'system' gradually assumed its modern meaning (Bassett 1991, pp. 16-20; Rudwick 1985, p. 446). The use of systems (with the present definition) was pioneered by the British, who based most of them on British strata. The Carboniferous System was established by Conybeare & Phillips (1822) to include the Old Red Sandstone; its limits only developed later, when the Devonian and Permian systems had been defined. Most systems (though often with rather imprecise boundaries) were established within the following 20 years (Rupke 1983, pp. 128-9); these included Lyell's (1833) subdivision of the Tertiary, and the establishment of the Cambrian by Sedgwick in 1835 (published a year later in: Sedgwick & Murchison 1836), of the Silurian and the Permian by Murchison (1835, 1841b), and of the Devonian by Sedgwick & Murchison (1839). When established, most of these systems were known to have characteristic fossil assemblages, but at first the Cambrian was hard to recognize outside Wales because it lacked any well-documented diagnostic fossils. In the first volume of the Journal, Murchison & Verneuil (1845) emphasized how the Silurian, Devonian and Carboniferous each had distinct organic remains in the same superposition across much of northern Europe. They also showed how the Permian System could be defined with reference to known sequences in Germany and Russia, a stratigraphical method which most of us would applaud. Murchison & Verneuil commented on the similarities between the fauna and flora of the Permian and the Carboniferous, even though a marked unconformity was present at this level in many areas of Europe (now termed the Hercynian Orogeny). They (Murchison & Verneuil 1845, p. 82) also noted that 'The Triassic system does not contain a single Palaeozoic form, whether animal or vegetable'; there was a very marked change in the fossils above the Permian, even though strong stratigraphic breaks at this level were not common. By 1845, Murchison & Verneuil had recognized that unconformities are much less useful than faunal changes for international correlation; this elementary principle of global stratigraphy has taken over a century to be widely applied. Much earlier, in 1838, Sedgwick chose the term 'Palaeozoic' to denote the Cambrian and Silurian jointly (Rudwick 1985, p. 242). Later, Phillips (1840, 1841) redefined the term Palaeozoic and suggested the corresponding terms: 'Mesozoic'

85

and 'Kainozoic' [subsequently 'Cainozoic' and now 'Cenozoic'] (Harland et al. 1989, pp. 30-1; Rudwick 1985, p. 363). Phillips' new eras had apparently made Murchison and Verneuil look more closely at the faunal changes across the Permian/Triassic boundary. In the first paper published in the Journal, Sedgwick (1845) gave an account of the rocks of North Wales; this was a continuation of an earlier paper published in the Proceedings of the Geological Society. At this time, Sedgwick had temporarily changed his mind about the use of the term 'Cambrian' and used 'Protozoic' instead; this may have been related to the discovery, by the Geological Survey, of Llandeilo and Caradoc fossils within the supposed 'Cambrian' of Wales (Hallam 1989, pp. 78-9). The boundary between the Cambrian and the Silurian was first agreed (in 1834) by Sedgwick and Murchison to run through unmapped territory in Wales (Secord 1986, fig. 3.9; Bassett 1991, pp. 8, 15-16), but by 1845, Murchison, Sedgwick and the Geological Survey all agreed that similar fossils were present on both sides of this boundary. By the 1850s, Murchison's solution (Fig. 1) was to extend the Silurian downwards to include all the fossiliferous rocks from the Lingula Flags (now classed as Upper Cambrian) to the Ludlow, whereas Sedgwick claimed everything up to and including the May Hill Sandstone (recognized today as Upper Llandovery) as Cambrian (Hallam 1989, p. 82). These arguments on the Cambrian-Silurian boundary are well known, but there was a more significant difference between Sedgwick and Murchison: while Sedgwick continued to stress the importance of physical stratigraphy and structure, Murchison realized (and put into practice) his belief that fossils were the best method of long-distance correlation (Secord 1986; Hallam 1989, p. 83-4). Sedgwick (1852), writing on the rocks of southwest England, was still maintaining 'that no good classification either of subdivisions or systems, or of subordinate formation, can ever be attempted without a previous determination of the physical groups'. This paper was written after the 'Great Devonian Controversy', recounted by Rudwick (1985) was over, but at a time when much stratigraphical uncertainty still prevailed about the ages of many rocks in Devon and Cornwall. Nevertheless, Sedgwick eventually put some of Murchison's precepts into practice by recording every fossil available to him; in addition, Sedgwick recruited M'Coy as his palaeontological assistant. While both Sedgwick and Murchison were presumably moderately competent palaeontologists, they both used specialists whenever possible. Rudwick (1985, p. 444) points out that the 'clinching evidence' for the resolution of the Devonian controversy was 'not only the result of Murchison's competent but quite conventional field work in Devon, the Rhineland and Russia, but also the product of fossil specialists such as the impoverished Lonsdale and the less than gentlemanly Sowerby and Phillips ... more or less professional palaeontologists'. Murchison's 1845 geological map of eastern Europe and Russia was based on long-distance correlation by fossils (Johnson 1982). Palaeontology was also important in Murchison's subsequent travels, for example, his visits to Scandinavia and Russia (Murchison 1847), and to Germany and Bohemia in 1853 (Bassett 1991, p. 38) while he was compiling the first edition of Siluria (Murchison 1854). By the 1850s, the Silurian System was becoming global in extent. In the United States, James Hall had compiled a list of Silurian fossils from New York (Murchison 1841a), and D. D.

86

W . S . MCKERROW

Owen (1846) had mapped and described the Silurian System of the mid-west (Johnson 1977); and shortly after, Sharpe (1848) had listed shelly fossils from each formation in New York and compared them with equivalents in Britain. Charles Darwin (1846) reported on fossils from the Falkland Islands resembling Silurian forms (they are actually Devonian), and Strachey (1851) recognized Silurian fossils in the Himalayas. Murchison has a claim to be the first global geologist; his map (Murchison 1854, p. 475, reproduced by Bassett 1991, p. 41) of the geographical distribution of 'Palaeozoic formations' around the world was the first of its kind. Murchison's renown, though deserving of high recognition, was gained through numerous publications which did not always give due credit to others (Torrens 1990); he appears to have been following a contemporary custom (Flinn 1992). Murchison originally grouped together what we now call Caradoc and Llandovery beds. In 1852, M'Coy recognized that Murchison's 'Caradoc beds' contained two distinct faunas, so that what we now term 'Ordovician' and 'Silurian' could be distinguished palaeontologically (Hallam 1989, pp. 80-1). M'Coy and Salter also described fossils from the Cambrian (in its modern sense) of Britain. At about the same time, Barrande (1852-1911) distinguished three successive Early Palaeozoic faunas in Bohemia. Thus, by the mid-1850s, although not designated by any formal nomenclature, the importance of a tripartite division of the Early Palaeozoic was becoming recognized internationally (Hallam 1989, p. 83) and the boundaries (Fig. 1) were incorporated in the literature by such authorities as Lyell (1865). The Ordovician System was originally defined by Lapworth (1879) as extending from the base of the Arenig Series to the base of the Llandovery Series. Although these series were quite well defined by their faunas, Lapworth appears to have been influenced by the presence of two unconformities in Wales and Shropshire: one below the Arenig and another below the Llandovery, both reflecting geographically restricted tectonic events (Woodcock 1990; Toghill 1992). Subsequent international decisions have now classified the Tremadoc Series with the Ordovician on the basis of internationally recognizable faunal changes at its base (Norford 1991 and references therein). The very fact that system boundaries are the subject of so much discussion, illustrates their subjective nature. Modern work on the definitions of systems is now concerned with obtaining international agreement on precise definitions of the base of each system, using type sections where good zonal fossils are present. McLaren (1977) showed, for the Devonian, just how it should be done; since then, the base of the Silurian (Cocks 1988) and the Carboniferous (Paproth et al. 1991) have been agreed; the base of the Ordovician is close to settlement (Norford 1991), while work is still in progress on defining the base of the Cambrian (Cowie & Brasier 1989). The International Geological Congress at Bologna in 1880 recognized the distinction between stratigraphical and chronological divisions: the duration of a system was recognized as a period (Hancock 1977, p. 15). This reflected some new thinking about different types of stratigraphical units, which is still not universally agreed (see also Dunbar & Rodgers 1957, pp. 290-2).

Caradoc sandstone' (p. 13), but in the same volume Murchison & Verneuil (1845, p. 81) refer to 'the whole Palaeozoic series'. In these earlier papers, the term 'series' was used primarily in a lithological sense. Two years later, Murchison (1847, p. 2) compared some Swedish beds and their faunas with the 'limestones of Wenlock' and others with the 'Ludlow formation', but his only time-stratigraphical terminology comprised 'Lower Silurian', 'Upper Silurian' and 'the Old red (Devonian) system'. Eventually, the major divisions of the 'Cambrian' and 'Silurian' systems came to be termed 'series', while the major divisions of the Devonian and younger systems (or more accurately: periods) were called 'stages'. It was not until 1859, seven years after Sedgwick and M'Coy had distinguished the Caradoc and May Hill fossils, that Murchison (by then Director of the Geological Survey) proposed the term Llandovery as a series (Hallam 1989, p. 81; Bassett 1991, p. 38), as opposed to 'Llandovery rocks', the phrase he had earlier employed in The Silurian System (Murchison 1839). This proposal was also prompted by the recognition of a postCaradoc unconformity in Shropshire by Aveline and Salter (Bassett 1991, p. 31). Local unconformities like this Shelveian event (Toghill 1992), rather than faunal changes, continued to play intrusive roles in the definition of many series. Apart from in Norway (see below), smaller divisions than series were not, however, discerned during the first three-quarters of the nineteenth century, so, until Lapworth (1879-80) published a list of graptolite zones, the series (which were based primarily on shelly fossils) formed the main pillar of international correlation in the Early Palaeozoic. Because many benthic animals could not cross wide oceans, independent Ordovician series have been defined in North America and elsewhere using native shelf faunas, and it is only in recent years that these have been correlated, with moderate precision, with the British series (Ross et al. 1982; Fortey et al. 1991, fig. 8). The term 'stage' was first employed in stratigraphy for ammonite-rich sequences in the Mesozoic by d'Orbigny (184251) who defined it 'solely according to the identity in the composition of the faunas' (see Hancock 1977, p. 9). Stages were thus originally, as subsequently they always have remained, chronological terms, while the Early Palaeozoic 'series' started life as formations, and then with the growth ofbiostratigraphy, subsequently developed into time-rock units. Many think they still have a role as such, but this is not universally accepted (see Harland et al. 1989, p. 21). The work of Norwegian geologists allowed the immediate recognition of Murchison's Silurian series in the Oslo area (Johnson 1982); of these, Kjerulf (1857) is the most remarkable: he proposed a sequence of stages for the local Silurian, which has only been superseded in the past 15 years. During the last 60 years, following the lead of Bancroft (1933, 1945), the original series (some would now call them epochs) of Murchison and Sedgwick are today divisible into numerous stages, both in Britain and abroad. These are being redefined by relation to type localities, so that, even in the Ordovician, when many indigenous faunas prevailed, international correlations are becoming ever more precise (Williams 1969; Barnes & Williams 1991; Holland & Bassett 1989).

Step 5: series and stages

Step 6: graptolite zones

In Early Palaeozoic stratigraphy, the use of the term 'series' began haphazardly. Sedgwick (1845), in the first volume of the Journal, refers to the 'Bala series' (p. 11) and the 'series of

Oppel (1856-8) showed that the Jurassic Period could be divided into 33 zones based on pelagic ammonites. He pointed out that the recognition of zones 'involves exploring the verti-

GLOBAL S T R A T I G R A P H Y cal range of each separate species in the most diverse localities, while ignoring the lithological development of the beds' (Hancock 1977, p. 12). The application of Oppel's conception of zones to the Early Palaeozoic had to wait until Hall and Lapworth examined the pelagic graptolites. Geologists working on the Palaeozoic owe at least as great a debt to Lapworth as they do to Murchison and Sedgwick. Lapworth (1873) produced a series of papers on the palaeontology of graptolites. He then showed that many of his newly defined species had very short time ranges (Lapworth 1879-80). Subsequently, and most significantly, Lapworth (1878, 1889) applied his new graptolite zones to unravelling the stratigraphy and the structure of southern Scotland. The introduction of Lapworth's graptolite zones raised several problems in Early Palaeozoic stratigraphy. At first the zones were based almost entirely on sequences in the Southern Uplands, where brachiopods and trilobites are absent or rare; there was therefore great uncertainty in correlating the zones with the previously established series based on shelly fossils. Later, Lapworth (1889) described graptolites occurring with trilobites and brachiopods in southwest Scotland, where the pre-Ashgill benthic faunas are largely different (because of the wide Iapetus Ocean) from those in England and Wales where the series were defined. New correlations are still being proposed between the graptolite zones and the series and stages (e.g. Fortey et al. 1991; Holland & Bassett 1989), and studies in graptolite evolution are now allowing even finer chronological divisions to be discerned (Rickards 1989).

87

gradational changes occur only in a minority (?< 10%) of benthic lineages. A sudden appearance, followed by stasis, is the norm for many genera of trilobites and brachiopods, so most benthic taxa can never be expected to be used as fine time indicators. Numerous alternatives to the original graptolite zonal scheme are now available (see Cowie & Brasier 1989; Barnes & Williams 1991; Webby & Laurie 1991; Holland & Bassett 1989; Bassett et al. 1991). In North America and Australia some endemic taxa are employed as indices for graptolite zones. The development of micropalaeontology permits correlation of many sections previously considered to be barren. Conodonts are now almost as useful as graptolites (more so in some facies) and there is lively discussion (e.g. Barnes 1988; Norford 1991) on the relative merits of the two groups in biostratigraphy. Acritarchs (e.g. Fig. 2), chitinozoans and ostracodes also provide useful stratigraphical indices. In the Early Cambrian, various small shelly fossils are proving accurate guides to correlation, while in the Silurian vertebrates and plant spores can also be employed. These different schemes result in the duplication of zonal indices for the same time interval, but this should not lead to arguments about usage or priority. All correlation schemes must (eventually) be related to internationally agreed series and stages. Most of the finer subdivisions should, perhaps, remain informal, to be emended as our knowledge develops (e.g. Fig. 2).

Step 8: event stratigraphy Step 7: other palaeontological criteria for zonation Biostratigraphy, in the long run, must rely on evolutionary changes in animals and plants, but these do not always occur progressively. Moreover, many changes in fossil sequences are not related to evolution: they can be due to local or regional changes in facies or, in some cases, to migrations. Ziegler (1965) showed that similar Silurian benthic communities occur in similar environments at different times, and thus that benthic assemblages cannot be used p e r se to indicate fine time intervals. Subsequently, it has been shown (e.g. Ziegler et al. 1968; Cocks 1989) that it is only in lineages with gradational evolutionary changes, and which have been studied by quantitative methods, that benthos can be used as precise indicators of small time divisions (Fig. 2). The total range of a species can then be determined by recording the ancestors in the beds below and the descendants in the beds above. As early as 1945, Bancroft showed that enough changes occurred in some Late Ordovician brachiopods and trilobites for the subdivision of the series into stages (see appraisal in Williams 1969, p. 245). Williams (1948) and Sheldon (1987) have also described gradational evolution in a few families of Ordovician trilobites. In the Silurian, gradational evolution has been described in S t r i c k l a n d i a which enabled successive taxa of these brachiopods to be employed as time indicators within the Llandovery Series (Williams 1951; Baarli 1986); gradational evolution is also known in E o c o e l i a (Ziegler 1966). At present, using macrofossils, it is difficult to give precise ages to Llandovery shelly facies without reference to members of one of these two brachiopod lineages (Fig. 2). We now realise that, at least with shelf benthos, a few wellstudied lineages showing gradational evolution are the best basis on which to create fine time divisions. But appreciable

Some Early Palaeozoic eustatic changes in sea level are related to the widespread development of black shales (Leggett et al. 1981) and also to more local occurrences of red shales (Ziegler & McKerrow 1975). More significantly, lowering of sea-level can be correlated with some extinctions (e.g. Leggett et al. 1981; Fortey 1984; Johnson & McKerrow 1991; Nielsen 1992), especially in the pelagic realm. During the Early Palaeozoic, the only large sea-level change related to major extinctions in the shelly faunas was in the late Ashgill (Brenchley & Newell 1984; Brenchley 1984; Owen et al. 1991). Several sea-level changes can also be correlated with increased intercontinental migrations of benthic faunas (Scotese & McKerrow 1990; Sheehan & Coorough 1990). Some environmental changes are indicated by changing ratios in stable isotopes in beds relatively unaffected by diagenesis or metamorphism (e.g. Corfield et al. 1992; Kirschvink et al. 1991). We are still at the stage of trying to distinguish which sealevel changes are globally synchronous (e.g. Johnson et al. 1991). The methods suggested by Vail et al. (1977) and Haq et al. (1988) may eventually allow recognition of Early Palaeozoic stratigraphic sequences bounded by synchronous stratigraphical breaks on cratons, but in orogenic regions many unconformities are only regional in extent and some are diachronous (McKerrow et al. 1991; Toghill 1992). For example, Woodcock (1990) has shown clearly that, in the Welsh Basin, the three big stratigraphical divisions recognized by Lapworth (Woodcock rightly calls them 'supergroups' rather than 'systems') are bounded by regional unconformities. By contrast, on the cratons of North America (Ross & Ross 1992), the Russian Platform (Kaljo & Nestor 1990), the Andean Platform (Baldis et al. 1992), the Australian Platform (Nicoll et al. 1992) and the Yangtze Platform (Johnson et al. 1985), many stratigraphic sequences have synchronous boundaries and are likely to be related to eustatic sea-level changes.

88

W. S. M C K E R R O W

Llandovery area litho stratigraphy

Eocoelia evolution

Stricklandiid evolution (Williams1951 emended)

Graptolite zones

(Ziegler 1966

centrifugus

N

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~ c "= o O

emended)

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4,

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lira t a

crenulata

sulcata

4, Cerig Formation

griestoniensis

4 curtisi Stricklandia

crispus

la e vls

ii

turriculatus ,

I~Normwood Fm

Rhydings Formation Z

-

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convolutus

z O nuJ <

argenteus

lens

progressa

o

intermedia

I

S t r ! c k l a n d i a lens intermedia

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3b

~ hemisphaerica

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3a

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I

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~'magnus

r~

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Fm

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Formation

Z

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~Z <

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atavus

~la

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Formation

upernus

Magnetostratigraphy, now commonly employed as a stratigraphical tool in the Mesozoic and Cenozoic, is gradually being extended down into the Palaeozoic (e.g. Trench et al. 1991; Kirschvink et al. 1991). But magnetostratigraphy in the Early Palaeozoic has still a long way to progress. As in other types of event stratigraphy, we still need to employ conventional biostratigraphic data to correlate the events.

Conclusions: the aims and achievements of Early Palaeozoic stratigraphy The main aim of stratigraphy is to provide a chronology of Earth history. In the Early Palaeozoic, this has resulted in the development of different zonal schemes in different environments. Some of the finer zones may be as short as 300 000 years. To many geologists 150 years ago, the production of a geological map was the prime aim of stratigraphy. Biostratigraphy is still the basis for most regional geological maps, and a good

Fig. 2. The modern correlation of the Llandovery Series (from Cocks 1989, fig. 35). Stars indicate records from the type Llandovery area. The acritarch zones are from Hill (1974). Reproduced by permission of the Trustees of the Natural History Museum, London.

map is the first requisite for all types of mineral and petroleum exploration. Smith and Brongniart were able to produce geological maps based on their studies of the characteristic fossils of each formation, and it was to the credit of Murchison and his contemporaries that this method was extended to the Early Palaeozoic. Geophysics and deep borehole evidence now allow modern maps to show the distributions of formations at depth as well as on the surface. Modern biostratigraphy has seen advances in the determination of the stratigraphical age of physical events such as sealevel and climate changes, magnetic reversals and asteroid impacts, so that they can be developed for use in global correlation (see Whittaker et al. 1991). Many igneous and structural events can be dated by radioactive isotopes, but at present few boundaries of Early Palaeozoic series can be estimated to within 5 Ma (e.g. Snelling 1985). Some Early Palaeozoic global changes in sea level (e.g. in the late Ashgill) are associated with the spread of land ice and a

GLOBAL

STRATIGRAPHY

lowering of sea level; others are synchronous with orogenic events and there may be a causal connection (Johnson & McKerrow 1991, p. 164). Until we can determine the causes of such global changes, the full potential of sequence stratigraphy for intercontinental correlation is unlikely to be achieved. At present, plate movements appear to be more predictable than sea-level or climatic changes, but it is perhaps possible that, when the cycles of global changes in the past are better correlated and understood, geologists may even start looking forward as well as backward in time.

L. R. M. Cocks, R. A. Fortey, S. P. Hesselbo, M. E . J o h n s o n , Derek Siveter, E. A. Vincent and A. M. Ziegler kindly made suggestions which i m p r o v e d early drafts o f this paper.

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Received 3 August 1992: accepted 1 September 1992.

From QJGS, "1,5-6.

On the OLDER PAL~OZOIC (Protozoic) ROCKS OF NORTH WALES. By the Rev. A. SEDGWICK, M.A., F.R.S., V~%odwardian Professor of Geology and Fellow of Trinity College in the .University of Cambridge.

§ 1. Introduction. IN a paper read before the Geological Society in June, 1843, and intituled, " An Outline of the Geological Structure of North Wales,"* the author gave a description of those stratified rocks in the northern counties of the principality which are of anterior date to the mountain limestone. Those rocks he separated into the following three principal groups : - 1. Chlorite-slate and mica-slate. These form a band along the north-western side of the promontory of Carnarvonshire from Porth Dilleyn to Bardsea island. 2. Greywacke and roofing slate, often containing calcareous bands, and alternating with Plutonic rocks of cotemporaneous formation : and these rocks the author terms, in his present paper, the protozoic, group. They extend in an east and west direction, from the borders of Shropshire to the western coast of Carnarvonshire; and their north-western boundary, from the confines of Shropshire to Yspytty Evan, coincides nearly with the Holyhead road ; and from Yspytty Evan to Conway, with the Conway river. 3. An overlying and sometimes unconformable deposit of flagstone, &c., coterminous alo~lg the IIolyhead road and Conway river with the last-mentioned principal group; but bounded towards the north-west by an overlying range of mountain limestone.

From QJGS, 3, 1.

On t/,e Silurian and Associated Rocks in DALECARLIA,and on the Succession from Lower to Upper Silurian in SMOLAND, (~LAND, and GOTHLASD, and in SCANIA. By Sir RODERmK I. MuacHIsos, G.C.S., V.P.G.S. &c. PLAT~ I. HAWNa already communicated the additional knowledge I obtained last year (1845), concerning the drift and erratic blocks of Swedea*, l propose in. this memoir to give the results of the examination of those pal0eozoic and associated rocks in several parts of that country which fell under the joint observation of my friend M. de Verneuil and myself during the same period. Two of the districts under review have not been critically examined since they were described by Hi~inger; and his memoirs beiiJg in the Swedish language, with which few persons are familiar, and having been written before the paleeozoie clarification

From QJGS, 8, 1-2.

2. On t]/e SLATE ROCKS of DEVON and CORNWALL. By the Rev. A. SEDGWICK, F.R.S., G.S. &c. AFTER a painful interruption of three years, I resumed my geological work during the past summer, and revisited some portions of Devonshire and Cornwall, a small part of the typical Silurian country (of Sir R. I. Murchison), a part also of the Cambrian groups of North Wales, and lastly some groups of the newer fossiliferous slates of Westmoreland and Yorkshire. I rejoice to appear once more as a fellow-labourer, and to lay the first-fruits of my summer's task before the Geological Society. My present notice will be confined to Devonshire and Cornwall. It is well known to all who take any part in the working of our Society, that during the past year Sir R. I. Murehison, after an examination of certain fossils sent to him from Cornwall, has introduced some new colours into the geological maps of Cornwall and South Devon*. Thus, he colours the great headlands, between the Bays of St. Austell and Falmouth, Lower Silurian. Again, he colours a considerable part of the coast in the neighbourhood of Looe, &e., Upper Silurian; and the same colour is extended to a portion of the dates of South Devon which skirt the-north side of the metamorphic rocks of Bolt Head and Start Point. The first change of colour is grounded on good evidence; for in the great headlands S.W. of Austell Bay there is a development of rocks with a remarkable mineral structure, and with fossils which I should call Cambrian, and which Sir R. I. Murchison calls Lower Silurian. I f I mistake not, there is, however, rather too great an extension given to the new colour in this part of the map of Cornwall. As to the Upper Silurian colour, it was put in hypothetically, or on imperfect evidence; and I believe, that both from Devonshire and from Cornwall it must be expunged as erroneous.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 93-102 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 209-218

Charles Lapworth and the biostratigraphic paradigm RICHARD

A.

FORTEY

Department of Palaeontology, The Natural History Museum, Cromwell Road, London S W 7 5BD, UK

Abstract: Lapworth's paper on 'The Moffat Series' (1878) provided a model for deciphering the 'interminable greywackes' of the Southern Uplands, and one which lasted for a century. The same paper established graptolites in a dominant position in Lower Palaeozoic biostratigraphy. The changes in the biostratigraphic paradigm are discussed with reference to Lapworth's contribution; issues implicit in his 1878 paper are still contentious. Graptolites have exemplified the conflict that can arise between the use of fossils as stratigraphic ciphers, on the one hand, or as complex organisms to be interpreted biologically on the other. They have been subjected to the vicissitudes of stratigraphic fashion. It is shown that Lapworth's biostratigraphy has been enduring in contrast to his structural or palaeogeographic interpretations. However, the subsequent separation of litho- and chrono- from biostratigraphy, while conceptually necessary, has encouraged an idealistic pursuit of the perfect stratigraphic section for the purpose of defining stratigraphic boundaries. This has not always been constructive, not least because such boundaries often coincide with events which militate against the preservation of ideal sections. But Lapworth's close integration of biostratigraphic range with observations on lithology, 'barren beds' and fossil preservation may have a new lease of life in the context of event stratigraphy. 'The Moffat Series' was read by Charles Lapworth to the Geological Society on 21 November 1877, and published as pp. 240-343 of Volume 34 of the Quarterly Journal the following year. With this paper, the mysteries of the vast tract of 'interminable greywackes', the Southern Uplands of Scotland, seemed at last to yield to the scientific method. Here was an area of great structural complexity which had previously yielded only the most simplistic interpretations; furthermore, Lapworth 'solved' the problem by the application of a particular palaeontological method: the recognition of a new sequence of graptolite zones which could be used to trace out the complexities of structure with extraordinary reliability. The award of the Murchison Fund of the Geological Society of London to Lapworth hard on the heels of his paper shows that the significance of his work was quickly appreciated. One year later, in 1879, he was to publish his celebrated article in the Geological Magazine in which the concept of the Ordovician System was introduced, a concept nourished by his detailed work in southern Scotland. Lapworth's description remains a primary reference for those visiting Dob's Linn (this is the modern spelling of Dobb's Linn of the early accounts), Craigmichan Scaurs, or Muckra Burn. His acuity of observation was remarkable: while the interpretative context in which his structures have been placed has changed several times, the hard facts of his sections have almost all survived unchallenged. If we are celebrating the lasting influence of The Geological Society's Journal, Lapworth's (1878) paper is more than just a contender for longevity. It signalled a change in the understanding of a great area of our islands. It introduced a biostratigraphical approach to structural and historical interpretation. It confirmed the graptolites as a group of major importance in calibrating early Palaeozoic

time. Some of its conclusions still stand; others have been superseded. Why this should be tells us something about both the enduring and the ephemeral sides of geological enquiry. The issues raised in the Moffat Series are with us still, and are worth consideration now that the Journal publishes fewer biostratigraphic results than formerly.

Graptolites: fashion in biostratigraphic calibration 'Owing to the great rarity in the Moffat Series of fossils belonging to the well-understood families of the Brachiopoda and Crustacea, which are universally regarded as the most trustworthy exponents of the geological age of the containing beds, we are forced to rely almost exclusively upon such evidence as may be afforded by their Graptolithina' (p. 333). ' . . . e a c h species and variety of Graptolite &c has a definite range in the vertical succession of strata.' (p. 252). Lapworth may have been a trifle disingenuous in the first statement, because James Hall had already (1865) published a splendid monograph on the graptolites from Quebec, which had demonstrated a succession of species which were to prove of zonal utility. Lapworth himself had already published on graptolites in a stratigraphic context. However, he was correct in asserting the value of relying upon graptolites in the British 'Silurian'. The trilobites (Lapworth's 'Crustacea') had already been proved of worth by Murchison in his Silurian System (1839), and they had been subsequently monographed by John Salter (18641883). The brachiopods had been similarly treated by Davidson (1866-1883). The success of the zones proposed by Lapworth in unscrambling the structure of the central Southern Uplands served to shift attention to graptolites as a biostratigraphic key to problems yet unsolved. Most of 93

94

R.A.

LAPWORTH, 1878

maximus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ELLES & WOOD, 1913

turriculatus ~ . . . . . . . . . . . . . . . . . . . . . . . .

spinigerus

i i

maximus band

/ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

sedgwickii

(= sedgwickfi )

cometa

FORTEY

convolutus

i i

cometa band

CURRENT

turriculatus

convolutus

i argenteus magnus

gregarius

i itriangulatus Subzone fimbriatus Subzone

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cyphus 5:

vesiculosus

o

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modestus & vesiculosus

i

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acinaces atavus

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

acuminatus

acuminatus

acuminatus persculptus

anceps

anceps

extraordinarius i pacificus anceps i complexus

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Fig. 1. The persistence of Lapworth's biostratigraphic concepts (compare Figs 3 and 4), showing his original zonal scheme deduced in the Moffat Series (left), Elles & Wood's subsequent modification (centre), and modern usage (right). Note that despite the recognition of a few new zones and a change of name, Lapworth's concepts survive in some detail. However, he did list the 'Barren Beds' as a unit with time significance. Lapworth's zones are, with certain refinements, still in employment (Fig. 1); they have been a lasting contribution which seem likely to be permanent. The reasons for this are not hard to understand, at least in retrospect. The Moffat successions are almost entirely confacial, the product of sedimentation away from terrestrial perturbations, as Lapworth himself suggested. Furthermore, the succession of graptolitic bands is punctuated by barren intervals which tend to truncate vertical ranges of species; this accentuates the discreteness of zones, by giving comparatively neat concurrent ranges to species within a given zone. Graptolites then approached the ideal standard for biostratigraphic subdivision which had been set by the work of Oppel (1856) using ammonites in rocks of Jurassic age. In a word, they became fashionable. The acme of graptolite zones is probably the great monograph of British Graptolites by Elles & Wood (1901-1918); this work was edited by Charles Lapworth, according to the cover page. Thus it had his guidance and seal of approval. It provided the necessary documentary evidence for zones not merely for what was to become the later Ordovician and early Silurian, but for almost the whole of the both Systems. It was more than a match for the coarser correlations of Salter and Davidson. Moreover, Lapworth himself, in a running series of papers published in

the Annals and Magazine of Natural History had shown how graptolites were unsurpassed for international correlation of Lower Palaeozoic strata. If one adds to this that the Ordovician System itself was conceived by Lapworth as he began progressively to realize the importance of graptolite faunas in distinguishing major divisions in Lower Palaeozoic time, then there is a recipe for their unchallenged supremacy as biostratigraphical indicators; from Dob's Linn to the world at large. As if to confirm this, Rudolf Ruedemann (1904), working in the shale formations of the eastern United States, contrived a zonal system based on graptolites for much of the Ordovician which showed concordance with Lapworth's scheme. 'Gertie' Elles was to become the high priestess of the Lapworthian method. From the Sedgwick Museum, Cambridge, she performed an enormous number of determinations of graptolites, armed with the great book. Hardly a Memoir of the Geological Survey that touched on Ordovician or Silurian was published without the signature of G. L. Elles, or, before 1914, E. M. R. Wood upon the graptolites. This is a testimony both to Elles' industry, and to the successful application of the full set of zones established in the early years of the twentieth century, including her own (1904) Welsh additions to the Lapworth canon. But when she followed this work to attempt the

THE B I O S T R A T I G R A P H I C P A R A D I G M biostratigraphic unscrambling of complex structure (thereby following Lapworth's example in applying palaeontology to a geological problem) the results were less happy. Where better to apply Lapworth's system of deduction than to the complex and ill-understood ground of the Lake District? Elles (1933) proposed zones after studying the geology there, just as Lapworth had done, but on this occasion the passage of time has been less kind to them (Jackson 1962). Older graptolite faunas, unknown to Elles, have been added (Jackson 1979; Rushton 1985; Maletz et al. 1991), and the zonal scheme for the Arenig Series is being thoroughly reinterpreted. The graptolites were not the sole key to the complex structure of the Lake District, a structure which is still in the process of disentanglement. This is not to say that graptolites have no part to play in the process, because the new discoveries have played a vital part in the generation of tectonic models, but rather that Elles' application of the Lapworthian method was not an invariable guarantee of success in the absence of its brilliant originator.

Geology versus biology: palaeontology pulled two ways One of the compelling qualities of Lapworth's system as embodied in Elles & Wood's monograph was its simplicity; graptolites were recognizable from their overall form and from a few, simple measurements: thecal spacing, stipe width and colony size. Graptolites were probably the first fossil species to be quantitatively defined. These criteria were those used by Lapworth, and could even be applied in a general way in the field. The monograph was nothing less than a catalogue for the calibration of time, and one that was easily mastered. True to their name, the outlines of the graptolite colonies were, almost literally, written on the rocks for biostratigraphic correlation. It should be added that theories asserting the planktic habits of graptolites gained wide currency at about the same time as the heyday of Elles & Wood's monograph (e.g. Marr 1925), adding the final touch to their theoretical perfection as correlation tools. However, acknowledging planktic habits for graptolites was not a sine qua non for the successful application of the Lapworthian method. For example, there was much disagreement about the site of accumulation, and mechanism of the deposition of graptolite shales (e.g. Grabau 1929; Bulman 1964) which were not finally rationalized until the recognition that Lower Palaeozoic oceans differed in their distribution from those later in geological time. The graptolites portrayed in Elles & Wood were, in truth, little more than cartoons of the original organisms. The serrated outlines of the thecae revealed little of their true structure and nothing of the colony development. They were 'writing on the rocks' (as the Greek root of the name describes them) for the convenience of biostratigraphers. This did not matter for geologists, primarily concerned with the correlation of formations, but it did matter to zoologists. As the graptolites began to be taken seriously as organisms, so a wealth of fine structures were discovered. This process had already started by the 1890s when Gerard Holm had discovered that splendid detail could be revealed by etching whole colonies out of limestones. The description of these specimens suddenly turned graptolites from stratigraphic ciphers into complex animals. These could be studied without primary regard for stratigraphic utility. Palaeontology as a whole has continued to be pulled between these

95

two poles: as geological tool on the one hand, as palaeobiological discipline on the other. If there are rivals to Lapworth's contribution they might be found in the magnificent papers describing isolated graptolites by O. M. B. Bulman (commencing 1932) and Roman Koslowski (1949), which are as indispensible now as when they were published. It is significant that Bulman published his contributions in Arkiv fiir Zoologie, rather than a geological journal. Bulman was writing at the same time as Erik Stensio's magisterial works on fossil fishes were being published, works in which the anatomy was realized in almost as much detail as could be obtained from a dissection of a living fish. These papers set a new standard in the description of fossils, which has not been surpassed. Koslowski also recognized graptolites for what they were for the first time, an extinct clade related to the living hemichordates, and thus opened a new research programme in which zoological information might play a direct part. However, 95% of graptolites are preserved in the flattened mode, as in the Ordovician and Silurian at Moffat. They simply lack the rich information which can be obtained from isolated material (Fig. 2). Could they really be trusted as stratigraphic indicators, if they were so incompletely known? It was not long before differences in taxonomy arose between material studied in the 'classical' flattened style and the new isolated material preserved in full relief. Neither Elles nor Ruedemann really took on board the new morphological discoveries. There was a barrier, indeed, on occasion hostility, between those who used graptolites primarily as geological indicators, and those who sought out the secrets of their anatomy and biology. The former may have thought of themselves as the guardians of Lapworth's tradition, while the latter no doubt allied themselves with the new evolutionary biology following the 'Modern synthesis' of Darwinism in the 1930s. This tension between the palaeobiological role and the biostratigraphical role of fossils has not gone away; it is still there in the contrast between some of the authors who write in the journal Paleobiology and those who write in the Journal of

Paleontology. Conodonts become fashionable in the latter half of this century It is well-known that graptolites are rare in Lower Palaeozoic inner shelf deposits, such as the limestones which dominated deposition upon the Laurentian platform. Discovery of the ubiquity of conodonts in such deposits (and their comparative ease of extraction by way of acid dissolution) led to a 'boom' in conodont specialists in the 1970s and 1980s. Conodonts, in their turn, became fashionable. In these organisms there was less conflict between different preservational modes. From Lower Palaeozoic 'layer cake' successions (where no equivocal questions about the way up of strata exist) a succession of conodont zones to rival Lapworth's graptolite zones was proposed and promulgated (see, for example, Sweet & Bergstr6m 1971). These were conceived not in the contorted splendour of the Southern Uplands but in the flat-bedded quarries of Sweden and the Western USA. Such was their popularity that they became a focus for particularly intensive research. When the Working Group of the IUGS on the Cambrian-Ordovician boundary reached a decision on what kind or organism should be critical for the

96

R.A.

FORTEY

ill

! f~

+ definition of the Ordovician it was the conodonts, not the graptolites, that were chosen as the primary reference (Norford 1988). The place of the graptolite Rhabdinopora (formerly Dictyonema)flabelliformis, the first planktic graptolite, had been usurped by a modest but widespread conodont of the genus Cordylodus. It is interesting that the same kind of idealism has been claimed with regard to conodonts as stratigraphical indicators as has been applied to graptolites: they are not subject to diachronism, they are planktic, and so on.

Reconciling the biological and geological approaches Graptolites are now known in more detail than Lapworth could have imagined. Ultrastructural studies (Crowther 1981) have revealed even the fabric of their construction, at magnifications measured in thousands. Isolated material

Fig. 2. The fully anatomized graptolite skeleton (left) (courtesy of P. R. Crowther) compared with the drawings used in Elles & Wood's monograph (right) of the genus Climacograptus (sensu lato), showing the difference between the preservation commonly encountered in the field and that used for palaeobiological studies.

has now been described from many additional parts of the geological column (Cooper & Fortey 1982; Williams & Stevens 1987; Mitchell 1987). The knowledge of graptolite fine structure is now being fed back to the interpretation of the specimens on the rock. This has the positive effect of reconciling the morphological and the stratigraphical approach to the study of specimens with mutual benefit for both biostratigraphy and palaeobiology. The tell-tale signature of a complex thecal structure may be preserved even in the most recalcitrant of graptolite shales. The negative aspect, if it can be so described, is that the simple approach using the 'great book', armed with which any stratigraphic problem could be solved, applies no more. The straightforward, and somewhat idealistic, phase of research which Lapworth initiated has passed. Thus it is that apparently arcane work on the ultrastructure, or colony development of graptolite colonies

THE B I O S T R A T I G R A P H I C P A R A D I G M has now contributed to problems of practical stratigraphy. This has, if anything, only re-emphasized that graptolites are superb fossils for stratigraphic correlation; many apparent anomalies in ranges have been resolved thanks to new knowledge of fine structure. To cite one example, there was an apparent mismatch in age between Ordovician Didymograptus bifidus Zone of Europe and North America as determined using 'tuning fork' graptolites, the latter apparently being older than the former. Studies on isolated material revealed, within the first millimetre of the colony, such differences in structure as to show that there was no likely close relationship between these 'tuning forks' on either side of the Atlantic. On the other hand, genuinely reliable, widespread species were identified among other groups of graptolites (such as isograptids). It is becoming clear that there are some geographically restricted graptolites, including many of those that belonged to shelf biotopes, but it is also true that there were many genuinely pandemic taxa, even at times of high 'provinciality' elsewhere (Cooper et al. 1991). B i o z o n e s m o d i f i e d , a n d the r e s u r g e n c e o f g r a p t o l i t e s

The methods of graphical correlation have refined zonal usage, by using all the range data for fossils, especially first and last appearances of species, rather than 'lumping' them by zone (Shaw 1964; Edwards 1984). Conodonts were subjected to this method, yielding results that added to their lustre as correlation tools (Sweet 1984), with a five-fold increase in precision within a single craton. This method does not, of course, 'invalidate' zones, which remain valuable as a common language for international correlation, and are the appropriate level of resolution for many problems. Recent graphical correlations using graptolite range data (Cooper & Lindholm 1990) show that they are equally capable of refining the timescale. During the century or more after the publication of Lapworth's Moffat Series paper, the graptolites have been subject to several surges in biostratigraphic popularity. Now their peculiar virtues are respected alongside those of conodonts. Graptolites extended into oceanic deposits. They included species which were exceptionally pandemic in distribution, even at times when other elements in the fossil faunas were endemic (Fortey & Mellish 1992). Conodonts can both complement graptolites, and extend into limestone facies where they are absent. Correlation between zonal schemes based on these different organisms is becoming more secure (Bergst6m 1986). The complete rehabilitation of graptolites as biostratigraphic tools received its official imprimatur with the selection, in May 1985, of the graptolitic section at Dob's Linn as the base of the Silurian System, and hence the top of the Ordovician System (Cocks & Rickards 1988). This was at an horizon (based on the acuminatus Zone) only a part zone away from its original definition. Lapworth would no doubt have been delighted.

The durability of biostratigraphic data compared with contingency of structural and palaeogeographic inference 'Those who accepted the theory of the Llandeilo age of the dark shales and greywackes of the south of Scotland, and attempted to correlate them with their supposed equivalents

97

on the south of the Solway, have frequently expressed their astonishment that the Scottish deposits, which must have been laid down in a sea in some places less than 30 miles distant from the volcanic area of the Lake-district, yet showed no trace whatever of contemporaneous igneous action, whether in the form of trap-dyke, lava flow, or bed of volcanic ash.' (Lapworth 1878, p. 342) [Lapworth goes on to suggest that they may actually be of somewhat different ages]. 'Nor is this extraordinary north-westerly attenuation of the Lower Silurian rocks a phenomenon exclusively confined to Britain. On the contrary it is one of the most striking features of the Lower Silurians of Europe in general.' (p. 339). 'After undergoing innumerable repetitions among the desolate wilds of Eskdalemuir, in the contorted and inverted attitudes of their equivalents in the Moffatt area, the beds of this great group gradually roll over to the s o u t h w a r d . . . ' (p. 342). 'The Girvan district is here regarded as a distinct and separate area.' (footnote to p. 341). The four quotations just given relate to aspects of geology other than the detailed biostratigraphic exploration l, ~/'/

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98

R.A.

of the Moffat area itself and its succession of graptolites zones. They are all made with Lapworth's characteristic assertiveness, and the contemporary reader might well have accepted all these statements as just as true as the others relating to biostratigraphy. With the wisdom of hindsight it is clear that there are different ways to interpret all four of the statements today. In contrast, as we have seen, the biostratigraphic scheme proposed by Lapworth survives as 'ground truth' in its essentials. This point about the durability of biostratigraphic results is an important one, because, in the perception of some geologists, biostratigraphy is sometimes seen as a rather routine procedure by comparison with the grander science of structural deduction or palaeogeography. Possibly, it is perceived that the conceptual breakthrough was made by Lapworth, so that all that follows is in a sense, 'fine tuning'. What the biostratigrapher deals with is not so much falsification of rival hypotheses, the definitive mode of scientific reasoning described by Karl Popper, as progressive refinement of what is already known. The detailed revision of the late Ordovician to earliest Silurian part of Lapworth's sequence in Dob's Linn carried out by S. H. Williams (1982a, b, 1983) is central to the current meaning and correlation of biozones, for all that the techniques employed are the classical ones of bed-by-bed collecting and description. An internationally acceptable and recognizable base to the Silurian System depends on such meticulous biostratigraphy.

Changes in regional palaeogeographic setting The concept of 'extraordinary northwestern attenuation' of Ordovician and Silurian was an idea that was quickly falsified. For all Lapworth's disclaimer (in the footnote) that Girvan was a completely separate area, it was apparent that there was a great thickness of sediments there which included time equivalents of the Moffat succession (Williams 1962). Far from being a separate area, this Girvan-to-Moffat change became a textbook example of regional facies and thickness variation (Fig. 4), and was promulgated as such through the numerous editions of Wells & Kirkaldy's textbook of historical geology. The second 'attenuation' to which Lapworth referred was the succession in Scandinavia and the Russian Platform, which is indeed similarly condensed to that at Moffat, but would now be interpreted as part of a separate palaeocontinent, and truly decoupled from the Southern Uplands. Of Lapworth's original statements with regard to sediment distribution, probably the only one which survives unmodified is the idea that 'these strata must have been laid down in an area removed in some way from the irregular and disturbing influences of river-deposits and current action...'. Lapworth himself would have regarded this depositional setting as part of the general, northwesterly attenuation of 'Silurian' sequences across Europe. There have been some profound changes since then. Once the contrast with Girvan was established, the Moffat area was recognized as being far removed from sediment sources which, under this interpretation, then lay rather to the north ; this, coupled with the notion of crustal shortening represented by the regional isoclinal folding, permitted the problem of distance to be resolved. What is a comparatively short distance today was much greater at the time of deposition of the rocks. This view of the palaeogeographic

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place of the Moffat area persisted through two thirds of the present century, which might be termed the 'geosynclinal period': Moffat was an example par excellence of an early Palaeozoic starved geosyncline. As appropriate, Moffat was incorporated into Marshall Kay's (1951) monumental, if baroque, apotheosis of the geosyncline. Possibly the last appearance of the Southern Uplands in this vein was in A. Williams' (1969) review of brachiopod distributions. Current systems within the geosynclinal system were invoked to explain features of Ordovician brachiopod distribution, which included faunas extending from Girvan into Scandinavia. But the palaeogeography was, essentially, present-day geography with added geosynclines. The 'post-geosynclinal' period stems from the recognition of the vanished ocean Iapetus (Wilson 1966) and, subsequently, a mid-European oceanic tract termed Tornquist's Sea by Cocks & Fortey (1982). This change in narrative, whereby the principles of plate tectonics could be applied to the classic Caledonides, is now so familiar as not to require elaboration here. It solved, at a stroke, the dilemma of the profound differences between the Lake

THE B I O S T R A T I G R A P H I C P A R A D I G M

99

District and the Southern Uplands 'in some places less than thirty miles distant' which Lapworth had remarked (above). It provided a rationale for the differences between the Girvan district and Moffat, and an explanation for the suite of rocks at Ballantrae, so different from their contemporaries around Skiddaw. The introduction of terranes has allowed a further degree of freedom, and one that has directly affected the Moffat area. As this is written, the Atlas of Palaeogeography and Lithofacies has just been published by the Geological Society, and there (Ingham in Cope et al. 1992) Moffat will be found 'floating', rather uncertainly, as an oceanic terrane within Iapetus, and outboard of other terranes, carrying the legend 'relative position not known'.

collected which spans the same time interval. The modern versions of Lapworth's scheme have now been tested in this say in numerous localities around the world (Cocks & Rickards 1988), including China, USSR, Australia and the Americas; the sequence of species which appear in these sections is very similar. If anything, one might say that the biostratigraphic science contrasts with the tectonic, because the latter has been through several revisions, as earlier ideas have become falsified, often in the context of new theory. In truth, biostratigraphy acts as an independent monitor of tectonic theory, and both should collaborate in the generation of robust hypotheses; this, of course, is precisely the way Lapworth proceeded.

Changes in structural interpretation

The divorce of rock and fossils: end of a marriage of convenience or dissolution of a natural partnership?

One might repeat a similar history of change with regard to structural interpretation, but the briefest summary will demonstrate the point. Lapworth's demonstration of isoclinal folding, often with an inverted limb, at the local level, became a pervasive structural model at the regional level, much employed by Peach and H o m e , and other distinguished surveyors. After the introduction of plate tectonic interpretations, the hypothesis that the Southern Uplands comprised part of an accretionary prism (Leggett et al. 1979) emphasized the importance of regional thrusting in dividing the Uplands into different tectono-stratigraphic units. This intrepretation has itself not been without controversy, and the structure of the Southern Uplands is still under debate, although the terms of the debate are now invariably conducted with reference to plate tectonic models. Thus there have thus been several shifts in interpretation as was the case with the sedimentary setting.

Comparative durability of biostratigraphy There is no reason to suspect that the question of the site of deposition of the Moffat Series has been definitively settled. Paradoxically, its position is now so freed from constraints that it could have been located practically anywhere outboard of Laurentia. The re-interpretations of the structure of the Southern Uplands continue. As has been shown, the scientific method has offered change and change again since Lapworth's paper was published, depending upon which paradigm (structural, tectonic or sedimentological) was current at the time, and it cannot be supposed that this revisionary process has now stopped. Compared with these periodic conceptual fluxes, the biostratigraphy has been extraordinarily enduring. There have been certain changes to the nomenclature of the graptolites, which can be the cause of justifiable irritation to the non-specialist, as well as new discoveries and changes in the stratigraphic ranges of species. The addition of the extraordinarius Zone near the end of the Ordovician is possibly the most important change to Lapworth's sequence (Williams 1983), but even in this case the characteristic fauna is both improverished, and confined to a single band at Dob's Linn. It should not be claimed that Lapworth's biostratigraphic science is in some sense 'better' than his structural science, on account of its comparative durability. But the capacity of good biostratigraphic schemes to evade subsequent falsification is assuredly one of the achievements of this branch of geology. In the field, a zonal scheme has to run the gauntlet of being potentially falsified by every sequence subsequently

' . . . b l a c k rock showing the peculiar variegated lines of the

M. gregarius Zone, and affording M. tenuis, C. scalaris, and other of its commoner fossils...' (p. 266). ' . . . w e notice with much interest the extraordinary 'Clingani' band of our typical section. It is here nearly a foot in thickness, and is crowded with well-preserved examples of Monograptus Clingani (Carr.) and M. leptotheca (Lapw.).' (p. 271). Lapworth (1878) was able to describe the details of field relationships of rocks and fossils in a minute and leisurely way which would not be permitted in a modern journal. These intimate descriptions reveal how Lapworth conceived his zones, and how he was able to trace his shale 'bands' across the intensely folded country. Time and again the reader will be struck by the way Lapworth described the lithological details and the fossils together, an intimate association used in an almost forensic examination of the structure in the field. Although the fossils are dubbed with Latin names they are, in truth, little more than complex lithological signals. Their state of preservation, crowding and colour are mentioned along with their names. They are deployed in conjunction with other stratigraphical signals, such as peculiar colours produced by weathering, or the stripiness of shale beds. In contrast, evolution is hardly mentioned; I can find only one suggestion that one species might be ancestral to another in the paper. Lapworth did not neglect the biological aspects of graptolites (Lapworth 1897), but at this earlier stage in his career he seems to have regarded them primarily as geological indicators. To Lapworth, therefore, biostratigraphy was not divorced from lithostratigraphy; rather, the fossils were an intimate part of the whole aspect of the rock available for correlation purposes. The fossils were regarded almost as part of the lithology. The recognition of a distinction between chrono-, lithoand biostratigraphy has been one of the important changes to stratigraphic practice within the last half century. It would be difficult to leave out this consideration from a discussion of Lapworth's achievements. The major formational ~ames used by Lapworth: Glenkiln, Birkhill and Hartfell, survive into modern usage. As originally proposed, the 'Barren Mudstones' had the status of an 'interzone' towards what is now the top of the Ordovician (Fig. 1), a rock interval which was listed amidst the formally recognized zones. Lapworth's notebooks (see S. H. Williams in Cocks & Rickards 1988,

100

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Fig. 5. A modern interpretation of part of the section studied by Lapworth, after Williams (in Cocks & Rickards 1988, fig. 5), showing the 'range bar' convention applied to a confacial, but not completely continuously fossiliferous section. fig. 1) show another, unfossiliferous 'Belcraig Shale' sandwiched between the linearis and anceps zones, and given equal status to these zones. In .these cases, thickness of rock was taken as a surrogate for time, even in the absence of fossils. The chrono-, bio- and litho aspects were thoroughly intertwined. The 'unpicking' of these various concepts has resulted in greater clarity for biostratigraphy; the ranges of fossil taxa are now routinely shown as vertical bars extending upwards through the section; see, for example, the graptolite ranges shown through the Ordovician-Silurian boundary interval at Dob's Linn, Fig. 5 in Williams' paper. We are so used to seeing biostratigraphic data presented in this way that it is possible to forget that it is a convention, and one which conceals data which were of importance to Charles Lapworth. I identify two consequences which have not always been to the advantage of biostratigraphers.

The ideal section One consequence of the separation of the discipline of biostratigraphy has been an ambiguous achievement. This has been the spread of an idea of the ideal biostratigraphic section, a kind of platonic rock section equipped with perfect properties for international correlation: continuous, confacial and conformable, fossiliferous throughout, yet

with cryptic breaks minimized, replete with fossils of several groups, which are arranged in evolutionary series. Furthermore, such sections have an horizon suitable for hammering in a 'golden spike' for the base of a chronostratigraphic interval to immediate international satisfaction. Such sections rarely, if ever, exist in nature. Yet their pursuit has been one of the motivating forces behind various Working Groups of the International Geological Correlation Programme. It is curious to find such an idealistic concept holding sway in the geological sciences, which are so generally pragmatic. The importance of sound criteria for international correlation is not to be gainsaid, but this is, perhaps, a different matter from the relentless pursuit of a perfect section which is likely to prove a chimera. This is probably why the definition of the Cambrian-Ordovician has remained undecided after more than 15 years of intense work and debate (Norford 1990). The intensity of argument increases as smaller and smaller flaws are examined, and this is not surprising because the ultimate level of focus is upon such minutiae as subjective and minor taxonomic disagreements, where specialists are notoriously combative. To those outside these debates, the arguments must seem as esoteric as the medieval disputes as to how many angels could dance upon the head of a pin. For the definition of major boundaries, such as the base of a System, the chances of finding an ideal horizon in an

THE B I O S T R A T I G R A P H I C P A R A D I G M ideal section are still further slimmed by the fact that most boundaries were placed originally, and with good reason, at some important event in world history, thus generating precisely the circumstances under which ideal sections are likely to become corrupted by the sticky stuff of history. The odds are stacked against the existence of the ideal section. The end of the Ordovician, which is recorded in the Moffat Series, coincides with the major Hirnantian glaciation at the end of the Ordovician (Beuf et al. 1971) and is thus a case in point. Shelf sections spanning the Ordovician-Silurian boundary are invariably dramatically affected by this event, and it is unlikely that even the offshore palaeogeographic site at Moffat escaped its influence. Virtually every other Palaeozoic system and series boundary carries with it some eustatic or other event which affects the likelihood continuous confacial fossil faunas.

101

fleshed-out historical narrative of events in the heart of the Palaeozoic. Now that biostratigraphy has established itself as a discipline in its own right, it has taken a lesson from Charles Lapworth in applying its unique precision to wider geological problems. But this does not signal the end of the need for more and detailed study of classical rock and fossil sequences. Just because the principles of biostratigraphy have a long pedigree it does not diminish the need for their continued exercise.

I thank L. R. M. Cocks who read and improved the manuscript, and P. R. Crowther and S. H. Williams who allowed me to use their graptolite illustrations.

References

Loss o f information on rock-fossil interactions The range-bar convention in biostratigraphy may serve to deflect attention from some of those features which Lapworth so keenly observed, relating to the occurrence and lithological association of fossil material. At its worst, this dissociation means that the biostratigrapher is called in as a consultant to provide his determinations on isolated specimens, often without regard to any circumstances of field occurrence, essentially as a kind of technician. The biostratigrapher's job might be in danger of becoming no more than the provision of an inventory of names which can be added to other criteria (e.g. isotopes, trace elements) for synthesis. But details of field occurrence do contribute both to stratigraphy and to the biological knowledge of the fossils themselves. For example, the 'barren beds' in the Moffat succession a r e not merely inconvenient gaps in the fossil narrative, but reflected both ash from distant volcanoes and oceanic conditions at a time of climatic crisis, which was also one of the major turning points in graptolite evolutionary history. Equally, the complanatus or clingani 'bands' are likely to have had more significance than just being a part of the range of their respective species. Perhaps, now that the conceptual framework of biostratigraphy has been sufficiently clarified, the time has come to go back to Lapworth, and examine further the interactions between bio- and lithostratigraphy. The application of sequence stratigraphy to fossil bearing sequences has done this to some extent, although the 'ideal sequence' is potentially as intransigent a taskmaster as the 'ideal' section. Nonetheless, there is reason to suppose that the Ordovician-Silurian sequence in all its detail will be related to climatic and oceanographic events of which Charles Lapworth had little conception.

The way forward The stratigraphic endeavour started by Lapworth has, in a sense, come full circle. The biostratigraphy he initiated has survived, with additional refinement. These modifications have been introduced progressively over the last century, rather than being initiated by a profound change in conceptual framework, as has been the case with the structural and palaeogeographical views of the Southern Uplands. Now once again the intimate association between rocks and the fossils they contain, to which Lapworth devoted much scrutiny, is being re-examined to produce a

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MCKERROW, W. S. & LEGGETT, J. K. 1980. Silurian palaeogeography on the margins of the Iapetus Ocean in the British Isles. In: WONES, D. R. (ed) The Caledonides in the U.S.A. Memoir No. 2, Department of Geological Sciences, Virginia Polytechnic Institute, 49-55. COOPER, R. A. & LINDHOLM, K. 1990. A precise worldwide correlation of Early Ordovician graptolite sequences. Geological Magazine, 127, 293-305. , FORTEY, R. A. 1982. The Ordovician graptolites of Spitsbergen. Bulletin of the British Museum (Natural History) Geology, 36, 157-302. --& LINDHOLM, K. 1991. Latitudinal and depth zonation of early Ordovician graptolites. Lethaia, 24, 199-218. COPE, J. C. W., INGHAM, J. K. & RAWSON, P. F. (eds) 1992. Atlas of

Palaeogeography and Lithofacies. Geological Society, London, Memoir, 13, 1-153. CROW~ER, P. R. 1981. Fine structure of the graptolite periderm. Special papers in Palaeontology 26. DAVIDSOt~, T. 1866-1883. A monograph of the British fossil Brachiopoda. Palaeontrographical Society Monographs, London. EDWARt)S, L I E . 1984. Insights on why graphic correlation (Shaw's method) works. Journal of Geology, 92, 583-97. ELLES, G. L. 1904. Some graptolite zones in the Arenig rocks of Wales. Geological Magazine, 41, 199-211. --, 1933. The lower Ordovician graptolite faunas with special reference to the Skiddaw Slates. Summary of Progress of the Geolog&al Survey of Great Britain, for 1993, 91-111. -& WOOD, E. M. R. 1901-1918. A monograph of British graptolites. Palaeontographical Society, London. FORTEY, R. A. & MELLISH, C. J. T. 1992. Are some fossils better than others for inferring palaeogeography? Terra Nova, 4, 210-216. GRABAU, A. W. 1929. Origin, distribution and mode of preservation of the graptolites. Memoir of the Institute of Geology, National Research Institute of China 7. HALL, J. 1865. Graptolites of the Quebec Group. Figures and descriptions of Canadian Organic remains. Decade 2. Canadian Geological Survey, 1-151. JACKSON, n . 1962. Graptolite zones in the Skiddaw Group in Cumberland, England. Journal of Paleontology, 36, 300-313. ,1979. A new assessment of the stratigraphy of the Skiddaw Group along the northern edge of the main Skiddaw Inlier. Proceedings of the Cumberland Geological Society, 4, 21-31.

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KAY, M. 1951 North American Geosynclines. Memoir of the Geological Society of America, 48. KOSLOWSKI, R. 1949. Les graptolites et quelques noveaux groupes d'animaux de Tremadoc de la Pologne. Palaeontologica Polonica, 3, 1-235. LAPWORTIt, C. 1878. The Moffat Series. Quarterly Journal of the Geological Society of London. 34, 240-343. , 1879. On the tripartite classification of the Lower Palaeozoic rocks. Geological Magazine, 16, 1-15. ,1897. Die Lebenweiser der Graptolithcn. In: WALTHER, J., Lebenweise fossiler Meeresthiere. Zeitshcrift der deutsches geologische gesseUschaft, 49, 238-258. LEGGETI", J. K., MCKERROW, W. S. & EALES, M. H. 1979. The Southern Uplands of Scotland: a Lower Palaeozoic accretionary prism. Journal of the Geological Society, London, 136, 755-770. MALETZ, J., RUSHTON, A. W. A. & LINDttOLM, K. 1991. A new early Ordovician didymograptid, and its bearing on the correlation of the Skiddaw Group of England with the T0yen Shale of Scandinavia. Geological Magazine, 128, 335-343. MARR, J. E. 1925. Conditions of deposition of the Stockdale shales. Quarterly Journal of the Geological Society of London, 81, 113-133. MITCHELL, C. E. 1987. Evolution and phylogenetic classification of the Diplograptacca. Palaeontology, 30, 353-405. MURCHISON, R. I. 1839. The Silurian System, founded on geological researches . . . &c, Murray, London. NORFORD, B. S. 1990. Introduction to papers on the Cambrian-Ordovician boundary. Geological Magazine, 125, 323-6. OPPEL, A. 1856-1858. Die Juraformation Englands, Frankreichs und der Siidwestlichen Deutschlands. Stuttgart. RUEDEMANN, R. 1904. Graptolites of new York. Part 1. Memoirs of the New York State Museum Albany, 7, 457-803. RUSHTON, A. W. A. 1985. A Lanceficldian graptolite from the Lake district. Geological Magazine, 1122, 329-333. SALTER, J. W. 1864-1893. A monograph of British trilobites. Palaeontographical Society, London.

FORTEY

SHAW, A. B. 1964. Time in stratigraphy. McGraw-Hill, New York. SWEET, W. C. 1984. Graphic correlation of upper Middle and Upper Ordovician rocks, North American Midcontinent Province, U.S.A. In: BRUTON, D. L. (ed) Aspects of the Ordovician System. Universitetsforlaget, Oslo, 25-35. & BERGSTR6M, S. M. (eds) 1971. Symposium on conodont biostratigraphy. Memoirs of the geological Society of America 127. WELLS, A. K. & KmKALDY, J. F. 1966. Outline of historical geology. 5th edition (first edition 1937). Thomas Murby & Co. WILLIAMS, A. 1962. The Barr and Ardmillan Series (Caradoc) of the Girvan

District, south-west Aryshire, with descriptions of the Brachiopoda. Geological Society, London, Memoirs, 3. , 1969. Ordovician faunal provinces with reference to brachiopod distribution. In: WOOD, A. (ed.) The Precambrian and Lower Palaeozoic rocks of Wales. University of Wales Press, Cardiff, 117-154. WXLLIAMS, S. H. 1982a. The late Ordovician graptolite fauna of the Anceps Band at Dob's Linn, southern Scotland. Geologica Palaeontologica, 16, 29-56. ,1982b. Upper Ordovician graptolites from the top Lower Hartfell Shale (D. clingani and P. linearis zones) near Moffat, southern Scotland. Transactions of the Royal Society of Edinburgh (Earth Sciences), 72, 229-255. , 1983. The Ordovician-Silurian boundary graptolite fauna of Dob's Linn, southern Scotland. Palaeontology, 26, 605-639. , 1988. Dob's Linn--the Ordovician-Silurian boundary stratotype. In: COCKS, L. R. M. & RICKARDS, R. B. (eds) The Ordovician-Silurian

boundary: a global synthesis. Bulletin of the British Museum (Natural History) Geology, 43, -

& STEVENS, R. K. 1988. Early Ordovician (Arenig) graptolites of the Cow Head Group, western Newfoundland. Palaeontographica Canadiana, 5, 1-167. WILSON, J. T. 1966. Did the Atlantic close and then re-open? Nature, 676--681. -

Received 31 October 1992; accepted 10 November 1992.

From QJGS,34, 240-241. 19. The MOFFA~ S~Rrm. By C~ARL~S LArWORTB, Esq., F.G.S. (Read 1%v. 21st, 1877.) [PLAT~.SXI.-XIII.] CONTENTS.

Ir~trodu~tion. I. General characters of the Lower Silurian Rocks of the south of Scotland. II. General characters of ths strata of the Moffat district. III. History of previous opinion.

A, -Physical Relations of the Moffat Series. L Description of the typical sections of Dobb's Linn and Craigmiehan Scants. XI. Description of the black bands to the south of the Moifat Valley. (a) Black-shale bands south-west of St. Mary's Loeb. i. Muckra Band; ii. Riskinhope Band; iii. Whitehope Band; iv. Borrybush Band. (b) Black bands in the valley of the Yarrow. i. Mount-Benger Burn; ii. Eldinhope. (v) Black hand of ~Ettrick and Glenkiln. i. Ettriek; ii. Entertxona; iii. Belcraig ; iv. Glenkiln. III. Description of the sections of the Moffat Series to the north of the MoffatYarrow Yalley. (a) Basin of the Upper Annam i. Frenchland Burn; ii. Garple Spa; iii. Rittonside; iv. Headshaw T,inn ; V. Harti~ll Spa. (b) Basin of the Meggat Water. (c) Basin of the Moffat Water. IV. Summary of observations and conclusions regarding the physical relations of the Moffat Series.

B. Subditrisions, Lithology, and Pal~eon¢ology of the Moffat Series. L The GIenkiln Shales. II. The Hartfell Shales. (a) Lower Hart~ell. i. Zone of Clirna/,ograptus Wilsoni; ii. Zone of lh'cranograpCus Clinqani ; iii. Zone of PleurograTtus linearis. (b) Upper Hartfell. i. Barren Mudstones ; ii. Zone of l)icellograTtus anceps. III. The Birkhill Shales. (a) Lower Birkhill. i. Zone of .DilolograTtus acumi~gus; ii. Zone of Diplograplus vesiculosus ; iii. Zone of MonograTCus greqarius. (b) Upper Birkhill. i. Zone of JDiTlograpCuscomeCa; ii. Zone of Monoqraptus sTinigerus ; iii. Zone of RastriCes maximus. Table showing the vertical distribution of the Fossils of the Moffat Series.

C. Conclusion. I. Systematic importance of the divisions of the Moffa£ Series. II. Comparison of the :Faunas of the three divisions of the Moffat Series with those of their foreign equivalents. (a) Llandeilo :Formation ; (b) Bala or Caradoc ; (c) Lower Llandovery. III. General conclusions as to the geological age and relationships of the Moffi~t Series. IV. Bearing of the foregoing conclusions upon the general question of the suecession among the Silurian Rocks of the south of Scotland. INTRODUCTION.

§ I. General characters of the Lower Silurian _I~ocks of the south of Scotland. No single geographical r e , o n in B r i t a i n is m o r e clearly defined physically t h a n the broad tableland k n o w n as the S o u t h e r n H i g h lands or Uplands of S~otland. Cut off a b r u p t l y from the n o r t h of E n g l a n d by the shallow inlet of the Solway and t h e m o u n t a i n - w a l l of the Cheviots, and f r o m the m a i n mass of Scotland by the great central valley of L a n a r k and Mi.dlothian, it stretches like a vast zone across the entire b r e a d t h of t h e island from sea to sea. 0 c c a sionally some of its h i g h e r points are sufficiently grouped t o g e t h e r to be classed popularly u n d e r a c o m m o n title, such as the Moorfoots, L o w t h e r s , and L a m m e r m u i r s ; b u t the region, as a whole, m a y best be described as a rolling sea of broad r o u n d e d hills and deep n a r r o w valleys. T h e only level spots occur along t h e banks of its few really i m r o r t a n t rivers, where their lower valleys e x p a n d into the long fertile reaches of w h i c h the Merse, Nithsdale, and A n n a n d a l e are t h e m o s t familiar examples. The more elevated areas, which rarely exceed 2000 feet in height, show here a n d there strips of peat moss or h e a t h e r y moor-land. Nowhere, however, do we m e e t w i t h t h e crag, cliff, and rocky g r o u n d of the N o r t h e r n H i g h l a n d s , b u t hill a n d dale are clothed alike in a universal m a n t l e of soft green turf. T h e district is consequently p r e e m i n e n t l y pastoral, agriculture being almost entirely restricted to the low-lying, open dales.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 105-124 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 427-446

Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles N. J. RILEY British Geological Survey, K e y w o r t h , Nottingham N G 1 2 5 G G , U K

Abstract: Vaughan's (1905) zonation of the Carboniferous Limestone in the Bristol district was a pioneer biostratigraphical study, which because of its meticulous execution can be reinterpreted in a modern context. The replacement of his scheme with a chronostratigraphical one by George et ai. (1976) had a similar revolutionary affect on British Dinantian stratigraphy. However, it is now time to revise the British Dinantian stages so that they more closely correspond to biostratigraphical events. Dinantian biostratigraphy still requires considerable refinement, but it has now achieved a diversity of techniques and resolution far beyond that which was available at the time of these earlier proposals. It is the most pragmatic and closest approximation to widespread chronostratigraphical correlation available. This paper discusses these and related issues and presents a review and correlation of current biozonations.

This contribution is written as a tribute to Vaughan's paper on the 'Palaeontological Sequence in the Carboniferous Limestone of the Bristol area' published by the Geological Society in 1905. It was one of the most influential papers to have appeared in the Journal and pioneered biostratigraphical subdivision of the Carboniferous Limestone in Britain, whilst providing one of the first detailed commentaries on the principles of biostratigraphy. This review aims to summarize the d e v e l o p m e n t of biostratigraphical subdivision of the Dinantian Subsystem (Lower Carboniferous) in the British Isles and provide a commentary on the wide diversity of biostratigraphical schemes currently available in terms of their facies application, interrelationships and current problems. International correlation is not dealt with here, but is assessed in Brenckle (1991). As biostratigraphy is used to recognize chronostratigraphical units, this paper will also address the relationship of biostratigraphy to the British Dinantian stages proposed by George et al. (1976) and those aspects of sequence stratigraphy which are relevant to this classification. Aspects of geochronometry and palaeomagnetism have been summarized by Leeder (1988) and Hailwood (1989) respectively. Chemostratigraphy is a promising new area of stratigraphical research being applied to Carboniferous rocks, but there is little information in the public domain for this technique to be reviewed in the present context. For reasons of brevity authors of taxonomic names are not included in this account, however they can be found in the references quoted for each fossil group covered in the text and British Geological Survey memoirs. A summary of British Dinantian classification is given in Fig. 1.

to the biostratigraphical investigation of the British Carboniferous. Reports from this committee were issued through the BA by Garwood between 1896 and 1900 and by Hind, until 1907. Against this background, Vaughan's (1905) zonation, which was based on the distribution of corals and brachiopods in the Carboniferous Limestone of the Bristol district, was the first serious attempt to apply a biostratigraphical zonation to the Lower Carboniferous marine limestones of Britain. Prior to this classic work, Carboniferous fossils had largely been described more as a taxonomic exercise, with little regard for their utility in subdividing the very broad lithostratigraphical units then recognized. The state of Carboniferous biostratigraphy immediately prior to Vaughan's paper was illustrated by Hinde & Howe (1901, p. 388) who subdivided the Carboniferous Limestone throughout the British Isles into only two zones. Vaughan's scheme was heralded as a preliminary zonation (Vaughan 1905, p. 183), however, it influenced the correlation and subdivison of the Carboniferous Limestone throughout Britain and Ireland for the next seventy years with only minor refinement at its reference section (e.g. Reynolds 1921), until a formal chronostratigraphical scheme was proposed by George et al. (1976). This proposal defined a series of regional stages erected at various basal boundary stratotypes in Britain and Ireland. Yet even this standardized scheme has inherited boundaries related to some of Vaughan's zones; a major tribute to the accurate and careful observations made by Vaughan. Vaughan's grasp of the essentials of biostratigraphical theory was remarkably advanced, both in concept and practice. Today, when we take many of our stratigraphical procedures and concepts for granted, his account makes refreshing reading. It presents a remarkably mature and argued case for the erection and application of biostratigraphical subdivision of the Carboniferous Limestone which, to this day, serves as a key paper; not only for

Vaughan's zonation of the Avon Gorge, Bristol The sponsorship of a committee on the 'Life Zones of the British Carboniferous Rocks' by the British Association (BA) in the late nineteenth century added great momentum 105

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D I N A N T I A N BIO- AND C H R O N O S T R A T I G R A P H Y Carboniferous stratigraphers, but also as a pioneer example of general biostratigraphical philosophy (ibid., p.183-4). Vaughan was conscious of several important biostratigraphical zonal concepts (and their interrelationships) that we would now recognize as assemblage zones, genus zones, interzones, lineage zones and subzones. He was aware of the role of homeomorphy in taxonomic identification and demonstrated that some zones could be diachronous and facies dependent, whereas others indicated the diachroneity of certain lithostratigraphical units. His observations were meticulously linked to named sections allowing others to confirm and expand upon his zonal scheme. He was also aware that microfossils had great potential; however, these were ignored, probably because he intended that his zones could be recognized easily in the field. Vaughan was conscious that his scheme was parochial, because detailed description of other regions was unavailable, so his zones could not be tested at the time of their proposal. He predicted that his scheme would be improved as the results of studies elsewhere in Britain became clear. The first real test of this opinion was demonstrated through the work" of Garwood (1907, 1913, 1916) who described the faunal sequence in the Carboniferous Limestone areas bordering the southern Lake District in northwest England. Garwood erected his own zonal terminology for faunas underlying the D i b u n o p h y l l u m Zone, but did attempt a correlation with Vaughan's zones. This alternative zonation resulted from the discovery of stratigraphically significant faunas that were not present in the Bristol area. His studies were extended south and eastwards into the Settle area of Yorkshire Dales by Garwood & Goodyear (1924) and into the adjacent Craven Basin by Parkinson (1926). All these authors provided an uneasy correlation at certain horizons with Vaughan's zones; however, the broad framework of the Vaughanian scheme was recognizable and summarized by Garwood (1929). The faunal differences between northwest and southwest Britain were thought to result from zoogeographical provincialism. This position was embelished and adapted further by Hudson (1930) and other workers e.g. Turner (1950). This resulted in a subtle corruption of both Vaughan's scheme and Garwood's interpretation of it, with different workers using the zones in various senses, as faunas present in other regions but not recognizable in the southwest were integrated. Vaughan's scheme only addressed the subdivision of the Carboniferous Limestone facies. Basinal sequences generally lack rich coral/brachiopod faunas and zonation of these successions using ammonoids (goniatites) was initiated by Hind (1918), but it was Bisat (1924) who made the first significant breakthrough at establishing a workable zonation (see the later section on ammonoids). Micropalaeontological studies were rarely undertaken prior to the last three decades, however the impact of micropalaeontology has been to extend biostratigraphical zonation into non-marine facies and strata which lack macrofauna, or situations where only small rock samples are

107

available, such as in boreholes. These techniques have complemented and provided an independent means of testing the reliabilty of certain macrofaunal zonations.

Dinantian eustasy The modern debate on the eustatic controls upon British Dinantian stratigraphy was initiated by Ramsbottom (1973) who divided the Dinantian into six major cycles. Each cycle included a transgressive base and regressive top. Major biostratigraphical changes were introduced by each transgression. This approach echoed that invoked by Wright et al. (1927) for the Namurian (Millstone Grit) of the Rossendale area in northwest England. Ramsbottom's synthesis radically changed the way in which Dinantian sequences could be subdivided and correlated. The eustatic hypothesis was developed in subsequent papers (Ramsbottom 1974, 1977, 1981b) and lead to a hierarchical nomenclature with the original six major Dinantian cycles subdivided further into eleven mesothems (Dla, D l b etc.). Mesothemic boundary status was given to perceived regressive/transgressive boundaries, where such a boundary was associated with significant biostratigraphical change, as distinct from the numerous minor cyclical boundaries which pervade the Carboniferous Limestone facies in particular. A full critique and discussion of Ramsbottom's hypothesis and classification was given by George (1978) who was one of its main opponents. Ramsbottom's hypothesis provided a vehicle for advancing Dinantian stratigraphical practice, which still remains largely unrealized. Prior to his synthesis, only tectonic controls were considered important in producing depositional and faunal hiatuses in the Dinantian (Hudson & Turner 1930a, b; Rayner 1953, pp. 277-281). Indeed it is reasonable to regard Ramsbottom's classification as a pioneer attempt at a modern sequence stratigraphical framework for the British Dinantian. The main weakness was not the hypothesis itself, but some of the evidence and conclusions used to support it, such as the reliance on a superficial interpretation of diagenetic and sedimentological criteria which were used to recognize some of the eustatic boundaries. This was exacerbated further by insufficient or incorrect biostratigraphical data (for amplification of these points refer to the Chronostratigraphical Section of this paper). Another complication, largely ignored by Ramsbottom, is the variety of syndepositional tectonic controls which operated during the Dinantian which blur the distinction between eustatic and tectonically driven sedimentary sequences. An attempt to address this latter difficulty was advanced by Horbury (1989) in a detailed sedimentological study of a late Dinantian carbonate platform in the Morecambe Bay area of northwest England. Horbury, Concluded that it is possible to elucidate between short term glacioeustatic driven cycles (c. 0.02-0.1 Ma) and local pulsed tectonism (c. 0.4Ma). This work provided an insight into the possible glacioeustatic status of minor cyclicity, but not

Fig. 1. Dinantian classification and zonation applicable to the British Isles. The relevant range charts (Figs 2 and 3) give the key to ammonoid and conodont abbreviations. The seismic sequence boundaries are taken from interpretation of the biostratigraphical data given in the text of Ebdon et al. (1990), not their figures. Ramsbottom (1977) never clarified the boundaries of mesothems Dla, Dlb and Dlc, but Ramsbottom & Mitchell (1980) equated the Dlc mesothem with the Ivorian Stage of Belgium. Stipple ornament shows interzones (conodonts and miospores) or non-sequence (brachiopods).

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Ramsbottom's mesothems which occupy much longer time intervals, and which may have resulted from a longer term cycle of glacio-eustasy, or processes such as sea floor spreading rates or geoidal eustasy. There is still much to be gained in attempting to generate a eustatic based sequence stratigraphy for the Dinantian, if only to stimulate detailed multidisciplinary comparative studies in tectonicallyseparate terrains.

Chronostratigraphy One of the most significant conclusions of Ramsbottom (1973) was the dramatic illustration of the incompleteness of the Avon Gorge sequence, through the recognition of numerous stratigraphical gaps. Some of these hiatuses had previously been suspected by Butler (1973) and Mitchell (1972), but it was Ramsbottom's hypothesis which provided the mechanism for predicting why and where such stratigraphical gaps lay. Not only was this a demonstration of the predictive capacity of Ramsbottom's synthesis, but it also had major implications for Dinantian stratigraphy, since it explained, in terms of sequence stratigraphy, why Dinantian coral/brachiopod faunas present in northern Britain were not represented in Vaughan's zones derived from the Avon Gorge (this also affected the 'Avonian' conodont zonation proposed by Rhodes et al. 1969). Furthermore it exposed even greater differences between Garwood's (1913) and Vaughan's (1905) schemes, than those already recognized (e.g. Rayner 1953). Consequently the use of Vaughanian zones and their derivatives in different senses in separate regions, as well being used in a chronostratigraphical sense, was no longer acceptable. In this climate the resultant formal proposal for chronostratigraphical subdivision of the British Dinantian in the 'Dinantian Report' by George et al. (1976), was a great step forward. It provided the first formal chronostratigraphical classification of the British Dinantian, against which all the biozonal schemes could be compared. It also fulfilled the need to separate the conceptual principles of biostratigraphy and chronostratigraphy (although this has been ignored by many stratigraphers). This resulted in subdivision of the British Dinantian into six regional stages; the Courceyan, Chadian, Arundian, Holkerian, Asbian and Brigantian. Their characteristic fossils were summarized and each stage was defined at a basal boundary stratotype. A commentary on the recognition and correlation of the stages, supplemented with correlation charts, was also provided for each region of Britain and Ireland. Despite statements by George (1978) that the chronostratigraphical scheme was not based on Ramsbottom's cycles, it clearly was. Not only was this admitted by Ramsbottom (see discussion in George, 1978), who was a coauthor of the Dinantian Report, but the cycles were shown to correspond in the accompanying charts (George et al. 1976, table 1), and some of the stage boundaries were chosen at or adjacent to lithological horizons which fulfilled Ramsbottom's cycle boundary criteria, such as dolomites (Arundian), algal horizons (Chadian), and sandy strata (Holkerian and Brigantian). The stratotypes were located in settings where it could be predicted from Ramsbottom's model that the sequences were at their most complete in a Carboniferous Limestone facies. The only exception was the base of the Courceyan, which had to be chosen at a position which reflected the Heerleen definition (1935) of the base of

RILEY the Carboniferous (Jongmans & Gothan 1937). This definition is now superseded (Paproth et al. 1991) by the entry of the conodont Siphonodella sulcata at the recently defined basal boundary stratotype for the Carboniferous at La Serre, near Cabri~res in southern France. The then definitive ammonoid Gattendorfia subinvoluta was unknown from Britain and Ireland, so for pragmatic reasons the extinction of the late Devonian miospore Retispora lepidophyta (formerly Spelaeotriletes lepidophytus) was used. Ramsbottom (1977 & in George 1978) also considered that his mesothems were respon.sible for introducing the characteristic faunas used to recognize the stages and by implication deduced that established biostratigraphical boundaries reflected mesothemic boundaries. This approach conveniently married the chronostratigraphical framework with existing biostratigraphical zonation, particularly the Vaughanian zonation in the sense used by Garwood (1913 et seq.). Hence the base of the Chadian was chosen at what was thought to be the entry of Eoparastaffella, in continuity with the base of the Visran Series at its stratotype in Belgium. The base of the Arundian approximated to C2S~ and the bases of the Holkerian, Asbian and Brigantian stages with the $2, D1 and D2 zones respectively of Garwood (1913). It is because of this close correspondence to existing biostratigraphy that the stages can be recognized away from their stratotype sections without having to accept any mesothemic significance either conceptually or in facies interpretation. Problems do exist however if a close isochronous lateral correlation with the stratotypic stage boundaries is attempted, although recognising the presence of the stages themselves is generally easy. These problems are more severe than realized by George et al. (1976), but not as imposing as suggested by Ebdon et al. (1990), and are outlined as follows. Courceyan. The base was defined in a cliff section at the Old Head of Kinsale , Ireland (Irish Grid 16242 04069) at the junction with the Kinsale Formation and underlying Old Head Sandstone Formation. Problems with the stage do not relate to its stratotypic definition but to recommendations by Ramsbottom & Mitchell (1980) who proposed that the Courceyan be replaced with the Belgian, Hastarian and Ivorian stages, which they considered were equivalent. This practice has been adopted to some degree, but serious and valid objections were raised by Fewtrell et al. (1981a) and these are compounded further by the suspected diachroniety of Courceyan coral zones (Sevastopulo & Nudds 1987). Furthermore it is now known that the base of the Visran and the top of the Ivorian do not correspond (Conil et al. 1989, 1991). The erroneous equation of the bases of the Chadian and the Visran is still a common practice, which is further confused by the previous use of the conodont Mestognathus beckmanni as a basal Vis4an marker, now invalidated by its presence in late Tournaisian strata (Conil et al. 1991, fig. 2) and confusion of this species with the slightly stratigraphically earlier appearance of M. praebeckmanni. Chadian. The most serious difficulty of all arises with the Chadian. This stage was defined in the Craven Basin, in a well exposed road cutting at Chatburn [National Grid Refe-

D I N A N T I A N BIO- AND C H R O N O S T R A T I G R A P H Y rence SD 7743 4442], at the first lithological change below the entry of the foraminiferan Eoparastaffella. The basal boundary corresponded to what was believed to be the junction between the Horrocksford Beds and the overlying Bankfield East Beds within the Chatburn Limestone Group. Subsequent workers have failed to repeat this foraminiferal record (Fewtrell et al. 1981a, b; Riley 1990b, in press). Indeed the real entry of Eoparastaffella is in the lower part of the Hodder Mudstone Formation (Riley 1990b; Riley in Aitkenhead et al. 1991, pl.1, o,q), some 300m above the base of the Chadian Stage at the stratotype. Other diagnostic taxa, such as the brachiopod Levitusia humerosa, enter about 150 m above the base. There is no sequence boundary associated with the stage boundary, and even local lithostratigraphical correlation cannot be traced because the stage boundary does not, in fact, coincide with the base of the Bankfield East Beds (see Riley in press for a full discussion). Despite these difficulties with the early part of the Chadian, its base is widely correlated uncritically, but in reality a significant part of the stage cannot be distinguished from the late Courceyan. Riley (1990b) suggested that the term 'late Chadian' be used to convey Chadian recognized by the presence of Eoparastaffella and assessory taxa, such as Gnathodus homopunctatus, which are strictly Vis6an. Thus enabling clearly defined correlation and adherance to the original biostratigraphical concept of the Chadian Stage. Obviously there is a need for the Chadian Stage to be abandoned or restratotyped as discussed in Riley (1990b, in press).

Arundian. This was defined at Hobbyhorse Bay [SR 8880 9563], Dyfed, south Wales, in a cliff section at the junction between the dolomitized Hobbyhorse Bay Limestone and the overlying Pen-y-holt Limestone. The lowest 16 m of the Arundian stratotype lacks fauna diagnostic of this stage (Ramsbottom 1981a). The earliest Arundian is thus indistinguishable biostratigraphically from the late Chadian. This observation was confirmed by Simpson & Kalvoda (1987) who considered the dolomitic top to the Hobbyhorse Bay Limestone was secondary dolomite; however this interpretation does not necessarily detract from a primary depositional differentiation between these units. Whether the stratotype boundary is a sequence boundary remains to be confirmed, however Simpson & Kalvoda (1987) considered the entire section to represent progressive bathymetric deepening. It seems pertinent to redefine the base of the Arundian at the first entry of primitive archaediscids as suggested by Davies et al. (1989); such a procedure is possible without relocating the stratotype. Holkerian. This is defined at Barker Scar [SD 3330 7827], near Holker Hall, on the northern shore of Morecambe Bay, Cumbria, at the junction between the Dalton Beds and the overlying Park Limestone. Recent mapping by I. C. Burgess (pers. comm. 1992) suggests that the sandy strata referred to the Davidsonina carbonaria Beds by Garwood (1913) present in the Ravenstonedale district, within the lower part of the Ashfell Limestone, are absent due to non-sequence at Barker Scar. This non-sequence is represented 2 km to the west of the stratotype by a palaeosol, and it is suspected that this horizon lies in the sandy interval below the stratotypic boundary at Barker Scar (at the base of bed I, in Ramsbottom 1981a). This observation parallels

109

suspicions raised from BGS mapping in thick sequences in Northern Ireland and north Wales by W. I. Mitchell and J. R. Davies respectively. These areas contain corals intermediate in character between Siphonodendron and Lithostrotion ('cerioid tendency') associated with late Arundian foraminifera. This transition fauna is not recorded at Barker Scar. Further study is still required, however it seems likely that a considerable non-sequence is developed at Barker Scar and that the stratotype will require relocation.

Asbian. This is defined in a hillside outcrop at Little Asby Scar [NY 6988 0827] in Ravenstonedale, northwest England, at the junction of the Ashfell Limestone and the overlying Potts Beck Limestone. The original record of the diagnostic coral Dibunophyllum, from the basal bed has never been repeated, despite intense searching along a coral bed which is laterally well exposed for many hundreds of metres (Patterson pers comm. 1991). Strank in Ramsbottom (1981a) recognized the first characteristic Asbian foraminiferal entry at 19.6 m above the base of the section, and this corresponds to the earliest repeatable occurrence of Dibunophyllum in the section. Clearly there is a case to redefine the Asbian boundary at this horizon, but there is no need to relocate the stratotype. Brigantian. This is defined in a stream gorge at Janny Wood [NY 7832 0375], near Dent, northwest England at the base of the Peghorn Limestone. Several workers including; Burgess & Mitchell (1976); Pattison in Frost & Holliday (1980); Somerville & Strank (1984b) and Wilson (1989), have noted that some D2 Zone macrofossils occur in a transitional fauna in late Asbian rocks. Thus some of the characteristic Brigantian and D2 macrofossils listed by George et al. (1976) and Burgess & Mitchell, such as Lithostrotion maccoyanum, Lonsdaleia duplicata, L. floriformis and Pugilis pugilis are not restricted to that stage, or overlap with characteristic Asbian faunas. This problem is particularly characteristic north of the Askrigg Block and in Scotland, but has also been encountered in Derbyshire and north Wales (Chisholm et al. 1983; Somerville & Strank 1984b). However, it must be stressed that these difficulties usually relate to only a short interval around the Asbian/Brigantian boundary. Bearing in mind the excellent ammonoid stratigraphy available in the late Dinantian, it seems reasonable to relocate and redefine the Brigantian in an ammonoid bearing sequence, if this transition coral/brachiopod fauna is to be avoided in a definitive stratotype.

Seismic sequence stratigraphy A seismostratigraphical classification for the Dinantian of northern England was proposed by Ebdon et al. (1990), and further developed by Fraser et al. (1990) and Fraser & Gawthorpe (1990). These studies also provided an interpreted sequence stratigraphy, with seismic sequence boundaries corresponding to diastems or equivalent correlatable surfaces in laterally conformable successions. The overlying Silesian Subsystem was also treated in a similar manner. Ironically this scheme also subdivided the Dinantian into six sequences, given the notations EC1 to EC6, with EC1 commencing in the late Devonian.

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However, the boundaries between these sequences do not always coincide with those of Ramsbottom's (or between Ebdon et ai. 1990 and Fraser & Gawthorpe 1990) and are conceptually different, being derived largely from seismic evidence in the East Midlands, supplemented with stratigraphical data from key boreholes and surface sections. Indeed these authors attributed the driving mechanism for their sequences entirely to tectonic controls linked to syndepositional rifting in the basement ('syn-rift megasequence') and did not consider eustacy as relevant except at the subsequence scale, refering to Hubbard's (1988) study of Jurassic and Cretaceous passive plate margins as a justification for this point of view. These studies revitalized the subdivision of the Dinantian into tectonic sequences; a practice which was introduced by Hudson & Turner (1933a, b) but not accommodated into the scheme of Ebdon et al. (1990). Ebdon et al. (1990) suggested that their seismic sequences might provide a more applicable chronostratigraphical subdivision of Dinantian strata in northern England and the Midlands, with the possibility of using them to redefine the existing stages. However, this is unlikely to be realized because of the coarse resolution of seismic stratigraphy. For example the basal part of Sequence EC3 in the Craven Basin was correlated with the base of the late Chadian by Ebdon et al. (1990), but with the early Arundian by Frazer & Gawthorpe (1990). When compared with the depositional sequence stratigraphy described by Riley (1990b), the reason for this divergence of correlation becomes clear; there are two unconformity surfaces within this interval and these can be resolved biostratigraphically in borehole and field examination, but not seismically. This argument will be developed further elsewhere, but it is interesting to note that in the Namurian, the sequence boundary associated with the Mid-Carboniferous Boundary (Riley et al. 1987), which is a well constrained boundary in north Africa, Eurasia and the USA, providing the basis for international subsystem divison of the Carboniferous and which must be eustatic, is not recognized in the seismic sequence stratigraphy published by these authors. Thus seismic sequence boundaries, in the above examples, fail to achieve as near an approximation to chronostratigraphical boundaries as is currently reached using biostratigraphical markers and appear to miss sequence stratigraphical events which are internationally significant.

Biostratigraphy of the British and Irish Dinantian The following section is intended as a guide on the current status of biostratigraphical schemes. Ranges of taxa are derived principally from the published data referred to in the relavent sections, supplemented in the case of foraminifera and trilobites with the author's observations.

