Introduction to Metamorphic Textures and Microstructures Introduction to Metamorphic Textures and Microstructures Second edition A.J. Barker Lecturer in Geology University of Southampton, UK Stanley Thornes (Publishers) Ltd © 1998 A.J. Barker The right of A.J. Barker to be identified as author of this work has been asserted by him in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system, without permission in writing from the publisher or under licence from the Copyright Licensing Agency Limited. Further details of such licences (for reprographic reproduction) may be obtained from the Copyright Licensing Agency Limited, of 90 Tottenham Court Road, London WIP OLP. First edition published in 1990 by Chapman & Hall, London Second edition published in 1998 by: Stanley Thomes (Publishers) Ltd Ellenborough House Wellington Street CHELTENHAM GLSO 1YW United Kingdom 98 99 00 01 02 / 10 9 8 7 6 5 4 3 2 A catalogue record for this book is available from the British Library. ISBN 978-1-4615-7293-0 ISBN 978-1-4615-7291-6 (eBook) DOl 10.1007/978-1-4615-7291-6 Typeset in Sabon 10.5/12.5 by Cambrian Typesetters, Frimley, Surrey Preface Technological advances over recent years, and examines interrelationships between deforma­ a wealth of new research providing refreshing tion and metamorphism. It includes an exten­ new interpretations on many metamorphic sive chapter on strain-related microstructures, microstructures, encouraged me to embark on and others on the controversial but important this considerably expanded and fully updated topics of porphyroblast-foliation relation­ second edition. ships, and shear-sense indicators. The penulti­ My aim in producing this book on metamor­ mate chapter is on the topic of fluids and phic textures and microstructures has been to vemmg, and in the final, completely provide a detailed introduction to the thin­ redesigned, chapter, the characteristic reaction section description and interpretation of meta­ textures and microstructures associated with morphic rocks. Although primarily written for particular environments of metamorphism and the advanced undergraduate student, it should specific P-T -t trajectories are discussed. provide a useful first source of reference for This is done with reference to many of the any geologist dealing with metamorphic rocks. classic areas of metamorphism from around It is intended that the text should be both well­ the world. Appendices giving mineral (and illustrated and comprehensive, but at the same other) abbreviations (Appendix I), a glossary time concise and affordable. of terms (Appendix II), and a list of key The book is comprised of three parts. Part mineral assemblages for the major composi­ A provides an introduction to metamorphism tional groups of rocks at each metamorphic and metamorphic rocks, and compared to the facies (Appendix III) form an integral part of first edition has been enlarged to include more the book. detail on the basic interrelationships between A comprehensive list of references is given at equilibrium assemblages, mineral chemistry the end of each chapter. Wherever possible, the and microstructures in the interpretation of intention has been to refer to specific references metamorphic reactions. Part B introduces the relevant to individual topics covered in the fundamental textures and microstructures of text, as well as referring to key review papers metamorphic rocks, with emphasis on the on all the major subjects. It is important to conditions and processes responsible for their recognise that this book can provide only a formation. This section includes a chapter on glimpse into the complexities of textural and fabric development (including the use of microstructural development of metamorphic SEM), followed by chapters covering topics rocks, and so for a more comprehensive insight such as crystal nucleation and growth, inclu­ into a particular topic the cited references are sions, intergrowths and retrogression. Part C strongly recommended. My knowledge of v Preface metamorphic rocks has grown considerably tion to this fascinating area of study, and during the writing of this second edition, and it unlocks a few of the secrets of the rocks that is my hope that your knowledge and enthusi­ you are studying. asm will be enhanced by reading it. Without doubt, the interpretation of metamorphic rocks and their microstructures is a complex topic, Andy Barker and there are still many unanswered questions. Southampton I hope that this text provides a useful introduc- March 1997 vi Acknowledgements Acknowledgement is given once again to all for developing and printing the photographs, those postgraduate and