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Petrology of the Metamorphic Rocks Petrology of the metamorphic rocks Second Edition Roger Mason London UNWIN HYMAN Boston Sydney Wellington © R. Mason, 1990 This book is copyright under the Berne Convention. No reproduction without permission. All rights reserved. Published by the Academic Division of Unwin Hyman Ltd 15/17 Broadwick Street, London WIV IFP, UK Unwin Hyman Inc. 955 Massachusetts Avenue, Cambridge, MA 02139, USA Allen & Unwin (Australia) Ltd 8 Napier Street, North Sydney, NSW 2060, Australia Allen & Unwin (New Zealand) Ltd in association with the Port Nicholson Press Ltd Compusales Building, 75 Ghuznee Street, Wellington I, New Zealand First published in 1990 British Library Cataioguing in Publication Data Mason, Roger 1941- Petrology of the metamorphic rocks. - 2nd ed. I. Metamorphic rocks. Petrology I. Title 552.4 ISBN 978-0-04-552028-2 ISBN 978-94-010-9603-4 (eBook) DOI 10.1007/978-94-010-9603-4 Library of Congress Cataioging-in-Publication Data Mason, Roger, 1941- Petrology of the metamorphic rocks/Roger Mason. - 2nd ed. p. cm. Includes bibliographical references. 1. Rocks, Metamorphic. I. Title. QE475.A2M394 1990 552·.~c20 90-35106 CIP Typeset in 10 on 12 point Times by Computape (Pickering) Ltd, North Yorkshire Preface There has been a great advance in the understanding of processes of meta­ morphism and of metamorphic rocks since the last edition of this book appeared. Methods for determining temperatures and pressures have become almost routine, and there is a wide appreciation that there is not a single temperature and pressure of metamorphism, but that rocks may preserve, in their minerals, chemistry and textures, traces of their history of burial, heating, deformation and permeation by fluids. However, this excit­ ing new knowledge is still often difficult for non-specialists to understand, and this book, like the first edition, aims at enlightenment. I have concen­ trated on the interpretation of the plate tectonic settings of metamorphism, rather than following a geochemical approach. Although there is an impress­ ive degree of agreement between the two, I believe that attempting to discover the tectonic conditions accompanying rock recrystallization will more readily arouse the interest of the beginner. I have used a series of case histories, as in the first edition, drawing on my own direct experience as far as possible. This m~ns that some subjects are treated in more detail than others, and many important topics are barely mentioned at all. It also means that general concepts appear in a rather haphazard order in the text. To help my readers, I have provided a glossary of definitions of terms used in the book, which are indicated in bold type in the text. I discuss thermotectonic models, based on very simple 2-dimensio­ nal analyses, in describing the metamorphic history of many of my examples. Although many problems remain to be solved in this approach (and I do not claim that my models are sufficiently rigorous to be contributions to research discussion), its success in accounting for the shape of many P-T-t paths has convinced me that it is a useful framework to use in learning about meta­ morphisrri. I hope I have provided my readers with an interesting and sound basis for further exploration of this fascinating branch of the Earth sciences. The present form of the book arose from a course of lectures and demonstrations given at Wuhan College of Geology (since renamed the China University of Geosciences) in the People's Republic of China in 1986. I thank President Zhao Pengda and Professor You Zhendong for inviting me to give the course, Mrs Han Yuqing who worked very hard as my inter­ preter, and the students of the class who suffered the culture shock of being introduced to western-style discussion in lectures! I have been encouraged to PREFACE emphasize the tectonic theme in a thorough and constructive review of my manuscript by Dr M. J. Le Bas. In addition to those I thanked for advice in my first edition, I would like to thank Dr L. Ashwal, Dr A. P. Boyle, Professor K. Burke, Dr K. W. Burton, Mr A. J. Griffiths, Dr W. L. Kirk, Dr J. D. A. Piper, Dr G. D. Price, Ms K. V. Wright, Dr J. W. Valley, and Dr C. Xenophontos. I thank Dr P. W. Edmondson for compiling the index. I also acknowledge with thanks receipt of a travel grant from the Central Research Fund of the University of London to visit classic metamorphic localities in the USA, and also a grant from the Royal Society of London to the Sulitjelma '83 Ophiolite Expedition. Contents Preface page v List of tables IX 1 Introduction 1 Definition 1 The processes of metamorphism 2 The three field categories of metamorphic rocks 6 Studying thin sections of metamorphic rocks 8 Laboratory study of metamorphic rocks 9 Why study metamorphic rocks? 16 2 Rock and mineral compositions, and their relationship 18 Classification of metamorphic rocks 18 Chemical compositions of metamorphic rocks 26 The Phase Rule 28 Recognition of equilibrium in metamorphic rocks 32 Composition - assemblage diagrams 37 Metamorphic textures 44 Metamorphic facies 49 Thermotectonic modelling 53 3 Metamorphism associated with igneous intrusions 57 The Skiddaw contact aureole, Cumbria, England 64 The Beinn an Dubhaich aureole, Isle of Skye, Scotland 75 Compositions of fluids in contact metamorphism 84 Thermal models of contact metamorphism 86 viii CONTENTS 4 Dynamic metamorphism 94 The Mutki granitic mylonite, Turkey 95 The Lochseiten Mylonite, Switzerland 103 Dynamic metamorphism in relation to depth 105 5 Metamorphism associated with lithospheric extension 107 Metamorphism at spreading oceanic ridges 107 Metamorphism in sedimentary basins 119 6 Metamorphism in subduction zones 129 7 Metamorphism in collision zones 138 Metamorphic evolution of pelitic rocks of Connemara, Ireland 144 The Sulitjelma copper-mining district, Norway and Sweden 149 8 Metamorphism in stable continental crust 169 Metamorphism in Dengfeng County, Henan Province, China 175 Pyroxene gneisses of northwestern Scotland 177 The Adirondack Mountains, New York State, USA 184 Heat flow and thermal gradients in continental crust 189 9 Metamorphic rocks of the mantle 191 10 Extraterrestrial metamorphism 199 Impact metamorphism 199 Shock metamorphism at the Nordlinger Ries crater, Germany 200 Metamorphism in Moon rocks 205 Appendix 208 Glossary 209 References 220 Index 225 List of tables Table 1.1 Pressure units used in petrology. page 6 Table 2.1 Scheme of classification of metamorphic rocks in the field. 25 Table 2.2 Calculation of ACF values for an analysed basic igneous rock. 40 Table 2.3 Calculation of AFM values for a pelitic rock. 42 Table 2.4 Names of metamorphic facies and typical mineral assem- blages of basic rocks and pelitic rocks in each. 52 Table 3.1 Analysis of Skiddaw slate compared with an analysis of andalusite. 69 Table 3.2 Tectonic and metamorphic history of the Skiddaw contact aureole. 75 Table 3.3 Reactions in the system CaO--MgO--Si02-C02 at a pressure of 101.3 kPa. 79 Table 3.4 Maximum contact temperatures T max computed for intru- sion of magma at temperature Tm into dry country rocks at temperature Te. 86 Table 4.1 Textural classification of mylonites. 102 For Marion .
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