Ammonoids The species ranges of selected late Dinantian ammonoids in the British Isles are given in Fig. 2. Ammonoids characterize hemi-pelagic sequences. They are rarely associated with coral/brachiopod faunas except in bioherms, within limestone turbidite sequences and in some peritidal settings, where conditions favoured local post-mortem accumulations such as along strand lines. Their zones are globally

RILEY applicable, reflecting their nekto-pelagic habit and in the late Dinantian, as in the overlying Namurian, they provide the highest biostratigraphical resolution of any fossil group. Very little is known of the early Courceyan ammonoid faunas in Britain and Ireland. Ammonoid zonation of this interval has been refined by Kullmann et al. (1991), based mainly on German sections. The base of the Carboniferous is no longer defined under the terms of the Heerlen decision of 1935 (Jongmans & Gothan 1937), which used the entry of the ammonoid Gattendorfia subinvoluta, but by the entry of the conodont Siphonodella sulcata (Paproth et al. 1991). According to Korn (1986), this Lies within the Imitoceras prorsum Ammonoid Zone. Matthews (1983) recorded I. cf. prorsum from the basal Courceyan Castle Slate Member in County Cork, Ireland. Goldring (1955) reported Gattendorfia crassa from his faunal division 'B' in the Pilton Shale Formation (according to Bartzsch & Weyer 1988, this ammonoid falls within the lower part of the Siphonodella sandbergi Conodont Zone). In the overlying division 'C' he recorded lmitoceras sp.. Butcher & Hodson (1960) illustrated Hammatocyclus aft. homoceratoides from the overlying division 'D' of Prentice (1960), this lies within the Landkey Formation or the basal part of the Tawstock Formation (Heddon Member, Jackson 1991) of late Courceyan or early Chadian age. Riley (in Edmonds et al., 1985) recorded Protocanites from beds correlated by Jackson (1991) with the Landkey Formation. Undescribed Kazakhstania sp. is known from the uppermost part of the Courtmacsherry Formation at Ringabella Point in Ireland. This record is believed to lie within the upper part of the Siphonodella Conodont Zone (Sevastopulo in an unpublished field guide, Palaeontological Association 1987). Matthews (1970) described a unique fauna from east Cornwall which included Kazakhstania sp. (Gattendorfia of Matthews 1970), Muensteroceras complanatum, M. cf. rotella and Pericyclus princeps, together with unspecified pericyclids (incorrectly referred to Ammonellipsites and two new, undescribed genera; a Gattendorfiinid (Gen. nov. A) and a Pseudarietitinid (Gen. nov. B) (Bartzsch & Weyer 1988). No conodont fauna is associated, hence the precise age within the Courceyan is unknown. The type material of P. princeps is thought to come from the Calcaire de Vaulx et de Chercq in Belgium. Paproth et al. (1983) assign this to the Ivorian (late Tournaisian) on macrofaunal grounds. M. complanatum comes from the lower part of the overlying Calcaire de Calonne of late Ivorian age (Paproth et al. 1983). M. rotella is also known from these units, which were referred to Tn3c by Del6pine (1940), (includes the Scaliognathus anchoralis Conodont Zone and the upper part of the underlying Polygnathus communis carina Conodont Zone, down to the appearance of Eotaphrus cf. bultyncki). If Matthews' (1970) identifications are correct, then this implies a slightly younger age for the upper range of Kazakhstania than is currently accepted. There are unconfirmed records of Protocanites from the Lower Limestone Shales in the Gower region of South Wales (George 1969). Riley (1991) reviewed the British and global distribution of mid-Dinantian ammonoids (late Courceyan to Holkerian), revised the Fascipericyclus-Ammonellipsites Ammonoid Zone of Ramsbottom & Saunders (1985) and erected a new successive zone, the Bollandites-Bollandoceras Ammonoid Zone. The overlying Beyrichoceras Zone was redefined and

D I N A N T I A N BIO- AND C H R O N O S T R A T I G R A P H Y STAGES ZONES B. micronotum Beyrichoceras aranaeum Beych.redesdalensis Bt. sulcatum O. gilbertsoni E. @rimmed G. hudsoni Michiganites hested N. rotiforme N. vittiger Prolecanites discoides B. excavaturn

B. globosum B. mk:ronotoides B. submicronotum Beych. founieti Beych. implicatum Beych. stenolobus Beyrichoceras delicatum Beyrichoceras obtusum Beyrichoceras rectangularum Beyrichoceras vesciculiferum Bt. castletonensis Bt. umbilicatum G globostdatus Girtyoceras deani Girt.yoceras discus Girtyoceras simplex l lrinoceras omatissimum IN. spirorbis Parad. pseudodiscrepans Praedarelites culmiensis Pronorites cyclolobus Be),ch. truncatus G. crenistria G. fimbriatus Girtyoceras meslerianum Girtyoceras platiforme Girtyoceras premeslerianum Amsb. falcatus G. concentricus G. spirifer Girtyoceras cowdalense Parafl. striatus

Asb. (I.)

Brigantian

B2o B2b P l o Plb P1c Pld P2o P2b P2c X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X

STAGES ZONES Arnsl~. robustus Arnsb. sphaericostriatus Amsb. waddingtoni Arnsb. warslowensis D. kathleenae Girtyoceras brueningianum 14. carraunense H. waldeckense K. hawkinsi Neoglyphioceras spirale Parag. bisati Parag. elegans Parag. kaflovecense Pronorites ludfordi S. recdina S. turnen H. hibemicus H. mediocris H. posthibemicus H. ramsbottomi H. tumida Meta. hodsoni . Parag. koboldi Parag. rudis Lusit. granosus Neoglyph. caneyanum S. crenistriatum S. subtile Girtyoceras multicamera turn Girt),oceras weetsense Meta. varians Neoglyph. subcirculare Parad. marioni S. adeps S. delepinei S. newtonense S. ordinatum S- procerum S. splendens S. stolbergi Girtyoceras shorrocksi Girtyoceras waitei Lyrogoniatites georgiensis Meta. plicatifis

111 Asb. (I.)

Brigantian

B2o B2b Plo Plb =1cl Pld P2o P2b P2c X X X X X X X X X X X X X X off. X X X off. X X X X X X X X X X X X X X X X x X X X X X X X X X x x X X X X

Fig. Z. Ranges of selected ammonoids in the late Dinantian of the British Isles. Abbreviations: Arnsb., Arnsbergites; B., Bollandoceras; Beych., Beyrichoceratoides ; Bt., Bollandites, D., Dimorphoceras ; E., Entogonites ; G., Goniatites ; H., Hibernicoceras ; K., Kazakhoceras, Lusit. , Lusitanoceras ; Meta. , Metadimorphoceras, Neoglyph. , Neoglyphioceras ; Parad. , Paradimorphoceras ; Parag. , Paraglyphioceras ; S. , Sudeticeras, Asb (1.), late Asbian. Sources are given in the text.

revised by Riley (1990a). The account of Ramsbottom (in Earp et al. 1961) provides a full summary of the P~, to Pzc zonal sequence in it's type area, the Craven Basin, northwest England. This scheme was developed principally by Bisat (1924, 1928, 1934, 1952, 1955, 1957), Moore (1930, 1936, 1939, 1941, 1946, 1950, 1952, 1958) and Moore & Hodson (1958). Korn (1988) has revised the taxonomy of late Dinantian ammonoids. The stratigraphical distribution of late Dinantian ammonoids is summarized in Fig. 2.

Bivalves. As in the Namurian it is the marine bivalves present in the hemipelagic mudstone facies of the Dinantian which have the greatest stratigraphical value. These assemblages accompany the ammonoids and are subject to a similar distribution. The taxonomy and detailed stratigraphy of many Dinantian bivalves requires careful scrutiny before their stratigraphical potential can be realized. Non-arine bivalves, fundamentally important in the zonation of late Namurian and Westphalian sequences, are virtually restricted to the Scottish Midland Valley and the

Northumberland Trough, and are of little biostratigraphical value in a Dinantian context. The posidoniid Karadjalia is characteristic of some Courceyan hemipelagic sequences in southwest England. In the earliest Vis6an (late Chadian, FA Ammonoid Zone) the mytiloid Aviculomya occurs. Undescribed species of Posidonia and Dunbarella enter in the early part of the BB Ammonoid Zone. In the Beyrichoceras Ammonoid Zone, Posidoniella vetusta is characteristic of 'knoll reef' settings. In the hemipelagic mudstones, Actinopteria persulcata, Dunbarella persimilis, Posidonia kochi and P. corrugata enter in this zone; the latter species and A. persulcata extending into the Arnsbergian. Posidonia becheri is abundant in the Plb and Plc ammonoid zones. This species last occurs in the P~d Ammonoid Zone, where it is rare, but is joined by abundant P. membranacea and P. trapezoedra, which continue their range into the Pendleian.

Conodonts A range chart of selected conodonts is given in Fig. 3. Conodonts are present in nearly all Dinantian marine facies,

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STAGES

•

ZONES

spic.

P. sp~atus Pa. variab#is Ps. dentilineatus P. inornatus

B. acu/eatus aculeatus B. aculeatus plumulus CL unicomis

P. communis communis S. isosticha E, laceratus G punctatus G delicatus P. co~-amuniscafina G. cuneiformis Pr. owent H? cf. cristulus D. bassi B. sl~nulicostatus Ps. multistriatus Ps. pinnatus M. groessensi Ps. minutus P. mehli lMus Eo. bultJ/ncki P. mehli mehli D. bouckaeai ",4." petilus

i o,.(.,ich.(,., ,r. i

Courceyan

X X X X X X X X

I

I

in.

J

I

I

_ Ps. multistriatus

X X X X

X X X X

X

X

I=

lanc 'l, --'.

Polygnathus mehli

X

X

X

• I:I

I:I

X

I G. bilineatus

L. commutata

L. n'~Do.

coll.

X

X

X

~:~

I

X

X

I

X

X

X

X

X

X

X

X

X

X X

X X

X X

X X

X X

X X

X X

X X

X X

X X X X

X X X X

X X

X

X

X X X

X

X

X

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Era. asymmetricus C unicomis G. austini C. c#sratus C. regularis G. gin'yi girtyi G. praebilineatus G bilineatus L. mononodosa M txp/tm G. ginyi collinsoni

X X X X X

X X X X X

Fig. 3. Ranges of selected conodonts in the Dinantian of the British Isles (modified from Varker & Sevastopulo 1985). Stipple denotes interzones. Abbreviations: genera--'A.', Apatognathus ; B. , Bispathodus ; C. , Caousgnathus ; CI. , Clydagnathus ; Clo. , Cloghergnathus ; D., Dollymae ; Do., Doliognathus ; E., Elictognathus ; Eo., Eotaphrus ; Em., Ernbsaygnathus ; G., Gnathodus ; Ge., Geniculatus ; H. ?, Hindeodus ?; L. , Lochriea ; M. , Mestognathus ; N. , Neoprioniodus ; P. , Polygnathus ; Pa. , Patrognathus ; Pr. , Prioniodina ; Ps. , Pseudopolygnathus ; S., Scaliognathus ; Si., Siphonodella ; T., Taphrognathus. Zones---anch /b. ; S. anchoralis / P. bischo~fi ; bouc., D. bouckaerti; bul. ; Eo. bultyncki; cf. bul., Eo. cf. bultyncki ; coll., G. girtyi collinsoni ; has., D. hassi ; hom., G. homopunctatus ; in., P. inornatus / Siphonodella ; lat., Do. latus ; L. mono., L. mononodosa; prae., M. praebeckmanni; spic., P. spicatus Stages--Ch. (e), early Chadian; Ch. (1.), late Chadian; Ar.,

Arundian; Ho., Holkerian; As. (e.), early Asbian; As. (1.), late Asbian. but particular taxa are facies selective, leading to parallel zonation schemes in separate facies and regions. They do require digestion of considerable quantities of rock (on average greater than 1 kg) for a representative assemblage. The best preserved routine preparations come from limestone turbidites, but hemi-pelagic shales can generate

the highest abundances. As with miospores, conodonts change colour in response to their thermal burial history, rendering them useful in assessing hydrocarbon maturity. A very thorough review of the development of conodont zonation, stratigraphical ranges and facies distribution was given by Varker & Sevastopulo (1985) to which the reader is

D I N A N T I A N BIO- AND C H R O N O S T R A T I G R A P H Y referred. In addition the Scottish Dinantian has yielded exquisitely preserved material which has been vital in understanding the biology of these previously enigmatic chordates (Aldridge et al. 1986). Because conodonts were nekto-pelagic they have a global marine distribution rivalled only by the ammonoids, and are therefore particularly significant in international correlation. For this reason the base of the Carboniferous has been redefined, from the original Heerleen ammonoid based definition, to coincide with the entry Siphonodella sulcata as noted already. Several important studies have been published since Varker & Sevastopulo (1985), notably Austin (1987) and Stone (1991) on the Arundian stratotype and adjacent sections and Armstrong & Purnell (1987) on the Northumberland Trough. There have also been important taxonomic papers which modify late Courceyan and early Visran zonation, in particular that of Von Bitter et al. (1986), on Mestognathus, and Belka (1985) who introduced new gnathodid taxa. Conil et al. (1991) has revized conodont distribution in the Belgian Dinantian and these studies together with those on the Tournaisian/ Visran boundary interval in Britain by Riley (1990a and in Chisholm et al. 1988) have significantly improved our knowledge on the distribution of key taxa such as Scaliognathus anchoralis, Mestognathus praebeckmanni, and Gnathodus homopunctatus in relation to the Chadian and Vis6an boundaries in Britain. The accompanying range chart (Fig. 3) is a modification of that given by Varker & Sevastopulo (1985) and reflects these developments.

Coral /brachiopod zonation The ranges of selected coral and brachiopod taxa in the British Isles are given in Figs 4,5,6,7. Coral/brachiopod assemblages dominate the Carboniferous Limestone macrofauna. Their main advantage is that they have been recorded where relevant, in most stratigraphical studies published this century, especially memoirs accompanying maps by the British Geological Survey. These assemblages are usually easily seen in the field and in borehole core and are particularly suited to field mapping techniques. However, being large, non-mobile benthos they are very susceptible to facies controls and are often distributed at discrete horizons, being absent from much of the strata which may lie within their zonal range. As already stated, Vaughan's (1905) pioneer work established a zonation for southwest Britain, whereas Garwood (1913) initiated a zonation for northern England. Their schemes were embelished, modified and geographically extended to the rest of the UK and Ireland by subsequent workers. The reader is referred to George (1958, 1969), George et al. (1976), Green & Welch (1965, p. 15) and Rayner (1953) for a full summary of this phase in the development of British coral/brachiopod zonation and to George et al. (1976) for stratigraphical comparisons. The Courceyan zones were revised by Ramsbottom & Mitchell (1980), Mitchell (1981) and Sevastopulo & Nudds (1987) as assemblage zones. Mitchell (1989) gave a summary of the distribution of rugose coral faunas from the Chadian to Brigantian interval in Britain, giving alphabetical notations for each fauna. This study did not use the classical zonal divisons of Vaughan (1905) and Garwood (1913). The distribution of heterocorals was summarized by Sutherland & Mitchell (1980). Recent studies on brachiopods include

Courceyan

STAGE Zones

V. vetus Cyathaxonia comu F. densum F. omaliusi M. favosa M. konincki M. megastoma Sy. clevedonensis Sy. konincki Z delanouei Z vaughani AL burringtonensis Ax. simplex C. patulum greeni C. patulum patulum Ca. comucopiae Cravenia tela Cy. modavensis K. tortuosum Am. cravenensis Ca. gigantea Corwenia vaga F. ambi~uum K. praecursor SL cylindrica Sy. hawbankensis

113

V. vetus

early Chadian

Z. delanouei C. patulum

X X X X X x x x x x x x x..... x x x x

S. cylindrica

X X x x

x x x x x x x x x x

Fig. 4. Ranges and of selected corals in the Courceyan and early Chadian (Tournaisian) of the British Isles. The correspondence of the S. cylindrica Zone with the base of the Chadian is schematic. Abbreviations: AI., Aulophyllum; Am., Amplexicarina; Ax., Axophyllum; C., Caninophyllum patulum; Ca, Caninia gigantea; Cy., Cyathoclisia; F., Fasciculophyllum, K., Koninckophyllum; M., Michelinia; Si., Siphonophyllia; Sy., Sychnoelasma; V., Vaughania. The sources are given in the text.

Brunton (1984) which reviews earlier work, Brunton & Mundy (1986, 1988a, b) and Brunton & Tilsley (1991). Considerable data on coral/brachiopod distribution is included in the numerous BGS memoirs which describe Dinantian sequences. Where modern comparative work with other biostratigraphical schemes has been carried out, for instance by Sevastopulo & Nudds (1987), between conodont distribution and the Courceyan coral zones in southwest Britain and Ireland, a marked lateral diachroneity exists. These authors interpreted this as a limited facies tolerance of the corals, vindicating George's (1958, p. 236) awareness of the crude chronostratigraphical significance of certain Courceyan corals. Similar problems occur in the Vis6an, for example the entry of Dorlodotia briarti was interpreted as a late Arundian event (Nudds 1981); however, my own observations show that this entry at Ravenstonedale in northwest E n g l a n d is in fact accompanied by Cf4tr2 foraminiferal assemblages which indicate a late Chadian age (the correlation of the base of the Arundian below the Brownber Pebble Beds by George et al. 1976 was erroneous due to a wrongly labelled thin section supplied by R. Conil; M. Coen, pers. comm. 1991). Conversely the entry in Belgium is apparently associated with Cf46 Subzone foraminifera, upon which the original late Arundian interpretation of the British record was based. When one considers that D. briarti is found predominantly at inner carbonate ramp settings it is clear that these anomalies reflect the narrow facies tolerance for this coral.

114

N.J.

STAGES

Assemblages Ax. simplex Ca. oomucopiae Carcinophyllum simplex Carruthersella compacta Clis. r~idum Clis. incjletonense Cravenie tela Cyax. rushiana D. briarfi D. pseudovermiculare K. clitheroense K.. c2/atho~¥lloidee K. meathoper~e K. vesioulosum M. megastoma P. murchisoni Si. c)4indrica Spirophyllum praecurser Sy. urbanowitschi Clis. multiseptatum Cravenia lamellata Hap. subcinica Si, caninioides Si. ~arwoodi ~'. hefonensis Si ? ciliate Sph. martini Solenodendron horsfieldi Sy. konincki Amp/, enniekilleni K. cartyanense M. tanuisepta Siph. caswellense Amplexizaphrentis ashlellensis Ax. mendipense K. ashfellense K. Ir~ila Siph. sociale

Chodlan 0ate) A X X X X X X X X X X X X X X X X X X X

Aiur~lan B X

C

X

X

X

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X

E

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G

STAGES

I~'lgontlan H

I

J

K

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Assemblages C. bristeliense "Caninia"iuddi C/is. r~idum Di. smithi L. arachnoideum L. araneum L. portlocki Siph. muttiradiale Siph. scaleberense AI. redesdalense C/is. keyserlin~i Dib. bourtonense K. vau~hani Si. benburbensis Siph. pnceum Siph. pauciradiale Dib. biparfitum ~'ph. fasciculatum H~Dlolasmadense L. maccoyanum Solen. furcatum Act. floriformis AmpI. derbiensis Aul. pach,yendothecum Clis, delicatum Dph. lurcatum D~h. lateseptatum K. ma~lnificum K. pmprium Lor~dalia duplicate Palastraea retie Cotwenia ru~osa K. interruptum Nemistium edmondsi Orionastraea ensiler Orionastraea placenta Orionoastraea indivisa Slim. slimonianum

Chodlan Oate)

A

Arundlan B

C

D

Ho&lerlon E X X X X X X X X X

I~lganllan

Asblan F

G

H

I

X X

X X

X

X

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X X X X X X X X X X X X

X X X X X X X X X X X X X X el, X X

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J

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K

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X

X X X X X X X O~.

X X X X X

X

Fig. 5. Ranges of selected corals in the late Chadian to Brigantian (Visran) of the British Isles modified from Mitchell (1989). Abbreviations as Fig. 4, except: Act., Actinocyathus; Ampl., Amplexizaphrentis; Car., Carcinophyllum ; Clis., Clisiophyllum ; Cyax., Cyathaxonia ; D., Dorlodotia ; Di, Diphyphyllum ; Dib. , Dibunophyllum ; Hap., Haplolasma ; L. , Lithostrotion ; Siph. , Siphonophyllia ; Slim., Slimoniphyllum.

Sources are given in the text. These observations illuminate the fact that, as George (1958) pointed out, coral/brachiopod sequences have great local value (faunal bands of Garwood 1913), but because occurrences are linked strongly to lithology and are often the result of fortuitous, accidental preservation, precise correlation between disparate regions is difficult. This is borne out by the upward stratigraphical extension of many of Vaughan's original guide taxa by subsequent workers and the confusion of Vaughan's zonal notations as already discussed. A further problem is that Vaughan's scheme, was based on a sequence in which major non-sequences occur as already discussed in the Chronostratigraphy section.

Flora Calcareous algae are abundant in carbonate rocks but little is known of their stratigraphical distribution. The extinction of K o n i n c k o p o r a is used as a basal Brigantian marker event. Davies et al. (1989) demonstrated that bilaminar K o n i n c k o p o r a enters in the late Chadian and not, as was thought, in the Arundian. Riley (in p r e s s ) has discovered monolaminar K o n i n c k o p o r a in the late Courceyan at the Chadian stratotype; previously this form was used as a Chadian marker.

Terrestrial macrofloral stratigraphy is poorly known and was summarized by Scott (1984) and Scott et al. (1984). Important papers on the phytogeography of Dinantian floras include Raymond (1985) and Rowley et al. (1985). The most important floral components in a stratigraphical sense are miospores. These have the advantage of wide dispersal and are unique in that they can be used routinely to correlate between non-marine and marine sequences. They are particularly abundant in coal, palaeosols and finegrained terriginous clastics. Because they can be extracted from small volumes of rock, they are especially suited to borehole sampling. Offshore hemipelagic shales and most carbonate-dominated sequences however, tend to yield sparse or unidentifiable miospore assemblages. Knowledge about miospore palaeoecology and parent flora is improving but is not comprehensive. Where it is possible to compare miospore zonations with marine faunas the relationship between them appears to be consistent at least within Britain, Ireland and the nearby continent. Miospore zones in the early Courceyan and late Visran rival the resolution achieved by some faunal zones. Another feature, as with conodonts, is their colour preservation in response to thermal history during burial; this characteristic renders them important in assessing hydrocarbon maturity.

D I N A N T I A N BIO- AND C H R O N O S T R A T I G R A P H Y

STAGES

early Chadian

Courceyan

V. vetus X X X X X X X X X X X

Zones Av. schmidti, Ch. failandensis Cleiothyridina roysii M. mitcheldeanensis Or. spinulifera Plic. stoddarti Pr. fremingtonensis " Pu. subpustulosa Spinoc. b,,assa Str. paeckelmanni Unisp. tomacensis Br. wexfordensis CleL glabistra (P.) CleL glabistra (V.) .. Dict. multispinife..rus Pugilis vaughani Pu. tenuipustulosus Rug. vaughani Schw. aspis Syr. cyrtorhyncha Athyris expansa Eom. derbyensis Megach. magna Palaeocho. cinctus Tylothyris laminosa Acanth. mesoloba A vonia youngiana Composita ambigua De/. comoides DeL destinezi Del. notata Dict. semireticulatus Levitusia humerosa , .P.licatifera plicah'lis Pu. nodosus Pu. pyxidiformis Retie. bellmanensis 'Spir. furcatus Spir. bollandensis Spit. copIowensis Spit. konincld Syr. elongata .

Z. delanouei c. patulum

x x

....

....

x x x x x x x x x x x

x x x

.

x

.

, ,

,,,

......

.

x

....

.

si Cylindric.

x x x x x

x x

....

x x x x x x x x x x x x x x x x x x

Fig. 6. Ranges of selected brachiopods in the Courceyan and early Chadian (Tournaisian) coral zones (Fig. 4). Abbreviations: Acanth., Acanthoplecta; Av., Avonia; Br., Brochocarina; Ch., Chonetes ; Clei., Cleiothyridina; Del., Delepinea ; Dict., DictyocIostus ; Eom. , Eomarginifera ; M. , Macropotamorhynchus ; Megach., Megachonetes; Or., Ovatia; Palaeocho., Palaeochoristes; Plic., Plicochonetes; Pr., Productinella; Pu., Pustula; Retic., Reticularia ; Rug., Rugosochonetes ; Schw. , Scheilwienella ; Spinoc. , Spinocarinifera; Spir., Spirifer; Str., Strophonema; Syr., Syringothyris; Unisp., Unispirifer. Sources are given in the text.

A selection of miospore ranges and their zones is given in Fig. 8. A complete miospore zonation of the British Dinantian was first proposed by Neves et al. (1972) and expanded in Neves et al. (1973). Subsequently Clayton et al. (1977, 1978) and Clayton (1985) developed this scheme. Higgs et al. (1988a, b) proposed new zonal divisions for the Courceyan. The reader is referred to these original papers for details of the ranges of taxa and references to earlier publications. The following account serves to compare selected miospore zonal boundaries with faunal and chronostratigraphical ones.

115

(i) Vallatisporites verrucosus-Retusotriletes incohatus (VI) Zone. This zone correlates closely with the base of Carboniferous and is the definitive base of the Courceyan Stage. It is associated with Siphonodella sulcata (definitive of the basal Carboniferous) and the Acutimitoceras prorsum ammonoid fauna in the Rhenish Slate Mountains (Higgs & Streel 1984). In southern Ireland, Matthews (1983) recorded A. cf. prorsum from this zone at Nohoval Cove, close to the Courceyan Stratotype. (ii) Kraeuselisporites hibernicus- Umbonatisporites distinctus (HD) Zone. Higgs and Streel in Higgs et al. (1988a) note that the base of this zone lies in the Peracuta Shales (base Tn2a) in Belgium, and Higgs & Streel (1984) recorded H D zone assemblages in the lower part of the Siphonodella crenulata Conodont Zone in Germany. The upper limit of the zone is unknown in terms of faunal biostratigraphy. (iii) Spelaeotriletes balteatus-Rugospora polyptycha (BP) Zone. The precise age of the base of this zone is unknown; however the top lies in the upper part of the Polygnathus spicatus Conodont Zone in Ireland. (iv) Spelaeotriletes pretiosus-Raistrickia clavata (PC) Zone. In Ireland the base of the zone lies just below the Polygnathus inornatus Conodont Zone and its top within the Pseudopolygnathus multistriatus Conodont Zone. (v) Schopfites claviger-Auroraspora macra (CM) Zone. In Ireland the base of the CM Zone lies in the lower part of the Polygnathus mehli Conodont Zone, resulting in an interzone between the CM Zone and the underlying PC Zone. The miospore characteristics of this interzone are poorly known. The appearance of the eponymous taxon Auroraspora macra is in the underlying PC Zone, but it is uncommon until the CM Zone. (vi) Lycospora pusilla (Pu) Zone. The base of this zone remains undefined in terms of precise faunal biostratigraphy. The early part of the zone differs only from the underlying CM Zone by the appearance of the eponymous taxon Lycospora pusilla. The lack of accessory taxa to identify the zone means that its base is relatively more sensitive to facies; a situation exacerbated by the rarity of the zonal guide in the lower part of the zone. Certainly definitive basal Vis6an microfaunas in Denmark (Bertelsen 1972) and north Wales (Somerville et al., 1989) contain good Pu Zone assemblages, suggesting that the basal part of the zone in which L. pusilla is rare lies within the latest Tournaisian. Higgs et al. (1988b) gave this lowest interval with rare L. pusilla subzonal status. (vii) Knoxisporites triradiatus-K, stephanephorus (TS) Zone. This zone was introduced by Clayton (1984) and assigned a late Arundian to mid-Holkerian age. No supporting evidence for this correlation was submitted with the original definition. Higgs (1984, fig. 3) equated the zone with the Holkerian Stage in northwest Ireland', however comments (ibid. p.191) later in this paper appear to corroborate the correlation given by Clayton (I984) and Higgs et al. (1988b). (viii) Perotriletes tessellatus-Schulzospora campyloptera (TC) Zone. The base of this zone may lie within the late Holkerian as commonly followed; however Gueinn (in Frost & Holliday, 1980) quoting unpublished work by Williams, implied that the lowest TC Zone assemblages in the Archerbeck

116

STAGES Assemblages Acanth. meso/oba CIr. ~b~'ar,, (P.) Composita gregaria Eom. derbiensis Levitusia humerosa Plicatifera p/icatilis Refic. bellmanensis Spit'. bollandensis Lamdarina manifoldensis "Camarat. " fawcetfensis Cornposita ambi~ua Dict. multispiniferus Pu. pyxidiformis Spir. furcatus Steno. isorhyncha Synng. cuspidata Syring. elongata

Comp. flco~ea De/. carinata . Echinoconchus punctatus Lino. hemisphaerica Mecjach. zimmermanni Megach. papillionacea Prod. garwoodi Del. notata Davidsonina carbonaria Oaviesiella derbyensis Daviesiella langollensis Lino. corrugatohemisphaedca Lin~rotonia ashfellensis Broch. wexfordensis Gi~. maximus Gig. tulensis Latip. latissimus Punctospirifer scabricosta T),lothyris laminosa Alitaria panderi Davidsonina septosa Del. comoides Fluctuaria undata Gig. inflatus Gig. crassiventer Gig. dentifer Gig. edelbur~ensis Gig. janischewskina Gig. semiglobus Prod. reclesdalensis Prouctus Droductus Pugilis pu~ilis Pu~ilis scoticus Semiplanus semiplanus Eom. cambriensis Gig. 9aylensis Gig. gigantoides Gi9. okensis Striatifera striata Gig. elongata Gi~. varians

N.J. Ic~e Chodk:m A

X X X X X X X X X

AnJndlan B

C

D

Holke~lan E

RILEY Bdgonflan

Asl:~n F

G

I

J

K

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X X X X X X X X

X

X X X X X X X X

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lmmmlJ

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nunm-n

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X X X X X

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X X

X X

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X X

Borehole lay some 10 m below the Burns Beds, correlated by George et al. (1976) within the early Asbian. Owens (in Ramsbottom, 1981, p.l.9) listed a TS Zone assemblage l m below the base of the Asbian at Hassler Brow, near to the Asbian stratotype. The base of the TC Zone therefore probably lies within the early Asbian. (ix) Raistrickia nigra-Triquitrites marginatus (NM) Zone. The eponymous taxa and other accessories used to recognize this zone are rare in the early part, hence the precise

X X

X X

X

X

X

X

X X X

X X

X

Fig. 7. Ranges of selected brachiopods in the late Chadian to Brigantian (Vis6an) coral assemblages (Fig. 5) of the British Isles. The only Holkerian record of Palaeosmilia murchisoni is in Ramsbottom (1981a). Abbreviations as Fig. 6, except: Camarat., Camaratoechia; Comp., Composita; Gig., Gigantoproductus ; Latip. , Latiproductus ; Lino., Linoproductus ; Prod., Productus, Steno., Stenoschisma. Sources are given in the text.

chronostratigraphical position of the zonal base is somewhat diffuse. Clayton et al. (1978) subdivided this zone into the Tripartites distinctus-Murospora parthenopia (DP) and Murospora margodentata-Rotaspora ergonulii (ME) subzones. NM zone assemblages were recorded by Hibbert & Lacey (1969) from the Menai Straits area of North Wales, in basement beds which were thought to be overlain by early Asbian faunas, however according to J. Davies (pers. comm. 1992) the overlying limestones are of late Asbian, or early Brigantian age. Currently the base of NM Zone is correlated with the base of the late Asbian.

DINANTIAN

BIO- A N D C H R O N O S T R A T I G R A P H Y STAGE

117

I c,. I,,,. I.o. 1 , b,an 1

Courceyan

ZONE

Crassispora maculosa Cyrtospora cristifer I-I)/menozonotriletesexplanatus Lophozonotriletes malevkensis Lophozonotriletes trian~lulatus Spelaeotriletes obtusus Spelaeotriletes resolutus Umbonatisporites abstrusus Vallatisporites verrucosus Verrucosisporites nitidus Kraeuselisporites hibemicus Neoraistrickia c~mosa Umbonatisporites distinctus Spelaeotri/etes balteatus Vallatisporites vallatus Anaplanisporites baccatus Colatisporites decorus Crassispora t~chera Granulatisporites micro~lranifer Kraeusefisporites mitratus ,Prolycospora ru~lulosa Raistrickia clavata Raistrickia condJ/Iosa ,Spelaeotriletespretiosus Convolutispora circumvallata Schopfites clavi~ler Lycospora pusilla Vallatisporites ciliaris Knoxisporites stephanel~horus Knoxisporites triradiatus Chaetosphaerites pollenisimi/is Crassispora aculeata Cribrosporites cribellatus Dic~/otriletes sa~enoformis Leiotriletes tumidus Perotfilites tesselatus Potoniespores delicatus

PC I CM X X

VI

HD

BP

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x

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| l l l

Schulzosporaspp.

Fig. 8. Selected ranges of Dinantian miospore taxa derived from Clayton (1984), Higgs et. al. (1988a) and Neves et al. (1972). The interzone between the CM and PC zones is not shown due to lack of data. Abbreviations: zonal symbols see text, Ch., Chadian; Ar., Arundian; Ho., Holkerian.

Stenozonotriletes coronatus Triquitrites mar~/inatus...... Verrucosisporites baccatus Wa/tzispora p/anian~/u/ata Dic~/otriletes pacti/is Murospora mar~lodentata Murospora parthenopia Raistrickia ni~tra ,. Rem,ysporites ma~nificus Rotaspora eff/onu/ii .Tripartites distinctus Diatomozonotriletes saetosus Grandispora spinosa Rotaspora fracta Rotaspora knoxi Savitrisporites n u x Spencerisporites radiatus Tripartites vetustus Triquitrites trivalvis Cin(lulizonates cf. capistratus Bellispores nitidus ,Ret!.c.u/atisporitescarnosus ,Schopfipol/enites el/ipsoides

(x) Tripartites vetustus-Rotaspora fracta (VF) Zone. The best correlation achieved for the base of this zone is that of Higgs (1984) who recognized its base associated with the P~an, ammonoid zonal boundary in northwest Ireland. This compares well with the base of the VF Zone in the Spilmersford Borehole in the Scottish Midland Valley

NC(~)

....

.....

X

x

x x x

x

X x

x

x x X X x x x X X X X X

x x X

x

x x x x x X X x x x X x

(Neves & Ioannides 1974), which lies below the top of the Spilmersford Beds. This horizon has been correlated with the Dykebar Limestone by Wilson (1989), a horizon which yields the bivalve Posidonia becheri, which is found in strata no younger than the P1~ Ammonoid Zone, but is characteristic of the underlying P~b and Plc ammonoid zones.

118

N.J.

Recently, Ebdon et al. (1990) reported that VF Zone miospores occur in latest Asbian strata at the Brigantian stratotype. Traditionally the base of the Asbian has been inferred to equate with the base of the Pla Ammonoid Zone; however, this new evidence suggests that it is more likely to lie at the top of this ammonoid zone. (xi) Bellispores nitidus-Reticulatisporites carnosus (NC) Zone. Attempts by Owens (in Ramsbottom 1981a) to obtain identifiable assemblages at the Pendleian stratotype (basal Namurian) near Clitheroe have failed to yield any indication of the position of the base of the NC Zone with regard to ammonoids. The best evidence for the base of this zone is that of Marshall & Williams (1971) who recorded Cingulizonates cf. capistratus from their locality 11, between the Three Yard and Five Yard Limestone in Northumberland, this lies within the P2b Ammonoid Zone, since Sudeticeras ordinatum is known above the horizon of the Five Yard Limestone. This is a minimum age for the base of the NC Zone, since their underlying sample containing VF Zone assemblages (localities 9 and 10) lies some 100m below, above the Cockle Shell Limestone, which is of a P2a Ammonoid Zone age. The V F / N C zonal boundary therefore lies in the late Brigantian between the upper part of the P2a and P2b ammonoid zones. There is no palynological signature to the Visran/Namurian boundary. Fora m in ifer a The generic ranges of selected British Dinantian foraminifera are given in Fig. 9. All Dinantian foraminifera were benthonic and it is the free living forms which are particularly useful stratigraphically. Foraminifera are abundant in mid-ramp and platform settings, but their small size enabled considerable post-mortem distribution, hence they are also found interjected with ammonoid bearing basinal mudstones in limestone turbidite sequences and in tempestites within peritidal sequences. They are therefore an important group in correlation between basin and shelf settings and in deciphering reciprocal sedimentation. Dinantian carbonates are highly indurated hence foraminifera are identified in random orientation from thin section of the bulk rock. They have the advantage over other Dinantian microfossils in that they are examined routinely within the context of the enclosing sediment. Their small size makes them ideally suited in borehole exploration where they can be examined as thin sections in mounted cuttings of carbonate rocks. By the end of the nineteenth century Britain was at the forefront of Carboniferous foraminiferal research through the work of Brady (1876), after which time much of this research lay dormant, the initiative being taken by Soviet, American and Belgian workers. It is tempting to speculate whether it was the dominance of Vaughan's coral/brachiopod zonation and its elaboration by subsequent workers which inhibited the development of British Dinantian micropalaeontology during the first half of the the present century. The importance of foraminifera was again realized by Davis (in Hudson & Cotton 1945) and the first attempt at a foraminiferal zonation of the British Carboniferous was made by Cummings (1961), who proposed seven zones based on the sequence of foraminifera present in the Archerbeck Borehole, in the Northumberland Trough. He considered that his zones occupied the entire Dinantian down to the C2S1 Coral Brachiopod Zone (late

RILEY Chadian or Arundian equivalent). During the 1960s and 1970s Belgian workers, lead by Conil, established a foraminiferal zonation for the Dinant and Namur basins of Belgium which is the standard for northwest Europe. This zonation was progressively applied to British and Irish sequences, culminating in the first comprehensive illustration of British foraminifera, linked to the regional stages of George et al. (1976) by Conil et al. (1980). This remains a key reference not only with regards to the distribution of British foraminifera, but also as a guide to the taxonomy and morphological terminology developed by Belgian workers. One important conclusion of Conil et al. (1980) was the realization that the Archerbeck Borehole sequence lay entirely within the Cf6 Zone, and that Cummings' zonation was unworkable. Fewtrell et al. (1981b), provided another key publication on the stratigraphical distribution of British Dinantian foraminifera which included a review of previous publications. These authors did not adopt or propose any foraminiferal zonation, but documented generic ranges in relation to the British Dinantian stages and illustrated a selection of species supplemented with brief generic descriptions. They also pointed out some of the problems that were emerging with the Chadian and Asbian stratotypes in relation to foraminifera. Also significant was their observation that some guide taxa entered earlier in Britain than had been reported from Belgium. Subsequent publications have included: Athersuch & Strank (1989); Conil et al. (1981); Marchant in Charsley (1984); Riley (in Chisholm et al. 1988); Riley in Davies et al. (1989); Riley (1990b); Riley (in press); Strank (1982a, b, 1983, 1985. 1986); Strogen et al. (1990); Somerville & Strank (1984a); Somerville et al. (1992a, b); Strank (in Mitchell et al., 1986). A British-based zonation scheme has not yet emerged and very little is known about early and mid-Courceyan foraminifera in Britain. Progress needs to be made in documenting accurately the distribution of particular species; detailed taxonomic studies will be required to do this. Revision of the Eoparastaffella (Cf4) and Neoarchaediscus (Cf6) zones is desirable since the eponymous taxa are absent from the lower subzones of each zone. Ostracodes Ostracodes are common in Dinantian marine and certain non-marine settings. Despite their abundance little is known of their biostratigraphy. The most reliable zonation is that presented by Gooday (1983) for entomozoacen ostracodes across the Devonian/Carboniferous boundary in the hemipelagic facies of southwest England. That proposed for the shallow marine sequence in the Northumberland Trough (Robinson, 1978) and adjacent regions, appears to be only locally applicable, because of facies controls. The British Micropalaeontological Society is currently compiling a review of British Carboniferous ostracodes (Athersuch et al.) to which the reader is referred. Trilobites The ranges of selected trilobite species in the British Isles are given in Fig. 10. Trilobites are locally common in many Dinantian marine settings. As in previous periods trilobites adapted to a variety of habitats giving rise to a diverse array

D I N A N T I A N BIO- AND C H R O N O S T R A T I G R A P H Y Courceyanl Chadian J (part) [ eady I late

STAGES Zones & subzones

cf4cz2

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cf4~ ..... cf4~

late

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X

X

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X X X X X X X X X

X X .. X X X X X X X

X X X X X X X ,,, X X

X X X X X X X X

X X X X X X X X

Toumayella

X X X X X X X X X

Va~uHne#a

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X X X X

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X X

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X X X L. X

X X X X

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X

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X X X X. X X

X X X . X. X X X

Pseudotaxis

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X X

X X

lale

c~,~

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x

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x

x

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Cribrospira

x

x

x

x

x

Endostaffella Holkeria Koskinotextularia Millerellas.I. Mstinia Palaeotextularia(rnonolaminar) Pojarkovella

X X X X X

X X X X X

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X

X

X X

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x

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x

x

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x x x

x x

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x

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x

x

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x x

x

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x

x

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x x x x x

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.

.

B~bradya

Fig. 9. Ranges of selected late Courceyan to Brigantian foraminifera in the British Isles. The Cf6fl Subzone is not recognizable in Britain and there is confusion over its definition. Conil et al. (1991) state in their text that the base of this subzone coincides with the entry of Howchinia, however in their range chart they show the entry of this genus at the base of the overlying Cf6y Subzone. Sources are given in the text.

early

119

~a~'na Euxinita Howchinia ,Koskinobi~enerina Neoarchaediscus Asterarchaediscus Janischewskina Loeblichia Wamantella Monotaxinoides

'1

X X X X X X X X X X

x x x x . . . . x x x

....

of forms. Three principal ecological associations are represented, located in hemipelagic mudstones, bioherms and carbonate ramp/platform settings respectively (Riley 1984a, b). Riley (1982) established a series of zones for the mid-Dinantian of the Craven Basin, northwest England and some of these can be traced into southwest England (Jackson 1991), Ireland and the English Midlands (Tilsley 1988a, b). A formal proposal of this zonation awaits valid taxonomic description of some of its component species. Owens (in Thomas et al. 1984) provided a review of previous British research and presented a range chart. Subsequently he monographed British brachymetopids (Owens 1986). Osm61ska (1970) monographed shelf facies forms (except brachymetopids); her contribution remains a key reference.

.

,,,

x x x x x x,,, x x

Trilobites from the hemielagic facies give a similar degree of stratigraphical resolution as achieved by the ammonoids, but most information on them has come from sequences in Germany (e.g. Brauckmann 1973). The range chart presented herein gives a false impression of an apparent scarcity of trilobites in the mid-Courceyan and Arundian. It only reflects the lack of published data and taxonomic work.

Conclusions Vaughan's (1905) coral brachiopod zonation scheme has influenced Lower Carboniferous stratigraphy in Great Britain and Ireland for much of this century. The Dinantian stages proposed by George et al. (1976), replaced this scheme. For a variety of reasons they now require some

i

STAGES

Courceyan early middle

An~lustibole ? porteri Brachymetopus woodwardi Moscho~llossis decorata Phillibole drewerensis Phillibole duodecimae Phillibole hercules Piltonia fr~i Piltonia salted Phillipsia ornata Bollandia ~lobiceps Brachymetopus macco,yi Phillipsia ~lemmulffera

PhilIpsia kelll/i Bollandia columba Bollandia ru~iceps Cummin~/e/la raniceps Eoc}/phinium clitheroensis Namuropyge g/aphra Phillibolina worsawensis Reediella reedi Bo//andia persephone Coombewooclia spatulata Cummin~ella tubercul~enata Liobole castroi Namuropy~e decora Phillibole coddenensis Phillibole nitidus Reediella stubblefieldi Tawstockia Ion~lispina Weania colei Weania feltrimensis Winterber~lia hahnorum Aprathia morata Cummin~lella ionesi Griffithides hotwellensis Gnffithides Ioncjiceps Lir~uapbillipsia mitcbelli Linguaphillipsia scabra Tawstockia milled Bra(~hymetopus omatus Cummin~lella carrin~ltonensis Eocyphinium seminiferum Linguaphillipsia cumbriensis Phillibole polleni Vande~rachtia vander~rachti ArchecJonus antecedens Arche~onus laevicauda Bollandia obseleta Cumminc=lella au~e Cummin~lella insulae Cyrtoproetu8 craooensis Eocyphinium caslletonensis Griffithides acanthiceps Liobole erdbachensis Namuropyge acanthina Phillibole aprathensis Piltonia hum#is Piltonia pa ucita Reediella ~lranifera Weania an~llica Cummin~lella laticaudata Cyrtoproetus michlowensis Eocyphinium breve Paladin bakewellensis Paladin ~llaber Arche~onus tever~lensis Cummin~ella sampsoni Griffithides whitewatsoni Kulmiell3 leei Paladin barkei Paladin ~llaber Paladin mucronatus Particeps scoticus

Chadian late

early

late

X X X X

X X

X

Arundlan

Holkerlan

Asblan early

Brlgantlan

late

early

X X X X

X

late

X X X X X X X X X

X X X X X X X

X

X

X X X X X X X X X X X X X X X X X X X

X X X X X X X

X X X X X X X X X X X X X X X

X

X

X X X X X X X X X X X X X X

Fig. 10. Ranges of selected trilobites in the Dinantian of the British Isles. Sources are given in the text.

D I N A N T I A N BIO- A N D C H R O N O S T R A T I G R A P H Y redefinition at their boundary stratotypes so that they correspond as nearly as possible to biostratigraphical events. This is because biostratigraphy, although conceptually different from chronostratigraphy, is the most pragmatic and closest means of approximating chronostratigraphical correlation in the British and Irish Carboniferous. Thus the Arundian and Asbian stages require their basal boundaries moving to the bases of the Cf4fl and Cf6tr foraminiferal subzones respectively. This can be done without relocating these stratotypes. The transitional nature of the coral/brachiopod macrofauna and lack of distinctive microfauna associated with the basal Brigantian suggests that a definitive basal boundary stratotype for this stage should be taken in an ammonoid bearing sequence at the base of the Arnsbergites falcatus Plb Ammonoid Zone. There are strong suspicions that the Holkerian stratotype is affected by a non-sequence and if this is confirmed it will be necesary to relocate this stratotype. The most serious problem arises with the Chadian which has no biostratigraphical, lithostratigraphical or sequence boundary associated with its base. It is recommended that the only way the original concept of this stage can be retained is by taking the base of the stage at the base of the redefined late Chadian (sensu Riley 1990b). This will require relocation of the stratotype. Global data on Lower Carboniferous biostratigraphy is still not at an acceptable standard to identify a eustatic sequence stratigraphy conclusively. Stratigraphers need to be aware that both tectonic and eustatic processes contributed to Dinantian sequence stratigraphy. The resolution of seismic sequence stratigraphy so far published from the British Carboniferous misses internationally important sequence boundaries that can be recognized using direct observation and biostratigraphical techniques. All the biostratigraphical schemes available for the British Carboniferous are capable of further refinement. Some miospore zones are still not related precisely to other biostratigraphical schemes. Trilobites, Visran corals and brachiopods require the erection of zonations that are independent from chronostratigraphy. Courceyan ammonoids are still poorly known and the stratigraphy of ostracodes throughout much of the Dinantian is unknown. Foraminiferal zonation needs to be revised to reflect the entry of eponymous genera and further refinement will be achieved if a consistent taxonomy at species level can be realized. All biozonations need to have more objective scrutiny regarding the effect of facies on the distribution of their assemblages. This can be achieved by comparing different zonation schemes (e.g. Sevastopulo & Nudds 1987) and by using less widely known techniques such as correspondence analysis (e.g. Hennerbert & Lees 1991). Stratigraphers need routinely to use more than one biostratigraphical technique in order to reduce the influence of facies effects in correlation. Even if these recommendations are achieved, it should be stressed that biostratigraphical evidence used in support of chronostratigraphical correlation should always be presented in terms of zones and guide taxa. Future revision of chronostratigraphical interpretations as techniques and classifications change will therefore be possible. Vaughan's (1905) scheme has serious gaps in its zonal coverage, but it is only because of his careful and meticulous work that we are able to reinterpret and retain the value of his pioneering contribution to British stratigraphy.

121

Thanks are expressed to my collegues, particularly A. McNestry and N. Turner for discussion of miospore data and zonations. This paper is published by permisssion of the Director, British Geological Survey (NERC). References AITKENHEAD, N., McBRIDGE, D., RILEY, N. J. & KIMBELL, S. F. 1992. Geology of the country around Garstang. Memoir of the British Geological Survey, Sheet 67 (England and Wales). Her Majesty's Stationary Office, London. ALDRIDGE, R. J., BRIGGS, D. E. G., CLARKSON, E. N. K. & SMITH, M. P. 1986. The affinities of conodonts new evidence from the Carboniferous of Scotland. Lethaia, 19, 279-291. ARMSTRONG, H. A . & PURNELL, M. A. 1987. Dinantian conodont biostratigraphy of the Northumberland Trough. Journal of Micropalaeontology, 6, 97-112. ATHERSUCH, J. & STRANd, A. R. E. 1989. Foraminifera and ostracods from the Dinantian Woodbine Shale and Urswick Limestone, South Cumbria, U.K. Journal of Micropalaeontology, 8, 9-23. AUSTIN, R. L. 1987. Conodonts of the Arundian (Dinantian) stratotype boundary beds from Dyfed, South Wales. In: HART, M. B. (ed.) Micropalaeontology of carbonate environments. British Micropalaeontological Society Series. Ellis Horwood Limited, Chichester, 238-255. BARTZSCH, K. & WEYER, D. 1988. Die unterkarbonische Ammonoideaubfamilia Karagandoceratinae. Freiberger Forschungshefte, C419, 130-142. BELKA, Z. 1985. Lower Carboniferous conodont biostratigraphy in the northeastern part of the Moraviailesia Basin. Acta Geologica Polonica, 35, 1-60. BERTELSEN, F. 1972. A Lower Carboniferous Microflora from the Orslov No. 1 Borehole. Danmarks Geologiske Undersogogelse, 2, series 99, 1-78. BISAT, W. S. 1924. The Carboniferous goniatite zones of the north of England and their zones. Proceedings of the Yorkshire Geological Society, 20, 40-124. 1928. The Carboniferous goniatite zones of England and their continental equivalents. Premier Congr~s International de Stratigraphie et de G~ologie du Carbonif~re, Heerlen, 1927, Compte Rendu, 1, 347-353. -1934. The goniatites of the Beyrichoceras Zone in the north of England. Proceedings of the Yorkshire Geological Society, 22, 280-309. -1952. The goniatite succession at Cowdale Clough, Barnoldswick, Yorkshire. Transactions of the Leeds Geological Association, 6, 155-181. -1955. On Neoglyphioceras spirale (Phill.) and allied species. Publication

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Received 17 December 1992; accepted 21 January 1993

From

QJGS,6 1 ,

181 - 182. 11. The PAL2gONTOLOGICXLSEQUENCE i~t $h~ CARBONIFEROUSLI~csTo~. of tke BRISTOT. ARSa. By ARTHUR VAUQHAN,B.A., D.Se., F.G.S. (Read June 8th, 1904; rearranged, and additional matter incorporated, October 1904.)

[PLATESXXII-XXIX.] COMTE,N*TS.

Page l. Introduction ..................................................................... I I. Detailed Description of Continuous Sections and Isolated Exposur~ ill the Bristol Area ......................................................... i) Continuous Sections :-(a) The Avon Section ................................................ Analysis of Stoddart's Paper ................................. (b) Sodbu ry .......................................................... (c) The Failure{ ~rea (including Flax ]3otu'ton) ............... (d) The Tytherington Section ....................................... (e) The Clevedon Area ............................................. Q") The Portishead District .......................................... (ii) Isolated Exposures :-(A) In the Cliftou-Olevedon Ridge ....................... . ...... (B) In the Clifton-Westbury-King's Weston Ridge ......... (C) In the Wickwar-Sodbury Ridge .............................. (D) In the Olveston-Tytherington-Cromhall Ridge ............ (E) The Backwell-Wrington Mess................................. I !1. ]~ltngesand Maxima of the Corals and Brachiopods in the Bristol Area .......................................................................... IV. Comparison of the :Bristol Sequence with that in Neighbouring Areas ........................................................................... V. Compariso. with the Belgian Sequence ................................... V1. Summary and Analysis ......................................................... VI 1 Notes on the Corals and Brachiopods referred to in the Faunal Lists ...........................................................................

{

181 188 188 200 203 211 219 225 228 231 233 239 240 243 248 255 257 266

1. I.~'I'RODUCTIOI~. T n , s paper deals w i t h the fossil sequence in the Carboniferous Limestone of the Bristol area, and with the possibility of dividing that system into a series of palveontological zones. The general geology of the area has been most luminously expounded by Prof. Lloyd Morgan in the series of papers which he has contributed to the Proceedings of the Bristol Naturalists' Society, and to these I make constant reference. I am thus able largely to dispense with detailed accounts of topography and lithology, which would otherwise interrupt seriously the pala~ontological discussion. Mr. E. B. Wethered has contributed a most instructive paper ' On Insoluble Residues obtained from the Carboniferous-Limestone Series at Clifton, '~ and it is with his lithological divisions t h a t I have mainly correlated the pala~ontologieal zones suggested in this paper. To the late Mr. W. W. Stoddart ~" we owe the first attempt to compile a list of the fossil contents of the beds in the Avon Section. I have drawn up a complete analysis of his observations, so f a r only as the Corals and Brachiopods are concerned ; this will, for convenience, follow immediately upon the detailed account of my own observations on the Avon section. For my purpose, i t is obvious that the essential desiderata are good exposures, the relative stratigraphical position of which is unquestionable. Exposures which satisfy these two conditions are to be found in several parts Of the Bristol area, and, from them, the determination of the faunal sequence is merely a matter o[ accurate observation and careful tabulation. Since every fossil that i s recorded in the following lists was noted down at the instant at which it was observed, while all specimens which presented any difficulty in determination were extracted as completely as possible and carefully re-examined at leisure, I may claim t h a t these lists are absolutely reliable, provided that each name .presents exactly the same i d e a to t h o s e w h o r e a d it, as i t d o e s to m e i n w r i t i n g it.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 127-150

Time from fossils: S. S. Buckman and Jurassic high-resolution geochronology J.

H.

CALLOMON

University College L o n d o n , 20 G o r d o n Street, L o n d o n W C 1 H OAJ, U K

Abstract: Chronostratigraphical classification of rocks can be approached from two directions. The first is a 'top-down' process of subdivision of the geological column in a hierarchy of successively finer units. These units are therefore defined by their boundaries, which are time-planes, and form complete continuous series or scales. They are chosen to be widely recognizable, and hence correlatable, by means, in the Phanerozoic, of their contained guide-fossils, i.e. by their characteristic biozones. This was the approach of d'Orbigny and Oppel in the Jurassic, leading to a standard chronostratigraphy down to Subzonal level notably espoused by Arkell and widely adopted today. The second approach is one of 'bottom-upwards' integration: the assembly into time-ordered sequences of the most minutely distinguishable local faunal horizons--distinguishable in the sense of evolutionary change--which may or may not subsequently be found to have more widespread value for time-correlations and biochronology. This was the method introduced by Buckman a hundred years ago to describe the ammonite biostratigraphy of the Inferior Oolite of Dorset, in response to the need for the finest attainable time-resolution in phylogenetic palaeontology. The time-equivalents of such faunal horizons were termed hemerae. Polyhemeral chronostratigraphy went into abeyance with Buckman's death in 1929, but its equivalent, in terms of the faunal horizons themselves, has been revived. A faunal horizon is defined as a stratigraphical entity within which no further biochronological subdivision can be made, so that the bed or beds embodying that horizon must, on the evidence of the fossils alone, be regarded as internally isochronous. A succession of faunal horizons becomes the record of well-spaced instants: the record is presumed a priori to be full of gaps of unknown duration waiting to be filled by new discoveries. The measure of chronostratigraphical finesse is the average time-interval between the moments represented by the faunal horizons, 6t, the secular resolution. The relative ability of groups of guide-fossils to resolve time-intervals 6t in rocks of age t is their secular resolving-power, R = t/6t. The current state of Jurassic chronostratigraphy is reviewed. The guide-fossils of choice are the ammonites, whose secular resolving-power exceeds that of any other group and which can give time-resolutions of 150000 years in rocks of age 150 million years (R >1000). These figures are compared with those attainable elsewhere in the Mesozoic and Palaeozoic. Resolution-analysis of the Jurassic shows that, at the level of resolution of ammonite faunal horizons, the geological record is highly incomplete, nowhere more so than in the Inferior Oolite. As Buckman concluded, the more complete the fossil record of a system becomes, the more incomplete turns out to be its lithochronology. This has important consequences in sedimentology, and in sequence stratigraphy.

Rock-time duality

observation is that of beds specified by heights in a succession, thicknesses and compositions: their lithostratigraphy and, if c o m p o s i t i o n includes fossils, their biostratigraphy. T h e next step introduces Steno's Principle of Superposition (1669), which states that in a n o r m a l succession of sediments the higher lying are the younger. This transforms a static description in terms of height into a dynamic one in terms of local t i m e - - r e l a t i v e t i m e - - a n d is an interpretation. Specification of rocks in a stratal succession according to their relative ages we refer to today as their chronostratigraphy, although this t e r m appears to have b e e n first explicitly i n t r o d u c e d only by H e d b e r g (1954). The third step involves the linking of local successions t h r o u g h time-correlations. These allow the ages of rocks at one place to be c o m p a r e d with those at another. T h e fourth step t h e n b e c o m e s the synthesis of a standard t i m e - o r d e r e d succession of rocks, correlation with which allows any local rock to be dated in terms of its relative position in the implied conjugate chronostratigraphic time-scale. Only in the fifth and final step is this standard chronostratigraphic time-scale calibrated in absolute terms, in years, by radiometric methods. So m u c h for principles. In practice, everything d e p e n d s

T h e history of the E a r t h is r e c o r d e d in the rocks a r o u n d us. To reconstruct this history we n e e d to measure the ages of the rocks. This allows us to present the history as a compilation of what has occurred, and such compilations m a k e up the greater part of text-books on historical geology. But perhaps even m o r e interesting today, it allows us to estimate the rates of underlying processes--tectonic, plutonic, m e t a m o r p h i c , s e d i m e n t a r y and biological. T h e s e rates range over m a n y orders of magnitude, and being derivatives with respect to time, the ability to measure t h e m d e p e n d s not so m u c h on the m e a s u r e m e n t s of the ages themselves as on the ability to m e a s u r e small differences in ages, to resolve time-intervals, to distinguish closely-spaced m o m e n t s in time. Alas, as we all know, although we n o w have powerful m e t h o d s of directly dating rocks t h r o u g h the m e a s u r e m e n t of radioactive decay, they are applicable to only a minority of cases. In taking stock of the present position in the g e o c h r o n o l o g y of fossiliferous s e d i m e n t a r y rocks, on which so m u c h of our history of the E a r t h is based, it is relevant to recapitulate five steps in the argument. T h e first and basic 127

128

J.H.

CALLOMON

on successful time-correlations. But, being a matter of interpretation, how can they be assured? How precise can they be made? In the early stages of the evolution of our geological time-scale, correlation was largely lithostratigraphic, for sediments with bedding-planes are clearly at least locally synchronous. Formations were characterized, mapped and placed in succession. Such successions give the impression of being regionally complete but discretely subdivided records of geological time, for in their very nature non-sequences, representing time-gaps at formational boundaries or partings between beds, are not readily evident. There evolved therefore the concept of 'the standard geological column' as a complete and continuous record of geological time, subdivided into a succession of distinguishable segments. The outline of its major features was substantially complete a century and a half ago and is shown in Fig. 1. (For a recent review from a Palaeozoic viewpoint, see McKerrow this volume.) It continues to provide part of the basic vocabulary of our subject even today and calls for only a few comments. Firstly, it can be read in two ways. On the one hand, the boxes represent slices of the geological column: Systems, rocks. In this representation, the horizontal lines delimiting the boxes signify time-planes in t h e rocks. On the other

1

(Lyell. 1873)

RECENT I'LEISTOCI';NE CAINOZOIC (Phillips

....

(Lyell 1839)

I'LIOCEN E

(Lycll 1833)

MIOCENE

(Lyell 1833)

OLIGOCFNE

(Bcyrich

EOCENE

(Lycll 1833)

PALAEOCENE

(Scllimper 1874)

1841) 1854)

CRETACEOUS

(Omalius d'Halloy 1822)

JURASSIC

(i3rongl~ial'l 1829)

TRIASSIC

(Albel'ti 1834)

PERMIAN

(Murchison 1841)

CARBONIFEROUS

(Conybeare 1822)

I'ALAEOZOIC

DEVONIAN

(Sedgwick/Murchisola 1839)

(l:'hillips 1840-4 I)

SILURIAN

(Murchison 1833)

ORDOVICIAN

(Lapworlh 1879)

CAMBRIAN

(Sedgwick 1835)

MESOZOIC (l~hillips

I II III IV V VI VII

Rock Eonothem Erathem System Series Stage Zone Subzone

Time Eon Era Period Epoch Age Chron Subchron

hand, the boxes represent durations of time vertically (Eras) and the horizonal lines represent time-markers (instants) in a vertical time-scale. This rock-time duality may seem obvious, but failure to remember it can still cause confusion, for it is vital to one aspect of the all-important act of time-correlation implied in dating a rock. Whereas the time-markers and their conjugate time-planes define the chronostratigraphic units to which we assign rocks when we date them, the recognition of the unit to which a rock belongs is by means of what lies between the planes, not of the planes themselves; for, except perhaps very locally, time-planes can generally not be recognized other than at the point in a section at which they have been defined. The corollary is that all stratigraphical time-correlations are Secondly, the figure shows a classification that is made up of the top three tiers of a hierarchy of successive subdivision (Table 1): Eons (I), Eras (II) and Periods (Ill). Such a process of refinement can clearly be taken further. Thirdly, the foundations of this classification were predominantly lithostratigraphic--formations and their superposition. That individual formations had their own characteristic fossil assemblages was fully recognized and these assemblages were described in considerable detail. But, with some exceptions, their use as important tools in time-correlation came later.

1841)

skeletal rnacrofi)ssils appear 0

(Gulfflint Formalkm)

ol.,a~ .

Level

approximations.

111

lI

Table 1. Rock-time duality and the hierarchy of standard chronostratigraphy and geochronology

,:,o

Fig. 1. The standard geological column at the highest three levels of the chronostratigraphic hierarchy. Additional categories intermediate between levels II and III in use today include subdivisions of Cainozoic (Cenozoic) into Tertiary (as redefined by Lyell in 1833) and post-Pliocene Quaternary (Morlot 1854).

Biostratigraphy and time-correlation: William Smith to Leopold von Buch The limitations of lithostratigraphy in attempts to refine and extend time-correlations were soon recognized. Not only were rocks of similar lithology not necessarily of the same age, but also, conversely, rocks of the same age could be of different facies (Gressly 1838). The first practical use of fossils for time-correlation is usually attributed to William Smith (1816). It is based on what might be called the Principle of Biosynchroneity: rocks containing similar fossils are of the same age. But fossil species have ranges. 'Same' therefore means 'more or less the same', depending on these ranges, and on what is understood by species and how closely they can be identified. Some fossils are clearly better for correlation than others, and those whose distributions (biozones) approximate most closely to synchroneity were called 'Leit-Muscheln' by von Buch (1839, p.64): the guide-fossils of today. The context was a review of the Jurassic of Germany and Switzerland. He comments (p. 61) on earlier attempts to classify the German succession by means of correlations with the then much better-known succession in England, based on claims to have recognized a lithologically similar

TIME FROM FOSSILS order of formations. For instance, lithological comparisons led Murchison (1831) to assign the 'slates' of Solnhofen and those of Stonesfield to the same 'geognostic horizon' ]sic] and to conclude therefore that the equivalents of the whole of the Upper Oolites of England must be missing in Germany. Such correlations were refuted by the 'zoological character' of the formations, 'which alone should decide the identity ]equivalence] of the formation'. What was needed was a catalogue of reliable guide-fossils, and these yon Buch proceeded to enumerate. At the same time he refined the standard classification by subdividing the Jurassic further, into the three universal parts of Lower, Middle and Upper Jura. These correspond to our Series of today, at .level IV of the standard hierarchy. Of the 102 species of guide-fossil he lists for the Jurassic, 30 are ammonites. The pre-eminence of ammonites as guide-fossils in the Jurassic gave that System a lead in the development and testing of the techniques of chronostratigraphical refinement through biostratigraphy that it has maintained to the present day.

Standard chronostratigraphy: d'Orbigny, Oppel and beyond The next step was taken by d'Orbigny (1850, R~sum~ g~ologique, p. 600). He subdivided the Jurassic into ten Stages ('6tages', 'Stufen' in German). The importance of this work and some of its shortcomings in execution were reviewed by Arkell (1933, p.8), who also provided an English translation of the key passages in the introductory pages of the classical R~sum~. Arkell's analysis reads as freshly as when it was written but two points are worth re-emphasizing. The first is that the Stages are undoubtedly what we would today call chronostratigraphic units. They explicitly represent the record in rocks of 'successive distinct geological epochs', recognized by their characteristic fossils. The second is that they are subdivisions of a larger continuous unit, slices of the geological column, defined by the dividing time-planes. They are therefore standard chronostratigraphic units, the rock-equivalents of a standard geological time-scale, and constitute the next level downwards in the hierarchy of classification by subdivision, level V. Arkell makes great point, implicitly perhaps rather than explicitly, of this difference between d'Orbigny's Stages as part of a standard classification and numerous other entities masquerading under the same title as 'stages' already in the literature at the time, such as Marcou's Vesulien, Argovien and Sequanien, variously interpretable as local litho-, bio- or even chronostratigraphical units--a list that had grown in 1933 to over a hundred for the Jurassic alone. Standard classifications generate standard nomenclatures; and it is the analogy between stratigraphical and zoological nomenclatures that guided Arkell in his subsequent proposals (1946) for a Code of Rules of Stratigraphical Nomenclature, analogous to those of Linn6an zoological nomenclature, in which d'Orbigny's R~sum~ g~ologique of 1850 marks the starting-point for a Rule of Priority in naming Stages in the same way as Linnaeus' Systema Naturae, 10th Edition (1758) does for naming species in zoology. Finally, Oppel's Zones (1856-58). In the introduction to his seminal work, Oppel made it quite clear that he was following the principles laid down by William Smith and Leopold von Buch, but taking the refinement of the chronological classification of the Jurassic of NW Europe

129

even further, down to the level of Zones. And although there can be few works on Jurassic stratigraphy during the past century in which 'zones' are not used, professedly in the Oppelian sense, there has been a longstanding uncertainty as to precisely what it was that Oppel meant by the term, an uncertainty that has caused much confusion in the past and that persists even today (e.g. Harland et al. 1990, p.21; Guex 1987). As Arkell wrote (1933, p.16): 'It is remarkable that Oppel nowhere defined what he meant by a zone. He is frequently credited with the first use of the word, but it had in fact been employed by several French geologists before him [including d'Orbigny, as alternative to "6tage"], and a definite meaning was already attached to it. Oppel adopted the term and accepted its meaning and no doubt it seemed to him in consequence unnecessary to give a definition . . . . If he had given a definition.., it would have been in fact superfluous, for his meaning is apparent on almost every page of the book'. Apparent, perhaps, but clearly in different ways to different readers. For an authoritative second opinion, however, nothing could have been clearer than the re-statement by Oppel's student, Waagen. In the introduction to his article defining and characterizing 'die Zone des Ammonites sowerbyi' in the lower Middle Jurassic (1867, p. 511-13), he explains the purpose to be achieved. It is not merely to give detailed descriptions of any particular bed or of the organic remains it contains: rather, by means of such descriptions, 'ein neues, bestimmt fixiertes Glied in der Zeitskala des Bildungsprozesses des Jura nachzuweisen'--to demonstrate the presence of a new [i.e. hitherto unrecognized] segment of the time-scale of the formation of the Jurassic. He makes no claim either that this segment can always and everywhere be distinguished from those adjacent, or that its characteristic fauna will never be found mixed with those below or above; but he confidently asserts that whenever a bed with this fauna by itself is found, it will always lie at the same relative position in the succession. And finding this to be the case in most of central Europe, are we not justified, 'diese Schicht als einen Zeitabschnitt to betrachten,.., als solchen fur sich besonders zu beschreiben und als Zone mit einem Namen zu belegen': in regarding this stratum as [equivalent to] a time-interval to be explicitly described and named as a Zone? And in our attempts to refine ever further the zonation of the Jurassic in the face of variable lithologies ('Bestand') and localized or migrating faunas and their individual species, how are we to arrive at firm conclusions? 'Das Mittel i s t . . , die Feststellung einer NormalSchichtenreihe, nach welcher die Bildungen in anderen, entfernter liegenden Distrikten beurtheilt werden kSnnen': the answer lies in the determination of a standard succession [sic] with which formations at other, more distant localities can be compared, i.e. correlated. And 'ein solches Glied der Normal-Schichtenreihe m~Schte ich denn in der Zone des A. sowerbyi festzustellen suchen': it is as such a member of the standard succession that I seek to recognize the Zone of A. sowerbyi. '[It] falls into the time-interval [sic] between the occurrence of A. murchisonae and the appearance of A. sauzei . . . . The Zone is therefore bounded below by the beds containing A. murchisonae and above by those with A. sauzei'. (Waagen's Sowerbyi Zone spans today a succession of 15 distinguishable ammonite faunal horizons, see below). Oppel's meaning of 'Zone' is brought out unambiguously by careful attention to definitions in stratigraphical nomenclature (e.g. Callomon 1985a). There are two sources

130

J.H.

CALLOMON

of confusion. The first has already been alluded to, and derives from the distinction between definition of a unit (here an Oppelian zone) and its recognition and use in correlation. The definition is clear from the numerous tables to be found throughout Oppel's book, but particularly in the summaries in table 63 (p. 822, reproduced in part by Arkell, 1933, p.18, Table III) and table 64. The Zones are continuous subdivisions of d'Orbigny's Stages, which are themselves subdivisions of the three parts, Lower, Middle and Upper, of the whole of the Jurassic. Their definition lies in their bounding time-planes, which we are attempting today to fix objectively and typologically by means of markers ('Golden Spikes') in type-sections. Their function is to provide a standard scale of reference against which all the known formations of Europe could be classified (correlated) according to their relative ages. Oppel's Zones are therefore standard chronozones. Their recognition is by means of the guide-fossils they contain, i.e. the biozones of the fossils. In general, the biozones used are concurrent-range assemblage biozones, in modern parlance, for the simple statistical reason that, in handling approximate data, the larger the data-set the closer the estimate of the quantity to be determined from it, in this case geological age. The second source of confusion has been the failure to distinguish between guide-fossils and index-fossils. Whereas d'Orbigny's Stages were named after places, Oppel chose to name his Zones after species of fossils (p.813), to emphasize the biostratigraphical basis of his classification. 'I have named individual Zones each after one of its more important [sic] species...'. He immediately goes on to warn against the subconscious bias that the choice of one particular species to label a Zone might introduce into our interpretation of it, carrying over to the Zone as a whole what might have been largely a local accident of range or relative abundance. 'Important' is a relative concept. H e discusses briefly the alternative of place names, as for Stages, but concludes that the dangers of bias arising from the association of a zonal name with its particular development at the eponymous locality are even greater. The function of the index is purely name-giving. It helps if it is at the same time a good guide-fossil, but this is not essential. All that is required as a minimum is that it at least occurs, and preferably that its type horizon lies, in its nominal Zone. Unfortunately, Oppel's remarks have been widely ignored, especially in Germany. There has resulted an often impenetrable confusion of zonal nomenclature as authors changed the choice of index every time they thought another was more appropriate. Often, the index-species has become the guide-species, i.e. what purports to continue to be a standard chronozone has become transformed into a single-taxon total-range biozone. The subjective and ephemeral nature of such biozones has been stressed elsewhere (Callomon 1985a). Their use in standard chronostratigraphy is almost as unsatisfactory as is that of lithostratigraphical units. And it was mainly the need to bring order into this nomenclatural confusion that led Arkell to propose his Code of Stratigraphical Nomenclature (1946), the first attempt at what has become a flourishing multinational industry. Such, then, was the legacy of the founding fathers. The number of standard Zones into which the Jurassic had been divided at the end of Die Juraformation in 1858 was 33, of which 22 were named after ammonites. The tragic death at the age of 34 of Albert Oppel, the Mozart of the Jurassic,

cut short a career that would have given us many further contributions to Jurassic stratigraphy. The problems of biogeographic provincialism among the ammonite guidefossils, for instance, were becoming apparent as his interests spread further afield, to the Mediterranean and the Himalayas, indicating limits to the applicability of the standard classification he had created. The process of refinement was however carried forward by his successors and continues at the present day. As we have seen, it is a process of subdivision through the insertion of new boundary time-planes, reflecting the incorporation of new discoveries. This can be done in two ways: by delimitation of new Zones, or by splitting of existing Zones into finer subdivisions at a yet lower level in the standard hierarchy, level VII: Subzones. Oppel himself already introduced the category of Subzone a number of times, albeit somewhat tentatively, and their popularity has grown. The distinction between Zones and Subzones is largely a matter of choice, and taken by most authors to reflect the interrelation between the finesse of stratigraphical resolution that can be attained and the areal extent over which it can be recognized. By and large, the category of standard Zone is retained for units that can be recognized over at least sub-continental distances, whereas Subzones may be recognizable over lesser but still useful extents. At least one International Stratigraphic Guide (Hedberg 1976) makes provision for yet further subdivisions, as Zonules, but these have not been widely used. A modern example of an analysis of standard chronostratigraphy down to the level of Subzones, taking into account an assessment of biostratigraphy, correlation and Arkell's Rule of Priority in nomenclature, may be seen in the seminal classification of the Lias of NW Europe by Dean et al. (1961). It stops short only of the final step, the typological definition of the subzonal boundaries. Two further technical points remain to be made. The first is orthographic. Standard Oppelian chronozones and fossil biozones, including those of chronozonal index species, being conceptually different entities, the difference should be apparent from the way they are written. I have therefore strongly advocated (1985a) the retention of, or the return to, or, especially outside the Mesozoic, the adoption of a convention consistently followed by Arkell (e.g. 1956), even if perhaps not invented by him. He indicates standard chronozones named after an index-fossil by writing the Linn6an name of the fossil in non-italicized, ordinary roman letters, with capital initial and, similarly, initially capitalizing 'Zone' and 'Stage' when used in standard chronostratigraphic sense. Thus, the Upper Jurassic Cordatum Zone and cordatum zone are not the same thing. The latter lies within the former, to an extent depending on the interpretation of a zoological entity and the state of knowledge. The former is (or will be) defined in terms of two bounding time-planes fixed in type sections. The second point relates to the definition of standard chronostratigraphic units. Standard chronostratigraphy having a hierarchical structure, units at higher levels in the hierarchy should be defined in terms of those at the levels below them, as in zoological nomenclature (Callomon 1965). The ultimate and only unambiguous fixed point is that at the lowest level. A genus rests ultimately on the type specimen of the nominal sub-species of the type species of the nominal sub-genus. Similarly, a standard Stage should be defined in terms of the standard Zones and Subzones it contains.

TIME FROM FOSSILS Standard units being now, by general agreement, defined by their bases, the ultimate typological anchor of a Stage should be the basal time-plane, defined in a type section, of the lowest Subzone of the lowest Zone in the Stage; and so on, through Series and Stages upwards. A recent set of Guidelines from the International Commission of Stratigraphy (Cowie et al. 1986) purporting to advise numerous Boundary Stratotype Working-Groups on procedure, makes no mention of hierarchy and its consequences. The present state of subdivision of the Jurassic is illustrated in Fig. 2. It shows how its standard chronostratigraphical classification has evolved by a process of analytical, hierarchical top-down subdivision. The number of Zones recognized today in the classical areas of Europe is about 76, the number Subzones about 155. All are named after ammonites as indices. It is a process that may have worked exceptionally well in the Jurassic through the fortunate circumstance of its ammonites as guide-fossils, but the principles apply to all Systems whose chronostratigraphy is based on biostratigraphy. Does this process of refinement then take us to the limits of time-resolution that can be achieved? The answer in the

Erathems &

II

Jurassic is no, but further progress calls for a different approach. The first to introduce this was S. S. Buckman in the Geological Society's Journal a hundred years ago.

The limits of biostratigraphical time-resolution: the hemerae of S. S. Buckman and the classification of the Inferior Oolite Sydney Savory Buckman (1860-1929) was a remarkable man. The oldest son among five siblings, he was born in Cirencester where his father, James Buckman, was Professor of Geology, Natural History and Botany at the Royal Agricultural College, and an enthusiastic naturalist in the great Victorian tradition. Resigning from his post in 1863 after prolonged conflicts with a recently newly appointed Reverend Principal, conflicts arising almost certainly from James Buckman's overt support of Darwin's recently published theory of evolution (Torrens 1988), the Buckmans moved to Bradford Abbas, 5 km WSW of Sherborne Abbey in Dorset, to take u p farming at what is now East Farm. James' wife (n6e Savory) died when Sydney was only five years old, so that the boy was brought up largely under the

Series & Stages

Systems

IV

V

Zones

Subzones

VI

VII

Peterborough t

Cordaturn Tithonian

Cenozoic

Cordatum

E L

Kimmeridgian v

O

100 ._ Cretaceousl O

Oxfordian

N

° ~._//////./4 ~Jurassic/, • "////////~ 200 - ~

:3

"////////////~ gd//././//.///k. Callovian ,~///////////~ f/I/Ill/Ill/l~

Triassic

Bathonian o v

300 -

.£

0

I

Bajocian

\

"o Aalenian

o N o

400

Toarcian t~ "i

Q.

I

\

Pliensbachian Sinemurian

O .J

_ Hettangian

600

Faunal horizons

III

01

,00

131

I

Costicardia Bukowskii Praecordatum Mariae Scarburgense Lamberti Lamberti Henrici Spinosum ,L Proniae Athleta Phaeinum Grossouvrei Coronaturn Obductum Jason Jason Medea Enodatum Calloviense Calloviense Galilaeii Koenigi Curtilobus Gowerianus Kamptus Terebratus Herveyi Keppleri Discus Discus Hollandi Hannoveranus Orbis Blanazense

/

[~--;~i0--f3%] ~ g - - q 6 ~ 4 f3-5] ~Z--(o-8~ ;fo~T3]

/ r~---~;~7o-] / [2~---~zc-~-.1 / [~6---8-9-6-~] / r~-~---~-8~-1 / I ~ - - ~ - ~ -] [r~-- -8-5~:8-6~-] F?6----¢9-4-8-~- ] [~ .... ~--] [~

78~-792 1

[ 13 760-780] [I'2--- 691-759 ,]

fi~-- ~i-~~ ] [1-0--- %~-8~-1

r-9--- ~;~-~-] I--f----7~;(3g-]

r-6-----~-7-8--]

[-~----;o--~-- ] r-4-----2~:49--] .... ~.~-] -2- -- -~-£C

]

r~ . . . . . ~;2~--]

r ...........

1

Fig. 2. The subdivision of the Jurassic down to the lowest level of the standard chronostratigraphic hierarchy, the Subzone (VII), and thence to the limit of biochroological resolution in the example of the ammonite faunal horizons of the Oxford Clay at Peterborough as described by Brinkmann (1929; see also Fig. 6): numbers refer to stratigraphic heights in centimetres. The Stages are often further subdivided at a level intermediate between V and VI into Substages. Sometimes these are separately named, e.g. as in the Domerian (upper) and Carixian (lower) Substages of the Pliensbachian, but Lower, Middle and Upper are more usual. Time-scale at left from CTS89 of Harland et al. (1990).

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influence of his remarkable father. He was sent for his secondary schooling to nearby Sherborne School (1871-78), which, through the enthusiasm of some of its teachers, was building up strong interests in natural history and, given its location, in geology in particular. In this it was strongly supported by Buckman senior, co-founder in 1875 and first Honorary Secretary of the Dorset Natural History and Antiquarian Field Club (Torrens 1978). But besides nurturing his interest in geology, the school clearly gave young Buckman an excellent basic education, strong after the manner of the times in reading, writing and the classical Greek and Latin languages, a familiarity with which is apparent in much of his own later writings. Original intentions to follow school by University and then to enter the Church came to nothing and, encouraged by his father-in-law, he set off for a year's study and travel on the Continent, to see whether he might find an interest in chemistry, the basis of the occupation as pharmacists in London of his mother's family and even originally of his own father, who had started his working life as a pharmacist in Cheltenham. Sydney Buckman was therefore enrolled for two semesters in the Chemical Laboratory of the University of Wiesbaden, but it, too, was not to his taste. It marked the end of any further attempts at formal higher education and he returned to Dorset in 1880. He did however later acknowledge the value of having learned the German language (Davies 1930b). Buckman's subsequent career continued to be as varied and turbulent as had that of his father. After two years studying to become a land-agent, he married in 1882 and and took up farming near Andoversford, east of Cheltenham. 1889 saw the beginning of a new career as novelist, living near Stroud ('James Corin', as in Corinium, Roman Cirencester). In 1894 the family moved to Charlton Kings, today a part of Cheltenham, and in 1904, following a break-down in health, the Buckmans, now with five children, moved to Thame, SE of Oxford, where they lived for the rest of Sydney's life until his death in 1929. Although his strenuous field-work, much of it by bicycle, came to an end in 1904, his geological writings continued undiminished. The list of his publications runs to over 200 titles. His most famous work, started from Thame in 1909, must be the monumental Type Ammonites (1909-1930), running to seven volumes with over a thousand plates which, whatever may be said about its systematics, continues to be the most comprehensive description of British Jurassic ammonites. His active geological career spanned half a century. During this period, although he undertook occasional paid work for the Geological Survey and earned something from the sale of fossils, he never held a position of employment as a geologist either in a Survey, in industry, or in an academic institution. In retracing Buckman's career as a stratigrapher we can see how the development of his ideas was influenced by a combination of circumstances. Firstly, under the influence of his father, he had early become familiar with the general principles of stratigraphy, systematic palaeontology and the application of biostratigraphy to the unravelling of geological time. Secondly, the region in which he grew up provided almost unrivalled opportunities to apply these principles to innumerable quarries and outcrops that yielded an abundance of fossils from almost every bed. Thirdly, leading amongst these fossils were the ammonites. He was aware of their renown as guide-fossils through his father both directly and indirectly, for he had become familiar with

the literature, including Oppel. His acute sense of observation therefore quickly led him to appreciate the points stressed by Oppel, that just how good ammonites were for correlation and hence for time-resolution depended crucially on two factors: on how closely their species could be identified, and on how precisely their stratigraphic horizons were recorded. The first of Buckman's many papers to appear in the Quarterly Journal of the Geological Society (1881), published at the age of 21 and only his second publication, is prophetic. Under the innocuous title of 'A descriptive catalogue of some species of ammonites from the Inferior Oolite of Dorset', it begins firmly with a stratigraphical introduction, setting up a framework to which the systematic discussion of the ammonites is then referred. This framework has two parts. The first consists of field observations in the form of detailed sections and their ammonite faunas. The second is a chronostratigraphic classification in terms of the standard Zones of Oppel and Waagen, from the Zone of Harpoceras murchisonae to that of Amm. parkinsoni. But the notable observations relate to the thicknesses of some of the Zones and hence, conversely, of just how carefully a section has to be recorded. Writing of the Zone of Stephanoceras humphriesianum (p. 588), he notes: 'At O b o r n e , . . . its thickness is about 5 feet, while at Louse-Hill and Wyke quarries this zone is only represented by two thin l a y e r s , . . , the two being only about 6 inches thick, but containing nearly all the species that one finds at Oborne...'; and of the Inferior Oolite of Sherborne as a whole, that it 'can be very well divided into four zones, which are extremely well marked, but vary greatly in thickness at different localities; and it is probably this variation in thickness, and sometimes almost complete absence [his italics] of a zone, that has led to very much confusion.' In other words, ammonites can reveal not only those zones that are present but also those that are missing: they can reveal significant sedimentary non-sequences--the incompleteness of the geological record. This point was to assume ever more significance in his later work. Buckman's next major stratigraphical paper, also published in the Journal (1889a) was concerned with an old problem: the age and classification of the Cotteswold, Midford and Yeovil Sands of Gloucestershire, Somerset and Dorset respectively. These formations lie between the clays of the Upper Lias below and the limestones of the Inferior Oolite above: to which should they be assigned? They consist throughout their outcrop of very similar yellow, unfossiliferous fine-grained micaceous sands 30-50 m thick, interspersed with thin, harder bands of sandstone containing fossils, including occasional ammonites. These ammonite 'horizons' (p. 442) revealed a succession of clearly distinguishable faunas. While not all were found at any one locality, whenever several did occur they always did so in the same relative sequence. Their homotaxis therefore indicated synchronism. Their number and distinguishability showed that ammonite biostratigraphy could achieve time-resolutions that were much finer than those indicated by the lithostratigraphy. Hence it should be able to provide a way of analysing the problem: 'Now the questions arise, Do these series of Sands begin on the same horizon, and, including the Cephalopoda-bed, do they end on the same h o r i z o n ? . . . Are the sands all on one horizon, as stated by Wright; or are they on two different horizons, as Oppel and my father thought?'. The answer to the first question was emphatically no.

TIME FROM FOSSILS Buckman showed, firstly, that both the lower and upper boundaries of the Sands were highly diachronous; secondly, that the lowest and highest ammonite horizons found in the Sands at one locality could pass laterally into Upper Lias clays or Lower Inferior Oolite limestones respectively; and thirdly, that the thicknesses of the sands associated with or lying between adjacent faunal horizons could vary from 1 to 50 m. Hence, in his own words: It has been observed that attention to lithology is likely to ensure success in the matter of correlation. I am bound to confess, however, that my experience of Jurassic rocks tells me that in many cases this observation is quite incorrect. Within the limits of one basin it may happen that the same horizon can often be identified by the similarity of l i t h o l o g y ; . . , while in correlating the strata of one basin with those of another, such an idea will probably lead to very decided errors. The strata now to be discussed have suffered singularly in the matter of correlation from this similarity of lithology. . . . The celebrated Dr Oppel, who visited this country about 1855, comprehended the position of these sands with his usual acute perception; and had our English geologists given to his work the attention which it deserves, it ought to have been impossible for the discussion to be been maintained. As we would say today, lithostratigraphy can be a poor guide to chronostratigraphy. But this had been the central tenet both of d'Orbigny and of Oppel before him: the idea that a given faunal horizon or zone could transgress facies boundaries was hardly new. Was Buckman doing anything more remarkable than importing an idea already well established abroad, albeit perhaps into the culture of a somewhat sleepy and sceptical, local Establishment? In principle, probably not. But in practice, and perhaps implicitly rather than explicitly, yes: the important point is that 'diachroneity' implies a time-scale, that of attainable time-resolution. A n d the diachroneity of the boundaries of the Midford Sands became apparent only through a considerable refinement of the ammonite biostratigraphy. Where Oppel had divided the Upper Lias and lowest Inferior Oolite into only three Zones, those of Posidonia bronni, Amm. jurensis and Amm. torulosus, Buckman was able consistently to distinguish seven successive ammonite faunal horizons (still variously called 'beds' and 'zones'). His proposal henceforth to abandon lithology as the primary character in stratigraphic classification in favour of faunal successions based on ammonites was certainly quite definite, even if the word 'time' was not explicitly mentioned. And the drive to refine the ammonite record and to extend the refinement over the whole of the Jurassic was to become one of his main motivations during the rest of his life. These ideas surfaced explicitly in Buckman's famous paper on the Inferior Oolite of Sherborne (1893), whose appearance just a century ago this chapter commemorates. Buckman was by now well launched into his monograph on the ammonites of this formation (1887-1907). It had started very much along conventional lines, as little more than a descriptive, typological catalogue of the wealth of beautifully preserved specimens that had accumulated over the years, largely in the collections of his father and friends. But in looking for a more natural basis of classification, Buckman became increasingly aware of evolutionary connections, both through his own observations and through his increasing familiarity with the literature. The latter included, fortunately, the seminal work of Waagen (1869)

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and, less fortunately, that of Hyatt (1889 and earlier: see Donovan 1973). His complete conversion to phylogenetic classification appeared in print almost simultaneously in part (iii) of his Monograph (March 1889, 'Classification by descent', p. 125-41) and yet again in this Journal (1889b, November, 'The descent of Sonninia and Hammatoceras'). There was therefore now a second, palaeontological need for the closest possible dating of rocks, and of those of the Inferior Oolite of Dorset in particular. The purpose henceforth of stratigraphy became primarily to provide geochronology, and the classification of strata was to be according to their ages--chronostratigraphical. Buckman immediately realized the need of a dual nomenclature: one for rocks, another for their ages. He also understood the nature of Oppel's Zones, that they were rock-units, the distinguishable sedimentary expression of durations of time in a standard scale; that these durations of time should in principle be, and in his experience were, further subdivisible through refinements of biostratigraphy; and that there existed no technical term for the smallest time-units thus discernible. For this he introduced the word 'hemera' (as in 'ephemeral'). It is worth quoting the critical sentences again: On Zonal Correlation.--The geological unit for the correlation of strata has hitherto been the 'zone'. Gradually, however, it has been felt that either the zones must be increased in number, or some modification adopted, if the true faunal sequence is to be expressed with that accuracy that is now necessary . . . . The term 'Hemera'.--Its meaning is 'day' or 'time'; and I wish to use it as the chronological indicator of the faunal sequence. Successive 'hemerae' should mark the smallest consecutive divisions which the sequence of different species enables us to separate in the maximum development of strata. In attenuated strata, the deposits belonging to successive hemerae may not be absolutely distinguishable, yet the presence of successive hemerae may be recognized by their index-species, or some known contemporary; and reference to the maximum development of strata will explain that the hemerae were not contemporaneous but successive. The term ' h e m e r a ' . . . is designed as a chronological division, and will not therefore replace the term 'zone' or be a subdivision of it. Taken by itself, as Arkell remarked, it would be difficult to imagine anything stated much more clearly than this. But Buckman went further, driven by the palaeontological considerations alluded to above. Right at the beginning he states: I may, however, remark that the division of the Jurassic period on palaeontological g r o u n d s . . , is a necessity. Ammonites have been chosen as the indicators of horizons [sic], and their rapidity in development [evolution] makes them peculiarly suitable. Therefore, as far as possible, the chronological u n i t and the Ammonite-species should go together: and any system of grouping the chronological units should depend on the epacme, acme, and paracme of Ammonite-families. And later: The term 'Hemera' is intended to mark the acme of development of one or more species, lit] will therefore enable us to record our facts correctly; and its chief use will be in what I may call 'palaeo-biology'. Buckman then proceeded to describe and analyse the

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strata around Sherborne. He took into consideration some 20 quarries over a distance of only 12km of outcrop, recording 17 of them with their fossils, bed by numbered bed, down in some cases to thicknesses of only 1-2 inches (3-5 cm). The frame of reference for correlation was now a succession of 12 hemerae in place of the 4 0 p p e l i a n Zones used previously. The interesting conclusions were as follows. Firstly, the thicknesses of the 'strata deposited during the ... hemerae' (p. 493) could vary dramatically: those of the Garantianae hemerae, for instance, represented by 6 m (20 feet) of Sherborne Building Stone north of the town, had shrunk to a mere 5 cm (2 inches) at Bradford Abbas, only 6 km away. Secondly, what appears to be a single bed in one section can contain the faunas of several hemerae--a case of what today we would call a condensed deposit. The example Buckman quoted was the famous Fossil Bed of Sandford Lane, a single solid bed of limestone only 45 cm thick. Thirdly, strata representing one or several hemerae could be absent altogether in a section, and a common reason for this was synsedimentary erosion, sometimes directly reflected in the spectacular erosion-planes for which the Inferior Oolite of Dorset has become famous. 'The "incompleteness of the record" and the attenuation of the deposits are especially noticeable' (p.484). In either case, a faunal condensation or a total stratal non-sequence at one locality can be demonstrated only by correlation with the succession at another at which it is more complete. The logical conclusion of this argument, which Buckman seemed to wish to indicate in his introductory sentences quoted above, with their reference to 'the maximum development of strata', is that we can only be sure that our hemeral succession is complete when we have pieced together the complete stratal succession. The dilemma is immediately obvious: but how do we know when the stratal succession is complete? The further Buckman cast around for correlations the less complete the succession around Sherborne turned out to be. The hemeral succession of Sherborne could be successfully carried over to the outliers of Inferior Oolite at Dundry, near Bristol, and to the Cotswolds, with further dramatic changes of facies and thicknesses. They could even be identified in continental Europe. His observation (p.494), that in the Sandford Land Fossil Bed 'no other locality in England yields the same fauna as the lower part of this bed', remained true until two years ago; and his recognition that it does occur around Gingen in southern Swabia in sections described by Waagen (1867) was a brilliant act of correlation. (For the first modern description of the Swabian succession, see Oechsle 1958. The definitive guide-fossil (p. 105) is Shirbuirnia [sic] stephani (S. Buckman, 1882)). In what were to be his last two papers on the Inferior Oolite based on his own field-work, he takes in the successions of the Dorset coast and nearby localities. In the first of these papers (1910a, table III), the number of hemerae in the Inferior Oolite has grown to 18. But what is probably the most important general conclusion to emerge from the whole study of the Inferior Oolite is summarized in the introduction to the second paper (1910b, p. 90): A schoolboy once defined a net as a series of holes strung together, and the Dorset Inferior Oolite might be defined as a series of gaps united by thin bands of deposit . . . . the deposits are so local, the deposits of one place correspond to the gaps of another. Therefore many localities have to be placed together to produce the full tale of the Inferior Oolite.