undergraduate students and to Chris Forster, who converted many of who over the years have contributed to what is the line drawings of the first edition into now a most intriguing and diverse array of thin computerised images. All new diagrams were sections. The study of this worldwide collection computerised and/or hand drawn by the of metamorphic rocks has without doubt author. I should also like to thank those people broadened my knowledge of their textural and who supplied thin sections, and those authors microstructural features. It provided the back­ who supplied original photographs, or permit­ ground that induced me to write the original ted me to reproduce diagrams from their publi­ edition, and in the seven years since initial cations. Also, thanks to Barbara Cressey for publication, my enthusiasm has been main­ advice and assistance with SEM work. tained through the continued interest shown by I would also like to thank the many people students in providing me with new and inter­ who gave me feedback on the first edition, or esting thin sections to look at. have spared some of their time to review Once again, I would like to acknowledge the chapters of this new edition. The comments classic works of Harker, Spry and Vernon, and suggestions have all been useful in shap­ whose books greatly stimulated my own interest ing and improving on early drafts of the text. in metamorphic textures and microstructures. The people I would particularly like to thank Additionally, I would like to acknowledge the are Kate Brodie, Giles Droop, Scott Johnson, Metamorphic Studies Group, whose interesting Cees Passchier, Stephen Roberts, Doug programme of conferences and field meetings Robinson, Joan Soldevila, Peter Treloar, has, over the years, afforded me excellent discus­ Rudolph Trouw and Ron Vernon. Last but by sion with a truly international list of contribu­ no means least, I offer many thanks to Linda, tors and participants. Without doubt, this has for her continued patience and encourage­ maintained my enthusiasm for the subject and ment, especially during the final stages of greatly enhanced my understanding of metamor­ writing and production. phic rocks and metamorphic processes. Special thanks are extended to Barry Marsh AJB vii Contents Preface v Acknowledgements Vll Part A Introduction to metamorphism and metamorphic rocks 1 1 Environments and processes of metamorphism 3 1.1 Environments of metamorphism 3 1.1.1 Regionally extensive metamorphism 4 1.1.2 Localised metamorphism 6 1.2 The limits of metamorphism 7 1.3 An introduction to chemical processes of metamorphism 8 1.3.1 Equilibrium assemblages and the phase rule 8 1.3.2 The energy of the system 10 1.3.3 Reaction types 11 1.3.4 Reaction rates 14 1.3.5 Diffusion 15 1.3.6 Fluid phase 16 1.4 Physical processes acting during metamorphism 16 1.4.1 Volume changes during reaction 16 1.4.2 Deformation processes on the macro- and micro scale 16 1.4.3 Crystal defects and surface energy 17 1.5 Deformation-metamorphism interrelationships 18 References 18 2 Facies concept and petrogenetic grids 21 2.1 Metamorphic facies, grade and zones 21 2.2 Petrogenetic grids 23 References 25 3 Compositional groups of metamorphic rocks 27 3.1 Pelites 27 3.1.1 Medium-pressure 'Barrovian' metamorphism 28 3.1.2 Low-pressure assemblages 31 3.1.3 High-pressure assemblages 32 ix Contents 3.2 Metacarbonates and calc-silicate rocks 32 3.3 Quartzofeldspathic metasediments 33 3.4 Metabasites 33 3.5 Metamorphosed ultramafic rocks 36 3.6 Meta-granitoids 37 References 37 Part B Introduction to metamorphic textures and microstructures 39 Definition of texture and microstructure 39 Equilibrium and equilibrium assemblages 39 Reference 40 4 Layering, banding and fabric development 41 4.1 Compositional layering 41 4.2 Introduction to stress, strain and fabric development 42 4.3 Classification of planar fabrics in metamorphic rocks 43 4.4 Processes involved in cleavage and schistosity development 45 4.5 Processes involved in formation of layering in gneisses and migmatites 51 4.5.1 The nature and origin of gneissose banding 51 4.5.2 The nature and origin of layered migmatites 53 References 55 5 Crystal nucleation and growth 57 5.1 Nucleation 57 5.2 Growth of crystals 60 5.3 Size of crystals 61 5.4 Absolute growth times 65 5.5 Shape and form of crystals 65 5.6 Twinning 75 5.6.1 Introduction 75 5.6.2 Primary twins 76 5.6.3 Secondary twins 77 5.7 Zoning 79 References 83 6 Mineral inclusions, intergrowths and coronas 85 6.1 Growth of porphyroblasts to enclose residual foreign phases 85 6.2 Exsolution textures 89 6.3 Inclusions representing incomplete replacement 93 6.4 Symplectites 93 6.5 Coronas (of