To paraphrase the introductory motto on the fly-leaf of Vol. III of Type Ammonites (Buckman 1923), the more complete the faunal record becomes, the less complete the sedimentary record turns out to be. Buckman's field observations have stood the test of time down to the smallest detail (see e.g. Parsons 1974, 1976, 1980). Almost the only changes have involved even further refinement rather than revision--the number of successive ammonite faunas now recognizable in the Inferior Oolite has grown from the original 12 of 1893 to over 40 (Callomon & Chandler 1990). Buckman's analysis stands as one of the all-time classical landmarks of stratigraphy. Its wider contributions to the subject as a whole were two-fold. Firstly, it showed how detailed biostratigraphy of ammonites could be used to resolve geochronology across a complex mosaic of disparate lithological units formed in a tectonically quiescent regime, that of Jurassic southern England. This evokes the interesting parallel with the contemporary achievements of Lapworth (recently reviewed by Fortey 1993), who showed how the biostratigraphy of graptolites could be used to resolve geochronology across a complex mosaic of disparate tectonic units in a monotonous sedimentary regime, that of the Siluro-Ordovician Southern Uplands. Biostratigraphy succeeded where lithostratigraphy had failed. Secondly, it had shown how the time-resolution of the geological record could be carried well beyond the conventional limits of Oppelian standard zonal chronostratigraphy. It remains only to make one further important point. This concerns the basic principles behind the refining of time-resolution by polyhemeral analysis. As was shown in the introduction, conventional refinement of standard chronostratigraphy proceeds by successive subdivision of a geological column that is regarded from the outset as being always complete. The act of delimiting additional smaller units within larger ones by inserting newly-defined time-planes does not enlarge the duration of geological time being classified. It merely provides ways of characterizing that which was assumed to be already there, even if it had not been previously recognized. In contrast, polyhemeral refinement proceeds through the new discovery of hemerae that are then inserted into the previously known succession. It makes no assumptions as to what might be there before it is discovered: the implication is in fact the opposite, that the record is a priori incomplete, the gaps waiting to be filled. There is no theoretical limit to the number of hemerae that could in principle be discovered. The process is therefore one of successive additions rather than subdivisions, of what might be called bottom-upwards synthesis. This leads immediately to the question of the time-duration of a hemera, and this is discussed further below.

Polyhemerai chronology: difficulties and objections Buckman's paper of 1893 started a vigorous debate that was to last for 40 years, terminating in another masterly and comprehensive review by Arkell (1933, pp. 17-37). Most of this debate is now of little more than historic interest. There were objections based largely on misinterpretation, or non-comprehension, of what Buckman had written. Some of these are to be found in the reports of discussions following the reading of Buckman's papers at the Society's meetings. Their only residual value is as entertainment. Mr H. B. Woodward had some difficulty with the concept of a

TIME FROM FOSSILS zone (Buckman 1889a, p. 473, repeated in Woodward 1892, p.298): 'Zones are assemblages of organic remains of which one abundant and characteristic form is chosen as index'. To which Buckman is said to have replied that the fossils in a museum would fit this definition. Professor Blake (in Buckman 1893, p. 522), on learning of the highly attenuated beds assigned to a zone or hemera in some sections, thinner than the ammonite that was its index, foresaw that in such cases the ammonite would have some difficulty in fitting into its own zone. Buckman duly showed how this trick could be done by producing a specimen of an ammonite, probably of the same species of cadicone Teloceras that can still be collected at Oborne today, planed off by 'the same erosion-plane that had attenuated the bed (Davies 1930b, p. 227, and see Fig. 4 below). Even more telling would have been the pebble lags made up largely of rolled, bored and epifaunaUy encrusted ammonites found at numerous horizons, evidence of hemerae whose former sediments had been totally removed, probably by winnowing. Mr A.J. Jukes-Browne (1903) and others unnamed had a general difficulty with time-rock duality. Buckman tried to come to their help (1902, 1903) by appeal to every-day experience, including the famous parable of the Dorset labourer whose lunch--the tangible manifestation of lunch-time--had been eaten by a dog: would we have said, 'My dinner is gone: one o'clock is absent'? Further complaints at this level of discourse clearly reached him from the editors of journals (ours included) in the form of anonymous referees' comments, for Hugh Torrens has discovered a previously unpublished epigram in Buckman's papers that we both feel should be shared by a wider public, especially in the climate in which we toil today: Speak kindly of the referee Forgive him if he teases He doesn't do it to annoy He really thinks it pleases.--S.S.B. (A not altogether dissimilar passage is to be found in Dodson 1865). An illuminating account of the contemporary state of geological opinion on zonal palaeontology that Buckman so robustly invaded, has been given by Davies (1930b). More serious criticisms arose from two causes: incompleteness, in a failure to follow arguments to their conclusion and to realize their implications; and overspecification. This led to contradictions and uncertainties, some of which persist in the practice of stratigraphy to this day. The main misunderstanding stemmed from the failure to distinguish explicitly between the time-duration of a hemera and the time-intervals between hemerae. It lies at the heart of the distinction already made above between refinement by subdivision of a continuum and refinement by addition through insertion into a discretely-spaced series. Trueman (1923) realized that even after lengthy debate there was still no term for the rock-equivalent of a hemera. He therefore introduced the term epibole, which has led to further confusion that need not be reviewed here. Then, how to recognize hemerae? Hemerae of what? Here we come to problems generated by overspecification. Buckman referred his hemerae explicitly to the 'acme of development of one or more [ammonite] species' (see quotations above). How are such species defined? Few attempts at zoological classification can have aroused greater

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controversy at the time than had Buckman's own taxonomy of ammonites. In successive species of the same lineage, phyletic classification in palaeontology runs into well-known problems arising from the dimension of time, discussed for instance by Bather (1927) who introduced the useful term transient for a segment of a phyletic chronospecies (see also the discussion of a particular case among thre ammonites by Callomon 1985b, p. 557). And what is meant by the 'acme' of an evolving species? And even if we could define it, how would we recognize it in the rocks? All we can see are 'ammonite horizons': beds containing ammonites. And if successive acmes involved members of different co-existing lineages, could such hemerae overlap? These points were taken up by Trueman and others. Some beds contain ammonites, many do not. Could the absence of ammonites be due to ecological factors? In other words, could biostratigraphical gaps reflect not only stratigraphical but also faunal non-sequences? Could fossil horizons be diachronous? In which case, would the hemera of a species become something that varied from place to place, and hence merely a synonym of a local range biozone? Or would what we see at one place be merely a local manifestation of part of a hemera, a sort of 'teil-hemera'? And how long, in years, was the duration of an ammonite hemera and how long the intervals between them? And why only ammonites? And so on. It was uncertainties such as these that contributed to the demise of further attempts to develop Buckman's polyhemeral methods in Jurassic geochronology. But the greatest contributor must have been Buckman's attempts in his last years to extend them over the rest of the Jurassic. This made him depend almost entirely on the field observations of others, when available, or on his own intuitive deductions of phyletic relationships based largely on Hyattian 'biogenetic laws' when not. Morley Davies' final compilation of all the hemerae coined by Buckman, published in an editorial appendix to the last volume of Type Ammonites (Davies 1930a), lists some 375. Unfortunately, most of them, particularly in the Upper Jurassic, are not based on established fact. As Arkell wrote (1933, p. 36): 'It is one of the great misfortunes for Jurassic geology that when increasing age and frailty prevented Buckman from continuing active field-work, he lost sight of the distinction between results obtained with hammer, collecting bag and field notebook, and those arrived at by speculation and deduction from matrix at home . . . . the two kinds of results are almost inextricably interwoven in his later published works. Only those with intimate local knowledge of the English Upper Jurassic rocks can hope to distinguish the two'. Like the clock that struck thirteen: an event not only raising disbelief but also casting doubt on all that went before. Arkell's epitaph in effect brought to an end a chapter in the development of methods of geochronological refinement.

The limits of biostratigraphic time-resolution: characteristic faunal horizons Although Buckman's presentation of geochronology based on biostratigraphy may have been faulty, his methods of recording field observations, both of rocks and of fossils, were admirable. So was his primary goal: to 'mark the smallest consecutive divisions [of time] which the sequence of different species enables us to separate'; and the purpose

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to which he wished to apply the results, the analysis of evolutionary 'palaeo-biology', seems more appealing than ever (see the apposite reviews by Paul (1985) and Fortey (1985)). The needs for high-resolution chronostratigraphy have only widened (e.g. Hailwood & Kidd 1993), for instance in the combination of sedimentology and sequence stratigraphy as analytical techniques in the study of basin evolution tied to the history of eustatic sea-level. Interest in the refinement of Jurassic chronostratigraphy to the limits attainable by means of ammonite biostratigraphy, following Buckman's methods, was revived 30 years ago (Callomon 1964), when the idea of a characteristic faunal horizon as the ultimate time-diagnostic infrastandard-subzonal stratigraphical unit was explicitly reintroduced. It has therefore been a relatively simple matter to revive the core of Buckman's methods, stripping away what is unnecessary and filling in what had been missing. The basic approach, already outlined above, is a positivist one: to start with the record of what is observable in the field as stratigraphy and then to interpret it with minimal assumptions of time. The argument that follows has been generalized to apply not only to Jurassic ammonites, but equally to other groups of fossils, with modifications that depend on their time-diagnostic characteristics. The greatest contrast is probably to be found between the nekto-benthic ammonites and planktonic micro- and nannofossils, and their applications to stratigraphy have been compared elsewhere (Callomon 1994). (1) Rocks and stratigraphical horizons. The rocks continue to provide the basic observations from which all else must be derived, including time. (This is the opposite of Buckman's approach, in which the epiboles were derived from the hemerae). A succession of strata in a section define the stratigraphical horizons at which fossils are found. All sedimentary successions are at one scale or another lithologically discontinuous and it is usually convenient (but not essential) to break them up into distinguishable units, or beds. How beds are differentiated in practice is a matter of lithostratigraphical judgment and need not concern us here. (2) Fossils and faunal horizons. Fossils, singly or in assemblages of taxa and individuals, may occur at many horizons in a section at one locality. Taking the taxa one by one, their known ranges span the strata that are their local range biozones. ('Known', because we have resolved to exclude from the argument that which is unknown. Hence biozones change with the state of knowledge (Callomon 1985a). The 'total range biozone' much cited in theoretical discussions, which is represented in a section by a local partial-range biozone, or teilzone, can never be known). Local range biozones of different taxa usually begin and end at different levels. Local concurrent-range biozones are therefore shorter than individual range biozones, and total-assemblage concurrent-range biozones are the shortest. Every first and last appearance of a taxon in a section then marks the boundary of a total-assemblage concurrentrange biozone (the unitary association biozone of Guex 1987) and the faunal succession (or floral: but faunal hereafter for short) as a whole becomes a time-ordered sequence of distinguishable, non-overlapping totalassemblage concurrent-range biozones. Conversely, applying the Principle of Biostratigraphic Synchroneity, all the stratigraphical horizons (parts of a bed, a whole bed, or

several beds) making up such a biozone are, as far as can be judged from the fossils alone, effectively of the same age. They may therefore be treated as equivalent to a single, faunally indivisible bed representing the sediments of a short period of t i m e - - a geological instant. They may therefore be called simply a faunal horizon. The local biostratigraphical succession has become a quantized, time-ordered sequence of distinguishable faunal horizons. But what makes successive faunal assemblages distinguishable? And which distinctions are of interest? (3) Guide-fossils and characteristic faunal horizons. Not all faunal horizons as defined above are of practical value for geochronology. We are interested only in the subset of fossil taxa in an assemblage that we believe to be of value for at least regional time-correlations between sections and hence for the construction of a regional geochronology--the guide-fossils. But how do we know which they are? Distinctions between fossil assemblages may be ascribed to five factors. The first two are experimental. (a) Quantity and quality of available material: quantity includes total absence-collection-failure. Deficiency on both counts can be made good by more work: improving the state of knowledge. (b) Taxonomic skill: the observer's ability to distinguish taxa. The next two are ecological. (c) Ecoenvironmental factors: those factors that determine habitat, seen in changes of faunal compositions of fossil assemblages (relative numbers of different species, sexes or ontogenetic stages), including the limiting case of total absence. (d) Ecophenotypic factors: environmental factors that can induce somatic changes in organisms seen as changes of shape in fossils. Both ecological factors are time-reversible and likely to be local; fossils of organisms sensitive to either of them are facies-dependent. Lastly, and most importantly, (e) Genotypic evolution: seen as the phenotypic time-evolution of the morphological characters of the fossils we collect. It is effectively irreversible (although the evolution of some characters may be reversible, leading to homoeomorphies) and likely to be widely independent of geographical distributions, hence of greatest value in the selection of guide-fossils. The identification of time-diagnostic guide-fossils is an art and proceeds by trial and error, the trials including tests to assess the importance of all five of the factors enumerated above. Such trials usually involve tests of conjectural regional correlations and depend on the characteristics of the fossil group under consideration. Once the guide-fossils have been selected, we apply to them the same arguments as before and describe their successions in terms of their own effective faunal horizons: the concurrent-range biozones of restricted subsets specified by the guide-fossils. These faunal horizons of .guide-fossils are therefore the sought-for minimally chronologically distinguishable regional stratigraphical units in the stratal succession. They may be referred to as characteristic faunal horizons--characteristic of the specified group of guide-fossils:

A characteristic faunal horizon is a bed or series of beds, characterized by a specified taxon or assemblage of time-diagnostic guide-fossils, within which no further stratigraphical differentiation of the fauna can be made.

TIME FROM FOSSILS A characteristic faunal horizon may be recognized in a single section if its guide-fossils are already known. It may be recognized further afield:

Two local faunal horizons at different places are effectively of the same age if their guide-fossil faunas cannot be distinguished. This is the Principle of Synchroneity restated in terms of faunal horizons and, as before, forms the basis of all biostratigraphic time-correlations. Such correlations are clearly subject to the uncertainties of 3(a-b) above, which are what make them approximations. (4) Characteristic faunal horizons and geochronology. Faunal horizons as defined above are rock-units in the same class as other types of biozone. The periods of time represented by their fossils and sediments are a priori unknown. They are the shorter of either the minimum time needed for one characteristic fauna to have evolved into another distinguishable one, within the experimental uncertainties implied by factors 3(a-b) above, or of the times of formation of the sediments. Any realistic estimates of what the periods actually were must come from other sources. The periods of time between faunal horizons cannot be deduced from the fossils alone either. It becomes important, therefore, to distinguish clearly between (i) the time-duration, 6ti, represented by the ith faunal horizon, and (ii) the time-interval, 6tij, between the times of formation, ti, tj (the ages), of the horizons i, j. There seem to be as yet no technical terms for precisely these quantities, although there is no shortage of terms that come close. A selection was given by Arkell (1933, pp. 21-22). All of them are deficient in one way or another, but this is not the place to add to them. Buckman's 'hemera', stripped of references to 'species' and 'acmes' conveys the right spirit; but I am attracted by the term 'biochron' introduced by Williams (1901). It seems sufficiently general to encompass what is required: the biochron (6t~) of a faunal horizon. An interval 6tij represents a non-sequence or sedimentary hiatus: stratigraphical, faunal, or both. Geochronology deduced from successions of faunal horizons makes no a priori assumptions, however, concerning the relative magnitudes of durations 6t~ and intervals 6tij, least of all the special assumption that 6t~j is zero, that the faunal record is complete and that 6tij therefore represents the evolutiontime of distinguishable fossil assemblages. Refinement of biostratigraphic geochronology proceeds in two ways: reduction of the experimental uncertainties inherent in the characterization of faunal horizons; and discovery of new horizons and their insertion into hitherto unrecognized gaps in the sequence. We can never know when the process is complete, but is useful to devise some measures of success, some indices of geochronological finesse, for comparing the power of one technique with that of another. There are two that come to mind: (i) the secular resolution: the smallest time-interval 6t in the geological record that can be resolved; and (ii) the secular resolving-power, R,: the inverse of the secular resolution as a fraction of the age of the rocks in which the time-interval is being resolved,

R, =t/6t In the geochronology of faunal horizons we assume the

137

duration dti is negligible and that what we resolve is 8t~j,: R, (faunal horizons) = t~.i/6t~i, Some estimates are given below. In summary, the construction of a biostratigraphical geochronology based on the differentiation of characteristic faunal horizons differs very little from what Buckman actually did in practice, as, indeed, have many others, even if not articulated in these terms. It is a universal method, one that proceeds from the minimal premise of an incomplete record that is to be refined by addition at the lowest levels of observation--the process of bottom-upward synthesis.

Jurassic ammonites as guide-fossils Many fossil groups have been successfully used for high-resolution geochronology in the Jurassic, but almost always only as substitutes for ammonites when these fail. What are the factors that combine to make ammonites such pre-eminent guide-fossils? Firstly, an average, individual, well-preserved specimen, treated as an ideogram, is rich in morphological characters that convey a lot of information. It is the ability to grasp minute distinctions between such ideograms that singles out the human eye as a device for pattern-recognition, far outstripping any combination of ruler and computer currently available in digitized biometry. This is why passports continue to carry both the photographs and signatures of their legitimate bearers. It follows that even among ammonites, strongly sculptured forms such as Kosmoceras are better guide-fossils than smooth, featureless ones. But for its potential to be realized, the human eye has to be trained, and the limit of what can be achieved with ammonites as guide-fossils thus depends strongly on the taxonomic skill of the stratigrapher, perhaps more so than in many other groups (3(b) above). Secondly, the number of specimens hence needed to characterize a stratigraphically diagnostic assemblage is relatively small. The specimens found at one level can usually be readily divided into groups differing strongly in morphology, assigned to different families or genera and belonging to separate lineages. Phyletic diversity is usually low, and more often than not the time-diagnostic forms are those of a single genus or family. The distinguishable taxa making up the characteristic assemblage tend in fact to be the variants of single biospecies, the transients of a single lineage (Callomon 1985b), but such an assignment to a particular zoological category is irrelevant for stratigraphical purposes. Typically, in ammonites, 5-10 specimens, if well-preserved, should suffice to characterize a faunal horizon, and a similar number to recognize it. Very often, even a single specimen can limit the possibilities to a very narrow range. Thirdly and most importantly, the changes of morphology with time seen in successions of assemblages are determined almost entirely by genotypic evolution (3(e) above), which, for reasons still entirely unknown, was so much more rapid in ammonoids, from their earliest days in the Palaeozoic, than in any other group of invertebrates (except perhaps monograptids). The changes one is looking for are now the smallest detectable changes in the composite ideogram of whole assemblages of variants of transients of an evolving lineage, not just of individuals. Sometimes the

138

J.H.

CALLOMON

changes involve no more than a shift of the centre of gravity of the distribution of the variability. At others, they can affect mainly one character, such as adult size, in all variants. Often, however, they involve nuances of sculpture too subtle to quantify. This point was well brought out in an attempt to apply Buckman's descriptions of one of the dominant family of ammonites from the lower Inferior Oolite, the Graphoceratidae, to their biostratigraphy in the much more complete successions of southeastern France (Caloo 1971). Such delicate changes in morphology can only be relied upon as time-indicators if alternative explanations can be ruled out. The most likely would be ecophenotypic (3(d) above), but ammonites appear to have been remarkably resistant to such influences. The disproof again comes from homotaxial correlations, in finding the same successive changes at localities sufficiently far apart for identity of biofacies to have been highly unlikely, and this is almost invariably what has been observed. Ammonite successions of a single lineage are found to be the same within the whole of a biogeographic province, which means over distances of at least 1000-2000 km. I know of only one indisputable example of ecosomatic modification of Jurassic ammonites, found in the Bajocian pelagic carbonate sea-mount facies of the Venetian Alps (Sturani 1971) and perhaps the related fills of Neptunian dykes in Sicily (Wendt 1971). It takes the form of dramatic dwarfing, the fully mature adults being only half or a third as large as usual elsewhere. Yet, remarkably, this is the only effect. The normal course of their anatomical ontogeny, including the considerable modifications seen in their sexual dimorphism, is retained intact--as are all the morphological nuances characterizing their faunal horizons. More serious are ecoenvironmental factors. These can be local or distant. Locally, the relative compositions and abundances of assemblages can vary rapidly from place to place and level to level, as Buckman discovered. In many of the well-bedded epeiric or shelf-sea sediments with which most ammonite biostratigraphy has been concerned, ammonites are in fact rare or absent. Ammonites were therefore strongly facies-dependent in their local distributions. The problems this creates can be largely overcome by hard work, as Buckman also showed, relying on the converse of the Principle of Synchroneity: if assemblages at two nearby localities are of similar composition but morphologically distinguishable, it implies that they are of different ages. On the intercontinental scale, a further problem emerges. Ammonites were all more or less provincial in their habitats. Bioprovincialism could be extreme, when evolving lineages were restricted to regions of endemism, such as those traditionally referred to as Boreal or Tethyan. In such cases, long-distance interprovincial correlations rarely go beyond the level of zonal precision. But there are many other groups of so-called cosmopolitan ammonites in whose evolution rough parallels can be perceived all over the globe, down to generic level, yet which persistently differ in details of just the kind that would, within a province, be ascribed to small differences in age. Here we are almost certainly concerned with geographic subspeciation. Hence if two distant assemblages differ slightly in aspect, e.g. in the Lower Lias of Britain and the Andes, the explanation could be either age-difference, or geographic subspeciation, or both, and the two factors may never be separable. Such

uncertainty imposes limitations on the use of ammonites for correlation. In summary, time-resolution down to the finesse of ammonite faunal horizons can be achievable within the areal extent of a faunal province but not beyond--distances of a few hundreds or thousands of kilometres. Hence, each faunal province has to have its succession of faunal horizons worked out separately.

A m m o n i t e faunal horizons of the Jurassic Biostratigraphy by faunal horizons was actively taken up in France by Gabilly in an important and wide-ranging review of stratigraphical methods at the second Luxembourg Colloquium in 1967 (not published till seven years later: Gabilly 1974) and systematically applied by him to a revision of the Upper Liassic Toarcian Stage at its type-locality at Thouars (Gabilly 1976). Since then, the method has been increasingly adopted in the Jurassic both of Europe and further afield. It can often be applied equally well also to a re-analysis of older stratigraphical descriptions, many of which have never been fully evaluated. Few major regional reviews in recent times have ended without a zonal synthesis of one kind or another, although precisely what kind is rarely stated: usually presented in tabular form to resemble a continuous, standard, chronostratigraphical, Oppelian scale but, more often than not, revealed by the text to amount to no more than a succession of selected local biozones of unknown chronological extent or completeness. Such stratigraphical information can be recast in the form of a succession of faunal horizons with no loss of information. Some selected examples of stratigraphical analyses based on characteristic faunal horizons are listed in Table 2. There is room to expand briefly on only two of them: the Inferior Oolite where it all began, and the Oxford Clay at Peterborough, made famous by Brinkmann (1929).

The Inferior Oolite of southern England The list of faunal horizons characterized today is shown in Fig. 3. It is set against a standard zonation that is becoming widely accepted, although none of it has yet been formally defined in terms of boundary stratotypes ratified by international agreement. The Dorset Inferior Oolite spans the Aalenian and Bajocian Stages, and the basal Bathonian. The number of faunal horizons now stands at 56, compared with Buckman's 18 of 1910 (and the 55 hemerae, largely conjectural, in the final compilation by Davies in 1930). Horizons A a - I - B j - 3 are based on the evolutionary transients of one family, the Graphoceratidae; Bj-4-Bj-12, on Sonniniidae; Bj-13-20, on Stephanoceratidae; and Bj-21 Bt-1, Garantianinae and Parkinsoniinae. The list is even now incomplete, for there are indications both in Britain and abroad of further assemblages and horizons to be differentiated. The faunal succession of the Garantiana and Parkinsoni Zones, for instance, has so far received little more than cursory attention. Figure 3 therefore summarizes the biochronology of the Inferior Oolite based on ammonites, presented, as we have seen, as a series of effectively instantaneous, well-separated snapshots. But how instantaneous, and how well separated? How complete is the record of geological time? To attempt to answer these questions required evidence from other sources, such as sedimentology.

Table 2. Some Jurassic chronostratigraphical classifications down to characteristic ammonite faunal horizons Standard

Europe L Jurassic, Sinemurian L. Pliensbachian Toarcian M Jurassic, Aalenian (a) (b) Bajocian Bathonian L. Callovian M. Callovian Callovian U. Jurassic, Kimmeridgian Arctic M Jurassic, U. Bajocian-L. Callovian U Jurassic, U. Callovian-M. Volgian M - U Jurassic, U. Bajocian-Kimmeridgian America M - U Jurassic, U. Bajocian-Oxfordian U Jurassic, Oxfordian-Tithonian

Stages V

Zones VI

Subzones VII

Faunal horizons

1

6

17

61

1

1

2

6

18

2

1 1 1 1 1

6 3 5 8 8

15 4 9 17 11

22 11 16 37 16

3 4a 4b 5 6

1 1

3 2

8 4

16 21

7 8

1 1

6 6

14 11

23 28

9 10

2 3 4½

12 28 29

----

37 46 100+

11 12 13

3½ 3

---

---

47 22

14 15

Notes

(1) Page (1992): Great Britain. Traditionally one of the most finely subdivisible and widely correlatable parts of the Jurassic, now probably approaching the attainable limits of time-resolution. Many of these horizons can be recognized all over Europe west and north of the Alps. The number of Zones and Subzones is unchanged from those of Dean, Donovan & Howarth (1961). (2) Phelps (1985): Ibex-Davoei Zones of Britain and France. Analysed in terms of 'zonules', here interpreted as faunal horizons. Areal extent as note (1). Approaching completeness. (3) Gabilly (1976): western France. Standard zonation differs in detail from that adopted in Britain by Dean et al. (1961), summarized in slightly revised form by Howarth (1992); number of Zones and Subzones almost the same. Still scope for further refinement. (4a) Contini (1970): eastern France. Most horizons identical with those of Dorset, with some additions and omissions; close to complete. (4b) Callomon & Chandler (1990). For comparison, the number of Buckman's hemerae in 1910 was 6. Extent as note (1), especially Scotland and Iberia. (5) Callomon & Chandler (1990) with additions: southwestern England. Most horizons recognizable here and there all over Europe, but successions elsewhere indicate still considerable gaps in the English succession. (6) Westermann & Callomon (1988). A compilation and review based on the works of many authors all over Europe, reflecting unusually sparse and scattered occurrences of ammonites. Scope for considerable further refinement in the Middle and Upper Bathonian. (7) Callomon, Dietl & Page (1989), Page (1989). The standard Subboreal succession as seen in Britain and Germany, based on the evolution of only two lineages, the Macrocephalitinae and Kosmoceratidae; probably now close to the attainable limit. (8) Brinkmann (1929), as reinterpreted by Callomon (1984a). Subboreal based on the evolution of the Kosmoceratinae. See Fig. 2. (9) Cariou (1985): Submediterranean, western France. Widely recognizable at this level of resolution, especially in Iberia, but still scope for considerable further refinement. (10) Hantzpergue (1'989): western France, Aquitaine Basin. Faunal provincialism in ammonites became acute from this level upwards; analysis of the immensely rich successions of faunal horizons in the Subboreal Province (including Britain) and the Rhodano-Franconian Submediterranean Province (including the classical White Jura of the Jura, Swabia, Franconia and southern Poland) has hardly begun. (11) Callomon (1993b): East Greenland. Independent standard Boreal zonation still not closely correlatable with that of Europe, but applicable over the whole of the Arctic. Subdivision into Subzones not yet attempted. (12) Callomon & Birkelund (1980, 1982); Birkelund, Callomon & Ftirsich (1984): East Greenland. Partly Subboreal and Boreal Provinces. (13) Callomon (1985b). The faunal horizons of all the known transients of a single lineage, the Cardioceratidae, wherever found in the Boreal Realm, arranged in time-ordered sequence. (14) Callomon (1984b): western North America, spanning the craton and at least three allochthonous terranes in the Cordillera. An almost extreme case of highly discontinuous, fragmentary successions at widely scattered localities, yet providing a quite respectable Jurassic biochronology in terms of ammonite faunal horizons. Scope for almost unlimited refinement in principle, severly restricted by non-fossiliferous facies in practice. (15) Callomon (1993a): Mexico. Based entirely on re-analysis of previous accounts.

139

140

J.H.

CALLOMON

Standard zonation

(a) LOWER BATHONIAN

Zones

Subzones

(c)

Standard zonation

AALENIAN

Zones

Bt-3

[ Oxycerites yeovilensis

Bt-2

[ Morplugceras macrescens

Bt-1

Zigzag

[ Parkirtsonia convergens

Yeovilensis

Aa-16

Euhoploceras acavthodes

Macrescens

Aa-15

Graphoceras formosum

Aa-14

Graphoceras concavum

Aa-13

Graphoceras cavatum

Aa-12

Brasilia decipiens

Aa-ll

Brasilia gigantea

Aa-10

Brasilia bradfordensis, similis

Aa-9

Brasilia bradfordensis, baylii ..

Convergens

UPPER BAJOCIAN Bj-28

[ Parkinsonia bomfordi

Bj-27c

[ Parkinsonia pseudoferruginea

Bj-27b

[ Parkinsonia parkinsoni

Bj-27a

l Strigoceras truellei

Bj-26b

I Parkinsonia rarecostata

Bj-25

J Garantiana tetragona

Bomfordi Parkinsoni Truellei

Garantiana

... ..

Bj-24 Bj-23 Bj-22

[ Garamiana dichotoma [ Leptosphinctes davidsoni I Caumontisphinctes polygyralis

Bj-21

[ Caumontisphinctes aplous

Bj-20

J Teloceras banksi

.._ .....

Aa-8

Brasilia bradfordensis, subcornuta

Aa-7

Ludwigia murchisonae

Teu-agona

Aa-6

Ludwigia patellaria

..............

] ]

Bj-I 7 Bj- 16 Bj-15

[ Steph. . . . . . . . blagdeniforme 1 I Stephanoceras gibbosum l ] Stephanoceras humphriesianum 7

Bj-lnb

[ Chondrocera . . . . ighti

l

Bj-laa Bj-13

I I

Chond. . . . . . . . delphinum

]

bilicum

]

Bj-12

[. Steph. . . . . . . . . 'rhytum

Bi-1 Ib Bj-I la Bj- 10 Bj-9

[ I ] [

Stepha. . . . . . . . . .

J

Otoites sauzei

]

Witchellia laeviuscula ' Witchellia ruber

Ancolioceras opalinoides

Polygyralis

Leioceras bifidatum

1

Aa-2

Leioceras lineatum

I

Aa-I

Leioceras opalinum

Bj-8b

[ Shirbuir'nia trigonalis

Bj-8a

L. Witchellia nodatipinguis

.]

Bj-7b

1. Witchelli . . . . . . . ta

]

Bj-7a

[ Witchellia gelasina

[

Bj-6c

I Witchellia "pseudoromani" MS

Bj-6b

[ Fissilobi. . . . gingense

Humphriesianum

Sauzei

] 'i.]

Laeviuscula Laeviuscula

Trigonalis

Sayni

] "']

Bj-6a

[ Euhoploceras zugophorum

]

Bj-5

l Witchellia romanoides

I

Bj-4

['" Bradfordia inclusa

]

Bj-3 Bj-2b

] Hyperlioceras subsectum [' Hyperlioceras rudidiscites

I I

Bj-2a Bj-1

I'" Hyperlioceras walkeri

I

[ Hyperlioceras politum

[

Murchisonae Murchisonae

Aa-3

Romani

]

Bradfordensis

Baculata

1

Namuna evohaa

Bradfordensis

Ludwigia obtusiformis

Blagdeni

Humphriesianum

Gigantea

Aa-4

Fig. 3.

[ Teloceras blagde,,i

Concavum

Aa-5

LOWER BAJOCIAN

Bj- 18

Collcavurn

Garamiana

Banksi

(b)

Formosum

Acfis

Subfurcatum

Ovalis

Discites

Fig. 3. The ammonite faunal horizons of the Inferior Oolite of Dorset-Somerset. (a) Upper Inferior Oolite; (b) Middle Inferior Oolite; (c) Lower Inferior Oolite. Note: the labelling of some horizons with additional letters a, b, c . . . reflects the insertion of further horizons recognized since the first list was drawn up in 1990, so as not to have to change the main framework of numbering introduced in that list. The letters imply no reduction or other inequality of rank and importance.

Subzoncs

Obtusiformis

Haugi Scissum

Opalinum

(Continued.)

What the faunal horizons look like in the field may be seen in Fig. 4, which illustrates three sections in weathering profile. Their details provide the evidence for the reconstruction of a long and complicated lithochronology, the residual record left by many competing processes, each with its characteristic rate acting for a specific duration, often in cyclic and repetitive sequences. Some important processes are listed and categorized in Table 3. The Inferior Oolite shows records of all of them, although no systematic analysis appears yet to have been published. Unfortunately, in any attempt to assess the durations (6t) the effects of the destructive processes dominate, largely erasing those of the constructive processes. Although the successions are all well bedded, in almost no beds have any of the finer sedimentary structures survived intensive bioturbation by deep burrowers ($7). It is therefore often not possible even to claim that the top of a bed must be younger than the bottom, and all of it must be regarded as effectively synchronous with the last bioturbational turnover. The effects of $9 and S10 are reflected in the sharp partings and erosion-planes that separate beds, but again, there is no way of estimating their durations. Attempts to assess the 'stratigraphic completeness of the record' in the Inferior Oolite sedimentologically by such methods as those proposed by Schindel (1980, 1982) or Sadler & Strauss (1990), which base their time-scales on sedimentation-rates (S1 and $2), are therefore inappropriate. H o w the succession has been built up from the records of individual sections is shown in Fig. 5. The 'gaps united by thin bands of deposit' are evident. The time-durations that left no record ($8) or whose records have been destroyed ($9, 10) are often greater than the time intervals •tij, between the biochrons of adjacent faunal horizons. What is less evident, however, is any coherent relationship between the lengths of the gaps and their positions, such as might be explicable by simple sequence stratigraphy--and this across a distance of only 8 0 k m in a single basin. A large non-sequence at one locality may be correlated with several

TIME FROM FOSSILS

141

HORN PARK

BURTON BRADSTOCK

OBORNE

m

~S"

5--

":.-__~

19,b ~ _ ~-1~ .

.

~

-

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9

~

. ,

Bathonian

12 i d ~ _ . - ~ - - ~

Bt-2

® m-4 ~Bt-3

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c6_ B t - l , 2 _

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.

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-88--'"-- ~

3 -..,',

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®

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9

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® ~a-~ ~ Aa-, -

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Bj-6,7 Jan

Fig.

4. Three sections in the Inferior Oolite of Dorset shown in weathering profile (nos 1, 4 and 11 in Fig. 5).

Table

3. Ten important processes in sedimentary lithochronology

Positive evidence S1 Constructive: accumulation of sediment by local chemo- or biogenesis--autochthonous $2 Constructive: accumulation of sediment by transport---=allochthonous $3 Constructive: encrustation, chemical or epibiontic $4 Diagenetic: compaction $5 Diagenetic: differential concretionary cementation, e.g. of body-fossil or burrow infills $6 Diagenetic: general induration by cementation or recrystallization $7 Destructive: bioturbation Negative evidence $8 Neutral: non-deposition, omission surfaces $9 Destructive: differential removal of unconsolidated sediment, winnowing: lag deposits, conglomerates $10 Destructive: erosion of consolidated sediment: erosion planes, non-sequences, pebble conglomerates

The time-duration of each process may be indicated symbolically as 6t($1), 6t($2), 6t($3)... etc.

142

J. H. C A L L O M O N

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,

i.,l., a5

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Fig. 5. The ammonite faunal horizons recognized in 13 sections of the Inferior Oolite of Dorset and Somerset. The horizons are labelled at the left as in Fig. 3. The localities, across the top, are as follows: 1 BB: Burton Bradstock (Fig. 4). 2 Ch: Chideock, Quarr Hill. 3 WH: Waddon Hill, W of Beaminster. 4 HP: Horn Park, W of Beaminster (Fig. 4). 5 Be-CF: Cockroad Farm, W of Beaminster. 6 Se: Seavington St Mary. 7 LH/HH: Louse Hill and Halfway House, W of Sherborne. 8 BA: Bradford Abbas, railway-cutting and East Hill, SW of Sherborne. 9 SL: Sandford Lane, N of Sherborne. 10 CI: Clatcombe, Upper and Lower, N of Sherborne. 11 Ob: Oborne, Frogden Quarry, NE of Sherborne (Fig. 4). 12 Br-L: Bruton, Lusty railway-cutting. 13 Du: Dundry, SE of Bristol. Question-marks: probably present but not yet identified. Small circles: former presence indicated by pebble lags in conglomerates.

Brinkmann's epic study of the Oxford Clay of Peterborough (1929) remains another of the all-time classics of stratigraphy, undiminished both in its exposition of principles and in its relevance today. It gains additionally in interest when looked at in comparison with Buckman's analysis of the Inferior Oolite. Both shared the same goals: the biochronology of ammonites at the attainable limits of time-resolution and its interpretation in terms of biological evolution. But they differ greatly in the methods used, imposed by differences of stratigraphical facies that lie at almost the opposite extremes of the range. The two studies therefore strongly complement each other in contrasting techniques but arrive at general results that turn out to be very similar. The Lower Oxford Clay (now Peterborough Member) around Peterborough is 17 m thick. It consists at first glance of monotonous, grey, fine-grained siliclastic silts and clays (process $2 of Table 3) with subordinate carbonate and organic matter (mainly $1). Its structure ranges from fissile paper-shales to structureless mud-rock, in both of which macrofossils, notably ammonites, although crushed ($4), are still largely unbroken and horizontal. Bioturbation ($7) was thus not sufficiently vigorous to destroy the microbiostratigraphy. Looked at more closely, however, the succession is subdivided into more or less sharply bounded beds differing in colour and other details of lithology, ranging in thickness from 4 cm to 4 m. A recently revised description (Hudson & Martill 1994) lists a succession of 55 beds. The boundaries are often marked by shell-accumulations that certainly indicate sedimentary breaks, probably of omission ($8) and possibly winnowing ($9), but erosion proper (S10) does not appear to have been important. The sedimentology is therefore quite different from that of the Inferior Oolite, and the thickness of a bed may well be related to the time-duration of its formation. Through some 13 m of these clays, Brinkmann and an assistant over a period of seven weeks collected 3000 ammonites of the genus Kosmoceras, recording the level of each to the nearest centimetre. (Only those who have attempted to repeat this exercise can appreciate the prodigious labour involved. Arkell, in private, did sometimes wonder whether the results were as real in the rocks as they appeared to be in print, just as others had expressed doubts about Buckman's descriptions of the Inferior Oolite. A n d just as in Buckman's work, re-examination has fully confirmed Brinkmann's). In 2000 of these shells, biometric characters such as dimensions and numbers of ribs were measured. The data were however not sufficiently numerous at centimetre height-intervals to be statistically significant. They were therefore combined into lots over larger stratigraphical intervals: either, in most cases, over the whole of the bed or, in a few others, over ranges of 1 - 4 0 c m within a bed. These lots were then evaluated statistically to give sets of mean values. One of these sets is shown in Fig. 6, that of the mean maximum diameter of the adult macroconchs, Zugokosmokeras. The horizontal lines represent the boundaries between beds that were lithostratigraphically not further divisible. These beds have been numbered here for

TIME FROM FOSSILS A, Der Stature

gNgokosrt~'¢~.

108

Tabelle 89 (hierzu Abb. 98 u. 99)t). Die phylogenefisehe Entwicklung des Enddurchmessers im Stsmrn.

g.go~srao¢~¢~-

,n6lt°/o l 2 3

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Fig. 6. The evolution of the adult diameter d of macroconch Kosmoceras (Zugokosmokeras) through the Oxford Clay of Peterborough (from Brinkmann 1929). Columns from the left: arbitrary numbering of beds as in Fig. 2; stratigraphic heights in cm; number of specimens measured; mean value of diameter, d; standard deviation. At the right: standard chronostratigraphic classification.

convenience (although this is not the numbering used elsewhere) and their positions in the standard zonation of the Callovian are shown in Fig. 2. Successions of such mean values were then tested for continuity and linear regression ('trends') with stratigraphical height. There were statistically quite clear examples of breaks in the former, e.g. at +135/136cm and at + 5 5 9 / 5 6 0 c m . But neither statistically significant breaks nor linear trends could be found within a bed, e.g. bed 8, 136-460 cm. The beds are therefore also biostratigraphically not further subdivisible within the sensitivity of digitized single-character statistical biometry. This is confirmed by the non-measurable, visual characteristics assessed by eye. The assemblage of shells from each bed has its own subtle characteristic aspect that no measurement has revealed, so that even assemblages that were biometrically unresolved can readily be distinguished by eye. The beds conform almost ideally to the definition of faunal horizons, and the ammonite biochronology of Peterborough

143

can thus be discussed in the same terms as that of the Inferior Oolite. It is however more direct. The faunal horizons are all in immediate succession at one locality, instead of having to be assembled from many, and they are all based on the phyletic transients of only a single dimorphic lineage, that of the genus Kosmoceras. The question whether the morphological discontinuities at bed boundaries reflect stratigraphical gaps, or evolutionary punctuation, or both, and if the former, how large the gaps would have to be, were addressed by Brinkmann himself and revived by Raup & Crick (1981, 1982). They could not be resolved on the evidence of Peterborough alone, but the intercalation of further faunal horizons elsewhere (Callomon 1968) showed that at least in many cases, non-sequences are the major factor. The faunal horizons at Peterborough must therefore also be regarded biochronologically as a series of discrete snapshots well spaced in time. Almost identical analyses have been applied to successions of trilobites in the Ordovician of south Wales (Sheldon 1987, 1988). Pygidial rib-counts and measurements of carapace width were made on over 3300 specimens, representing eight parallel lineages, collected in fine-grained shales from 400 stratigraphical intervals of average thickness 23 cm, totalling 90 m of sediment. Yet, as at Peterborough, the data had to be combined into lots over more extended intervals to give statistically well-defined means. These were finally presented in the form of clumped values from eight discrete, effectively instantaneous faunal horizons. Comparing the mean values from what are now effectively successive transients of the eight lineages, in some cases there are significant discontinuities, in many others not. Whether results from such a punctuated stratigraphical record can be claimed to support gradualistic evolution becomes very much a matter of the definition of 'gradualistic'.

Estimates of biostratigraphical time-resolution We come now to the final step, the estimation of time-intervals in absolute terms, in years. There have been many attempts to date the standard geological column radiometrically and revisions appear almost annually. They differ among themselves for a variety of reasons, principally the residual uncertainties in individual age-determinations and the still sparse framework of secure anchor-points in the record. In the Phanerozoic, these differences can lie in the range of 1-5%. The scale shown at the left in Fig. 2 is chosen arbitrarily to be the Cambridge Time-Scale (CTS 89) of Harland et al. (1990). Radiometric age-determinations within the Jurassic are still not good enough to date with any reliability the boundaries of lower subdivisions, at Stage level (V) and below; so these are shown diagrammatically on an equal-interval approximation. In calculating durations in the Jurassic, only System boundary-ages have therefore been used. Elsewhere, estimates may be based on Series boundaries and, exceptionally, Stage boundaries as well, where their dating appears to be sufficiently reliable. Estimates of some time-durations and time-intervals are collected in Table 4. In reading this, the distinctions between three kinds of time-estimates must be borne in mind. The first of these is the time-duration, At = (t2-tl) , of the largest of the units being considered (System or Series) derived from the radiometric estimates of the ages of its beginning (tl) and end (t2). These ages are the basic

144

J.H.

CALLOMON

Table 4. Estimates of biochronological time-intervals

Unit

Number of units n

Average duration Ate, atn

Secular resolving power /~' =-{/At, ate (b)

Jurassic (146-208 Ma B P )

Standard, N W Europe System Stages Zones: ammonites Subzones: ammonites Horizons: ammonites Cf S. S. Buckman (1893-1929) Ages: ammonites Hemerae: ammonites Other groups Zones: dinoflagelates (a) nannoplankton (a)

1

At = 62 Ma

11

At,, = 5.6 Ma

76 c. 155 say 450

820 ka 400 ka at, = 140 ka

220 440 1260

47 375

16

At,, = 3.9 Ma

22

2.8 Ma

45 63

1 12 c. 56 37 25

At = 81 Ma At, = 6.7 Ma 1.4 Ma 2.0 Ma 3.0 Ma

73 52 34

2 13 21

At, = 4.0 Ma 615 ka 380 ka

220 360

6 54

At,, : 5.3 Ma 590 ka

140

1

At = 37 Ma At,, = 4.6 Ma 1.2 Ma

195

At = 45 Ma At,, --- 7.5 Ma 6.5 Ma 5 Ma

41 54

Cretaceous ( 6 5 - 1 4 6 Ma B P )

Standard, Europe, all System Stages Zones: ammonites foraminifera (a) calcareous nannoplankton (a) Standard Valanginian-Hauterivian, N Europe Stages Zones: ammonites (c) Subzones: ammonites Standard Upper Cretaceous, N America Stages Zones/Subzones: ammonites (d) Triassic (208-245 Ma BP)

Standard, Tethys-N America System Stages Zones: ammonites (e)

8 32

Permian (245-290 Ma BP)

Standard, Russia System Stages Zones (o): fusulinids, Japan (a) ammonites, Canada (a)

1

6 7 9

Carboniferous (290-363 Ma BP)

Standard, all System Stages (a) Zones: goniatites (a) forams, Donets (a) Regional, Namurian, England 'Zones' (Stages) Horizons: goniatites (f)

1

25 20 29 7 45

At = 73 Ma At,, = 2.9 Ma 3.6 Ma 2.5 Ma At = c. 15 Ma At,, = 2.1 Ma at,, = 330 ka

90 130

975

D e v o n i a n (363-409 Ma BP)

Standard, all System Stages (a) Zones: conodonts (g) Upper Devonian (a) Zones: conodonts (h) Horizons: conodonts (h) Middle- Upper Devonian Zones: ammonoids (i) Horizons: ammonoids (j)

1

7 28 15 30 19 36

At = 46 Ma At,, = 6.6 Ma 1.6 Ma At = 14 Ma At,, = 930 ka at, = 470 ka At 23 Ma At, =1.2 Ma at,, = 420 ka

235 395 790 310 88O

TIME FROM FOSSILS

145

Table 4. (Continued.)

Number of units

Secular resolving power Average duration At,,, fit,,

(b)

1 8 30 12

At = 30 Ma Atn = 3.7 Ma 1.0 Ma 2.5 Ma

425 170

Ordovician (439-510 Ma BP) Standard, all System Stages (a) Zones: trilobites, UK (1) graptolites (a)

1 19 c. 34 21

At = 71 Ma At,, = 3.7 Ma 2.1 Ma 3.4 Ma

230 140

Cambrian (510-545? Ma BP) Middle-Upper, Standard (n) Stages Zones: trilobites, UK (m) trilobites, Australia (m) trilobites, USSR (m)

6 35 46 27

At = c. 25 Ma At,, = 4.2 Ma At,, = 710 ka 540 ka 930 ka

730 970 560

Unit Silurian (409-439 Ma BP) Standard, all System Stages (a) Zones: graptolites (k) conodonts (a)

n

(a) Harland et al. (1990); (b) i- (the average age of a System) = (t2 + tl)/2; (c) Kemper (1978) N Germany; (d) Obradovich & Cobban (1975); (e) Tozer (1984); (f) Ramsbottom (1977) and Riley (in Cope 1993); (g) cited in (a), see also Aldridge (1987), Sweet (1988); (h) Ziegler (1974), Ziegler & Sandberg (1990); (i) cited in (a), and House & Price (1985); (j) Becker (1993); (k) Rickards (1976); (1) cited in (a), and Thomas et al. (1984); (m) see (1) and Palmer (1977); (n) post-Tommotian trilobitiferous Cambrian only, from recent radiometric revisions by Bowring et al. (1993), Landing (1994), that depart significantly from the estimates in (a) and retaining the older age for the base of the Ordovician unchanged; for a recent revision of the whole of the Phanerozoic chronometric time-scale, see Odin (1994); (o) estimates of zonal durations in the Permian are still determined almost wholly by the fragmentary and highly incomplete state of the biostratigraphic record, and do not therefore say much about the intrinsic biochronological resolving-power of its guide-fossils. numerical input to what follows. Their average gives the mean age of the unit, T. The duration At is the homologue of what in lithochronological resolution-analysis has come to be called the temporal scope of an analysis (Schindel 1982, summarized e.g. by Skelton 1993). The second, intermediate kind is the mean duration At~ of the n finer standard chronostratigraphic units into which At may be subdivided (i = 1 , . . . n): the mean duration of a Stage, Zone or Subzone within a System, etc. The third kind is the smallest time-interval that can be resolved by fossils, the secular resolution of the biochronology, the mean time-interval 3t 0 = At~n, where n = n ( m a x ) , the maximum number of moment that can be resolved. It is the analogue of acuity in microstratigraphical analysis of lithochronology (Schindel 1982, or Skelton 1993). (The analogy is limited, for the microstratigraphical acuity m has the physical dimensions of a time-duration, that of the accumulation-time of an observed thickness h of sediment: m=h/(dh/dt), where the accumulation-rate d h / d t is assumed to be constant and continuous. In contrast, the secular resolution t~tij is a time-interval between identified instants, a time-duration of effectively negative evidence). Lastly, it is interesting to calculate the secular resolving-power R,

=

tij/ ~tij

of the guide-fossils used to distinguish the faunal horizons i

and j, of mean age tij, as a means of comparing them in the rocks of similar ages, or at different times during the Phanerozoic. The concept can be extended to the higher chronostratigraphical units, the Zones and Subzones, by noting that their mean durations Ati are the same as the mean intervals of their mean ages,

ati

= (r,-

rj)

Finally, as ti, the age of a faunal horizon, changes during the course of a System, so does R , even at constant resolution 6tij. It suffices, therefore, also to average R , over the whole of the largest unit considered, of mean age t: R, = t / 6tij The values are given in the last column of Table 4. They show how fossils of only moderate resolving-power can achieve time-resolutions in the Cretaceous comparable to those of high-powered trilobites in the Cambrian. The resolving-power of magnetostratigraphy in the Tertiary is even less. Fine time-resolution and time-correlation are now also being achieved in various parts of the geological column by physical methods not directly dependent on radiometric dating. They fall into two classes. In the first class, the time-dependence of the quantity being measured is periodic and the time-resolution that of

146

J.H.

CALLOMON

the periodicity. Best-known is magnetostratigraphy. The signal is a binary N-R bar-code whose elements vary in relative lengths that have to be determined against some other time-dependent process, assumed to be linear and continuous - in this case, sea-floor spreading (see for example Mussett & McCormack 1993). It has found greatest application in the Tertiary (reviewed in Harland et al. 1990). Time-resolution lies in the range of 0.1 - 1 Ma which, in rocks 50 Ma old, can give resolving-powers of 50-500. But the limitations on practical applications lie in being able to identify the chron in which the age of a particular bed lies. This requires additional evidence: either from the pattern of an extended N-R sequence, assuming it to be free of gaps or, again, from fossils (e.g. Ali et al. 1993). The other method in this class is the periodic chemostratigraphy of the stable isotopes of the organic metabolic elements, carbon and oxygen, reflecting climatic fluctuations that are at least in part the response to Milankovitch cycles of insolation. The time-resolution achieved in the Pleistocene is spectacular: 20 ka in rocks up to 1.6 Ma old (resolving-power 800; Weaver 1993). But the general limitations are similar to those of magnetostratigraphy--the need to locate individual levels in the sequence. Both methods are therefore best suited for use in quasi-continuous pelagic sedimentary successions, such as deep-sea sediments or polar ice-caps (Anklin et al. 1993; Dansgaard et al. 1993) Evidence of Milankovitch cyclicity is growing in pre-Pleistocene sediments (Cretaceous, Kemper 1987; Lias, Weedon & Jenkyns 1990; and see review by Schwarzacher 1993). In the second class, the value of a measurable physical quantity varies smoothly, although not necessarily linearly, with time. The best examples are again chemostratigraphic: stable isotope compositions of elements sequestered from sea-water and subsequently preserved unchanged in the sediments, usually in calcareous fossils. ~3C and ~sO have been successfully used in the Upper Cretaceous (Jenkyns et al. 1994) but appear to be more useful for correlating short anomalous 'events' than for general purposes of dating and time-resolution. The most promising element so far is strontium. The marine ratio of ~7Sr/~6Sr in the reservoir of the world's oceans is expected to be least sensitive to short-term climatic fluctuations and, in such a heavy element, the ratio as then recorded in calcareous fossils to be undistorted by metabolic kinetic isotope-effects. Curves for the Late Cretaceous (McArthur et al. 1993, 1994) and Early Jurassic (Jones et al. 1994) are indeed found to be smooth. Time-resolutions attainable from their gradients are estimated to be 0.5-0.8 Ma in the former (resolving-power c. 100-160 in the Campanian) and 0.5-1.0 Ma in the latter (resolving-power 200-400 in the Sinemurian)--comparable to 1-2 standard ammonite Subzones.

Practical applications The refinement of biochronology to its attainable limits, with the finesse of geological time-resolution it makes possible, continues to find three important applications, to each of which Buckman made seminal contributions. The first is the historical one of general stratigraphical time-correlation, transcending lithostratigraphical boundaries and facies-changes. Such correlations play probably their most important role today in basin-analysis, particularly in sequence-stratigraphies, in which sharp time-controls on facies-equivalences and non-sequences are crucial. The

rocks provide the primary evidence of sedimentary processes spanning an enormous dynamic range of time-scales. Their characteristic periods range from those reflected in sequence-boundaries and systems-tracts at the upper end (105-107 years), to those of cross-bedded foresets, tempestites and slumps at the lower (10 2-10 +2 years). As we saw earlier, one of the first to demonstrate how the biochronology of ammonites can descend into this regime of time-scales was Buckman himself, in the case of the Bridport-Yeovil-Midford Sands of southern England. One of the most ambitious and detailed modern sequencestratigraphic classifications of a whole System must be the recent description of the Jurassic of the Normandy-Wessex Basin by Rioult et al. (1991). It rests entirely on ammonite biochronology at zonal and subzonal resolution. The second application is to another age-old problem, that of estimating the 'completeness of the geological record'. By this is meant theoretically the fraction of the total, continuous time-duration At between two specified moments tl, t2 actually recorded in the rocks. But as we have seen, bio- and chrono-stratigraphy do not give us methods of measuring continuous time-durations: they allow us only to distinguish events at minimum time-intervals apart, 6t. The operational definition of completeness has therefore to be modified (Sadler & Strauss 1990): it is the fraction of all the time-intervals ~t between t~ and t2 that have left a recognizable record in the rocks--any record, no matter how short in itself. In lithostratigraphy, in the special case of discontinuous but still 'complete' stratified sequences, the measure of 6t is the acuity, referred to above. It is calculated assuming uniformitarian estimates of rates of sedimentation ( d h / d t ) taken from recent observations (Schindel 1982). The completeness is then deduced from the ratio of the observed total thickness of all the strata between time-planes tl, t2, to what it would have been, had sedimentation been continuous. And perhaps not surprisingly, it is found that the greater the relative acuity, A t / 6 t , the less complete the record appears to become. In biostratigraphy, the analogue of the acuity is the secular biochronological resolution of successive faunal horizons, 6t~j, or of whatever coarser unit is used. The numbers taken from the record of the Inferior Oolite shown in Fig. 5 are collected in Table 5. If by means of fossils the presence or absence only of Stages or Substages could have been recognized, the record would have appeared to be everywhere 100% complete. If standard Zones could be resolved, the record would have appeared on average to be only 73% complete. And at the resolution of the faunal horizons recognized today, the record is on average only 40% complete. But there is here an additional difficulty in that there is no independent way of estimating the maximum number of distinguishable faunal horizons ultimately to be expected between any two given time-planes tl, t2. The number depends on the state of knowledge: as we have seen, in the Inferior Oolite it has grown from 18 to 56. A section with the same nine faunal horizons would have changed from being biochronologically 50% complete in 1910 to only 16% complete today. As the biological record becomes more complete as a whole, so the geological record becomes more incomplete, which is precisely what Buckman said. How do biochronological estimates of completeness based on ammonites compare with those obtained from

T I M E F R O M FOSSILS

147

Table 5. The 'completeness of the geological record' in the Inferior Oolite as indicated by ammonite biochronology

Localities (Fig. 5) Resolution: Stages scope* number % completeness Resolution: Zones scope number % completeness Resolution: faunal horizons scope number % completeness

1

2

3

4

5

6

7

8

9

BB

Ch

WH

HP

Be-CF

Se

LH/HH

BA

SL

3 3 100

3 3 100

3 3 100

3 3 100

3 3 100

14 8 57

14 11 78

14 9 64

3 3 100 t 9 8 89

3 3 100

14 11 78

3 3 100 t 11 6 43

14 9 64

14 9 64

3 3 100 t 10 8 80

56 20 36

56 18 32

54 21 39

56 23 41

45 14 31

37 10 27

56 21 38

56 22 39

42 20 48

10 C1

11 Ob

12 Br-L

13 Du

Average

t

t

1 1 100

2 2 100

3 3 100

3 3 100

100

8 8 100

7 7 100

14 9 64

14 11 78

74

32 22 69

29 20 69

56 22 39

56 29 52

43

* Only the Lower Bathonian is represented in the Inferior Oolite. But even at Substage level (Lower and Upper Aalenian, Lower and Upper Bajocian, Lower Bathonian), at which the maximum scope would be 5, the representation would be everywhere 100% complete. t These sections have exposed only parts of the Inferior Oolite, either cut off at the tops by erosion or covered at the base. Numbers of faunal horizons as in Fig. 5; those shown as queried taken as present. microstratigraphical lithochronology? Both kinds of estimate can be made on the Oxford Clay of Peterborough (Figs 2 and 6). Taking the figures of average durations from Table 4, the faunal horizons of the Jason and Coronatum Zones would represent time-intervals 6tij of 70 000 years. But this succession of faunal horizons is here so far the most detailed we have. At the time-resolution of faunal horizons, therefore, the Middle Oxford Clay of Peterborough appears to be biochronologically 100% complete. A microstratigraphical analysis of the same succession has been given by Schindel (1982), at time-resolutions of 10 000, 1000 and 100 years. The estimates of lithochronological completeness are 14%, 4% and 3% respectively. As Buckman would have put it, large gaps joined by exceedingly thin layers of sediment, even when the fossil record appears to be complete. Finally, there is the third field of enquiry to which the ultimate refinement of biochronology makes an indispensable contribution. It is the mapping of patterns of biological evolution in the fossils themselves, the reconstruction of their lineages in phylogenetic classification: its use, as Buckman put it, 'in what I may call "palaeo-biology'". But that is another story. Conclusion

I have tried in this review to show how the refinement during the last hundred years of Jurassic geochronology by means of fossils has followed two distinct routes. In the first, going back to the founding fathers of geology, a geological column regarded to be at all times a complete representation of a continuous passage of time is subdivided successively into ever-thinner slices, the units of a standard chronostratigraphy that forms the basis of reference against which rocks are dated through correlation by means of fossils. It is therefore most widely applied to problems involving time-correlations over distances, and the precision with which this can be done at subzonal resolution is impressive. To stand on beds, never more than a few feet thick, of clays at Brora on the east coast of Scotland, or on sands at South Cave in Humberside, or shales at Peterborough and Weymouth, or limestones near Poitiers,

on the Meuse or in the Ard~che, or ironstones near Hanover, or clays near Bayreuth, or ironshot oolites in the Argovian Jura, or sandstones on the Vistula, or clays in the Oka valley, east of Moscow, or marly limestones in the Caucasus or trans-Caspian T u r k m e n i s t a n - - k n o w i n g that in each case one is in the E n o d a t u m Subzone of the Calloviense Zone of the Lower Callovian of the Middle Jurassic, in sediments whose age spanned perhaps only 400ka: who could fail to be moved? Clearly, such time-control is adequate for most meso- and macrogeological problems. The emphasis in the second route is not so much on time-correlation as on time-resolution. It was introduced by Buckman just a hundred years ago, and its results are equally impressive. To go into any quarry showing a few metres of Inferior Oolite in Dorset or Somerset, and to be able to assign any of its beds yielding ammonites to one or other of some 55 chronologically distinct levels, must also be cause for wonder. Both methods in the end imply an ability to distinguish geological events, and the finesse with which they can do so, discussed above, is comparable. The distinction between them may appear to be more a theoretical one, of principle, than one of immediate practical consequence. This is reflected in much of the stratigraphical literature, in which the distinction is rarely considered and even more rarely acted upon, with apparently little loss. Failure to bear this distinction in mind may however dictate, even if unconsciously, a choice that can fundamentally prejudice our whole approach to a geological problem. It relates to the question of completeness, discussed above, and how we deduce it (or visualize it) f r o m graphical representations of field evidence. W h e n we date beds in a section, such as those s h o w n in Fig. 4, by assigning them to standard Zones, a particular Zone is either present, shown as a bed or series of beds extending over some lithostratigraphical range, or absent, shown as a horizontal line of zero thickness. The temptation is to assume that the Zones that are recorded are tolerably complete. Looking at a graphical representation, the impression is one of long periods of sedimentation separated by brief nori,sequences in the partings between beds. This is

148

J.H.

CALLOMON

the representation that appears in chronostratigraphical compilations of regional stratigraphy, such as the Geological Society's Correlation Charts (e.g. Cope 1980a, b for the British Jurassic). It is the graphical representation that appears in many sequence-stratigraphical analyses, such as for instance that of the Jurassic of the Normandy-Wessex Basin by Rioult et al. (1991). It also surfaces widely in conventional range-charts of fossils. In contrast, the method of faunal horizons makes no a p r i o r i assumptions about completeness: rather the reverse, that thicknesses of rock notwithstanding, most of the time lies in the gaps, in the intervals between the brief events that are recorded (Fig. 5). The choice of interpretation bears some resemblance, therefore, to that of another classical problem. Faced with the same evidence, the optimist claims that the glass is half full, the pessimist that it is half empty. It was Buckman's achievement to have shown by means of fossils that in the Jurassic with which he was familiar, the pessimists have it. I fear his conclusions apply to Phanerozoic stratigraphy quite generally. I a m i n d e b t e d to H.S. T o r r e n s for m u c h b i o g r a p h i c a l i n f o r m a t i o n on the B u c k m a n family. T h e details s h o w n in Figs 3 - 5 i n c o r p o r a t e m a n y n e w a n d as y e t u n p u b l i s h e d results o b t a i n e d in r e c e n t y e a r s d u r i n g fieLd-work c a r r i e d out in c o l l a b o r a t i o n with R.B. C h a n d l e r , A . E . E n g l a n d , W . E . J o n e s ( L o n d o n ) , J.G. H u x t a b l e ( T a u n t o n ) and D. Sole ( L y m e Regis), w h o s e h e l p is gratefully a c k n o w l e d g e d .

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From QJGS,47, 479-480. 37. The BA~OClA~ o f the SKBRBORNE DISTRICT: its R~LATION f0 SVBJACENT and SVPE]UACm~T STRATA. By S. S. B v c x ~ , Esq., F.G.S. ( R e a d J u n e 7th, 1893.) CONTENTS,

Page

Introduction .................................................................. I. Section at Stoford, Somerset ............................................. ,, Bradford Abbas, Dorset .................................... II. . . . . (near Vicarage), Dorset ........................ III. ,, Halfway House, Dorset ....................................... IV. . . . . (in field), Dorset ........................... V. ,, Louse Hill, Dorset VI. ,, Marston Road, Dorset VII. ,, Holway Hill, Dorset .......................................... VIII. ,, Sandford Lane, Dorset ....................................... IX. ,, Combe (Limekiln Quarry), Dorset ........................ X. ,, Redhole Lane, Dorset .......................................... XI. ,, Clatcombe (disused quarry), Dorset ........................ XII. . . . . (Farmhouse), Dorset .............................. XlII. ,, (on Farm), Dorset ................................. XIV. Frogclen, Dorset ................................................ XV. • ,, • ,, Oborne Village, Dorset ....................................... XVI. ,, Milborne Wick, Somerset .................................... XVII. ,, Dundry, Somerset ............................................ xvni. ,, Leckhampton Hill, Gloucestershire ........................ XlX. Correlation of the Strata ................................................... Conclusion ..................................................................... Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Table I. Table II. Table III. Table IV.

Analysis of Sections .......................................... Stratigraphical Diagram ........................... facing Correlation of the Strata ........................... facing Correlation of the Zones and Hemerm ..................

479 484 485 486 486 487 488 490 49]~ 492 496 496 496 497: 498 500 502 502 508 5ll 507 518 520

506 508 514 519

[NTROD UCTION.

Definition of the Term 'Bajocia,~.'--For the strata w h i c h are e q u i v a l e n t to the u p p e r p a r t of the I n f e r i o r Oolite, w i t h a portion of the Fullers' E a r t h , d'Orbigny proposed the t e r m ' B a j o c i e n . ' Like our own terms ' I n f e r i o r O o l i t e ' and ' Fullers' E a r t h , ' its boundaries w e r e s o m e w h a t u n c e r t a i n ; and like them, too, it w o u l d seem t h a t the same palmontological horizon has received different geological names at different l o c a l i t i e s - - f o r this, probably, a difference in lithologieal characters m a y be blamed. I n the present paper I use the t e r m ' Bajocian ' in a m e r e l y conventional sense---for t h e l o w e r beds of t h e upper part of the ' I n f e r i o r Oolite '; b u t I do not express any opinion as to its merits. I m a y , however, r e m a r k t h a t the division of the Jurassic period on palmontological g r o u n d s - disregarding t h e details of its i n c o n s t a n t and m e r e l y local lithology - - i s a necessity. A m m o n i t e s have been chosen as the indicators ot horizons, and their rapidity in development makes t h e m peculiarly suitable. Therefore, as far as possible, the chronological u n i t and the Ammonite-species should go t o g e t h e r ; and any system of g r o u p i n g the chronological units should depend on t h e epacme, acme, and paracme of Ammonite-families. No doubt, in practical application, the epacme of one family would be found contemporaneous w i t h the p a r a c m e of another, so t h a t possibly it m i g h t be necessary to consider only two of the developmental phases. Such terms as Bajocian, Toarcian, etc., m i g h t be used from the chronological point of view only, to express the successive portions of time of w h i c h the developmental phases of Ammonite-families gave evidence. T h e y could be used for palmontological purposes, and only indircctly would have rcfcrcnce to such strata as m i g h t have been "dcl)osited d u r i n g . t h e times tl,cy represent. The details of this schcme, c a n n o t bc discussed now. A t present I use t h e t e r m ' B a i o c i a n ' simply because it is t h e most exactly dcscril)tive t e r m we possess for the strata intended. I n a f o r m e r paper laid before the Society, I advocated a p a r t i c u l a r use of the t c r m ' T o a r e i a n . ' ~ I n t h e present paper the Bajocian will commence w h e r e the Toarcian, as then defined, finished.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 153-162 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 1025-1034

Vertebrate fissure faunas with special reference to Bristol Channel Mesozoic faunas ROBERT

J.

G.

SAVAGE

Department o f Geology, The University, Bristol BS8 1R J, UK Abstract: Throughout the Mesozoic and Cenozoic, tectonically activated joint fissures and karstic cave systems have acted as reservoirs for terrestrial vertebrate remains. They yield concentrations of microfaunal elements rarely if ever preserved in other situations. In the 1850s, Charles Moore realized their potential and exploited them, to discover the earliest known mammals in late Triassic fissure infillings in the Carboniferous Limestone of Somerset. The history of that discovery and its associated problems of interpretation are recorded. His work greatly influenced succeeding generations of palaeontologists and, as techniques developed, fossil fissure discoveries have made a major impact on our understanding of vertebrate evolution.

oldest known mammals, which Moore had isolated from the sediment. Second, the sediment was no ordinary bedded deposit; it was a terrestrial Mesozoic infilling in vertically jointed Carboniferous Limestone at Holwell quarry near Frome on the Mendip Hills. While not totally original, the combination was highly innovative, and over the past century have others built on that work to make some remarkable palaeontological discoveries. It is pertinent first to examine the context in which Moore did his work. Charles Moore was born in 1815 in the Somerset town of Ilminster. From exposures in the Upper Lias along Strawberry Bank on the northern side of the town, the young boy tracked down a six-inch bed (the Cephalopod bed) which yielded him abundant ammonites and concretions; the latter he split open to find cuttle-fish, fish and reptiles. At an early age Moore began work in his father's book-selling business and in 1837 moved to Bath to work in Mr Meyler's book shop. Moore stayed in Bath for seven years and in that time would have become acquainted with the galaxy of early geologists in the city who attended meetings of the Bath Royal Literary and Scientific Institution. On his father's death in 1844 Moore returned to Ilminster and for nine years ran the family bookshop for his sisters. In 1850 he read a paper to the Somerset Archaeological and Natural History Society on microscopic fossils; in 1853 he presented another paper, this time on Fossil Infusoria (i.e. foraminifera). The papers were not published and no details are recorded. His marriage in 1853 to Miss Eliza Deare of Widcombe brought him back to Bath where he remained for the rest of his life. In 1854 he was elected a Fellow of the Geological Society. Marriage provided Moore with ample means and time to pursue his fossil collecting, and soon after he began work at Dundry Hill, south of Bristol and about 12 miles from Bath. The hill is an outlier of Inferior Oolite, recognized by William Smith. The limestones which cap the hill have been worked for centuries as a local building stone,

In the 1860s, the Mesozoic was seen as the age of reptiles and particularly of dinosaurs. Many reptiles were known to have become extinct at the end of the Mesozoic, and the Cenozoic was the age of mammals. Darwin's work had made it clear that mammals evolved from reptiles, but the fossil evidence for a transition was lacking. Primitive marsupial-like mammals were known from the Eocene beds of the Paris basin, but their ancestors were not recognizable there. The Middle Jurassic Stonesfield Slate in Oxfordshire had yielded toothed jaws of animals which had all the appearance of mammals. Although .found around 1812, it was not until Cuvier had pronounced upon them that they were accepted to be both Jurassic and mammalian. The discovery of vertebrate concentrations in Mesozoic fissures pushed back the origin of mammals some 45 million years to the Late Triassic, almost as far back as the dinosaurs themselves. While no substantially earlier mammals have since been found, later workers have built on this breakthrough to make large collections of the Late Triassic tetrapods and demonstrate their place on the boundary between reptile and mammal. It is due to those fissure faunas that this transition between two major animal groups is one of the best documented in the fossil record. Sediments filling underground cavities and open fissures represent a major source of fossil vertebrate remains. Their significance is hugely disproportionate to the relatively small volume of sediment they contain and arises from the enhanced preservation, compared to surface sites, of both bones and sediments, and the operation of one or more concentrative processes (Simms 1993).

Charles Moore and the discovery of vertebrate fissure faunas In 1867, the Geological Society published a 120-page paper by the amateur geologist Charles Moore which was outstanding on two counts. First it recorded the discovery of microscopic teeth of Triassic mammals, the geologically 153

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comparable to Bath stone. Moore collected bags of sandy clay, which occurs in thin seams in the limestone. This yielded him a rich fauna of minute brachiopods; of the twelve species he recorded, ten were species n o v a (Moore 1855). The question is, how did he get started on separation and examination of microfossils; the archives do not hold the answer. Nowhere in his publications does Moore explain how he prepared the material. The most we get are a few passing references to washing and looking at every grain with a lens. If the matrix was clay, washing would allow the clay to be floated off and he would then be left with a residue of grains, mostly quartz, with pyrite and organic particles. He makes no mention of using size fractionation or density separation techniques to concentrate the residue further. Although Moore appears to have had a microscope for the Infusoria, he mentions only a lens for the brachiopods and vertebrates. This is certainly to be expected, as in the nineteenth century microscopes were not built to give very low power magnification (×20) with wide field of view and a depth of focus to enable the complete specimen to be placed on the stage using reflected light. This

perhaps explains Moore's assertion that he examined over a million grains. It was at Vallis Vale near Frome, some 12 miles south of Bath, that De la Beche (1846) described the striking unconformity of the Inferior Oolite resting horizontally on inclined Carboniferous Limestone. He further noted the occurrence of oyster shells of Inferior Oolite age adhering to the surface of Carboniferous Limestone at nearby Holwell (Fig. 1). This was enough to encourage Moore to investigate the scene. Having got to Holwell (probably in 1855 or 1856) he found not only a richly fossiliferous seam separating the Oolite from the Carboniferous, but also numerous fissures in the quarry which were infilled with sandy clay and pieces of oolite. Moore had three cubic yards of the clay removed to the cellars of the Royal Literary and Scientific Institution in Bath. Moore presented a paper to the 1858 British Association Meeting on the organic remains from the Holwell Triassic fissure; he listed his finds of fish and reptile taxa, comparing them to species discovered in Bristol by Riley & Stutchbury (1840). When Moore's paper was published the following year, he added in a footnote that since reading the paper he had found three mammalian

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V E R T E B R A T E FISSURE FAUNAS teeth, identical with Microlestes antiquus recorded by Plieninger. For several years afterwards he spent several hours each day sifting through the clay, extracting almost a million individual fossil fragments; these include over 45 000 Acrodus teeth and 29 mammalian teeth belonging to the genus Microlestes (Fig. 2). W. H. T von Plieninger had processed the local Rhaetic bone bed at Degerloch in Wiirttemberg to yield a large collection of teeth, one of which belonged to a mammal which he named Microlestes (Plieninger 1847). The author describes how the widely distributed bone bed marks the boundary between the Keuper and the Lias, extending over shallow coastlands of the Lias sea, with enormous masses of teeth, scales, coprolites and unrecognizable skeletal elements of fish and reptile remains. Pleininger had much material available to him; he records how he washed and freed blocks from the sandy matrix. The sand thus removed, he elutriated carefully and examined with great pain in small portions with a lens. Here he found one completely preserved molar tooth with two roots and a well-preserved crown with six cusps; the tooth he compared to a marsupial from the Paris basin illustrated by Cuvier, concluding his was also a small insectivorous mammal; hence the name, meaning 'little thief', a name later found to be preoccupied by a beetle and changed to Microcleptes, meaning 'little brigand'. This in turn was also found t o be preoccupied by another beetle and so it changed for a third time, to Haramiya, being the same thing in Arabic rather than Greek. We do not know when Moore first became acquainted with Plieninger's work. The Moore papers preserved in the Society's archives reveal that he was in correspondence with Richard Owen as early as 1848 about the finding of Liassic ichthyosaurs. On 6 November 1858 Moore wrote to Owen enclosing three Microlestes teeth, adding 'I believe you know that I have from the same bed, teeth of the Muschelkalk Placodus'. The mention of Muschelkalk

155

faunas, and the correspondence with Owen, suggests that Moore was well acquainted with current continental work. It seems probable that Moore went to Holwell primarily to examine the unconformity and was immediately intrigued by the fissure infills, 'Liassic dykes' as he called them. Moore clearly recognized the nature of these 'abnormal' deposits and their potential to yield microfossils. In his seminal 1867 paper to the Geological Society he noted that a deposit of clay 12 feet thick and containing Liassic shells, occurred at a depth of 270 feet in the Carboniferous Limestone at Charterhouse (p. 492). He showed remarkably clear and perceptive judgement in his assessment of the development of the folded Carboniferous strata of the Mendips, their denudation to a peneplain by Triassic times and their island status in early Mesozoic with the seas lapping the lower slopes. He further recognized that the sedimentary dykes, while mostly Liassic in age, had a wide time range from Keuper to Oolitic. He had a clear picture of the Mendip island inhabited by reptiles and the primitive mammal Microlestes, whose remains were washed into the dykes, to be sealed in by later Oolitic sediments. Today that interpretation can hardly be improved on: only fine tuned to clarify detail. When Moore read his 'Abnormal secondary deposits' to the Geological Society on 20 March 1867, his paper followed one by H. W. Bristow of the Geological Survey 'On the Lower Lias or Lias Conglomerate of Glamorganshire'. Bristow's paper was accepted for publication in the August number of the Quarterly Journal. Moore's paper had to wait until December when it appeared as a supplement to the volume for 1867. The editor noted that the paper was 'unavoidably deferred'. Winwood (1892) recorded that the delay was to allow time for survey officers to examine the sites and report back. The Survey regarded the establishment of the stratigraphica| succession as their property, and it must have been galling for them that an amateur should be the first to demonstrate the existence of the

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Fig. 2. (a) Crown and side view of tooth (2 mm long) of Microlestes, RhaetoLiassic mammal from Holwell fissure. (after Simpson 1928). (b) Upper cheek dentition of two forms of the mammallike reptile Oligokyphus from Windsor Hill fissure and (e) reconstructed skeleton (50 cm long) of Oligokyphus. (from Kiihne 1956).

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Rhaetic in Britain and point out their errors in stratigraphical correlation. While it is suggested above that Moore probably chanced upon the Holwell fissures, his acquaintance with local geology was thorough and he would certainly have known of the previous discovery of saurian remains en Durdham Downs in Bristol (Riley & Stutchbury 1840). That discovery was announced to the Geological Society in 1836 in a paper communicated by the President (Charles Lyell), yet four years elapsed before the paper appeared in the Transactions. The discoverer was the local amateur Samuel Stutchbury and to ensure credibility he asked his friend the eminent Bristol medical doctor, Henry Riley, to join him; unhappily no record of the geological occurrence was kept. The jaw fragment and teeth they recovered were by them allocated to two species of saurian, named Thecodontosaurus antiquus and Palaeosaurus platyodon. Moore (1881), in his last paper to the Society, discussed the provenance of these enigmatic reptiles. He noted that the Dolomitic Conglomerate in which they were found was believed by Riley & Stutchbury to be Permian; it rested on Carboniferous Limestone and the authors equated it with the Magnesian Limestones. This age meant that the reptiles were the stratigraphically oldest known, yet they belonged to an advanced stock of saurians. Etheridge (of the Geological Survey) proposed that the Dolomitic Conglomerate was Keuper in age on the basis of its stratigraphic relationships (1870). Moore (1881) pointed out that the conglomeratic facies, sometimes dolomitized, was of variable age, Keuper, Rhaetic or Liassic. However as Moore had found the same two reptiles in the Rhaetic of Holwell and Vallis on Mendip, he deduced that the Durdham Down site in Bristol was also Rhaetic in age. Moore visited the Durdham quarry, and although the precise site of the reptile discovery was unknown, he found adhering to the sides of a vein enough matrix to extract from it numerous fragments of bone and teeth. He concluded that the situation was very comparable with that at Holwell, with a series of veins whose infillngs derived from different geological ages. Moore had formulated a very clear and extraordinarily accurate appreciation of the taphonomy of the deposits. The only aspect he did not consider was the time range of the reptile taxa, assuming they had sufficiently short ranges for Rhaetic taxa not to be found in Keuper or Liassic strata. It would appear that the Durdham Down reptile locality is the first true fissure fossil vertebrate site to be recorded. In defining it this way we exclude numerous bone accumulations at cave entrances and pothole infills, mostly no older than Pleistocene. In his lifetime Moore amassed a vast collection of fossils, from microfossils to complete ichthyosaurs, and almost all from the county of Somerset. Moore left his collections to the Royal Literary and Scientific Institution in Queen Square Bath. In the century since his death, the institution and his magnificent bequest have gone through traumatic times and their future is not, even at the present time, securely assured.

The legacy--after Moore Despite this promising beginning to a new area of palaeontology, there was no systematic application of the principles for over 50 years from the time of Moore's death until interest was again aroused again in the 1930s. In that

SAVAGE time gap vertebrate fossils were recovered from fissure deposits, but they did not involve the microvertebrates and processing skills which Moore had pioneered. For example in 1878, coal miners at Bernissart in Belgium working 322 m below the surface came across a clay filled pocket in which they found large dinosaur bones. The site was to yield 29 more or less complete skeletons of the bipedal herbivorous dinosaur Iguanodon and many other vertebrate remains. About t h e same time the phosphorites of Quercy in southwest France were being exploited commercially. The phosphorite occurred in 'pockets' in the Jurassic limestone. There was much debate on the origin of the phosphorite, until is was recognized that the abundant vertebrate remains found in some of the pockets were those of animals which had fallen into karstic fissures or caves where they had died and been preserved, or had been transported there by running water. The exploitation of the phosphorites as a source of fertilizer led to the discovery of abundant remains of amphibians, reptiles, birds and mammals. Articulated skeletons were very rare and the mammals found ranged through carnivores, ungulates, bats, rodents and primates. However it soon became apparent that the mammals were not all of one age and ranged through Eocene and Oligocene. It was not until the 1960s when the quarries were reopened that the ages could be elucidated (Buffetaut 1987). It was in the 1930s in Germany that Richard Dehm began systematically to explore the Tertiary infillings in the fissured Franconian and Schwabian Jura. The matrix he washed and sieved from many sites yielded rich mammal faunas of Oligocene and Miocene ages (Dehm 1935). At about the same time that Dehm was beginning his studies, Walter Kiihne left Germany and came to Britain. His first objective was to repeat the work of Charles Moore on the same site. He went to Holwell in 1939 and collected 2 tons of fissure infill, which on washing and sieving yielded him some 20 mammalian teeth, all but two of them the same that Moore had found (Kiihne 1946). However Kiihne did not work the same fissure that Moore had; Moore's fissure had been quarried away and Kiihne was well aware that the fissures were not all of the same age. The successful repetition of Moore's work some 80 years later was the stimulus needed for systematic and intense field work to begin. Kiihne simply took a geological map and noted where Carboniferous Limestone was exposed adjacent to Triassic continental sediments. He went to the areas, searched in the local Carboniferous Limestone quarries for fissures and examined their infillings for vertebrate fossils. He extended the search beyond the Mendip plateau, intending then to continue across the Bristol Channel into south Wales where the same relationships showed up on the maps. However it was August 1939 and the war intervened. Kiihne was interned on the Isle of Man for the duration, but had sent to him half a ton of matrix from another fissure he had discovered, that at Windsor Hill near Shepton Mallet. K/ihne recovered over 2000 bones and teeth of the mammal-like reptile Oligokyphus from the fissure, which he dated on the invertebrate content as Liassic. (Kiihne 1956). Oligokyphus was the only vertebrate in the infilling and Kiihne in his clear account of the biostratonomy argued that the animal was completely terrestrial and rodent-like, its remains accumulating around springs. When the river flooded, bones were swept into the nearby sea and some entered the open fissure. Kiihne was searching for the elusive transition between

VERTEBRATE FISSURE FAUNAS reptile and mammal. Microlestes appeared to be a mammal but only teeth were known. Now with Oligokyphus he had virtually complete skeletal material. It was very close to dividing line, combining characters from both major stocks. The final verdict was that it is taxonomically a reptile, but very mammalian in appearance and mode of life. Despite this refreshing new start to vertebrate fissure studies in Britain at the end of the war, Ktihne in 1951 decided to return to Germany. From Berlin he continued his palaeontological field work, making major Mesozoic mammal discoveries in Portugal, although in brown coal deposits and not in fissures. However he left behind him at University College London two palaeontologists who would carry the torch, Pamela Robinson and Kenneth Kermack.

157

teeth from the Carboniferous. The vertebrate contents comprise a range of small reptiles, sometimes though rarely as partly associated skeletons. Robinson interpreted the small size of the animals preserved as due to the sparse vegetation on the hot dry Mendip uplands. Dead lizard-like reptiles would mummify in the heat; these lightweight packages could be readily carried in flood waters into the caves. The absence of mammals was considered to be due to ecological unsuitability of the habitats on the uplands. Dating these fissures is difficult; sometimes a sedimentary cover seals the entrance, as at Emborough where the cave is capped by Rhaetic beds. In other instances dating is by comparison of the fauna with those from other parts of the world where they can be more precisely dated. Robinson was of the opinion that fissures of this type were all of Triassic age and probably Keuper (Upper Triassic). Her work has provided a major breakthrough in our understanding of fissure formation and the mechanics of infill. It was in the cave faunas that Robinson herself went on to specialize, collecting from the rich deposits at Slickstones Quarry near Cromhall in Gloucestershire and from Emborough Quarry on Mendip (Fig 3). The latter quarry, like Windsor Hill, contained essentially a one species fauna; in this case it was a gliding lizard, which Robinson named Kuehneosaurus after her mentor (Robinson 1962). Kuehneosaurus was the first aerial vertebrate, gliding in Late Triassic times long before the pterosaurs came on the scene. Much of the matrix from this fissure is highly calcified and does not break up in water; fossil extraction required other techniques. Fortunately about that time, staff in the Palaeontological Laboratory at the Natural History Museum in London were developing methods of freeing vertebrate fossils from calcareous settings, using acetic and formic acids. Weak solutions of these acids will selectively dissolve the calcium carbonate in the matrix but will not attack the calcium phosphate (apatite) of which bones and teeth are composed (Toombs 1948; Rixon 1949). However once freed of the matrix, the fossils are very fragile and need protection; this was provided by coating them with acid resistant plastics (Rixon 1976). The pioneering work of the Natural History

T h e search for n e w faunas Robinson was primarily a geologist and approached the fieldwork with a determination to create order out of chaos. The mixture of marine and terrestrial faunas i n fissure infillings had led to confused explanations of their origin. Surveying the Mesozoic geology in the west country, Robinson identified two major types of fissure formation, each with a characteristic type of infill, a characteristic fauna and distinctive ages (Robinson 1957). One type is the Neptunian dyke; these in the Mendip region characteristically infill east-west tension clefts formed in the Carboniferous Limestone while it is covered by the sea; the sea floor sediments get swept into the fissure along with remains of terrestrial animals living close to the nearby shore-line and carried into the sea by flood waters. The Holwell Microlestes fissures of both Moore and Kiihne belong here, as does the Windsor Hill Oligokyphus fissure. These fissures can be dated on their faunas, and range in age from Rhaetic to Inferior Oolite. The second type is the underground watercourse. In Triassic times the Mendip hills stood out as islands in the early Mesozoic sea. Karstic features included the formation of pot-holes and joints opening to create fissures, with phreatic and vadose cave systems developing. The infillings contain few invertebrates; only the non-marine crustacean Euestheria along with derived crinoids and bradyodont shark

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Museum staff in developing these techniques has revolutionized palaeontology and made possible the exploitation of many fissure faunas. While Robinson concentrated on Triassic cave systems in Somerset and Gloucestershire, Kermack devoted himself to the Neptunian dykes on the west side of the Severn estuary in south Wales. Here a series of Carboniferous Limestone quarries in the Bridgend area of Glamorgan contain numerous marine Rhaeto-Liassic fissures, which yielded Kermack spectacularly rich reptile and mammal faunas. Kermack is primarily a zoologist and his field objective has been to maximize the yield of fossils. He collected very large quantities of matrix and back at University College London these were washed and the residues concentrated by a variety of techniques to reduce the non-organic elements. A major problem with this sort of preparation is that while there is a richness of fragments, literally thousands of specimens, there is virtually no associated material. For example the skulls of these small reptiles and early mammals rarely have fused bones, so that in fossilization, all are preserved as individual bones or parts of bones. Similarly teeth seldom remain in place in the jaws. With many species of about the same size, this makes it exceedingly difficult to allocate specimens to their taxon. The collection forms the basis of a series of papers by Kermack, his colleagues and students. Evans has published on reptilian elements, giving detailed accounts of an early eosuchian (Evans 1980, 1981). The most exciting species has been the early mammal Morganucodon watsoni (Fig. 4). Kermack was able to identify virtually all the individual bones in the skull and mandible and so make a complete reconstruction of the shrew-sized insectivorous mammal. The animal possesses essential mammalian features in the

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In other parts of Europe fissure faunas have been exploited, for both micro- and macro-vertebrates. Dehm and his colleagues in Munich have produced a vast range of micromammal faunas from the southern German fissure sites, which he began to exploit nearly 6o years ago. In southern heel of Italy at Gargano are fissures infilled with Mid-Miocene mammals; the fauna is highly endemic, the area being an island in Miocene times and the mammals having acquired gigantism--including giant hedgehogs, rodents and owls.

Back to the laboratory Exploitation of the full potential of fissures as vertebrate rich sites required advances on two fronts; understanding the geomorphic, tectonic and taphonomic processes involved in their formation as traps for vertebrates, and also development of cheap and efficient methods of concentrating the microfauna. Advances in the first area have come through studies in karstic geomorphology; the most relevant are two recent review volumes on palaeokarst (James & Choquette 1988; Bosak et al. 1989). For example Smart et al. (1988) discussed the factors controlling the initiation, development and sedimentation of neptunian dykes and cavern infills, together with comment on their importance in palaeoenvironmental reconstruction and interpretation. They carried out a field study of the Blue Holes (underwater caves) on the Bahama Banks, which they consider to be modern examples where active sedimentation processes can be studied. Ford (1984, 1989) has made extensive studies of the occurrences of palaeokarsts in Britain. Simms (1990, 1993) has researched Triassic palaeokarsts in Britain, in particular those which have yielded vertebrate faunas and has demonstrated the crucial importance of climatic factors in their formation and infill. With the growth of taphonomy as a serious study over the past two decades, attention focused on vertebrate fissure faunas has brought with it the experimental approach of testing models. Andrews (1990) has made a detailed study of the taphonomy of vertebrate cave faunas; his examples are Pleistocene, but the principles hold for other periods. Andrews points to the major role played by predators, especially owls and other birds of prey, in the origin of the vast rodent accumulations found in some Cenozoic caves. The Middle Pleistocene cave fauna from the Carboniferous Limestone cave infill at Westbury-sub-Mendip has been carefully analysed in a major contribution by Bishop (1982). Just as the field practices have become refined over the years with the systematic exploitation of fissure faunas, so the processing of the matrix changed. With the frequent need to process tons of the payload, methods had to be found to do this quickly and cheaply. While the details differ with each site, the basic approaches are the same. If the material will break down in water, it can often be done in the field and so greatly reduce transport costs. If acids are needed to break down carbonates, then laboratory facilities will be needed. While fissure faunas have not been a major feature of North American discoveries, soft matrices from Eocene sites in Wyoming were being washed and screened by Wortman as early as the 1890s. By the 1930s, Hibbard was using similar techniques to process Cenozoic sediments in Kansas for small vertebrates and molluscs. Hibbard developed a screening system using mesh-bottomed boxes which were

SAVAGE suspended in the nearby streams to process tons of matrix in situ (Hibbard 1949). Over the following decades, Hibbard and his students made vast collections of micro-vertebrates from many parts of of USA, mainly in Pliocene and Pleistocene sequences. These have given us a detailed view of the life of small vertebrates, especially rodents, and added greatly to the overall understanding of their ecology. The next major step in the screening process was taken by Ward, who introduced a mechanical means of bulk processing. A 3501itre polythene tank is fitted with two sprinklers, one oscillatory and the other rotary. These are fed by a continual 24 hours a day water supply which can wash 10-15 kg of matrix per hour. Stainless steel mesh screening of various sizes down to 500 ~um can be used (Ward 1981). Wetting agents, hydrogen peroxide and hexametaphosphate are sometimes added to the water to aid the disaggregation of the sediment prior to screening. The most obstinate problem in the whole recovery process has long been that of recovering the vertebrates from the residue. The fossils invariably make up a minute fraction of the residue, a few percent at most. Moore described how over three years he hand picked over a million particles with the aid of a hand lens. Kiihne had a similar experience in recovering Microlestes teeth, although he had the advantage of a simple binocular microscope. The great advances in microscopy over the past half century have included instruments which are ideal for hand picking the residue; stereoscopy, wide angle, low power and good depth of focus are the prime essentials. Chemical and physical methods have been tried to produce vertebrate concentrates from the residues. Chemical approaches have used acids to dissolve the non-organic elements in the residue selectively; acetic or formic acid to reduce the calcareous particles, thioglycollic acid to reduce the limonitic particles (Howie 1974). Physical approaches have used density fractionation methods. Quartz, which is often very abundant in residues, has a specific gravity (2.6) slightly less than bone (2.7-3.0). Hematite (5.2) is much higher than bone. The problem with high density liquids for separation is that they are expensive and dangerous substances to handle; in consequence they have not found general acceptance. Variations on jigging or panning techniques used in mineral dressing have been tried, but again not found widespread favour. A different and innovative approach has been introduced by Freeman (1982) using the interface method, which exploits the lipophilic surface properties of apatite (the major constituent of vertebrate bone and tooth). The residue is placed in a two phase mixture of water and a water insoluble organic liquid. The organic liquid wets the phosphate particles in preference to gangue minerals, which are wetted by the water. To recover the phosphate, the mixture can be briefly agitated and allowed to settle; the bone will be concentrated at the interface and can recovered by decanting through a fine mesh. Alternatively a substrate can be used to attract the phosphate selectively. Freeman suggests a polystyrene substrate with an aromatic or gelatinous hydrocarbon; a second possibility is to use a paraffin wax or petroleum jelly substrate with kerosene as the hydrocarbon liquid.

The future Future technical advances will undoubted bring changes in the ways we process the fissure infillings to obtain the

VERTEBRATE

vertebrate concentrate. At the present time the most persistent problems centre around the residue sorting. Mesozoic mammal teeth like those of Microlestes are barely one millimetre across. Holwell clay yields one such tooth for every 100kg of matrix. In the Scottish Middle Jurassic Ostracod Limestone (Kilmaluag Formation) of Skye, such teeth turn up at a rate of about one tooth per 100 tons. With yields as low as this, the prize has to be extremely valuable to justify the time, effort and expense required. But such efforts are indeed essential if we are to make the big breaks. Our knowledge of the fossil record can never be complete, but some of the yawning gaps can with dedication be narrowed. It is no longer a case of using the swipe and hope technique, to set out aimlessly and see what turns up. Successful field work requires careful planning, research of all available data and a defined objective. For example one of our biggest gaps in the record of mammalian evolution lies in Mid-Cretaceous times. We have some (though limited) knowledge of faunas at the beginning and the end of the Cretaceous. At the outset of Cretaceous times the small mammals in the dinosaurian world were descendants of stocks that stretched back to Rhaeto-Liassic times. The mammals we see, as the dinosaurs disappeared at the end of the Cretaceous, were almost all new stocks: the marsupials and placental mammals that were soon to inherit the Earth. During the 70 million years that separate these two faunal sequences, we have only the merest fragments of evidence of mammal evolution, yet in that period the mammals underwent radical changes. Continental Middle Cretaceous sediments are all too poorly known, anywhere in the world, but the search must continue. Another approach is to use the original Moore method; identify areas where limestone plateaus were exposed during Cretaceous times to act as potential sources for fissure and cave development, to be sealed off in later Cretaceous or early Cenozoic times. Recently the Deccan traps of India have yielded Cretaceous mammals in sediments intercalated with the lavas; though these are end and not mid-Cretaceous, such sites are prime tragets for investigation (Prasad & Sahni 1988).

The author is grateful to J. Thackray for assistance with the Moore papers in the Society's archives and to M. Simms for discussions on palaeokarsts.

References ANDREWS, P. 1990. Owls, Caves and Fossils. Natural History Museum, London. BISHOP, M. J. 1982. The mammal fauna of the early Middle Pleistocene cavern infill site of Westbury-sub-Mendip, Somerset. Palaeontological Association Special Papers, 28, 1-108. BOSAK, P., FORD, D.C., GLAZEK, J. & HORACEK, I. (eds) 1989. Paleokarst; a systematic and regional review. Elsevier, Amsterdam. BRISTOW, H. W. 1867. On the Lower Lias or Lias Conglomerate of Glamorganshire. Quarterly Journal of the Geological Society of London, 23, 199-207. BUFFETAUT, E. 1987. A short history of Vertebrate Palaeontology. Croom Helm, London. DE LA BECHE, H. T. 1846. On the formation of the rocks of south Wales and southwest England. Geological Survey Memoir, 1, 1-296. DEHM, R. 1935. Llber terti~ire Spaltenftillungen im Fr~inkischen und Schwabischen Jura. Abhandlungen der Bayerischen Akademie der

Wissenschaften. MathematischMiinchen. N.F. 29, 1-86. E'mER~DGE, R.

1870.

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Distribution of the Reptilian or Dolomitic Conglomerate of the Bristol Area. Quarterly Journal of the Geological Society of London, 26, 174-192. EVANS, S. E. 1980. The skull of a new eosuchian reptile from the Lower Jurassic of south Walcs. Zoological Journal of the Linnean Society, London, 70, 203-264. 1981. The postcranial skeleton of the Lower Jurassic cosuchian

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Gephyrosaurus bridensis. Zoological Journal of the Linnean Society, London, 73, 81-116. FORD, T. D. 1984. Palaeokarsts in Britain. Cave Science, 11, 246- 264. 1989. Paleokarst of Britain. In: BOSAK, P., FORD, D.C., GLAZEK, J. & HORACEK, I. (eds) Paleokarst; a systematic and regional review. Elsevier, -

Amsterdam. 51-70. FRASER, N. C. 1982. A new rhynchocephalian from the British Upper Triassic. Palaeontology, 25, 709-725. 1988. The ostcology and relationships of Cievosaurus. Philosophical Transactions of the Royal Society, London, 321, 126-178. FREEMAN, E. F. 1982. Fossil bone recovery from sedimcnt residues by the 'Interfacc Method'. Palaeontology, 25, 471-484. HIBBARD, C. W. 1949. Techniques of collecting microvertcbrate fossils.

Contributions from the Museum of Paleontology, University of Michigan, 7-19. HOWlE, F.M.P. 1974. Introduction of thioglycollic acid in preparation of vertebrate fossils. Curator, 17, 159-165. JAMES, N.P. & CHOQUETTE, P.W. (eds) 1988. Paleokarst. Springer Verlag. KERMACK, D.M., KERMACK, K. A. & MUSSE'IVl', F. 1968. The Welsh Pantothere Kuehneotherium praecursoris. Zoological Journal of the Linnean Society, London, 47, 407-423. KERMACK, K.A., MussEl"r, F. & RIGNEY, H.W. 1973. Thc lower jaw of Morganucodon. Zoological Journal of the Linnean Society, London, 53, 87-175. --, -& -1981. The skull of Morganucodon. Zoological Journal of the Linnean Society, London, 71, 1-158. K/JHNE, W.G. 1946. The Geology of the Fissure-filling 'Holwell 2'; the Age-determination of the Mammalian teeth therein; and a Report on the Technique Employed when Collecting the Teeth of Eozostrodon and Microcleptidae. Proceedings of the Zoological Society, "London, 116, 729-733. 1956. The Liassic Therapsid Oligokyphus. British Museum (Natural History), London. MOORE, C. 1855. On new Brachiopoda from the Inferior oolite of Dundry. 8 ,

Proceedings of the Somerset Archaeological and Natural History Society, 5, 107-128. 1867. On Abnormal Conditions of Secondary Deposits when connected with the Somersetshire and South Wales Coal- Basin; and on the age of the Sutton and Southerndown Series. Quarterly Journal of the Geological Society of London. 23, 449-568. 1881. On Abnormal Geological Deposits in the Bristol District. Quarterly Journal of the Geological Society of London, 37, 67-82. PLIENINGER, W. H. T VON, 1847. Zahne aus der oberen Grcnzbreccie des Keupers bei Degerloch und Steinenbronn. Jahresheft des Vereins fiir Vaterliindische Naturkunde in Wiirttemberg, Stuttgart, 3, 164-167. PRASAD, G.V.R. & SAHNI, A. 1988. First Cretaceous mammal from India. Nature, 332, 638-640. RILEY, H. & STUTCHBURV,S. 1840. A Description of various Fossil Remains of three distinct Saurian Animals, recently discovered in the Magnesian Conglomerate near Bristol. Transactions of the Geological Society of London, Series 2, 5, 349-357. RixoN, A. E. 1949. The use of acetic acid and formic acid in the preparation of fossil vertebrates. Museums Journal, London, 49, 116-117. 1976. Fossil Animal Remains: their preparation and conservation. Athlone Press, London. ROBINSON, P. L. 1957. The Mesozoic fissures of the Bristol Channel area and their vertebrate faunas. Zoological Journal of the Linnean Society, London, 43, 260-282. 1962. Gliding lizards from the Upper Keuper of Great Britain. Proceedings of the Geological Society of London, No. 1601, 137-146. 1973. A problematic reptile from the British Upper Trans. Journal of Geological Society, London, 129, 457-479. SIMMS, M. J. 1990. Triassic Palaeokarst in Britain. Cave Science, 17, 93-101. 1993. Emplacement and preservation of vertebrates in caves and fissures. Zoological Journal of the Linnean Society, London. (in press). SIMPSON, G. G. 1928. A Catalogue of the Mesozoic Mammalia in the Geological Department of the British Museum. British Museum (Natural History), London. SMART, P.L., PALMER, R.J., WHITAKER, F. & WRIGHT, V.P. 1988. Neptunian Dikes and Fissure Fills: an overview and account of some modern examples. In: JAMES, N.P. & CHOOUETTE, P.W. (cds) Paleokarst. Springer Verlag. 149-163.

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-

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-

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162

R.J.

SWINXON, W. E. 1939. A new Triassic Rhynchocephalian from Gloucestershire. Annals and Magazine of Natural History, Series 11, 4, 591-594. TooMBs, H. A. 1948. The use of acetic acid in the development of vertebrate fossils. Museums Journal, London, 48, 54-55. WARt), D. J. 1981. A simple machine for bulk processing of clays and silts. Tertiary Research, 3, 121-124. WmTESIOE, D. I. 1986. The head skeleton of the Rhaetian sphenodontid

SAVAGE

Diphydontosaurus avonis Gen. et sp. nov. and the modernizing of a living fossil. Philosophical Transactions of the Royal Society, London, B312, 379-430. WlNwooI~, H. H. 1892. Charles Moore, F.G.S., and his work. With a list of the fossil types and described specimens in the Bath Museum, by E. Wilson. Proceedings of the Bath Natural History and Antiquarian Field Club, 7, 232-292.

Received 19 July 1993; revised typescript accepted 14 August 1993.

Addendum Since the article was written more information has come to light (Crane 1993) about the little known Bristol geologist and naturalist Samuel Stutchbury (1789-1859), who with Riley described the first dinosaurs from a Triassic fissure in Bristol in 1836. Stutchbury began his career in 1822 as assistant to Wm. Clift in the Hunterian Museum of the Royal College of Surgeons. He resigned in 1825 (and was soon after succeeded by Richard Owen) to sail as naturalist on a Pearl Fishing boat to coral reefs in the Pacific Ocean. Returning two years later he worked as a dealer in natural history

specimens until his appointment in 1831 as curator of the museum of the Bristol Institution (later to become the Bristol City Museum). Dr Riley was also a keen anatomist and a founder of the Bristol Zoological Gardens in 1835 on a site a few hundred metres from the dinosaur fissure quarry.

Additional reference CRANZ, M. D. 1993. Samuel Stutchbury (1798-1859). In: Dictionary of National Biography; Missing Persons. Oxford University Press, 648-49.

Added November 1994.

From QJGS,23, 449-450. 1. On ABnORmAL COI~I)ITIOZCSOf SZCO~'I)ARY D~.POSITS when connec2ed with the SOMERSETSHIR:E a ~ d SOUTH WAL:ES COAL-BAsIN; ~/Rd O~ the age of the SUTTO~ and SOUTHm~I)OW~¢ S:F.a~. :By CKav~,s :M:ooR~., Esq., F.G.S. (Read March 20, 1867.~) [PLATES XIV.-XVII.]

Co~zz~zs. I. Introduction. II. The Mendip Hill~ 1. Old Red Sandstone. 2. Carboniferous Limestone. 3. Basaltic Dyke. 4. Date of Upheaval. 5. Denudation. 6. System of Secondary veins. 7. Ago of the Conglomerates. III. Strai.ifled Rocks subsequent to the Mendip upheaval. I. The Trias. a. South of the Mendips. b. Within the Coal-Basin. c. Batheaston Section. 2. The Rh~tic Beds. a. Section of Keuper, Rhmtic, and Liassic Beds at Camel. $. Organic renlains in the Rhmtic White Lisa.

c. Insect and Crustacean Beds. d. Saurian and Ostrea-beds. e. Ammoni~s-Tlanerbis Beds. f. Beer-Crowcombe and Hatch Sections. 3, Rhseticand Lia~icBeds within the Coal-Basin. a. Section of Keuper, Rhsetic, Lower, Middle, and Upper Lias, and Oolite at Cameramen. b. Section in Mungor Road Quarry. 4. Relative thicknes~ of Secondary Beds South and North of the Mendips. 5. Whatley Lisa and Fontaine. dt~upe-Four. 6. Mells Middle Lia~ and Coal.

8. Value of Zones of Zoological IV. Abnormal Secondary deposits on Life. the Carboniferous Limestone. VI. The So,th Wales District. 1. Marston Road Section. 1. Penarth Rhmtic and Liassic 2. ]tolwell Carboniferous LimeSection. stone and Liassie Dykes. 2. Bridgend Liassie Sections. 3. The Microlestes Quarry. 3. Cowbridge Section. 4. Sections in the Vallis. 4. Llanbethian Quarries. 5. Gurney Slade Liassic Dykes. 5. Laleston Quarry. 6. Charter-House Liassic Lead6. Stormy Quarry. mine. 7. Section at Ewenney. 7. Terrestrial and Freshwater 8. Section at Brocastle. Fauna. 9. The Sutton Stone and the V. The Bath District. Southerndown series. 1. Pinch's Well. a. Local Deposition of the Sut2. The Ammonites-Bucklandi ton Stone. Beds. b. Organic remains from the 3. Willsbridge Section of Lower Sutton Stone. Lias, "~euper, Rh~etic, and c. The Ammo~zites-Bucklandi Coal-measures. Bed.~. 4. Sections near Bristol. 10. Langan Lead-mine. 5. Sections at Keynsham and 11. Inadmissibility of the term Stout's Hill. " Infralias." 6. Rhmtic White Lias and Carboniferous Limestone at VII. Conclusion. VIII. Description of Organic reBroadfield Down. Sections near Shepton Mallet. reruns. IX. List of Fossils.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 165-172 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 219-226

Triassic pebbles, derived fossils and the Ordovician to Devonian palaeogeography of Europe L.R.M.

COCKS

Department of Palaeontology, The Natural History Museum, Cromwell Road, London S W 7 5BD, UK Abstract: Papers published by Salter (1864) and Davidson (1870) on the faunas from pebbles in a Triassic conglomerate at Budleigh Salterton, Devon, are reviewed. After modern reassessment, these pebbles, although of apparently similar quartzites, have been found to be of four different ages, two Ordovician (mid-Arenig and late Llandeilo) and two Devonian (Lochkovian-Pragian and Frasnian). By comparing these four faunas with those contemporary in adjacent palaeocontinents, it can be shown that, apart from the earliest one, they have affinities closer to those of the Armorican peninsula of Brittany and Normandy than to the rest of Britain and that these Armorican faunas are in clasts which were transported northward by Triassic rivers. Consideration of all the various faunas in the whole of northwest Europe reflect the earliest Ordovician of southern Britain as part of the vast Gondwanan continent, from which it became detached by the mid-Ordovician, with a widening Rheic Ocean between the two palaeocontinents; and the subsequent merging of Avalonia with Baltica and Laurentia to form Laurussia by Mid-Devonian times. New palaeogeographical maps depicting phases from the Ordovician to the Devonian are presented.

and described by the leading brachiopod specialist of the nineteenth century, Thomas Davidson, who published his results firstly in the Society's journal (1870) and subsequently in greater detail in a Palaeontographical Society Monograph (1881). It was Davidson who recognized that the faunas first described by Salter were in fact of two quite different ages, one Ordovician and the other Devonian, and that individual pebbles with more than one fossil on them never contained any mixture of the two faunas. This removed the anomaly of the 'oldest' spiriferid. However, this triumph of palaeontology was somewhat marred (1870, p. 73-74) by Davidson's reluctance to endorse Salter's separation of the Ordovician faunas between those of the Gr~s Armoricain and the May Sandstone. Nevertheless, Davidson's work was all that was required to resolve any apparent palaeogeographical problems, particularly in the light of the 'fixist' biogeographies then believed. So matters rested until comparatively recently, when firstly Sadler (1974), in reviewing trilobites from a tectonically complicated belt near Gorran Haven, Cornwall (Fig. 1), asserted that one of the Budleigh Salterton trilobites originally described by Salter was Neseuretus tristani (Brongniart), characteristic of inshore midOrdovician facies across much of Europe. Then Cocks & Lockley (1981) reassessed the Ordovician part of the Budleigh Salterton brachiopod faunas and confirmed Salter's original findings, that there were in fact two separate Ordovician faunas in the pebbles, one of Arenig, probably middle Arenig, age (Fig. 2), in the same lithological facies as the Gr~s Armoricain of Brittany, Normandy and Sarthe, France, and the second of latest Llandeilo age (Fig. 3), comparable to that from the Gr~s de

The Budleigh Salterton Pebble Bed is of Triassic age, about 25 m thick, and is well displayed in cliffs along the southern English coast around Budleigh Salterton, Devon (Fig. 1), and the eroded pebbles from it largely make up the local beaches. The pebbles are of indurated quartzite and an average geologist might spend an afternoon there thinking them to be unfossiliferous. However, in about 1835 a Mr Carter first found some fossils in a Budleigh Salterton pebble and brought them to the attention of a local amateur geologist, W. Vicary. Vicary paid local workmen to find more and by 1863 was able to read to the Geological Society a paper on the Pebble Bed (Vicary 1864), attached to which was a substantial note on the fossils (Salter 1864) figuring 30 species and mentioning four more. Salter's work was shrewd, since he realised that these fossils were unique in Britain; quite different from faunas at the 'Silurian System' sites in Wales and the Welsh Borderlands, and different again from faunas from western Scotland which had American affinities. The Budleigh Salterton fauna was like those of central Europe (France, Spain and Bohemia) and the age was 'Lower Silurian', what we would now term Ordovician, and Salter equated them directly (1864, p.287) with the Grrs Armoricain and May Sandstone of France. However, Salter was puzzled by a spiriferid brachiopod, which he named Spirifer antiquissimus, which was considered by far the oldest of its group to have been found anywhere. After that communication to the Society, collecting proceeded apace and by 1869 over 400 specimens had been collected by dedicated amateurs, chiefly Vicary and R.H. Valpy, both of whom subsequently bequeathed their collections to the British Museum (now the Natural History Museum). The brachiopods in these collections were revised 165

166

L. R. M. COCKS ]/ ._I/

COR~WALL~

~

° tBR, AN

0:o

-~

Salterton pebbles, all of them in an apparently identical indurated quartzite facies; two Ordovician and two Devonian--Figs 2-5 show characteristic elements of the fauna and full faunal lists are in Cocks & Lockley (1981) and Cocks (1989). So how may these four faunas (all transported in ~Triassic times at least some distance, and perhaps as much as 300 km, from their original place of deposition) be used to elucidate the palaeogeography of Europe? Each one is of separate significance, as will now be reviewed.

DEVON Budleigh Salterton

~ut~

W

~X../ -

tNORMANay .

Le Mans

Fig. 1. Southwest England and the Armorican peninsula, showing the position of Budleigh Salterton and other localities. petit May, for example at May itself, near Caen, France (Fig. 1). Trilobites occurring with the second brachiopod fauna have been revised (S.F. Morris pers. comm.) to include Eohomalonotus vicaryi (Salter, 1865), Iberocoryphe serrata (Tromelin, 1877), Neseuretus tristani (Brongniart, 1817), Kloucekia cf. mimus (Salter, 1864) and Crozonaspis aft. incerta (Deslongschamps, 1825). However, a surprise came when the Devonian brachiopods from Budleigh Salterton were analysed in further detail (Cocks 1989) and were discovered also to be of two quite separate ages (a) Lower Devonian (Lochkovian-Pragian), and corresponding in facies and fauna (Fig. 4) to the Gr~s ~ Orthis monnieri (now termed the Landrvennec and Gahard Formations) of Brittany; and (b) Upper Devonian (Frasnian). These Frasnian forms, whilst corresponding in some species to faunas of similar age from France, are found in a quartzite facies not exactly matched by described faunas from northwest France in rocks of that age (Fig. 5). Thus, to summarize, there are no fewer than four quite separate faunas and ages represented in the Budleigh

Lower Ordovician In the Early Ordovician (Tremadoc-Arenig), what is now western Europe was divided into three separate palaeocontinents (Fig. 6). Most of the area formed one corner of the vast Gondwanan continent which stretched half way round the world to include South America, Africa, India, most of Arabia, Antarctica and Australia (Cocks & Fortey 1988). The European corner of Gondwana was at high latitudes, completely lacking carbonate deposits and with widespread but low diversity faunas in shallow-water clastic facies, consisting of a few trilobites and bizarre inarticulate brachiopods such as the oldest fauna at Budleigh Salterton (Fig. 2). These brachiopods are also known from France (from Brittany, Normandy, Sarthe and the Montaigne Noire), the Iberian Peninsula, Czechoslovakia, Morocco, Libya and Algeria. Although these bizarre inarticulates are not known from elsewhere in southern Britain, the Stiperstones Quartzite of Shropshire is developed in a similar facies to the Gr~s Armoricain, and the contemporary Arenig trilobite faunas of South Wales show strong Gondwanan affinity (Fortey & Owens 1987). Sedimentological studies of the Armorican quartzites also support the integrity of both southern Britain and Amorica as being attached to the north African part of Gondwana (Noblet & Lefort 1990). Gondwana was separated from the tropical continent of Laurentia (principally North America, but including Scotland and northwestern Ireland) by the Iapetus Ocean, and from Baltica (which included all of northern Europe eastwards to Novaya Zemlya and the Urals) by the Tornquist Sea. These separations were originally identified on faunal criteria (Wilson 1966; Cocks & Fortey 1982) at a time when palaeomagnetic data were equivocal; but more

Fig. 2. Inarticulate brachiopods from pebbles of Arenig age in Triassic conglomerate, Budleigh Salterton, Devon: (a) Lingulepis crassipyxis Havli~ek, B 21675 x 2; (b), Ectenoglossa,.lesueuri (Rouault), B 14419 × 1; (¢, d), Lingulobolus hawkei (Rouault), ventral and lateral views, B 14327 x 1.5, W. Vicary and T. Davidson Collections.

O R D O V I C I A N TO D E V O N I A N P A L A E O G E O G R A P H Y

167

(a)

(b)

(c)

(d)

(e)

(~

Fig. 3. Articulate brachiopods from pebbles of Llandeilo age in Triassic conglomerate, Budleigh Salterton, Devon: (a, b, d) Corineorthis erratica (Davidson), (a, b) latex cast of exterior and natural mould of ventral interior, B 20936 × 1.5; (d) latex cast of dorsal interior, B 20936 x 2; (¢) Salopia? pulvinata (Salter), latex cast of dorsal interior, BB 70910, x 2.5; (e, f) Tafilaltia valpyana (Davidson), (e) latex casts of dorsal exterior and interior, BB 95940, x 3, BB 95941, x 2.5.

recent palaeomagnetic results have confirmed them, with the surprising additional information that Baltica had rotated through nearly a right angle during the Ordovician (Torsvik & Trench 1991). Whether Southern Britain was truly in the place shown in Fig. 6a is quite uncertain; perhaps a more realistic position would have been further along the African coast, maybe even as far as Mauretania, since a preliminary analysis of late Cambrian and Tremadoc facies and faunas show some affinities which indicate such positioning (R. Feist, W.S. McKerrow pers. comm.). However, at some time during the Ordovician, Avalonia became detached from the main continent and was carried northwards. This was a segment or segments of Gondwana, comprising some of the eastern seaboard of the United States; part of the maritime provinces of Canada, particularly Nova Scotia and eastern Newfoundland; southeast Ireland; England; and Belgium and adjacent areas. Opinions are divided on the date of the separation; some believe it to be as early as Tremadoc, but it is shown here (Fig. 6) as post-Arenig, and was most probably Llanvirn; although positive evidence for separation on faunal grounds is somewhat later. The divorce of Avalonia from Gondwana resulted in the progressive narrowing of both the Tornquist Sea and also the Iapetus Ocean, so that Avalonia and Baltica merged in the late Ordovician and both collided with Laurentia in the late Silurian and early Devonian. The widening area between Avalonia and Gondwana was the site of the Rheic Ocean. Opinions differ as to whether the western Avalonian area of what is now North America, and eastern Avalonia (southeast Ireland, England and the Brabant massif) were originally one microplate or more, for example Ziegler (1990, p. 27) regards them as three separate microcontinents; however, the Cambrian facies and faunas were the same in both South

Wales and Newfoundland, and there were certainly the same ostracode and fish faunas in Nova Scotia and Wales in the Silurian, and thus the two halves of Avalonia are shown together in the reconstructions presented here (Fig. 6). During the past ten years much discussion has centred on the detailed timing of these events and also on the relative configurations and positions of the main land masses, including Armorica (northwest France). For example, the shallow-water marine benthic faunas of eastern Avalonia are quite different from those of Baltica during the early Ordovician, but from Mid-Ordovician times onwards they became progressively more similar. The time when Tornquist Sea finally closed as Baltica and Avalonia collided is less certain. A calc-alkaline volcanic arc is now known from northeast of the Anglo-Brabant massif and this was active during the late Caradoc and possibly the early Ashgill. The collision may have caused the intrusion of the buried granites along Tornquist's line and running eastwards from the southern North Sea (Ziegler 1990 p. 19 and fig. 2) which have been dated at about 440 Ma, close to the Ordovician-Silurian boundary; the ostracodes were essentially the same in Avalonia and Baltica by the early Llandovery (Berdan 1990). Woodcock (1991) has reviewed the hidden Caledonides under eastern England and Belgium. The Iapetus closed progressively from late Llandovery times in the north (northern Norway and Greenland) to early Mid-Devonian, with a strike-slip component of considerable but unknown dimensions.

Middle Ordovician In contrast, the second Budleigh Salterton fauna (Fig. 3), of late Llandeilo age is not Avalonian or Baltic, but of Armorican aspect, with identical brachiopod species known

168

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Fig. 4. Brachiopods from pebbles of Lower Devonian (Lochovian-Pragian) age in Triassic conglomerate, Budleigh Salterton, Devon: (a) Salopina adventita Cocks, natural mould of ventral interior, BC 6576 × 3; (b) Platyorthis monnieri (Rouault), natural moulds of several ventral and one dorsal interiors, B 21586 x 1.5; (c) Zthyris? incerta Davidson, natural mould of dorsal interior, B 21711 × 2; (d), Mclearnites rouaulti (Davidson), natural mould of ventral interior, B 21600 × 1.5; (e) Leptostrophia etheridgii (Davidson), latex cast of dorsal interior, B 21539 × 1.8; (f) Shaleria vicaryi (Davidson), natural mould of ventral interior, BC 6088 × 2; (g, h) Howellella cortazari Carls, natural mould of dorsal interior, BB 70944 × 3; (i, j) Katunia? vicaryi (Davidson), natural mould of conjoined valves, B 21530 x 3; (k) Nucleospira vicaryi Davidson, natural mould of dorsal interior, B 21549 × 2. not only from Normandy but also from Czechoslovakia, and with the trilobite fauna matched with the Botella Quartzite of Spain (S.F. Morris pers. comm.). This helps to establish that the shallowest-water biofacies of the two continents were distinct by the Mid-Ordovician, and that the southern boundary of Avalonia lay to the north of the Armorican quartzites. This deduction is supported by the small fauna described by Bassett (1981) from boulders within a Devonian tectonic and sedimentary melange at Gorran Haven, Cornwall (Fig. 1); although none of the five brachiopod genera there are actually conspecific with Budleigh Salterton forms and they appear to be of a slightly different (and probably a little older) Llandeilo age. Thus the Budleigh Salterton pebbles may be regarded as 'Brittany in Britain'. Probably the products of a Triassic fluvial braided stream environment (Warrington & IvimeyCook in Cope et al. 1992, p. 98), they had a southerly provenance (Audley-Charles 1970, pl. 7) and were perhaps eroded from a mid-Channel position (as postulated by Davidson more than a century ago). Their origins would,

like the rest of the Armorican craton, have lain to the south of the putative Rheic suture and the mid-European Caledonides. Evidence from the Brabant Massif (which was in the same tectonic block as the London platform and to the north of the Rheic suture) dates an initial phase of compressional deformation along the southern margin as mid-Caradoc, after which there was back-arc extension which triggered tholeiitic basaltic magmatism (Ziegler 1990, p.23 and encl. 2). The faunas subsequently reflect this, with the deeper-water Foliomena brachiopod fauna known from the Ashgill of Belgium to the north of the Rheic suture (Sheehan 1987; Fortey & Cocks 1992). Turning further afield from Budleigh Salterton, in the past few years much collatory work has been published on the Lower and Middle Palaeozoic of the mainland of Europe. Apart from the relatively undeformed Bohemian massif, these areas have been subsequently heavily overprinted by the compressive Variscan (late Visean to late Westphalian) and Alpine (Tertiary) orogenies, leaving the true palinspastic early Palaeozoic reconstructions of the area

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Fig. 5. Brachiopods from pebbles of Upper Devonian (Frasnian) age in Triassic conglomerate, Budleigh Salterton, Devon: (a) Petrocrania transversa (Davidson), natural internal mould, B 21544 × 2; (b, ¢) Douvillina edgelliana (Davidson), natural internal moulds of dorsal and ventral valves, B 21534 x 2; B 21541 x 1.5; (d) Douvillina? budleighensis (Davidson), natural internal mould of ventral valve, B 21538 × 2; (e) Anoplia sp., latex cast of dorsal and ventral interiors, BC 21550 and B 21725 x2; (f, g) Productella vicaryi (Salter), internal moulds of ventral valves, B 21550 and B 21725 x 2; (h) Cryptonella? sp., natural dorsal internal mould, BC 6435 x 1.5; (i) uncinuliform rhynchonellide, natural ventral internal mould, BC 21528 x 1.5; (j) "Camarotoechia' valpyana (Davidson), natural mould of conjoined valves, B 20984 x 3; (k) Cyrtospirifer verneuili (Murchison), natural ventral internal mould, B 21542 x 1.5; (!), Cyrtospirifer? micropterus (Davidson), natural dorsal internal mould, BC 6090 x 1.5. rather problematical (Coward 1990). Ziegler (1990) reviewed the whole area in a masterly fashion and this been augmented by reviews by Erdtmann (1991) Germany, Verniers & Grootel (1991) on Belgium, Sch6nlaub (1992) on Austria.

has has on and

Silurian The Silurian (although not represented at Budleigh Salterton) was a period of cosmopolitan faunas, since the major continents were not far enough apart to encourage provinciality in the common brachiopods, trilobites and other benthic fauna. The exception was the ostracodes (Berdan 1990), which were the last group of organisms to reflect the separate sides of the Iapetus Ocean, and continued to do so until the early Devonian. Exceptions to the general cosmopolitanism are the two peripolar faunas, the Clarkeia fauna to the south and the Tuvaella fauna to the north (Cocks & Scotese 1990). In northwest Europe the Rheic Ocean was widening, which was indicated first by

different brachiopod species in comparable facies (e.g. between Shropshire and Bohemia) in the Wenlock and some different genera by Ludlow times.

Devonian The two younger faunas in the Budleigh Salterton pebbles are of Lower Devonian (Pragian-Lochkovian) and Upper Devonian (Frasnian) age. There are very comparable Lower Devonian quartzites bearing similar brachiopods in northwestern France, for example the Land6vennec and Gahard Formations of Brittany (formerly known as the Gr~s Orthis monnieri, after the same common brachiopod which is also abundant at Budleigh Salterton, Fig. 4b). However, there are no known late Devonian described faunas from quartzites in Armorica comparable to those forming the Frasnian Budleigh Salterton pebbles, although there are some sandstone beds, known as the Gr~s de Goasquellou, within the Frasnian Traonliors Formation which crops out sporadically some 20 km east of Brest. The only brachiopods

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~ L A N D O V E R y Fig. 6. The palaeogeography of northwest Europe, (a) in Mid-Arenig times, with southern Britain attached to Gondwana, (b) in early Caradoc times, with Avalonia, including the London-Brabant massif, detached from Gondwana, (¢) in late Llandovery times, showing the Avalonian fusion with Baltica and the narrowing Iapetus Ocean and (d), in mid-Devonian times, after the closure of Iapetus. Arm., Armorica; Ib., Iberia; Fla., Florida.

O R D O V I C I A N TO D E V O N I A N P A L A E O G E O G R A P H Y listed from the Gr~s de Goasquellou are Apousiella cf. bouchardi, 'Atrypa' and Douvillina dutertrii, but Productella subaculeata, Cyrtospirifer cf. verneuili, and various other phyla are known from the surrounding Traonliors Formation, which is described as very poorly fossiliferous (Babin et al. 1982). Thus it seems likely that these two Devonian faunas from Brittany are also the same as those in the Budleigh Salterton pebbles, although the actual source area for the latter may well have come from a now-eroded northward extension of the present Armorican outcrops. The palaeogeography of late Devonian times has not yet been satisfactorily elucidated in the region; in southern Devon itself there were deep-water sediments being deposited in a very unstable tectonic situation (Bluck et al. in Cope et al. 1992, p. 65); however, the quartzites forming the Budleigh Salterton pebbles themselves clearly originated elsewhere. Various published palaeogeographies (e.g. Dreesen 1989) show a 'Condroz Shelf' of shallower-water sediments in a fringing belt lying to the south of the London-Brabant high, but of course those deposits lay on the northern margin of the Rheic Ocean in contrast to the Budleigh Salterton pebbles which originated to the south. Certainly the contemporary faunas found in quartzites in Belgium and Germany, which lay to the north of the Rheic Ocean, are quite different from those of Budleigh Salterton.

Conclusions It is not so long since the criteria underlying the correct use of fossils to elucidate palaeogeography have been clarified (Cocks & Fortey 1982). They are; (1) similarity of faunas alone does not necessarily indicate geographic continuity; (2) recognition of facies belts parallel to the edges of former continents can be used in conjunction with the differences displayed by faunas of the inner shelf (assuming comparable sediments) to distinguish separate continents; (3) planktonic or epipelagic faunas are primarily related to palaeolatitude and not to palaeocontinental distribution; (4) larval dispersal differences cause different faunal groups to cross oceans and other barriers at very different times; and (5) faunas from oceanic islands may have mixed affinities compared with those from neighbouring continents. Thus some key fossil groups are more useful than others (Fortey & Mellish 1992). Through the careful analysis of the Lower Palaeozoic faunas, such as those from Budleigh Salterton, the relative positions of palaeocontinents can be deduced, and these results can be combined with palaeomagnetic and other data generated by quite different branches of the Earth Sciences. The four palaeogeographical maps presented here (Fig. 6) are ostensibly based on a variety of relatively recent sources, including Cocks & Fortey (1982), McKerrow et al. (1991), Torsvik & Trench (1991), and Ziegler (1990). However, the truth is that they are actually built upon a marvellous variety of previous work, stretching well back into the nineteenth century and continuing forward into the active present. From the original concept of rocks dated by fossils, stratigraphy was born; but the fossils have always also been used to try to determine relative relationships; what would now be called ecology, basin analysis and palaeogeography. This palaeontological and biostratigraphical work has become progressively teamed with other disciplines of geology ranging from geophysics and palaeomagnetic studies through structural geology to

171

petrology and mineralogy, so that the end results can be seen as a real contribution to the understanding of the history of the Earth. The Geological Society of London, with its substantial volumes of varied publications over many years, has always been at the centre of this learning progression and looks likely to remain so. I am grateful to S. Morris for re-evaluating the Budleigh Salterton trilobites, to Claude Babin for information on French localities, and to R. Fortey and S. McKerrow for discussion. The figured specimens are deposited in The Natural History Museum, London (B, BB and BC).

References AUDLEY-CHARLES, M.G. 1970. Triassic palaeogeography of the British Isles. Quarterly Journal of the Geological Society of London, 126, 49-89, pls 7-13. BARIte, C., MELOU, M., PLUSOUELLE¢, Y & MORZADEC, P. 1982. Carte g~ologique de la France h 1/50, 000: 275- Le Faou. Bureau de Recherches g6ologiques et mini~res. BASSE~r, M.G. 1981. The Ordovician brachiopods of Cornwall. Geological Magazine, 118, 647-664, pls 1-4. BERDAN J.M. 1990. The Silurian and Early Devonian biogeography of ostracodes in North America. In MCKERROW, W. S. & SCOTESE, C. R. (eds) Palaeozoic Palaegeography and Biogeography. Geological Society London, Memoirs, 12, 223-231. Coci~s, L.R.M. 1989. Lower and Upper Devonian brachiopods from the Budleigh Salterton Pebble Bed, Devon. Bulletin of the British Museum (Natural History), London, (Geology), 45, 21-37. -& FORTEY, R.A. 1982. Faunal evidence for oceanic separations in the Palaeozoic of Britain. Journal of the Geological Society, London, 139, 465-478. -& --, 1988. Lower Palaeozoic facies and faunas around Gondwana. In: AUDLEY CHARLES, M. G. & HALLAMA. (eds) Gondwana and Tethys. Geological Society, London, Special Publications, 37, 183-200. -• LOCKLEY, M.G. 1981. Reassessment of the Ordovician brachiopods from the Budleigh Salterton Pebble Bed, Devon. Bulletin of the British Museum (Natural History), London, (Geology) 35, 111-124, figs 1-35. -& SCOTESE, C.R. 1991. The global biogeography of the Silurian period. Special Papers in Palaeontology, 44, 109-122. COPE, J.C.W., INGHAM, J.K. & RAWSON P.F. (eds) 1992. Atlas of Palaeogeography and Lithofacies. Geological Society, London, Memoirs, 13, 1-153. COWARD, M.P. 1990. The Precambrian, Caledonian and Variscan framework to NW Europe. In: HARDMAN, R. F. P. & BROOKS, S. (eds) Tectonic Events Responsiable for Britain's Oil and Gas Reserves. Geological Society, London, Special Publications, 55, 1-34. DAVIDSON, T. 1870. Notes on the Brachiopoda hitherto obtained from the 'Pebble-bed' of Budleigh Salterton, near Exmouth, in Devonshire. Quarterly Journal of the Geological Society of London, 2,6, 70-90, pls 4-6. ,1881. Monograph of the British Fossil Brachiopoda. Vol. I V , Part IV. Devonian and Silurian Brachiopoda that occur in the Triassic Pebble Bed of Budleigh Salterton in Devonshire. Palaeontographical Society [Monograph], London, 317-368, pls 38-42. DREESEN, R. 1989. Oolitic ironstones as event-stratigraphical marker beds within the Upper Devonian of the Ardenno-Rhenish Massif. In: YOUNG, Z. P. t~ TAYLOR, W. E. G. (eds) Phanerozoic Ironstones. Geological Society, London, Special Publications, 46, 65-78. ERDTMANN, B.D. 1991. The post-Cadomian early Palaeozoic tectonostratigraphy of Germany. Annales de la Societ~ G~ologique de Belgique, 114, 19-43. FORTEY, R.A. & COCKS, L.R.M. 1992. The early Palaeozoic of the North Atlantic regions as a test case for the use of fossils in continental reconstruction. Tectonophysics 206, 147-158. -& MELLISH, C.J.T. 1992. Are some fossils better than others for inferring palaeogeography? Terra Nova, 4, 210-216. -OWENS, R.M. 1987. The Arenig Series in South Wales. Bulletin of the British Museum (Natural History), London, (Geology), 41, 69-307. McKERROW, W.S., DEWEY, J.F. & SCOTESE, C.R. 1991. The Ordovician and Silurian development of the Iapetus Ocean. Special Papers in Palaeontology, 44, 165-178. NOaLET, C. & LEFORT, J.P. 1990. Sedimentological evidence for a limited separation between Armorica and Gondwana during the Early Ordovician. Geology, 18, 303-306.

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SADLER, P.M. 1974. Trilobites from the Gorran Quartzites, Ordovician of south Cornwall. Palaeontology, 17, 71-93, pls 9, 10. SALTER, J.W. 1864. Note on the fossils from the Budleigh Salterton Pebble-bed. Quarterly Journal of the Geological Society of London, 20, 286-302, pls 15-17. SCHONLAUB, H.P. 1992. Stratigraphy, biogeography and paleoclimatology of the Alpine Paleozoic and its implications for plate movements. Jahrbuch der Geologischen Bundesanstalt, Vienna, 135,381-418. SHEEHAN, P.M. 1987. Late Ordovician (Ashgillian) brachiopods from the region of the Sambre and Meuse rivers, Belgium. Bulletin lnstitut Royal Sciences Naturelles de Belgique, 57, 5-83. TORSVIK, T.H. & TRENCH, A. 1991. Ordovician magnetostratigraphy: Llanvirn-Caradoc limestones of the Baltic Platform. Geophysical Journal International, 107, 171-184.

COCKS

VERNIERS, J. & GROOTEL, G.V. 1991. Review of the Silurian in the Brabant Massif, Belgium. Annales de la Societ~ G~ologique de Belgique, 114, 163-193. VICARY, W. 1864. On the Pebble-bed of Budleigh Salterton. Quarterly Journal of the Geological Society of London, 20, 283-286. WILSON, J.T. 1966. Did the Atlantic close and then re-open? Nature, London, 676-681. WOODCOCK, N.H. 1991. The Welsh, Anglian and Belgian Caledonides compared. Annales de la Societ~ G~ologique de Belgique, 114, 5-17. ZIEGLER, P.A. 1990. Geological Atlas of western and central Europe. Shell International Petroleum Maatschappij BV.

Received 2 October 1992; accepted 29 October 1992

From QJGS,20, 116, 286. l. On the P~BBLZ-BZDof BUDr.~.IOHSXLTERT01~. By W. VICARr, Esq., F.G.S. With a Norp. on the FossiLS; by J. W. SALTER, Esq.,

F.G.S. [The publication of this paper is unavoidably deferred.] (Abstract.) THE south coast of Devonshire from Petit Tor, near Babbacombe Bay, to a little beyond Sidmouth, exhibits cliffs of New Red Sandstone, one of the beds of which, near Budleigh Salterton, is composed of pebbles of all sizes and of a flattened oval form ; this bed attains a maximum thickness of about 100 feet, and some of the pebbles composing it were found by Mr. Vicary to contain peculiar fossils. Mr. Vicary gave a description of the physical features of the area over which the pebble-bed extends, and entered into the stratigraphical details of this and the associated strata, referring to Mr. Salter's Note for information upon the affinities of the fossils. In his Note, Mr. Salter observed that, on comparing the fossils of the Budleigh-Salterton pebbles with those from the Caen sandstone in the Society's Museum, he found that all the species contained in the latter collection were also represented in the former. The general aspect o f the fossils was stated to be quite unlike that exhibited by English Lower Silurian collections ; and Mr. Salter therefore suggested that the exact equivalent of the Caen sandstone does not exist in England. This difference in the two faunas appeared to him to favour the theory of the former existence of a barrier between the middle and northern European regions during the Siluri,~n period. Note on the FossILs from the BUDI,EIOH SALTERTONPEBBLE-BF~.

By J. W. SXLXZR,F.G.S., A.L.S. W m ~ I first examined the pebbles from the Budleigh Salterton beds in the choice cabinet of Mr. Vicary, of Exeter, the impression made upon me was that anything and everything might be expected on British soil. Familiar as we had long been with the great variety of forms displayed by our own Silurian series, there had, nevertheless, been so far among them a great uniformity of type, and that a type shared by the fossils of the whole of the northern or Scandinavian area, as Sir R. I. Murchison and others have long ago indicated. We knew that the principal forms found in Russia and Sweden were represented more or less perfectly in the sandstones and shales of the Border-counties, and the slates of our Welsh and Cumbrian series. Nor would it have surprised any student of the palaeozoic rocks to find a large development of North American forms in our western limits, as, for instance, the Canadian fossils found by Sir R. I. Murchison in the West Highlands, or the New England types discovered and described by General Portlock in the county of Tyrone.

From QJGS,26, 70-71. 1. Notes on the BR/tCrrIoPoDA hitherto obtained from the " P~.BBL~BED " o f BUDLEIOK-SALTERTOI~, near EXMOUTH, "in DEVONSHIRE.

By Trro~xs Dxv~so~r, Esq., F.R.S., F.G.S., &c.* (PLArZs IV.-VI..) I~oDu~IO~. O~ the 16th of December, 1863, ~fessrs. W. ¥icary and J. W. Salter made an important communication to the Geological Society on the "pebble-bed" at Budleigh-Salterton, wherein some thirty-six different fossils were described and illustrated; of these, ten or twelve were Brachiopoda. Since that period Messrs. Vicary, ¥alpy, Edgell, Box, Winwood, * Thispaper was read at the Exeter meeting of the British Association, in August 1869, but has subsequentlyundergone considerablerevision. and others have been zealously at work collecting additional information, and every specimen that might assist in determining the age of the rock from which these drifted pebbles were derived has been carefully preserved*.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 175-183 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 417-425

Sedimentary structures: Sorby and the last decade J.R.L.

ALLEN

Postgraduate Research Institute for Sedimentology, The University o f Reading, P.O. Box 227, Whiteknights, Reading R G 6 2AB, UK Abstract: Henry Clifton Sorby pioneered in the last century the description and especially the hydrodynamic interpretation of sedimentary structures, together with their use as palaeocurrent indicators. Research completed since the last syntheses were published a decade ago shows that work along these lines continues to be necessary and relevant, particularly as regards the physical explanation of structures, and to present significant challenges and opportunities. Perhaps the most pressing needs are for a better understanding of (1) bedforms in gravels, silts and carbonate sediments, (2) tidal and especially sand-wave bedding, (3) hummocky and swaley cross-stratification, and (4) soft-sediment and dewatering structures in turbidites. Many sedimentary structures present a little-exploited opportunity to quantify process-rates and define short time-periods from the rock record.

Henry Clifton Sorby (1826-1908) of Sheffield may with every justification be called the 'Father of Sedimentology'. In his great Presidential Address to the Geological Society of London (Sorby 1908), he drew together into one bright beacon some of the glimmers of sedimentological thought which had guided his independently financed scientific career lasting more than half a century. In papers dating from the 1850s, he had pioneered the descriptive and hydrodynamic study of sedimentary structures, together with their application to palaeocurrent and palaeogeographic analysis, the latter topic receiving no general synthesis until another century had passed (Potter & Pettijohn 1963). He had also introduced new ways of observing and of reasoning about sediments and sedimentary rocks in general, from the experimental, to the chemical, to the petrographic using the microscope. Always his approach was quantitative, and it is perhaps partly this which, while his brilliance was acknowledged by his contemporaries, delayed the widespread acceptance of his viewpoint and methods. The description and especially the hydrodynamic interpretation of sedimentary structures was what Sorby (1908) emphasized in his Presidential Address. Others at about this time took up work on these questions (Cornish 1901; Gilbert 1914; Kindle 1917; Bucher 1919), but interest was short-lived. In 1948, however, Shrock published his Sequence in layered rocks. Although written from the standpoint of the structural geologist concerned to establish way-up in deformed strata, this book had the incidental effect of drawing attention in an emphatic way to the variety and richness for environmental interpretation of sedimentary structures, sparking off a tradition of structure atlases (e.g. Pettijohn & Potter 1964; Gubler et al. 1966; Conybeare & Crook 1968; Ricci Lucchi 1970). In parallel, there has grown up an interest in marrying sound descriptions of sedimentary structures to an understanding of their origin and interpretation, in term of natural environmental factors, loose-boundary hydraulics, and the

fluid mechanics of disperse systems (e.g. Middleton 1965; Allen 1968, 1982a; Collinson & Thompson 1982). Allen's (1982a) two-volume work aimed to be a comprehensive and critical synthesis of the field which would serve to focus research activities for some time to come. Although the work of description cannot be described as complete, the emphasis today is unquestionably on obtaining a fundamental understanding of sedimentary structures, through instrumented field work, critical laboratory experiments, and theoretical (including numerical) studies. With his appreciation of the value of precise measurement in geology, Sorby would not have felt out of place intellectually among those researching today on sedimentary structures, but he would almost certainly have expressed amazement at the number of investigators now active, the sophistication of their techniques, and the extent of their understanding. In the decade or so since the preparation of the last syntheses (Allen 1982a; Collinson & Thompson 1982), many more advances worthy of attention have been made in the field pioneered by Sorby. The following introductory outline, while defining the nature of these advances, cannot claim to be exhaustive.

Aeolian bedforms and bedding The publication of McKee's (1979) A Study of Global Sand Seas heralded a substantial renewal of interest in aeolian processes, landforms and bedding structures, expressed partly through major reviews (Greeley & Iversen 1985; Pye & Tsoar 1990). To these may be added Fisher & Schmincke's (1984) account of the sedimentary features generated by sediment-laden gaseous flows related to explosive volcanicity. Fryberger (1979) made an important advance by linking dune types to wind regimes, evaluated so as to reveal the way the potential for sand transport varied with wind direction and frequency. Typically, barkhan and transverse dunes depend on unimodal sand-transport regimes of little 175

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directional variance. Such dunes can preserve in their internal cross-bedding a record of daily fluctuations of wind strength and direction (Hunter & Richmond 1988). Dome-shaped dunes, beginning to be detected in the rock-record (Karpeta 1990), also arise under conditions of unimodal sand transport. Longitudinal (seif) dunes, with their bimodally distributed patterns of internal crossbedding, are related to a range of sand-transport regimes (high-variance unimodal, bimodal or complex) and in some instances form where there are only small, but distinct, seasonal shifts of wind direction (Tsoar 1982, 1983; Rubin & Hunter 1985). Complex transport regimes may create longitudinal dunes capable of oblique movement (Sneh 1988; see also Clemmensen & Blakey 1989). Star dunes, the most complicated in terms of internal structure, arise in complex sand-transport regimes, in which the wind either more or less boxes the compass or shows two diametrically opposed, commonly seasonal modes (see also Clemmensen 1987; Lancaster 1989). Aeolian cross-bedding patterns (Rubin 1987) have now been evaluated regionally for a number of epochs (Glennie 1983; Peterson 1988), to the great benefit of palaeoclimate studies. Zibar and granule ripples are now better known (Neilson & Kocurek 1986; Fryberger et al. 1992), and Anderson (1987) has shown theoretically that the wavelength of ballistic ripples in sand depends on grain size and sand transport rate. Adhesion structures take many forms and are important indicators of aeolian deposition on damp surfaces (Kocurek & Fielder 1982; Olsen et al. 1989), such as playas and the intertidal zone of beaches. Sediment drifts accumulated at obstacles in the path of the wind are common features not only in terrestrial deserts (Hesp 1981; Clemmensen 1986; Gunatilaka & Mwango 1989) but also on other planets possessing an atmosphere (Greeley & Iversen 1985). Experimental work by Paola et al. (1986) has refined our understanding of the complex patterns of flow and surface drag force which shape structures of this general class.

Aqueous sandy bedforms in unidirectional currents Continuing attention is rightly being given to the naming, classification and general hydraulic relations of sandy bedforms generated by unidirectional aqueous currents, a topic in which Sorby (1908) was a pioneer, with his categories of 'ripple-drift' and 'drift-bedding'. Ashley (1990) presented a wide-ranging scheme of classification which clarifies a number of issues and could help international communication. To Allen's (1982a) general review and synthesis of bedforms and their hydraulic stability fields may be added that of Southard & Boguchwal (1990a, b), in which new experimental information is included (Costello & Southard 1981; Boguchwal & Southard 1990). Southard and his associates present the data partly in velocity-depthgrain size graphs and, like Allen (1982a), stress the role of temperature (viscosity) in controlling the positions of boundaries between stability fields. The role that water temperature plays in influencing the character of bedforms in aqueous environments certainly is too little appreciated, when it is recalled that the temperature of a sandur stream is little above 0 °C, whereas that in a tropical lagoon can be as high as 25 °C. Mantz (1992) made an important study of bedforms generated in quartz silts and very fine sands; a wide variety of ripple marks, for example, can arise in these

ALLEN by no means uncommon grades of sediment. Sumer & Bakioglu (1984) provide a rigorous theoretical explanation for the well-known grain size-limitation of current ripples, and Gyr & Schmid (1989) link the initiation of ripples to sweep events in the innermost turbulent boundary layer. Better models have been proposed for the shape of, and flow over, ripples and dunes (Haque & Mahmoud 1985; Wiberg & Nelson 1992). Miiller & Gyr's (1986) work suggests how 'boils' on the surface of a river may be linked to the two- transforming to three-dimensional vortices generated in the mixing layer of the separated flow downstream of dunes. A hydraulic sequence of bedforms similar to that in quartz sands is reported from detrital halite (Karcz & Zak 1987). The major gaps in knowledge, however, remain the character and hydraulics of bedforms in gravels and in carbonate sediments. Although well known to arise during exceptional events, such as breakout floods, gravel dunes (Fig. 1) are proving to be common structures in macrotidal estuaries and in ephemeral stream systems where intense flows are more normal. However, these structures--and the rock-record affords many examples of them---cannot yet be interpreted hydraulically in the same way as their counterparts in sand-grade sediments, because of a lack of experimental data. Despite much recent attention, upper-stage plane beds, and the parallel lamination linked to them, remain enigmatic and controversial. The seemingly invariable association of parallel lamination with upper-stage plane beds means that the sedimentary surface is only nominally plane, and that the lamination records the existence and passage over the bed of extremely fiat sediment waves (Alien 1985a). What remains unclear from these almost exclusively experimental studies is whether these waves should be attributed to a bed-water surface interaction, turbulence effects (burst-sweep events, large coherent structures), sediment grading and sorting, or to some combination of these factors (Allen 1985a; Bridge & Best 1988; Cheel & Middleton 1986; Paola et al. 1989; Cheel 1990; Best & Bridge 1992). The first possibility could be further explored by experiments in closed, rectangular conduits. Plane beds could arise when the degree of sediment suspension reaches a critical stage and dunes are 'washed out' (see below) as local shear stresses become excessive (Bridge 1981; Fredsoe 1981; Johns et al. 1990). The work of Weedman & Slingerland (1985) suggests that some parting lineations found on plane beds--Sorby's (1908) 'graining in the line of the current'--may be more widely spaced than predicted by 'sediment-free' models, because of the significantly enhanced viscosity (effective viscosity) of the bed-load layer over plain water under conditions of intense grain transport. An experimental study by Arnott & Hand (1989) provoked renewed interest (Allen 1991) in massive bedding and the long-standing contention that, at sufficiently high rates of sediment deposition, lamination cannot form because of grain occlusion (review in Allen 1982a). Under conditions of very unsteady flow, such as typify flooding rivers and turbidity currents, the sediment deposition rate is effectively independent of the instantaneous hydraulic conditions (Allen 1982a; Lowe 1988). As Sorby (1908) stated, it is essential to regard such structures as climbing ripple cross-lamination as recording geologically significant processes operating on time-scales of minutes or hours (see also Ashley et al. 1982). This concept of the time-scale of

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Fig. 1. Dunes in pebble gravel, Hills Flats, Severn Estuary. Ebb tidal stream from upper right, with spade for scale.

many sedimentological events has been accepted for some time, but his statement must have greatly surprised many of Sorby's contemporaries. Studies in the rock-record have further strengthened a recognition of the complexity of bedforms in rivers (Allen 1983a; Haszeldine 1983; Kirk 1983), and that many fluvial channel sand-bodies present a composite of mainly bar- and channel-related 'architectural elements' (Allen 1983a; Miall 1985). Many gravel-bed rivers are steep enough for supercritical flow to occur locally or during flood stages. Transverse ribs (Rust & Gostin 1981), a common feature of their beds, may record an interaction between the current and the gravel in transport such that zones of alternately subcritical and supercritical flow arise along the stream (Allen 1983b).

Tidal bedding Shallow-marine tidal environments are complex, partly because of the multiplicity of periods on which the tide varies, but also on account of the seasonal but otherwise random, frequently substantial influence of storm surges (Pugh 1987). Progress in understanding tidal bedding patterns continues on some fronts to be slow. Work on contemporary and sub-fossil sandy bedforms confirms intraset discontinuities in all their variety in cross-bedding units as a major criterion of tidal sedimentation and guide to tidal regime (Dalrymple 1984a; De Mowbray & Visser 1984; Langhorne & Read 1986; Terwindt & Brouwer 1986; Dalrymple et al. 1990). In addition to spacing patterns indicative of the spring-neap cycle, it is now possible to recognize evidence of the diurnal inequality typical of semidiurnal tides (Allen 1985b; De Boer et al. 1989). However, with subtidal bedforms, particularly the larger and deeper-lying ones, little progress has been made on the question of internal structure; this is likely to present as equal a variety as the tidal regimes that are recognized. Langhorne (1982) was able to show in some detail how a particular sandwave varied in position and shape over short periods in relation to changing tidal (and wave) conditions, but his method is highly labour-intensive and provides only

a general insight into the internal structures generated. Some subtidal sand waves are known to include seismic reflectors pointing to an internal 'master bedding' (Bern6 et al. 1988), as suggested by some models of sandwave internal structures (Allen 1980). From the rock record, Allen (1982b), P.A. Allen & Homewood (1984), Kreisa & Moiola (1986), and Uhlir et al. (1988) described tidal cross-bedding with internal features indicative of the operation of semidiurnal or diurnal tides and of the spring-neap cycle. Some of the sequences described showed clear evidence of the diurnal inequality. Partly because of their importance for a n understanding of the Earth-Moon system over geological time, great attention has been paid to lamination patterns in mixed muddy and sandy sediments accumulated vertically under tidal conditions. Work in modern environments shows that a single tide can give rise to a variety of complex patterns of lamination, but that, from a succession of laminae, semidiurnal or diurnal, fortnightly, and annual (seasonal) layerings are nonetheless recognizable (Van den Berg 1981; Tessier et al. 1989; Allen 1990; Dalrymple et al. 1991; Roep 1991). In rocks, in many respects more easily handled than loose, contemporary sediments, sophisticated techniques of time-series analysis have allowed several tidal periodicities to be isolated among complex patterns of vertically accreted laminae (Kvale et al. 1989; Tessier & Gigot 1989; Tessier et al. 1989; Brown et al. 1990; Kuecher et al. 1990; Kvale & Archer 1990, 1991). Undoubtedly the most penetrating and rigorous of these studies is that of Williams (1989) on the reinterpreted, late Precambrian Elatina Formation of South Australia. In the complex signal represented by the vertical sequence of lamina thicknesses preserved in the Elatina mudstones, he could identify the semidiurnal, diurnal, fortnightly and monthly periodicities, as well as the lunar apsides and nodal cycles.

Lee-side processes and stratification The processes of grain settling (fallout) and avalanching (grainflow) on the leeward side of steep bedforms strongly influence the profile of these structures and the kinds and

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relative importance of the stratification they preserve internally (Allen 1982a). Sorby (1908) took a keen interest in these processes and, in particular, made a detailed study of angles of repose and their variation with grain size and packing; he noted that the slope of a heap of grains was significantly greater when failure occured than after avalanching had ceased. Chakrabarti & Lowe (1981) and Anderson (1988) devised theoretical models of grain settling, and Hunter & Kocurek (1986), Anderson (1988) and Hand & Bartberger (1988) further explored the process experimentally. They confirm that the deposition-rate profile becomes less concave-up with increasing sediment transport rate, and also with decreasing bedform height, patterns which affect the shape of the leeward profile and may bear on the conditions under which, as noted above, subaqueous dunes pass into upper-stage plane beds (Chakraborty & Bose 1992). Grain size also declines with increasing distance from the bedform crest. Anderson (1988) found a deposition-rate maximum very close to the crest of aeolian dunes, and a similar maximum of grain size, known experimentally, was predicted by Chakrabarti & Lowe (1981). Lobe-shaped sandflow cross-strata are now known to occur within large subaqueous as well as aeolian bedforms (Buck 1985; Hunter 1985a), and prove overwhelmingly dominant over fallout laminae. Hunter (1985b) has succeeded in producing a rigorous theory for the frequency and duration of avalanches as a function of sediment transport rate.

position sketched a decade ago (Allen 1982a). Mainly theoretical studies have confirmed the intrinsic instability of an erodible bed beneath an oscillatory current (Blondeaux 1990), the transition from rolling grain to vortex ripples with increasing bed relief (Vittori & Blondeaux 1990), and the unsteady vortex but steady, cellular streaming currents associated with the vortex bedforms (Du Toit & Sleath 1981; Longuet-Higgins 1981; Hara & Mei 1990a; Blondeaux & Vittori 1991). Brick-pattern ripples, formed at high Reynolds numbers (Vongissessomjai 1984), turn out to be associated with a three-dimensional pattern of steady, cellular streaming (Hara & Mei 1990b; Vittori 1992; Vittori & Blondeaux 1992), as sketched for the bed in Fig. 2, and seem to be due to centrifugal instability (Hara & Mei 1990b). The character of ripples in combined oscillatory and steady currents remains poorly understood, but there is some evidence that crests become sinuous in cross-flows (Lee Young & Sleath 1990). Field studies have added to the record of large wave-related ripples on the continental shelf in depths up to 160 m (Forbes & Boyd 1987; Leckie 1988). Because several controlling factors are involved, it is impossible to make unique inferences about ancient wave conditions from the size, shape and grain size of wave-related ripples preserved in rocks. Within this constraint, P.A. Allen (1984), Clifton & Dingier (1984) and Diem (1985) offer some useful refinements on the older interpretative models.

Bedforms related to wind waves Several important advances in knowledge, understanding and application of these structures have been made on the

While there is a growing appreciation from the rock-record of the abundance and distinctiveness as a facies of storm beds, typified by hummocky and/or swaley crossstratification (Kreisa 1981; Brenchley & Newall 1982; Hunter & Clifton 1982; Mount 1982; Brenchley 1985; Surlyk & Noe-Nygaard 1986; Leckie 1988; P.A. Allen & Underhill 1989; Smith & Ainsworth 1989; DeCelles & Cavazza 1992), progress in understanding has been slow, partly because it is very difficult to measure and record complex, high-energy storm processes in the contemporary environment. Field evidence indicates a link between hummocky cross-stratification, two- or three-dimensional wave-related ripples of large size, and surface wind-waves (typically three-dimensional and, if only for this reason, generating three-dimensional bottom-current patterns) of storm scale (Swift et al. 1983; Duke 1985; Greenwood & Sherman 1986; Leckie 1988; Cheel & Leckie 1992). Experimentally, oscillatory flows similar to those expected from storm waves can generate large bedforms and internal structures resembling hummocky cross-stratification, under conditions apparently equivalent broadly to the dune-upper plane bed transition (Arnott & Southard 1990; Southard et al. 1990; Myrow & Southard 1991). A decline in wave-current strength as a storm abates can be expected to generate a bed with a distinctive vertical pattern of structures and textures (see also Allen 1982a), including a possible, early massive portion (DeCelles & Cavazza 1992) due to a high rate of sediment fallout (Allen 1982a, 1991; Lowe 1988). The available evidence, such as it amounts to, is against the storm-surge ebb (Mount 1982) and unidirectional flow (P.A. Allen & Underhill 1989) models for hummocky and related bedding, but in favour of combined flow ( e . g . P . A . Allen 1985; Duke 1985; Nottvedt & Kreisa 1987; Leckie 1988; Duke et al. 1991; DeCelles & Cavazza 1992). Fabric studies

woves

Fig. 2. Schematic pattern of calculated streamlines representing the steady streaming at the bed associated with a field of brick-pattern wave ripples. The streamlines converge toward the crests of the bed features. Based on Vittori (1992).

Marine storm bedding

SEDIMENTARY STRUCTURES (e.g. Cheel 1991) could go far toward proving the expected strongly three-dimensional character of the storm-wave currents involved in making hummocky and swaley bedding. Myrow (1992) argues that pot and gutter casts are related to storm conditions.

Secondary flows and related bedforms Sets of flow-aligned, counter-rotating bottom vortices, that is, secondary flows, have many causes (Allen 1982a; Dyer 1982), and have been held to account in a number of ways for a variety of flow-aligned bedforms in aqueous and terrestrial environments (Allen 1982a; Flood 1983). Direct field evidence, however, of a link to secondary flows has been hard to obtain. Allen (1987a) found water-surface features suggestive of secondary flow in association with streamwise erosional furrows on estuarine mudflats. Viekman et al. (1992) measured cross-currents converging on debris-filled furrows on the floor of Lake Superior, as had been hypothesized (Flood 1981, 1983; Allen 1982a).

Desiccation fractures These common structures typically of muddy sediments are not just proofs of short-term atmospheric exposure but, when examined systematically, at least in intertidal settings (Plummer & Gostin 1981; Allen 1986a, 1987b), begin to reveal hitherto unsuspected details concerning the duration of, and controls on, the processes of formation. Fracture systems (Fig. 3) can evolve over periods of up to weeks or months, the stress field changing as the fissures ramify, thus altering the directions of crack growth, and their very varied geometry is as well strongly affected by sediment and environmental conditions. A related important advance is the careful dismissal by Astin & Rogers (1991) of the origin by either synaeresis or an intrastratal process of the fracture patterns widespread in the Devonian lake sediments of Scotland. Giant fracture patterns, attributable to either

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desiccation (Loope & Haverland 1988) or thermal contraction (Tucker 1981; Tucker & Tucker 1981; Kocurek & Hunter 1986), have recently been described from playa and evaporite sequences. The geometry of the giant fractures in the British Trias locally has a tectonic control. It may be suspected that desiccation and related fractures hold many unexploited clues to depositional patterns in environments subject to repeated submergence and emergence. That a sandy surface was once exposed to the atmosphere has always been difficult to prove (but see Allen 1982a). Microdeltas due to runoff are increasingly recognized as a potential exposure indicator in cross-bedded sandstones (Dalrymple 1984b).

Soft-sediment deformation and dewatering structures Sorby (1908) not only pioneered research on sedimentary structures due to the action of sediment-transporting currents, but he also described, from the Langdale Slates, structures which recorded soft-sediment deformation, noting that the affected sediments must have been semi-liquid. Wide-ranging interest in the deformation of unconsolidated and partly consolidated sediments has led to renewed attempts to classify soft-sediment and dewatering structures, and to identify their causes and controls (Owen 1987; Leeder 1987). Although unsophisticated by comparison, shaking tables similar to those long-employed by earthquake engineers are now being used by sedimentologists to understand soft-sediment deformation (Owen 1992). Allen (1986b) exploited empirical studies by seismologists and earthquake engineers to develop a model showing that, in fault-bounded sedimentary basins, the stratigraphic frequency of (externally generated) softsediment deformation structures (e.g. Bartsch-Winkler & Schmoll 1984; Clague et al. 1992; Roep & Everts 1992) declines steeply with distance from the fault zone (see also

Fig' 3. Desiccation cracks spreading over the surface of an intertidal mudflat, Berkeley Pill, Severn Estuary. Arrows show directions of crack propagation. Note the evidence for shear along some of the fractures; observe the growing tips and the young fractures that turn orthogonally towards established ones. Scale box measures 50 mm square.

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Leeder 1987). The rock-record provides some support for the model (Leeder 1987; Ord et al. 1988) and further research is justified. Descriptions of wave-related softsediment deformation structures, however, are reminders of what even modest, internal triggers can perform (Dalrymple 1979; Allen 1985c). In shallow-marine and lacustrine environments, for example, storm waves could be a more important cause of soft-sediment deformation than an external seismic trigger. However, many aspects of the mechanics of these aseismic soft-sediment deformations remain puzzling. Wrinkle marks (Allen 1985c) have on some sloping intertidal mudflats been seen to take the general form of groups of huge circular 'dendrites' each more than a metre across. Ourtward growth from a series of nuclei can be inferred, but the slope of the sedimentary surface seems to have had no significant influence on the process. The question of the true origin(s) of dish structures remains unresolved. Allen's (1982a) stoping model calls only for the hydraulic injection of water into an existing sequence of unlithified beds, and is not discounted by Hurst & Buller's (1984) finding that the clay minerals in dish structures apparently can be intrastratal in origin. Cheel & Rust (1986) found dish structures associated with ball-andpillow in a late Quaternary subaqueous outwash deposit. This association points to the formation of dish-structures some while after rather than during deposition, as is also demonstrated in some turbidite contexts by either the presence of a veined roof to the dish-structured zone or evidence of the prior formation of convolute lamination (Allen 1982a). Fluidization, however, does not seem to be the answer, for in their experiments Tsuji & Miyata (1987) could not reproduce the closely clustered, almost en echelon dishes so typical of field examples. True fluidization is in any case a highly disruptive process, after which little if any primary lamination will have survived. Consideration should be given to the possibility that columnar to sheet-like pillar structures (Lowe & LoPiccolo 1974; Lowe 1975) can be explained in terms of the instability of a settling dispersion of ill-sorted particles. Work by chemical engineers has afforded some suggestive parallels (Whitmore 1955; Weiland & McPherson 1979; Fessas & Weiland 1981 1984; Weiland et al. 1984; Batchelor & Janse van Rensberg 1986; Law et al. 1988; Cox 1990; Revay & Higdon 1992).

Conclusion Rightly, sedimentary structures remain a focus for research, in terms of their intrinsic interest and the light they can shed on environmental conditions and on processes inherently difficult or at present impossible to observe directly. They present perhaps five immediate challenges and one major opportunity. Although great attention has been paid to bedforms in unidirectional aqueous currents, the character and hydraulic relations of these features in gravels and in silts, neither by any means uncommon, remain largely ignored. There is little understanding of the transport and bedforms of carbonate sediments, which present many contrasts, especially in particle shape and overall density, from typical siliciclastic debris. The intensification of research on tidal bedding has many spurs. However, there is more speculation about the internal structures of subtidal sand waves than there is empirical knowledge. It is not sufficient

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to claim a sand-wave origin from some rock sequence; what we need are comparative data based on some technique for exploring very large samples from contemporary sand waves, perhaps by freezing and then raising large volumes of sediment from forms on the sea bed (cf. the sinking of mine shafts). Equally challenging are the problems of hummocky and swaley cross-stratification, where again today there is more speculation than knowledge founded in the contemporary environment and on (very difficult) experiments. The widespread and volumetrically important turbidite facies presents many soft-sediment and dewatering structures which are poorly understood at a fundamental level. A better understanding of these structures, as the result of experimental and theoretical approaches, could increase our appreciation of a set of processes observable with great difficulty if at all in the natural environment. There is growing interest among sedimentary geologists in the sedimentological expression of time, for example, orbital and related cyclicities. Sedimentary structures have a largely untapped contribution to make to this work, for very many of them represent a direct record of the passage of time and of process-rate, for example, climbing ripple cross-lamination, a point which Sorby (1908) was quick to appreciate. This paper is Reading University PRIS Contribution No. 261.

References ALLEN, J.R.L. 1968. Current ripples. North-Holland, Amsterdam. -1980. Sand waves: a model of origin and internal structure. Sedimentary Geology, 26, 281-328. -1982a. Sedimentary structures. 2 vols. Elsevier, Amsterdam. 1982b. Mud drapes in sand-wave deposits: a physical model with application to the Folkestone Beds (early Cretaceous, southeast England). Philosophical Transactions of the Royal Society, A306, 291-345. -1983a. Studies in fluviatile sedimentation: bars, bar complexes and sand sheets (low-sinuosity braided streams) in the Brownstones (L. Devonian), Welsh Borders. Sedimentary Geology, 33, 237-293. -1983b. A simplified cascade model for transverse stone ribs in gravelly streams. Proceedings of the Royal Society of London, A385, 253-266. -1985a. Parallel lamination developed from upper-stage plane beds: a model based on the large coherent structures of the turbulent boundary layer. Sedimentary Geology, 39, 227-242. -1985b. Principles of physical sedimentology. Allen & Unwin, London. 1985c. Wrinkle marks: an intertidal sedimentary structure due to aseismic soft-sediment loading. Sedimentary Geology, 41, 75-95. -1986a. On the curl of desiccation polygons. Sedimentary Geology, 46, 23-31. 1986b. Earthquake magnitude frequency, epicentral distance, and soft-sediment deformation in sedimentary basins. Sedimentary Geology, 46, 67-75. -1987a. Streamwise erosional structures in m u d d y sediments, Severn Estuary, southwestern UK. Geografiska Annaler, A69, 37-46. -1987b. Desiccation of mud in the t e m p e r a t e intertidal zone: studies from the Severn Estuary and eastern England. Philosophical Transactions of the Royal Society, B315, 127-156. 1990. Salt-marsh growth and stratification: a numerical model with special reference to the Severn Estuary, southwest Britain. Marine Geology, 95, 77-96. -1991. The Bouma Division A and the possible duration of turbidity currents. Journal of Sedimentary Petrology, 61, 291-295. ALLEN, P.A. 1984. Reconstruction of ancient sea conditions with an example from the Swiss Molasse. Marine Geology, 60, 455-473. 1985. Hummocky cross-stratification is not produced purely under progressive gravity waves. Nature, 313, 562-564. -& HOMEWOOD, P. 1984. Evolution and mechanics of a Miocene sand wave. Sedimentology, 31, 63-81. UNDERHILL, J.R. 1989. Swaley cross-stratification produced by unidirectional flows, Bencliff Grit ( U p p e r Jurassic), Dorset UK. Journal of the Geological Society, London, 146, 241-252.

SEDIMENTARY

ANDERSON, R.S. 1987. A theoretical model for aeolian impact ripples. Sedimentology, 34, 943-956. 1988. The pattern of grainfall depostion in the lee of aeolian bedforms. Sedimentology, 35, 175-188 ARNOTr, R.W.C. 8£ HAND, B.M. 1989. Bedforms, primary structures and grain fabric in the presence of suspended sediment rain. Journal of Sedimentary Petrology, 59, 1062-1069. 8£ SOUTHARD, J.B. 1990. Exploratory flow-duct experiments on combined-flow bed configurations, and some implications for interpreting storm-event stratification. Journal of Sedimentary Petrology, 60, 211-219. ASHLEY, G.M. (Chairperson and others) 1990. Classification of large-scale subaqueous bedforms: a new look at an old problem. Journal of Sedimentary Petrology, 60, 160-172. , SOUTHARD, J.B. & BOOI~IROYD, J.C. 1982. Deposition of climbing-ripple beds: a flume simulation. Sedimentology, 29, 67-79 ASTIN, T.M. & ROGERS, D.A. 1991. 'Subaqueous shrinkage cracks' in the Devonian of Scotland reinterpreted. Journal of Sedimentary Petrology, 61, 850-859. BARTSCH-WINKLER, S. 8£ ScliMOLL, H.R. 1984. Bedding types in Holocene tidal channel sequences, Knick Arm, upper Cook Inlet, Alaska. Journal of Sedimentary Petrology, 84, 1239-1250. BATCHELOR, G.K. & JANSE VAN RENSBERG, R.W. 1986. Structure formation in bidisperse sedimentation. Journal of Fluid Mechanics, 166, 379-407. BERNt~, S., AUFFREr, J.-P. & WALKER, P. 1988. Internal structure of subtidal sandwaves revealed by high-resolution seismic reflection. Sedimentology, 35, 5-20. BEST, J. & BRIDGE, J. 1992. The morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae. Sedimentology, 39, 737-752. BLONDEAUX, P. 1990. Sand tipples under sea waves. Part I. Ripple formation. Journal of Fluid Mechanics, 218, 1-17. -& Vn'roaL G. 1991. Vorticity dynamics in an oscillatory flow over a rippled bed. Journal of Fluid Mechanics, 226, 257-289. BOGUCHWAL, L.A. & SOUTHARD, J.B. 1990. Bed configurations in steady unidirectional water flows. Part I. Scale model study using fine sand. Journal of Sedimentary Petrology, 649-657. BRENCHLEY, P.J. 1985. Storm influenced sandstone beds. Modern Geology, 9, 369-396. 8£ NEWALL, G. 1982. Storm-influenced inner-shelf sand lobes in the Caradoc (Ordovician) of Shropshire, England. Journal of Sedimentary Petrology, 52, 1257-1269. BRIDGE, J.S. 1981. Bed shear stress over subaqueous dunes, and the transition to upper stage plane beds. Sedimentology, 28, 33-36. & BEST, J.L. 1988. Flow, sediment transport, and sediment dynamics over the transition from dunes to upper-stage plane beds: implications for the formation of planar laminae. Sedimentology, 35, 753-763. BROWN, M.A., ARCHER, A.W. & KVALE, E.P. 1990. Neap-spring tidal cyclicity in laminated carbonate channel-fill deposits and its implications. Journal of Sedimentary Petrology, 60, 152-159. BUCHER, W.H. 1919. On ripples and related sedimentary surface forms and their palaeogeographical interpretation. American Journal of Science, 149-210, 241-169. BUCK, S.G. 1985. Sand-flow cross-strata in tidal sands of the lower Greensand (early Cretaceous), southern England. Journal of Sedimentary Petrology, 895-906. CHAKRABARTI, C. 8£ LOWE, D.R. 1981. Diffusion of sediment in the lee of dune-like bedforms: theoretical and numerical analysis. Sedimentology, 28, 531-545. CHAKRABORTY,C. 8£ BOSE, P.K. 1992. Ripple/dune to upper stage plane bed transition: some observations from the ancient record. Geological Journal, 27, 349-359. CHEEL, R.J. 1990. Horizontal lamination and the sequence of bed phases and stratification under upper-flow-regime conditions. Sedimentology, 37, 517-529. 1991. Grain fabric in hummocky cross-stratified storm beds: genetic implications. Journal of Sedimentary Petrology, 61, 102-110. & MIDDLETON, G.V. 1986. Horizontal lamination formed under upper flow regime plane bed conditions. Journal of Geology, 94, 489-504. & RUST, B.R. 1986. A sequence of soft-sediment deformation (dewatering) structures in late Quaternary subaqueous outwash near Ottawa, Canada. Sedimentary Geology, 47, 77-93. & LECKIE, D.A. 1992. Coarse-grained storm beds of the Upper Cretaceous Chungo Member (Wapiabi Formation), southern Alberta, Canada. Journal of Sedimentary Petrology, 62, 933-945. CLAGUE, J.J., NAESGAARD, E. 8£ SY, A. 1992. Liquefaction features in the Fraser delta, evidence for prehistoric earthquakes. Canadian Journal of Earth Science, 29, 1734-1745. -

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Received 14 December 1992; revised typescript accepted 21 January 1993

From QJGS,64, 171. 10. Or* lhe APPLICATION of QUANTITATIVB METHODS tO the STUDr of the STRUC'rURE and HlsTOSr 03" RocKs. By the late HZNRr CLI~'TON SORBZ, LL.D., F.R.S., F.L.S., F.G.S. ( R e a d J a n u a r y 8th, 1908.) [Pr.ATZS XIV-XVIII.] CONTENTS. Io Introduction

II. III. IV. V. VI. Vii. VIII. IX. X. XI. XII. XlII. XlV. XV. XVI. XVlI. XVIII. XlX.

Page ................................................... 171

Final Velocities ............................... ....... .......... Angles of Rest of Sand and of Small Pebble8............ The Effects of" Currents ....................................... Ripple-Drift .............. Varying Size of'the Grains'"~iiiiiii~ii~ii~i~iiiiiiii:iill Drift-Bedding ................................................... Joints of Encrinites, etc..................................... Very Fine-Grained Deposits. ................................ The Green Slates of Langdale .............................. Washing-up, eta. of Clays .................................. ... On the Interspaces between the Constituent Grains of Deposited Material ....................................... Segregation ..................................................... Contraction of" Rocks after Deposition .................. Concretions ...................................................... 5pots in Welsh Slates ....................................... Slip-Surfaces ................................................... Surfaces of Pressure-Solution .............................. Determination of the Pressure to which Rocks have been Subjected ............................................

172 174 17ti 181 185 186 189 189 196 199 200 203 214 215 ~20 222 2~4 227

I. INTRODUCTION.

I~ the case of nearly all branches of science a great advance was made w h e n accurate q u a n t i t a t i v e m e t h o d s were used instead of m e r e l y qualitative. One great advantage of this is t h a t it necessitates more accurate t h o u g h t , points out w h a t r e m a i n s to be learned, and sometimes small residual quantities, which o t h e r w i s e would escape attention, indicate i m p o r t a n t facts. Since it applies to nearly all branches of geology, it is necessarily a wide subject, but so connected t o g e t h e r t h a t it seems undesirable to divide it. My object is to apply e x p e r i m e n t a l physics to the study of rocks. At least six different kinds of physical questions are involved, some of w h i c h have been sufficiently studied, b u t others require e x p e r i m e n t s which would be very difficult to carry out, and all t h a t I can n o w do is to endeavour to deduce plausible results from w h a t is k n o w n . I n doing this, it may be necessary to assume cases sufficiently simple for calculation, which may . b u t imperfectly correspond to natural conditions, so t h a t the results may be only a p p r o x i m a t e l y correct. Ill some cases, facts seem to show t h a t there are important properties connected w i t h subsiding material

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 185-193 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 801-809

Structure and origin of limestones B.

W.

SELLWOOD

Postgraduate Research Institute for Sedimentology, The University, Whiteknights, Reading R G 6 2AB, UK Abstract: Sorby's Presidential Address of 1879 on the structure

and origin of limestones was essentially an interim report of research in progress. His petrographic approach to limestones, stemming from three decades of research, laid the foundations for a wide range of research lines, some of which were not fully exploited for almost a century, such as fluid inclusion studies in diagenesis. The significance of many of his discoveries (e.g. that some Jurassic ooids and Palaeozoic corals were originally calcitic) had geochemical implications that have only recently achieved research prominence (e.g. CO2 and the 'greenhouse' Earth). In addition, Sorby's legacy was the example of his peerless approach to research, applying ruthless empiricism to the problem at hand. In the case of limestones, this involved an application of meticulous descriptive petrography and innovative experimentation. His approach to understand more fully the complex problems posed by carbonates remains unsurpassed, involving a thorough integration of detailed observation, imaginative thinking and judicious use of analytical techniques. By the time of his monumental Anniversary Address, as President of the Geological Society of London in 1879, Henry Clifton Sorby had researched for more than 30 years upon 'various questions essential to the proper elucidation' of the structure and origin of limestones, and yet he felt 'painfully conscious how much still remains to be learned'. His 1879 Address is rightly regarded as a landmark, seminal to much later research on the petrography of limestones. The strength of Sorby's work came from the lucidity of his deductive reasoning based upon a wealth of detailed observations and a wide range of experiments. And all this from a wealthy amateur who received no formal university training and never held a geological post. Folk (1972) regarded reading Sorby's Address of 1879 a being almost like reading the Bible. Sollas (1909) drew analogy between Sorby and Faraday, noting that both these great men had published their first scientific contributions on the analysis of 'a piece of lime'. Sorby's discourse had not been spontaneously generated. It drew upon his own vast observational experience, his invention of techniques, and his adaptation of methods developed by others. Of the utmost significance was the method of preparing thin transparent slices of rocks and minerals, devised around 1830 by William Nicol and first described in Witham (1831). Indeed, shortly before Sorby's death, at the age of 81, Geikie (1908) paid glowing tribute to his pioneering works on optical petrography which, Geikie proclaimed, had transformed petrography from the neglected branch of geology to 'the dignity of an almost independent science'. The Geological Society of London, according to Geikie, could take pride in the fact that Sorby had announced the birth of optical petrography 'within our walls'. He was referring not merely to Sorby's first paper, published in the Quarterly Journal in 1851, but to a series of papers published over the next eight years culminating in an epoch-making demonstration (Sorby 1858) of the application of optical microscopy to rock description, and to the

interpretation of rock genesis. This masterpiece, although setting out to prove whether particular crystalline materials were deposited from solution in water or from igneous fusions, also laid the foundations for fluid-inclusion analysis which today is a major technique in diagenetic studies. His last paper (Sorby 1908), published posthumously by the Society, underlined the importance of quantification in rock studies. By applying experimental physics to the study of rocks he also realized that results obtained on artificial and pure end-members would only approximately relate to the complex systems of the real world. Nonetheless, he attempted to determine the porosity of a wide variety of rocks, including many limestones, using both optical and boiling water techniques. These techniques are now both outmoded, the one involving the weighing-out of components scrupulously drawn on card from a camera-lucida projection, the other measuring the loss in weight of an artificially water-saturated sample. But the important thing was Sorby's visionary appreciation of what each method showed. The petrographic method gave an evaluation of how the rock had been changed by the emplacement of cement, whereas the water-porosity method measured the porosity 'as it now exists'. The former thus permitted quantification of parageneses, the latter provided practical data; the stuff of petrophysics. Sorby was a great empiricist. Cemented shell sands from a Quaternary raised beach near Torquay were described alongside samples from Bermuda and the deep-sea Challenger Expedition. He knew that limestones had the potential to form in a wide range of settings, and that they were not just tropical in origin. But researches into modern carbonate environments have, since the earliest regional investigations, such as those of the Bahamas (Agassiz 1894, 1896) and Florida (Vaughan 1910), concentrated on tropical areas. Only over the past few years have temperate-water carbonates been seriously studied (e.g. Chave 1967; Lees & Buller 1972; Nelson 1988). However, it is now realized that 185

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such calcite-dominated carbonates may be more comparable, in both mineralogical and diagenetic terms, with some of the carbonate systems of the past (James & Bone 1989, 1991). A Sorby-like approach might have brought this realization 70 years earlier. The last section of his Address consists mostly of descriptions of the major British limestones and it would be appropriate at this point to refer to British monographs, following up Sorby's works, comparable with those produced in France by Cayeux (1929, 1935); but there are none. Cayeux (1935) makes brief reference to Sorby's works, referring to his discussion of the origin of concretions, dolomite, ooids and cone-in-cone structure. In contrast, the mighty works of Murray & Renard (1891) from the H.M.S. Challenger expedition, and the results of the Funafuti boring (e.g. Judd 1904) are prolifically cited. It is curious to note that Sorby is himself a little short (3½ lines, p. 77) in dealing with deep-sea sediments, stating that so much attention had been paid by others (presumably a reference to the Challenger scientists, Murray and Renard) to deep-sea sediments that he would use his time describing less well known subjects, such as the Tertiary limestones of the Isle of Wight, and the Chalk! I have organized this tribute to Sorby broadly in the manner of his own address, and with some of his own headings. Within sections I have attempted to show some of the modern derivatives of his research and a few of the significant intervening works. This has had to be a very selective procedure. In place of an account of British limestones (which comprises the last 18 pages of Sorby's Address), I chart some of the post-Sorby developments in the study of the origin of limestones, and briefly consider some possible future trends. Sorby excluded a general discussion of dolomitic rocks from his Address, and so I have omitted dolomites from this review.

of the plate, or parallel to the axis of the spine or j o i n t . . . in perfect crystalline continuity'. Such fragments were usually cemented by singly-crystalline overgrowths of calcite optically continuous with the host grain. Microtexture, as well as mineralogy, strongly influence preservation potential in carbonate grains, not only of petrographic detail, but also of geochemical signals useful today in diagenetic or palaeotemperature investigations (e.g. Marshall 1992).

Microscopical structure and mineral nature of shells

(1) grain still aragonite; (2) original feature now a mosaic* of generally calcite spar

Sorby's 1879 Address begins with a bold overview: 'Limestones, being mainly derived from broken-up and decayed shells and corals, it is in the first place necessary to understand the structure and mineral constitution of modern calcareous organisms'. It is a fitting tribute to this approach that Bathurst's (1971) masterwork also begins with the petrography of skeletal materials. Before Sorby, the detailed microscopical character of modern shells had been researched from the biological viewpoint by Carpenter (1844, 1847)~ In addition, Sorby measured hardness and specific gravity, and used optical petrographic techniques (e.g. determining optic figures etc.), to become the first to document which shells were aragonitic and which were calcitic. In most cases, and without the special help afforded by stains (Friedman 1959; Dickson 1966) and X-ray diffraction techniques available today, his determinations closely match those currently accepted (Tucker & Wright 1990). He noted the similarities between the skeletal structure of crabs and those of trilobites. He comments on how the peculiar fibrous structure of Sepia might 'have interest in connexion with the fossilization of Belemnites', and he was particularly struck by the intimate interrelationship between mineral and organic structure in the calcitic hard-parts of echinoderms. 'Each plate, each spine and each joint of a single crystal of calcite, having its principal axis perpendicular to the plane

Influence of original mineral constitution on preservation Sorby was the first to realize that mineralogy and structure had an impact on the mechanical durability and preservation potential of shells, having already noted (Sorby 1862) that during weathering aragonite shells selectively dissolve, whereas calcitic shells are more stable. His experiments (e.g. keeping shells at 145 °C in the boiler of a high-pressure steam-engine for a month) had shown that, upon heating, aragonite could convert to calcite but he did not 'know of any process by means of which calcite can be changed into aragonite'. It is also clear that he had realized the full significance of finding well-preserved calcitic fossils associated with 'the mere casts of others', having related his observations on modern skeletal materials to fossil counterparts. He observed that when an originally aragonitic shell is changed to calcite it 'passes into a mass of crystals' (a mosaic) whereas originally calcitic components retain their true original structure (figs 3 and 4 of Sorby 1879). He suggested criteria whereby originally aragonitic components might be recognized. His optical criteria (* below), when supplemented by more modern approaches, are still valid and are included in Sandberg's (1983) criteria by which former aragonite may be recognized. These are, in order of descending reliability:

containing orientated aragonite relicts; (3) original feature now a mosaic* but lacking aragonite relicts; high Sr 2+ contents relative to levels reasonably expected in primary calcite (i.e. thousands of ppm); (4) original feature now a calcite mosaic* but S r 2+ values are either low (hundreds of ppm) or not measured; (5)* particular grain types selectively dissolved (moulded) or comprise calcite-filled moulds, this criterion can be strengthened if skeletal grains known to have been originally aragonitic, by analogy with modern forms, have behaved similarly in the same rock (i.e. most marine gastropods and nautiloids). Sorby demonstrated that Palaeozoic corals were originally calcitic and thus different from their modern Scleractinian counterparts. The significance of such observations, particularly the implications of changes in sea-water chemistry through time, was not realized for a century (e.g. Wilkinson 1979). Debate on this important issue continues today. Thus, by 1879 Sorby had laid the foundations of diagenetic studies in carbonates. A few significant edifices arose on these foundations, like that of Cullis (1904) who documented the petrography of carbonates in the Funafuti borehole (Cullis also used stains to distinguish between aragonite, calcite and dolomite), but several decades were

LIMESTONES to elapse before significant progress was made (e.g. Bathurst 1958, 1959).

Disintegration of shells and the origin of lime-mud Sorby deduced that lime-mud could be generated through the organic degradation (including micro-boring) and physical abrasion of skeletal material and pre-existing limestones. He also knew that minute crystals of calcite and aragonite could precipitate directly from certain waters. Fine mud, he decided, could never be generated from the degradation of crinoids, oysters and brachiopods, whereas he showed that corals kept for a few weeks in water gave rise to such minute particles that the water became 'like dilute milk'. The origin of lime-mud became controversial eight decades later when Cloud (1962) argued that evaporative precipitation alone explained Bahamian limemud. Later studies (Stockman et al. 1967; Neumann & Land 1975) showed that disintegration of aragonitic codiacian algae also generated vast quantities of lagoonal aragonitic mud. Direct precipitation of aragonite needles has also been demonstrated (Shinn et al. 1989), so both skeletal degradation and precipitation may contribute to the reservoir of lagoonal, and peri-platform, lime-mud. But in modern temperate waters, skeletal disintegration is probably the most important source of fine-grained carbonate (e.g. Lees 1975). Sorby thought that aragonitic muds could change to calcite, thus explaining ancient finely crystalline limestones. The origins of micrite-grade matrix in ancient limestones, although becoming more easy to investigate with the general application of the SEM (e.g. Loreau 1972), are still to some extent problematic. Using a combination of optical petrography, Feigl stain, SEM, X-ray and electron diffraction techniques, Lasemi & Sandberg (1984) demonstrated that welMithified Pleistocene 'micrites' had resulted from the alteration of predominantly aragonitic precursors, the mechanism involving a one-step neomorphic process of calcitization. Further application of geochemical techniques, such as an investigation of the Sr content (e.g. Sandberg 1983; Tucker & Bathurst 1990; Tucker & Wright 1990), can help to resolve whether significant amounts of aragonite were originally present. Sorby does not appear to differentiate between high magnesian calcite (calcite with 4 mole% MgCO3; HMC) and low magnesian calcite (LMC). It is only over about the past three decades that our awareness has increased with regard to the diagenetic significance of such differences, in particular the greater potential for HMC to undergo diagenetic change (e.g. dissolution, Land 1967; and micro-dolomitization, Lohmann & Meyers 1977).

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He concluded that where originally aragonitic shells were represented by casts, dissolution of their aragonite had led to the re-precipitation of calcite cement. In many cases, however, he noted that crystallization had obliterated original textures and grain outlines. That such processes could not always be attributed to 'the effect of heat-metamorphism' were proven by the occurrence of apparently similarly lithified modern sediments and reefs in the Bahamas and Bermuda. Thus, he recognized that early lithification and cementation typified many carbonates, but it was to be many decades before readily acceptable criteria were established whereby cementation textures could be distinguished from those produced by recrystallization (Bathurst 1958, 1959). Some recrystallization phenomena are still difficult to identify, particularly at the ultra-fine scale of individual coccoliths and foraminiferans. Early cementation in carbonate beach rock (Ginsburg 1953; Tracey in Emery et al. 1954) and observations on early diagenesis in south Florida carbonates confirmed Sorby's original view that carbonates need not be deeply buried in order to become cemented. Subsequently, submarine cementation was described in reefs (e.g. Ginsburg et al. 1968; Macintyre et al. 1968; Land & Goreau 1970) and on the sea floor (Shinn 1969). It is now accepted that penecontemporaneous cementation took place in many Phanerozoic reefs (e.g. James 1983; Schroeder & Purser 1986) and sea floor cementation in now known from a wide range of settings from carbonate sand belts (e.g. Dravis 1979), the deep sea (Dix & Mullins 1988) and in submarine caves (Whitaker & Smart 1990). Even though complicated classifications of spar types were proposed (e.g. Folk 1965), the wider realization that particular types of early cement could be environmentally specific has really come over the past 25 years (e.g. foregoing references and Dunham 1971; Halley & Harris 1979; Longman 1980; Tucker & Bathurst 1990). Given & Wilkinson (1985) have attempted to explain why aragonite, HMC or LMC precipitation is favoured, and why cements have a particular morphology, by reference to the interrelationships between Mg/Ca ratio, and rate of carbonate ion supply. Much current research is directed towards increasing our understanding of the processes of deeper burial cementation and pressure-dissolution (Scholle & Halley 1985; Choquette & James 1987). Particular problems currently being addressed are the role of pressure-dissolution in the development of bedding in limestones (Simpson 1985; Bathurst 1991), the origin of deep-burial cements and the interrelationships between cementation, the evolution of formation fluids and basin evolution.

Consolidation of limestones The porosity of newly deposited carbonate sediment is very variable. According to Sorby (1879), if grains are 'nearly spherical, and of the same size, it could not be much less than ½ of the whole volume'. He found, by experiment, that varying the grain shape and the amount of infiltrated fines, significantly altered the porosity. Experimental studies have subsequently shown that compaction affects grainy limestones less than muddy ones and that a porosity loss of 30% may occur in shelly lime muds without significant shell breakage taking place (Bhattacharya & Friedman 1979). In limestones, Sorby noted that what had been original pore spaces were generally lined, or filled, with crystalline calcite.

Oolitic grains Sorby's type examples of oolitic grains were 'Sprtidelstein' from a mineral spring at Carlsbad and were what we commonly now term cave pearls (Dunham 1972) or cave pisolites (Bathurst 1971). As such they might not be considered ideal, even though he regarded recent oolitic grains from 'Bahama and Bermuda' as being of the same general character. Nonetheless, his meticulous observations (fine concentric structure, a well-defined positive pseudouniaxial figure and appropriate specific gravity) shows that the grains he studied consisted of aragonite. Sorby suggested that the thin concentric layers within the grains (Figs 1 and

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B.W.

Recent

Marine

Ooids

,:'~

Random

SELLWOOD

Radial

- - Tangential

Nucleus / Ancient

Marine

~IConcentric laminae Ooids

Pore-filling sparry calcite

Calcitised aragonite with relic sructure --,...

t ---

Micrite

Radial fabric I Fig. 1. Major types of microstructure seen in modern and ancient ooids (from Tucker & Wright 1990). 2) did not result from the direct precipitation of crystals from solution, but by the mechanical accumulation of minute prismatic crystals with their long axes parallel to the surface of growth. They were, he proposed, 'mechanically accumulated round the centre, something like the layers in a large rolled snowball'. Bahamian ooids had similar characteristics, but less perfectly developed. He suspected that this was because they had formed in water rich in the mud derived from decayed shells, and that the purely chemical deposit served to collect minute aragonitic granules onto the spheroidal grains. The dominant modern view, following many subsequent studies, favours a precipitational rather than accretionary origin for ooids (e.g. Bathurst 1971; Loreau & Purser 1973), this view being supported by experimental studies (e.g. Davies et al. 1978).

Fig. 2. Concentric structure in Carlsbad cave pearl (left); calcitic Jurassic ooid with radial microstructure (centre); originally aragonitic ooid now preserved as a calcite mosaic (right) (from Sorby 1879).

Fig. 3. SEM micrograph of porecast (epoxy replica of microporosity within originally calcitic ooids), Great Oolite (MidJurassic) grainstone, 1406 m depth, Weald Basin.

Ooids, their mode of formation and significance, continued to provoke interest (even of petrographers) through the post-Sorby years and several memorable papers were published (e.g. Brown 1914). The mineralogy of ooids influences both their microfabric and their subsequent diagenesis, with most modern marine ooids being aragonitic in composition. Aragonite is the favoured precipitate wherever Mg:Ca ratios are high, whether in the sea or lakes (Richter 1983), marine ooids generally forming on shallowly submerged high energy shoals or shorefaces in tropical seas. HMC ooids are rare in modern systems whereas LMC ooids are known from modern streams, caves, lakes and soils. Various types of microstructure are commonly seen in ooids (Fig. 1) and individual grains may retain original internal microporosity to considerable burial depths (e.g. Fig. 3). Using the petrographic criteria already established for shell material, Sorby points out quite clearly that some ancient ooids were aragonitic whilst others (e.g. some from the British Jurassic) were originally calcitic and had their principal negative axis arranged in a radiate manner. Cayeux (1935, p. 225) considered this 'structure radire' to be a secondary feature, and this remained the favoured interpretation until the 1970s (see above). Research on radial aragonitic ooids of the Great Salt Lake Utah (Sandberg 1975) raised fundamental questions on whether the compositions of non-skeletal marine precipitates have remained similar to those of the Recent throughout Phanerozoic time (e.g Sandberg 1983, 1985). It is now well established that there were time intervals in the past when marine precipitates were broadly comparable with those of the present (e.g late Carboniferous to Permian) and others when significant differences existed. During the early and mid-Palaeozoic (Cambrian to early Carboniferous) and most of the Mesozoic (Jurassic and Cretaceous) LMC appears to have dominated both cements and non-skeletal grains. There is an apparent absence, globally, of late Cretaceous ooids. These changing temporal abundances of aragonite, HMC and LMC, and grain-types, record significant changes in the composition not only of seawater, but also of the whole ocean/atmosphere system (e.g. Wilkinson 1979; Wilkinson et al. 1985; Sandberg 1985; Berner 1992).

LIMESTONES

The Sorby scientific legacy: the origin of limestones T o w a r d s e n v i r o n m e n t a l interpretation To Sorby the origin of limestones was very much bound up with questions such as: what are the constituents within limestones and how have they been put together? By 1926, Twenhofel (p.152) was already convinced that to interpret the origin of any sediment required a reconstruction of the environment in which the sediment was deposited. Interpretation of sedimentary environments, facies analysis, requires a refined stratigraphic framework, but many limestone successions lacked the fossil groups then (and still) vital for precise correlation. Twenhofel was also able to underline the importance of diagenetic modifications. These alteration processes had, by then, been specifically addressed by such workers as Andrre (1911) and Schuchert (1920). Stemming ultimately from the classic observations on reefs by Darwin (1842), and subsequent expeditions such as that to Funafuti (Bonney 1904), some of the most convincing attempts at carbonate environmental interpretation were made on ancient reef systems (e.g. Grabau 1903, on Palaeozoic reefs in general; Munthe 1910 on the Silurian reefs of Gotland and those in the English Wenlock by Crosfield & Johnston 1914). More generalized environmental studies on Recent systems, such as that of Vaughan (1910) in Florida and the Challenger reports (Murray & Renard 1891), allowed interpretations to become integral to regional stratigraphic papers, among the classics being Dixon & Vaughan (1911) on the Carboniferous succession in the Gower Peninsula, S. Wales. Carbonates still posed intractable problems, however, especially chalks. Although they had been discovered to contain coccoliths (by Sorby), and planktic foraminiferans (Jukes-Brown & Hill 1903, 1904), Twenhofel (1926, p. 295) considered chalks to be shallow-water deposits that accumulated 'under conditions which are still not understood'. This is an oddly opaque statement, particularly in view of the detailed descriptions of pelagic oozes then available in the Challenger reports. Indeed, these reports were so thorough in their coverage that no comparably exhaustive survey of oceanic sediments was undertaken for 50 years (the Deep Sea Drilling Project and the Ocean Drilling Program; reviewed in Jenkyns 1986). Sorby (1879), however, had already suggested that Chalk, although far from being identical with Globigerina-ooze, was analogous to 'deep ocean mud comparatively free from volcanic and other mechanical mineral impurities'. He had recognized the pelagic nature of this shelf-sea sediment, even though his implication that it accumulated at great depth is not now accepted (Jenkyns & Hsu, 1974; Hancock 1975). The discovery of major oil accumulations in North Sea chalks, preferentially porous in re-sedimented facies (Scholle et al. 1983; Hancock et al. 1987; Taylor & Lapr6 1987) has led to further re-evaluations of onshore chalks, some exhibiting slumps and debris flows (e.g. in Normandy by Quine & Bosence 1991). Rhythmic bedding in chalks, and other hemi-pelagic carbonates, are providing convincing evidence (through integration of petrographic and oxygen isotopic data) for short-term cyclical change in Water temperature and productivity consistent with Milankovitch fluctuations (e.g. Ditchfield & Marshall 1989). A major impetus towards a better understanding of both depositional and diagenetic processes came when substantial

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hydrocarbon discoveries were made in carbonate reservoirs, especially between 1920 and 1930 (e.g. late Jurassic, Smackover oolites in Arkansas; the mid-Cretaceous Golden Lane and Poza Rica rudistid reefs in Mexico; the Yates Field of W. Texas; and the vast Kirkuk Field of Iraq in which there were several reservoirs, some in Cretaceous chalks and others in Tertiary limestones). In the Middle East the work of Henson (e.g. 1950) on Cretaceous and Tertiary reefs signalled a re-birth of carbonate study, the reef resurgence continuing in the USA with the classic study by Newell et al. (1953) of the Permian Reef Complex of Texas and New Mexico. In hydrocarbon exploration reefs began to rival anticlines as exploration targets, even though many of the reefal reservoir facies turned out to be adjacent to, rather than in, the ancient reefs themselves. In the 1950s began a series of classic studies on modern carbonate systems. These researches re-laid the foundations upon which facies models could be refined, and against which they could be tested (e.g. Illing 1954; Emery et al. 1954; Newell et al. 1959; Ginsburg 1956; Cloud 1962; Logan et al. 1970; James & Ginsburg 1979 and many others). A major difference in approach has distinguished such studies from parallel studies on terrigenous clastic systems and this has been the necessity, in carbonates, to integrate a very wide range of biological, petrological and physico-chemical observations. Only with the realization, in the 1970s, that reservoir sandstones undergo profit-damaging diagenesis, did some siliciclastic practitioners re-discover petrography. But with carbonates, and particularly using borehole ditch cuttings, it was soon established that microfacies interpretations could provide a valuable predictive tool in subsurface exploration. The microfacies schemes evolved by Wilson (1969, 1975) and Fliigel (1982) are frequently cited as giving a first-order means of interpreting carbonate interrelationships in frontier areas. Such approaches are useful in subsurface studies because wireline logs are often less informative in carbonates than in terrigenous clastic successions. L i m e s t o n e classification Understanding required classification but, as Folk (1972) observed, the literature of the 1920s to 1940s suggests that carbonate knowledge had actually regressed from Sorby's time. Folk's (1959) classification, inspired by Krynine's treatment of sandstones, recognized that carbonates could, in simple terms, be considered to consist of mechanically transported grains (allochems), carbonate mud matrix (micrite) and pore-filling calcite cement (spar). The classification requires a petrographic approach. Not only does the scheme provide an accurate description, it is also genetic, the rock class relating to depositional hydraulic energy (high energy oosparite; low energy micrite etc.). Folk's classification was re-stated in the volume edited by Ham (1962) in which the most widely used of all carbonate classifications also appeared (Dunham 1962). The Dunham scheme emphasized depositional texture but did not propose formal terminology for grain types. Dunham's scheme (defining grainstone, packstone, wackestone and mudstone) could be used in the field and on whole core, whereas Folk's scheme mostly required more detailed petrography. Neither classification worked well for reefal carbonates nor for carbonates that had been severely altered by diagenesis. A derivative of the Dunham scheme, that of Embry & Klovan

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(1971), has been generally adopted for reefs and more recently Wright (1992) has proposed a petrographicallybased scheme that attempts to address the problem of altered and crystalline carbonates. R i m m e d shelves and ramps, and sequence stratigraphy During the 1950s and 1960s, and in a growing body of literature, consideration of the origin of limestones involved many additional examples being described, and interpretations of ancient depositional environments being refined. Carbonate successions were usually interpreted by reference to the newly described modern systems. This was an essentially uniformitarian approach. A typical example is that of Roehl (1967) who explained most of the Ordovician and Silurian facies present in oilfields in Montana and the Dakotas by direct analogy with the Bahamas. There were many other such accounts (reviewed in Scholle et al. 1983; Sellwood 1986). Steep-sided, shoal- or reef-rimmed platforms (such as those of the Bahamas, Florida and atolls) were used as depositional models for many ancient limestone successions. In such systems facies distributions are generally concentrically arranged, reflecting a concentration of wave and current energy at the rim. However, as many stratigraphers knew, ancient seaways in which limestones originated spread over the heartlands of continents and had minimal slopes ('epeiric seas' of Shaw 1964; Irwin 1965). Ahr (1973) termed such systems ramps and these were shown to be dominant at times when reef-constructing organisms were either absent or inhibited (James 1983). Some very relevant data on the workings of a modern low-slope carbonate systems came with studies of the Persian (Arabian) Gulf (Purser 1973), but most environmental interpretations involving ramps are still conceptual. Ramp-like carbonate platform successions have now been interpreted from all parts of the geological record (Read 1985; Burchette & Wright 1992), having been best developed in gently subsiding situations such as foreland and interior basins, and along passive margins. The broad-scale interpretation of carbonate successions now involves a thorough integration of geophysical, sedimentological, geochemical and stratigraphic data so that the evolving dynamics of such systems may be visualized, often using computer models (e.g. Sarg 1988; Aigner & Dott 1990). Although a far cry from Sorby's meticulous petrographic analysis of limestones, many of the largely conceptual approaches of recent years have depended upon such studies. Modern interpretation of the origins of carbonate successions does not only involve the recognition of either ancient ramp or rimmed shelf depositional environments. It also requires a consideration of sequence stratigraphic context. This is because ramps and rimmedshelves would be expected to behave differently as a result of sealevel changes, both in terms of the architectural response of their sediment bodies, and their susceptibility to the diagenetic effects of meteoric waters (e.g Wilgus et al. 1988; Crevello et al. 1989; Tucker & Wright 1990; Schlager 1991). These concepts, although prominent in current carbonate research, go well beyond Sorby's essentially pragmatic approach to limestones. Limestone diagenesis As discussed above, it is possible to interpret some types of cement as formed by near-surface processes, and to relate

them to the early burial history of the limestone. It is even possible, in favourable cases, to map the distribution of palaeo-meteoric lenses within ancient carbonate platforms (e.g. Meyers 1978; Emery & Dickson 1989). The relative timing of the emplacement of particular cement phases can be related to basin evolution, fluid (including oil) migration and burial history (e.g. Prezbindowski 1985; Sellwood et al. 1993). Refined diagenetic interpretations have been inspired, in part, by an extension of essentially petrographic techniques such as staining (Friedman 1959; Dickson 1966), cathodoluminescence (e.g. review in Emery. & Marshall 1989), SEM and UV luminescence, and are truly part of the Sorby legacy. Very significant advances have also been made as a result of sound petrography being supported by the application of a wide range of geochemical techniques. These include stable isotopic analyses (e.g. Hudson 1977 and recent review by Marshall 1992), electron probe microanalysis (e.g. Reeder & Paquette 1989), laser microprobe (Smalley et al. 1989) and many others (reviewed in Morse & Mackenzie 1990). The key to sound interpretation, however, has often been founded upon a thorough understanding of both the petrography and the geological context in which the limestone occurs. 'Stable isotopic analysis is the last thing anyone should do!' (M.L. Coleman pers. comm.). Fluid inclusion analysis, is a technique with which Sorby would have felt familiar, it was in great measure his own (Sorby 1858). Sorby had investigated 'fluid-cavities' in rock salt, and also calcite crystals from veins, recognizing the potential of inclusions as palaeothermometers in a wide range of minerals. This technique has become increasingly used in carbonate geothermometry over the past decade (e.g. Lee & Friedman 1987; Sellwood et al. 1989). Pressure-corrected homogenization temperatures from undeformed inclusions can be compared with isotopicallyderived palaeotemperature evaluations to help constrain fluid compositions during crystal growth. Freezing temperatures obtained from fluid inclusions can provide an independent check on fluid salinities (wt % NaC1 equivalent). The use of such techniques, when combined with thorough petrography and interpretation of burial histories, is beginning to define fluid movement within significant portions of basins (e.g. McLimans 1987; Sellwood et al. 1989, 1993). Carbonate diagenesis in the deeper parts of basins may not remain an 'out of sight and out of mind' issue for much longer (Scholle & Halley 1985).

Origin o f limestones: not just h o w but when ? The inadequacy of a high resolution stratigraphic framework has often hampered the refinement of both depositional and diagenetic interpretations. Sorby did not ask 'when?' in his 1879 Address, but it is a key question today. Not only the crucial questions of when did deposition occur and how confidently may units be correlated? We also seek to know about the time-frame within which depositional conditions changed, the possibility of short-term fluctuations (Milankovitch cyclicity) and shorter term storm frequency. Interpretations of basin evolution, and especially those put to practical use (in the petroleum industry for example), demand the absolute dating of depositional, diagenetic and structural events. Without an adequate answer to the

LIMESTONES 'when?' question, other interpretations may become suspect. To these ends new methods have been applied. Biostratigraphic techniques, now vastly improved since Sorby's day, can be used in conjunction with a range of chemostratigraphic techniques such as carbon-isotope stratigraphy (Gale et al. 1993) and 87Sr/86Sr stratigraphy (e.g. Koepnick et al. 1985; Elderfield 1986). Such advances have been made as a result of innovative application of laboratory techniques, scrupulous observations and imaginative thought: the Sorby approach. Conclusions In the historical development of our subject Sorby was unique. He saw both what others had not seen, and what they had seen. However, not only did he think what others had not thought (Folk 1972), he also comprehended the potential of his observations, methods and insights, and many of their implications. His was inspired empiricism. I am grateful for many useful discussions with colleagues at PRIS, and in particular to R. Goldring, A. Parker, G. Price and K. Ziegler. The paper was greatly improved by two anonymous referees whose comments I greatly appreciate. PRIS Contribution Number 280.

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Received 16 March 1993; revised typescript accepted 5 May 1992.

From QJGS,35, 39, 56. THE ANNIVERSARY

A D D R E S S OF T H E P R E S I D E N T ,

:I-IsNRr CLIFTO~X SORDY, Esq., F.R.S.

T A B L E OF CONTENTS.

Introduction ......................pag(e 56 Recent Limestones ............... page Tertiary Limestones of the Isle of Microscopical structure of Shells, Wight ................................. &e. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 ~[ineral nature of SheDs, &c. . . . . . . 58 Chalk ..................... Structure of Crystals ............ ~..... 6, Kentish Rag Wealden and Purl~'ec~:'~c"l~;":::::: General structure of various groups of Shells, &o......................... 6x ~Portland Oolite ........................ CorMline Oolite ........................ Difference in the properties of Calcite and Aragonito ............ 64 Kelloways Rock ........................ Cornbrash .............................. The influence of original mineral Forest Marble ........................ constitution on the preservation of fossils.: ............................ 65 Great Oolite ........................... Structure of Fossils .... ~,............ 66 Inferior Oolite ........................ Disintegration of Shells, &c.......... 69 The Oolites as a whole ............... Lias ....................................... Preservation of tim external forms of altered SheD-fragments ...... 7x Cleveland-hill Ironstone ............ Chemical deposition of Carbonate 7x Magnesian Limestone ............... Limestones of the Coal-measures... of Lime .............................. Consolidation of Limestones ...... 7z Carboniferous Limestone ............ Devonian Limestones ............... Re&.l~e~ment of Lime by Ferrotm ide, Magnesia, &c................ 73 Silurian Limestones .................. StTucture of different Limestones.. 7 3 Metamorphic Limestones ............ Travertine and Tufa .................. 73 General Conclusions .................. Oolitic grains ........................... 74 Tabulated results .....................

76 77 78 79 79 8o $o 8I 8l 8z 8~. 83 83 84 84 85 86 85 87 89 9o 9I 94

Introduvtion. I ~ n o w proceeding to the more special portion of m y Address, I propose to t r e a t on the s t r u c t u r e and origin of limestones, relying m a i n l y on m y o w n observations, b u t incorporating ~eneral facts derived f~om o t h e r sources. I have n o w for n e a r l y t h i r t y years been s t u d y i n g various questions essential to t h e proper elucidation of m.y subject, and y e t I feel painfully conscious how m u c h s~ill r e m a i n s to be learned. Some of these questions are not strictly geological, but y e t are as necessary in studying limestone rocks as a n a t o m y is for pal~eontology. I shall, therefore, not scruple to e n t e r into t h e m so far as appears desirable to establish, on a good foundation, the more specially geological conclusions.

Microscopical structure of living Shells, dfc. Limestones being m a i n l y derived from b r o k e n - u p and decayed shells and corals, it is in the first place necessary to u n d e r s t a n d the structure and m i n e r a l constitution of m o d e r n calcareous organisms. T h e i r general microscopical characters have already been well described by Carpenter and other w r i t e r s ; but they have looked upon t h e m far too m u c h from a biological point of view for m y present purpose. The p u r e l y mineral structure of the c a r b o n a ~ of lime plays a m u c h more important part t h a n has often been ascribed to

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 195-202

Flood basalts versus central volcanoes and the British Tertiary Volcanic Province GEORGE

P.

L.

WALKER

Department of Geology & Geophysics, SOEST, University of Hawaii, Honolulu, Hawaii 96822, USA Abstract: A controversy in the final decades of the last century developed between Geikie and Judd over the nature of the volcanoes in the British Tertiary Province. Geikie regarded the lavas as in Antrim and Skye as plateau basalts (today called flood basalts) erupted from widely scattered fissures, while Judd regarded the intrusion complexes as in Central Mull and Skye as eroded stumps of major central volcanoes from which the lavas originated. Soon after, instigated by the controversy, British volcanology flowered for several decades in what may be called its 'Classic Period'. Detailed mapping projects were undertaken, and new concepts were developed that are basic to volcanology. This paper views the controversy and its aftermath, and also briefly reviews recent conceptual developments in the North Atlantic Volcanic Province of which the British Tertiary rocks are a part. The paper discusses the present understanding of flood basalts and central volcanoes, and presents new criteria based on structural features of lava flows (such as the dependence of lava thicknesses, and the occurrence of pipe vesicles, on ground-slope angle) to distinguish between them. Magnetic fabric study of magma-flow directions in intrusions and lava flows, and palaeomagnetic study of postemplacement tilting of igneous rocks, have great unrealized potential.

Controversies can be healthy in the advancement of science. They direct attention to the issues involved, and provoke others to enter the arena and contribute new data and ideas. Controversies are, however, not always resolved within the lifetime of the principals. The pages of the Journal of the Geological Society record many controversies, some of them very spirited and having varied outcomes. Quite commonly the truth, or more correctly the eventual consensus view, proves to be a compromise. This was the outcome of the controversy discussed in this article. This controversy developed just over a century ago between Archibald (later Sir Archibald) Geikie and John W. Judd over the exact nature of the piles of lava flows and associated intrusive igneous rocks (notable in Antrim, Skye and Mull) in the British Tertiary Volcanic Province. About a century earlier, the same area had been an arena of a much more embittered controversy which was fought between the Huttonians or Plutonists and the Wernerians or Neptunists regarding the igneous or sedimentary origin of basalt. It is a matter of history that the Huttonians prevailed. There are two aims here: to present a personal view on the G e i k i e - J u d d controversy and the flowering of British volcanology that was its immediate sequel; and to review the present state of understanding of the British Tertiary Volcanic Province, together with that of the North Atlantic Volcanic Province of which it is an important part.

'successive sheets of basalt have proceeded from no one centre of eruption. They die out now towards one quarter, now towards another, yet everywhere retain the universal regularity and gentle inclination of the whole volcanic series' (Geikie 1897, p. 194). Furthermore, he observed, the dykes and volcanic necks that are likely to represent feeders of the lava flows are widely scattered. These reasons are equally valid today. Judd on the other hand directed attention towards the intrusive complexes such as those in Central Mull and the Cuillin Hills of Skye, and postulated that these were 'the denuded cores and basal wrecks of great volcanoes' (Judd 1874, 1881, 1886, 1889). Judd was less persuasive than Geikie, and partly spoiled his case by apparently getting wrong the sequence in which the rock-types were emplaced. Geikie also made mistakes and for a short time considered that the Cuillin gabbros in Skye were metamorphic rocks of Archaean age. Geikie had lived as a youth in Scotland and had wandered extensively through the Hebrides. He was understandably annoyed by this outsider entering the field and forestalling his own plans for publication. H e was also somewhat resentful because Judd, being a university professor, seemingly had more leisure to pursue his studies than had Geikie as a Geological Survey geologist.

The Classic Period of British volcanology The Geikie v. Judd controversy

Following closely upon and provoked by the G e i k i e - J u d d confrontation, British volcanology flourished as never before. I think it is appropriate to call the first three or four decades of the present century the Classic Period of British volcanology. It was a period of intensive research rewarded by important discoveries. The efforts of a few dedicated, creative and hardy individuals opened up new vistas in volcanology. The period commenced with publication of the memoir by H a r k e r (1904) on the Tertiary igneous rocks of Skye,

Geikie (1871,1984,1897) wrote eloquently about the basaltic plateaus of Antrim and the Hebrides. H e regarded these as magnificient remnants of a much more extensive plateau possibly coincident with the far-ranging Tertiary dyke swarms of the British Isles and extending as far as Iceland and the Faeroes. Plateau basalts are today more commonly called flood basalts, following Tyrrell (1937). Geikie's reasons for regarding the basaltic lava piles as remnants of extensive plateaus or flood basalts were that 195

196

G.P.L.

which was a model of careful observation and clear description, and it strongly influenced many, including me. Harker made many discoveries. One was that the gabbro intrusions of the Cuillins and Blaven have a banding (today it would be called layering) which dips towards a focus. Another was that a great complex of intrusive sheets (later called 'cone-sheets') occurs in the Cuillin Hills, inclined towards the same focus as the inward-dipping layering in the gabbros. He also recognized that magma-flow directions in dykes are not necessarily vertical, and can sometimes be inferred from structural features of the dykes. Harker followed this with his memoir on the Geology of the Small Isles of Inverness-shire (Harker 1908) in which he described the great ultrabasic intrusive core of the Island of Rum. Two masterpieces of volcano mapping soon followed, namely Ben Nevis and Glencoe (Clough et al. 1909; Bailey & Maufe 1916) and Mull (Bailey et al. 1924). The concept of cauldrons as subsidence features bounded by ring faults and ring dykes was developed at Glencoe and applied in Mull, and has proved to be of fundamental importance in volcanology. The described cauldrons are relatively deep-seated structures but their probable relationship to surface calderas was recognized. Cauldrons and calderas were seen as two manifestations of the same igneous events exposed at different levels. Regarding Mull, never before had such a complex mapping project been achieved. The 'one-inch to the mile' (now published at 1:50,000) map of Mull revealed the details of the core of a major central volcano as had never been seen before and emphasized how incredibly numerous are small intrusions in this situation. The ring complexes in Mull were recognized to comprise broadly two kinds of intrusions having a ring-like plan view, namely vertical or steep outwardly dipping ring-dykes, and inwardly dipping cone-sheets, emplaced respectively by a relaxation of or an increase in pressure in the magma chamber (Anderson 1936; Richey 1932). It is now known that probably all major basaltic and central volcanoes have great intrusive complexes (the 'coherent intrusion complexes' of Walker 1992) in their core. Like the innumerable basaltic flows that form the superstructure of a major volcano, these complexes of small intrusions are generated by innumerable and frequent magma excursions out of the magma chamber. Studies of other intrusive centres in the British Tertiary Volcanic Province followed, as summarized by Richey (1932) and Emeleus (1982). Each centre showed distinctive features. Thus the structures of the Mourne Mountain Granites (Richey 1928) were such as could be accounted for by being emplaced as subterranean cauldron subsidences; the Goatfell granite in Arran (Tyrrell 1928; recently restudied by England 1992) is a superb example of a diapir, partly bounded by a ring fault inside which uplift occurred; and the intrusion-centre of Ardnamurchan (Richey & Thomas 1930) is remarkable for the large number of annular gabbroic intrusions. Meanwhile the finding of great thicknesses of lavas in East Greenland by Wager (1934) and by others in West Greenland, more or less doubled the size of the North Atlantic Tertiary Volvanic Province. This account concentrates attention on structural studies but acknowledges that the Classic Period also contributed greatly to petrological thought, as ably summarized by Thompson (1982a, b) and Wilson (1993 and this volume).

WALKER The Geikie-Judd controversy followed closely on the birth of microscopic petrography: Sorby's important paper on the microscopic structure of rocks was published in 1858 and Zirkel's 'Mikroscopische Gesteinsstudien' followed in 1863. Prior to about 1890, microscopic studies were mainly descriptive petrography, but from about 1890 onward attention was increasingly directed at chemical relationships. Harker (1909) and Judd were pioneers in this field. The mapping in Mull provoked significaant advances in petrology. The concept of magma type was introduced by Bailey et al. (1924). The question of the genetic relationships of Hebridean rocks was pursued by Bowen (1928), and a petrogenic scheme was proposed by Kennedy (1930, 1933) and by Kennedy & Anderson (1938) that is a clear antecedent of today's concepts of basalt magma types. Meanwhile Wager & Deer (1939) recognized that a remarkable story was told by the Skaergaard intrusion, and when they presented this story--the story has been retold since (e.g. Stewart & DePaolo 1990)--they provoked a worldwide interest in gabbroic intrusions that is still very much alive (see for example, the recent study of the Kap Gustav Holm intrusion by Bernstein et al. 1992). S u b s e q u e n t research in the British Tertiary P r o v i n c e

The past three decades have seen steadily continued research in the British Tertiary Province, mostly as a period of consolidation and quantitative documentation. The big revolution in volcanology during this period came with the study of pyroclastic rocks and explosive volcanism, and took place mostly on young volcanoes elsewhere. A recent topic has been the relation of the Hebridean volcanic centres to the Mesozoic basins of the region; Butler & Hutton (1994) have made a structural analysis of this for the Skye centre.

The North Atlantic Province Increasingly in recent years attention has tended to shift from the British Tertiary Province to the larger entity of the North Atlantic Province of which it is a small but important part. This change in emphasis has been associated with the development of ideas on hotspots and mantle plumes, and with geophysical exploration of the ocean floor. Geikie recognized that Tertiary dykes are distributed over roughly half of the area of the British Isles, and speculated that basaltic flows may originally have covered a similar area. He considered that the existing lavas as in Skye and Antrim were remnants of a previously much greater area that extended to the Faeroe Islands and Iceland. Tyrrell (1949) pursued this theme, pointing to the basalts in East and West Greenland and postulating that a vast basaltic plateau had extended from England to Baffin Land, a plateau that in East and West Greenland, Iceland and the Faeroes consists of enormously thick basalt piles. A prodigious volume of basalts, enough to stretch the most vivid imagination, was implied. Tyrrell realized however that Greenland and Scotland were contiguous before continental drift and hence that the area and volume of basalts, although great, was less than might appear. Tyrrell did not know that the Tertiary basalts in Iceland are significantly younger than those of Antrim, Skye, and Greenland; non-steady-state conditions were implied, early Tertiary volcanism being on a much bigger scale than recent volcanism in Iceland.

F L O O D BASALTS VERSUS C E N T R A L V O L C A N O E S About this time, studies made on zeolite zonation (Walker 1960a, b) showed that the lavas of Antrim and eastern Iceland thin up-dip and hence probably form remnants of more or less isolated lava lenses (Fig. lg-3), and with less certainty that the lavas of Mull are remnants of an eroded upstanding central volcano broadly similar to the lava shields of Hawaii (Walker 1970, Fig. lg-1). The up-dip thinning in eastern Iceland is particularly striking and was detected because the zeolite zones, inferred to be parallel with the top of the lava pile, demonstrably cut across the lava stratigraphy (Walker 1960b). Bodvarsson & Walker (1964) attributed the up-dip thinning and strong tilting to isostatic sagging, as new lavas were superposed on preceding lavas that were being conveyed away from the rift zone by spreading caused by dyke injections. Palmason (1980) successfully modelled the mechanism for this process on the basis of steady state volcanism. The development of the plate tectonics paradigm including the concept of hotspots and mantle plumes, and the dating of volcanic rocks in the North Atlantic area by radiogenic methods and magnetic stratigraphy, together with the geophysical exploration of the North Atlantic in recent years, have critically changed our views on the Province. More or less steady-state hotspot volcanism and spreading since about 16 Ma have created Iceland, and are in process of enlarging it. Over a more extended period of 60 Ma, they created the other volcanic accumulations of the North Atlantic Province. Among recent new ideas, that of thinspots (Thompson & Gibson 1991) explains well the isolation of the Hebridean Province from other volcanic areas in the North Atlantic region and concentration of the volcanism in sedimentary basins; that of incubating plumes (Kent et al. 1992) explains well the uprise of large silicic diapirs early in the volcanic history of Mull and other centres. Also the utilization by magma of structures such as tectonic pull-aparts (Hutton 1988; Butler & Hutton 1994) well explains the localization of the Mull and Arran centres so close to major faults. The finding of seaward-dipping reflectors under the North Atlantic, thought to be basalt accumulations, and the possible large-scale underplating of the crust by basaltic intrusions, are among the latest and most exciting developments (White & Mackenzie 1989; White 1992). The seaward-dipping reflectors are consistent with the seaward dip and down-dip thickening of lava piles in East and West Greenland, and also the down-dip thickening towards the spreading axis in Iceland. If interpretation of these deep structures is correct, a truly prodigious output of basaltic magma from the Province is implied, besides which the volume inferred by Tyrrell in 1937 and even more so by Geikie in 1897 fade almost into insignificance.

Central volcanoes: a m o d e r n view The controversy between Geikie and Judd concerned the identification of plateau (flood) basalts and central volcanoes in a setting of deep erosion. Here it is appropriate to consider the present state of understanding of these volcanic structures and the nature of the distinction between them. Walker (1993a) recognized five kinds of basaltic-volcano systems, namely lava-shield volcanoes, stratovolcanoes, central volcanoes, flood-basalt fields and monogenetic

197

volcano fields. Volcanoes of the first three kinds erupt more than once and are termed polygenetic. Individual volcanoes in the last two types are monogenetic and erupt only once. Central volcanoes differ from the others in having voluminous silicic as well as basaltic volcanic rocks. Active polygenetic volcanoes possess a high-level magma chamber, which is sustained by (a) a sufficient input rate of magma from the source and (b) a sufficient frequency of upcoming magma batches, to keep hot the magma pathway from the source to the chamber. The high-level chamber modulates magma excursions into the volcanic edifice and inter alia heats groundwaters and sustains high-temperature geothermal fields; fossil geothermal fields are recognized from the occurrence in them of such secondary minerals as epidote. The concentration of magma input to the magma chamber of a polygenetic volcano has several consequences: (a) magma excursions from the chamber are channelled into narrow rift zones where coherent dyke complexes may form, or alternatively into coherent intrusive-sheet (cone-sheet) complexes, and (b) a cumulate prisms grows under the chamber. Intrusion complexes and cumulate prisms are responsible for the large localized positive Bouguer gravity anomalies such as coincide with rift zones and summit calderas of volcanoes in Hawaii (Kinoshito 1965; Strange et al. 1965) and Reunion (Rancon et al. 1989), and the intrusive centres of Rum, Ardnamurchan and Skye (McQuillin & Tuson 1963; Bott & Tuson 1973). Shield volcanoes in Hawaii and the Galapagos have sometimes been regarded as close analogues of Mull and Skye. They form upstanding edifices having common sub-aerial slopes exceeding 4 °. The British Tertiary centres however occur on continental crust; they include great volumes of silicic intrusives, and of silicic volcanics now largely removed by erosion (Bell & Emeleus 1988). Shield volcanoes in Hawaii and the Galapagos have negligible amounts of silicic rock. Good analogues in rift or hotspot environments occur for example in Afar and the Red Sea/Gulf of Aden area, notably the volcanoes of Aden (Cox et al. 1970) and Jebel Khariz (Gass & Mallick 1968). Other examples such as the Tweed volcano (Stevens et al. 1989) occur in the eastern part of Australia. These examples are sufficiently young that the form of their volcanic edifices may still be discerned, and the Tweed volcano is sufficiently eroded to reveal the intrusive core. All are probably smaller and more alkaline than were the Mull and the Skye Cuillin volcanoes. Larger analogues occur in Tibesti and Manchuria. In Manchuria, a flood-basalt field (the Gaima Plateau) covering about l l 0 0 0 k m 2 occurs astride the China/North Korea border. Near the middle of the field and rising some 1500 m above the basaltic plateau is the central volcano of Changbaishan, which is an eruption centre for silicic lavas, ignimbrites and fall deposits (Machida et al. 1990) with a caldera 7 km wide. Flood-basalt fields: a m o d e r n view Most reviews on flood basalts (e.g. Cox 1980; Yoder 1988) concentrate attention on the geochemistry. Here attention is directed at other aspects. A flood-basalt field is an accumulation of overlapping or superposed tabular lava sheets that is erupted from scattered

198

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WALKER

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Fig. 1. Maps of the Antrim basalts. (a) Structural features. Arrows give dip, in part measured from attitude of prismatic cooling joints in the lavas. Sub-sea level parts are speculative, mostly extrapolated. (b) More detailed map of Islandmagee area where faulting is anomalously intense and throw direction is inconsistent, possibly because of dissolution of underlying Triassic salt. (e) Distribution of dykes and volcanic plugs (after Walker 1959). Plugs are elongate parallel with the dyke trend, and some consist of several discrete bodies on this trend; they probably evolved by local widening of dykes• (d) Generalized contours of the intensity of the dyke swarm, expressed as the percentage of dykes in the total rock. (e) Outlines of some volcanic plugs, drawn on the same scale. (f) Distribution and elevation of the top of the analcime/natrolite zone which embraces roughly the lowest 100 m of the basalts. Sea-level contour mostly extrapolated. The zone is down-bowed towards the subsidence axis although less strongly than the lavas. (g) Possible relationships of zeolite zones to basaltic piles shown in cross-section. Diagram 1 best fits the basalts of Mull. Diagram 3 best fits the Antrim basalts.

F L O O D BASALTS VERSUS C E N T R A L V O L C A N O E S vents and lacks any centralized vent system. The individual flows tend to have greater volumes (commonly >0.5 km 3) than are normally erupted from polygenetic volcanoes, and the whole accumulation has the aspect of a low plateau or plain. Where lavas infill and flow down valleys, they present the aspect of flooding the topography. Eruptions tend to be either from fissures or from point-source vents. It should be borne in mind that fissure-vents evolve with time into single-point vents (where volcanic plugs may develop) as wall-erosion locally widens the fissure (Bruce & Huppert 1990). In accounts of flood-basalt fields, attention is usually directed at the giant fields exceeding 100 000 km 2 in area and 100 000 km 3 in volume that are distributed sparsely through the geological record. Many small to moderate-sized flood-basalt fields also occur and are better analogues to the British Tertiary basalts. Good examples of moderate-sized fields are Rahat and K h a y b a r / I t h n a y n / K u r a in Saudi Arabia, both 20 000 km 2 (Camp & Roobol 1989; Camp et al. 1991) and the McBride and Nulla fields in Queensland, 5800 and 6600km 2 respectively (Stephenson et al. 1980). The volcanism in each field has been spread over the past 5 to 10 Ma and each field has the potential to erupt again. Some of the lava flows particularly in the Queensland fields are very large (Stephenson & Griffin 1976).

central volcano on the rift zone (Saemundsson 1986). Active central volcanoes include Hekla, Askja, and Krafla. Several scores of extinct central volcanoes are known amongst the Tertiary lava piles of eastern, northern and western Iceland, each more or less enclosed by flood basalts (Fig. 2).

Studies o f lava -flow structures Lava flows constitute the bulk of the British Tertiary and North Atlantic Provinces. In a logical scheme of things they would attract the greatest attention and might reasonably be expected to help distinguish flood basalts from central volcanoes. Strangely however most of the structures described by Geikie (he commented that 'a more detailed description of them seems to be required') still have not been explained. A notable exception from this neglect is the columnar-jointing shown by a few lava flows. The occasional spectacularly columnar jointed lavas as at the Giant's Causeway and Staffa attracted much attention from early geologists, some of whose explanations for the columnar rock would be classified today as fanciful. Tomkeieff (1940) recognized the multi-tiered character of columnar-jointed flows and initiated the modern terminology of colonnade and entablature for the tiers. The regularity of the prisms in the colonnade is now attributable to solidification of lava under static conditions, as when it infills a depression, and the closer joint spacing in, and common greater thickness of, the entablature are due to water cooling (Saemundsson 1970). Recent morphometric research (yet unpublished) on lava flows in Hawaii has an important bearing on the environment of basalt effusion. The thickness of basaltic lava is quite sensitive to the ground-slope angle. On slopes exceeding about 4° the average flow unit is about 1 m thick whereas on slopes under 2° it is 5 m or more thick. The lavas of Antrim and north-western Skye have thicknesses generally indicative of slopes of <2 ° (Table 1). Recent research shows that the internal structures of lava

Close associations o f flood basalts and central volcanoes Distinct types of volcanic systems, notably flood-basalt fields and central volcanoes, commonly occur in close association with one another, as in the example in Manchuria cited above. It is supposed that a central volcano may develop if and where magmatic activity in a flood-basalt field becomes sufficiently concentrated. Iceland illustrates the close association particularly well. A number of active rift zones from which occasional flood-basalt eruptions take place, occur in a zone crossing the country, and several have a

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200

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Table 1. Average thickness of lava flow units No of units

Rock type

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Total thickness (m)

Average thickness (m)

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7.05" ll.00t 9.80

* Walker (1959) t Preston (1982)

flows are also sensitive to ground-slope angle. In Hawaii, pipe vesicles and related vesicle cylinders and segregation veins are virtually absent from lava erupted on groundslopes of 4° or more (Walker 1987), and are most abundant in lava on slopes of <2 °. These features are extensively developed in the lavas of Antrim and north-western Skye, and also in Iceland, indicating that emplacement took place on prevalent very shallow ground-slopers and favouring the view that those lavas are flood basalts. Studies o f m a g m a - f l o w directions Harker's discovery that magma-flow directions in dykes can be inferred from structural features of the dykes has not yet been followed up, apart from the study by Macdonald et al. (1988) which showed lateral magma flow in the Cleveland dyke of N England. Meanwhile a powerful technique, based on the anisotropy of magnetic susceptibility of rocks, has become available that permits flow directions to be readily determined from the magnetic fabric. In dykes in the rift zones of the Koolau volcano in Hawaii, magma flowed upwards and sideways at an angle varying from 20 ° to 70 ° to the horizontal (Knight & Walker 1988). On a bigger scale, dykes of the continent-ranging Mackenzie swarm in Canada (Ernst & Baragar 1992) flowed vertically within 500 km of the centre of magmatism, and horizontally farther out. It is not known what magma-flow direction will be revealed by this magnetic method in the British Tertiary rocks. A study of flow directions in the Skye dyke swarm and cone-sheets of the Cuillin Hills has recently been initiated (Walker 1993b). The anisotropy of magnetic susceptibility technique also successfully yields the flow direction in lava flows (Canon & Walker in press) and might be very revealing when applied to a large basaltic field such as that of Antrim. Our study of the Cuillin Hills can significantly contribute to understanding major basaltic volcanoes. Volcanoes in Hawaii, built on oceanic crust, subside so much and so rapidly during their first two million years that erosion can never penetrate deeply into the volcanic core. Rafted as it is on continental crust however, the core of the Cuillins volcano is dramatically revealed for study. A n t r i m : the largest f l o o d - b a s a l t r e m n a n t in Britain The largest basalt remnant and the most clear example of a flood-basalt field in the British Tertiary Province is that of

Antrim. As a boy, the writer discovered the challenge and joys of fieldwork in Antrim. The Antrim basalts cover over 4300 km e and originally covered probably at least twice that area. The maximum thickness is not known: 769-780 m was penetrated in two drill holes near the centre of the basin. Studies of zeolite distribution contribute to knowledge of the lava structure. In the basaltic pile of eastern Iceland, the top of the analcime zeolite zone is at a depth of 600 m. In peripheral parts of Antrim, analcime characterizes a nearly-continuous zone along the base of the lavas that is about 100 m thick, and the original lava pile thickness was therefore 700 m thick. In the Langford Lodge drill-hole the zone containing analcime and its proxy, stilbite, is 460m thick, and the lava thickness was therefore originally about 1060 m. The basalts of Antrim lack any visible intense dyke concentrations. Instead dykes are scattered through an area that in south Antrim is 3 0 k m wide, and the maximum dyke-intensity is only about 5%. No localized large positive Bouguer anomaly is known that might be interpreted to be a coherent complex or cumulate prism. No epidote-bearing altered rocks of fossil high-temperature geothermal fields are seen at the present exposure level, and rhyolites are present only in minor amount. The Antrim lavas have dips commonly of 5° to 15°. If, as now seems probable, the lavas formed mostly on slopes <2 ° then the present dip must largely be due to tilting. In general the dip is centripetal and delineates a saucer-shaped structure elongated NW-SE. The axis of maximum subsidence lies some distance west of the volcanic axis along which intrusions are most concentrated and rhyolites occur.

This structure raises the question whether the dip was caused by a general isostatic sagging. The attitude of the zeolite zones shows that the lavas thicken down-dip towards the middle of the saucer, and lends credence to sagging caused by the load, but the apparent displacement of the subsidence axis from the volcanic axis (Fig. lc) would then be anomalous. There is an alternative, preferred explanation for the saucer shape, namely tilting due to a general late-volcanic or post-volcanic regime of crustal extension. Evidence in favour of this comes from the orthogonal systems of N W - S E and N E - S W faults that cut the basalts (Preston 1982; Parnell et al. 1989). These faults in general downthrow

F L O O D BASALTS VERSUS C E N T R A L V O L C A N O E S outward away from the centre of the basalt outcrop and the several faults in each system repeatedly step in the same direction. This and the considerable dip of the lavas suggest that the faults are of listric type. One consequence of this faulting is that the maximum known depth to the base of the lavas--780m in the Langford Lodge borehole--is significantly less than the 2500 m that would be calculated by extrapolating the dip of the surface lavas and assuming no faulting. There is an exceptionally high concentration of faults in a narrow zone of subsidence in Islandmagee, and the fault-throw is not consistent in direction (Fig. lb). It is in this area that the underlying Trias contains thick beds of halite, and the suggestion is made that the anomalous faulting here is related to subsidence due to localized dissolution of salt by ground-water circulation induced by magmatic activity. The zeolites that occur in this same area are also anomalous and include an abundance of the rather rare sodic species, gmelinite.

201

however go some way towards disentangling the relationships. The regional Hebridean linear dyke swarms span both the flood basalts and the central volcanoes and extend far outside both. No general model of dyke behaviour is yet available. Did the magma ascend vertically near volcanic centres, and horizontally farther away as in the Mackenzie swarm of Canada (Ernst & Baragar 1992) or, conversely, did the magma travel almost horizontally in and near volcanic centres and almost vertically farther away, as maintained in Iceland by Gudmundsson (1992)? The availability of the magnetic fabric technique will help resolve this problem. This is SOEST Contribution number 3819.

References

Conclusion

ANDERSON, E. M. 1936. Dynamics of formation of cone-sheets, ring-dykes, and cauldron subsidences. Proceedings of the Royal Society of Glasgow, 61, 128-157. BAILEY, E. B. & MAUFE, H. B. 1916. The geology of Ben Nevis and Glen Coe. Memoir of the Geological Survey of Scotland. , CLOUGH, C. T., WRIGHT, W. B., RICIIEY, J. E. & WILSON, G. V. 1924.

Geikie and Judd, the two principals in the historic controversy regarding the exact nature of volcanism in the British Tertiary Province, were probably both right although a full evaluation of the issues has not yet been made. The extensive basalts were no doubt erupted from fissures, as Geikie maintained, and the dykes that mark these fissures extend far from the volcanic centres. The great concentrations of dykes found in volcanic centres as in the Cuillin Hills and central Mull mark the rift zones of central volcanoes. Some of the basalts were no doubt erupted from these rift zones and built large volcanic edifices. Where flood-basalt lavas end and central-volcano lavas begin is not yet resolved. Geikie emerged from the fray better than Judd, and of the books they published (presumably spurred by the controversy), Geikie's 'Ancient Volcanoes' (1897) was a more enduring and scholarly work than Judd's rather inconsequential volume on Volcanoes (Judd 1881). More important, the controversy instigated mapping by exceptionally gifted geologists in Skye, Mull, Glencoe and other centres and this mapping (that, incidentally, did most to prove Judd's case) and the important discoveries in volcanology that resulted is perhaps the protagonists' greatest legacy to volcanology. Antrim and northern Skye are certainly flood basalts, and the Harrats east of the Red Sea/Dead Sea rifts are close modern analogues. Uncertainties exist for example on where the foci of eruption and deposition were, and how primary depositional dips are distinguished from volcanically induced tilts, or tilts related to postvolcanic extension. Some uncertainties are readily resolved by physical volcanology. Thus, depositional groundslope angles can be estimated to the nearest 5° or better from lava structures, and magnetic fabric (magnetic anisotropy susceptibility) study yields lava-flow direction. The Hebridean magmatic activity also generated central volcanoes as in Mull. Here a complex pattern of updoming by silicic diapirs, subsidence caused by volcanic and intrusive loading, and the deformation needed to accommodate great swarms of dykes and conesheets, may prove less tractable to analyse. The lava-flow structures, magnetic fabric study of flow directions, and palaeomagnetic study of tilting, may

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Queensland. In: JOHNSON, R. W. (ed.) Volcanism in Australasia. Elsevier, Amsterdam, 41-52. , GRIFFIN, T. J. & SUTHERLAND, F. L. 1980. Cainozoic volcanism in Northeastern Australia. In: HENDERSON,R. A. & STEPHENSON,P. J. (eds) The geology and geophysics of northeastern Australia. Geological Society of Australia, 349-374. STEVENS, N. C., KNUTSON, J., EWART, A. & DUGGAN, M. B. 1989. Tweed Volcano. In: JOHNSON, R. W., KNUTSON, J. & TAYLOR, S. R. (eds) lntraplate volcanism in eastern Australia and New Zealand. Cambridge University Press, 114-115. STEWART, B. W. & DEPAOLO, D. J. 1990. Isotopic studies of processes in mafic magma chambers. II. The Skaergaard Intrusion, East Greenland. Contributions to Mineralogy and Petrology, 104, 125-141. STRANGE, W. E., MACHESKY,L. F. & WOOLLARD, G P. 1965. A gravity survey of the island of Oahu, Hawaii. Pacific Science, 19, 354-358. THOMPSON, R. N. 1982a. Magmatism of the British Tertiary volcanic province. Scottish Journal of Geology', 18, 49-107. 1982b. Geochemistry and magma genesis. In: SUTHERLAND,D. S. (ed.) Igneous rocks of the British Isles. Wiley and Sons, Chichester, 461-477. & GIBSON, S. A. 1991. Subcontinental mantle plumes, hotspots, and pre-existing thinspots. Journal of the Geological Society, London, 148, 973-977. TOMKE[EFF, S. I. 1940. The basaltic lavas of the Giant's Causeway district of Northern Ireland. Bulletin Volcanologique, series 2, 6, 89-144. TYRRELL, G. W. 1928. 7~e geology of Arran. Memoir of the Geological Survey of Scotland. 1937. Flood basalts and fissure eruption. Bulletin Volcanologique, Series 2, 1, 89-111. 1949. The Tertiary igneous geology of Scotland in relation to Iceland and Greenland. Meddelelser fra Dansk Geologisk Forening, 11,413-440. WAGER, L. R. 1934. Geological investigations in East Greenland. Part 1. General geology from Angmagssalik to Kap Dalton. Meddelelser om Grcnland, 105. -& DEER, W. A. 1939. Geological investigations in East Greenland. Part -

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3. The petrology of the Skaergaard intrusion, Kangerdlugssuaq region. Meddelelser om Gr~nland, 105. WALKER, G. P. L. 1959. Some observations on the Antrim basalts and associated dolerite intrusions. Proceedings of the Geologists' Association, 70, 179-205. 1960a. The amygdale minerals in the Tertiary lavas of Ireland. III. Regional distribution. Mineralogical Magazine, 32, 503-527. 1960b. Zeolite zones and dyke distribution in relation to the structure of the basalts in eastern Iceland. Journal of Geology, 68, 515-528. 1970. The distribution of amygdale minerals in Mull and Morvern (Western Scotland). West Commemoration Volume, University of Saugar, India, 181-194. 1987. Pipe vesicles in Hawaiian basaltic lavas: their origin and potential as paleoslope indicators. Geology, 15, 84-87. 1992. 'Coherent intrusion complexes' in large basaltic volcanoes. Journal of Volcanology and Geothermal Research, 50, 41-54. - - - - 1993a. Basaltic-volcano systems. In: PRITCI-IARD,H. M., ALABASTER,Y., HARRIS, N. B. W. & NEARY, C. R. (eds) Magmatic processes and plate tectonics. Geological Society, London, Special Publications, 76, 3-38. 1993b. Re-evaluation of inclined intrusive sheets and dykes in the Cuillens volcano. Isle of Skye. In: PRITCHtARD, H. M., ALABASTER, T., HARRIS, N. B. W. & NEARY, C. R. (eds) Magmatic processes and plate tectonics. Geological Society, London, Special Publications, 76, 489-497. WHDTE, R. 1992. Crustal structure and magmatism of North Atlantic Continental margins. Journal of the Geological Society, London, 149, 841-854. & MCKENZIE, D. 1989. Magmatism at rift zones: the generation of volcanic continental margins and flood basalts. Journal of Geophysical Research, 94, 7685-7729. WILSON, M. 1993. Magmatic differentiation. Journal of the Geological Society, London, 150, 611-624. YODER, H. S. 1988. The great basaltic 'floods'. South African Journal of Geology, 91, 139-156. ZIRKEL, F. 1863. Mikroskopische Gesteinsstudien. Sitzungbericht Akademie der Wissenschafien Wein, Mathematisch-naturwissenschafien Klasse, 47, 226-270. -

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From QJGS,27, 279. 2. On the T~RrXARYVo~.cA~c RocKs of the Bm~rls~ IS~DS. By ±RC~A~D GEixr~., Esq., F.R.S., F.G.S., Director of the Geological Survey of Scotland, and Professor of Geology in the University of E d i n b u r g h . - - F i r s t Paper. [PL~T~ XlT.] IN the present communication I propose to offer to the Society the first of a series of papers descriptive of those latest of the British volcanic rocks which intersect and overlie our Palaeozoic and Secondary formations, and which, from fossil evidence, are to be regarded as of miocene, or at least of older Tertiary, date. Materials for this purpose have been accumulating with me for some years past. I n bringing forward this first instalment of them, I wish to preface t h e subject with some general introductory remarks regarding the place which the rocks seem to me to hold in British geology, and on the nomenclature which I shall use in describing them. These remarks will be followed by a detailed description of the first of a succession ~f districts where the characteristic features of the rocks are well displayed. Other typical districts will be described in future memoirs. GENERAL I~TROI~UCTIO~.

1. Area occ~t2ied by the _Pocks. The rocks to which I propose to direct attention cover m a n y hundreds of square miles in the British Islands. They spread over the north-east of Antrim, from Belfast to Loch Foyle, forming there a great plateau or series of plateaux, with an area of fully 1200 square miles and an average thickness of 550 feet. F r o m Ireland the same rocks are prolonged northwards through the I n n e r Hebrides. They form nearly the whole of the islands of Mull, Rum, :Eigg, Canna, and Muck. They cover fully three-fourths of Skye, and extend even as far as the Shiant Isles. But far beyond our own area they reappear with all their characteristic features in the Farce Islands, and again in the older volcanic tracts of Iceland. I n

From QJGS,30, 220-221. 23. The S~'.CONDARY ROCKS of SCOTLAND. Second Paper °. On the ANCIENT VOLCANOES of the HIGHLANDS and the RELATIONSof their Pl~oDvcrs to the MEsozoic STI~A~A. By JOHN W. JUDD, Esq., F.G.S. (Read January 21, 1874.) [PLATES XXII. & XXIII.] CONTENTS. I. Introduction. 1. History of Previous Opinion on the subject. 2. Volcanic Origin of the rocks constituting the great plateaux of the Hebrides and the North of Ireland. 3. Subaerial Origin of these old Volcanic rocks. 4. F vidcnces of the Former Elistence of great Volcanic mountains in the district.

15. Connexion between the Tertiary Volcanoes of the Hebrides and those of other districts. 16. General conchzsions from the relations of the Volcanic and Plutonic rocks of the Tertiary period.

III. The Newer-Pal~eozoicVolcanoes. II. The Tertiary Volcanoes. 1. Lavas of torn and the adjacent islands. 1. Classification of the Tertiary Volcanic rocks. 2. Characters of the Volcanic rocks of Lorn. 2. Nature and origin of the great Volcanic rock-mas~s :--Lavas, Intru3. Relations'of the Volcanic rocks of torn. sive masses, Volcanic agglomerates and Volcanic breccias. 4. Succession of rocks in Lorn. 3. Relations of the Volcanic rocks to one another and to the o',der 5. Conditions under which the Volcanic series of torn was deposited. deposits in the island of Mull. 6. Age of the Volcanic series of Lorn. 4. Sections illustrating the structure of the island of Mull;--Beinn 7. The Newer-Pahrozoic ]avas of the Lowlands of Scotland. Greig, Beinn Uaig, Craig Craggen, Beinn More. 8. The Eruptive masse~ of the Grampian nlotzn(aln.~. 5. Proofs that the central mountain-group of Mull constitutes the relic 9. :Relations of the igncolls rocks of Beinn Naris and Glencoe. of a great volcano. 10. Physical Features of Northern Scotland during the Newer-Palaeozoic 6. The Volcano of Ardnamurchan. periods. 7. The Volcano of Rum. 8. The Volcano of Skye. IV. Conclusion. 9. The Volcano of St. Kilda. 1. Comparison of the two great periods of Volcanic activity in Scothmd. 10. Comparison of the great Tertiary Volcanoes. 2. Influence of Volcanic action in determining the Characters and Rela11. Dimensions of the great Tertiary Volcanoes. tions of the Secondary rocks of Scotland. 12. Series of later Volcanic eruptions in the Hebrides, resulting in the 3. The " Geological Record " in the Highlands. formation of "Puys." 4. Light thrown upon some problems of Physical Geology by the 13. Subterranean Phenomena of the Tertiary Volcanoes. Volcanic rocks of the Highlands. 14. Ages of the several Volcanic outbursts already described.

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 205-218 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 611-624

Magmatic differentiation MARJORIE

WILSON

Department o f Earth Sciences, Leeds University, Leeds L S 2 9JT, UK Abstract: During the past 150 years, a wide range of processes have been invoked to explain the mechanism by which magmas differentiate.These may be divided into those which operate essentially in the liquid state, such as liquid immiscibility and thermogravitational diffusion, and those which involve some form of crystal-liquid fractionation. It is now generally accepted that the latter are the most important. Many of the models developed during the past twenty years to explain magmatic differentiation have their roots in ideas first proposed in the early years of this century. This review presents some of the historical background to the subject and attempts to summarize some of the more recent developments. Alfred Harker's perceptive study of in-situ crystallization within a high-level intrusion (Carrock Fell in the English Lake District), published in volume 50 of the Quarterly Journal of the Geological Society in 1894, clearly laid the foundations for many modern theories. In 1900 he introduced the oxide-oxide variation diagram, which is still widely used to depict the geochemical variations within suites of cogenetic igneous rocks. Studies of phase equilibria in synthetic and natural systems by Bowen and his contemporaries, during the 1920s and 30s, provided the theoretical background needed to understand the complex processes involved in fractional crystallization of magmas. In the past decade mathematical modelling has allowed a more quantitative approach to the problem. In 1909, in his seminal publication The Natural History of Igneous Rocks, Harker commented that 'the most fundamental problem in modern petrology (is) that of the origin of the great diversity of rock types actually found'. Eighty one years later Nielsen (1990) was to note that 'one of the primary goals of igneous petrology is the definition and evaluation of the roles of the processes responsible for chemical differentiation'. In the past century an immense number of publications have appeared on the subject of magmatic differentiation, a brief, but by no means comprehensive review of which will be presented here. Have we actually made any real progress? Of course the answer to this must be 'yes'. Have we built on the work of our predecessors ? To this the answer must be 'sometimes'. In the past decade there has been an ever-increasing tendency to dismiss anything published in the preceding decade as 'out of date', let alone anything produced in the early part of this century. Clearly the present generation of petrologists cannot read everything previously published on a particular topic. However by dismissing older publications there is a grave danger that inadvertently we may 're-invent the wheel'. Reading the classic works of Harker (1909) and Bowen (1928) does not leave one feeling trapped in a time warp; rather with an impression that these were men light years ahead of their time. What kind of progress would they have made had they had access to the vast geochemical databases presently available? This of course is entirely philosophical. However, it should serve to remind us all to conduct thorough literature searches as a precursor to new research projects. A wide range of processes have, over the past 150 years, been advocated as potential causes of magmatic differentiation (Fig. 1). As we shall see in subsequent sections some (e.g. thermogravitational diffusion, liquid immiscibility, assimilation) have gone in and out of fashion,

while others (e.g. fractional crystallization) appear to have stood the test of time. Other processes, such as gaseous transfer, are probably only significant during the latest stages of magmatic differentiation and will not be considered further here.

Early ideas about the causes of magmatic differentiation By the turn of the century, it was already well established (Harker 1909) that the great diversity of igneous rocks and the compositional variation within many individual rock bodies was mainly attributable to processes of differentiation. This had been a common basis for discussion for more than 60 years, since Darwin's classic 1844 publication Geological observations on the volcanic islands... Harker's ideas about magmatic differentiation were profoundly influenced by his studies of the Carrock Fell gabbroic intrusion in the English Lake District, reported in 1894 in volume 50 of the Quarterly Journal of the Geological Society. He was clearly aware of two different types of differentiation: the in-situ differentiation of a single magmatic body; and the differentiation, prior to intrusion or extrusion, responsible for the production of cogenetic suites of intrusives or extrusives. In the latter case H a r k e r (1909) reasoned that, as we only see the finished product, we have to speculate about the processes involved. However, in the in-situ case both the stages of the variation and to some extent the nature of the processes themselves may be studied directly. The significance of this was to gain fundamental importance, more than thirty years later, in studies of the great layered mafic intrusions (e.g. Wager & Deer 1939). Harker, however, was uncertain as to how conclusions based on a study of in-situ differentiation could be applied to the more obscure question of the 205

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M. WILSON

thermogravitational diffusion

liquid %

liquid immiscibility

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immiscibility was widely discussed as a potential liquid state differentiation mechanism at this time (e.g. Harker 1909; Bowen 1928) but was not considered to be particularly important. Magma mixing was also viewed as a significant process in contributing to the chemical diversity of magmatic systems, although, as noted by Harker (1909) it is actually a 'reverse differentiation mechanism' in that two different magma compositions become mixed to form one.

Crystal-liquid fractionation Whilst agreeing that liquid state differentiation mechanisms could generate localized compositional variations within magma bodies, both Harker (1894, 1909) and Bowen (1928) strongly favoured processes of crystal-liquid fractionation as the main cause of magmatic differentiation. Bowen noted that 'differentiation in a crystallising mass may be brought

about in two ways: through the localisation of the crystallisation of a certain phase or phases and through the relative movement of crystals and liquid'. Eighty five years later we are still debating about the precise mechanism by which this occurs! Harker (1894, 1909) explained his ideas about the mechanism of fractional crystallization in the context of progressive crystallization in a d y k e . He envisaged that as a result of crystallization at the margins of the intrusion the remaining magma would become progressively depleted in those components incorporated into the solid phase, as long as diffusion could keep pace with crystallization. Clearly this model depends very heavily on the ability of components to diffuse through a silicate melt, which we now know is actually a very slow process (e.g. Hofmann 1980). Indeed as early as 1928, Bowen argued that the rates of diffusion are too slow to have a n y significant effect in producing compositional gradients in silicate melts. Harker, however, believed that diffusion would be important in the early stages of crystallization but would decrease with falling temperature as the residual magma became more viscous. He (Harker 1909) placed great importance on crystallization in the marginal zones (walls, roof) of magma chambers as a major mechanism for differentiation. Becker (1897) first introduced the term fractional crystallization to describe this mechanism of igneous differentiation, but, unlike Harker, dismissed diffusion in favour of transportation of components in the liquid phase by convection currents. Seventy five years later Sparks et al. (1984) were to reach a similar conclusion, though based on rather different reasoning, using the term convective fractionation. The gravitational settling of minerals in a fluid magma had already been proposed by Darwin (1844) as a principal cause for magmatic differentiation, based upon his observations of phenocryst accumulations in the bases of lava flows. The viability of the process was to remain a matter of debate for the next 150 years! Harker (1909) realized that the viscosity of a magma might be sufficient to overcome the natural tendency of crystals to sink by virtue of their greater density and as a consequence did not regard crystal settling as a particularly important mechanism of differentiation. However, he was well aware of the time element which meant that while crystal settling to the base of a small sill might not be very important, it could be in a large crustal magma chamber cooling very slowly. Harker had studied the layered mafic-ultramafic igneous complex of the island of Rhum in the Tertiary volcanic

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differentiation of magmas in an unknown deep-seated magma reservoir. It may be argued that this is still as true today as it was in 1909! In the early part of this century ideas about magmatic differentiation were divided between processes of differentiation in the liquid state and those of crystal-liquid differentiation or fractional crystallization (Fig. 1).

Differentiation in the liquid state As part of his studies of the Carrock Fell intrusion, Harker (1894) discussed the possibility that gravitational stratification of denser components might occur in magma bodies, but concluded that such a process was incapable of explaining the symmetrical nature of the compositional zonation which he observed in the gabbro. He also considered (and dismissed) the possibility for differentiation in the liquid state due to temperature differences between the central and marginal parts of the intrusion (the Soret effect). By 1909, he had extended his ideas to much larger scale phenomena, envisaging the possibility of large crustal magma chambers, in which denser (more mafic) magma underlay less dense (more acidic) magma, from which simultaneous eruptions of constrasting magma compositions might emanate. Models involving such stratified magma bodies were to come back in vogue more than seventy years later (e.g. Huppert & Sparks 1980). Silicate liquid

MAGMATIC D I F F E R E N T I A T I O N province of NW Scotland (Harker 1908, 1909) but did not recognize the cumulate origin of these rocks and thought that the banding was due to repeated intrusion combined with deformation. Had he been aware of the spectacular evidence for crystal accumulation, which would be provided by the layered rocks of the Skaergaard intrusion, Greenland (not to be discovered by Wager and co-workers until thirty years later), might he have concluded otherwise? It is interesting to note in this context how our ideas can sometimes come full circle. Eighty years after Harker's original study, Bedard et al. (1988) concluded that his ideas about the origin of layering in the Eastern Layered Series of Rhum were broadly correct, overturning the models of the previous 25 years, which proposed crystal accumulation on the floor of a periodically refluxed magma chamber (e.g. Wager & Brown 1968). By the early 1930s (Daly 1933) ideas that layering in some plutonic bodies was the product of gravitational crystal settling were gaining widespread acceptance and these were reinforced by Wager & Deer's classic description o f the Skaergaard intrusion in 1939. However it was not until after the Second World War that the implications of these layered mafic intrusions for models of magmatic differentiation really became apparent (e.g. Wager & Brown 1968). Gravitational crystal settling models were to dominate most discussions of magmatic differentiation during the 1960s and 1970s. However, since McBirney & Noyes (1979) reevaluation of the evidence for crystal settling in the Skaergaard intrusion, more recent models have favoured in-situ crystallization. This is yet another illustration of the ways in which our ideas have come full circle. Like many of his fellow scientists, Harker (1909) considered that assimilation of crustal rocks could be an important process in the compositional diversification of magmas, particularly in deep crustal magma reservoirs where extensive melting of wall rocks might occur. However 'the enormous amount of heat needed to raise the solid (wall) rocks to the point of melting and to melt them' and the lack of evidence for superheated magmas to provide the necessary heat source concerned him. Bowen (1928) was also impressed with the amount of superheat necessary to assimilate significant quantities of crustal rocks and, whilst accepting that limited assimilation undoubtedly did contribute to the compositional variability of magmatic rocks, doubted 'whether the presence of foreign matter is ever essential to the production of any particular type of differentiate'. In the 1980s and 90s, models of wall rock assimilation have come back into favour with the recognition of assimilation coupled with fractional crystallization (AFC) as an important process in the petrogenesis of many continental magmas. Bowen (1928), building on the earlier ideas of Harker, stressed the importance of developing models for magmatic differentiation that were consistent with the fundamental principles of physical chemistry. He showed how a knowledge of phase equilibria in synthetic silicate systems, when used in conjunction with detailed field observations and mineralogical studies of igneous rocks, could be helpful in interpreting their petrogenesis. In addition, building on the work of Fenner (1926), he developed the use of the oxide-oxide variation diagram, first introduced by Harker (1900, 1909), as a graphical means of interpreting the chemical relationships within cogenetic suites of rocks. Bowen also introduced the idea of a Reaction Principle to

207

explain the mineralogical changes which occur during the progressive crystallization of mafic magma. Fifty years after the publication of Bowen's classic book The Evolution of Igneous Rocks (1928), Osborn (1979) reviewed the Reaction Principle and concluded that the basic idea was still valid. Whilst the war years (1939-45) undoubtedly slowed the pace of research, by the time Tilley gave his presidential address to the Geological Society of London in 1950 on 'Some aspects of magmatic evolution', it was clear how our understanding of magmatic differentiation processes had developed since the 1930s. Major steps in establishing the concept of magmatic differentiation series related to different parental basalt compositions (alkali and tholeiitic) had been made by Bailey et al. (1924) in their classic study of the Tertiary and post-Teriary geology of the Isle of Mull, Scotland. These ideas were developed further by Kennedy (1933). Tilley (1950) believed that the alkaline magma series could be derived from a tholeiitic basalt parent by fractional crystallization. However, Yoder & Tilley (1962) were later to show that a low pressure thermal divide precludes this and a fractionation relationship between these two primary magma compositions is not generally considered today. Tilley clearly demonstrated the different evolutionary trends of the tholeiitic and alkaline magma series on a total alkalis v. silica Harker variation diagram, still widely used today to classify volcanic rocks (Le Baset al. 1986). He discussed the variable nature of the iron-enrichment trends which could be generated by differentiation of tholeiitic basaltic magma in contrast with the trend of iron-depletion displayed by the calc-alkaline series. As we shall see in subsequent sections, this remains a subject for debate. Additionally he was concerned about the role of crustal contamination in the petrogenesis of the calc-akaline association of orogenic belts; again a subject at the forefront of discussion in the 1980s and 90s.

Mechanisms for magmatic differentiation: a review of current ideas M a g m a - m i x i n g processes in open system m a g m a chambers Magma mixing is clearly an important differentiation process contributing to the overall geochemical and petrological diversity within and between magmatic suites (Philpotts 1990). First proposed by Bunsen (1851), it was considered to be an important petrogenetic process by many workers during the later part of the ninteenth century and the beginning of this (e.g. Harker 1909; Bowen 1928). One of the more extreme theories was that all magmas were mixtures of basalt and rhyolite (e.g. Fenner 1938). A wide range of phenomena have been recognized ranging from incompletely mixed magmas (Furman & Spera 1985) to hybrids in which mixing appears complete (e.g. Sparks & Marshall 1986; Oldenburg et al. 1989). Clearly, in nature, magma mixing must be common but we rarely see its effects. For example, repeated injections of new batches of primitive magma into long-lived, open magma chamber systems undergoing crystal fractionation undoubtedly controls the variety of eruptive products we observe at many volcanic centres.This was recognized at the turn of the century by Harker (1909). Osborn (1959) showed that oxidation state plays a fundamental role in the course of

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M. WILSON

magmatic differentiation and that this may be directly related to whether the chamber is open or closed to new magma batches. In theory, a series of magmas generated by mixing of two parental liquids should define straight lines on oxide-oxide (i.e. Harker) variation diagrams, so long as they were not concurrently fractionating. In contrast, suites of rocks related by crystal fractionation should, in general, define curved trends on Harker variation diagrams. Complex mineralogical relationships can occur if two partially crystallized magmas are mixed and equilibrium is not completely re-established in the mixed magma. The chemical diversity of the 1959 eruptions of Kilauea Iki, Hawaii, have been cited by many workers as a classic example of magma mixing (e.g. Murata & Richter 1966; Wright 1973; Helz 1987). Recently, however, Russell & Stanley (1990) have suggested that crystal sorting is sufficient to explain the range of chemical compositions observed. Eichelberger (1975) proposed that many andesites and dacites in the western USA are the products of mixing of primary basaltic and rhyolitic magmas; reviving an idea more than 100 years old. However McBirney (1980) critically evaluated the concept and claimed that most of the petrographic and chemical features proposed as evidence for magma mixing (e.g. disequilibrium) could be explained by the eruption and mixing of magmas from compositionally zoned magma chambers. The idea that most magma chambers are episodically replenished by new pulses of magma is widely accepted and based on a range of evidence from both volcanic and plutonic rocks. A number of workers have shown that a variety of phenomena may occur during replenishment, depending upon the relationships between the densities and viscosities of the incoming and chamber magmas and the input rate of new magma (e.g. Huppert & Sparks 1980; Campbell & Turner 1986; Huppert et al. 1983, 1986). O'Hara (1977) and O'Hara & Mathews (1981) developed mathematical models to predict the course of magmatic differentiation in magma chambers which are periodically replenished with new batches of primitive magma, periodically tapped and continuously fractionated. Over the past decade a range of more complex models have been devised (e.g. Nielsen 1988, 1990). These will be discussed further in a subsequent section. Russell (1990) presents a comprehensive review of the thermodynamics of magma mixing, using a forward modelling approach to calculate synthetic data sets for mixed magma systems. The model data can then be compared against data for natural systems as an aid in interpreting their petrogenesis. Nielsen (1990) has shown, on the basis of model calculations that, in a magmatic system undergoing paired recharge and fractionation, the liquid line of descent (LLD) for major elements is similar to that produced by fractional crystallization. In recharged systems the magma chamber fractionates only the liquidus phases, and the liquid is constrained to evolve a l o n g the cotectics in some n-dimensional phase diagram. As an example, in a simple ternary system (Fig. 2), if the magma in a chamber is crystallizing ol + cpx + plag, adding a pulse of more primitive magma will push it back into the olivine phase field from which it will evolve back towards the ol-cpx cotectic. In such a situation the long term amount of ol + cpx removed from the system will be greater than that in a closed system.

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Fractional crystallization Gravitational crystal settling and differentiation in layered mafic intrusions. Much of our understanding of the differentiation of basic magmas is based on the record of crystallization preserved in layered mafic intrusions (e.g. Skaergaard, East Greenland; Stillwater, Montana; Bushveld, S Africa). Our knowledge of the petrogenesis of these bodies is grounded in the classical works of Wager & Brown (1968), Jackson (1961) and Hess (1960). However, in recent years, it has become clear that many of the petrographic, chemical and textural features of layered intrusions cannot be modelled adequately using the classic assumptions of gravitational crystal settling on to the floor of a magma chamber (McBirney & Noyes 1979; Shirley 1987). Many of the controversies have revolved around the site of crystallization. Throughout the 1960s and 1970s models were dominated by ideas of cumulus crystallization within the main body of magma and sedimentation by convection currents (e.g. Wager & Brown 1968). However McBirney & Noyes (1979) proposed that the layering is actually produced by in-situ crystallization on the floor (and walls) of the chamber; a complex process involving both chemical and thermal diffusion, nucleation and crystal growth. One of the oldest controversies in igneous petrology (Bowen 1928; Morse 1980) concerns the path of differentiation in tholeiitic magmas. It is generally accepted that a tholeiitic magma crystallizing at constant bulk composition (i.e. in a closed system) will generate an extreme iron-enrichment trend on an AFM diagram (Fig. 3) depicting the compositions of its derived liquids (Osborn

MAGMATIC DIFFERENTIATION

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Fig. 3. A (Na20 + K20)-F (FeO + Fe203)-M(MgO ) diagram to show the contrasting Bowen and Fenner trends of magmatic differentiation, of alkali and of iron enrichment respectively. 1959, 1962; Yoder & Tilley 1962). Such a trend is known as the Fenner trend and is characteristic of large, apparently closed, intrusive systems such as Skaergaard. In contrast, if the magmatic system is open to oxygen, in particular if f 02 is maintained at a constant value during crystallization (Ghiorso & Carmichael 1985), such that the system becomes progressively more oxidized with falling temperature, fractional crystallization ot a tholeiitic liquid is presumed to generate a residuum rich in silica and low in Fe (the Bowen trend), due to the precipitation of large quantities of spinel. The classic study of Osborn (1959) demonstrated that the difference between the Bowen and Fenner trends appears to be controlled by the timing of the onset of magnetite fractionation, which is, in turn, controlled by the oxidation state of the system. Simplistically we can think of the oxidation state of a magma in terms of the oxidation-reduction equilibrium: 4FeO + 02 = 2Fe203. From the thermodynamic equilibrium relation for this reaction we can deduce that the fugacity of oxygen ( f O2), which is a measure of the degree of oxidation of the system at a given temperature, should be proportional to the amount of Fe203. During closed system crystallization the early extraction of olivine and pyroxenes, which contain no appreciable FeaO3, should increase the concentration of FeaO3 in the residual liquid relative to FeO, leading to an increase in f 02 (Juster et al. 1989). However, once magnetite begins to crystallize the Fe203/Fetotal ratio should decline in the more evolved liquids. The magnitude of the effect will clearly depend upon the composition of the magnetite as well as that of the other iron-bearing phases in the crystallizing mineral assemblage. If olivine is in a reaction relationship with the magma to form pigeonite, FeO will be added to the liquid as the olivine dissolves which will counterbalance the tendency for f 02 to increase as olivine + cpx + plagioclase crystallize (Juster et al. 1989).

209

In certain circumstances the oxygen fugacity of a magma may be buffered by the surrounding country rocks (e.g. Ghiorso & Carmichael 1985). This probably involves hydrogen diffusion; the fugacity of hydrogen being linked to that of oxygen through the breakdown of water. Oxygen fugacity may also be internally buffered by magmatic oxidation-reduction equilibria, for example $2-SO2 and CO-CO2. Ghiorso (1985) calculates that for crystallization along an oxygen buffer (e.g. quartz-fayalite-magnetite (QFM) or nickel-nickel oxide (NNO)) the ratio of ferric to total iron in the residual liquid should remain approximately constant. Ghiorso & Carmichael (1985) suggest that, to generate the typical Fenner differentiation trend, a tholeiitic basalt must crystallize essentially along an oxygen buffer. In contrast the Bowen trend is generated by continuous oxidation of the melt (maintenance of constant f 02 as temperature decreases) which results in early magnetite saturation in the evolving liquid. The Skaergaard intrusion, East Greenland, has, for the past 50 years been cited as the classic example of in-situ differentiation of basic magma. This was first described by Wager & Deer in 1939 (after its discovery in 1930) and subsequently by Wager & Brown in 1968 in their classic book on Layered Igneous Rocks. Useful reviews of the structure and average composition of the Skaergaard Layered Series are given by McBirney & Noyes (1979) and McBirney (1989). Wager & Deer (1939) proposed that convection currents of variable velocity played a significant role in the cooling and differentiation of the intrusion, reintroducing the idea (proposed by Darwin 1844 and by Bowen 1928) that magmatic differentiation could be induced by crystal settling. The nature of convection in high-level magma chambers has occupied the minds of workers in this field ever since and our ideas have been strongly influenced by studies of the Skaergaard. However in more recent years several authors (e.g. McBirney & Noyes 1979; Naslund 1984) have come to question the importance of crystal settling, and have proposed new models to account for the development of layering, so spectacularly developed in the Skaergaard intrusion. Wager (1960) proposed that the differentiation of the tholeiitic Skaergaard parent magma was characterized by a trend of iron enrichment throughout, with very little silica enrichment until very late stages (Fig.4). This is in marked contrast to the normal trend of differentiation observed in tholeiitic volcanic suites. However, Hunter & Sparks (1987) triggered a controversial series of short papers (McBirney & Naslund 1990; Morse 1990; Brooks & Nielsen 1990; Brooks et al. 1991) by proposing that earlier calculations of the Skaergaard differentiation trend (e.g. Wager & Brown 1968) were incorrect, and that the magmatic differentiation sequence actually follows the more normal eruptive trend of early iron-enrichment (Fig. 4), with magmas evolving from tholeiitic basalt to ferrobasalt in the Lower Zone (LZ), from a ferrobasalt to an iron-rich tholeiitic andesite (icelandite) from the Middle Zone (MZ) to the Upper Zone (UZ) and from icelandite to an iron-rich rhyolite at the Sandwich Horizon. Hunter & Sparks argued against the Fenner-type of differentiation for Skaergaard partly on the grounds that iron-rich magmas are rare as eruptive compositions. However Brooks et al. (1991) pointed out that iron-rich silica-poor dykes and plateau lavas do in fact occur in the area of the Skaergaard and cited additional examples from

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Fig. 4. (a) : F e 2 0 3 (total Fe) v e r s u s SiO2 wt % variation diagram for the Skaergaard intrusion, Greenland, showing the liquid lines of descent for the chamber magma predicted by Wager & Brown (1968) and Hunter & Sparks (1987). Also shown are the average compositions of the cumulates from each of the major zones of the Layered Series from McBirney (1989). Abbreviations : HZ, Hidden Zone; LZ, Lower Zone; MZ, Middle Zone; UZ, Upper Zone; SH, Sandwich Horizon. (h) Fe20 3 (total Fe) versus SiO2 wt % variation diagram showing the liquid line of descent (LLD) of the Thingmuli tholeiitic volcanic series, Iceland ( Carmichael 1964) and an experimentally determined LLD for a tholeiitic basalt from the Galapagos Ouster et al. 1989). Both evolved in a relatively oxidizing environment, close to the QFM oxygen buffer. Also shown for comparison is the Skaergaard LLD from (a). oceanic environments (e.g. Melson & O'Hearn 1986; Sinton et al. 1983). Brooks et al. (1991) proposed that the Skaergaard iron-rich liquids evolved from a MORB-like tholeiitic parent magma by simple crystal fractionation in a closed or nearly closed system. They concluded that the iron-rich Fenner differentiation trend may actually be the normal liquid line of descent of tholeiitic magmas, and that the Bowen trend of silica enrichment arises only when magmas come under the

influence of oxidizing conditions in near surface (probably open) magma chambers. This has been confirmed by experimental studies on material from the Galapagos area by Juster et al. (1989). Thus the requirement of closed system differentiation may be essential to develop the Fenner trend. Indeed Morse (1990) suggests that plutonic processes will tend to enhance the Fenner trend of iron enrichment, and thus that we should expect to see differences in the differentiation trend of tholeiitic magmas between the volcanic and plutonic environment. Problems arise of course because layered intrusions such as the Skaergaard predominantly preserve the crystal extract, whereas the volcanic successions preserve the liquids. Indeed the more efficient the process of crystal-liquid fractionation the more difficult it is to define the liquid line of descent of the magmas from which a series of plutonic rocks has formed. This is one of the major points of contention between Hunter & Sparks (1987) and their opponents. Brooks et al. (1991) raised the possibility that liquid lines of descent constructed from volcanic sequences may not actually represent the fractionation path of the magmas in the underlying chambers, even though least-squares modelling calculations can be performed which give good residuals. Magma compositions may depart from the theoretical liquid line of descent as a consequence of open-system magmatic processes, including magma chamber refluxing and in-situ differentiation. This is a view echoed in the theoretical modelling of magmatic differentiation processes by Nielsen (1990). If correct, this may challenge many of our traditional perceptions of magmatic differentiation based on Harker diagrams and studies of volcanic rocks. Hunter & Sparks (1987) suggested that the Skaergaard magma chamber might not have been completely closed and could have lost significant volumes of silicic magma to the surface by eruption or lateral intrusion. Since most of the roof rocks of the intrusion have been removed by erosion, there is no way of knowing whether there was indeed any connection to the surface. In addition Stewart & DePaolo (1990) have measured the Nd-Sr isotopic composition of the cumulates from the Layered Series , which reveals that the Skaergaard magma was actually assimilating small amounts (2-4%) of Precambrian gneissic wall rock, at least during the early stages of crystallization. Thus even the type example of closed system magmatic differentiation was probably not completely closed to external influences. Defant & Nielsen (1990) noted that for simple systems undergoing fractional crystallization the instantaneous bulk composition of the fractionating mineral assemblage begins to approach the bulk composition of the magma from which it is crystallizing when the magma becomes multiply saturated i.e.when it reaches a eutectic. In simple terms this means that at the mafic end, the extracted cumulates should be most different in composition from the crystallizing magma, and at the evolved (acidic) end most similar. Such relationships may change, however, if the system becomes open to recharge or assimilation. This could have important implications with respect to the interpretation of the composition of the Skaergaard cumulates (McBirney 1989; Fig.4). The isotopic evidence for assimilation (Stewart & DePaolo 1990) may mean that the early cumulates are not what would normally fractionate from an evolving tholeiitic magma. Addition of the assimilant would probably have driven the magma composition off the cotectic along which

MAGMATIC DIFFERENTIATION it was evolving, changing the fractionating mineral proportions as well as the mineral assemblage (Nielsen 1989). Marsh (1988) and Sparks (1990) have continued the debate about whether the plutonic and the volcanic record reveal fundamentally different styles of magmatic differentiation. Marsh investigated the dynamic evolution of a sheet-like basaltic magma chamber and concluded that most of the crystallization should occur near the roof, but that descending plume-like convection currents would transport crystals down to the chamber floor. In this model the more differentiated liquids are always trapped in the downward crystallizing roof zone and therefore the residual magma never differentiates to any considerable extent. In many respects the physical aspects of this model have strong similarities to that proposed originally by Wager & Deer (1939) for the Skaergaard intrusion. However Sparks (1990) argues that this model is not applicable to large magma chambers in which crystallization occurs predominantly at the floor, while cooling occurs predominantly through the roof. It is clear that this remains an area for further study! C o n v e c t i o n in m a g m a c h a m b e r s

Until the late 1970s our ideas about the physical processes that allowed fractional crystallization to take place were based on very simple concepts and differed little from those of Bowen (1928). Crystals were considered to nucleate and grow within a magma and then to settle out under the influence of gravity to form cumulate rocks. Although many other processes potentially responsible for magmatic differentiation had been recognized by the turn of the century (e.g. Harker 1909), including in-situ crystallization on the margins of the magma chamber, magma mixing, crustal contamination, immiscibility and liquid state diffusion, by the late 1920s these were all regarded as subordinate to crystal settling. In the past decade, the dynamics of magma chamber processes have become an important theme, with increasing importance attached to the role of convection in fractional crystallization. This represents a considerable shift of emphasis from previous studies of magmatic differentiation, based on experimental determinations of phase equilibria in silicate systems (e.g. Hess 1989). It is interesting to note that the idea of thermal convection in magma chambers was first proposed by Becker (1897). The convective system established in a particular magma body is necessarily a transient condition, because convection will enhance the rate of cooling. Thus the life span of a convection system depends on the amount of heat that is lost by conduction through the walls and roof and the amount of new magma (if any) periodically injected into the chamber. Flow rates calculated for magmas undergoing convection are comparable to, if not higher than, the settling velocities predicted by Stokes Law. Thus convection will either cancel out or enhance the effects of crystal settling. It is important to remember, however, that Stokes Law relates to the movement of small spheres in Newtonian fluids. Magmas that contain more than a few percent crystals and those that are highly polymerized are more likely to behave as non-Newtonian or Bingham fluids which have a finite yield strength (McBirney & Noyes 1979). From recent studies (for reviews see Sparks et al. 1984; Turner & Campbell 1986) it is clear that heat and mass

211

transfer processes in multicomponent fluids such as silicate melts are complex. Calculated thermal Rayleigh numbers range between 10 9 for viscous rhyolite to 10 23 for basaltic magmas. Thus given a value of 10 3 for the onset of turbulent convection, we might conclude that all magmas, regardless of their chemical composition, stored in high-level magma chambers should be in a state of vigorous thermal convection (Hess 1989). This should have fundamental implications for models of crystal settling in magma chambers for, as pointed out by Sparks et al. (1984), the convective motions are usually sufficiently vigorous to keep crystals in suspension. However crystal settling may occur from within boundary layers at the margins (floor, walls) of the magma chamber. In recent years most models for crystallization in magma chambers have favoured in-situ growth on the floor and walls of the chamber (McBirney & Noyes 1979; Irvine 1980 a, b; Turner & Campbell 1986; McBirney et al. 1985; Nilson et al. 1985). However, Sparks et al. (1993) have recently returned to gravitational crystal settling models to explain the modal and rhythmic igneous layering which typifies many large layered mafic intrusions. They propose that each mineral phase has its own critical concentration which must be exceeded before sedimentation can occur. Sparks et al. (1984) proposed a mechanism for magmatic differentiation, termed convective fractionation, in which in a crystallizing boundary layer the less (or more) dense liquid fraction convects away from the residual crystals (Fig. 5). Langmuir (1989) and Tait & Jaupart (1990) propose that the evolved melt from the boundary layer may then be mixed back into the main body of the magma (Fig. 6). This means that magma in the central part of the chamber may exhibit the effects of differentiation without ever having crystallized directly itself. Nielsen (1990) has incorportated this concept into a complex mathematical model of in-situ crystallization. His results show that the course of differentiation (liquid line of descent) can be strongly influenced by the amount of crystallization in the boundary layer and on the mineral

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Fig. 6. Schematic weight % CaO versus MgO variation diagram to show the effects of in-situ crystallization. For the case of perfect fractional crystallization the initial chamber magma m evolves along the LLD m-p-s. The marked inflection corresponds to the point at which plagioclase becomes a major fractionating phase. Highly fractionated melt evolving in-situ in the solidification zone (Fig. 3) has the composition s. If a small amount of s mixes back into the main body of magma (m) the new chamber magma will have a composition m'. This will then evolve along a slightly different LLD ( m ' - p ' - s ' ) . Mixing-in of the new solidification zone melt s' with m' will yield a new chamber magma m". The chamber magma thus evolves along a path m-m'-m" which contrasts markedly with the LLD predicted for perfect fractional crystallization. assemblage crystallizing. A consequence of in-situ fractionation is that the system evolves by mixing in a small amount of magma with an extreme composition (Fig. 6), particularly with respect to its trace element characteristics. This simple graphical representation demonstrates that the magma chamber composition will evolve towards the composition of the solidification zone liquids, not along any specific cotectic. Rather surprisingly models of convective fractionation are not new. In 1918, Grout proposed a two phase model of convection in which crystal laden melt from the chamber roof sinks to the floor where crystals settle, and residual liquid rises as it is less dense and is recycled into the zone of crystallization. Integration of this effect could result in the gravitational differentiation of considerable volumes of magma. An important property of multicomponent fluids such as silicate melts is that individual components (including heat) can have different diffusivities. As a consequence such fluids may become vertically stratified with respect to density, composition and temperature (Irvine 1980a; McBirney & Noyes 1979; Rice 1981; Sparks et al. 1984; Turner & Campbell 1986). If opposing gradients of two (or more) components with different diffusivities are set up, the system may separate into a series of independently convecting layers, bounded by sharp diffusive interfaces, across which heat and chemical components are transported by molecular diffusion. This p h e n o m e n o n is known as double (multiple)diffusive convection, and Sparks et al. (1984) consider that this will inevitably occur in silicate magmas. Indeed Irvine (1980a) considered that multi-diffusive convection is probably one of the principal mechanisms in the fractional crystallization of magmas. This process may also be effective in transmitting the effects of magma mixing and assimilation through a cooling body of magma. There is substantial evidence to suggest that many silicic magma chambers are

Thermogravitational diffusion The diffusion of chemical species in silicate melts governs the kinetics of most magmatic processes including partial melting, fractional crystallization, magma mixing and crystal growth. Hofmann (1980), Watson & Baker (1990) and Lesher & Walker (1991) give excellent reviews of this complex subject and the reader is referred to these articles for a more detailed discussion of the principles and governing equations. In the late ninteenth century a particular type of diffusion known as Soret diffusion was regarded as being one of the main causes of magmatic differentiation. This refers to the tendency of non-convecting homogeneous solutions to develgp concentration gradients when subjected to a temperature gradient. Hess (1989) presents an excellent review of the p h e n o m e n o n . The governing equation has the form:

( C c - C.)/Co= o A r where Cc and CH are the temperatures at the cold and hot ends of the system respectively and Co is the initial concentration, a is the Soret coefficient and AT is the temperature difference between the two ends of the system. The compositional gradient which can develop in a system will depend upon the magnitude of the Soret coefficient, which can vary in both sign and magnitude from component to component, and on the temperature gradient. Components with positive Soret coefficients accumulate at the cold ends of temperature gradients whereas those with negative coefficients concentrate at the hot ends. Harker (1894), in his study of the Carrock Fell intrusion, gave careful consideration to the possibility that Soret diffusion could have been responsible for the chemical variation he observed. He came to the conclusion that diffusion controlled gradients in a liquid magma were not the major control, favouring instead a model which combined crystallization and diffusion. However it must be noted that Harker had only one complete major element chemical analysis and one partial analysis of the gabbro on which to base his ideas. Additionally, since he was basing his interpretations on Soret's original model of diffusion in saline solutions, he did not consider the possibility that different components of a silicate melt might diffuse in different directions in the same temperature gradient. In 1981, Walker et al. demonstrated experimentally that a basalt magma held at several hundred degrees above its liquidus and subjected to a steep temperature gradient developed strong chemical gradients in about a week, becoming broadly andesitic at the hot end and a low silica basalt at the cold end (Fig. 7). It is interesting to note that this is the converse of normal crystal-liquid differentiation trends, in which the low temperature differentiation products are silica rich. These experiments triggered renewed interest in the potential for the development of diffusion controlled chemical gradients in magma bodies. Large temperature gradients are likely to exist at the margins of magma chambers and these will control the effective role of thermal diffusion in the fractionation of chemical species within the boundary layer (Carrigan & Cygan 1986; Cygan & Carrigan 1992). Numerous researchers (e.g. Hildreth 1979, 1981; Koyaguchi 1989) have

MAGMATIC DIFFERENTIATION Soret effect 20.0

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Fig. 7. Weight % oxide variations as a function of temperature, generated as a consequence of the Soret effect in an experimental charge. Data from Walker et al. (1981). The mid-points of each curve represent the initial starting composition of the system. suggested that thermal diffusion (Soret fractionation) may be a significant process contributing to chemical zonation in magma bodies, particularly silicic ones. However other workers (e.g. Michael 1983; Stix et al. 1988) have argued that crystal fractionation processes and not Soret diffusion can account for the observed chemical gradients in silicic magmas. Indeed recent experimental determinations of the Soret coefficients for a variety of silicate melt compositions (e.g. Lesher 1986; Lesher & Walker 1988) have suggested that the effect would be small and often of the opposite sense to that observed in natural magmatic systems. In addition, processes of crystallization, crystal settling and convection will tend to destroy any large scale chemical gradients developed by diffusion within the liquid magma. However in stagnant boundary layers near the walls and roof of a magma chamber, Soret diffusion may locally modify the pattern of magmatic differentiation (Hess 1989). Cygan & Carrigan (1992) have developed a numerical model to examine the effect of non-linear and time dependent temperature fields upon the mass flux associated with thermal diffusion. Their results suggest that thermal diffusion of magma in a time-dependent thermal field is minimal and thus that this cannot be considered a significant chemical fractionation process. It it interesting to note, however, that their predicted changes in the silicate melt induced by thermal diffusion in a boundary layer are orders of magnitude lower than those observed in experimental (e.g. Walker et al. 1981; Lesher 1986) and field (Hildreth 1979) studies.

Assimilation and fractional crystallisation Since the classic work of Bowen (1928), it has generally been accepted that crystallizing magmas may simultaneously assimilate the surrounding wall rock. Even Harker (1909) conceded that extensive melting may occur adjacent to magma reservoirs in the deep crust, although he did not envisage that the effect would be significant due to the absence (as he thought) of sedimentary strata in the deep crust. The heat required for assimilation must clearly be derived from the heat contained within the magma itself and from the heat of crystallization liberated by mineral precipitation. Most authors now agree that assimilation, combined with fractional crystallization, is an important mechanism of magmatic differentiation in all continental magmatic settings, particularly in zones of thickened continental crust. As noted by McBirney (1984) the major element

213

composition of a contaminated magma does not necessarily reflect the effect of an added component in any simple way. In many cases the evolving magma will continue to follow its original liquid line of descent. However a magma assimilating felsic crustal rocks will differentiate to produce much larger volumes of more evolved products than if it had not assimilated any crust. Increasingly complex theoretical models to decribe the trace element and isotopic evolution of systems undergoing paired assimilation and fractional crystallization (AFC) have been devised since the late 1970s (e.g. Allegre & Minster 1978; Taylor 1980; DePaolo 1981; Ghiorso & Kelemen 1987; Nielsen 1989, 1990). The older models (e.g. DePaolo 1981) involve several important simplifying assumptions , the most critical of which is that all partition coefficients (D) are constant over the range of fractionation being modelled, and that there is a constant ratio (r) between the rate of assimilation and the rate of fractional crystallization. The recent development of phase equilibria based differentiation models (e.g. Ghiorso & Kelemen 1987; Nielsen 1988) has allowed formulation of more complex, and more realistic, models of AFC processes. The most recent (Nielsen 1989, 1990) permit calculation of liquid lines of descent for Sr and Nd isotopes and evaluation of the effects of assimilation on temperature, fractionating mineral proportions and melt composition. Nielsen (1989) has demonstrated that in systems undergoing AFC the bulk partition coefficients for Sr and Nd are strongly dependent on the chemistry of the assimilant and on the rate of mixing. Virtually all silicic assimilants will increase the bulk partition coefficient for network modifying cations. Additionally he has confirmed that assimilation will generally cause a change in the fractionating mineral proportions. For example, a peraluminous assimilant will increase the proportion of plagioclase crystallizing from a mafic magma, while a peralkaline assimilant will increase the proportion of augite.

Liquid immiscibility Silicate liquid immiscibility occurs whenever a single melt splits into two coexisting melts in response to changes in pressure, temperature or composition. The idea of liquid immiscibility as a differentiation mechanism was probably first proposed by Scrope (1825) and given serious consideration by Harker (1909), Daly (1914) and Bowen (1928). In 1951 Roedder reported the results of experiments which demonstrated the existence of a large field of silicate liquid immiscibility in the system K20-FeO-A1203-SiO2. Experimental evidence for the existence of a miscibility gap between carbonatite and silicate magmas was presented by Freestone & Hamilton (1980) and Kjarsgaard & Hamilton (1988). Roedder (1979) reviewed the evidence for liquid immiscibility in a wide range of magmatic rocks including low-K ultrabasic and basic komatiites, high-K feldspathoidbearing basalts, high-A1 olivine-bearing subalkali basalts, normal and high Fe subalkalic basalts, nephelinites and high Ti Lunar mare basalts. He suggested that immiscibility in silicate systems usually yields a felsic alkali-aluminosilicate melt and a mafic melt rich in Fe, Mg, Ca and Ti. Philpotts (1982) considered that immiscible liquids are present in sufficient amounts that this should be considered a viable means of magmatic differentiation during the late stages of crystallization of common magmas. Indeed McBirney & Nakamura (1973) proposed that immiscibility in the later

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M. WILSON

stages of differentiation of the Skaergaard intrusion was responsible for the formation of granophyres. Petrologists have from time to time postulated that the common association of basalt and rhyolite, without transitional rock types, is evidence for silicate liquid immiscibility. However the simple juxtaposition of two contrasting magma types is obviously insufficient to prove them immiscible. As noted by Bowen (1928), coexisting immiscible liquids should be in equilibrium with a common mineral assemblage. Le Bas & Handley (1979) used this logic to demonstrate an immiscible relationship between ijolites and sovites from the East African Rift.

The experimental approach to understanding magmatic differentiation In a review of this brevity it is impossible to describe the experimental approach to studies of magmatic differentiation in any detail. Bowen (1928) was clearly one of the pioneers of modern experimental petrology, although his work concentrated largely on phase equilibria in synthetic binary and ternary systems. Such data have provided a tremendous insight into phase equilibria in silicate systems, although they have liitle quantitative application to the understanding of magmatic differentiation processes. Excellent reviews of the interpretation of phase equilibria data may be found in Cox et al. (1979) , Yoder (1979) and Hess (1989) and will not be reiterated here. Over the past 30 years, experimental studies have provided important constraints on the nature of the crystallizing phases in basaltic systems as a function of pressure and temperature (e.g. Yoder & Tilley 1962; Green & Ringwood 1967; Holloway & Burnham 1972; Bender et al. 1978; Elthon & Scarfe 1984; Gust & Perfit 1987; Baker & Eggler 1987; Longhi & Pan 1988 ). Such data provide the experimental basis for the most recent quantitative models of magmatic differentiation (e.g. Ghiorso & Carmichael 1985; Nielsen 1988). Milestones in the intepretation of experimental data are the classic papers of Yoder & Tilley (1962) and O ' H a r a (1968).

Representing the data: variation diagrams H a r k e r diagrams Oxide-oxide weight percent variation diagrams illustrating the compositional variation within magmatic suites have been in use, essentially unmodified, for almost a century. They were first introduced by Harker (1900, 1909) and for this reason bear his name. It was Bowen (1928), however, who realized their full potential for interpreting the processes of magmatic differentiation and their physical meaning in the context of experimental phase petrology. Bowen showed how effective variation diagrams are in illustrating the chemical relationships among the members of a rock association. They enable us to explore in a simple graphical way the compositions and quantities of phases that have to be added or subtracted from an evolving magma to produce the next magma in the liquid line of descent (LLD). Cox et al. (1979) present an excellent review of the interpretation of major and trace element variation diagrams which should be compulsory reading for anyone unfamiliar with the subject.

Pearce element ratio diagrams In 1968 T.H. Pearce introduced a new method for the representation of chemical data on variation diagrams, in

part to overcome the constant sum effect inherent in oxide-oxide weight percent variation diagrams. At the time the method received little attention in the literature. However there has recently been a resurgence of interest in their use (Pearce 1987; Ernst et al. 1988; Nicholls 1988; Russell & Nicholls 1988; Russell & Stanley 1990; Pearce & Stanley 1991), although admittedly restricted to a group of Canadian geoscientists closely associated with their originator. Pearce element ratios provide an interesting way of using major element data for cogenetic suites of volcanic rocks to test hypotheses on liquid lines of descent in evolving magma chambers. The method is based on the conversion of weight per cent oxide data for a suite of rocks to element fractions :

ei = WiAi/MWi where W~, Ai and MW~ are the weight percentages, the number of cations in the oxide formula and the molecular weight of oxide i. The Pearce element ratio (ri) of an element i is then defined as: ri = edez where z is a conserved element whose amount does not change during the differentiation process being investigated. Typically P, Ti and K are chosen as conserved elements, at least during the initial stages of differentiation of basaltic magmas. Complex ratio diagrams using axes constrained to be sensitive to the fractionation of a particular mineral (e.g. 0.5 ( M g + F e ) / K versus Si/K for olivine) are used to evaluate the role of that mineral in the petrogenesis of a suite of cogenetic rocks related by fractional crystallization. The supporters of the method argue that Pearce element diagrams can yield insights into igneous processes that are not obvious or quantitatively expressed when portrayed on other variation diagrams (e.g. Harker diagrams). All agree that the method is sensitive to analytical error and to the assumption that the chosen conserved elements are effectively excluded throughout the entire crystallization sequence. Defant & Nielsen (1990) have used a forward modelling approach to generate synthetic data sets with which to evaulate whether Pearce element ratio diagrams can correctly predict the proportions of phases involved in magmatic differentiation. Their results show that for cases of homogeneous crystallization and in-situ crystallization, with or without magma chamber recharge, Pearce element ratio analysis gives quite consistent results. However the method breaks down when any kind of assimilation has occurred.

Differentiation indices Running through much of the older geological literature is the idea that analyses of igneous rocks, if plotted on the appropriate type of variation diagram, can be arranged in an evolutionary sequence. To this end a variety of differentiation indices have been devised. The Harker index (SiO2 as abscissa) depends upon the commonly observed increase in SiOz in successive liquids with progressive fractional crystallization and has been widely used for much of this century. Similarly, for basaltic compositions MgO is commonly used as the abscissa in variation diagrams. In addition indices based upon the magnesium-iron ratio have been widely used (e.g. 100MgO/MgO + FeO or Mg2+/Mg2+ + Fe2+). More complex differentiation indices, including the Solidification Index of Kuno (1959) and the Differentiation Index of Thornton & Tuttle (1960), have been devised but are rarely used these days. For a more

MAGMATIC D I F F E R E N T I A T I O N complete discussion of this subject the reader is referred to Cox et al. (1979) and Ragland (1989).

Modelling magmatic differentiation Over the past 20 years a number of attempts have been made to model the process of differentiation mathematically. Quantitative models for the geochemical evolution of magma chambers have become increasingly more complex since the basic Rayleigh fractionation equation was first applied to the problem of fractional crystallization (Neumann et al. 1954). Models may be divided into two groups : (a) those based upon graphical analysis of synthetic systems or projections (e.g. Presnall et al. 1979; Walker et al. 1979; Grove et al. 1982) and (b) those based on a numerical approach (e.g. Allegre et al. 1977; Minster et al. 1977; Allegre & Minster 1978; Nathan & Van Kirk 1978; Hostetler & Drake 1980; Langmuir & Hanson 1981; O'Hara & Mathews 1981; DePaolo 1981; Nielsen & Dungan 1983; Ghiorso 1985; Ghiorso & Carmichael 1985; Nielsen 1988, 1989, 1990; Defant & Nielsen 1990). Early numerical models (pre-1985) were based on statistical evaluation of experimental data to derive empirical expressions to describe the phase equilibria. In contrast the later models (e.g. Ghiorso 1985; Nielsen 1988, 1989, 1990) are all based on a thermodynamic approach. Most of the pre-1985 models rely on assumptions which are known to be invalid over extended periods of differentiation. For example partition coefficients (D) were generally asssumed to be independent of pressure, temperature and system composition. The more recent models of Nielsen (1988, 1989) use variable partition coefficients, which are functions of temperature and composition, to simulate liquid lines of descent in differentiating magma systems. The simplest models have their roots in Harker -type major element variation diagrams, using a linear least squares approach to deduce the proportions of minerals (of specified composition) which might fractionate from a particular magma composition to yield a more evolved daughter (e.g. XLFRAC, Stormer & Nicholls 1978; GENMIX, Le Maitre 1981; Wright & Doherty 1974). It is however, important to note that such mass balance calculations are able to produce a number of arithmetically feasible solutions (all with low sums of squares of residuals, or2). For meaningful results, it is essential that in the system being modelled there are fewer mineral phases than chemical components (oxides). In general, while major element modelling can provide some useful insights into the nature of magmatic differentiation processes, the variation of major elements can tell us little about more complex processes such as refluxing of magma chambers with batches of more primitive magma or crustal assimilation (Nielsen 1990). In such cases isotopic and trace element behaviour may be much more sensitive. Nielsen (1990) has developed one of the most complex simulations of magmatic differentiation thus far. No doubt even more complex models will emerge in due course. At present, such models are limited by an inadequate experimental data base, with which to constrain the crystallization behaviour of the complete spectrum of magma types under crustal and mantle conditions. They are also limited by our lack of understanding of the thermodynamics of silicate melts. One of the most useful aspects of these numerical

215

calculations is the ability to forward model a particular petrogenetic process or series of processes. Starting with a chosen parental magma composition, different liquid lines of descent can be modelled for combinations of homogeneous (perfect) fractional crystallization, in-situ fractionation, refluxing of the chamber, crustal assimilation and periodic eruption. In addition the oxygen fugacity of the system can be varied. These theoretical models can then be compared to an actual data set, for example for a suite of cogenetic volcanic rocks. This forward modelling approach may be particularly useful for evaluating complex petrogenetic models which cannot be evaluated by experimental techniques. Nielsen (1990) also uses this forward modelling approach to evaluate the effectiveness of other modelling techniques (e.g. linear least squares calculations, trace element ratio diagrams and Pearce element ratio diagrams) with some rather interesting results. For example he shows that linear least squares mass balance calculations and Pearce element ratio diagrams, for systems which have evolved by in-situ fractionation, reveal the phase assemblage crystallizing in the solidification zone. This may explain those cases where the phase assemblage predicted by least squares modelling is inconsistent with observed phenocryst assemblages. Nielsen (1990) also uses this approach to demonstrate that log-log trace element diagrams (Allegre et al. 1977) used in the analysis of fractional crystallization processes are only valid if fractional crystallization is the only process responsible for differentiation.

Summary It is as clear today, as it was a century ago to Harker, that magmatic differentiation must be the result of a complex series of processes. Most petrologists now agree that some form of crystal-liquid fractionation is the dominant driving mechanism, although the manner in which this occurs remains a subject for debate. Nevertheless, liquid-state differentiation mechanisms, including themogravitational diffusion, liquid immiscibility and magma mixing are clearly capable of generating significant compositional variations within magma bodies. As noted by Harker (1894, 1909) it is important to differentiate between the in-situ differentiation of a single magma body and the processes of differentiation in deep seated magma reservoirs responsible for the formation of cogenetic suites of intrusives or extrusives. Layered mafic-ultramafic intrusions provide unique natural laboratories in which to study the former. The latter, by comparison, are in some respects almost as elusive now as they were to Harker in the 1890s. However, unlike Harker, we clearly have a much greater understanding of the physico-chemical processes which must operate in high-level magma bodies. Accepting that some form of crystal-liquid separation provides the dominant control for magmatic differentiation, one of the major controversies remaining is the mechanism by which this actually occurs. Is it primarily induced by localized crystallization at the walls, roof and floor of a magma reservoir (in-situ differentiation) or through the relative movement of crystals and liquid (gravitational crystal settling). In the past hundred years we have seen gravitational crystal settling go in and out of favour several times. It dominated most discussions of magmatic differentiation during the 1960s and 70s. In contrast, in the 1980s models involving in-situ crystallization gained

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M. WILSON

popularity, following McBirney & Noyes (1979) reevaluation of the evidence for crystal settling in the Skaergaard intrusion. Currently (e.g. Sparks et al. 1993) gravitational crystal settling seems to be back in vogue, but one may wonder for how long? Geochemical and S r - N d - P b isotopic studies of cogenetic suites of magmatic rocks have provided powerful support for models of magmatic evolution by the combined processes of crustal assimilation and fractional crystallization (AFC). This must be a common cause of magmatic differentiation in most high-level magma chamber systems. However it is most easily detected within the continental crust when the chamber magma and the wall rocks have strongly contrasting isotopic and trace element characteristics. From the classic studies of Bowen in the 1920s up to the 1970s most discussions of magmatic differentiation relied heavily upon interpretations of phase equilibria in natural and synthetic systems (e.g. Bowen 1928; Philpotts 1990). While this approach greatly enhanced our understanding of the processes involved, it was limited in its ability to make quantitative predictions about the course of evolution in natural magmatic systems. In contrast, in the past decade increasingly sophisticated thermodynamic modelling techniques have been applied, allowing predictions to be made about the liquid line of descent for a given magma composition, evolving under a specified set of conditions. Unfortunately too few petrologists have adopted this forward modelling approach in petrogenetic studies of cogenetic suites of igneous rocks. This is clearly one of the most powerful ways in which we can quantify the various processes involved in magmatic differentiation and will undoubtedly dominate discussions for the remainder of this decade. Looking back over the past century, we can identify several distinct periods when rapid advances were made in our understanding of the processes involved in magmatic differentiation. In many instances these resulted from detailed field based studies. The description of igneous layering in the Skaergaard intrusion, Greenland, by Wager et al. in the 1930s and its re-interpretation in the late 1970s by McBirney & Noyes (1979) were clearly important milestones. In addition the tremendous increase, during the past thirty years, in the volume of high quality geochemical and isotopic data available for cogenetic suites of magmatic rocks has been of fundamental importance. During the past decade quantitative modelling of these data has enabled us to evaluate the viability of the various differentiation mechanisms which have been proposed over the years, although this is not an easy task given the number of variables involved. In this respect it is interesting to note how many of the 'new' models proposed during the past decade to explain magmatic differentiation have actually been around for more than seventy years, some for more than a hundred. I would like to thank H. Downes and M.J. Norry for their thoughtful reviews. References ALLEGRE, C.J. & MINSTER, J.F. 1978. Quantitative models of trace element behaviour in magmatic processes. Earth and Planetary Science Letters, 38, 1-25. , TREUIL, M., MINSTER, J.F., MINSTER, B. & ALBAREDE, F. 1977. Systematic use of trace elements in igneous processes. Part I: Fractional

crystallisation processes in volcanic suites. Contributions to Mineralogy and Petrology, 60, 57-75. BAILEY, E.B., CLOUGH, C.T., WRIGHT, B.A., RICHLY, J.E. & WILSON, G.V. 1924. The Tertiary and post-Tertiary geology of Mull, Loch Aline and Oban. Geological Survey of Scotland, Memoir, 53, Edinburgh. BAKER, D.R. & EGGLER, D.H. 1987. Compositions of anhydrous and hydrous melts coexisting with plagioclase, augite and olivine or low-Ca pyroxene from latm to 8 kbar: Application to the Aleutian volcanic centre of Atka. American Mineralogist, 72, 12-28. BECKER, G.F. 1897. Fractional crystallization of rocks. American Journal of Science, 4, 257-261. BEDARD, J.H., SPARKS, R.S.J., RENNER, R., CHEADLE, M.J. & HALLWORTH, M.A. 1988. Peridotite sills and metasomatic gabbros in the Eastern Layered Series of the Rhum complex. Journal of the Geological Society, London, 145, 207-224. BENDER, J.F., HODGES, F.N. & BENCE, A.E. 1978. Petrogenesis of basalts from the project FAMOUS area: Experimental study from 0 to 15 kbars. Earth and Planetary Science Letters, 41, 277-302. BOWEN, N.L. 1928. The evolution of the igneous rocks. Princeton University Press, New Jersey, (reprinted in 1956 by Dover Publications, N e w York). BROOKS, C.K. & NIELSEN, T.F.D. 1990. A discussion of Hunter and Sparks (Contrib. Mineral. Petrol. 95: 451-461). Contributions to Mineralogy and Petrology, 104, 244-247. --, LARSEN, L.M. & NIELSEN, T.F.D. 1991. Importance of iron-rich tholeiitic magmas at divergent plate margins: A reappraisal. Geology, 19, 269-272. BUNSEN, R. 1851. Ueber die prozesse der vulkanischen Gesteinsbildungen Islands. Annalen der Physik (Leipzig) 2nd series, 83, 197-272. CAMPBELL, I.H. & TURNER, J.S. 1986. The influence of viscosity on fountains in magma chambers. Journal of Petrology, 27, 1-30. CARMICHAEL, I.S.E. 1964. The petrology of Thingmuli, a Tertiary volcano in Eastern Iceland. Journal of Petrology, 5, 435-460. CARRIGAN, C.R. & CYGAN, R.T. 1986. Implications of magma chamber dynamics for Soret-related fractionation. Journal of Geophysical Research, 91, 11451-11461. Cox, K.G., BELL, J.D. & PANKHURST, R.J. 1979. The interpretation of igneous rocks. Allen & Unwin, London. CVGAN, R.T. & CARR1GAN, C.R. 1992. Time-dependent Soret transport: Applications to brine and magma. Chemical Geology, 95, 201-212. DARWIN, C.R. 1844. Geological obervations on the volcanic islands visited

during the voyages of H.M.S. Beagle, with brief notices on the geology of Australia and the Cape of Good Hope, being the second part of the Voyage of the Beagle. Smith Elder & Co., London. DALY, R.A. 1914. Igneous Rocks and their Origin. McGraw Hill, New York. -1933. Igneous Rocks and the depths of the Earth. McGraw Hill, N e w York. DEFANT, M.J. & NIELSEN, R.L. 1990. Interpretation of open system petrogenetic processes: Phase equilibria constraints on magma evolution. Geochimica et Cosmochimica Acta, 54, 87-102. DEPAOLO, D.J. 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallisation. Earth and Planetary Science Letters, 53, 189-202. EICHELBERGER, J.C. 1975. Origin of andcsite and dacite: Evidence of mixing at Glass Mountain in California and at othcr circum-Pacific volcanoes.Geological Society of America, Bulletin, 86, 1381-1391. ELTHON, D. & SCARFE, C.M. 1984. High-pressure phase equilibria of a high magnesia basalt and the genesis of primary oceanic basalts. American Mineralogist, 69, 1-15. ERNST, R.E., FOWLER, A.D. & PEARCE, T.H. 1988. Modelling igneous fractionation and other processes using Pearce diagrams. Contributions to Mineralogy and Petrology, 100, 12-18. FENNER, C.N. 1926. The Katmai magmatic province. Journal of Geology, 34, 675-772. -1938. Contact relations between rhyolite and basalt on Gardiner River, Yellowstone Park, Wyoming. Geological Society of America Bulletin, 49, 1441-1484. FREESTONE, I.C. & HAMILTON, D.L. 1980. The role of liquid immiscibility in the genesis of carbonatites. An experimental study. Contributions to Mineralogy and Petrology, 73, 105-117. FURMAN, T. & SPERA, F.J. 1985. Comingling of acid and basic magma and implications for the origin of I-type xenoliths. I- Field and petrochemical relations of an unusual dike complex at Eagle Lake, Sequoia National Park, California, U.S.A. Journal of Volcanology and Geothermal Research, 24, 301-318. GHIORSO, M.S. 1985. Chemical mass transfer in magmatic processes. I. Thermodynamic relations and numerical algorithms. Contributions to Mineralogy and Petrology, 9tl, 107-120. -& CARMICHAEL, I.S.E. 1985. Chemical transfer in magmatic processes.

MAGMATIC

DIFFERENTIATION

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Received 15 January 1993; revised typescript accepted 4 March 1993.

From

QJGS,50, 311-31 2. 21. CA~ocx F p ~ : a S ~ in the VARrATION O/ Io~n~ovs RoeKMAssss.--PAa'r I. Tn~ GABSRO. By ALFma~ HA1~K~, Esq., M.A., F.G.S., Fellow of St. John's College, Cambridge. (Read May 9th, 1894.) [PLAT~ XVI. & XVII.] CONTSNTS. Page 1. Introduction ............................................................ 311 2. Mineralogical Characters of the Gabbro ........................... 316 3. Minor Textural and Mineralogical Variations .................. 319 4. Orderly Variation from Centre to Margin ........................ 320 5. Discussion of the Causes of such Variation ..................... 324 6. Some Deductions from the Phenomena ........................... 329 7. Reactions between Gabbro and Enclosed Masses of Lava ...... 33l 8. Conclusion ............................................................... "J34 Section across Carrock FeD ............................................. 314 1. INTRODUCTION.

DuR~6 the last two years I have devoted some attention to the igneous rocks of Carrock Fell and the hills west of that well-known summit. Occurring in a somewhat critical situation on the border of the English Lake District, they were examined by Mr. J. E. Marr and myself, partly with reference to their bearing on the general geology of the district ; but, apart from this, they offer in themselves some features which are of sufficient interest to be worthy of record. [ have had the advant~tgc of my colleague's co-operation, more especially ill the field-work, and take this opportunity of acknowledging my iudebt,edness to him. The carlit,s|, conziected ~ccoutJt of the Carrock Fell rocks was given by the late Mr. Clit'ton Ward ~ in 1876. He recogni'~ed three general types of igneous rocks in the district : - (a) Spherulitic felsite of Carrock Fell and Great Lingy; (b) Diorite (?) of Miton Hill and Round K n o t t ; (c) Hypersthenite of Mosedale Crags and Langdale. He gave a brief account of their characters in the field and under the microscope, with chemical analyses of the first and last, and put forward a view of their mutual relations and mode of origin. In his opinion the several types pass into one another in the field, and he regarded them as produced by the metamorphism of part of the volcanic series, on the strike of which they occur. Dr. C. O. Trechmann, '~ in 1882, pointed out that the dominant pyroxene in the so-called hypersthenite is not hypersthene, but diallage, and the rock would therefore be more correctly described a,q a gabbro. Mr. J. J. H. Teall, 3 in 1885, briefly noticed the spherulitie felsite of Carrock Fell as a typical example of a granophyre in the sense of Rosenbusch. Later he described both this rock and the gabbro (a quartz-bearing variety), stating that the one passes into the other by insensible gradations, t In 1889 Mr. T. T. Groom 2 pointed out, the occurrence on Carrock Fell of another type of rock, a tachylyte, in thin veins, cutting the gabbro, but considered to be connected with it. The same writer reasserted the existence of all transitional stages between the acid granophyre and the basic gabbro, and this passage seems to have been generally accepted. ~

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 221-235 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 1009-1023

Granite magmatism MICHAEL

P.

ATHERTON

Department o f Earth Sciences, University o f Liverpool, Brownlow Street, Liverpool L 6 9 3BX, UK

Abstract: Read's two presidential addresses to the Geological Society (1948, 1949) heralded the end of the coherent rearguard action by the 'granitizers' against the 'magmatists'. They were the distillation of his thoughts on the genesis of granite and culminated in his concept of the 'Granite Series'. In this, he identified a continuity from metamorphic through migmatitic rocks to granite. Although he was wrong on granitization, the general idea remains intact and granites are produced by high-temperature metamorphism leading to partial melting. However the role of migmatites is still contentious. Not all granites belong to the granite series as he presented it; this particularly applies to Cordilleran (Andean) type granites. The genesis of this type will be discussed in the context of the earlier, classic work of Nockolds (1940) on the Garabal Hill complex where he demonstrated fractional crystallization was a major process in producing the diversity present and was similar to that seen in volcanic rocks, which had clearly been liquids. He also proposed that the source was basaltic with the implication, strongly supported by modern isotopic studies, that granites of this type are essentially mantle derived. Nockolds contribution was a geochemical confirmation of Bowen's belief in the importance of closed-system fractional crystallization in the differentiation of plutonic rocks. Most authorities today would accept this but would not necessarily follow Bowen in the belief that major granitic batholiths formed by differentiation of basalt or that the system was closed. Recent models of the generation of the two main types of granite are presented, incorporating many of the ideas of Nockolds and Read. The discussion focuses on high-level differentiation, partial melting and intrusion in an extensional regime, high-T/low-P metamorphism associated with the magmatism, and the relation of granite to the plate tectonic setting. The papers by Read (1948, 1949) and by Nockolds (1940) marked the end of one round and the beginning of a new one in the long but exhilarating debate on granite. In a Popperian sense the limitations of the old w e r e revealed with stark logic by the 'Chief Pontiff' Bowen, summarized in 'The granite problem and the method of multiple prejudices' (Bowen 1948). Here the 'soaks' or 'granitizers', including Read, were confronted with questions they could not answer. However, as is common in geological science, much of the old was modified during the dialectic and incorporated in the new synthesis, as shown below. So much so that even Eskola (1955) who had started as a convinced magmatist was later able to accept that 'metasomatic granitization' could be important in the formation of some granites. To most geologists today it would appear that the magmatists won, although even with geochemical arguments such as those of Nockolds lending crucial support, it is still not unanimous (see Mehnert 1987; Kresten 1988).

modern authorities (McBirney 1984, p. 278) find difficulty with the 'room problem', but cite assimilation, stoping or plastic deformation as able to account for the displacement necessary. In batholiths, assimilation and stoping does not create space and in any case are commonly confined to roof zones, while plastic deformation is often absent or confined to the margins of plutons (Pitcher et al. 1985). To some extent the problem of room is based on the inherent assumption that most orogenic granites are intruded under compression. Thus Clarke (1992, p.19) states 'within Phanerozoic continental crust, granitoids occur preferentially but not exclusively in compressional tectonic regimes (dynamic orogenic belts) as opposed to extensional or stable cratonic belts'. Although this is in part true, two examples of important granite types in mountain belts will be described in which extension, and hence a space-producing tectonic regime is the important ingredient in granite formation, intrusion and ascent. One of the most important aspects of the room problem is the more recent understanding of the shape of large batholiths and plutons (Brun et al. 1990). Most are thin bodies which do not have massive roots. Such geometries can be the result of intrusion into upwardly arching or 'sag'-type structures. Related is the significance and origin of 'ghost stratigraphy' where crustal rocks have apparently been granitized, according to some authorities, with imperceptible gradation between the two rock types. Thus, in Donegal, wedges of the Main Granite granitized the country rock to produce the superb 'ghost stratigraphy' (McBirney 1984 after Pitcher & Berger 1972). More likely, it seems that

The Granite problem The 'granite problem' started with Hutton and W e r n e r (see Pitcher 1993) and only the emphasis has changed with time. Four aspects of the problem, in part solved, will be considered here. The first is the 'room problem'. This was a major prop in the argument of the transformists who found accommodation of very large bodies of magma in the crust unacceptable without complementary, concomitant deformation in the country rocks, a feature which is certainly not evident in large Cordilleran batholiths (Pitcher et al. 1985). Even 221

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ATHERTON

the essential form of that granite is sheeted, and is the result of multiple injection in an extensional shear zone (see Hutton 1982), an explanation compatible with extension producing space. A second problem, which remains undecided in detail, was that between the experimentalists who maintained granite was produced from basalt by fractional crystallization (Bowen 1948) and those who considered granite to be a primary magma formed by melting metasedimentary material (Winkler 1965). Certainly some granites are the product of differentiation from basalt, i.e. some plagiogranites of ophiolitic suites (Pederson & Malpas 1984) and perhaps even some Cordilleran granites (Le Bel et al. 1985). However the disproportionately large volume of relatively acid rocks and lack of consanguineous intermediate and basic rocks in granitic provinces makes it more likely that partial melting of common crustal rocks is the main process in granite production. This includes both sedimentary and igneous, and even more particularly y o u n g basaltic material, or some composite. The third problem relates to source. Until recently it was a commonplace view that granites, being restricted almost entirely to the continents, were the product of partial melting of continental crust, specifically crust which was chemically evolved (Leake in Leake et al. 1980). McBirney (1984) even goes so far as to point out that sedimentary processes were so efficient at concentrating lithophile elements that it was logical to conclude 'many if not most granites were from crustal rocks already enriched in Si and AI and alkalis by surficial means' (McBirney p.378). Certainly some granites are, such as the 'S' types of Chappell & White whose characteristics are those of material which has passed through a sedimentary cycle (Chappell & White 1974). But most granites originate indirectly from the mantle or are melts showing a mixture of continental crust and mantle components. To complicate things, in some cases there may also be components from the slab and the sediment it may carry down with it. This may be considered a composite of mantle and crust, and as a result is difficult to decipher (see De Paolo 1981; Clarke 1992). The fourth problem relates to the diversification of compositions in granitic batholiths and, specifically, zoning in plutons. This has commonly been related to closed system in-situ fractional crystallization (Atherton 1981). Although this may be the case for certain plutons, isotopic evidence suggests many plutons are not simple closed systems (see Clarke 1992). In the course of this article, aspects of these problems which were considered by Read and Nockolds will be addressed and expanded on, taking onboard present day thinking. Discussion of their work in a modern context inevitably leads me to specific aspects of the subject I have found interesting. This is, therefore, not a comprehensive review, rather a working of the strands of thinking which I consider important and which were first voiced or articulated by Read and Nockolds. Much is left out, so I include in the references two general texts which put granites in a modern context (Clarke 1992; Pitcher 1993).

Read and 'Place in Plutonism' The two presidential addresses which Read produced for the anniversary meetings of the Geological Society of London in

March 1948 and April 1949 on Place and Time in Plutonism, were the last in a series of addresses where he meditated at length on current beliefs both 'mine and everyone else's about fundamental aspects of metamorphism' (Read 1948, p.156). These two addresses completed his meditations, so much so, that in the last one he could conclude that such 'meditations, commentaries, contemplations and such-like exercises begin to lose their savour' (Read 1949, p.151) and it was time to be refreshed 'at these bounteous springs of field w o r k . . , and by the discipline of laboratory study' (Read 1949, p.151). In this he thought he had the good wishes of those at least who had been at the end of 'these interminable and often pedagogic harangues' (Read 1949, p.151). The quotations indicate a major aspect of Read's writing. He was a superb communicator, often tongue in cheek. As Eskola said, 'I cannot remember having ever heard this man utter a sentence without witticism' (Eskola 1955). This is clear in all his writings, which have been widely read and enjoyed, stimulating students with their erudition and clarity. Although as Eskola said, in spite of being 'wrong in his chief argument' we 'have learned much of great value from these papers' (Eskola 1955, p.120). I intend to concentrate on his contributions of 1948 and 1949 in the light of present-day thinking and to point out some of the frailties of his arguments. However, it is important to emphasize at the beginning the use of the term plutonism. Read, following Lyell, considered this as 'all these operations that give rise to the plutonic rocks, these as I define them comprising the vast transitional assemblage of the metamorphic, migmatitic and granitic rocks' (Rea, 1948, p.156). He saw, quite rightly, the distinction made between regional and contact metamorphism as an academic convenience obscuring the integrating processes of granite production and emplacement at whatever level with the rock transformations appropriate to that setting. The connecting link, migmatite, being a mixture of 'granite however formed and metamorphic rock' (Read 1948, p.157). Had he used 'granite-like material' rather than 'granite', I think there would have been no dissension. He appreciated that the general validity of the plutonic series may not be acceptable to those 'who regard the igneous rocks as an inviolate self evident and self-consistent class' (Read 1948, p.157). Whatever the unity of the plutonic series as he saw it determined the content of the two addresses and was the major theme. Although he divided the addresses into two, one concerned with time, the other with place in plutonism, he considered these two aspects were inseparable and that rocks were not instant snapshots but 'would be cinema films if we had the right projector', each rock holding the history of that rock through time, the totality of the rocks of an area integrating the 'plutonic' history. Modern P - T - t studies are our attempts to do just that, and his much-used analogy with the movies suggests a clear appreciation of what modern metamorphic studies aims to do (see M. Brown, this volume). In the first few sections of 'place' in plutonism, he dealt with zones, facies, pressure and depth noting the early insistence on depth marking major zones, i.e. an upper fracture zone and a lower flowage zone through a transitional zone showing both types of behaviour (Van Hise 1904). This relates in modern thinking to an upper/lower division of the crust where rock behaviour can be simply and

GRANITE MAGMATISM perhaps incorrectly, described as brittle and ductile respectively, a point returned to later. Read was perhaps overly impressed by the French school with their 'ectinites' (isochemical metamorphic rocks) and migmatites (rocks which suffered an influx of 'alkaline feldspathic substances'), although he thought this classification was arbitrary, but did take on board the idea of 'a depth variation in the nature of granitization and granite contacts; contact metamorphism in the higher levels passed into regional metamorphism in the deeper' (Read 1948, p.159). In general, he followed Barrow (1893) favouring temperature as the important factor in metamorphism and granite formation, either causing zonations (Barrow 1893) or enhancing the metamorphism at depth or even at high level (Goldschmidt 1912; Eskola 1915; Turner 1933; Cloos & Hietanen 1941). Modern thinking is in line with this, emphasizing that orogeny, metamorphism and plutonism are all linked (see Brown, this volume). In discussing facies and equilibrium, Read was more impressed by the lack of equilibrium in rocks supplying a history, but was not too clear on the relation between facies, zones and equilibrium. Nonetheless these ideas foreshadowed the study of high-T, low-P metamorphism and of P - T - t paths which form an important part of modern studies of metamorphism and partial melting. Later sections in 'place' on original composition and equilibrium as well as open/closed systems tended to review these aspects, and where Read voiced an opinion, it was determined to some extent by his overall view of the unity of the plutonic series. Thus following Balk & Barth (1936), he thought there was a gradual and continuous change in composition through the series sediment --~ slate ~ schist----~ gneiss--~ augen gneiss--* intrusive granite. This is of course granitization, and as a major process, together with other related aspects such as 'fronts', lit-par-lit injection and large scale diffusion, could be considered perhaps dead ends and mainly products of general conjecture. In Read's conclusion to his 1948 address, he emphasized the unity of 'plutonic' activity as did many contemporary workers (see above), identifying a continuity from metamorphic through migmatitic rocks to granite. Read on 'Time in Plutonism' In the second address, Read again uses the idea of maps and rocks being films rather than stills but now emphasizing time which he attempts to integrate with deformation: 'la kinrmatique n'est que de la geomrtrie dans le temps' (Goguel 1943). Right at the beginning he asserts 'all granites belong to o n e series' (Read 1949, p.103). This is boldly stated, as is his second assertion, that the use of the term 'progressive' in metamorphic description is misleading and unwarranted. He emphasized an important feature of metamorphic rocks: that sequence may be one of place, n o t time. This is as important to emphasize now as it was then, as it conflicts with the orthodox thermodynamic interpretation of metamorphism (Turner 1981). In fact, Read visualized plutonic rocks showing evidence 'for their sojourn in a succession of thermodynamic envelopes constituting unified history of changing conditions throughout their life' (Read 1949, p.104). In this, his thinking was in tune with modern concepts of metamorphism and magmatism (for a collection of examples of P - T - t paths in classic areas see Daly et al. 1989). After a discussion of aspects not germane to this paper, he moved on to discuss time and

223

migma-magma (migma being a mixture of melt plus extraneous crystalline solid). Although Read appreciated that the idea of the production of migma-magma invoked the operation of a 'nebulous host' (Read 1949, p.132) of ichors, juices and emanations, which were viewed by many rather critically (Bailey 1958; Eskola 1955), he maintained that there were migmas and magmas, and that the latter could come from the former (cf. White & Chappell 1990). He further felt that the whole of plutonism should be discussed within the context of 'the time-relations of crystallization, deformation, granitization, migmatization, metamorphism, intrusion and orogeny' (Read 1949, p.132). Such ideas were not that new; Barrow (1912) had erected a complex sequence of deformations, magma intrusion and crystallization and metamorphism in the Dalradian. However, Read was concerned to emphasize the relation of magmatic phases and orogeny, particularly the relation of granites to the history of orogenesis, i.e. whether syntectonic or early or late, stressing that many granites are passive bodies which have been deformed (Read 1949, p.140). In the 1949 paper, Read presented the 'Granite Series', the culmination of half a dozen presidential addresses, in which he 'attempts to relate plutonic phenomena at the various levels of exposure and to give a unity to the processes of granitization, migmatization and metamorphism at depth and successively at higher positions and at later times' (Read 1949, p.143). Read had lyrically embraced the French concepts but had added time. Following the French, deep granites are mostly syntectonic, often impregnated and associated with migmatites while intrusive granites, at a higher level, were mostly post-tectonic, with narrow contacts forming intrusive sheets which could be displacive. However, he felt this essentially dual classification fades away when time is added to give a unified sequence of plutonic events linked with place. Here he says 'granitization needs no longer to be argued, our end terms are secure' (Read 1949, p.146). He did not feel it incumbent on him to specify any mechanism whether wet or dry, migmatitic or magmatic, suffice it to say that he envisaged 'a state of extreme chemical mobility, during which arises the plutonic series' (Read 1949, p. 147). In a sense, his caution over exact mechanisms was correct because it is only now with more recent quantitative work that we can begin to understand how 'granite' came into being in terms of critical melt fraction, melting conditions and transport (see review by Wickham 1987). Read initially envisaged that the 'series' began with coherent movement of the whole 'plutonic segment' (Read 1949, p.147), maybe with some differential movement, which was followed by the granitized core parting company with its envelope of 'metamorphite'. At this stage, intrusion can occur and it 'may vary from the nebulous migma to the equally nebulous magma' (Read 1949, p.147). He envisaged that this material moved to lower pressure (extensional zones) or was pressed into (and through) compressional zones. Some straining off of the fluids, as well as solution of more readily melted material, could change composition on ascent. Finally, the migma-magma would free itself of its plutonic associates and form 'cross-cutting diapir granites' (Read 1949, p.147). Here emplacement would be in extensional systems involving faulting and doming, as well as 'pushing and shoving' along weak belts. The Granite Series could be diagrammatically viewed on

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a place-time graph (Fig. 1), and a classic example was the Hercynian Belt of western Europe where the Cornubian Batholith represented the high level, divorced (dead) magmas, while the Massif Central with its dominant migmatites and anatectites represented the source. This large scale recognition of granites connected in time and space (over 600 miles) was Read at his boldest and most imaginative.

The validity of the 'Granite Series' Without begging the question of classification, we now see Read was correct in that the Granite Series approximates to the evolution of one volumetrically minor but important type of granite, the 'S' types of Chappell & White (1974), in which the initial mush or mash is a melt phase plus restite phases (residual source material). However, we have no need now to invoke ichors and such like. For granitization we now say partial melting with or without vapour. Nonetheless, descriptions of such granites paraphrase Read's description briefly described earlier (and see Fig. 1), although he did not dwell on restite phases. The idea 'per migma ad magma' was essentially field based, and now we can use other means to argue through the problem of how granites of this type formed. Much discussion on this subject

has revolved round granites such as Cooma in the Lachlan Fold Belt in SE Australia (White & Chappell 1990). Here there is a rapid change from migmatite with coherent structures to a mobile mixture of melt plus unconnected inclusions (---restite?). This occurs at about 30-40% melt fraction. This important idea of critical melt fraction relates to a series of melting experiments such as those by Van der Molen & Paterson (1979) who found a sudden change in differential stress during deformation of partly molten granites between 26-36% melt. In granitic systems, melt percentages within the range 30-50 may allow mobilization of zones hundreds of metres in extent, convection ultimately producing homogeneous compositions (Wickham 1987). Somewhat similar to the Cooma granite is the Trois Seigneurs massif in the Pyrenees (Wickham 1987). Simplified cross-sections are shown for comparison in Fig. 2. Here metamorphic rocks pass into migmatitic biotite sillimanite gneisses then, always gradationally, into a biotite granite-quartz diorite body ('deep biotite granite'). Compositions and textures of the latter are heterogeneous, and local meta-sedimentary enclaves are extremely common. The pluton is an 'S'-type with 25-40% biotite and some cordierite. Within t h e metamorphic sequence there are pods of muscovite granite l m - l k m across. They are SiO2-rich

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(leucogranite) and do not contain cordierite, although they are peraluminous. Muscovite is a retrogressive phase while tourmaline can reach 10% by volume. Compositionally they are homogeneous and are considered to be mobilized teucosome, as among other things (according to Wickham) they have identical compositions and, in contrast to Cooma, are unmodified melt compositions. Melting at 670-700 °C, 3.0-4.0 kbar under water-rich conditions was followed by segregation, compaction or movement of melt into fractures. When large enough, these bodies intruded the overlying metasediments, but were quickly halted as they were water-saturated. The 'deep biotite granite', was, according to Wickham, generated by partial melting and homogenization of the same source material and both these bodies were considered to form in-situ. However, the 'biotite granite' has a compositional variation that clearly indicates a more primitive magmatic component is also present (see ~80 data, Wickham & Taylor 1985). Together with the presence of quartz diorite, it seems there is no simple correlation of the leucogranite and the deep 'granite' as implied in the petrogenetic model (Wickham, p.163). Commonly, there are significant differences between leucosome chemistry and mineralogy and that of an apparently associated granitic body. For example, Ca-rich cores in plagioclase in the granite are frequently not present in the restite, and compositions of the leucosomes are not those of predicted melts, although they could have been modified by back-reactions involving H20 (White & ChappeU 1990). It appears that the hiatus between leucosome and 'granite' bodies remains, and that leucosome arguments are a cul-de-sac (see Leake 1983). We still need to find the true route between melting and massive granite bodies; it emphasizes our lack of thorough understanding of segregation and plumbing systems. We can be more specific than Read, however, and divide the latter into three: (a) zone of partial melting, segregation and collection, (b)

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transport zone and (c) an emplacement zone, where freezing occurs (Fig. 3). With regard to (a), we can be certain that most if not all 'granites' are crustally derived as partial melts either of basaltic material or of sedimentary material, or of some mixture (see Leake 1990; De Paolo 1981). To date the segregation mechanism of granitic-like material in the crust has focused on migmatites (see Atherton & Gribble 1983 and Ashworth 1985) using field, geochemical and experimental studies. As important is an understanding of the mechanical properties of rock melt which, with increasing melt proportion, changes from linearly-elastic to elastic plastic to viscous, which will be reflected in the flow behaviour (Shaw 1965; Murase & McBirney 1973; Shaw 1980 and see Wickham 1987 for a review of physical and related properties). Flow behaviour will therefore vary with melt fraction. For low melt fractions as seen in migmatites, a variety of possible segregation mechanisms can be considered based on melt fraction. Compaction may be important as is extensional fracturing, filter pressing capillary and small volume buoyant ascent (see Wickham 1987) up to the metre scale, but these cannot explain larger bodies. Furthermore, hydrous low melt fractions would crystallize rapidly on decompression. The generation of high melt-fraction material (>30%) is poorly understood but must involve pervasive melting followed by melt separation and collection which may be complete or partial. As viscosity approaches the liquid state (Shaw 1965), convection may produce homogeneous distributions within large bodies. This lack of behavioural continuity between high melt fraction and low melt fraction material is general, and most granitic rocks cannot be linked to a source at the intrusion level (Clemens 1984) nor can migmatites be related to particular plutons. Thus only 0.1% of 'S' type granites in the

226

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A modern Readean analogy In a very recent example published in the Journal o f the Geological Society, D'Lemos et al. (1992) discuss granite magma generation, ascent and emplacement within a

Fig. 3. Granitic plumbing system in Cordilleran settings, showing the three important zones: source and MASH zone; transport and MAGIC zone; intrusion and ACID zone. Note the narrow conduit (dyke-like) through which magmas arrive at the intrusion level and the thin form of the batholith, here exaggerated. Source and other zones may move closer or overlap in different settings.

transpressional orogen. With the exception of a few technical terms not in use in 1948/49, the paper could and might well have been written by Read. Furthermore it is concerned with the Armorican rocks of northwest France which were used by Read in his type example. It indicates the powerful albeit unconscious influence of his thinking, even today, strongly emphasizing aspects of time and place. The Late Precambrian (Cadomian) belt of northwest France in the region of St Male is made up of mylonitized and folded migmatites and deformed, anatectic granites interleaved within medium-grade Brioverian rocks (Fig. 1). To the east, the Mancellian region is made up of greenschist Brioverian facies intruded by undeformed (late), long, linear ( E - W ) granitic plutons (Fig. 1), with mariolitic cavities and well defined contact aureoles. Regional metamorphic grade decreases eastward. The whole suggests a deep source region for the St Male area and a shallower regime to the east, the present juxtaposition of contrasting crustal levels being the result of regional transpression. Similar chemical and isotopic data convinced D'Lemos et al. that the two regions were different crustal levels but belonged to the same tectonic unit. The familiar story (as Read might have told it) is of melting, partial melt separation and convection, followed by transport through ductile diatexite and metatexite, to form the veins and diapiric kilometre-sized bodies of the St Male region (Fig. 1). P - T estimates indicate a depth of 3.8km and temperatures between 650 and 750°C. Anatexis was the result of structural inversion of the basin, followed by high temperature metamorphism consequent on crustal thickening. The Mancellian granites to the east are biotite granodiorites with minor cordierite and muscovite, and are 'larger volumes of homogenized magma represented by the

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strike-slip was initiated along zones, which were thermally softened by anatexis. Thus regional tectonics and granite formation may be 'inextricably linked processes' (D'Lemos et al. 1992, p.490), and the associations of shear zones and granites is no surprise; even more so with the demise of diapirism as a major method of magma ascent through the crust. Indeed, many workers have considered long lineaments and major fault structures may dissect the whole crust, cause melting, as well as being magma conduits (Pitcher 1978; Leake 1983). The generalized model for the Cadomian belt shown in Fig. 1 follows all major aspects of Read's ideas on the granite series, particularly those on space and time. Perhaps like all good ideas there are problems to this neat Readean picture. Thus Power (1993) commented that there are in fact statistical differences between the two granite sets and the case for geochemical similarity should be rejected. However it does not preclude a similar petrogenesis, although Power's analysis demands more careful consideration of the petrogenetic significance of these differences. Seemingly, the lack of continuity in the 'Granite Series' still tantalizingly exists.

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ASTHENOSPHERE

b Fig. 4. Cartoons showing the tectonic setting for: (a) The Hercynian high-T/low-P metamorphism in the Pyrenees near the Trois Seigneurs Massif. Deep seawater circulation occurs to the bottom of Palaeozoic (PZ) basin, with anatexis at 700 °C and 10-12 km depth. Hot, upwelling asthenosphere heats the lower crust sufficiently to produce large granodiorite magma bodies (modified after Wickham & Oxburgh 1985). (b) The Cretaceous-Tertiary marginal basin and Coastal Batholith, Peru. Splitting of the crust, and form of the basin and Batholith are taken from geophysical evidence (Jones 1981) and evidence for lithosphere stretching and basin content are given in Atherton & Webb (1989) and Atherton (1990). v, volcanogenic rocks.

antectic granites of the St Malo region' (D'Lemos et al. 1992, p.488). The upward transport in the St Malo belt was considered by D'Lemos et al. to be from transpressional to transtensional zones, giving rise to apparent flower structures. Magma entering ductile 'extensional jogs' within shear zones (compare Read 1949, who had envisaged magma moving into extensional zones from compressional zones). In contrast, the Mancellian granites were emplaced into brittle-deforming upper crust within structurally controlled extensional features. The two zones were linked by a sigmoidal anastomosing strike-slip shear zone/fault system from St Malo to the elongate B o n n e m a i n / Vires-Carolles granites (Fig. 1). This system controlled migration, ascent and emplacement of the granitic magmas. Furthermore, the close temporal relation between peak anatexis and regional strike-slip displacements indicates

There were two areas which today we think are important and in which Read was mistaken. Firstly, he thought 'granites to be for the most part genetically unconnected to basic magma' (Read 1949, p.149) so 'plutonic granitic rocks are not near blood-relations of the volcanic, basaltic rocks and this opinion is by now commonly accepted by all reasonable men' (Read 1949, p.104). Secondly, that 'though there may be granites and granites, most of them are of one kind and all of them may likely be of one connected origin' (Read 1944, p.90). This quote is reiterated at the end of his 1949 paper where he presented the Granite Series of the Hercynian belt of western Europe as the archetypal example. Thus he envisaged his Granite Series was sufficient to explain all granites, and such granites apparently had their sources in metamorphosed/migmatized zones of sialic material. It is now clear that the most voluminous granites e.g. Cordilleran granites, do not belong to his Granite Series and furthermore these are related to basalt. They are commonly the product of partial melting of dominantly basaltic material or less commonly the differentiation of basalt.

The application of geochemistry In essence Read was against the experimentalists who supported the proposition that basalt was primary and 'granite' was produced by fractional crystallization, i.e. it was the residual liquid. In contrast to the beliefs of the Read school, and indeed many continental schools (see Read 1944, 1948, 1949), this interpretation was supported by physico-chemical principles and data. Powerful support was given to the Bowen model by Nockolds and co-workers in studies of element distributions in granitic rocks. However, Bowen's thesis that differentiation from basalt produced granite may well work for small bodies, but it is apparent that it does not explain large batholiths. Nockolds' paper on Garabal Hill published by the Geological Society in 1940 has served for many as a model explanation of the variation in basic-acid complexes and represented a confirmation of the

228

M.P.

ATHERTON

Bowen model. This has particular significance here in that although Garabal Hill and the Highland granites as a whole were thought by Read to belong to his Granite Series, we now see that Garabal Hill represents the other much more abundant class of granites, that is, those whose source is basaltic or igneous crust (i.e. the Cordilleran or T types of White & Chappell 1983). It is in these rocks that differentiation is common and extensive, as well as mixing and assimilation. Interestingly, the earliest account of the Garabal Hill complex by Dakyns & Teall (1892) suggested that the rocks formed by differentiation at depth! Two other scenarios had been put forward, one that the ultrabasic and acid magmas were separate, and the other, which Nockolds had originally espoused, was that the acid magma was contaminated by basic material and the former derived from an original basic magma by 'contrasted' differentiation. All very modernsounding models. The rocks of the complex vary from ultrabasic (peridotite, pyroxenite, hornblendite) through gabbro to pyroxene-mica diorite and various appinitic, fine-grained diorite to medium-grained granodiorites ending with a porphyritic granodiorite which forms the bulk of the complex. The granodiorite contains dark fine-grained basic xenoliths with apparent K-feldspar phenocrysts and small rounded quartz crystals rimmed by dark minerals. In modern parlance these are almost certainly incorporated phenocrysts (Vernon 1986), the quartz forming 'ocelli' structures, a clear indication of magma mingling accompanied by some reaction. Nockolds presented a table showing appearance and disappearance of minerals---which conforms to and indeed was used to confirm Bowen's reaction series. Major element variation diagrams were an important and critical lynchpin in his argument and, in order to avoid conclusions on the basis of a single complex, he cannily also plotted Caledonian and Lower ORS calc-alkali rocks of W Scotland including volcanic rocks. He noted a 'perfectly continuous and regular variation to the more acid compositions' (1940, p.494) and a scatter in the more basic rocks. Notably the continuous trends are almost linear and follow those of the lavas. He considered 'gradational differentiation' at depth, with the pyroxene-mica diorite, because of its position, as the most basic rock of the continuous series, to be the parent. The pyroxene-mica diorite compared well to andesite and mica diorites from elsewhere in W Scotland and this satisfied Nockolds it was a definite magma type. It is certainly a common type and early in any complex. This being so, he considered the continuous variation shown by the rock compositions represented stages in the liquid line of descent while compositions more basic than the pyroxene mica diorite represent accumulative types. He anticipated the recent work on granite mineral assemblages, and considered many rocks to be mixtures of crystallized parent liquid plus accumulated crystals (compare McCarthy & Hasty 1976). Furthermore he considered that the early stages of differentiation might involve gravity separation of crystals, e.g. olivine, but later at more acid compositions magma can move relative to crystals in what he describes as filtration differentiation. The latter is compatible with today's thinking where most workers would invoke some sort of boundary layer differentiation (Bateman & Chappell 1979; Atherton 1981; Sawka et al. 1990) or convective fractionation (Sparks et al. 1985) to explain the evolution in

composition of the magma which gives rise to zoned intrusions. However, even with the advent and application of fluid dynamic theory, there is a some debate about how liquids evolve and separate. Thus Mahood & Carnejo (1992) describe leucogranite layers and dykes which are considered to be trapped ascending differentiated liquids. Crystallization across the deep levels of the magma chamber, produced residual melt which segregated by flowage upwards into tensional features of various kinds including fractures, which may be favourable for separation of small melt fractions. Crystallization on the floor of the magma chamber is likely to be an important process, as the adiabat in a convecting magma chamber is steeper than the liquidi (McBirney & Noyes 1979; Mahood & Carnejo 1992), and marginal wall crystallization has not been convincingly demonstrated in the field. Furthermore, present research by the author shows that most batholithic intrusions are thin, flat bodies with aspect ratios (thickness to width) up to 1:15. In such a scenario, sidewall differentiation is likely to be of minor consequence. Although crystal settling was considered a possibility by Nockolds, he clearly appreciated other processes were possible. Nowadays settling is considered unlikely (see for example Sparks et al. 1985) and we see no evidence of it in the field. Layering in granite is rare and when present e.g. Hinchinbrook (Stephenson 1990) the subhorizontal layering is clearly produced by unidirectional crystallization downwards. Thus buoyant and residual liquid removal from the crystallizing phases (Rice 1981) in a variety of scenarios seems probable. Another important argument which Nockolds elucidated, related to the hybrids of the basic and acid components of the differentiation series that may also lie on the liquid line of descent. To some extent, Nockolds anticipated the recent revival of interest in enclaves and the relation to hybridization, e.g. Vernon (1983); Didier & Barberin (1991). However, the role of synplutonic dykes, which I think account for most of the fine-grained mafic enclaves in Cordilleran high level granites (see also Pitcher & Bussell 1985), was not appreciated at that time, neither was their role in the initial injection and mixing and later convective mixing. This appreciation of the interaction of contemporaneous basic and acid magmas is also seen in the role of basic magma as a heat source (Huppert & Sparks 1988) and volatile supply (Whitney 1988). The basic magmas are clearly of mantle origin and may interact with crustally derived anatectic silicic melts to form hybrid calc-alkali rocks, ultimately, according to some petrologists, producing very large bodies of tonalite (Castro et al. 1990). Nockolds' detailed and well written paper was read in 1940, some eight years before Read's penultimate address. It reflects the powerful influence of Bowen (1928) and the application of experimental phase diagrams, which was in stark contrast to the Read school who adhered to the less rigid, albeit more imaginative concept of granitization and an upper crust enveloped in ichors and such like nebulous notions, based largely on field and petrographic work. Today it is clear that most igneous petrologists accept fractional crystallization as an important, perhaps dominant process, albeit modified by magma mixing and contamination. In Fig. 5, Nockolds' data for Garabal Hill are plotted on an AFM diagram and compared to that for the Coastal Batholith, Peru. The gabbros in Peru are chemically

GRANITE MAGMATISM

229

F

6arabal

F

Hill / ~ /

/

/

oGranitorocks id

~

\

.

/

// ~

Coastal

Batholith,

Peru

\

O ~1~:~~ OUltramafir°cks c / r~r~c~q~°-~

/

"0o.:..

A

IVl

A

M

Fig. 5. AFM diagrams for Garabal Hill and the Coastal Batholith showing a calc-alkaline trend for both. Note the ultramafic rocks from Garabal Hill have no equivalents in the Coastal Batholith (data from Nockolds 1940 and Atherton et al. 1979). unrelated to the Batholith (Atherton et al. 1979) and anyway are earlier. They are not cumulates related to any exposed acid rocks. The Garabal Hill sequence is very similar except it lies lower in the diagram (greater arc maturity?) with the peridotites and pyroxenites lying in a separate field along the F - M join. The latter have no equivalents in Peru. The trend from diorite/tonalite to granite in the Peru rocks has been interpreted to be the result of high level fractionation (Atherton & Sanderson 1985), following Nockolds' model for the acid rocks of Garabal Hill. Plots of extract polygons using Nockolds mineral data, analyses and modes (Fig. 6) are revealing. On all plots the liquid lineage defined by Nockolds remains intact while the basic rocks, apart from one or two grossly 25 - - -

-

15 CaO 10

0

10

20

30

40

Fig. 6. CaO v. MgO diagram with extract polygon for the rocks of Garabal Hill. Plagioclase-olivine-clinopyroxene extract polygon shown shaded. In the more gabbroic rocks, orthopyroxene is common so an extract polygon for plagioclase-clinopyroxeneorthopyroxene is shown dashed, hb is the hornblendite which is close to the hornblende composition; hyg is the hypersthene gabbro mentioned in the text.

contaminated types, lie in the mineral field indicating they could be cumulitic, as envisaged by Nockolds. However, most lie below the 'liquid line' and its extension, indicating they are variably olivine and/or orthopyroxene enriched. This particularly applies to the gabbros, apart from one hypersthene gabbro which could be a cumulate (Fig. 6) with 41% plagioclase, 30% augite (+little hornblende), 23% orthopyroxene, 2.8% olivine and 2% ore (modal data from Nockolds). Thus in the Garabal Hill complex, the connection between the basic and acid rocks is more complicated than Nockolds perceived, and associated basic rocks are not cumulates. They may be contemporaneous or slightly earlier than the acid rocks as in the Coastal Batholith O f Peru, emphasizing the acid/basic association common to Cordilleran plutonism (Atherton 1990). Recent work on the chemistry of late Caledonian granitoid plutons show they have mantle and crustal components. Garabal Hill, for example, has no inherited zircons, and the end data indicate a largely mantle-derived origin, the Cr-and Ni-rich pyroxene-mica diorites being the most primitive (Stephens & Halliday 1984) with little crustal component. Summerhayes' (1966) earlier work indicates the isotopic coherence of all rock types in the complex, and he concluded that the source of the acid rocks was probably basaltic in composition, cf. Nockolds (1940). In short, the Garabal Hill work emphasizes four important aspects of diorite-tonalite-granodiorite-granite sequences, much of which Nockolds (1940) anticipated. (1) The primitive liquids are commonly quite basic, i.e. 55% SiO2, the source almost certainly was basaltic (cf. Nockolds 1940). (2) Crystal fractionation accounts for rocks with compositions >55% SiO/, while some of the more basic rocks may be viewed as partly cumulate (gabbros) but many, including the ultra-basic rocks (i.e. peridotites, diorites, pyroxenites) are unrelated directly to the acid rocks and are olivine- and/or orthopyroxene-enriched. (3) The association of peridotite with basalt and the lack of a recognizable sialic component (e.g. Dalradian, Moine

230

M.P. ATHERTON

or Lewisian) suggests the source was located at the base of the crust, probably within the crust/mantle boundary zone, and was essentially of mantle material (Stephens & Halliday 1984). (4) Intrusion of the late orogenic Garabal Hill Complex took place along a major E-W lineament considered to extend to mantle depths. The association of granite magmatism with deep faults, reflected the change from a ductile compressional to a brittle extensional regime in which melting and intrusion took place (Watson 1984; Atherton & Plant 1985).

Senal Blanca i Linga/ ]

]

t Santa Rosa

/

.uaora

Rb/Sr 1.o

Granites and linear structures Of the points listed above, one is of particular significance. Granites are frequently intruded into various vertical and horizontal extensional features. Thus Leake (1990) stresses the importance of faulting and strike-slip in the British Tertiary Province and Irish Caledonides, suggesting that major crustal fracturing assists in triggering magma melting, forming conduits and freezing sites, and that 'major batholiths probably occupy holes created by pulling apart the crust' (1990, p.579). Such arguments have of course been put forward by Pitcher (1978) for the Coastal Batholith of Peru, by Hutton (1982) for the Main Donegal granite, by Petford & Atherton (1992) for the Peruvian Cordillera Batholith and by many others including Watson (1984), Atherton & Plant (1985), Harrison et al. (1990) and Schmidt et al. (1990). Lameyre (1988) went so far as to suggest that structure was probably more important than plate tectonic setting.

The Coastal Batholith, Peru: an example of Cordilleran plutonism The Coastal Batholith of Peru, which is similar in some respects to Garabal Hill is a good example of a Cordilleran plutonism. Here a recent model for its genesis is compared to the crustal melting model of Wickham & Oxburgh (1985) for the Trois Seigneurs massif in the Pyrenees. Both models have aspects in common combining some of the important features and seminal ideas of Read and Nockolds: specifically the relation of metamorphism, extension via major plate movements and orogeny, as well as the importance of source and differentiation, to granite genesis and intrusion. The formation of the Coastal Batholith of Peru (Atherton 1990) relates to a major cycle of crustal growth which started with (1) extension at right angles to the coast (Andean), then (2) subsidence and formation of the Albian marginal basin as the continental lip split or rifted, (3) dyke intrusion with spreading, producing high-T metamorphism and a thermal high under the spreading centre, (4) basin filling, (5) gabbro intrusion, (6) mild compression with inversion followed by cratonization. Metamorphic facies analyses of the basinal volcanic fill indicate local thermal gradients of up to 300 °C km-' as well as a downward increase in grade (Aguirre et al. 1978; Offler et al. 1980). The metamorphism is similar to that seen in rifting environments e.g. in Iceland, or hydrothermal ocean-floor metamorphism at a spreading centre, and is due to basaltic magma at high levels in the crust. Spreading/subsidence with rifting (crustal accretion) was modelled quantitatively by Palmason (1986). The thermal,

~0

55

60

65

70

75

SiO 2

Fig. 7. Rb/Sr v. SiO2 for some superunits of the Coastal Batholith, Peru showing the characteristic form due to fractional crystallization with plagioclase as the major precipitating felsic phase.

spreading/subsidence parameters for the marginal basin (Fig. 4b) are similar to those used by Palmason indicating that melting of hydrous basalt occurs at very shallow depths (5-7km) and that the products are tonalite, granodiorite, granite depending on the temperature. Modelling the melting of the lowest enriched basinal basalts reproduced the chemistry of the batholith rocks (Atherton 1990). In this model, tonalite-granodiorite-granite are produced immediately after basin inversion and cratonization from newly accreted hydrous basaltic crust. Plumbing was controlled by a mega-lineament within the marginal basin that formed on extension related to dextral strike-slip parallel to the present coast (Atherton & Plant 1985). A similar but larger rifting/spreading structure is seen in central Chile (Levi & Aguirre 1981) where Jurassic to Palaeogene rocks are symmetrically exposed along an Andean trend and plutons intruded axially within this mega-structure, suggesting the model for Peru may be extended to other parts of the Andes. In the Coastal Batholith of Peru, the dominant rock-type is tonalite (c. 60% SiO2), and the more evolved rocks, culminating in granite (sensustricto) (<10%), formed by high-level differentiation (Atherton et al. 1979; Atherton & Sanderson 1985; Fig. 7). The most basic rocks are diorites with 57% SiOz similar to, but slightly more siliceous than the primitive pyroxene-mica diorite of Garabal Hill (Nockolds 1940; Fig. 8). Fractionation in all the super-units is dominated by plagioclase, hornblende and clinopyroxene, with accessory minerals becoming important at high-SiO2 levels (general ranges from trace element modelling (Atherton & Sanderson 1985) are: 45-60% plagioclase, 10-45% clinopyroxene, 20-45% hornblende, 10-20% biotite with minor zircon, allanite and apatite). The values are similar to those of the hypersthene gabbro 'cumulate' from Garabal Hill mentioned earlier, but the assemblage is more hydrous. With these phases crystallizing, near linear trends will be produced on differentiation (cf. Nockolds) as one would expect from the AFM plot i.e. M/FM does not change radically throughout the differentiation (Fig. 8). The rocks are juvenile with positive end values and Sri near 0.704. Generally the crustal component is small or

GRANITE MAGMATISM

these differences relate to local conditions of extension, and therefore subsidence, as well as the presence of adjacent continent above sea level. The basin in Peru is at the edge of the continent which was below sea level, while the basins in the Pyrenees were intercontinental, but both are the product of rifting/spreading systems and strike-slip, presumably related to major plate movements. Both have been compared to a modern analogue e.g. Gulf of California and the Salton Sea (Wickham & Oxburgh 1986; Atherton & Webb 1989), where extensional strike-slip is associated with a spreading system. In such situations thermal anomalies may well be extreme and give rise to high temperature/low pressure metamorphism with very hot middle and lower crust.

ml

Lima s e g m e n t

m

mmm

I ,,,,,, ° MgO °

s I m

~mO o Nmln o

1

55

60

70

65

i

i

75

80

Si02

Experimental work and granite source

Lima s e g m e n t m ~

mm •

-

CaO

4

°o,~k

0 55

60

65

231

70

75

80

Fig. 8. CaO and MgO v . S i O 2 with percent variation diagrams for granitic rocks from the Lima Segment of the Coastal Batholith, Peru. Filled circles are for rocks of the major superunit of that segment (Santa Rosa; Atherton et al. 1979). Note the coherent linear trends typical of fractional crystallization unmodified by marked crustal contamination. These are very similar to the plots of Nockolds (1940).

negligible, and when present is associated with high level fluid interactions (Beckinsale et al. 1985; Mukasa & Tilton 1984). In Peru, the crustal structure based on geophysical sections is well defined (Jones 1981; Couch et al. 1981) with major crustal rifting and 3.0 gm cm -3 density material in a high-level arch within the continental lip (Fig. 4). The association of rifting in continental settings with exceptionally high thermal gradients and granitic rocks was noted earlier in the Trois Seigneurs massif (Wickham & Oxburgh 1985). In that case it involved migmatite, S-type granite and regional ductile subhorizontal extension; Read (1948) also related large scale flat schistosities to regional extension. Although the rifting, metamorphic gradients and flat lying isograds are similar in both examples, basaltic magma was responsible for the thermal structure in Peru while granodioritic magmas are thought to be responsible in the Pyrenees (Wickham & Oxburgh 1985). Further differences are in the rift-basin fill, which was entirely volcanogenic in Peru and mainly sedimentary in the Pyrenees. However,

One aspect which neither Nockolds nor Read could consider, and which is of critical importance in defining source, is the experimental information relating to likely sources and conditions of melting (e.g. Wyllie 1983). On the whole, most modern workers agree that crustal melting takes place in fluid-absent conditions involving the breakdown of hydrous silicates (Burnham 1979; Clemens & Vielzeuf 1987). However in the Pyrenean example discussed here, because of the large bulk of leucogranite and the isotopic evidence, large scale influx of groundwater during metamorphism was invoked (Wickham 1987). Near saturated acidic melts unfortunately do not travel far as they quickly cross their solidus on ascent, so it seems unlikely that large bodies of leucogranite divorced from source formed in this way. Fluid-absent melting is fairly well understood for pelitic compositions (Thompson 1982; Powell 1983; Vielzeuf & Holloway 1988) and has been applied to high grade terrains. However, the conditions for fluid-absent melting in marie compositions has not been so well studied, but apparently occurs over a wide temperature range: 850-1000°C, and melts vary from granitic in composition at lower temperatures to tonalitic at higher temperatures (Rushmer 1991). The high temperatures present at s h a l l o w depths during rifting are sufficient to melt hydrated basalt producing tonalitic melts e.g. Coastal Batholith, Peru while trondhjemitic melts characterize the strike-slip transtensional environment over hot, inboard, thickened crust e.g. Cordillera Blanca, Peru (Atherton & Petford 1993). Experimental work is consistent with melting producing 'granite' in extensional environments, either major rifts, strike-slip or shear zones in thin or thick crust and with the associated high-temperature low-pressure metamorphism marking hot zones in the continental crust below which lithospheric thinning may be extensive. This conjunction of high temperature metamorphism, rifting (with horizontal extension), volcanism and granite formation (and high level differentiation) often related to major plate realignment and uplift has a distinctly Readian look to it. We have yet to understand fully processes in the melt zone and the fate of the accompanying restite; these are topics for future research.

Where are we going? In spite of the fact that the contest--tranformist v. magmatist--is more or less settled, major problems highlighted in it are still with us. Before briefly discussing

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M.P.

ATHERTON

these mention should be made of the new techniques, not available in the 1940s, which are and will be important in solving these problems. The ion-probe and similar grain-scale techniques now allow spatial isotopic and trace analysis e.g. microscale isotopic analysis of zircon crystals using SHRIMP (sensitive high resolution ion microprobe) on several generations each with discordance, record provenance, age and metamorphic history of a single population (Williams 1992). Here is the tool to deconvolute mixed isotopic systems relating to the complex history of granite and the crust. These will allow proper consideration of episodicity in plutonic magmatism, exact timing of acid/basic magmatism and perhaps a complete timebase to relate to major plate movements. It should also help us to understand the nature and age of the deep crust and the role of the igneous underplate e.g. Bega Batholith (Williams 1992), which is one of the major problems defying an understanding of the granite genesis. There are three related problems. (1) Is MASH (melting, assimilation, storage, homogenization, Hildreth & Moorbath 1988) an important process in plutonic magmatism or merely an artifice hiding our incomprehension of the actual processes going on in the lower crust? (2) Do trondhjemities over deep crust really indicate delamination (Kay & Kay 1991), and what is the significance on the continent scale? (3) Where and what are the P - T conditions of those magma chambers at depth where significant early differentiation apparently occurs e.g. Pitcher (1993)? Answers to these questions will need to be integrated more fully with structural studies and deep geophysics. In chemical modelling of high-level processes, it is important to determine 3D shapes of plutons and to relate this to plumbing systems (Vigneresse 1990). In batholiths, present research by the author shows that magma chambers with aspect ratios (width to thickness) up to 15 will have different convection and crystallization regimes than deep carrot or cylindrical-shaped chambers which may lie beneath calderas. Recently we have been made aware that emplacement structures provide little information on magma ascent mechanisms. It is also apparent that in a simple way the 'space problem' so beloved of the transformists, may be a chimera and Read, paradoxically, was right in that magma fills 'holes' i.e. tectonically created cavities abetted by internal magma buoyancy (Hutton 1988). However he was, along with many others, wrong in considering crustal level and time as the main factors in emplacement behaviour i.e. 'permitted' and 'forceful' intrusion occurring at different levels (equivalent to a brittle/ductile transition with depth). Ironically, in Donegal where Read ended his field career, granites showing both types of behaviour were intruded at the same depth (Naggar & Atherton 1970) and at the same time. Modern studies of granites must include metamorphic and structural analysis. All granites have structure often related to the plumbing system, which integrates source, ascent and high level processes. Turning to chemistry, it is obvious that micro-probe analysis of accessory and major phases for trace elements with particular attention to heterogeneities and zoning will be important (together with textural studies) in elucidating the chemistry of evolving liquids and defining possible cumulates. With analysis of lower crustal minerals from xenoliths and zircons from all these components, it should be possible to be more precise about source and evolution of the granite system e.g. Miller et al. (1992). Chemical

modelling is already at a new stage using trace element data across crystals and in fluid inclusions, and will include new KD data specifically determined for granitic systems. Values for rare earth (REE) and large ion lithophile (LIL) elements, are at present poorly defined and have commonly been determined from volcanic systems of doubtful comparability. Future modelling with these new data will be process-orientated, although it is apparent from earlier comments that the physical processes producing chemical diversity in rocks is as yet not fully understood. Neither has this been linked to texture development, although recent 3D work on the generation and evolution of a crystal framework texture and its subsequent infilling has been started (Bryon et al. pets. comm.). This approach must be integrated with the phase petrology and chemical evolution and deformation on a microscale. Study of 3D textures in granitic rocks should also produce a proper classification of granitic textures similar to that devised for basic rocks. This should generate textural criteria for the recognition of 'cumulate' and possible restite (sensu lato). Hand in hand with the textural and detailed modelling is the need for more experimental work on kinetics e.g. Johannes & Holtz (1992) and on possible sources and melting conditions, e.g. Rapp et al. (1991) and Rushmer (1991). Such work will give residue, melt and mineral compositions in detail as well as the relevant phase relations. It should be possible to explain why tonalites which require outside heat to form within continental crust dominate in the crust while granite (ss), which may be the normal metamorphic product of melting of lower crust, is volumetrically insignificant. The discussion on possible sources brings us to classification. Modal and chemical classifications without genetic implications have been the mainstay in granite description. They will remain so and perhaps increase in use while those relating to source or tectonic environment are on less firm ground. Thus the classification of Chappell & White (1974) which relates granites to their source has had a shifting base with time which looks unlikely to stabilize. 'S' type granites recently redefined are almost continent specific (White et al. 1986), while the proliferation into M (mantle), l-Cordilleran (igneous, infracrustal), A (anorogenic) and now even C-type (crustal) seem to me to negate the whole reason for setting up the system in the first place. Furthermore, there is no provision for transitional types which contradict the gradational variation seen in granite suites across the world (Lameyre & Bowden 1982). Although the T , 'S' typology can be very useful at the beginning of granite studies and has inspired a vast reaction (a good thing), as petrogenetic shorthand it tends to a rigidity in attitude and sterility in use. Better would be the idea of 'series' or 'lineages' which may be loosely grouped as propounded by Lameyre & Bowden (1982) with the emphasis on the integration of modal and chemical data with the phase petrology and textures (Atherton 1988). Granites (ss) in such a system with very similar compositions can clearly be seen to be the product of a variety of lineages, which themselves mirror the different sources but also emphasize the continuity of the granite system. With regard to granite typologies related to tectonic setting, I cannot follow Pitcher in concluding that 'the different tectonic regimes will provide different source rock assemblages' (Pitcher 1987). Certainly some granites appear to relate to a specific structural environment, but the

GRANITE

chemistry of the source is often the result of a long history of mantle and crust enrichments/depletions which may have no relation to the geotectonic setting prevailing on magma genesis. Finally, there is the association of basins, granites and thermal highs. Many granites are associated with basins which have high thermal gradients (Wickham & Oxburgh 1986; Atherton 1990; D'Lemos et al. 1992). Metamorphism is characteristically high-T/low-P type. The association is not exactly as Read might have thought, and is certainly not a freak of nature (De Yoreo et al. 1991), but a full understanding of the relationship is important in studies of granite and the crust, and could be realized in the near future, although there are a variety of options and perhaps processes (see Brown this volume). Although regional scale contact metamorphism may be a cause of high-T/low-P metamorphism (cf. Barrow 1893; see De Yoreo et al. 1991), it is now clear that the association with extension is also important in a variety of tectonic situations, often where earlier tectonic thickening is only moderate or absent. As outlined here, the heat source is mantle upwelling, consequent on rifting. In this model, granite is the end product of mantle advection and/or basalt insertion via dykes high in the crust, and subduction or thrust loading are not responsible (see the modelling of the thermal state of the source of the Boulder Batholith, Zen 1992). Perhaps Read was partly right and there is a separation of plutonism and volcanism, particularly of the eRic-alkaline variety, in that many batholithic granites are not directly related to subduction while many volcanic rocks are! I acknowledge the help and support given to me by D. Bryon, N. Petford, K. Lancaster, K. McNally, A. M. Fioretti and my thanks to W. S. Pitcher for introducing me to granites.

MAGMATISM

& WEBB, S. 1989. Volcanic facies, structure and geochemistry of the marginal basin rocks of ccntral Peru. Journal of South American Earth Sciences, 2, 241-261. , McCouR'r, W.J., SANDERSON, L.M. & TAYLOR, W.P. 1979. Thc geochemical character of the segmented Pcruvian Coastal Batholith and associatcd volcanics. In: A rltERTON, M.P. & TARNEY, J. (cds) Origin of Granite Batholiths: Geochemical Evidence. Shiva Press, Cheshire, 45-64. BARLEY, E.B. 1958. Some aspects of igneous geology, 1908-1958. Transactions of Geological Society of Glasgow, 23, 29-52. BALK, R. & BAR'rH, T.F.W. 1936. Structural and petrological studies in Dutchcss County, New York. Part 1. Geological Society of America Bulletin, 47, 775-850. BARROW, G. 1893. On an intrusion of muscovite-biotite gneiss in the south eastern Highlands of Scotland and its accompanying mctamorphism. Quarterly Journal of the Geological Society of London, 49, 330-358. 1912. On the geology of lower Dcesidc and thc southern Highland Border. Proceedings of the Geologists" Association, 23, 268-284. BATEMAN, P.C. & CIIAPPELL, B.W. 1979. Crystallisation, fractionation and solidification of thc Tuolumnc intrusive scrics, California. Geological Society of America Bulletin, 90, 465-482. BECKINSALE, SANCIlEZ-FERNANDEZ,A.W., BROOK, M., fOBBING, E.J., TAYLOR, W.P. & MoorE, N.B. 1985. Rb-Sr whole rock isochron and K-Ar determination for the Coastal Batholith of Peru. In: PrrCUER, W.S., ATltER'rON, M.P., COBBING, E.J. & BECKINSALE, R.D. (cds) Magmatism at a Plate Edge: The Peruvian Andes. Blackic Halstcad Press, Glasgow, 177-202. BOWEN, N.L. 1928. The evolution of the Igneous Rocks. Dovcr, New York. 1948. The granite problem and the method of multiple prejudices. Geological Society of America Memoirs, 28, 79-90. BROWN, M. 1993. P - T - t cvolution of orogenic belts and the causes of regional metamorphism. Journal of the Geological Society, London, 150, 227-241. BRUN, J.P., GAPAIS, D., COGNE, J.P., LEDRU, P. & VIGNERESSE, J.L. 1990. Thc Flamanvillc granitc (north wcstcrn Francc): an unequivocal example of a syntectonically expanding pluton. Geological Journal, 25, 271-286. BURNItAM, C.W. 1979. Magmas and hydrothcrmal fluids, in: BARNES, H.L. led.) Geochemistry of Hydrothermal Ore Deposits (2nd cdition). John Wiley, New York, 71-133. CASTRO, A, MORENO-VENTAS & DE LA ROSA, J.D. 1990. Microgranular enclaves as indicators of hybridization processes in granitoid rocks, Hercynian Belt, Spain. Geological Journal, 25, 391-404. CHAPPELL, B.W. & WtlrrE, A.J.R. 1974. Two contrasting granite types. Pacific Geology, 8, 173-174, CLARKE, D.B. 1992. Granitoid Rocks. Topics in Earth Sciences 7. Chapman &ttail, New York. CLEMENS, J.D. 1984. Water contents of silicious to intermediate magmas. Lithos, 17, 273-287. & VIELZEUF, D. 1987. Constraints on melting and magma production in the crust. Earth and Planetary Science Letters, 86, 287-306. CLODS, E & HIETANEN, A. 1941. Geology of the 'Martic Overthrust" and G&narm Series in Pennsylvania and Maryland. Geological Society of America Special Papers, 35. Couot, R., WInTSETr, R., HUEItN, B. & BRICfiNo-GUARUPE, L. 1981. Structures of thc contincntal margin in Pcru and Chile. In: KULM, L.D., DYMOND, D., DASCII, E., & HUSSONG, D.M. (eds) Nazca Plate: Crustal formation and Andean convergence. Geological Society of America Memoirs, 154, 703-726. DAKYNS, J.R. & TEALL, J.J.H. 1892. On the plutonic rocks of Garabal Hill and Meall Brcac. Quarterly Journal of the Geological Society of London, 48, 104-121. DALY, J.S., CLWV, R.A., YARDLEY, B.E.D. (cds) 1989. Evolution of Metamorphic Belts. Geological Society, London, Special Publications, 43. DE PAOLO, D.J. 1981. A neodymium and strontium isotope study of the Mesozoic eRic-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California. Journal of Geophysical Research, 86, 10470-10488. DE YOREO, J.J., Lux, D.R. & GUIDOI'II, C.V. 1991. Thermal modelling in low-pressure/high-temperature metamorphic belts. Tectonophysics, 188, 209-238. DIDtER, J. & BARBARIN, B. 1991. Enclaves and Granite Petrology. Developments in Petrology 13, Elsevier, Amsterdam. D'LEMOS, R.S., BROWN, M. & SrRACUAN, R.A. 1992. Granite magma generation, ascent and emplacement within a transprcssional orogcn. Journal of the Geological Socie(v, London, 149, 487-490. ESKOLA, P. 1915. On the relationship between the chemical and mineralogical composition in metamorphic rocks of the Orijarvie region. Bulletin dc la Commission Gologiquc dc Finlandc, 44. 1955. About the granite problem and some mastcrs of the study of -

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Received 21 July 1993; revised typescript accepted 24 August 1993.

From QJGS,96, 451.

THE BY

GARABAL

HILL-GLEN COMPLEX

FYNE

STEPHEN ROBERT NOCKOLDS~ PH.D.

IGNEOUS B.SC.

F.G.S.

Read 1 May 1940 [PLATE X X I V ]

CONTENTS I. I n t r o d u c t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I I . P e t r o g r a p h y of t h e c o m p l e x : - (a) T h e u l t r a b a s i c r o c k s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) T h e g a b b r o s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) T h e p y r o x e n e - m i c a - d i o r i t e . . . . . . . . . . . . . . . . . . . . . (d) T h e c o a r s e a p p i n i t i c d i o r i t e a n d a p p i n i t e ... (e) T h e m e d i u m a p p i n i t i c d i o r i t e . . . . . . . . . . . . . . . . . . (f) T h e x e n o l i t h i c d i o r i t e . . . . . . . . . . . . . . . . . . . . . . . . . . . (g) T h e f i n e - g r a i n e d q u a r t z - d i o r i t e . . . . . . . . . . . . . . . (h) T h e m e d i u m g r a n o d i o r i t e . . . . . . . . . . . . . . . . . . . . . . . . (j) T h e p o r p h y r i t i c g r a n o d i o r i t e a n d a s s o c i g t e d xenoliths .......................................... (k) T h e a p l i t e s a n d p e g m a t i t e s . . . . . . . . . . . . . . . . . . . . . I I I . Letter s h e e t s a n d d y k e s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IV. T h e r m a l m e t a m o r p h i s m of t h e s u r r o u n d i n g schists V. C o n t a m i n a t i o n of t h e igneous r o c k s w i t h s e d i m e n t s V I . T h e f o r m of t h e c o m p l e x . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V I I . P e t r o g e n e s i s of t h e c o m p l e x . . . . . . . . . . . . . . . . . . . . . . . . . . . V I I I . S u m m a r y and conclusions .............................. I X . L i s t of w o r k s r e f e r r e d to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

From QJGS, ] 04, 155.

Page 451 452 457 462 465 467 468 470 471 477 480 481 482 486 487 488 507 508

A COMMENTARY ON PLACE IN PLUTONISM THE ANNIVERSARY ADDRESS OF T H E P R E S I D E N T , PROFESSOR H E R B E R T HAROLD READ, D.SC. A . R . C . S . F . R . S . F . R . S . E . , D E L I V E R E D AT T H E ANNUAL G E N E R A L M E E T I N G OF THE SOCIETY ON 17 MARCH, 1948 CONTENTS I. II. III. IV. V. VI. VII. VIII.

Introduction .................. Zones ........................... M e t a m o r p h i c facies . . . . . . . . . Depth ........................... Pressure and depth ......... Stress a n d a n t i s t r e s s ...... P l a c e on t h e m a p . . . . . . . . . . . . T h e original c o m p o s i t i o n effect . . . . . . . . . . . . . . . . . . . . . . . . IX. Equilibrium ..................

Page 156 158 164 166 169 170 172 173 175

X. XI. XII." XIII. XIV. XV. XVI. XVII. XVIII. XIX.

Closed or o p e n s y s t e m s ... C o m p o s i t i o n series . . . . . . . . . Diffusion in m e t a m o r p h i s m Privileged paths ............ Lit-par-lit ..................... S c h i s t o s i t y o n b e d d i n g ... Metamorphic differentiation Fronts ........................ T h e u n i t y o f p l u t o n i s m ... List of references ............

Page 176 177 181 185 186 187 191 193 196 201

SUMMARY

A g e n e r a l e x a m i n a t i o n is c o n d u c t e d i n t o t h e p r o p o s a l s c o n c e r n i n g p l a c e a n d t i m e in the m a k i n g of t h e p l u t o n i c r o c k s - - t h e s e c o n s t i t u t i n g t h e m e t a m o r p h i c , m i g m a t i t i c a n d g r a n i t i c classes. Such n o t i o n s as zones, levels, fronts, facies a n d a s s e m b l a g e s , a n d t h e whole m o d e r n a p p a r a t u s of m e t a m o r p h i s m , are d e a l t w i t h m a i n l y f r o m t h e space aspect.

From QJGS, "105, 101.

A CONTEMPLATION TIlE

OF TIME IN PLUTONISM

ANNIVERSARY ADDRESS OF T H E P R E S I D E N T , PROFESSOR H E R B E R T HAROLD READ, D.SC. A . R . C . S . F . R . S . F . R . S . E . , D E L I V E R E D AT THE A N N U A L GENERAL MEETING OF THE SOCIETY ON 27 APRIL, 1949. CONTENTS I. II. III. IV. V. VI. VII. VIII. IX.

Time and time again ................................................... Time and crystallization ................................................ Time, crystallization and deformation .............................. Inversion .................................................................. The interpretation of metamorphic history ........................ P o l y m e t a m o r p h i s m or m o n o m e t a m o r p h i s m ..................... Time and migma-magma ............................................. T h e G r a n i t e Series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Page 102 105 110 118 123 130 13"2 143 152

SUM MARY

A general s u r v e y is first m a d e o f c e r t a i n a s p e c t s o f t i m e in p l u t o n i s m . T h e s e include t h e d u r a t i o n o f p l u t o n i c t i m e s , t h e i r l i m i t s a n d s u b d i v i s i o n s , t h e different d a t e s c o n c e r n e d in t h e m , a n d t h e u n i t y or o t h e r w i s e o f p l u t o n i c processes. More specific inquiries b e g i n w i t h t h e c o n s i d e r a t i o n o f t i m e a n d c r y s t a l l i z a t i o n . T h e c r i t e r i a for t h e d e t e r m i n a t i o n o f t i m e - s e q u e n c e s in t h e c r y s t a l l i z a t i o n o f t h e p l u t o n i c rocks are r e v i e w e d a n d e x a m p l e s of t h e i r a p p l i c a t i o n g i v e n . T h e m o r e c o m p l e x q~mstion o f t h e t i m e - r e l a t i o n s o f c r y s t a l l i z a t i o n a n d d e f o r m a t i o n is n e x t t a k e n up, c h i e f a t t e n t i o n being g i v e n to the e v i d e n c e for t h e r o t a t i o n of p o r p h y r o . bhLqts. T h e diagnosti(, r e q u i r e m e n t s fl)r p r o - c r y s t a l l i n e , p a r s - c r y s t a l l i n e a n d p o s t c r y s t a l l i n e d e f o r m a t i o n s ~Lrc c x a m i l m d .

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 237-247

Hydrothermai orefields and ore fluids A.

H.

RANKIN

School o f Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames KT1 2EE, UK Abstract: The development of Hydrothermal Theories of Ore Genesis during the past 150 years owes much to the pioneering work of two eminent British geologists, H. C. Sorby and K. C. Dunham, who published benchmark papers in the Quarterly Journal of the Geological Society on the classic mining districts of Cornwall and the English Pennines. Sorby's paper of 1858 laid the foundation for fluid inclusion studies. Despite considerable scepticism or indifference that has lasted well into the twentieth century, fluid inclusions are now widely regarded as the best way of establishing the nature and composition of ancient mineral-forming fluids. Dunham's paper of 1934 provides a classic account of mineral zonation away from a focus of mineralization in the North Pennine orefield. By analogy with the Cornish deposits this focus was considered to be the result of ascending mineralizing fluids from a hidden granite at depth. Although subsequent drilling confirmed the existence of granite (the Weardale granite) directly beneath the North Pennine Orefield it was older than the Carboniferous Limestone which hosts the mineralization and therefore cannot be its direct source. With the 'coming of age' of fluid inclusion techniques in the 1960s, widespread and systematic studies were carried out on samples from the Cornubian and Pennine orefields and other classic mining districts of the world over the succeeding three decades. These, together with stable isotope studies, established that hydrothermal, ore-forming fluids cover a wide temperature range (50° to >500°C) and compositional range (0 to >60 wt % dissolved salts), and are of diverse origin. The composite and protracted nature of hydrothermal events in Cornwall, and indeed in areas of the world where mineralization is spatially associated with granites, is now evident. Most authors agree that, whilst components of the early mineralization may be due to metalliferous fluids directly evolved from cooling granite bodies, much of the later base-metal mineralization is due to thermal convection and pulsation of meteoric fluids and basinal brines within the intrusion and surrounding country rocks. The basinal brine expulsion theory is generally favoured for carbonate-hosted, epigenetic, base metal deposits of the Mississippi valley type. The nature, geological setting and fluid inclusion characteristics place the Pennine ores clearly in this broad class. However, most recent work suggests that the mineral zonation patterns in the northern part and temperature differences between the northern and southern part are still best explained by the hydrogeological and geochemical influence of granite at depth. Prior to the d e v e l o p m e n t of m o d e r n techniques for the bulk mining and processing of large tonnage, low grade ores in the 1950s and 1960s, most of the world's base metal production came from small mineral veins typically clustered in well-defined geographical areas known as 'orefields'. The British Isles has b e e n particularly well e n d o w e d with a rich variety of mineral veins exploited since p r e - R o m a n times from small surface and underground workings. Three main phases of mining and development during the R o m a n occupation, the Elizabethan era and the Industrial Revolution, resulted in the delineation of two major ore districts in England; one centred around the Hercynian granites of D e v o n and Cornwall (the Cornubian orefield), and one centred on the Carboniferous limestones of the English Pennines (the Pennine orefields) (Fig. 1). At various times during their development, these districts have provided the bulk of the old world's lead and silver in the case of the Pennine orefields, and most of it's copper and tin in the case of the Cornubian orefield (Table 1). Much has been written about the mining history of these areas, and of their socio-economic importance as training grounds and repositories of knowledge for successive generations of miners who took their technical skills and 'know how' to develop newly discovered mining districts elsewhere in the world. Inevitably, there has been a rapid decline in the U K metalliferous mining industry since its peak in the nineteenth century (Fig. 2). At the time of

writing the South Crofty tin mine in the C a m b o r n e - R e d r u t h area of Cornwall is the only surviving non-ferrous metal mine in England. Much less has b e e n written about the considerable contributions made by British scientists over the years to general theories of ore genesis based on geological, mineralogical and fluid inclusion studies on these two orefields. This contribution aims to redress this imbalance through a review of the d e v e l o p m e n t of current states of knowledge of the nature, origin and composition of mineralizing fluids with special reference to these orefields and the role of granites in their genesis. The quintessential theme and framework for this review is e m b o d i e d in two seminal papers which appeared in the Quarterly Journal of the Geological Society of London. The first is H. C. Sorby's classic paper of 1858 in which he described the use of fluid inclusions as indicators of the nature and composition of ancient ore-forming fluids. The second is K. C. D u n h a m ' s b e n c h m a r k paper of 1934 on mineral zonation and hydrothermal ore genesis in the north Pennine orefield.

Mineralization of the Cornubian and Pennine orefieids There is an immense volume of literature on the geology and mineralization of both the Pennine and Cornubian orefields, with recent reviews provided by Colman et al. 237

238

A.H.

RANKIN Hercynian orogeny These five masses are believed to be connected at depth to a larger subterranean granite batholith whose axial trace approximately delineates the extent of the ore field (Jackson et al. 1982). Pervasive and fracture-controlled alteration of the host granites and, to a lesser extent the surrounding country rocks, has given rise to a variety of hydrothermal alteration styles: notably tourmalinization, greisenization and kaolinization. Extensive kaolinization of the granites has led to the development of economic china clay deposits in the orefield, notably within the western lobe of the St Austell pluton and the southwestern part of the Dartmoor granite. Because of the close spatial association between granite intrusions and mineral veins, the area is frequently cited as a classic example of mineralization associated with acid magmatism (e.g. Guilbert & Park 1986).

o

~ 4oN OREFIELD

Alston Pb F Ba (Ag) (Zn)

The P e n n i n e orefields

Pennine Orefield

Pb (F) (Ba)

South Pennine Orefield

~J

"~ I t)

Cu Sn (Pb) (Zn) (W)

Fig. 1. Location of the Pennine and Cornubian ore fields of the UK showing major and minor (in parentheses) production of minerals. Modified, in part, from Plant & Jones (1989).

(1989), Jackson et al. (1989), Willis-Richard et al. (1989), Alderton (1993) and Ixer & Vaughan (1993) to whom the reader is referred for detailed descriptions. In essence, both orefields are dominated by structurally-controlled mineral veins seldom more than a few metres wide and a few kilometres along strike. T h e C o r n u b i a n orefield

The polymetallic mineral veins of Devon and Cornwall have contained a variety of metals dominated by tin, copper, arsenic and tungsten, but also containing iron, uranium, copper-nickel-arsenic and lead-zinc paragenetic assemblages. The orefield occupies an area of at least 3800 km 2 (Jackson et al. 1982) and is dominated by five major granite masses, post-tectonically intruded into a sequence of deformed metasediments and volcanic rocks of Devonian and Carboniferous age, towards the end of the

The Pennine orefields of England comprise three distinct mineralized areas. From north to south these are the Alston block and the Askrigg block of the north Pennine orefield, located mainly in the counties of Durham and Yorkshire, and the south Pennine orefield centred on the Derbyshire dome. The mineral deposits and geology of these ore fields are broadly similar, but subtle differences in chemistry and mineralogy are evident from north to south (as summarized by Colman et al. 1989a). Mineralization mostly occurs as a series of steeply-dipping veins composed mainly of fuorite, galena, barite and calcite with local enrichments of copper and zinc. Although the ore fields are centred on outcrops of the Visean carbonates, ore shoots within the veins are restricted to a small number of competent limestone and sandstone horizons. Most authors, as will be shown later, believe that these deposits belong to the class of mineral deposits broadly referred to as Mississippi Valley Type (MVT). However, the anomalously high quantities of fluorite present has led Dunham et al. (1983) and Russell & Skauli (1991) to classify the Pennine ores as a 'fluoritic' or 'high enthalpy' subtype of Mississippi Valley Type deposits, respectively.

Early development of ore-genetic theory, 1500-1900 The historical development of ore-genetic theories really started with the seminal work of George Bauer, De Re Metallica (1556), published under his latinized name of Georgius Agricola. Agricola's lavishly illustrated work was a philosophical, as well as a practical account of the genesis, exploration and exploitation of ore deposits based on his experience in the Erzgebirge region of Germany. His principal contribution to the scientific study of ore deposits was the recognition that ores are not random phenomena but can be classified into many different types according to their form and origin; a remarkable insight, bearing in mind that even to this day students of ore geology still use classification schemes based on this concept. The ore deposits in Agricola's native Saxony were mostly fissure veins which he believed to have formed by deposition from solutions circulating through open joints and fractures. According to the historical review of 'theories of ore deposition' by Guilbert & Park (1986), many scientists over the last 200 years acknowledged the importance of hot

HYDROTHERMAL

OREFIELDS AND ORE FLUIDS

239

Table 1. Comparative estimates of total mineral production from the Cornubian and Pennine orefields and its economic value at December 1993 prices (major metals and minerals, excluding china clay)

Production (millions of tonnes)

Value (£ millions)

Cornub~n orefield (Devon and CornwaH)* (Jackson 1979) Sn >2.0 6366 Cu 1.3 1544 Pb 0.35 109 Total value (millions) 8019 Pennine orefield (AIston, Askrigg, Derbyshire)t (Dunham 1983) Pb >6.0 1404 Zn 0.34 165 CaF 2 6.5 618 BaSO 4 1.5 75 Total value (millions) £2261

Production (millions of tonnes)

Value (£ millions

(Alderton 1993) 2.5 2.0 0.25

7958 2314 78 10350

(Ixer & Vaughan 1993) 7.5 1755 0.36 174 6.8 646 1.6 80 2655

* Values based on London Metal Exchange Prices December 1993. t Values based on 75% of London Metal Exchange prices for December 1993 for Pb and Zn and on prices quoted for metallurgical grade fluorite and drilling-mud grade barite in November 1993 issue of Industrial Minerals.

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aqueous solutions (hydrothermal fluids) in the formation of mineral veins in the Erzgebirge. But it was the French scholar, Ellie de Beaumont who really promulgated the hydrothermal theory of ore genesis in a series of papers in the mid-nineteenth century, citing in his paper of 1847 the presence of large fluid inclusions in support of this theory. The presence of relatively large, but very rare fluid-filled cavities, clearly visible to the naked eye had been described and scientifically investigated earlier by several eminent British scientists of the time, including Sir H u m p h r e y Davey (1822) and Sir David Brewster (1823), but their geological significance was not appreciated at the time.

S o r b y ' s contribution o f 1858 [

1900

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1950

2000

1950

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LEAD

50 40 30 20 10 0 1850

1900 YEARS

Fig. 2. United Kingdom mine production of base metals: ten-year averages, 1850-1990. Modified from Highley et al. (1991). Note the rise in tin production from 1970 to 1990 following the opening of the Wheal Jane mine in Cornwall. Since its closure in 1991, South Crofty is the last remaining underground tin mine in Cornwall and the only significant producer of base metals in England.

The impetus for Sorby's seminal work on fluid inclusions published in 1858 was the acquisition of a new scientific instrument for studying very thin sections of rocks and tiny crystals grown in the laboratory: the optical microscope. The preconceived notion that fluid inclusions were rare, scientific curios was soon dispelled when Sorby discovered that under the microscope ' . . . it is easier to see that the proportion of many millions to the cubic inch is very common in some minerals.' Through a series of careful experiments on laboratorygrown, water-soluble crystals, including potassium chloride, sodium chloride and potassium bichromate, Sorby beautifully demonstrated the following features. (1) Tiny droplets of mother liquor may be trapped and preserved as fluid inclusions during the growth of crystals. In natural samples they provide a unique record of the nature and composition of ancient mineral-forming fluids. (2) When crystals are formed from aqueous solutions at elevated temperatures differential contraction of the contained fluid takes place in a manner similar to the development of the head-space in a mercury-in-glass

240

A. H. R A N K I N

thermometer. This results in the development of a 'vacuity' or contraction vapour bubble in the inclusion fluid on cooling. (3) The relative size of the vacuity varies depending on the temperature at which the crystal grew. The important conclusion was that, by determining the temperature at which the liquid and vapour components become homogeneous again (the homogenization temperature, T,), natural fluid inclusions could also be used as geothermometers for a variety of rocks and minerals. Sorby then carried out a further series of simple experiments on the thermal expansion of various salt-water solutions contained within glass tubes and established the following empirical relationship between vapour-liquid ratios in fluid inclusions and the temperatures and pressures at the time of trapping: v = (Bt + Ct2)(1 - 0.00000271p) - 0.00000271p.

where: v -- relative size of the vacuity t = temperature in degrees centigrade p -= pressure in atmospheres B and C = constants whose values depend on the nature and strength of the salt solution in the cavity. Sorby took care to warn his readers that these equations were accurate only for moderate values of temperature and pressure, and they were advised to adopt them provisionally. He also recognized some of the potential pitfalls of his new geothermometric method which could lead to erroneously high estimates of temperatures. These included: (i) heterogeneous trapping of discrete vapour bubbles (air in his experiments); (ii) leakage if the crystals are subsequently subjected to higher temperatures through, for example, the use of Canada balsam as the mounting medium. Sorby's observations (Fig. 3) were not restricted to crystals grown from aqueous solution. He recognized and described glass inclusions in crystals of iron silicates, and of Humboldtilite in slags formed from copper-nickel and iron smelting, and good examples of ~stone cavities' in pyroxene

from blast furnaces at Masborough in his native Sheffield. He inferred that both were products of trapped silicate melts. Thus, by simple microscopic examination of natural crystals, Sorby had devised a method of determining whether rocks and minerals had been formed from igneous fusion or the action of water, an issue that was still being hotly debated at the time. Sorby applied this new-found method to rocks and minerals from a number of geological environments including the granites, elvans (quartzporphyry dykes) and mineral veins of Cornwall. He recognized the existence of stone cavities in both the elvans and granites of St Austell and Land's End, but noted their absence in quartz from associated mineral veins where aqueous inclusions predominated. He estimated a temperature of around 200°C for the mineral veins from St Michael's Mount and the Camborne area. These were only slightly lower than his estimate for the granites themselves ( m e a n = 216°C). Whilst the inclusion evidence suggested hydrothermal processes were important in the formation of the mineral veins, the apparent co-existence of aqueous and stone inclusions in the granite led Sorby to postulate the development of a separate aqueous phase during cooling of the granite melt. In this respect Sorby appeared to be amongst the first committed magmatic hydrothermalist as far as mineral deposits were concerned, and a strong supporter of Ellie de Beaumont's views (1847) on the matter. S o r b y ' s o w n w o r k ' u n d e r the m i c r o s c o p e '

When Sorby delivered his classic paper to a meeting of the Geological Society in London on December 16 1857 it received considerable attention, not least from its chairman at that time, Leonard Horner. According to Sorby (reported in Judd 1908), Horner commented that he ' . . . had been a member of the Geological Society ever since its foundation, and during the whole of that time he did not remember any paper having been read which drew so largely on their credulity'. There was considerable scepticism and criticism of Sorby's work, as Sorby himself explained (see Judd 1908): 'In those early days people laughed at me. They quoted

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/'~ Fig. 3. Original sketches by Sorby (1858) of fluid inclusions in laboratorygrown crystals.

H Y D R O T H E R M A L O R E F I E L D S AND ORE FLUIDS Saussaure that it was not a proper thing to study mountains through a microscope'. The fate of further development of the subject, at least in the UK, was really determined through publication of another paper in the Quarterly Journal of the Geological Society by an equally eminent geologist of his time, J. A. Phillips, who by a quirk of fate happened to be one of Sorby's few supporters! Phillips carried out extremely detailed studies on Cornish veins and granites and obtained very wide temperatures using Sorby's fluid inclusion calculation method. Phillips even constructed a crude microscope heating stage but failed to homogenize many inclusions. He, therefore, concluded that because, gas-liquid inclusions showed such considerable variations in the relative size of their vacuities, Sorby's thermometric method was fallacious (Phillips 1875). What both authors failed to realize, and what left the subject in limbo for more than 100 years, was that fluid inclusions may be secondary, reflecting post-depositional recrystallization, as well as primary in origin.

241

outer barite zone (Fig. 4), provides a landmark in British ore geology. It initiated debate, speculation and scientific enquiry on the nature and origin of mineralizing fluids and the role of granites in ore genesis, which has extended over 50 years. Strongly influenced by previous work on zonation of the mineral lodes of Cornwall, and their association with granite (Davison 1927), Dunham postulated that: (i) falling temperature away from a focal point for mineralization was 'the prime cause of the zone of distribution' for the Pennine ores; (ii) mineralization was the result of juvenile fluids related to concealed granite at depth coincident with the fluorite zone outcropping at surface. Remarkably, gravity surveys (Bott & Masson-Smith 1957) and drilling (Dunham et al. 1965) confirmed the existence of a granite cupola directly under the orefield (Fig. 5), although it was soon apparent that the granite far from being co-eval with the overlying mineral deposits (post-Carboniferous) was very much older (Devonian) and could not possibly be the direct source of the mineralization

Development of ore genetic theory 1900-1965 The first half of the present century saw rapid advances in the application of fundamental principles of physics and chemistry to the study of ore deposition, and the growth in field observations arising out of an increasingly scientific, as opposed to serendipic, approach to mineral exploration. The most influential work in the first part of this period was that of W. Lindgren in the U S A who formulated a comprehensive and eloquent classification scheme (Lindgren 1933) based on field observations and the inferred physico-chemical conditions of formation. Lindgren recognized that hydrothermal deposits were the most important class of base and precious metal deposits, so much so that he further sub-divided them into hypothermal, mesothermal and epithermal classes (Table 2). Many modifications have subsequently been made to Lindgren's original proposal but many of the concepts and terms originally introduced still remain (Guilbert & Park 1986). Occasional attempts by the magmatists to challenge the hydrothermalist's view of ore deposition (e.g. Spurr 1923) were soon dispelled, although Spurr is credited by Guilbert and Park (1986) as being the first to provide a generalized statement of the theory of mineral zones, so well illustrated in Cornwall and the north Pennines. The first detailed accounts of primary zonation in these areas were provided by Davison (1927) and Dunham (1934) respectively.

Contribution o f Dunham's 1934 paper The recognition and delineation by Dunham (1934) of a distinctive pattern of mineral zonation in the north Pennine orefields in which a central fluorite zone is surrounded by an Table 2. Lindgren's (1933) classification of hydrothermal

ore deposits according to temperature and depth (pressure) or formation Sub-division

Temperature range

Hypothermal Mesothermal Epithermal

500-300 ° 300-200 °C 200-50 °C

Developments in fluid inclusion techniques and methodology Little interest was shown in the use of fluid inclusions as geological indicators during the early part of the twentieth century. However, during the period 1940-1965 a steady stream of pioneering papers in North America (notably by Roedder and Smith), the USSR (notably by Kalyuzhnyi, Lemmlein and Yermakov) and France (notably by Deicha), culminated in the reconciliation of many of the problems which had given cause for such doubt following Sorby's early work (for reviews see Deicha 1955; Smith 1953; Lemmlein 1956; Kalyuzhnyi 1960; Yermakov 1965; Roedder 1972). These included: (1) the recognition that several generations of inclusions, representing both primary and secondary crystallization processes, may be preserved in a single crystal; (2) the realization that both necking-down and leakage can alter the liquid-vapour ratios of fluid inclusions, and the recognition that such effects can give rise to erroneous temperature estimates; (3) the development of suitable microscope heating stages for accurately recording homogenization temperatures of fluid inclusions, and the realization (at least in North America and Europe) that the alternative decrepitation method of fluid inclusion geothermometry was unreliable. After some 100 years of inactivity and apparent disinterest in fluid inclusions within the UK following Sorby's 1858 paper, studies on British ore deposits suddenly erupted. The catalyst for this was K. C. Dunhan who invited a young researcher from the USA, F. J. Sawkins, to his department in Durham in the 1960s to apply the new-found methods of fluid inclusion studies to the Pennine and south west England mining districts (Sawkins 1966a, b).

Depth Great Intermediate Shallow

Characterization o f the Pennine orefluids: fluid inclusion and related studies Sawkin's pioneering fluid inclusion study of mineralization from the Alston block showed that, as Dunham (1934) had

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Yennine t y p e veins, t h i n b l a c k lines. G r e a t S u l p h u r Vein t~,'pe veins, d o t t e d lines. Z o n a l b o u n d a r i e s , h e a v y b l a c k lines.

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Fig. 4. Map reproduced fom Dunham's paper of 1934 showing the inner fluorite zone and outer barite zone of the North Pennine orefield (Aiston block)•

postulated, formation temperatures for the outer barite zone (less than 130°C) were significantly lower than those recorded for the central fluorite zone (up to about 177 °C for fluorite and from 181-216 °C in early quartz). Sawkins thus proposed a two-fluid model for ore genesis involving the mixing of granite-derived fluids and cooler barium-rich fluids derived from surrounding sediments. A two-fluid model was also proposed by Solomon et al. (1971) on the basis of isotopic evidence, although these authors postulated that mineralization in the fluorite zone was due to circulation of connate brines rather than juvenile fluids. Sawkins' preliminary results were soon substantiated by more detailed fluid inclusion studies on the Alston Block (Smith 1974; Smith & Phillips 1974) and extended over the next 15 years to include the mineralization of the Askrigg Block and Derbyshire Dome (Roedder 1967; Rogers 1977, 1978; Smith 1973, 1974; Greenwood & Smith 1977; Small 1978; Atkinson et al. 1982; Christoula 1992). Salinities from all three orefields typically cluster around 20-25 eq. wt% NaCI, but a

decrease has been noted from north to south in the homogenization temperatures of fluid inclusions in fluorite from these three areas (Rogers 1977; Atkinson et al. 1982; see also Table 3). Recent unpublished results based on more refined methods have essentially confirmed these trends (Christoula 1992) as shown in Fig. 6. However, a high temperature ( T h = l l 0 - 1 6 0 ° C ) , low salinity fluid (05 eq. wt% NaCI) is also discernable in apparently primary inclusions in fluorite from Derbyshire. This is in agreement with the preliminary results presented by Roedder (1967) and by Moser et al. (1992). Liquid-hydrocarbon-bearing fluid inclusions are also reported from the Derbyshire orefield. Detailed studies by these authors have revealed that the hydrocarbon inclusions have all the hallmarks of natural petroleum seepages in the area, but coexist only with the anomalously high temperature, low salinity fluids that have a restricted occurrence in the northeastern part of the orefield (Fig. 6). First melting temperatures between - 6 5 and - 5 0 ° C ,

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Table 3. Summary of published homogenization temperatures of fluid inclusions in fluorite from the Pennine orefield

Alston

110-177 °C

Askrigg

92-143 °C 92-164 °C

Derbyshire

70-140 °C 74-127 °C 92-127 °C 74-158 °C

Sawkins (1966a) Smith & Phillips (1974) Smith (1973) Rogers (1978) Small (1978) Roedder (1967) Smith (1973, 1974) Rogers (1977) Atkinson et al. (1982)

indicative of calcium (with or without magnesium)-bearing brines (Roedder 1984), are commonly reported from 'normal' high salinity, aqueous inclusions from the Pennine orefields. Recent estimates of the ratio of sodium to calcium based on careful micro-thermometric studies of these inclusions have revealed a remarkable uniformity throughout the ore field in terms of their calcium contents (Christoula 1992). The above fluid characteristics are typical of those reported from a number of Mississippi Valley Type deposits worldwide (see Table 4). This was one of the main reasons why consensus opinion gradually changed from an ore genetic model involving a juvenile, granite-derived source, to one in which basinal brines, analogous to modern day oil-field brines (Carpenter et al. 1974) was favoured; but see Russell & Skauli 1991). The other was that the underlying Weardale granite clearly postdated mineralizing events by at least 100 million years and could not possibly have been the direct source of the mineralizing fluids. D u n h a m was amongst the first to advance the basinal brine theory, as opposed to his original 'juvenile' source theory, for the Pennine mineralization. He also proposed that the ore field represents a fluoritic sub-type of the Mississippi Valley Type class of deposit (Dunham et al. 1983).

km

I I

I

DURHAM

',,"iiiiiiiiiiii ...--.___...--i ~" ~,

243

I

~

~ ~

Fluoritezone

~.

"\

OREFIELDS AND ORE FLUIDS

' e l

Cusp within Granite

Fig. 5. Location of the fluorite zone of the Alston block, North Pennine orefield, in relation to Carboniferous basins, basement structures and the underlying Weardale granite. Modified from Greenwood & Smith (1977) after Bott (1967) and Dunham (1934).

Apart from variations in homogenization temperatures from north to south, and also in the potassium content of the inclusion fluids, other mineralogical and geochemical differences are apparent between different areas of the orefields (Dunham 1983; D u n h a m & Wilson 1985; Brown et al. 1987; Colman et al. 1989a). The most significant of these are a granitic association of trace elements and minerals, observed within the Alston Block mineralization, but absent within that of the Derbyshire dome. The favoured explanation for such a difference is the variable influence of granites beneath different areas of the orefield (Brown et al. 1987; Rankin & G r a h a m 1988; Colman et al. 1989a). Current theories of ore genesis for the north Pennine orefield thus link directly back to Du n h a m ' s classic study in which he suggested that the epigenetic vein mineralization is associated with buried granites. The major difference between Dunham's original hypothesis and the one generally accepted today is based on the fact that the underlying Weardale granite is substantially older than the overlying mineralization. Rather than acting as direct sources of mineralizing fluids, it is now generally considered that the granites have simply acted in variable ways as heat engines and loci for channelling mineralizing fluids from a sedimentary source (e.g. Brown et al. 1987). Ore fluids o f Cornubia: f l u i d inclusion a n d related studies Although Dunham's (1934) recognition of mineral zonation in the North Pennine orefield post-dated its recognition in south west England (Davison 1927), the first major publication on fluid inclusions in minerals from the Cornubian orefield for almost a century also came from Sawkins (1966b) working at D u r h a m University, Dunham's stamping ground. Sawkins (1966b) published only a preliminary account of his findings based on a few samples but they clearly showed a temperature decrease from the early tin-tungsten mineralization (Th = 300-450 °C), to the

244

A. H. R A N K I N 240

'

'

,

I

'

Table 4. Typical fluid inclusion characteristics of Mississippi Valley

I

type deposits compared with those of the Pennine fluorite deposits Alston

200 CL

E

Block

160

o

tO t~

120

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1'0

20

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I

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'

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80

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South Pennines m a.

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200

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Pennine Fluorites?

100-150 °C

70-180°C

>l.O

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15 to >20

15-27

Na, Ca, K C1 Liquid and gaseous hydrocarbons

Na, Ca, K C1 Oil present in inclusions from Derbyshire and light hydrocarbons from Askrigg and Alston$

* Based on summary by Roedder (1984). t Temperature, salinity, density and solute data from sources quoted in Table 2 and also in Christoula (1992). $ From Ferguson (1991) and Moser et al. (1992).

.N_ O~ 0

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4O

Askrigg Block

200

0 ,,,,., co '.~ t~

3'0

Temperature (Th) Fluid density (g cm 3) Salinity (eq. wt% NaCI) Major cations Major anions Organic matter

Typical MVT Deposit*

8o

0

40 0

10

20

30

40

Salinity (Equiv. wt. % NaCI)

Fig. 6. Homogenization temperature versus salinity plots showing the 'fields' for fluid inclusion data from each of the three Pennine orefields. The main fields show constant salinities but decreasing temperatures of homogenization southward. Compiled from recent data from Christoula (1992). Note the presence of a separate, atypical grouping of low salinity inclusions restricted to the Castleton area of Derbyshire. Their origin and significance is not yet fully understood (see Moser et al. 1992). later copper-iron-arsenic-zinc-sulphide mineralization (Th--200-350°), to the final lead-zinc-fluorite mineralization (Th = 100-180°C). Further evidence that temperatures decreased away from centres of intense hydrothermal activity and mineralization within the granites, the so called 'emanative centres' of Dines (1956), was provided by a novel use of fluid inclusions as exploration guides for blind ore bodies in the region by Bradshaw & Stoyel (1968).

Major contributions to our understanding of metaUogeny and fluid processes associated with the Cornubian granites were made in the 1970s in three doctoral theses by Alderton (Kings College, London), Charoy (CRPG, Nancy, France) and Jackson (Kings College, London) in 1976, 1979 and 1977 respectively. A steady stream of papers on the fluid inclusion characteristics of specific mineralization and alteration styles followed (Halls et al. 1985 give the bibliographic details). By now, the necessity of distinguishing between primary and secondary inclusions, which had so hampered Sorby's early advances in the area, had been realized (e.g. Jackson et al. 1977). Sawkins' preliminary findings were substantiated. Based on studies at St Michael Mount (Jackson & Rankin 1976) and at Cligga Head (Jackson et al. 1977; Charoy 1979) in Cornwall, the fluid inclusion evidence for main stage tin-tungsten-sulphide mineralization pointed to high temperature, moderately saline fluids (Th = 200-450°C, salinity-- 5-20eq. wt.% NaC1). The fluids responsible for late-stage, cross-course veins, either barren of mineralization or carrying minor iron, lead and zinc minerals, were confirmed to be of much lower temperature and salinity, sometimes approaching that of pure water. In contrast, studies of fluorite from a number of cross-course lead-zinc deposits showed that the fluids responsible for this stage of low temperature mineralization (Th=100-150°C) were substantially more saline (2025eq. wt% NaC1) with low eutectics indicative of high calcium contents (Alderton 1978). This led Alderton to propose a separate basinal brine source (cf. Mississippi Valley Type-fluids in Table 3) for fluids responsible for this style of mineralization compared to the magmatic and/or meteoric source generally envisaged for the earlier mineralization. More comprehensive and systematic fluid inclusion studies, coupled with stable isotope studies of hydrogen and oxygen, were carried out in the 1980s on mineralization and alteration assemblages from other parts of the Cornubian orefield (notably by Alderton & Rankin 1983; Jackson et al. 1982; Shepherd et al. 1985). These results, recently

H Y D R O T H E R M A L O R E F I E L D S AND ORE FLUIDS

245

Conclusions and wider implications 400

.,,.. ~ . . ~? .

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.

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o

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.¢

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-

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;0

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Fig. 7. Homogenization temperature versus salinity plot showing 'fields' for fluid inclusions from the Cornubian orefield, based on compilations of data from various sources as summarized by Jackson et al. (1989) and Wilkinson (1990b). Note the large spread of data for 'main-stage' Sn-W-Cu mineralization and the similarity between data from the 'cross-course' lead-fluorite mineralization and the Pennine fluorites (see Fig. 6). The separate low salinity, low temperature field marked 'kao' is characteristic of late stage quartz in intensely kaolinized granite (Alderton & Rankin 1983). The field for main-stage mineralization may be extended to higher salinities when the examples from the Dartmoor area are considered.

supplemented and reviewed by Jackson et al. (1989) and Wilkinson (1990a), have not substantially altered earlier views of the temperature and salinity ranges of the fluids, except that the temperature range has now been extended down to less than 100°C for late-stage fluid processes associated with the formation economic china clay deposits (Fig. 7). The range of fluids previously envisaged as responsible for mineralization has also been extended to include a component from metamorphic sources (as confirmed by Wilkinson 1990b). In essence, the fluid inclusion, isotopic and geochronological evidence points to a multi-stage process involving fluids from a variety of sources and a protracted period of mineralization from the time of the emplacement of the granite (270-300 million years ago) extending to Mesozoic and even up until the present day. What about the granites themselves? After all, Sorby's original 1858 contribution to the development of fluid inclusion studies in the UK was directed mainly towards the granites from Aberdeenshire and Cornwall. His original observation that fluid inclusion abundances vary markedly from one area to the next provided the main impetus for systematic studies of fluid inclusion populations in granite quartz from Devon and Cornwall (Alderton & Rankin 1983; Rankin & Alderton 1983). These studies have revealed an assemblage of mainly secondary aqueous inclusions whose abundance and distribution reflects the extent to which different parts of the batholith have been affected by early and late-stage fluids of diverse origin; a feature which explains why Phillips' (1875) comments on Sorby's methods were so valid in the early days.

Historically, the role of granites was thought to be of paramount importance in providing a source of both fluids and metals for both the north Pennine (Dunham 1934; Sawkins 1966a) and Cornubian orefields (Dines 1956). Magmatic differentiation processes and the exsolution of metal-bearing, hydrothermal fluids from the cooling granites of Cornubia are still believed to play an important role in early tin-tungsten-copper mineralization in this area (Jackson et al. 1989). Geochemical evidence of such fluids may be found trapped and preserved in inclusions in tourmaline veins (Bottrell & Yardley 1988; Wilkinson et al. 1994), in breccia pipes (Halls et al. 1985) and in the granites themselves (Rankin & Alderton 1983; Alderton et al. 1992). However, convective fluid flow models involving the episodic circulation of fluids from different sources over long periods of time, are now generally accepted for at least some of the main-stage mineralization and most of the late-stage mineralization and alteration including kaolinization (Sams & Thomas-Betts 1988; Jackson et al. 1989). It appears that the high heat production capacity of the radioelement-rich granites provided the driving force for convection of late-stage meteorite waters and basinal brines in the region (Tammemagi & Smith 1975). In the north Pennine orefields the underlying Weardale granite is no longer considered to be the direct source of mineralizing fluids. High homogenization temperatures of the fluid inclusions and the presence of 'granitic' minor minerals in surface veins (Brown et al. 1987) has led to the suggestion that the underlying granite has acted beyond its previously conceived role as a broad structural control on the mineralization; it is now believed to have supplied at least some of the trace elements (Rankin & Graham 1988: Christoula 1992) and radio-thermal heat to the mineralizing fluids. Hydrothermal theories for ore genesis have come a long way since the early pioneering work of Sorby and Dunham. In the past 20 years fluid inclusion studies have become firmly established as the most important source of information on the physical and chemical properties of ancient mineral-forming fluids. A range of modern instrumental methods are now available for the geochemical analysis of the tiny droplets of fluids contained within the inclusions (Roedder 1990; Rankin et al. 1993), even to the extent that variations in fluid inclusion geochemistry may be used in mineral exploration (Alderton et al. 1992). Systematic and integrated geological, mineralogical, geochemical and fluid inclusion studies have now been carried out on a range of mineral deposit types worldwide. Based on these studies and consideration of experimental and thermodynamic data for mineral transport and deposition in high temperature fluids, it is now widely accepted that hydrothermal processes are the most important of all primary ore-forming processes in the Earth's crust (Roedder 1984; Guilbert & Park 1986). Our definition of a hydrothermal ore-forming fluid has changed substantially since they were first envisaged by the early ore geneticists as 'emanations form cooling granites'. Based on fluid inclusion and stable isotopic evidence we now recognize that such fluids are of diverse origins encompassing the whole temperature range between those responsible for diagenetic and igneous processes (Fig. 8). Most hydrothermal ore-fluids are demonstrably alkali-chloriderich brines, of variable salinities, and with ore metal

246

A.H.

NATURE

AND

FLUIDS

ORIGIN IN THE

OF HYDROTHERMAL EARTH'S

CRUST

"Any hot aqueous fluid that exists in the Earth's crust"

T°C = ~50 to >500°C

RANKIN

M. W e s t e r m a n , who are continuing the traditions set out by Sorby in investigating, in yet m o r e detail, the further mysteries and scientific importance of fluid inclusions in the granites of south west England. The helpful c o m m e n t s of an a n o n y m o u s referee and the Editor, M. J. Le Bas, are also gratefully acknowledged. I am also i n d e b t e d to M. J. Le Bas for first introducing me to fluid inclusions, as a p o s t g r a d u a t e student u n d e r his supervision at the University of Leicester, thus opening up my lifelong interest in the subject and h y d r o t h e r m a l processes in general.

Composition = Na - K - Ca - CI - S O 4

References

Salinity = 0 to >50 weight % salts Ore metals = generally at ppm levels

Origin = various (see below)

/,/'#/f///,~#,

SURFACE

';"';";;';';

METAMORPHIC

IGNEOUS INTRUSION

Fig. 8. S u m m a r y diagram illustrating the impact of recent fluid inclusion studies (see summaries by R o e d d e r 1984) on our u n d e r s t a n d i n g of the nature and origin of h y d r o t h e r m a l ore-forming fluids in the Earth's crust. T e m p e r a t u r e s span the whole range predicted by Lindgren (Table 2). H o w e v e r , the original concept that all h y d r o t h e r m a l ore deposits were linked to magmatic processes has been substantially modified in recent years mainly in the light of stable isotopic studies. It is n o w recognized that h y d r o t h e r m a l fluids, and the metals t h e y contain, m a y be derived from a variety of sources.

contents measured in parts per million (ppm) rather than percentages. The importance of basinal brines (connate waters in Fig. 8) and magmatic and meteoric waters in the genesis of the Mississippi Valley Type and granite-associated ores, as exemplified by the Pennine and Cornubian orefields, is now universally recognized. It is equally apparent, from comparable studies on other deposit types elswhere in the world, that seawater plays a dominant role in the formation of volcanic-hosted massive sulphide (VMS) deposits and that carbon-dioxide-rich metamorphic fluids developed during dehydration and decarbonation processes accompanying metamorphism are, at least in part (but see Boyle 1991 for discussion) responsible for the formation and modification of gold-quartz veins in Archaean greenstone belts. I am grateful to all the former staff and students at Imperial College, L o n d o n for much stimulating discussion on the role of fluid inclusion studies in relation to metallogeny. I also thank my colleagues at Kingston, especially my g r a d u a t e students W. Cox and

ALDERTOND.H.M. 1976. The geochemistry of mineralisation at Pendarves and other Cornish areas. PhD Thesis, University of London. 1978. Fluid inclusion data for lead-zinc ores from South-west England. Transactions of the Institution of Mining and Metallurgy, B87, B132-135. 1993. Mineralisation associated with the Cornubian granite batholith. In: PATTmCK, R.A.D & POLYA,D.A. (eds) Mineralization in the British Isles. Chapman-Hall, London, 270-354. -& RANKIN, A.H. 1983. The character and evolution of hydrothermal fluids associated with the kaolinized St. Austell granite, southwest England. Journal of the Geological Society, London, 140, 297-309. --, -& THOMPSON,M. 1992. Fluid inclusion chemistry as a guide to tin mineralization in the Dartmoor granite, south-west England. Journal of Geochemical Exploration, 46, 163-185. ATKINSON, P., MOORE, J. t~ EVANS, A~M. 1982. The Pennine orefields of England with special reference to recent structural and fluid inclusion investigations. Bulletin of the Bureau de Recherche Gites Mineraux, Section II, no. 2, 149-156. BAUER, G. [Published under his latinized name of G. Agricola] 1556. De Re Metallica. (Translated into English in 1950 by: H.C. and L.H. Hoover, Dover New York). BOTT, M.H.P. 1967. Geophysical investigations in the northern Pennine basement rocks. Proceedings of the Yorkshire Geological Society, 36, 139-168. -8z MASSON-SMITH, D. 1957. The geological interpretation of a gravity survey of the Alston block and the Durham coalfield. Quarterly Journal of the Geological Society of London, 113, 93-117. BOTI'RELL, S.H. & YARDLEY, B.W.D. 1988. The composition of granite derived ore fluid from SW England determined by fluid inclusion analysis. Geochimica et Cosmochimica Acta, 52, 585-588. BOYLE, R.W. 1991. Auriferous Archean Greenstone-Sedimentary Belts. Economic Geology Monograph. 8, 164-191. BRADSHAW, P.M.D. & STOYEL, A.J. 1968. Exploration for blind orebodies in southwest England by the use of geochemistry and fluid inclusions. Transactions of the Institution of Mining and Metallurgy, 77, 437-448. BREWSTER, D. 1823. On the existence of two new fluids in the cavities of minerals, which are immiscible, and possess remarkable physical properties. Edinburgh Philosphophical Journal, 9, 268-270. BROWN, G.C., IXER, R.A., PLANT, J. • WEBB, P.C. 1987. Geochemistry of granites beneath the North Pennines and their role in ore formation. Transactions of the Institution of Mining and Metallurgy, 96, 65-76. CARPENTER, A.B., TROUT, M.L. & PICKET~, E.E. 1974. Preliminary report on the origin and chemical evolution of lead- and zinc-rich oilfield brines in central Mississippi. Economic Geology, 69, 1191-1207. CHAROY, B. 1979. Definition et importance des phenomenes deuteriques et des fluides associes dans les granites. Consequences metallogenique. Science Terre, Nancy, Memoire, CHRISTOULA, M. 1992. Fluid inclusion geochemistry of selected epigenetic, low temperature mineralization in the U.K. Ph.D. Thesis, University of London. COLMAN, T.B., FORD, T.D. & LAFFOLEY, N.D'A. 1989a. Metallogeny of Pennine orefields. In: PLANT, J.A. & JONES, D.G. (eds) Metallogenic models and exploration criteria for buried carbonate-hosted ore deposits---a multidisciplinary study in eastern England. Institution of Mining Metallurgy, London, 13-24. , JONES, D.G., PLANT, J.A. & SMITH, K. 1993. Metallogenic models for carbonate-hosted (Pennine and Irish-style) mineral deposits. In: PLANT, J.A. & JONES, D.G. (eds) Metallogenic models and exploration criteria for buried carbonate-hosted ore deposits--a multidisciplinary study in eastern England. Institution of Mining Metallurgy, London, 123-133. DAVISON, E.H. 1927. Recent evidence confirming the zonal arrangement of minerals in the Cornish lodes. Economic Geology, 22, 475-479. DAVEY, H. 1822. On the state of water and aeriform matter in cavities found in certain crystals. Philosophical Transsactions Royal Society of London, part 2, 367-376. -

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37.

HYDROTHERMAL

OREFIELDS

DEICHA, G. 1955. Les lacunes des cristaux et leurs inclusions fluides. Masson et Cie, Paris. DINES, H.G. 1956. The metalliferous mining region of south-west England. Memoirs of the Geological Survey of Great Britain, (HMSO London). DUNHAM, K.C. 1934. Genesis of the North Pennine ore deposits. Quarterly Journal of the Geology Society of London, 90, 689-720. 1983. Ore genesis in the English Pennines : A fluoritic subtype. In: KISVARSANYI, G., GRANT, S.K., PRATT, W.P. & KOENIG, J.W. (eds) -

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International conference on Mississippi Valley type, lead-zinc deposits. Rolla Conference Proceedings Volume, Univ. Missouri-Rolla Press, 85-112. - t~ W I L S O N , K. 1985. Geology of the Northern Pennine orefield: Volume 2. Stainmore to Craven. Economic Memoir of the British Geological Survey. , HODGE, B.C. & JOHNSON,G.A. 1965. Granite beneath visean sediments with mineralization at Rookhpe, northern Pennines. Quarterly Journal of the Geology Society of London, 121, 383-414. ELL1E DE BEAUMONT, J.D. 1847. Note sur les emanations volcaniques et metalliferes. Societe Geologique de France, Bulletin, 2e series, 4, 1249-1 FERGUSON, J. 1991. The organic geochemistry of hydrocarbon gases in fluorite from Northern England. Journal of Petroleum Geology, 14~ 221-228. GREENWOOD, D.A. & SMITH, F.W. 1977. Fluorspar mining in the northern Pennines. Transactions of the Institution of Mining and Metallurgy. 86, 181-190. GUIEBERT, J.M. & PARK, C.F. (JR.). 1986. The geology of ore deposits'. Freeman and Company, New York. HALLS, C., EXEEY, C.S. & BRUTON, E.V. 1985. A bibliography of magmatism and mineralization in southwest England. Occasional Paper No. 5, Institution of Mining and Metallurgy, London. HIGHLEY, D.E., SEATER, D. t~z CHAPMAN, G.R. 1991. Minerals Extraction-positive and negative trends. Bulletin of the Institution of Mining and Metallurgy (Minerals Industry International), No 998, 15-20. IXER, R.A. & VAUGHAN,D.J. 1993. Lead-zinc-fluorite-baryte deposits of the Pennines, North Wales and the Mendips. In: PATTRICK,R.A.D. & POLYA, D.A. (eds) Mineralization in the British Isles. Chapman & Hall, London, 355-395 JACKSON, N.J. 1977. The geology and mineralisation of the St. Just mining district, west Cornwall. Ph.D. thesis, University of London. -1979. Geology of the Cornubian tin field. Bulletin of the Geological Society of Malaysia, 11, 209-237. & RANKIN, A.H. 1976. Fluid inclusion studies at St. Michael's Mount. Proceedings of the Ussher Society, 3, 430-434. , HALLIDAY, A.N., SHEPPARD, S.M.F. & MITCHELL, J.G. 1982. Hydrothermal activity on the St. Just mining district, Cornwall, England. In: EVANS A.M. (ed.) Metallization associated with acid magmatism. Wiley & Sons, Chichester 137-179. , MOORE, J. McM. & RANKIN, A.H. 1977. Fluid inclusions and mineralisation at Cligga Head, Cornwall. Journal of the Geological Society, London, 134, 343-349. - - , WILEIS-RICHARDS,J., MANNING,D.A.C. & SAMS, M. 1989. Evolution of the Cornubian Ore Field, Southwest England : Part II. Mineral Deposits and Ore-froming Processes. Economic Geology, 84, 1101-1133. JUDD, J.W. 1908. Henry Clifton Sorby and the Birth of Microscopical Petrology. Geology Magazine, Decade V, 5, 192-204. KALYUZHNYI, V.A. 1960. Methods of study of multiphase inclusions in minerals. Kiev Izdatel. Akad. Nauk. Ukrainskoy RSR [in Russian, cited in Roeder 1948 below]. LEMMLEIN, G.G. 1956. Formation of fluid inclusions and their use in geological thermometry. Geochemistry International, 6, ]Translated from Russian]. LINDGREN, W. 1933. Mineral Deposits, 4th edition. McGraw-Hill, New York. MOSER M. R., RANKIN, A.H. & MILLEDGE, H.J. 1992. Hydrocarbon-bearing fluid inclusions in fluorite associated with the Windy Knoll bitumen deposit, UK. Geochimica et Cosmochimica Acta, 56, 155-168. PHILLIPS, J.A. 1875. The rocks of the mining districts of Cornwall and their relation to metalliferous veins. Quarterly Journal of the Geologaical Society of London, 31,319-345. PLANT, J.A. & JONES, D.G. 1989. Introduction. In: PLANT,J.A & JONES, D.G. (eds) Metallogenic models and exploration criteria for buried carbonate-

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hosted ore deposits--a multidisciplinary study in eastern England.

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ORE

FLUIDS

247

the Northern Pennine Orefieid, England and their genetic signifcance. Transactions of the Institution of Mining Metallurgy, 97, 99-107. --, HERRINGTON,R.J., RAMSEY, M.R., COLES, B., CHRISTOULA,M. & JONES, E. 1993. Current developments and applications of ICP-AES techniques for the gochemical analysis of fluid inclusions in minerlas. Proceedings of the Quadriennial IAGOD Symposium, 8, 185-198. ROEDOER, E. 1967. Environment of deposition of stratiform (Mississippi Valley type) ores deposits, from studies of fluid inclusions. Economic Geology Monograph, 3, 349-362. 1972. Composition of fluid inclusions. U.S. Geological Survey Professional Paper 440-JJ. 1984. Fluid Inclusions. Mineralogical Society America, Reviews in Mineralogy, 12. 1990. Fluid inclusion analysis--prologue and epilogue. Geochimica et Cosmochimica Acta, 54, 495-507. ROGERS, P. 1977. Fluid inclusion studies on fluorite from the Derbyshire orefield. Transactions of the Institution of Mining and Metallurgy, 76, 128-132. 1978. Fluid inclusion studies on fluorite from the Askrigg block. Transactions of the Institution of Mining and Metallurgy, 87, 125-131. RUSSELL, M.J. & SKAULI, H. 1991. A history of theoretical developments in carbonate-hosted base metal deposits and a new tri-level enthalpy classification. Economic Geology Monograph, 8, 96-116. SAMS, M.S. & THOMAS-BEVTS, A. 1988. Models of convective fluid flow and mineralization in southwest England. Journal of the Geological Society, London, 145, 809-817. SAWK1NS,F.J. 1966a. Ore genesis in the north Pennine orefield in the light of fluid inclusion studies. Economic Geology, 61, 385-391. 1966b. Preliminary fluid inclusion studies of the minerlization associated with the Hercynian granites of SW England. Transactions of the Institution of Mining and Metallurgy, 75, 109-112. SHEPHERD, T.J., MILLER, M.F., SCRIVENER, R.C. & DARBYSHIRE,D.P.F. 1985. Hydrothermal fluid evolution in relation to mineralization in southwest England with special reference to the Dartmoor-Bodmin area. In: HALLS, C. (ed.) High heat production (HHP) granites, hydrothermal circulation and ore genesis. Institution of Mining Metallurgy, London, Special Publication, 345-364. SMALL, A.T. 1978. Zonation of P b - Z n - C u - F - B a mineralization in part of the Yorkshire Pennines. Transactions of the Institution of Mining and Metallurgy, 87, 9-14. SMITH, F.G. 1978. Historical development of inclusion thermometry. Univ. Toronto Press, Canada. SMITH, F.W. 1973. Fluid inclusion studies on fluorite from the North Wales orefield. Transactions of the Institution of Mining and Metallurgy, 82, 174-176. -1974. Factors governing the development of fluorspar orebodies in the North Pennine orefield. Ph.D. Thesis, University of Durham. • PHILLIPS,R. 1974. Temperature gradients and ore deposition in the north Pennine orefield. Fortschrifie Mineralogie, 52, 491-494. SOLOMON, M., RAFTER, T.A. & DUNHAM, K.C. 1971. Sulphur, and oxygen isotope studies in the Northern Pennines in relation to ore genesis. Transactions of the Institution of Mining and Metallurgy, 80, 259-275. SORBY, H.C. 1858. On the microscopical structure of crystals, indicating the origin of minerals and rocks. Quarterly Journal of the Geological Society of London, 14, 453-500. SPURR, J.E. 1923. The Ore Magmas. McGraw-Hill, New York. TAMMEMAGI, H.Y. & SMITH, N.L. 1975. A radiogeological study of the granites of SW England. Journal of the Geological Society of London, 131, 415-427. WILKINSON, J.J. 1990a. The origin and evolution of Hercynian crustal fluids', South Cornwall, England. PhD thesis, University of Southampton. - - 1 9 9 0 b . The role of metamorphic fluids in the development of the Cornubian orefield: fluid inclusion evidence from south Cornwall. Mineralogical Magazine, 54, 219-230. --, RANKIN, A.H., MULSHAW, S.C., NOLAN, J. & RAMSAY, M. 1994. ICP-linked laser ablation for the determination of metals in fluid inclusions: an application to the study of magmatic ore fluids, S.W. England and New Mexico. Geochimica et Cosmochimica Acta, in press. WILL1S-RICHARDS, J. & JACKSON, N.J. 1989. Evolution of the Cornubian orefield Southwest England: Part I. Batholith modelling and ore distribution. Economic Geology, 84, 1078-1100. YERMAKOV, N. P. 1965. Research on the nature of mineral-forming solutions with special reference to data from fluid inclusions. International Monographs in Earth Sciences, 22. Pergammon Press, New York. -

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-

From QJGS, | 4, 453.

On the MICROSCOPICAL STRUCTURE 0 f CRYSTALS, indicating the ORIGtN of MINERALS and ROCKS. B y H . C. SORSY, E s q . , F.R.S., F.G.S., Corresponding Member of the Lyceum of Natural H i s t o r y o f N e w Y o r k , a n d o f t h e A c a d e m y o f N a t u r a l Sciences o f P h i l a d e l p h i a , &c. [Read December 2, 1857.]

[PLAT~-S XVI.-XIX.] CONTENTS. § 2. Water contained in Crystals. § 3. Minerals contained in Secondary Rocks. a. Rock-salt,Calcite,&c. b. Quartz-~'eins. § 4. Metamorphic Rocks. § 5. Minerals and Rocks formed by cooling from a stateof igneous fusion. § 6. Minerals and Rocks formed by the combined operation of Water and Igneous Fusion. a. Minerals in the blocks ejected from Vesuvius. b. Granitic Rocks. c. Temperature and Pressure under which GraniticRocks have been formed. Description of the Plates.

History of the Subject. I. Structure of Artificial Crystals. § l. Crystals formed from Solution in Water. a, Mode of Preparation and Examination ; general and special characters. b. Number, size, form, and arrangement of Cavities. e. Expansion of Fluids by Heat. d. Effects of Pressure. e. The Elastic Force of the Vapour of Water. § 2. Crystals formed by Sublimation. § 3. Crystals formed by Fusion. § 4. General Conclusions.

lI. Structure of Natural Crystals. § 1. Methods employed in examining Minerals and Rocks.

IN this p a p e r I shall a t t e m p t to p r o v e t h a t artificial a n d n a t u r a l crystalline s u b s t a n c e s possess sufficiently c h a r a c t e r i s t i c s t r u c t u r e s to p o i n t o u t w h e t h e r t h e y were d e p o s i t e d f r o m solution in w a t e r or crystallized f r o m a m a s s in t h e state o f igneous fusion ; a n d also t h a t in some cases an a p p r o x i m a t i o n m a y h e m a d e to t h e r a t e at, a n d t h e t e m p e r a t u r e a n d p r e s s u r e u n d e r w h i c h t h e y were f o r m e d .

From QJGS,?0, 689.

THE

GENESIS

OF THE NORTH ORE DEPOSITS I

PENNINE

BY KINGSLEY CHARLES DUNHAM, P H . D . B . S C . Read

February

[PLATES X X I V

7th,

F.G.S.

1934

& XXV.]

CONTENTS

I. II. III. IV.

V. VI. VII.

VIII.

Introduction ................................................ H i s t o r y of i n v e s t i g a t i o n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scope of t h e present s t u d y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deposits o f t h e P e n n i n e t y p e . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) F o r m o f t h e deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) L a t e r a l d i s t r i b u t i o n o f minerals ... (i) F l u o r s p a r a n d b a r i u m minerals ......... (ii) Chalcopyrite, sphalerite, galena ......... (iii) P y r i t e a n d m a r c a s i t e . . . . . . . . . . . . . . . . . . . . . (iv) R a r e cobalt a n d nickel minerals ...... (v) Q u a r t z a n d c h a l c e d o n y . . . . . . . . . . . . . . . . . . (vi) A r a g o n i t e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (vii) Dolomite, siderite, calcite ............... (c) Vertical distributior~ o f minerals ............... (i) Sulphides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (ii) B a r y t e s a n d fluorspar . . . . . . . . . . . . . . . . . . . . . (d) H y p o t h e s i s o f zonal d i s t r i b u t i o n ............ (o) T e x t u r a l relations o f t h e minerals ............ (i) B a n d e d veins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (ii) G r a n u l a r veins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (iii) U n i t y o f t h e mineralization ............... (iv) Paragenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Deposits of t h e Great S u l p h u r Vein t y p e ............ (a) F o r m a n d m i n e r a l o g y . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) Age-relations w i t h t h e P e n n i n e t y p e ......... Ago of t h e mineralization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ore genesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (a) L a t e r a l secretion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (b) F r o m t h e W h i n Sill . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (c) F r o m a s u b - P e n n i n e m a g m a .................. List of Works to w h i c h reference is m a d e ............

Page 689 690 692 693 693 694 694 695 699 699 699 699 700 700 700 702 703 706 706 708 708 708 710 710 711 712 713 714 714 715 716

From Le Bas, M. J. (ed.), 1995, Milestones in Geology, Geological Society, London, Memoir No. 16, 249-263 First published in Journal of the Geological Society, London, Vol. 150, 1993, pp. 637-651

Carbonate magmas D.K.

BAILEY

Department o f Geology, University of Bristol, Wills Memorial Building, Queens Road, Bristol, BS8 1RJ, UK Abstract: For 40 years, the case for the existence of carbonate magmas rested on field observations of carbonatite intrusions, in which the lack of thermal effects raised an apparent conflict with the high melting temperatures of pure carbonates. Since 1960, the position has changed, with the growth of experimental studies and increasing observations of effusive carbonate rocks. A nephelinite/phonolite volcano in Tanzania is currently erupting Na-Ca-K carbonate magma (around 600 °C). This is unlike all other intrusive and effusive carbonatites (350 examples worldwide) which are dominantly composed of Ca, Mg, Fe carbonates, and have negligible alkali contents. Although a number of effusive calcio-carbonatites are considered to be degraded alkali carbonatites, there are several examples (including one magnesi0-carbonatite) which are close to their erupted composition, and substantiate the existence of high T carbonate magmas lacking essential alkalis at the time of eruption. In these associations silicate magmas are absent (or minor), and in most the effusive carbonatites have been erupted directly from the mantle (with entrained peridotite debris and minerals). They provide a link with the ultramafic association (peridotite and pyroxenite), seen in some carbonatite intrusions, with the commonly associated ultramafic lamprophyres (which may also carry mantle xenoliths), and with carbonate-rich kimberlites. Many carbonatite intrusions also have little or no associated silicate magmas, putting in question a popular view that carbonatites normally form only minor parts of alkaline igneous complexes of nephelinite/phonolite type. The corollary, that the carbonatites are normally differentiates is even less sound, because in alkaline complexes the carbonatite is always last in the eruption sequence. Here the carbonatite may represent the final residua expelled from the source region. Most large carbonatite intrusions seem to have been emplaced at lower T than effusives, probably as a near-solidus mush, with the interstitial fluid metasomatizing the country rocks. A wider perspective of carbonate magma genesis is called for, to encompass various kinds of differentiation from alkaline silicate magmas, and primary carbonate magmas from various depths in the mantle (with or without silicate melts). The strongly bimodal composition distribution of calcic and dolomitic carbonatites is a further factor awaiting explanation. Half of the known carbonatites are in Africa, and their timing and distribution indicate that the activity is a response to lateral forces acting across the plate. Carbonate magmatism is waiting to be unleashed. This activity demands attention because it is now clear that carbonate magmatism is a crucial surface expression of deep mantle processes.

able to list 32; by 1966, when two books were published (Tuttle & Gittins 1966; Heinrich 1966), the total had risen to nearly 200, and at the last published count (Woolley 1989) there were around 330 (about half of which are in Africa). In choosing 11 examples from the 23 known in Africa at that time Campbell Smith (1956) could essay an all-embracing compass, summing up the salient facts, introducing the rival hypotheses, and presenting a fascinating snapshot of the state of the 'art'. A similar review now, of the same length, could do scant justice to the subject, because quite apart from the sheer increase in research, and fundamental new discoveries, the whole context has changed. Of the seven special features perceived by Campbell Smith as 'subsidiary problems' (pp. 212-213), five (magmatic associations, multiphase activity, multiple intrusion, explosive activity, and range of vent diameters) are now widely recognized as general, although not necessarily invariable, features of carbonatites. Minerals peculiar to carbonatites have received much attention since 1956 (e.g. Hogarth 1989). The concommitant high levels of mobile and silicate incompatible elements, and the extensive metasomatism

A turning point in carbonatite studies was marked in November 1956, with the simultaneous publication of reviews by two major geological societies (Campbell Smith 1956; Pecora 1956). From the time of the original use of the term 'Karbonatite' (to describe the carbonate rocks of Fen, S. Norway) a controversy had smouldered over Br6gger's (1921) proposal that these were products of carbonate magmas. By 1956 the n e e d for a review in the wider geological forum, of these hitherto rare rocks, had become evident in the rapidly increasing discoveries of carbonatites in the stable parts of the continents. Campbell Smith's Presidential Address to the Geological Society of L o n d o n was both apt and timely, and appropriately titled ' A review of some problems of African carbonatites'. Most of the new discoveries following the second world war were in Africa, and Campbell Smith had played a crucial part in the first positive recognition of carbonatites in that continent (Dixey et al. 1937). Both reviews, of course, have many parallels, and a number of interesting differences. Each highlighted the growing pace of new carbonatite discoveries, and predicted that this would not slacken. In 1956, Pecora was 249

250

D.K.

(fenitization) are intrinsic features, which must always have a bearing in any consideration of carbonatite magmatism. Fenitization has continued as an important strand in carbonatite studies, but within the larger context of carbonate magmatism can only be considered in terms of alkali mobility. A summary of fenitization, with leading references is given by McKie (1989). One of the 'two main problems' for Campbell Smith was carbonatite 'mode of emplacement', and the 'ultimate problem' (p.216) was the source of the CO2. These constituted the main questions for Pecora (1956) also, and in one shape or form they have always been, and are still the nub of the argument concerning igneous carbonates. But, as mentioned above, the context of the argument has changed radically since 1956. Not only can we now draw on extensive evidence of effusive carbonatite activity, and experiments on carbonate melting and crystallization, but entirely new concepts of melt generation colour petrological thought. When Campbell Smith (and Pecora 1956) allude to source regions for carbonatite, they refer to the deep crust, or the deeper parts of the Earth--neither uses the word 'mantle'. The dawn of our present intellectual milieu was still to break, with the recognition that the primary characteristics of most magmas are determined by partial melting in the Earth's mantle (Yoder & Tilley 1962). Regardless of whether a carbonatite is primary, or a differentiate of an associated silicate melt, considerations of carbonatite magmatism are now constrained by knowledge of mantle compositions and physical states, together with experimental evidence on mantle melting conditions. The 'two main problems' of Campbell Smith must, of course, be re-examined with this perspective, and the intention in the present paper is to do this firstly, through present knowledge of magmatic rocks and experimental melts, and secondly, through what may be termed the ultramafic connections. In his review, Campbell Smith selected his examples of African carbonatites in a series of increasing age, to illustrate different erosion levels. As a result he was able to point out (p.216) that t h e r e is a general pattern in carbonatite activity, produced repeatedly. He goes on to cite Dixey that 'carbonatites are a normal part of the magmatic history . . . of Africa over a long period of geological time'. This aspect of carbonatite activity has potentially profound implications in petrogenesis, and will be explored after the main discussion of carbonate magmas.

Carbonate magmas Underlying Campbell Smith's examination of the 'Problem of mode of emplacement' was the question of whether or not carbonatite was truly igneous or magmatic, i.e. what was the physico-chemical regime at the time of carbonatite formation. Large, central carbonatite masses have given rise to the largest range of proposals, and his perceptive discussion looks at all the then suggested mechanisms, from replacement, hydrothermal deposition, plastic flow (akin to salt domes), crystal mush, to purely magmatic intrusion. For Campbell Smith the crucial evidence lay in the dykes and cone sheets, which seemed to require a 'carbonatitic magmatic liquid' as envisaged by von Eckermann (1948).

BAILEY

Magmatic evidence from intrusive carbonatite petrography Many of the features touched on by Campbell Smith have been substantiated in later studies, especially the importance of fluidization for fragmental intrusions. Evidence of replacement is widespread, but no longer seen as necessarily an obstacle to a magmatic origin, because it mostly applies to replacement of carbonate, e.g. dolomitization, or sub-solidus recrystallization of a carbonatite protolith. Replacement has been specifically addressed by Barker (1989, 1993) who points out that many (if not most) plutonic silicate rocks have undergone sub-solidus reequilibration such that any original igneous texture may be largely a palimsest: he argues that such a process is even more likely to affect easily recrystallized carbonate rocks. Equally, the formation of some carbonatite bodies by hydrothermal/carbothermal deposition (e.g. Mountain Pass, California) does not vitiate magmatic evidence in other complexes: a continuum from magma to fluids may be expected in the natural environment. Field relations are all important, and have been fully reviewed by Barker (1989). But there must be a distinction between convincing magmatic field relations (which are likely to survive sub-solidus changes) and finding critical evidence of the nature of the original magma. The igneous characteristics of the regime may still be discernible, but the composition and original phase relations may be lost, or at best obscured. Flow structures, for instance, may be interpreted as the fabric from an intrusive magma, but replacement may have destroyed the original composition and texture. The same caveat may apply even to porphyritic texture, although here the original phenocrysts may survive to provide valuable information. More serious, however, is the fact that most intrusive carbonatites also show signs of chemical exchange with their wall rocks, as seen most dramatically in alkali metasomatism (fenitization). Even if this exchange could be accurately quantified, the facts of exchange and replacement, reveal a great residual uncertainty about how much of the material that was emplaced still remains. Still more perplexing is the unknown quantity of material that passed through, and out of, the conduit prior to and after it was filled. Any intrusion and its wall rocks are but remnants of volatile-rich systems and the cautionary reminder that igneous rocks are dead magmas must surely be imperative for intrusive carbonatite. Hence, the closest approach to original magma must be sought among the rapidly quenched effusives. These can provide better understanding of magmas and processes, and guide the interpretation of existing laboratory studies and the design of new experiments. In his gentle advocacy of carbonatite magmas, Campbell Smith differed from Pecora (1956) who leaned more towards a spectrum of dense fluids, a difference surely attributable to Campbell Smith's extensive personal contact with studies of the sub-volcanic complexes in East and Central Africa. His selected African examples are listed in order of increasing depth of erosion, and at the head is Kerimasi volcano, Tanzania, where James (1956) had just reported carbonatite on the flanks and in the crater. Effusive carbonatites could resolve many of the questions about the nature of carbonatite intrusions but neither Campbell Smith, nor Pecora (who also highlighted

CARBONATE MAGMAS the volcanic connections) felt able to speculate on what form the activity might take. In this they were undoubtedly restrained by the limited evidence on experimental carbonate-melt relations, which was then irreconcilable with geological observations (and where Pecora rightly perceived new experiments would be pivotal). By one of the ironies of science, the twin volcano of Kerimasi, Oldoinyo Lengai, had been recorded as erupting carbonate ashes several times earlier this century (Hobley 1918; Richard 1942; Guest, 1956) with the added irony that the 1917 eruption receives special mention by that great advocate of limestone syntexis, S.J. Shand in his text book, Eruptive Rocks (1927, p. 36). In 1956 the significance was missed, or had been lost from sight, and some of the essentials needed to carry forward the discussion of effusive activity (and experimental melting) would have to wait until 1960. That year saw the eruption of alkali carbonatite lavas in the crater of Oldoinyo Lengai, (Dawson 1962), the laboratory production of Ca carbonate melts at low T and P (Wyllie & Tuttle 1960), and the description of effusive dolomitic carbonatites of Cretaceous age in Zambia (Bailey 1960). There can be little question that widespread doubts about the existence (or even possibility) of carbonate melts were largely dispelled by the experimental results, closely followed by reports of the Lengai lavas, although new questions were raised by both. The experimental melts were compositionally close to calcio-carbonatite, but strongly hydrous, while the Lengai lavas were anhydrous, but quite unlike other carbonatites in bulk composition. New lines of research were opening, to be supported by growing developments in analytical methods, especially for trace elements and isotopes.

251

A l k a l i carbonate m a g m a s Oldoinyo Lengai, in northern Tanzania, is the only volcano at which flowing carbonate magma has been observed (Dawson 1962). By weight the rock is nearly 60% sodium carbonate, 30% calcium carbonate, and 10% potassium carbonate, a representative analysis being given in Table 1. Any losses, to wall rocks prior to eruption, and in gas and sublimates during eruption, are at present unknown but, regardless of what these may have been, the composition is unique. All other carbonatites are essentially Ca, Mg, Fe carbonates, with very low alkali contents (Table 1). Wall rock contamination at Lengai must be minimal because the silica content of the natrocarbonatite is extremely low. The marked difference between natrocarbonatite and all others gave rise to new concepts, including the proposal that loss of alkalis to wall rocks at depth would yield the non-alkali types (Dawson 1964), and carbonate-silicate liquid immiscibility, arising from experiments in alkali carbonate-silicate systems (Koster van Groos & Wyllie 1966). New controversies were born, most of which have at their root the question of whether natrocarbonatite is parental to virtually all other types. The question is natural and needs examination, but the debates have tended to polarize, choosing to ignore the alternative that natrocarbonatite may be simply one type among many. Fortunately, a new multi-author volume with Lengai as its central theme, is in press (Bell & Keller 1994), so that here it is appropriate to focus on the characteristics of the magma and its possible phase relations, so that it may be considered within the framework of other magmatic evidence. Petrographically, the lava is porphyritic with phenocrysts

Table 1. Compositions of effusive carbonatites, as examples of carbonate magmas SiO 2 TiO 2 AI20 3 Fe20 3 FeO MnO MgO CaO Na20 K20 PzOs H20 1. Natrocarb. (Lengai) 2. Calcio-c (Kerimasi) 3. Calcio-c (Ft. Portal) 4. Calcio-c (Polino) 5. Calcio-c (Kaiserstuhl) 6. Calcio-c (Emirates) 7. Magnesio-c (Rufunsa)

0.11

0.09

0.28

0.04

0.53

13.9

32.2

8.27

0.90

0.39

tr.

0.07

0.34

0.22

0.21

54.0

0.10

0.05

1.82

41.8

13.0

1.74

3.03

7.93

0.40

8.55

36.0

0.73

0.20

3.32

3.45 14.8

16.2

0.52

3 . 9 1 3.69

1 . 3 1 0.07

7.31

38.7

0.05

0.50

0.60

3.12 24.1

0.45

0.03

0.15

0.41

0.36

52.6

0.04

0.50

1.56

1.16 39.8

7.45

1 . 0 1 1 . 7 5 8.30

3.12

0.46

3.27

40.5

0.23

0.14

7.00

1.2

0.49

1.56

19.0

28.8

4.44

0.98

tr.

CO 2 34.7

F

CI

SO 3

SrO

2.93

4.21

2.18

1.53 1.04 0.40

0.2

0.08

0.63

0.15

0.54

0.67

0.15

24.58

References 1. Dawson 1989, table 11.3, p. 269 (Anal. 1). 2. Mariano & Roeder 1983, table 1, p. 451 (Anal. 3). 3. Barker & Nixon 1989, table 5, p. 172 (Anal. 1) F value calculated from mode and mineral analyses. 4. Lupini & Stoppa in press. 5. Keller 1989, table 4.1, p. 79 (Anal. KB2). 6. WooUey et al. 1991, table 3, p. 1160 (Anal. Type 1, Mean). 7.'Bailey 1989, table 1, p. 416 (Anal. 3: dolomite melt droplet). c, carbonatite.

BaO

1.10

252

D. K. B A I L E Y

of nyerereite (NaCa carbonate) and gregoryite (Na, K, Ca carbonate) in a matrix of the same minerals. Details of the compositions are given in Gittins & McKie (1980). Their diagram showing the natural phase compositions in the synthetic system, Na2CO3-K2CO3-CaCO3 at lkbar, is a helpful presentation, followed here in Fig. 1. Estimates of eruption temperatures suggest a small range, with the 1960 flows being reported as not incandescent at night, while incandescence was observed in 1988, and Dawson (1989) records that temperatures were consistently between 560 and 580°C, which must be close to the lower limit of incandescence. These temperatures are similar to a value around 600 °C for the gregoryite-nyerereite cotectic at 1 atmosphere pressure, estimated by Cooper et al. (1975) on the basis of experiments on the natural lava. This temperature is over 100 °C lower than those in the synthetic system (Fig. 1), presumably due to the high levels of fluxing components (especially halogens) in the natural lava. Flow morphology has varied from highly mobile, vesicular flows that reached the crater walls (200-250 m), to very viscous, short, and slow moving extrusions (Dawson 1989). All these observations are consistent with a melt near a cotectic. Reference to Fig. 1 shows that the lava compositions plot close to, and sub-parallel with, the 1 atmosphere cotectic of Niggli (1919). Especially significant is the position of the lava groundmass in the midst of the bulk compositions and close to the tie line between its phenocryst compositions. These features are precisely those to be expected from a cotectic melt, and suggest that the natural cotectic is recording a low pressure of equilibration, around 150-200 bar, for the natrocarbonatite, i.e. within 500m of the surface. It is noteworthy that Cooper et al. (1975), when reporting their experiments on a natural sample, concluded that the bulk composition is 'probably close to the cotectic'. When considering the status of the natrocarbonatite, it is useful to recall that the lava eruptions (within the crater) are a tiny fraction of the 2000 m volcano composed largely of fragmental nephelinite and phonolite (c. 60 km3: Dawson, 1989). The unusual composition has given rise to the widest diversity of hypotheses about its origin. (1) Immiscible separation of carbonate melt from a parental silicate melt in the range nephelinitephonolite. Le Bas (1989) has reviewed the petrological case, and Kjarsgaard & Hamilton (1989) the relevant experimental evidence. A corollary is that if the initial carbonate melt is more calcic, fractionation of calcite can yield more sodic residual melts.

(2) Prolonged fractionation of an original primary calciocarbonatite under anhydrous conditions (Gittins 1989). (3) Melting of troniferous sediments (akin to those around the neighbouring Lake Natron) in the volcanic substructure (Milton 1968, 1989; Peterson & Marsh 1986). With the exception of 3, all hypotheses place natrocarbonatite as the product of extended differentiation: fractionation in the silicate line culminating in phonolite, and then natrocarbonatite separation; nephelinite/ carbonatite immiscibility, followed by carbonatite fractionation; or extended fractionation in primary calciocarbonatite. The fact that the erupted melt at Oldoinyo Lengai corresponds to a low pressure cotectic is not taken into account, perhaps largely because Niggli's (1919) results have been ignored. The simplest interpretation of the phase relations would be that the lavas are minimum melts from bulk compositions close to the gregoryite-nyerereite join. All other propositions require qualifying assumptions about the P T X history of any supposed earlier melt. Fractional crystallization of calcio-carbonatite would require some yet to be defined step to effect the transposition into the natrocarbonatite system (see Fig. 2), and even then eruption of the minimum melt composition alone would still need explanation. Equally unexplained at present is how, or why, a melt with cotectic characteristics should form by immiscible separation from a high T silicate melt. Of course our present view of the natural magmatism may be blinkered; on a geological timescale we have only a single instantaneous 'freeze frame' of the activity, but this stricture applies equally to all hypotheses advocating natrocarbonatite as a general model for carbonatite petrogenesis. Purely on the basis of the phase relations, the melting hypothesis is more plausible but as many have pointed out, e.g. Cooper et al. (1975), the overall chemistry, and especially the stable isotope chemistry, is not compatible with sediment melting. A powerful consensus in favour of this view may be presumed from the absence of any mention of the hypothesis in the latest multi-author book on carbonatites (Bell 1989). There is, however, another possibility that is free from the difficulties raised by the melting of sediment, namely that the source might be older carbonates in the volcanic pile. These may form initially as high T sublimates from continued CO2-rich exhalation from silicate magmas. Subsequent ascent of magma within the volcano could mobilize these, driving the resultant low viscosity melts ahead through the axial zone. These alkali carbonate melts would not invariably be erupted as flows: in

K2CO3

;9

!

.'/

,. Na2CO3

C NY

CC

CaCO3

Fig. 1. Adapted from Gittins & McKie (1980), showing the positions of analyses for natrocarbonatite (open circles); nyerereite and gregoryite phenocrysts (triangles); and groundmass (solid square); projected onto the phase diagram for N a 2 C O 3 - K 2 C O 3 - C a C O 3 at I kbar. The broken curve C-C is the cotectic at 1 atms. calculated from Niggli (1919).

CARBONATE MAGMAS 1400

i

-

I

i

1400

i

P= 1 K b a r

/

7

/ /

1200

1200

/

L

/

oo

/

/ /

< rr

1000 NCss+L

u.l b.-

T

4¸ NY+L

1000

CC+L C

800

/

I

//

600 /

1 60

I NY

800

--

600

CC+NY

NC+NY I 80

--

I 40

Na2CO 3

I 20 CaCO 3

WT, PERCENT

Fig. 2. Schematic binary diagram to illustrate the constraints of temperatures, phase relations, and compositions on the natrocarbonatite erupted from Oldoinyo Lengai (corresponding to a melt with cotectic characteristics) adapted from Cooper et al. 1975. In the binary the natural cotectic is represented by the eutectic O. A melt near O cannot produce calcite nor a calcitic residuum, even by a hypothetical loss of alkalis, which could proceed only in the direction of NY (nyerereite) and no further. Any carbonatite with calcite phenocrysts could not have evolved from natrocarbonatite as currently erupted. The most alkaline liquid possible with calcite phenocrysts would be E, and the bulk compositions of calcite-phyric 'magmas' must lie between E and C, i.e. much less alkaline and higher in T than natrocarbonatite O. A specimen containing calcite (Dawson et al. 1987) may fall in this category, but the mantling of calcite by nyerereite suggests that it may be xenocrystic. The diagram shows that where calcite phenocrysts are abundant, e.g. bulk composition X (50% phenocrysts) the liquidus T must be very high, and the alkali content correspondingly small. Such melts would indicate markedly different conditions from those necessary to produce the natrocarbonatite magma currently being erupted from Lengai. Preliminary reports on the effects of F (Gittins 1989; Jago & Gittins 1991; Gittins & Jago 1991) show general lowering of the calcite liquidus by as much as 200 °C (at 8%F). In one of these (Jago& Gittins 1991) it is stated that CaF2 or F 'break the thermal barrier caused by nyerereite'. Critical phase relations are given only in outline, in which there are disparities (between diagrams and in the data) and informed discussion must therefore await publication of the defining experimental data (e.g. run compositions, stable equilibrium phase assemblages). In any case, whether or not the barrier is breached by equilibrium crystallization, calcite would have to be perfectly fractionated (even approaching the solidus) by which point the liquid compositions are already much richer in F than their postulated derivatives, natrocarbonatite lavas. See text and Tables 1 and 3, for F levels and temperature effects in carbonatites. a complex series of activity cycles they may accumulate in and choke the upper part of the conduit, to be intermittently erupted as ashes, which during declining activity fall back and fill the crater. The modern lava activity may represent only a quiet stage when melts are able to ooze out onto the crater floor. This mode of generation would be in keeping with the phase relations, the highly constricted and very small volume activity, the totally anhydrous composition, the U-Th disequilibrium results (Williams et al. 1986), and the unique trace element characteristics (Keller 1992).

253

Because the natrocarbonatite lavas at Lengai are the only flowing carbonate magmas observed, this composition has naturally featured strongly in attempts to account for the formation of non-alkaline carbonatites, most of which are calcitic. Links with natrocarbonatite have been sought for calcitic carbonatites in two contrasting modes: (a) for calcitic lavas and pyroclasts, by calcification of alkali carbonatites through the action of surface waters, and (b) for calcitic intrusions by loss of alkalis from alkali carbonatite, metasomatizing the wall rocks (fenitization). Calcification of original alkali carbonatite volcanics enjoys considerable support, and depends entirely on the recognition of nyerereite pseudomorphed by calcite. Gittins (1989) has provided a review of the position in terms of the phase relations, urging restraint against the over-enthusiastic adoption of the surface replacement processes. He also shows that where (as in many examples) monocrystalline calcite phenocrysts are present, the original alkali contents (if any) need not have been great. A simplified example is illustrated in Fig. 2, where it may be noted also that temperatures on the calcite liquidus are higher than those for natrocarbonatite, meaning that the observed magma is not a possible parent for any carbonatites with calcite phenocrysts. Obviously, alkali melts with a calcite liquidus may exist, but the melt temperatures will be considerably higher than natrocarbonatite, and the alkali content much lower. Actual alkali carbonate melts with calcite phenocrysts have yet to be observed in nature, and their existence still awaits confirmation. No intrusive carbonatites with pseudo-nyerereite have been reported, so that any inference about their earlier alkali content must at present be based on alkali metasomatism of the wall rocks. As noted before, the rocks are but remnants of a volatile-rich, open system, where the total through-put of material is unknown. Alkali activity is evident, but the alkali content of the original magma must be conjectural: the only certainty is that natrocarbonatite could not have been the original melt, because it is a low energy, cotectic composition from which loss of alkalis (if it were possible) could only transpose the material into sub-solidus field of nyerereite (see Fig. 2). If the assumption is that calcitic carbonatite is a residuum after alkali loss, then it may be presumed that the original melt had calcite on the liquidus. From Fig. 2 it may be seen that this would set a maximum alkali carbonate content on the bulk composition around 45%, ranging down to virtually zero. An initial melt with greater than 45% alkali carbonate would be effectively super liquidus (superheated), which entails an unconstrained presumption about the nature of the system: even this melt, however, must instantaneously lose its superheated status once it is in reaction with wall rocks. Hence the general case is represented by an intrusive magma with calcite on the liquidus. The best test of the likely alkali content lies in freshly quenched (unaltered) calcitic effusive rocks. Effusive calcio-carbonatite

Surprisingly perhaps, records of calcitic volcanic rocks slightly predate that of the landmark eruption of natrocarbonatite lava (Dawson 1962). James reported Recent calcitic carbonatite layers on the flanks of Kerimasi volcano (1956); ripple-marked calcitic tufts were reported from the Cretaceous carbonatitic volcanoes of Zambia

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D. K. B A I L E Y

(Bailey 1960, appendix V, plate V); and by 1961 the Recent carbonatite lava flow at Fort Portal had been described (von Knorring & Du Bois 1961). Dawson (1962) also pointed out that calcitic ashes were widely distributed in East Africa, making the valid suggestion that many of these might be degraded natrocarbonatite. This possibility, however, inevitably raised a question about the original composition of any effusive calcio-carbonatite. In those cases where such rocks contain calcite phenocrysts, the erupted bulk composition cannot have been the same as the Oldoinyo Lengai lavas, as indicated above, and in Fig. 2. The whole question of effusive calcio-carbonatites has fortunately been given new perspectives in reviews by Barker (1989) and Keller (1989). Barker, concentrating on field relations combined with petrography, is able to conclude (1989, p.44) that 'srvite liquids can erupt as lavas of low viscosity'. Keller presents detailed evidence on the calcite lapilli tufts of the Kaiserstuhl, showing that a replacement origin is ruled out by the magmatic chemistry combined with 'open inter-clast frameworks and open vesicles in lapilli' (1989, p.85). Some localities where calcitic effusive rocks are closest to magmatic compositions are listed in Table 2. Many other examples of calcitic volcanics are known, especially in East Africa, but are not listed here because they have been designated as calcitized alkali carbonatites (e.g. Clarke & Roberts 1986), but in the light of the Kaiserstuhl experience (Keller 1989) it would seem prudent to keep an open verdict. In terms of texture and composition the air-fall tear drop lapilli of the Kaiserstuhl (Keller 1981) are apparently determined by surface tension at a liquid-gas interface, the droplets having the composition of almost pure calcium carbonate (Table 1). As there seems little scope for major

losses or exchange after eruption, the present composition must be close to that of the original erupted melt. In the absence of a flux (e.g. H20) such a melt would be at a high temperature and above its 1 atmosphere dissociation point, leading to the conclusion that melt fragmentation occurred under pressure in the volcanic vent, and that droplet shaping was effected while the melt was supercooled in flight. Calcite microphenocrysts indicate that the melt was on the liquidus immediately prior to disintegration; the lack of strong concentricity of phenocrysts parallel to the droplet boundaries would be consistent with very rapid undercooling and quenching. Unless substantial amounts of a component (or components) have gone from the Kaiserstuhl lapilli, leaving no trace, high melt temperatures must apply. Calcite crystals in calcitic liquid would require a minimum temperature of 1230 °C at a minimum pressure of 40 bar (Point Q, fig. 20.1, Wyllie 1989). Pressure presents no problem, the melt could rise to 150 m below the surface; indeed, melt ascent to a critically shallow depth might be a plausible fragmentation/eruption trigger. Lowering of the melt temperature by the small amounts of non-calcitic components still present would not be dramatic, suggesting values similar to mafic igneous melts (1000-1200 °C): such values would be consistent with the deduction (Keller 1989) of a genetic link with melilite nephelinite. Similar conclusions emerge from consideration of other associations. At Kerimasi there is a close association of lapilli with melilite phenocrysts and those with calcite phenocrysts, and there are melilitite lapilli tufts cemented by calcium carbonate. In the Fort Portal lava flow, olivine, diopside and phlogopite xenocrysts, are enclosed in a groundmass of calcite, periclase, perovskite, apatite and spurrite, consistent

Table 2. Effusive calcio- and magnesio-carbonatites where the observed compositions are closest to erupted magma Ref & Locality

Cognate minerals

Xenocrysts

A Calcio-carbonatites 1. Ft Portal Cc, Cs, P, Ap, Mo O1, Di, Phi, Per, Ap (Uganda) 2. Catanda Carbonates. O1, Cr-Di, Cr(Angola) Mainly Cc Sp, Phi, Kaer, Ap 3. Polino (Italy) Cc, Zr, (Mo) O1, Phi, OI + Phi 4. U.A. Emirates Cc, Cr-Sp 5. Khanneshin Cc, Ank, Ba (Afghanistan) 6. Rufunsa Cc, Cr-Sp San 7. Kaiserstuhl Cc, Mt, Ap (Germany) 8 Kerimasi Cc (Tanzania)

Silicate melts in Complex

Province

None

K u/m

None

Tinguaite (?) K u/m None

None None Lc tephrite (minor) None Melilitite*

None

Melilitite*

Inference

Primary, direct eruption from mantle Primary, direct Primary, direct Primary direct Primary direct Primary, direct Differentiate, high T Differentiate (?) High T

B. Magnesio-carbonatite

6. Rufunsa (Zambia)

Dol, Cr-Sp

Phi

None

None

Primary Direct

* Silicate melt most closely related to carbonatite Abbreviations: Cc, Calcite; Cs, spurrite; P, periclase; Ap, apatite; O1, olivine; Di, diopside; Phi, phlogopite; Per, perovskite; Sp, spinel; Zr, zirconium garnet; Mo, monticellite; San, sanidine; Mt, magnetite; Dol, dolomite; Ank, ankerite; Ba, barite; Kaer, kaersutite; Lc, leucite; K u/m, potassic ultramafic lava. References: 1. Barker & Nixon 1989; 2. Silva 1973; 3. Stoppa & Lupini 1993; 4. Woolley et al. 1991; 5. Alkhazov et al. 1978; 6. Bailey 1990; 7. Keller 1981; 8. Mariano & Roeder 1983.

CARBONATE MAGMAS with high-T eruption. Although there are no associated silicate magmas at Fort Portal, the olivine-diopsidephlogopite constitutes a heteromorph of olivine leucitite, and in the volcanic fields further south (Katwe-Kikorongo) the characteristic eruptives are olivine leucitite and melilitite lapilli tufts cemented by calcium carbonate (Lloyd 1985). Similarly the only silicate magmas recorded from the Khanneshin carbonatite volcano (Afghanistan) are minor leucite tephrites (Alkhazar et al. 1978). Hence, a close connection in time and space seems to exist for effusive calcio-carbonatites and alkaline ultramafic melts in which low silica activity is marked by the appearance of melilite, leucite, kalsilite and perovskite, and there is no evidence to indicate that carbonate and silicate melt temperatures were radically different. Furthermore, the anhydrous character of the silicate rocks and minerals (and the effusive carbonatites), when fresh, points to low activity of H20 in the larger system. Further insights are provided by the Fort Portal lava which is a small single flow ( 1 - 5 m thick, covering 0.3km z) in a much larger field of carbonatite pyroclastic rocks, and clearly constitutes a special form of eruption. Barker & Nixon (1989, p. 167) say that the flow was apparently fed by lava fountains from a fissure marked by a spatter rampart. Either the additional components in the groundmass (especially SIO2) reduced the dissociation pressure of calcite, or the melt was undercooled, or largely crystalline before it gained access to the surface. Olivine and diopside have rims of monticellite, presumably formed by reaction with CaCO3, which in any formulation, in the absence of dolomite, releases CO2: the presence of periclase in the groundmass would also be consistent with degassing prior to, and during eruption. Even though it seems necessary that this material was near its solidus prior to final eruption, the mineral assemblage in effusive calcio-carbonatite is consistent with high temperatures (in contrast with alkali carbonatite) and low activity of SiO2 and H20. At Kerimasi there is coarse grained srvite (plug?) within the crater that may be analogous, consisting of calcite, monticellite and periclase (Mariano & Roeder 1983). In this case the periclase is late as it mantles earlier magnesio-ferrite, and in view of the near surface emplacement may plausibly be attributed to final crystallization below the dissociation point for Mg carbonate. A similar mineral assemblage is reported from Polino in the Umbria-Latium province in Italy, where calcitic vent tuffisite, carrying mantle-derived olivine and phlogopite, has much of the olivine replaced by monticellite: again the volcanic association is one of leucite- and melilite-bearing silicate rocks, (Stoppa & Lupini 1993). At present there is a strong current of support for the proposition that many carbonatites form by low-P liquid unmixing from a carbonated silicate melt (Le Bas 1989; Kjaarsgard & Hamilton 1989), but the fact that the above effusive calcio-carbonatites carry mantle debris (despite very low melt viscosity) denies the general applicability of low-P unmixing, and points to their formation in the mantle. A direct source in the mantle for some calcio-carbonatites is also indicated elsewhere. In the Zambian volcanic rocks, minor amounts of calcio-carbonatite appear in a dominantly magnesio-carbonatite assemblage (Bailey 1960), where the primary nature of the activity is recorded in melt droplets containing high Cr magnesio-chromite (Bailey 1989). Extrusive calcio-carbonatite containing similar chromite has been reported from the United Arab Emirates also

255

indicating a direct origin from the mantle (Woolley et al. 1991).

Liquid immiscibility: general considerations Before discussing magnesio-carbonatites, it is appropriate to examine the case for liquid immiscibility as a means of generating carbonatite magmas, because this hypothesis is currently in favour to explain alkalic and calcic carbonatites. Although the concept applied to carbonatites is of long-standing (e.g. von Eckerman 1961) experimental demonstrations of silicate-carbonate melt immiscibility were needed for a growing general acceptance of the possibilities of the hypothesis (Koster van Groos & Wyllie 1966; Freestone & Hamilton 1980). Recent reviews of the experimental results have been given by Wyllie (1989) and Kjaarsgaard & Hamilton (1989). Le Bas (1989) has explored the possible applications to natural examples, and puts the case for immiscibility as a general explanation for a diversity of N a + Ca carbonatites. An opposite view is propounded by Gittins (1989), who advocates fractionation of primary olivine srvite (calcio-carbonatite) under hydrous and anhydrous conditions as the central petrogenetic process. With so much written on the subject, it is beyond the scope of this review to evaluate the arguments for and against immiscibility versus fractionation. Present evidence does not rule out either, so that both mechanisms may be valid, and, as indicated later, even con-jointly the two are probably only part of the whole panoply of carbonatite genetic processes. More evidence is needed on both fronts. Carbonatite/nephelinite/phonolite volcanism is typically explosive and yet good natural examples of incomplete unmixing are still scarce. The most convincing example, mixed phonolite glass/calcite ash flows (Mt Suswa, Kenya; Macdonald et al. in press) still does not provide carbonatite compositions of general applicability. Appeals may be made to the low viscosity of carbonate melt, which permits perfect melt segregation, but this requires a quiescent 'magma chamber' for which there is no great evidence in sub-volcanic sections. Indeed in sub-volcanic complexes, the carbonatite is widely reported as last in the main intrusion sequence: the carbonatite obviously cannot be derived from the silicate complex it intrudes, so that if a parent magma chamber exists it awaits discovery (or recognition) in deeply exposed sections. Differentiation of any traditional type may not even apply; the carbonatite may be simply the final residua expelled from the source region. None of this rules out immiscibility, but it does mean that the case for its wider applicability must remain open. If there were doubt about the need for such caution it must be removed by the latest information on Spitzkop, S. Africa. This was one of the complexes featured by Campbell Smith (1956) as having the classic pyroxeniteijolite-foyaite-carbonatite assemblage, and nowadays would be an obvious candidate for explanation in terms of immiscibility, but Harmer (1993) argues from the isotope chemistry that the carbonatite cannot be derived from the silicate magmas. Extrusive calcio-carbonatites, such as those at Fort Portal, are crucial in this regard. They lack any associated silicate magmas, they carry dense xenocrysts of possible mantle origin, and dense xenoliths of deep crust and mantle. An explanation by low pressure, liquid unmixing is wholly inappropriate, and such cases (together with Spitzkop) serve

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BAILEY

to show that for each carbonatite the origin must be judged on its merits. Only in the objective approach will the full range of carbonatite origins be uncovered.

Calcio-carbonatite intrusions: a temperature enigma Calcitic (s6vite) intrusive rocks are the most abundant carbonatites, and they pose some of the same problems identified by Campbell Smith (1956) in spite of subsequent growth of knowledge. Most examples have field relations and mineralogies indicative of emplacement temperatures lower than might be expected from the evidence in the extrusive rocks. In 1956, the problem was still in the form that the melting temperature of calcite was too high (1339 °C at 1025 atmospheres CO2: Campbell Smith 1956, p. 190), and although later experiments (Wyllie & Tuttle 1960) showed that H20 profoundly lowered the temperatures of calcitic liquids, the dilemma re-emerges in another form if account be taken of the effusive calcio-carbonatites, with low contents of H20. In alkaline silicate magmas generally, H20 plays a subordinate role to CO2, S, F and CI (Bailey & Hampton, 1990). In carbonate melts, fluorine would have a similar effect to H20, and indeed in anhydrous alkali carbonatite (Anal. 1, Table 1) halogens are very high, such that a lowering of melt temperature by as much as 200 °C may be anticipated (Gittins 1989). But effusive calciocarbonatites are not characterized by high F contents (e.g. Anals 3 & 5, Table 1). For all carbonatites the average levels, and ranges, for different elements have been most recently compiled by Woolley & Kempe (1989), and the levels for the fluxing components are abstracted here in Table 3: their chief modal expression is in early-crystallizing apatite (P,F,C1,OH), pyroxene (Na), amphibole (Na,C1,OH), phlogopite (K,F,OH) and sulphides, and in late-crystallizing barite, fluorite and fluorcarbonates. At the levels in Table 3, these constituents would not dramatically lower magmatic temperatures; some fluxing components must have been lost from intrusions but the amounts are uncertain. Fluorite in late-stage vein deposits, presumably results from build up of F in residual fluids, but in most carbonatites such fluorite would make only a tiny fraction of the total mass. Temperature estimates for intrusive carbonatites show wide variation, with many values reflecting sub-solidus re-equilibration (as might be anticipated in carbonate rocks pervaded by fluids) and it has long been recognized that

calcitic assemblages form a continuum through to hydro/carbothermal deposits, hence Pecora's (1956) leaning towards a spectrum of carbonatitic fluids. Other indicators of low temperatures of final emplacement are the general lack of thermal effects on country rocks and accidental wall rock xenoliths. Textures frequently indicate final emplacement as a fragmental or crystal suspension, and the fact that most intrusions carry dense minerals such as magnetite, in large crystals, and in aggregates, also points to higher viscosities than those of carbonate melts. One possibility that has been widely mooted is that the original intrusion was alkali carbonatite and lost its alkalis in metasomatizing (fenitizing) the country rocks. Alkali metasomatism around carbonatites (McKie 1989) varies in type and extent, and the alkalis in the wall rocks may be the integrated product of sustained passage of carbonatite (and other fluids) through the conduit. Consequently, it is not possible to quantify the amount of alkali loss from the carbonatite that remains in the intrusion. Little if any alkali remains in the intrusions now and the former presence of alkalis in some effusive calcic carbonatites is at present conjectural, based on inferred calcification of previous alkali carbonate. While such a possibility cannot be excluded, the existence of non-alkalic effusive calcio-carbonatites (with levels of other fluxing components similar to those of intrusive carbonatites) requires another explanation for some intrusions. High temperature melts unable to reach the surface may be expected to have a protracted crystallization, during which components not accommodated in the carbonates (and early crystallizing phases) become progressively concentrated in the residual liquid/fluid. Such a residuum would lubricate the crystal mush during final emplacement, and fenitize the wall rocks. In various forms, such an emplacement mechanism has long found favour (Campbell Smith 1956, p.203 [also citing Chayes 1942, p.506, for the interstitial alkalic fluid to explain the carbonate intrusions of Bancroft-Haliburton]). Once again, it would perhaps be unwise to look for one general explanation to cover all cases of carbonatite intrusion. Alkali metasomatism adjacent to carbonatites is variable in type (K or Na), reflecting in part at least the history of the carbonatite prior to emplacement, e.g. a primary mantle source versus a differentiate from carbonate or silicate parents: new observations and techniques will ultimately provide tests for distinctions, or possible inter-relations, between different kinds of intrusion.

Table 3. Carbonatites: averages and ranges of analyses for hyperfusible elements (wt% )

Na20 K20 H20 +

P205 F Ci S SO3

Ferro-carbonatite

Magnesio-carbonatite

Caicio-carbonatite Av.

No.

Range

Av.

No.

Range

Av.

No.

Range

0.29 0.26 0.76 2.10 0.29 0.08 0.41 0.88

102 105 78 119 31 8 23 15

0.0-1.73 0.0-1.47 0.0-4.49 0.0-10.41 0.0-2.66 0.0-0.45 0.02-2.29 0.02-3.87

0.29 0.28 1.20 1.90 0.31 0.07 0.35 1.08

44 44 36 51 21 1 12 13

0.0-2.23 0.0-1.89 0.08-9.61 0.0-11.30 0.03-2.10 -0.03-1.30 0.06-2.86

0.39 0.39 1.25 1.97 0.45 0.02 0.96 1.08

46 51 35 54 20 3 12 14

0.0-1.52 0.0-2.80 0.04-4.52 0.0-11.56 0.02-1.20 0.01-0.04 0.12-5.40 0.06-3.00

(From Woolley & Kempe 1989).

C A R B O N A T E MAGMAS

Effusive magnesio-carbonatite Extrusive magnesio-carbonatite has so far been reported only from the Rufunsa volcanoes in SE Zambia (Bailey 1960). These are of Cretaceous age, and the sub-aerial deposits mantle an old rift valley floor, which was cut into Karoo (Jurassic) and Precambrian basement. Based on their location in a complex intersection of major rifts and the absence of silicate magmas, it was proposed that the carbonatites had a direct origin from the underlying mantle (Bailey 1960). At that time, understanding of the relationships between igneous activity and the mantle was still at an early stage of development, which meant that the Rufunsa volcanic/mantle connection could not be pursued. The earlier deduction of a mantle origin for the Rufunsa volcanics was substantiated by the analyses of quenched melt droplets in vent tuffisite, which were composed of virtually iron-free, high Mn, high Sr, dolomite (Table 1) containing microphenocrysts of high Cr magnesio-chromite (comparable with spinels in deep mantle samples) (Bailey 1989). All the subaerially erupted material is fragmental, and most is agglomeratic, composed of debris from the vents and vent-walls, and earlier pyroclastic deposits. The matrix is largely very fine carbonate ash of dolomitic/ankeritic composition, heavily stained red-brown with finely disseminated iron oxide/hydroxide. Unequivocal melt droplets are composed of colourless dolomite, or dolomite plus calcite, although primary calcite contents are hard to quantify due to accidental incorporation of calcite from calcio-carbonatite intrusions penetrated by the volcanic vents. Some rocks contain drop-like fragments of iron-stained carbonate, adding to the sense (derived from the calcite distribution) that the erupted melts were variable (although the bulk composition of the deposits overall is dolomitic/ankeritic). Most of the melt droplets in thin section enclose accidental grains as cores, i.e. they are essentially small autoliths, showing that, after fragmentation, the still-fluid melt coated the entrained solids before it was quenched. Experimental data on dolomitic liquids is limited, and in the absence of obvious fluxing components (e.g. H20 or alkalis) or evidence of their presence at the moment of quenching, the required melt temperature and pressure would have to be high. Dolomitic melt would require quenching at temperatures >1000 °C, at c. 10 kbar, if data on the pure carbonate systems were applicable (Wyllie 1989). While some amelioration of these values may be anticipated from the small amounts of extra components (e.g. Sr, Mn, P) in the melt, the implication must be that melt fragmentation and quenching took place at high temperatures, deep in the volcanic vent: hence, melt droplets had become part of the entrained solids long before reaching the surface. Other components that may have conditioned the original melt are iron (now in the groundmass) and potassium (based on the ubiquitous phlogopite, and phlogopitized fragments in the pyroclastic rocks, and extensive K metasomatism) (Bailey 1989): but if present originally, K and Fe had been largely segregated before the melt droplets were formed, so that on present evidence high T, high P quenching seems inescapable. If so, it may be concluded that (a) similar melts could not exist at the surface, and that (b) melt fragmentation (and quenching) took place at great depths, with the tuffisite pipe extending possibly into the mantle. Although undoubted

257

mantle xenoliths are as yet unreported from Rufunsa, the possible deep tuffisite formation, the spinel compositions, and the high K activity, suggest analogies with kimberlites (or lamproites), especially as the mid-Zambezi-Luangwa rift has been a locus of kimberlite/lamproite activity (Bailey 1989). In terms of Sr and Nd isotopes, the Rufunsa volcanic rocks are also highly unusual for carbonatites generally, being transitional to Group II kimberlites (Zeigler 1992). Such a relationship gives special interest to the Rufunsa province as a whole, because the intrusive carbonatites are closely similar to the contemporaneous intrusions of the classic Chilwa carbonatites of Malawi (Bailey 1960).

Magnesio-carbonatite intrusions Although replacement of calcitic carbonatite by magnesiocarbonatite is commonplace in intrusive complexes, as pointed out by Campbell Smith (1956 p.202) and recently emphasized by Barker (1989), unequivocal intrusive relations abound (Campbell Smith 1956, p. 203). In the Rufunsa sub-volcanic sections, both intrusion and replacement are clearly displayed, and in common with many intrusive complexes the general sequence is calcio-, followed by magnesio-, grading into ferro-carbonatite. Later activity took the form of replacement by silica-iron hydroxide, and veining by calcite-quartz-barite-fluorite (Bailey 1960). In places the intrusive carbonatites show transitions into intrusive tuffisite, which led to the conclusion that all the pyroclastics bore this relationship to the intrusions: the discovery that the dolomitic melt lapillae were coming directly from a mantle source rules out such a general explanation. Some magnesian pyroclastic eruptions might have originated from shallow intrusions, but they could have carried melt only if the residual liquid in the intrusion was rich enough in fluxes to lower the temperature and the dissociation pressure. So far, no evidence has emerged to identify such a melt. Most magnesio-carbonatite intrusions show little evidence of very high temperatures, and the fact that they appear 'intermediate' between calcio- and ferro-carbonatite, which has links with late-stage mineralization in many complexes, means that they share the intrusion temperature problem, discussed above, of the calcitic intrusions. Furthermore, pure dolomitic liquid would introduce another complexity, due to its high dissociation pressure. For these reasons, the case for emplacement as a crystal mush, lubricated perhaps by fluid, is even stronger for magnesiocarbonatite in shallow intrusions.

The enigma of bimodal compositions Another question, rarely (if ever) raised, and certainly never properly aired, is posed by the compositional dichotomy between calcio-and magnesio-carbonatites. A traditional view, based on the common intrusion sequence, is that magnesio-carbonatites are differentiates from primary calcitic magmas, but a whole battery of questions is set in train by the relative scarcity of intermediate compositions. A bimodal composition distribution has been evident from the outset, in the ease with which an original two-fold classification was accepted, i.e. s6vite (calcitic) and rauhaugite (dolomite) (Br6gger 1921): it appears clearly in the statistical diagrams of Woolley & Kempe (1989, figs 1.1-1.4) with two peaks emerging in the histogram for CaO.

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Few geologists who have worked on carbonatites would be likely to suggest that the 'gap' in distribution is a sampling artefact; indeed, it may be more likely that the bimodality of magma types has been partly blurred by intermediate samples that represent partial dolomitization or intrusion mixing. If dolomitic carbonatite succeeds calcitic carbonatite in intrusive sequences, and each was largely crystalline on final emplacement, it is hardly possible that they are in a crystallization sequence. There is no experimental evidence available with which to account for the contemporaneous development of two such contrasting magmas from a common source. May be it will be necessary to look to distinctive modes of melt generation in the mantle source. Providing a solution to this dichotomy represents a major challenge to carbonatite geology; it is time that it is clearly recognized.

Ultramafic and mantle connections An intimate connection between carbonatites and ultramatic, alkaline ultramafic and lamprophyric rocks, was perceived at the outset (Br6gger 1921) and was set out by Campbell Smith (1956), who was by then able to include examples where peridotite and pyroxenite were major parts of the association. A growing understanding of the relationships between igneous activity and the mantle, and the finding of low initial 87Sr/e'6Sr in carbonatites (see Powell et al. 1966 for review) effectively put the seal on the mantle source. At that time there was an unresolved question as to whether the ultramafic xenoliths in igneous rocks were cognate or accidental fragments of the mantle. Such xenoliths, reported from the classic complexes of Fen and Aln6 (Griffin & Kresten 1987) and in different forms in other examples, such as Catanda, Fort Portal, Polino, now provide independent evidence of a mantle source region for the activity, regardless of any subsequent roles assigned to differentiation in carbonatite genesis. There remains, however, a major area awaiting exploration, namely the connection between peridotite and carbonatite where revealed in deeply eroded complexes such as Shawa and Dorowa (Zimbabwe). As Campbell Smith (1956) was aware, an understanding of the peridotite connection is essential to our appreciation of carbonatite activity. Not a great deal of further progress has been made in field and petrographic investigation, although our background knowledge of the mantle now indicates this could be a fruitful area. Carbonatites a n d kimberlites

A link between carbonatite and kimberlite was also hinted at by Br6gger (1921) as noted by Campbell Smith (1956), who gave prominence to Daly's (1925) work on the carbonate dykes in the Premier kimberlite. Possible connections are manifold and have been suggested subsequently by many auihors (see especially, Barker 1989, pp. 54-56, for a review); indeed, yon Eckermann (1963) assigned a parental role for kimberlite, with carbonatite forming by immiscible liquid separation. Petrographically there are strong analogies between kimberlite and the typical melilite lamprophyres of the carbonatite sub-volcanic association (aln6ite/damptjernite) in the plutonic complexes. In one sense the link was made in the first description of kimberlite when Lewis (1897 in Yoder 1975)

suggested that 'melilite basalt' was a heteromorph. More recently the similarities were highlighted in Rock's (1991) proposed classification of lamprophyres, where kimberlite is effectively used as the basis of definition, and close links with other highly undersaturated lamprophyres, e.g. aln6ite are thereby emphasized. A genetic link between kimberlite and carbonatite has been robustly challenged, however, by Mitchell (1979, 1986) drawing attention to the differences in spinels and ilmenites between the two groups. Gaspar & WyUie (1984) questioned this in showing that the compositional ranges from the two groups overlap, but Wyllie (1989, p.539) still found 'Mitchell's arguments are persuasive'. An important additional point made by Mitchell is the occurrence of carbonatites (and their associated lamprophyric rocks) in alkalic complexes, in contrast with kimberlites: a view echoed by others, including Wyllie (1989, p.500, p.537) when he writes, 'carbonatites are normally found as small bodies associated with much larger volumes of silicate rocks'. This view also emerges in many generalized descriptions of carbonatite complexes in igneous petrology texts, but whether this represents the 'normal' arrangement is another matter. Of the 84 unquestionable carbonatite complexes listed by Gittins (in Tuttle & Gittins, 1966) for Africa, 54 are recorded with silicate magmas either absent or minor in amount. Five of the seven effusive calcio-carbonatites listed in Table 2 are in the same category (Catanda, Khanneshin, Polino, Fort Portal and Rufunsa). As noted before, the last four show evidence of connections with potassic magmatism or high K activity, and four of the five give evidence of a direct mantle source. In the Fort Portal area, diamond indicator minerals are reported from alluvial deposits (Barker & Nixon 1989). Isotope chemistry in the Rufunsa carbonatites is transitional to Group II kimberlites, spinels are the same as those in kimberlites, and there are no associated silicate magmas. It may be that many carbonatites form parts of sodic or sodi-potassic alkaline complexes, but others (notably with potassic attributes) emphasize the possible kimberlite connection. The matter is not academic: rather than rejecting this possibility, more research could be devoted to exploring it, and the differences between carbonatites in the sodic and potassic associations. As well as scientific value the results may have commercial application. High K activity is marked in the Rufunsa province by intense and extensive K feldspathization around the vents, and abundant phlogopite in the volcanics, leading to the view that the total material flux from the mantle was characterized especially by carbonates, K and Fe (abundant in the volcanic matrix). No sodic igneous rocks or minerals have so far been discovered. Melt inclusions in diamonds (Navon et al. 1988) show a number of compositional analogies to the estimated primary flux from the Rufunsa mantle (Bailey 1989), such that this would make a more favourable comparison than with the lamproite and Group II kimberlite chosen by Navon et al. (1988). This in turn indicates another reason for keeping open the links between carbonatites and kimberlites: perhaps the melt inclusions in the diamond are giving another sample of the melt/fluids involved in the generation of these rocks.

Mantle source and the primary flux In his preface to the most recent multi-author volume on carbonatites, Bell (1989) writes 'most contributors favour

CARBONATE MAGMAS the formation of carbonatites from differentiation of a carbonated silicate melt'. Only one strongly advocates, as a general case, carbonatite magma generation by direct melting of the mantle (Gittins 1989). Using other lines of argument, based on experimental studies, Eggler (1989) and Wyllie (1989) reach conclusions similar to each other, but different from Gittins, namely that typical carbonatites are derived from primary nephelinitic/melilititic magmas, with effectively only kimberlites representing carbonate-bearing magmas coming directly from the mantle source. Both Wyllie (1989) and Eggler (1989) concede that primary carbonatites are possible, but unlikely. For Wyllie (p.500), 'the high ratio of silicate:carbonatite in most alkalic complexes argues against this origin' while Eggler (p.575) having set requirements including high Mg number, essential alkalis and magmatic isolation, concludes 'Few, if any, carbonatites fulfil these criteria'. Even Gittins (1989) does not indicate an example where his proposed primary carbonatite has been erupted directly and he envisages (p.588) 'nephelinite and carbonatite magmas forming sequentially, or possibly simultaneously'. In essence, all three views try to relate carbonatite genesis to a generalization about the supposed ubiquity of the 'carbonatite/nephelinite' association (largely divorced from kimberlite generation) but as indicated in previous sections there are plenty of natural examples that show this perception to be too narrow. The notion of differentiation of carbonatites from alkaline ultramafic parent magmas has a long history (reviewed by Campbell Smith 1956, p.213) and the possibilities receive support from modern experimental studies, but there are good examples of direct eruption of carbonatite from a mantle source, and the latest isotopic data on Spitzkop (Harmer 1993) suggests that even carbonatite in the characteristic silicate association may not be derivative (possibly the reverse). Given that both primary and derivative options may be valid, what may be deduced about the mantle source? Alkaline magmas generally, whether associated with carbonatites or not, are characterized by high levels of volatiles and incompatible elements so that if generated from the mantle, some mechanism of enhancing the levels of mobile elements (compared to more common magma types) is necessary. In the case of alkaline ultramafic magmas, enhancement in the source mantle region is required, and because the activity is repetitive in continental interiors there is a case for pervasive metasomatism of the source mantle as a precursor to magmatism (Bailey 1972). Xenoliths of metasomatized mantle in alkaline eruptions permitted initial estimates of the minimum bulk additives needed as a precursor to high K magmatism, namely 'calcite (cc) and kalsilite (kp)' (to produce the typical metasomite, biotite clinopyroxenite) (Lloyd & Bailey 1975). Although subsequent studies allow refinement of the requirements in individual cases, the need to produce alkali clinopyroxenite from peridotite still leaves 'cc + kp' as a useful basic model (Bailey 1987). Although arrived at by an independent line of enquiry, there are close analogies with the deduced mantle flux through the Rufunsa carbonatite volcanoes, as well as with melt inclusions in diamonds, suggesting that fluids akin to ' c c + k p ' may be active in the mantle generation of alkaline and carbonate magmas. During the early 1970s there were parallel developments in experimental petrology, with Eggler (1974) showing that CO2 in ultramafic systems produced melts of

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nephelinitic/melilititic affinities. Predictions from Wyllie & Huang (1976) and Eggler (1976) were that the first melts from the deep mantle (in the presence of CO2) would be dolomitic. Both fields, mantle metasomatism and melting at the vapour saturated mantle solidus, have seen huge developments in the intervening years, such that the concepts are now firmly entrenched, as evidenced in many contributions in Bell (1989). Haggerty (1989) has developed the notion that metasomatic preconditioning is a prerequisite of carbonate and alkali rich magmas; Jones (1989) too looks to mantle 'enriched in carbonate components and incompatible elements'; and Gittins (1989) refers to a source 'from carbonated mantle', for his primary melts. Eggler (1989, p. 561) proposes that primary dolomite-rich carbonatites would be the initial primary melts from 'phlogopite-carbonate peridotite' at pressures below those he proposes for kimberlite generation (55-60 kbar): but, as noted above, both he and Wyllie et al. (1989, 1990) consider most carbonatites to be differentiates from alkaline magmas, generated by asthenospheric circulation/plume activity. In these scenarios the lithosphere is an inert membrane, and metasomatism an accessory process (largely post-magmatic). In fact, Eggler, specifically rejects volatile flux as a possible part of carbonatite genesis, saying (p.575), 'No magmatic precursory events that decouple volatiles from more refractory peridotite elements are necessary'. All the source requirements are inherent in OIB (ocean island basalt) mantle, subject to partial melting and fractionation (p.561). Mantle with ocean island basalt characteristics is, of course, a model composition deduced from the chemistry of ocean island magmas, which include nephelinites and rare carbonatites, so that its choice as a source for continental carbonatites is perhaps unsurprising. It is an artefact of the need to explain island magmatism in ocean sectors dominated by MORB generation, and is widely considered to be the characteristic composition of deep mantle plumes penetrating the oceanic asthenosphere. Similar plumes are figured in sub-continental sections by Eggler (1989, fig. 22.6) and Wyllie (1989, fig. 20.12). Both proposals, volatile flux and plume generated magmas, require energy and materials released from the deep mantle and through the lithosphere. Volatile fluxing, however, will be dependent on lithosphere structure and dynamics, whereas deep mantle plumes should be independent of the plate that lies above. Unlike the oceans, the continents reveal abundant carbonatite activity, which can be examined more directly in the context of the structure and history of the lithosphere. As about half the known carbonatites are in Africa, the greater part of which has been a stable plate for more than 550 Ma, this forms a suitable example, and one that appropriately reconnects with Campbell Smith's (1956) title theme. Eruption ages, lithosphere structures and tectonic events, have been examined elsewhere (Bailey 1992) and can be summarized here.

Lithosphere structure and eruption ages Rifting and magmatism are largely controlled by ancient zones of weakness in the African lithosphere, this long acknowledged relationship being clearly realized in the most recent maps in Kampunzu & Lubala (1991). When the ages of post-Jurassic igneous activity are compiled, four major peaks emerge (Early Cretaceous, Late Cretaceous, Eocene-

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D . K . BAILEY Oligocene, and Miocene-Recent); in most areas, activity was repeated at least once. The localization of activity by old lesions in the lithosphere, the plate-wide synchroneity, and the repetitions, rule out the possibility of a source in deep mantle plumes impinging at random on the base of the African lithosphere. Instead, the plate-wide igneous episodes are found to correlate with external events such as Africa/Europe collisions and a global change in plate motion directions (Bailey 1992). Igneous activity peaks and collision chronology are summarized in Fig. 3. Black & Liegeois (1993) also emphasize the lithosphere control over the location of alkaline magmatism in Africa, but they prefer to attribute the magma chemistry to earlier plume activity which enriched the mantle source. In this case the plume provides only precursor enrichment, not the thermal and mechanical driving forces of the magmatism. Once it is recognized that melt generation is contingent on tectonic stresses acting across the plate, it is evident that any such earlier plume enrichment of the source is needed only if all other processes of enrichment can be completely eliminated from consideration. Even then, repetition of activity at the same sites, and the mechanism of melt generation are left unexplained. Connections between alkaline magmatism and plume activity may continue to enjoy wide favour, but the necessity for a connection remains to be demonstrated.

60

(a) 30

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Ma Fig. 3. Radiometric ages of igneous activity compared with collision rates of the Africa/Europe closure. (a) Histogram of all igneous rock ages in Africa (Cahen et al. 1984). The broad peak around 190 Ma corresponds to the continental flood basalt (CFB) magmatism that marked the break-up of Gondwanaland. All subsequent ages refer to alkaline/kimberlite/carbonatite activity following break-up. Note the two Cretaceous peaks, the stepped profile in Tertiary to Recent rise in the histogram, and especially the low activity period (70-50 Ma) compared with the collision record (b), below. (b) Collision rates for Africa/Europe closure using the relative movement path depicted in Dewey et al. (1989, fig. lb). Note the zero closure interval compared with the lull in igneous activity across Africa between late Cretaceous and late Eocene. (c) Histogram of carbonatite ages in Africa (Woolley 1989) The difference in resolution compared with (a) is probably attributable to the longer time intervals used in the original compilation. The line O through the diagrams marks the initiation of Africa/Europe collision according to Olivet et al. (1987).

Given that episodic intraplate magmatism in Africa is a consequence of external forces acting laterally across the plate, it follows that small volume melts have been permitted to erupt by stress release in the lithosphere. The melts, or the volatiles, or the appropriate melt sources (probably all three) must be continually available (Bailey 1993). Lithosphere strain is ameliorated by the re-opening of old fissure systems (and some warping) allowing the release of volatiles and fluids from the deep mantle. Channelling focuses the volatiles from a large reservoir of underlying mantle into relatively narrow zones through the lithosphere. The ambient geotherm along the release path controls the nature, volume, and source-depth of the magmas (Bailey 1980) giving rise to the observed spectrum of kimberlite/carbonatite/alkaline, magmatism, with progressively steeper geothermal gradients, which in general reflect the pre-existing lithosphere thickness. Relationships to continental rifting, and with melt transport and style of eruption have been examined elsewhere (Bailey 1986). Mantle metasomatism is an integral part of the scheme, offering the means by which incompatible elements can build up in previously depleted mantle. Carbonatephlogopite peridotite has enjoyed considerable favour as a model mantle source for carbonatite magmatism, but it should be recognized that unless this source were primordial (which is not usually proposed) the carbonate-phlogopite components must have been added, at or below the solidus, subsequent to any previous melting event (Bailey 1986, p. 450). Within the magma genesis regime, carbonate magmas are of multiple origins; some may separate from kimberlite, some may differentiate from melilitite/nephelinite, some are certainly primary, some from mantle characterized by phlogopite, some from amphibole-bearing mantle. Carbonatites associated with syenites may be sourced in shallow upper mantle, within the felsic mineral stability ranges (Fig.

CARBONATE MAGMAS

800

I000

I

I

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- 20

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30

Fig. 4. Effects of melt/fluid percolation through the mantle to the solidus, along an initial geothermal gradient G, appropriate to off-craton regions (taken from Bailey, 1987). G1 is the initial geotherm, G2 is the perturbation from crystallization as percolating melt/fluid approaches the solidus. Crystallizing phases may include amphibole, mica, felsic minerals and carbonates, enriching the mantle in incompatible elements (fine stipple). Carbonatite melts" could accumulate near the solidus (over the depth range indicated), their low viscosity and low density would facilitate separation with the possibility of carrying mantle debris (olivine, diopside, phlogopite) if erupted directly to the surface. Melt accumulation is possible until the geotherm is steepened to N. Melts would be calcitic in this P T range (Dalton & Wood 1992), and dolomitic at higher pressures (along less steep geothermal gradients in cooler lithosphere). Potentially syenitic protoliths (and magmas) could develop below the peridotite solidus(coarse stipple): nephelinite/melilititemagmas represent melts that segregate and ascend from points on the geothermal gradient above the solidus. Solidus OE, in the presence of H20 and CO 2 (limited) (Olafssen & Eggler 1983). 4). Formation of primary carbonatite, as in Table 2, is shown schematically in Fig. 4, which also indicates possible links with nephelinite and with phonolite magmatism. Campbell Smith (1956) brought into a wider geological perspective what had hitherto been a petrological curiosity, thus helping to stimulate interest in what may now be perceived as a key rock type in understanding deep Earth processes. Important new information has come, and will continue to come, from areas such as experimental petrology and isotope geochemistry, but it is worth recalling that Campbell Smith's insight was informed by petrographic experience, harnessed to the field studies of his collaborators. Future advances can also be expected through modern techniques in microanalysis, and in volcanological research, thence adding an appropriate tribute to all the early pioneers in the field of carbonate magmas. Conclusions

(1) Most carbonatites are intrusions (mainly calcic) and the controversy aroused by Brrgger's (1921) original proposal of carbonate magma persisted for 40 years. Evidence from effusive carbonatite is essential for understanding the wider aspects of carbonatite magmatism, and for identifying the most relevant applications of the results from experimental petrology. Three types of effusive carbonatite are available for this purpose: natro-, calcio- and magnesio-carbonatite. (2) Natrocarbonatite is the only present-day example of flowing magma, but this composition is still unique. The

261

melt corresponds to a low pressure cotectic composition, presenting difficulties in relating it to other carbonatites, and to explanations requiring that it is comagmatic with its associated nephelinite/phonolite. Geochemistry also distinguishes this type from others. Its status as a composition from which other carbonatites may be derived is therefore questionable. (3) Of seven examples of effusive calcio-carbonatite, at least four carry evidence of direct eruption from the upper mantle. These may be primary. Present compositions show low, or negligible, contents of fluxing components, and the mineralogy is consistent with high T eruption. In four cases, links with leucite and melilite-bearing silicate magmas are indicated. (4) In the only known example of effusive magnesiocarbonatite, there is a similar lack of evidence of fluxing components, which would indicate quenching in the deep parts of a tuflisite system extending into the mantle. Other possible links with kimberlite activity, are seen in Cr minerals, high activity of K, and in the Sr/Nd isotope chemistry. There are no silicate magmas, and all the characteristics are consistent with a primary origin. Smaller amounts of effusive calcio-carbonatite in this association add weight to the possibility that some calcio-carbonatites may be primary. (5) While many small carbonatite intrusions, e.g. cone sheets, may have been emplaced at high temperatures, large intrusions, especially large plugs, indicate emplacement at temperatures below those inferred for surface eruptions. In the absence of flux components in the sampled compositions, the long-standing question of mode of emplacement remains. Final emplacement as a mush still seems the most reasonable answer. (6) A compositional gap exists between calcio- and magnesio-carbonatites, which is not explicable in terms of fractionation (based on present experimental data). The compositional gap may reflect primary differences in sources, or in melt generation mechanisms. This remains a crucial area for further research. (7) An important magmatic association is that between carbonatites and nephelinite/melilitite, but this is not universal. Associations with syenites, or with ultramafic rocks may be equally important (and overlapping), and some carbonatites are erupted in isolation. Final eruption of carbonatite through alkaline complexes may represent the expulsion of residua from the source region. (8) Hypotheses of origirr strongly favour a relationship between nephelinite (s.l.) and carbonatite, stemming from a prevailing perception of the ubiquity of the association. Similar reasoning leads to the separation of kimberlite and carbonatite genesis. Many carbonatites are not minor parts of alkaline complexes, and to overlook this fact may create an unnecessary stumbling block to progress. More research on the ultramafic connections is vital. (9) Analogies between carbonatite magmatism and oceanic island volcanism (and thereby sub-lithosphere plumes) is put in question by the observation of plate-wide activity triggered by external events. This implies an origin linked to permissive release of an energy and materials flux from the deep mantle into the lithosphere. (10) Carbonatites are of multiple origins, reflecting different aspects of carbon activity in the mantle, and any attempt to explain all the phenomena in a single hypothesis may prove futile.

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D.K.

One obvious prognostication, from looking at the aftermath of Campbell Smith's (1956) review, is that now, as then, new and unexpected data are still to come, to add to the questions that still remain. Meeting these challenges promises to unlock further secrets about the deep Earth. M y o w n e n j o y m e n t o f the c a r b o n a t i t e challenge is c o n t i n u a l l y q u i c k e n e d by the s h a r e d k n o w l e d g e o f friends a n d colleagues, w h o s e continuing c a m a r a d e r i e I h e r e salute. E a r l y e n c o u r a g e m e n t c a m e f r o m no less t h a n W. C a m p b e l l Smith, a n d it was a f u r t h e r pleasure that the invitation to c o n t r i b u t e o n ' C a r b o n a t e m a g m a s ' should c o m e t h r o u g h M. J. L e Bas. M y t h a n k s also to A . R. W o o l l e y , for calling m y a t t e n t i o n to the r e f e r e n c e s on the C a t a n d a volcanoes, in A n g o l a . Final text revisions b e n e f i t t e d f r o m the constructive c o m m e n t s o f D. S. S u t h e r l a n d a n d M. J. L e Bas.

References ALKHAZOV, V.Yu., ATAKISHIYEV,Z.M. & AZIMI, N.A. 1978. Geology and mineral resources of the early Quaternary Khanneshin carbonatite volcano (Southern Afghanistan). International Geology Review, 20, 281-285. BAILEY, D.K. 1960. Carbonatites of the Rufunsa valley, Feira District. Bulletin 5, Geological Survey of Northern Rhodesia. 1972. 'Uplift, rifting and magmatism in continental plates'. Journal of Earth Sciences (Leeds), 8, 225-239. 1980. Volatile flux, geotherms, and the generation of the kimberlite-carbonatite-alkaline magma spectrum. Mineralogical Magazine, 43, 695-699. 1986. Fluids, melts, flowage and styles of eruption in alkaline ultramafic magmatism. Alkaline and Alkaline-Ultrabasic Rocks and their Xenoliths. Transactions Geological Society South Africa, Special Issue, 88 (2), (for 1985), 449-457. 1987. Mantle metasomatism: perspective and prospect. In: FITroN, J.G. & UPTON, B.G.J. (eds) Alkaline Igneous Rocks. Geological Society, London, Special Publications, 30, 1-13. 1989. Carbonate melt from the mantle in the volcanoes of south-cast Zambia. Nature, 388, 415-418 (and 374). 1990. Mantle carbonatite eruptions: Crustal context and implications. Lithos, 26, 37-42. 1992. Episodic alkaline igneous activity across Africa: implications for the causes of continental break-up. In: STOREY, B.C. ALABASTER,T. & PANKHURST, R. J. (eds) Magmatism and the Causes of Continental Break-up. Geological Society, London, Special Publications, 68, 91-98. 1993. Petrogenetic implications of the timing of alkaline, carbonatite, and kimberlite activity in Africa. South African Journal of Geology, 96, 67-74. & HAMPTON, 1990. Volatiles in alkaline magmatism. Lithos, 26, 157-165. BARKER, D.S. 1989. Field relations of carbonatites. In: BELL, K. (ed.) op.cit., 38-69. 1993. Discriminating magmatic features in carbonatites: implications for the origins of Mg- and Fe-ricb carbonatites. South African Journal of Geology, 96, 131-138. -& NIXON, P.H. 1989. High-Ca, low-alkali carbonatite volcanism at Fort Portal, Uganda. Contributions Mineralogy Petrology, 103, 166-177. BELL, K. (ed.) 1989. Carbonatites: genesis and evolution. Unwin Hyman, London. -& KELLER,J. (eds) 1994. Carbonatite volcanism--Oldoinyo Lengai and the petrogenesis of natrocarbonatite. IA VCEI Proceedings in Volcanology. Springer, in press. BLACK, R. & LIEGEOIS,J.-P. 1993. Cratons, mobile belts, alkaline rocks and continental lithospheric mantle: the Pan-African testimony. Journal of the Geological Society, London, 150, 89-98. BROGGER, W.C. 1921. Die Eruptivgesteine des Kristianiagebietes. IV. Das Fengebiet in Telemark, Norwegen. Norsk Videnskapsselskapets Skrifter, I. Math Naturv Klasse, 9. CAHEN, L., SNEELING, N.J., DEEHAE, J. & VAIL, J.R. 1984. The geochronology and evolution of Africa. Clarendon, Oxford. CHAYES, F. 1942. Alkaline and carbonate intrusives near Bancroft, Ontario. Geological Society, America Bulletin, 53, 449-511. CLARKE, M.G.C. & ROBERTS, B. 1986. Carbonated melilitites and calcitized alkali carbonatites from Homa Mountain, western Kenya: A reinterpretation. Geological Magazine, 123, 683-92. -

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COOPEg, A.F., GITrINS, J. & TUTrLE, O.F. 1 9 7 5 The System Na2CO3-K2CO3-CaCO 3 at 1 kilobar and its significance in Carbonatite Petrogenesis. American Journal of Science, 275, 534-560. DALTON, J.A. & WOOD, B.J. 1992. The effects of Fe/Mg ratio and pressure on carbonate stability and melt compositions in peridotite assemblages. Transactions of the American Geophysical Union, 73, 616. DALY, R.A. 1925. Carbonatite dikes of the Premier Diamond Mine, Transvaal. Journal of Geology, 33, 659-684. DAWSON, J.B. 1962. Sodium carbonate lavas from Oldoinyo Lengai, Tanganyika. Nature, 195, 1075-1076. 1964. Carbonatitic ashs in northern Tanganyika. Bulletin Volcanologique, 27, 81-92. 1989. Sodium carbonatite extrusions from Oldoinyo Lengai, Tanzania: implications for carbonatite complex genesis. In: BELL, K. (ed.) op.cit., 255-277. - - , GARSON, M.S. & ROBERTS,B. 1987. Altered former alkalic carbonatite lava from Oldoinyo Lengai, Tanzania: Inferences for calcite carbonatite lavas. Geology, 15, 765-8. DEWEY, J.F., HEEMAN, M.L., TURCO, E., HuTroN, D.H.W. & KNO'Iq', S.D. 1989. Kinematics of the western Mediterranean. In: COWARD, M.P., DIETRICH, D. & PARK, R.G. (eds) Alpine Tectonics. Geological Society, London, Special Publications, 45, 265-283. OlXEV, F., CAMPBELLSMITH, W. & BISSET, C.B. 1937. (revised 1955). The Chilwa series of southern Nyasaland. Nyasaland Geological Survey Bullletin 5. ECKERMANN, H. YON 1948. The alkaline district of Alni5 Island. Svertiges Geologiska Undersokning, Series Ca. No. 36. 1961. Contributions to the knowledge of the alkaline dikes of the Aln6 region. IV. Arkiv frr Minereralogi och Geologi, 3, 65-68. 1963. Contributions to the knowledge of the alkaline dikes of the Aln6 region. IX. Carbonatitic Kimberlite from Sundsvall. Arkiv fiir Mineralogi och Geologi, 3, 397-402. EGGLER, D.H. 1974. Effect of CO 2 on the melting of peridotite. Carnegie Institution Yearbook, 73, 215-24. 1976. Does CO 2 cause partial melting in the low-velocity layer of the mantle? Geology, 4, 787-788. 1989. Carbonatites, primary melts, and mantle dynamics. In: BELL, K. (ed.) op.cit., 561-579). FREESTONE, I.C. & HAMILTON, D.L. 1980. The role of liquid immiscibility in the genesis of carbonatites---an experimental study. Contributions to Mineralogy and Petrology, 73, 105-17. GASPAR, J. & WYEEIE, P.J. 1984. The alleged kimberlite-carbonatite relationship: evidence from ilmenite and spinel from Premier and Wesselton Mines and the Benfontein Sill, South Africa. Contributions to Mineralogy and Petrology, 85, 133-1 40. GITrlNS, J. 1989. The origin and evolution of carbonatite magmas. In: BELL, K. (ed.) op.cit., 580-600. & JAGO, B.C. 1991. Extrusive carbonatites: their origins reappraised in the light of new experimental data. Geological Magazine, 128, 301-305. & McKIE, D. 1980. Alkalic carbonatite magmas: Oldoinyo Lengai and its wider applicability. Lithos, 13, 213-215. GRIFFIN, W.L. & KRESTEN, P. 1987. Scandanavia--the carbonatite connection. In: NIxoN, P.H. (ed.) Mantle Xenoliths. John Wiley & Sons, New York, 101-106. GUEST, N.J. 1956. The volcanic activity of Oidoinyo Lengai, 1954. Tanganyika Geological Survey Records 1954, 4, 56-59. HAGGERTY, S.E. 1989. Mantle metasomes and the kinship between carbonatites and kimberlites. In: BELL, K. (ed.) op.cit., 546-560. HARMER, R.E. 1993. The petrogenetic association between carbonatite and alkaline magmatism isotopic constraints. Terra Abstracts, 3, 20. HEINRICH, E. WN. 1966. The Geology of Carbonatites. Rand McNally and Co. Chicago, USA. HOBLEY, C.W. 1918. A volcanic eruption in East Africa. Journal of the East Africa and Uganda Natural History Society, 4, 339-343. HOGARTH, D.D. 1989. Pyrochlore, apatite and amphibole: distinctive minerals in carbonatite. In: BELL, K. (ed.) op.cit., 105-148. JAGO, B.C. & GITrINS, J. 1991. The role of fluorine in carbonatite magma evolution. Nature, 349, 56-58. JAMES, T.C. 1956. Carbonatites and rift valleys in East Africa. Tanganyika Geological Survey. Unpublished report, TCi[34. JONES, A.P. 1989 Upper-mantle enrichment by kimberlitic or carbonatitic magmatism. In: BELL, K. (ed.) op.cit., 448-463. KAMPUNZU, A.B., & LUBALA, R.T. 1991. Magmatism in extensional structural settings. Springer-Verlag, Berlin. KELLER, J. 1981. Carbonatitic volcanism in the Kaiserstuhl alkaline complex: Evidence for highly fluid carbonatitic melts at the earth's suface. Journal of Volcanology and Geothermal Research, 9, 423-431. 1989. Extrusive carbonatites and their significance. In: BELL, K, (ed.) op.cit., 70-88. 1992. Alkalicarbonatites and Ca-carbonatites: similarities differences

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Received 22 December 1992; revised typescript accepted 15 February 1993

Addendum Since this contribution was in press, other papers have appeared relating to carbonate (and carbonate fluid activity) in the mantle, which bear directly on some of the central issues (see especially, Ionov et al. 1993, and references therein). Mantle carbonates must obviously be seen in the context of effusive carbonatites erupted directly from the mantle, and in the carbonatite ultramafic connections (Conclusions 3, 4 and 7 above). Mantle carbonate trace element signatures are reported as akin to those in crustal carbonatites, which is welcome news but must be greeted with some reservations. No fresh effusive carbonatites are used in the comparisons, nearly all the examples being carbonatite intrusives from a wide range of geological environments (with very wide-ranging trace element levels). Most authors accept the prevailing consensus (as in Bell 1989) that primary carbonatites are

unlikely, so discrepancies in trace element patterns are attributed to low P differentiation and contamination in carbonatites. Such processes undoubtedly contribute to carbonatite chemical variations, but there is the additional (and widely disregarded) factor that some variations may relate more directly to differences in carbonatite genesis (Conclusion 10). Relating the direct evidence of carbonate activity in mantle rocks to erupted carbonatites is clearly an imperative, and should provide a catalyst for more rigorous re-appraisal of the whole spectrum of erupted carbonate magmas.

Additional reference IONOV, D.A., DueuY, C., O'REILLY, S.Y., MAYA, G., KOPYLOVA,M.G., & GENSHA~, Y.S. 1993. Carbonated peridotite xenoliths from Spitsbergen: implications for trace element signature of mantle carbonate metasomatism. Earth and Planetary Science Letters, 119, 283-297.

Added November 1994.

From QJGS, 1 12, 189. A REVIEW

OF SOME

PROBLEMS

OF AFRICAN

BY WALTER CAMPBELL SMITH, C . B . E . M . C . T . D .

CARBONATITES

S C . D . M . A . , PRESIDENT

ANNIVERSARY ADDRESS DELIVERED AT THE ANNUAL GENERAL MEETING OF THE SOCIETY ON 25 APRIL, 1956 CONTENTS

Page I. II. III. IV. V. VI. VII. VIII. IX. X.

Introduction .................................................................. Mineral composition of carbonatites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonatites in eastern and central Africa . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure of the complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The problem of the m o d e of e m p l a c e m e n t . . . . . . . . . . . . . . . . . . . . . . . . . . . Associated igneous rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The fenitized rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theories of the origin of carbonatites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ls9 191 194 198 202 205 208 212 216 217

SUMMARY Carbonatites are known at m a n y places in eastern and central Africa, from near Mount Elgon in U g a n d a to Spitzkop and Palabora in north-eastern Transvaal. T h e y range in age from pre-Karroo, post-Waterberg in the south to Miocene-Pliocene in the north. Owing to this great age difference d e n u d a t i o n has revealed carbon. atites at various levels of erosion, the deepest corresponding perhaps with the carbonatites exposed at Aln6 in Sweden and Fen in Norway. The composition and structure of the carbonatites, the associated alkaline igneous rocks, and the altered country rocks known as fenites are all described. The carbon. aces are mainly pure calcite but some are ankeritic and dolomitic and, locally, sideritic and manganiferous. T h e y carry characterisbic accessory minerals, particularly pyrochlore. The associated igneous rocks are ijolites with, less frequently, nephelinesyenite and at some centres pyroxenite. Fenitization results in t h e formation of aegirine-felspar rocks, nearly pure felspar rocks and felspathic breccia. Evidence as to chemical changes involved in fenitization is n o t always consistent, b u t addition of K or Na, or both, and loss of SiO s are satisfactorily d e m o n s t r a t e d . Current theories are reviewed. The carbonatites are believed to owe their origin to concentration of carbon dioxide or of carbonatitic fluid of m a g m a t i c origin, derived perhaps from pyroxenite highly charged with volatiles, a m o n g which carbon dioxide played the most i m p o r t a n t part, associated with phosphoric acid, fluorine, water etc., a n d in which the elements niobium a n d cerium, a m o n g others, were also concentrated. I. INTRODUCTION T ~ E r o c k s n o w g e n e r a l l y s p o k e n o f a s c a r b o n a t i t e s m a y be b r i e f l y d e s c r i b e d as r o c k s w h i c h , t h o u g h in g e n e r a l m i n e r a l c o m p o s i t i o n s i m i l a r to limestones and marbles of known sedimentary origin, yet appear to b e h a v e as i n t r u s i v e r o c k s a n d a r e c l o s e l y a s s o c i a t e d w i t h a l k a l i n e i g n e o u s rocks. In composition they consist mainly of calcium carbonate with subordinate amounts of carbonates of magnesium and iron. They occur within complexes of alkaline rocks believed to be igneous and occasionally at volcanic centres. Their situation, habit and structures and their r e l a t i o n t o t h e i g n e o u s r o c k s a r e all s u c h as t o s u g g e s t t h a t t h e y h a v e b e e n b r o u g h t i n t o t h e i r p r e s e n t p o s i t i o n in a t l e a s t a " p l a s t i c " c o n d i t i o n a n d a r e in f a c t i n t r u s i v e .

Index Badcallian metamorphism, 26, 38-41, 42, 54 Badnaban dyke, 42 Bahama Banks, fissure fauna, 160 Bailey, E. B., 196 Ballantrae Ophiolite, 59 Baltica, 166, 167, 170 Banks, J., 5 Barberton greenstone belt, 18 barkhan dunes, 175-6 Barrande, J., 86 'barren beds', 101 barren intervals, 94 'Barren Mudstones', 99 Barrovian metamoprhism, 68, 78 Barrow, G., 74, 78, 223 'Barrow's zones of progressive regional metamorphism', 68 base metal production, 237, 238, 239 basinal brine theory, 243 Bather, F. A., 135 batholiths, 221,228, 230, 232, 233 Bauer, G. (Agricola, G.), 238-9 Beannach dyke, 42 Beartooth mountains, 32 Beaumont, E. de, 239, 240 Becker, G. F., 206, 211 bedforms and bedding aeolian, 175-6, 178 aqueous and subaqueous, 176-7, 178-80 related to wind waves, 178-9 'Belcraig Shale', 108 Belgian Stage, 108 Belingwe region, Zimbabwe, 17, 28 Bellispores nitidus-Reticulatisporites carnosus Zone, 118 Ben Nevis volcano, 196 Ben Vuirich, 58 granite, 47, 48, 49, 50, 63 Betrand, M., 58 Beyrichoceras Ammonoid ZOne, 110-11 Bighorn mountains, 31, 32 bimodal mafic magmatism, 29 Binneringie intrusions, 29 'biochron', 137 biostratigraphy Dinantian, 105-7, 110-21 Early Palaeozoic, 83-9 Moffat series, 93-101 time-resolution, Jurassic, see under Jurassic geochronology biozones, 136 Birimian orogen, 13, 14 Bisat, W. S., 107 bivalves, British Dinantian, 111 blind ore bodies, 244 'blocking temperatures', 46 Blue Holes caves, fissure fauna, 160 Bohemain Massif, 168 'boils' on river surface, 176 Bollandites-Bollandoceras Ammonoid Zone, 110 boninitic magmatism, Precambrian, 32, 33 Bonney, T. G., 189 Borrowdale arc, 60 Bosost massif, 76 Bou Azzer ophiolite, 12-13 Bowen, N. L., 205, 206, 207, 213, 214, 216, 221,222, 227, 228 Bowen trend of silica enrichment, 209, 210 Bowen's reaction series, 228 Brabant Massif, 168, 170 brachiopod faunas, Budleigh Salterton, 165-9, 171

Abitibi Belt, 17 Abukuma Plateau, 69 Acadian orogenic events, 70 accretionary orogens, origin, 11, 12, 13-14, 19 Achiltibuie ultramafic bodies, 40 ACID processes, 226 'acme' of an evolving species, 133, 135 acritarchs in biostratigraphic calibration, 87, 88 acuity, 145, 146 adhesion structures, 176 Adirondack Mountains, 74 aeolian bedforms and bedding, 175-6, 178 Africa carbonatites, 249-62 greenstone belts, 17 alkali metasomatism (fenitization), 249-50, 253, 256 alkaline magmatism, 251-3, 255, 256, 259, 260 Alpine Fault, New Zealand, 57 Alps, collision zones, 76, 77 Alston Block, 241-3, 244 Amassalik mobile belt, 30-1 Ameralik dykes, 26, 27, 28 Amitsoq gneisses, 26, 27, 28 ammonites in biostratigraphic calibration, 130, 131, 132-5, 137-43, 146, 147 ecosomatic modification, 138 ammonoids, British Dinantian, 110-11 analytical top-down subdivision, 131 Andr6e, K., 189 anisotropy of magnetic susceptibility, 200 anorogenic magmatism, Proterozoic, 15 Antarctica, 31, 32, 73 Antrim flood basalts, 195, 196, 197, 198, 200-1 Appalachian orogenic belts, 70, 74, 75 Applecross Formation, 45 aqueous bedforms and bedding, 176-7, 178-80 Arabian-Nubian Shield, 12, 16, 18 aragonite, solution and precipitation, 186-7 Archaean plate tectonics, 18-19 terranes, 16-18, 25-33, 246 Archaean-Proterozoic boundary, 29 Archaean-Proterozoic mafic suites, 27, 32 Archerbeck Borehole, 118 Ardgour gneiss, 44 Ardnamurchan intrusive centre, 196, 197 Ardnish pegmatites, 43 Arenig fauna, 166 Arenig Series, 86, 95 Arkell, W. J., 129, 133, 134 Armorica, 166, 167, 168 Armorican Massif, 224, 226 Arnsbergian Stage, 111 Arnsbergites falcatus Ammonoid Zone, 121 Arran Goatfell Granite, 196 Arundian Stage, British Isles, 108, 109, 110, 113, 114, 115, 116, 117, 119, 120, 121 Arunta Complex, 73, 74 Asbian Stage, British Isles, 108. 109, 114, 116, 118, 119, 120, 121 Asbian-Brigantian boundary, 109 Askrigg Block, 242, 244 assimilation of crustal rocks, 207, 210, 212 Australia, 17, 19, 20, 31, 32 Australian Platform cratons, 87 avalanching (grain flow), 177-8 Avalonia, 12, 167, 168, 170 Avon Gorge stratigraphy, 105, 107, 108 265

266

INDEX

brachiopod/coral zonation, 113-14, 121 Brady, H. B., 118 Brewster, D., 239 brick-pattern ripples, 178 Bridgend quarries, fissure fauna, 158 Bridport-Yeovil-Midford Sands, 146 Brigantian Stage, British Isles, 108, 109, 111,112, 114, 116, 117, 119, 120 Brinkmann, R., 142 Bristol Channel, Mesozoic fauna, 153, 156, 158 Bristow, W. H., 155 Britain, attachment to Gondwana, 166, 167, 170 British Isles, Dinantian stratigraphy, see Dinantian stratigraphy in the British Isles British Tertiary Province, 195, 196, 199, 230 see also North Atlantic Province Brittany, 69 Br6gger, W. C., 249, 258, 261 Brongniart, A., 84 Buch, L. yon, 128-9 Buckman, S. S., 131-4, 138, 142, 146 Budleigh Salterton Pebble Bed, fauna from, 165-71 Bulman, O. M. B., 95 Bushveld complex, 29, 31,208 Bute, Island of, 61 Cadomian Belt, France, 226, 227 Cadomian-Avalonian belt, 19 caicio-carbonatite, 253-5,256, 257,261 calcitization, 186, 187 Caledonides, 59, 60-2, 98, 167, 230 metamorphism, 43, 47, 49, 68, 75, 77, 229 Norway, 68, 77 Cambrian System, 86, 87, 144, 145 Cambrian-Ordovician boundary, 95-6, 100 Cambrian-Silurian boundary, 85 Cambridge Time-Scale, 143 Campbell Smith, W. C., 249-51,255, 256, 257, 258, 261 Canadian shield, 17, 29, 73, 74 gravity anomalies, 15-16 Canigou massif, 76 Caradoc beds, 86 carbonate magmas, 249-62 carbonatites, 249-50, 263 alkaline, 256, 259 effusive, 250-1, 253-5, 256, 257-8, 261,263 intrusive, 250-1, 256, 257-8 link with kimberlites, 258, 260, 261 mantle source and primary flux, 258-9, 261 Carboniferous Limestone fissure fauna, 153, 154, 155-60 stratigraphy, see Dinantian stratigraphy in the British Isles Carboniferous System, 85, 86, 110, 113, 144 Carn Chuinneag, 39, 43, 58 Carn Gorm pegmatites, 43 Carrock Fell intrusion, 205,212 Cashel-Lough Wheelaun intrusion, 48, 50 Catanda carbonatites, 254, 258 cave pearls (pisolites), 187, 188 cementation of carbonates, 187 'Cenozoic', 85 Cenozoic-style plate tectonic processes, 19 Centenary History o f the Society, H. B. Woodward, 8 Central Highland Division, 63 Central Metasedimentary Belt, Ontario, 13 central volcanoes, 197, 199, 200, 201 Chadian Stage, British Isles, 108-9, 110, 113, 114, 115, 116, 117, 118, 119, 120, 121 chalk, pelagic nature, 189 Challenger expedition, 185, 186, 189 Changbaishan volcano, 197 characteristic faunal horizons, 136 Charterhouse Carboniferous Limestone, 155 chemical modelling, 232

chemostratigraphy, 146 Chilas complex, 13, 17 Chile, 230 china clay deposits, 238, 245 chronostratigraphy, British Isles, 105, 108-9 Chugach Metamorphic Complex, 81 Churchill Province, 31 Circular, the, 8 Cleveland dyke, 200 climbing ripple cross-lamination structures, 176 clingani 'bands', 101 closed-system fractionation, 208-10, 222 Coastal Batholith, Peru, 226, 227, 228-9, 230-1 Coastal Range, British Columbia, 76 Code of Rules of Stratigraphical Nomenclature, 129, 130 collisional metamorphism, 75, 76 collisional orogens, origin, 11, 12-13, 14-15, 19 colonnade lava tiers, 199 Colonsay rocks, 63 columnar structures, formation, 180 complanatus 'bands', 101 'completeness of the geological record', 146 concurrent-range biozones, 136 Connemara schists, 46 conodonts, in biostratigraphic calibration, 87, 95-6, 97, 111-13 contact metamorphism, 68, 69, 222, 223, 233 continents, dispersal and growth, 59-60 convection in magma chambers, 209, 211-12, 231,232 convective fractionation, 206, 211,212 Conybeare, W. D., 85 cooling histories and mineral ages, 46-7, 48 Cooma Complex, 224, 225 Coral Brachiopod Zone, 118 coral]brachiopod zonation, British Isles, 113-14, 118, 119, 121 corals, composition changes with time, 186 Cordilleran granite magmatism, 222, 226, 228-30, 231 Cordilleran orogens, 11 Cornubian Batholith, 224 Cornubian orefields and orefluids, 237-8, 239, 243-5, 246 Coronation Supergroup, 14 Coronatum Zone, 147 Cotteswold Sands, 132-3 Courceyan Stage, British Isles, 108, 110, 111, 112, 113, 114, 115, 118, 119, 120, 121 Craven Basin, 107, 109, 110, 111, 119 Cretaceous System, 144, 145, 146 fauna, 161 critical melt fraction, 224 Cromhall Limestone Quarry, fissure fauna, 158 crustal accretion, Precambrian, 25-6, 32 'crustal accretion-differentiation superevent', 26, 38 crustal anatexis, 70 crustal assimilation, 207, 213, 216 crustal extension and metamorphism, 75-6 crustal fracturing, 230 crustal melting, 20, 69-70, 231 crustal temperature changes, causes, 73 crustal thickening and magmatism, 32, 69-70, 73 crystal fractionation, 206-7, 208-12, 214-16, 228, 229 crystal settling, 206, 207,208-11,213, 215-16, 228 crystallization ages, in dating, 47-51 crystallization in fluid inclusions, 239-40 Cuillin Hills, intrusions, 195, 196, 197, 200, 201 Cullis, C. G., 186 current ripples, 176 Cuvier, G., 84 Dabje Mountains, 77 Dalradian block, 60-1, 63 Dalradian Supergroup, geochronology, 46-51 Dana, J. D., 58 Darwin, C. R., 86, 189, 205,206 Davey, H., 239 Davidson, T., 165

INDEX Davies, A. M., 135 De La Beche, H. T., 84, 154 Dead Sea Rift Fault, 57 Deccan traps, fissure fauna, 161 'deep biotite granite', 224, 225 Degerloch Rhaetic bone bed, 155 Dehm, R., 156, 160 dehydration melting, 19, 70 Delhi orogen, 16 Derbyshire Dome, 242, 243 desiccation fractures, 179 destructive plate margins, movements caused by, 59-60 Devonian palaeogeography from pebble fauna, 169, 171 Devonian System, 85, 86, 144 dewatering structures, 179-80 Dewey, J. F., 59 Diabaig Formation, 45 diamond-bearing rocks, 77 diamonds, melt inclusions in, 258, 259 differentiation indices, 214-15 diffusion intercrystalline, 77 in magmas, 206 Dinant basin, 118 Dinantian stratigraphy in the British Isles, 105-6 biostratigraphy, 105-7, 110-21 chronostratigraphy, 108-9, 121 eustasy, 107-8, 121 seismic sequence stratigraphy, 109-10, 121 dinosaur bones, discovery, 156 'dirty window', 28-9 'disequilibrium', 72-3 dish structures, 180 diurnal inequality of tides, and bedding patterns, 177 Dixon, E. E. L., 189 Dob's Linn, 93, 97, 98, 99, 100 dolomitic carbonatite, 257, 258 Donegal, 232 Donegal Main Granite, 221 Dorset Inferior Oolite, 138, 141,142, 147 double (multiple)-diffusive convection, 212 Drumbeg ultramafic bodies, 40 Dundry Hill, 134, 153-4 dunes, 175-7, 178 Dunham, K. C., 237, 241,243 Dunham's limestone classification scheme, 189 Durdham Downs, Bristol, fissure fauna, 156

Early Palaeozoic stratigraphy, 83-9 East African Rift, 214 East Cornwall, biostratigraphy, 110 East Greenland lava flows, 196, 197, 208 Eastern Layered Series, Rhum, 207 ecosomatic modification of Jurassic ammonites, 138 Elles, G., 94-5 Elsevirian orogeny, 13 'emanative centres', 244 Emborough Quarry, fissure fauna, 157 Embry & Klovan's limestone classification scheme, 189 emplacement mechanism for carbonatites, 256 Enderby Land granulite terrane, 76 entablature lava tiers, 199 Eoparastaffella Zone, 118 'epeiric seas', 190 equilibrium, mineralogical, 72 Eras, statigraphical, 85 Eskola, P., 72, 221,222, 223 Etheridge, R., 156 Europe, Northwest, palaeogeography, 166-71 European Variscides granulite terrane, 76 event stratigraphy, 87 extraordinarius Zone, 99

267

Faeringhavn terrane, 28 Falkland Island fossils, 86 Fascipericyclus-Ammonellipsites Ammonoid Zone, 110 'fast exposure paths', 76 fault controlled sequences, 57-64 fauna, from fissures, 153-61 faunal horizons, 133, 135-43, 145, 146, 147 Feltar mass, ophiolitic assemblage, 63 Fen carbonatites, 249, 258 fenitization (metasomatism), 249-50, 253, 256 Fenner trend of iron enrichment, 109, 210 ferro-carbonatite, 256 filtration differentiation, 228 Finland, granulites, 76 Fiskenaesset-type layered complexes, 27, 28 fissure faunas, Southern England, 153-61 flood basalts (plateau basalts), North Atlantic Province, 195, 196, 197-9, 200-1 floral biostratigraphy, British Dinantian, 114-18 fluid inclusions, 185, 190, 239-40, 246 techniques and methodology, 241-5 fluid-absent melting, 231 fluorite, inclusions in, 242, 244 Folk's limestone classification scheme, 189 foraminiferal biostratigraphy, British Dinantian, 118, 121 Forfarshire, Northeast, map of, 67 Fort Portal carbonatites, 254-6, 258 forward modelling approach, 215, 216 fossil extraction techniques, 157-8, 160 fossils, importance in stratigraphy, 83-4, 85 fractional crystallization, 206-7, 208-12, 214-16, 228, 229 fracture patterns in bedforms, 179 Franciscan Complex, Calfornia, 77 Ftichsel, G. C., 83 fundamental fractures, 57, 58-9, 62 Gabilly, J., 138 Gahard Formation, 166, 169 Gaima Plateau, 197 Galapagos, volcanoes, 197 Galway granite, 47 Garabal Hill Complex, 227-9, 230 Gargano fissure fauna, 160 garnet, petrological studies, 77 Garwood, E. J., 107, 113 Geikie, A., 185, 195, 199, 201 geochronology of Scottish metamorphic complexes, 37-51 'Geological Inquiries', booklet of, 6 Geological Society, the, 5, 6 origins of the Journal, 5-8 Geoscientist, the, 8 Geraldton-Beardmore terrane, 29 'ghost stratigraphy', 221 Giant's Causeway lavas, 199 Giletti, B. J., 37, 38, 43, 46, 47 Gilluly, J., 58 Girvan, fault controlled sequence, 60 Girvan district, palaeogeography, 97, 98, 99 Glen Dessarry syenite, 43, 47, 48, 50 Glen Kyllachy granite, 48, 50, 51 Glencoe volcanoes, 196, 201 Glenelg inlier, 44 Glenfinnan area pegmatites, 43 gneiss terrane accretion models, Precambrian, 26-7 Goatfell granite, Arran, 196 gold-quartz veins, 246 'Golden Spikes', 130 Goldschmidt, V. M., 72, 223 Gondwana, 166, 167, 170 Gorgona Island komatiites, 18, 31 Gorran Haven, Cornwall, 168 Gower Peninsula Carboniferous Succession, 189 'gradational differentiation', 228

268 grain settling, 177-8 Grampian Group, 63 Grampian Highlands, 46, 50, 51 granite classification systems, 232 layering in, 228 magmatism, 70, 221-33 'Granite Series', the, 223-6, 227 granite-greenstone terranes, 17-18, 25, 26 granite rocks, composition change over Earth history, 19 granitization (partial melting), 223, 224, 225 granule ripples, 176 granulite metamorphism, 76, 77-8 granulite-gneiss terranes, Archaean, 18 graptolites, in biostratigraphic calibration, 86-7, 93-5, 96-7, 99 gravel dunes, 176, 177 gravel-bed rivers, 177 Graveyard dyke, 41, 42 gravitational crystal settling, 206, 207, 208-11,213, 215-16, 228 gravity anomalies, 15-16 Great Bear batholith, 15 Great Dyke, Zimbabwe, 29 Great Glen, 51 Great Glen Fault, 57, 58, 59, 62-4 Greenland, 28, 29, 31, 45, 196, 197, 208 Greenough, G. B., 6 'greenstone' belts, ancient, 27 greenstone terranes, 28-9, 246 greenstone-granite terranes, Precambrian, 17-18, 25, 26 Grenville orogen, 13 Grenville Province, 76 Grenvillian Belt, Labrador, 45 Grenvillian metamorphism, 44 Grenvillian Ocean, 13, 15 Grbs Armoricain, 165, 166 Gr~s de Goasquellou sandstone beds, 169, 171 Gr~s de petit May, 165-6 Gressly, A., 84 Grout, F. F., 212 Gruinard Bay, 40 Guettard, J. E., 83 guide-fossils, 128-9, 130, 131,132, 134, 136-8 Hall, J., 93 Harker, A., 69, 195-6, 205, 206, 207, 208, 212, 213, 215 Harker diagrams (variation diagrams), 208, 214 Harker index, 214 Hartville uplift, 31 Hastarian Stage, 108 Hawaii, volcanoes and lava flows, 197 heat production in the earth, 19, 20, 27 Hebridean basaltic plateaus, 195, 196, 201 Hebridean Province, 197 'hemarae', 133, 134 Hercynian Belt, Western Europe, 224 Hercynian orogeny, 85 Hibbard, C. W., 160 high-magnesium calcite, 187, 188 high-pressure metamorphic rocks, 77 high-temperature metamorphism, 69, 70 high-temperature-low-pressure metamorphism, 75-6, 77, 233 Highland Boundary Fault, 46, 49, 57-8, 60-2, 67 Highland granites, 228 Hill, A. J., 189 Himalayas, 11, 73, 75, 86 Hind, W., 107 Holkerian Stage, British Isles, 108, 109, 115, 117, 119, 120, 121 Holm, G., 95 Holwell quarry, fissure fauna, 153, 154-5, 156, 157, 161 homogenization temperature, 240 Hooke, R., 83 Horner, L., 240 Hottah island arc, 14, 15 hummocky cross-stratification, 178, 179, 180

INDEX Hutton, J., 11 hydraulics of bedforms, 176-80 hydrocarbon inclusions, 242 hydrocarbon maturity, 114 hydrocarbon reservoirs, 189 hydrothermal oilfields and ore fluids, 237-8, 245'6 ore-genetic theory, 238-45 Iapetus Ocean, 46, 98, 99, 166, 167, 170 Iceland, lava flows, 196, 197, 199, 200, 201 Imitoceras prorsum Ammoioid Zone, 110 immiscibility of liquids, 213-14, 215, 255-6 Inchbae facies, 43 index-fossils, 130 Inferior Oolite, Southern England, 132, 133, 134, 138-42, 146, 147 intercrystalline diffusion, 77 interface method of fossil extraction, 160 intrusions categories, 29 as cause of regional metamorphism, 69 Inverian metamorphism, 38, 40, 41 'inverted metamorphism', 75 ion-microprobe analysis, 50 Irish Caledonides, 230 Irish Dinantian stratigraphy, 110-21 Islay rocks, 63 isobaric cooling paths, 76 isoclinal folding, 99 isothermal decompression paths, 76 Ivorian Stage, British Isles, 108, 110 Jason Zones, 147 Jimberlana intrusions, 29 Johnny Hoe suture, 15 Jones, O. T., 58 Jormua ophiolite, 16 Journal, the, origins, 5-8 Journal des Mines, 6

Judd, J. W., 195, 201 Jukes-Brown, A. J., 189 Julianehaab batholith, 13 Jura, fissure fauna, 156 Jurassic geochronology, 129-31, 135, 147-8 biostratigraphic time-resolution, 127, 131-4, 135-7, 147 ammonites in, 130, 137-43, 146 estimates of, 143-6 polyhemeral chronology, 134-5 Jurassic Period, 86-7, 144 'juvenile' source theory, 243 K-Ar dating, 38, 40, 41, 43, 45, 47 Kaapvaal craton, 18, 19, 20 Kaapvaal shield, 18, 19 Kainozoic, 85 Kaiserstuhl lapilli, 254 Kangamiut dykes, 30 Kangmar dome, 75 kaolinization, 245 Kapuskasing terranes, 18 Karelian terrane, 14 Katwe-Kikorongo volcanic fields, 255 Kennedy, W. Q., 57, 58, 59 Kerimasi, Oldoinyo Lengai, carbonatites, 250, 251, 252, 253, 254, 255 Kermach, K., 157, 158 Ketilidian belts, Greenland, 31, 45 crust, Scotland, 63 orogen, Greenland, 13-14, 15 Keuper]Lias boundary, 155 Keweenawan rift, 13 Khanneshin carbonatites, 255,258 Kilavea volcano, 208 kimberlites, link with carbonatites, 258, 260, 261

INDEX Knoxisporites triradiatus-K, stephanephorus Zone, 115 Knoydart pegmatites, 43 'Knoydartian' metamorphism, 44 Kobberminebugt suture, 13 Kohistan arc, 13, 17 Kola suture zone, 14 Kola-Karelian orogen, 14 komatiitic magmatism, Precambrian, 28-9, 31, 32, 33 Koolau volcano, 200 Koslowski, R., 95 Kraeuselisporites hibernicus- Umbonatisporites distinctus Zone, 115 Krynine, P. D., 58 KUhne, W., 156, !60 Kun Lun orogen, 11 Kurunegala, granulite formation, 77 Kylesku gneisses, 38, 41

Lachlan Fold Belt, 224, 226 Lake District, 95, 97, 98-9, 107 Borrowdale arc, 60 Lambert, R. St. J., 37 lamination patterns in aqueous bedforms, 176, 177 Land6vennec Formations, 166, 169 Land's End mineral veins, 240 Lapworth, C., 86, 87, 93-4, 95, 96-7, 98, 99-100, 101,134 Laramie mountains, 31, 32 lateral displacement of faults, 57, 59 Laurentia, 12, 166 Laurentian platform limestones, 95 lava-flow structures, 199-200 Laxford Front zone, 54 Laxfordian metamorphic events, 27 radiometric dating, 38, 39, 40, 41, 54 layered mafic intrusions, 207, 208, 215,228 Lehmann, J. G., 83 Leny Limestone, 50 leucosome chemistry, 225 Lewisian Complex, 54 geochronology, 38-43 North West Scotland, 25, 26, 27, 29, 31, 32, 41, 45 Liassic fissure fauna, 155 lime-mud, origin, 187 limestones classification, 189-90 structure and origin, 185-91 Limpopo belt, 18, 20, 76 Lindgren, W., 241 liquid immiscibility, 213-14, 215, 255-6 lithosphere structure and eruption ages, 259-60 lithospheric extension and regional metamorphism, 77, 78 iithostratigraphic time-resolution, 146 Llandeilo age of Scottish shales, 97 Llandovery Series, 86, 87, 88 local range biozones, 136 Loch Torr an Lochain dyke, 42 Lochan a' Chairn facies, 43 London-Brabant massif, 170, 171 longitudinal (seif) dunes, 176 Louis, J., Count de Bournon, 5 low magnesian calcite, 187, 188 low-pressure-high-temperature metamorphic belts, 69 Lulefi-Kuopio suture zone, 13 Lycospora pusilla Zone, 115 Lyell, C., 11, 84-5 Lys-Caillaouas massif, 76, 77 Mackenzie dyke swarms, 15, 200, 201 marie magmatism, Precambrian, 25-33 MAGIC processes, 226 magma mingling, 228 magma mixing, 206, 207-8, 212, 215 magma-flow directions, 200 magmas and magmatism, 19 alkaline, 251-3, 255, 256, 259, 260

269

anorogenic, 15 carbonate, 249-62 granite, 221-33 mafic, Precambrian, 25-33 plutonic, 69 tholeiitic, 208-10 see also magmatic differentiation magmatic advection of heat, 70, 75, 78 magmatic differentiation, 205,208-11,212-16 early ideas, 205-7 mechanism, 207-12 modelling, 215 magnesio-carbonatite, 256, 257, 261 magnesium calcite, 187, 188 magnetostratigraphy, 88, 145, 146 Main Central Thrust System, 75 Malene metavolcanic rocks, 28 mammals, origin, 153 mantle metasomatism, 259, 260 mantle source and primary flux, 258-9, 261 mantle-plume-related magmatism, 27 Marathon dyke swarms, 15 marine bivalves in stratigraphy, 111 marine storm bedding, 178-9, 180 MASH processes, 226, 232 Massif Central, 224 Mberengwa aUochthon, 17 M'Coy, 86 medium-pressure regional metamorphism, 68, 69 melt fraction material, 224, 225 melt generation and tectonism, 260 Mendip Hills, fissure fauna, 155, 156 mesothemic boundary status, 107 'Mesozoic', 85 Mesozoic fissure fauna, Southern England, 153, 157 metal-bearing hydrothermal fluids, 245 metamorphism 'inverted', 75 related to extension, 75-6, 223 and tectonics, 71 see also geochronomogy of Scottish metamorphic complexes; regional metamorphism metasomatism alkali (fenitization), 249-50, 253, 256 mantle, 259, 260, 261 micro-probe analysis, 232 microstructural studies, 71 Mid-Carboniferous boundary, 110 Midford Sands, 132-3, 146 Midland Valley, Scotland, 58, 60, 61,111 migma-magma, 223 migmatites, 224, 225 mineral ages and cooling histories, 46-7 mineral isochron ages, 77 mineralization of Cornubian and Pennine orefields, 237-46 Minnesota River Valley terrane, 18 miospore zonation, British Dinantian, 114-18, 121 Mississippi Valley Type mineral deposits, 238, 243,244, 246 Mistassini dyke swarms, 15 Miyashiro facies series, 72 'mobile belts', 26 Moffat area, palaeogeography, 93, 94, 97-9, 101 Moine thrust, 43, 46, 58, 63 Moinian Supergroup, geochronology, 43-4, 45-6, 47, 49 Molson dyke swarms, 15 monogenetic volcanoes, 197 Moorbath, S., 37, 38 Moore, C., 153-6, 160 Morar Group, 44 Moray Firth,Old Red Sandstone displacements, 62 Morecambe Bay carbonate platform, 107 Mourne Mountain granites, 196 Mozambique belt, 12, 19 Mull, Island Of, intrusive complexes, 195, 196, 197, 198, 201

270 multiple-diffusive convection, 212 Murchison, R. I., 83, 85, 86, 129 Murospora margodentata- Rotaspora ergonulii Subzone, 116 Nagssugtoquidian mobile belt, 30, 31 Nahanni terrane, 15 Nain Province, 31 Namur basin, 118 Namurian boundary, 110 natrocarbonatite, 251-3, 261 Neoarchaediscus Zone, 118 Neptunian dykes, 157, 158, 160 Neptunist theory, 83 New England, metamorphism, 71, 73 New England Appalchians, 69 Newer Granites, 46 Newsletter, 8 Nicol, H., 185 Nockolds, S. R., 221,222, 227, 228, 229, 230, 231 noritic magmatism, Precambrian, 29-31, 32, 33 Normandy-Wessex Basin, 146, 148 North America cartons, 87 exotic terranes, 59 fissure fauna, 160 North Atlantic cratons, 26, 31 North Atlantic Province, 196, 197, 199 see also British Tertiary Province North Sea Chalks, 189 North West Europe, palaeogeography, 166-71 North West Scotland geochronology of Highlands, 37-51 Lewisian Complex, 25, 26, 27, 29, 31, 32, 41, 45 Scourie dyke swarm, 19, 27, 29, 31, 41-3 Northumberland Trough, 111, 113, 118 Norwegian Caledonides, 68, 77 Nfik gneisses, 26, 27 oceanic crust on the continent, 59 oceanic lithosphere, Archaean, 19 Oldoinyo Lengai volcano, Kerimasi, 250, 251,252, 253, 254-5 Onaman-Tashota terrane, 29 oolitic grain formation, 187-9 open-system magma chambers, 207-8 Oppel, A., 86, 94, 129-30, 133 Oppelian Zones, 129-30, 133, 134 Orbigny, A. d', 86, 129, 133 Ordovician series, North American, 86 Ordovician System, 86, 87, 145 Moffat Series, 93, 94, 95-6, 97, 98, 101 Ordovician to Devonain palaeogeography of Europe, 165-71 Ordovician-Silurian boundary, 100, 101 ore-genetic theory, 238-46 orogens, origin, 11-16, 19 orogeny and regional metamorphism, 68-9 P - T - t paths, 69-78 ostracodes, in biostratigraphic calibration, 87, 118, 167 Ottawan orogeny, 13 Outer Hebrides, 45 'outer limit' lines, 67 Oxford Clay, Peterborough, 131,138, 142-3, 147 oxide-oxide variation diagrams, 208, 209, 210, 214-15 oxygen fugacity, 209 oxygen isotope dating, 77

P - T - t paths, 69-78 'paired' metamorphic belts, 69, 81 palaeogeography of Northern Europe, 166-71 Palaeozoic, Early, stratigraphy, 83-9, 94, 95, 101 Pan-African belt, 12, 16, 19 partial melting (granitization), 223,224, 225 Payne River dyke swarms, 15 Payson ophiolite, 16 Pb-Pb dating, 38, 39, 40, 54

INDEX Pearce element ratio diagrams, 214 Pechenga Series, 14 Pecora, W. T., 249, 250, 256 Pennine orefields and orefuids, 237-8, 239, 241-4, 245, 246 Penokean orogen, 13, 15 Periods, statigraphical, 85, 86 Permian Reef Complex, 189 Permian System, 85, 144 Perotriletes tessellatus-Schulzospora campyloptera Zone, 115 Peterborough Member, 142 Phanerozoic, 143 tectonism, 11, 19 Phillipines, tectonic activity, 59 Phillips, J. A., 84, 85, 241 Philosophical Transactions, 5, 6, 7, 8 Pikwitonei granulites, 18, 73, 74 Pilton Shale Formation, 110 plane beds, 176 plate tectonic uniformitarian model, 11-21 plateau basalts (floor basalts), North Atlantic Province, 195, 196, 197-9, 200-1 Pleistocene, time-resolution, 146 Plieninger, W. H. T. yon, 155 plumbing systems, 226, 232 plutonism, 69, 222-3, 232, 259, 260 'place' in, 222-3 'time' in, 223-4 plutons, shape of, 221 Polino carbonatites, 254, 255, 258 Poll Eorna dyke, 42 polygenetic volcanoes, 197, 199 Polygnathus communis carina Conodont Zone, 110 Polygnathus inornatus Conodont Zone, 115 Polygnathus mehli Conodont Zone, 115 polyhemeral chronology, 134-5 Pongola Supergroup, 18 Port aux Basques Complex, 71 Portsoy beds, 48, 50 Precambrian crustal development, 25-37 plate tectonics, 19, 20 Preketilidian belts, Greenland, 45 Principle of Biostratigraphic Synchroneity, 128, 136, 137 Principles of Geology, 11 Proceedings, the, 7, 8 prograde metamorphism, 77 progressive regional metamorphism, 69 Proterozoic crustal development, 25, 29-31 plate tectonics, 11-20 protolith formation and Badcallian metamorphism, 38-41 Pseudopolygnathus multistriatus Conodont Zone, 115 punctuated orogeny, 58 Purtuniq ophiolite, 16 Pyrenees, 76, 231

Quarterly Journal the, 7-8 quenched dykes, 30 Quercy phosphorites, 156 radiogenic isotope dating, development, 77 radiometric dating of Scottish metamorphic complexes, 37-51 Raistrickia nigra-Triquitrites marginatus Zone, 116 ramps, 190 Ramsbottom, W. H. C., 107, 108, 111 Rb-Sr dating, 37-8, 40, 41, 44, 45, 47, 48, 49, 50, 77 Reaction Principle, 207 Read, H. H., 221,222, 227, 228, 230, 231,232, 233 regional metamorphism, 67, 68, 69, 78, 222, 223 orogeny and, 68-9 P - T - t paths, 69-74 recent advances, 74-7 retrograde metamorphism, 77 Rhaetic fissure fauna, 155, 156, 159

INDEX Rhegreanoch dyke, 41, 42 Rheic Ocean, 167, 171 Rhum, Island of, igneous complex, 196, 197, 206-7 Riley, H., 111,156 rimmed shelves, 190 ripples, sedimentary, 176, 178 Robinson, P., 157-8 rock-time duality, 127-8, 135 role of fault, 62 'room (space) problem', the 221-2, 232 Rossendale Millstone Grit, 107 Royal Society, 5 Ruedemann, R., 94 Rufunso carbonatites, 254, 257, 258, 259 Rule of Priority, 129, 130 Russian Platform, 87, 98 Ryoke Metamorphic Belt, 81 Sahara, collisional orogen, 13 St Austell mineral veins, 238, 240 Saint Barth616my massif, 76 St Malo Migmatite Belt, 224, 226, 227 St Michael's Mount mineral veins, 240, 244 Salter, J. W., 96, 165 San Andreas Fault, 57, 59 San-yo granitoids, 81 Sanbagawa Metamorphic Belt, 81 sand ripples, 176 sand waves, 177, 180 Sandford Lane Fossil Bed, 134 Sawkins, F. J., 241-2, 243-4 Scaliognathus anchoralis Conodont Zone, 110 Scandinavian succession, 98 Schopfites claviger-Auroraspora macra Zone, 115 Scotland, 63 metamorphic complexes, geochronology, 37-51 Southeastern Highlands, regional metamorphism, 67, 68, 74, 78 Southern Uplands, 58-64, 93-4, 97, 98-9, 101 see also North West Scotland Scourian (Badcallian) metamorphism, 27, 38-41 radiometric dating, 38-43, 54 Scourie dyke swarms, 27, 29, 31, 41-3 sea-level changes, stratigraphy related to, 87, 88-9 secular biochronological resolution, 137, 146 secular resolving power, 137, 145 Sedgwick, A., 83, 85, 86 sediment drifts, 176 sediment waves, 176 sedimentary structures, Sorby and the last decade, 175-80 sedimentation and faulting, 58 sequence stratigraphy, 101, 190 series, stratigraphical, 86, 87 Sgurr Breac pegmatites, 43 Sharyzhalgay complex, 72 sheet-like pillar structures, formation, 180 shells, in formation of limestone, 186-7 Shelveian event, 86 Sherborne Building Stone, 134 Sherborne Inferior Oolite, 133, 134 SHRIMP, 50, 51,232 Silesian Subsystem, 109 Silurian series, establishment of, 86, 87, 93 Silurian System, 85, 86, 87, 145 Moffat Series, 93, 94, 97, 98 Siphonodella crenulata Conodont Zone, 115 Siphonodella sandbergi Conodont Zone, 110 Skaergaard intrusion, 196, 207, 208, 209-11,214, 216 skeletal disintegration as source of carbonate, 187 Skye, Island of, lava flows, 195, 196, 197, 199, 200, 201 Slave Province, Canada, 14, 17, 18 Slickstones Quarry, 157 Sm-Nd dating, 39-40, 41, 42-3, 44, 47, 77 Smith, W., 83-4, 128-9, 153 soft sediment deformation, 179-80

solidification index, 214 Solomon, M., 242 Solway line, 58, 60 Sorby, H. C., 175, 178, 185-9, 191,237, 239-41,245 Soret coefficient, 212 Soret diffusion, 206, 212, 213 South America, dykes, 31, 32 South Australian noritic dyke swarms, 32 South East Greenland dyke swarms, 29, 30-1 South Harris complex, 39 South Kola belt, 14 South Tibetan detachment system, 75 South Wales Lower Limestone Shales, 110 South West Greenland, 26, 27-30, 31 Southern Brittany Migmatite Belt, 73, 74 Southern Uplands fault, Scotland, 58, 60 Sowerby, 85 Spelaeotriletes balteatus-Rugospora polyptycha Zone, 115 Spelaeotriletes pretiosus-Raistrickia clavata Zone, 115 Spitzkop carbonatites, 255,259 spring-neap cycle bedding patterns, 177 Spurr, J. E., 241 Staffa lavas, 199 Stages, stratigraphical, 86, 87, 130, 131 d'Orbigny, 130 Rule of Priority in Naming, 129 standard chronostratigraphic units, 129, 130-1,136, 145 'standard geological column', the, 128, 143 standard time-ordered succession, 127 star dunes, 176 Steno, M., 83 Steno's Principle of Superposition, 127 Stensio, E., 95 Stillwater intrusion, 29, 31,208 Stiperstones Quartzite, Shropshire, 166 Stoer Formation, 45 Stonesfield Slate, Oxfordshire, 153 storm bedding, 178-9, 180 Strathan dyke, 42 Strathmore syncline, 61 stratigraphical horizons, 136 Stratigraphical Nomenclature, Code of, 129, 130 stratigraphy, Early Palaeozoic, 83-9 Strichen granite, 48, 51 strike-slip faulting, 57, 59 Stutchbury, S., 156, 162 subaqueous dunes, 178 subduction geotherms, decrease in, 19, 21 subduction zone metamorphism, 76-7 Subzones, stratigraphical, 116, 130, 131, 145 Sudbury dyke swarms, 15 supercontinent, Proterozoic, 15 Superior Province, Canada, 17, 18, 29 suspect terranes, 59 Sutton, J., 25, 38 Svecofennian orogen, 13, 15, 16 swaley bedding, 179, 180 'syn-rift megasequence', 110 Synchroneity, Principle of, 136, 137 Systems, stratigraphical, 85-6, 100-1 Tarfside Culmination, 47 Tayvallich volcanic sequence, 50 Teall, 67 tectonic control on sedimentation, 58 tectonic processes of magmatism, 230, 232 and metamorphism, 77-8 tectonic transfer of heat, 75 tectonism and melt generation, 260 temporal scope of an analysis, 145 Tertiary System, 85, 145 textural analysis, 71 textural modelling, 232

271

272

INDEX

Theory of Earth, 11 thermal modelling of orogenic belts, 73 thermobarometric measurements, 77 thermogravitational diffusion, 212-13, 215 thermometamorphism, 67, 68 tholeiitic magmas, differentiation in, 208-10 tholeiitic magmatism, Precambrian, 30 Thompson, A. B., 72 throw of a fault, 59, 60, 62-3 Tibetan sedimentary sequence, 75 tidal bedding, 180 Tien Shan orogen, 20 Tilley, C. E., 67-8 time-correlations, 127, 128-9, 147 time-duration, 137, 143, 144-5 time-interval, 137, 145 time-markers, 128 time-planes, 128 time-resolution, biostratigraphic, see under Jurassic geochronology time-rock duality, 127-8, 135 time-scale of sedimentological events, 176-7 time-temperature trajectories, 47 Tornio-Koillismaa intrusions, 29 Tornquist Sea, 98, 166, 167 Torridonian sandstones, 43, 44-6 total range biozone-assemblage, 136 Tournaisian/Vis6an boundary, 113 Transactions, the, 6, 7 'transient', 135 transverse dunes, 175-6 Traonliors Formation, 169, 171 Tremadoc Series, 86 Triassic System, 85, 144 fissure fauna, 157, 158, 160 palaeogeography, 166-71 trilobites in biostratigraphic calibration, 118-19, 120, 121 Tripartites distinctus-Murospora parthenopia Subzone, 116 Tripartites vetustus-Rotaspora fracta Zone, 117-18 Trois Seigneurs massif, 76, 224, 225,227, 230, 231 Trueman, A. E., 135 Turner, F. J., 223 Twenhofel, W. H., 189 Tytherington Quarry fissure fauna, 158, 159 U-Pb dating, 37, 38-9, 40, 41, 42-3, 45, 48, 49-50, 51, 54, 77 Uchi-Sachigo terranes, 17 uniformitarianism, plate tectonic model, 11-21, 25 uhitary association biozone, 136 upper-stage plane beds, 176

Vallatisporites verrucosus-Retusotriletes incohatus Zone, 115 Vallis Vale, fissure fauna, 154, 156 vapour-liquid ratios, Sorby, 240 variation diagrams, 208, 209, 210, 214-5 Variscan belt, 74 Variscan massifs, 76

Vaughan, A., 105-7, 108, 113, 118 Ventersdorp rift system, 19 Verneuil, M. E., 85, 86 vertebrate fissure faunas, Southern England, 153-61 Vicary, V., 165 Vis6an Stage, 108, 113, 114, 115, 121 Vis6an/Namurian boundary, 118 volatile fluxing, 259 volcanic-hosted massive sulphide deposits, 246 volcanology, British, classic period of, 195-6 Waagen, W., 129-30 Wabigoon terrane, 17 Wales, 58, 143 wall rock assimilation models, 207 Walls Boundary Fault, 62 Ward, D. J., 160 Watson, J., 25, 38 wave-related bedforms, 178-9, 180 Wawa-Abitibi terrane, 17 Weardale granite, 243, 245 Welsh Basin, 87 Werner, A. G., 83 West African craton, 12-13 West Greenland granulite-gneiss terranes, 18 lave flows, 196, 197 Westbury-sub-Mendip fissure fauna, 160 Western Alp blueschist belts, 77 whole-rock ages, 49, 77 determination of, 39-41 'Wilson cycle', 13, 29 Wilsonian cycle of megacontinent growth, 59 wind waves, bedforms related to, 178 Windsor Hill, Shepton Mallet, fissure fauna, 156, 157 Witham, H., 185 Witwatersrand Supergroup, 18-19 Woodward, H. B., 8, 134-5 Wopmay orogenic belt, 14-15, 73 Wyoming craton, 31, 32 Yangtze Platform cratons, 87 Yeovil Sands, 132-3, 146 Yorkshire Dales, 107 Zambian volcanic carbonatites, 253-4, 255 zibar ripples, 176 Zimbabwean craton, 17, 18, 20 zircon grain analysis, 45, 49, 50, 51,232 zonal mapping, 68 Zones, stratigraphicai, 86-7, 94, 130, 131,133, 145, 147 Opellian, 129-30 Zonules, 130 zoogeographical provincialism, British Isles, 107