THE INTERPRETATION OF IGNEOUS ROCKS

Transcription

1 THE INTERPRETATION OF IGNEOUS ROCKS

2 TITLES OF RELATED INTEREST Boninites A. J. Crawford (ed.) Carbonatites K. Bell (ed.) Cathodoluminescence of geological materials D. J. Marshall Chemical fundamentals of geology R. Gill Crystal structures and cation sites of the rock-forming minerals J. R. Smyth & D. L. Bish The dark side of the Earth R. Muir Wood Deformation processes in minerals, ceramics and rocks D. J. Barber & P. G. Meredith (eds) Geology and mineral resources of West Africa J. B. Wright Igneous petrogenesis B. M. Wilson Image interpretation in geology S. Drury The inaccessible Earth G. C. Brown & A. E. Mussett Introduction to X-ray spectrometry K. L. Williams Komatiites N. Arndt & E. Nisbet (eds) Mathematics in geology J. Ferguson Metamorphism and metamorphic belts A. K. Miyashiro Perspectives on a dynamic Earth T. R. Paton Petrology of the igneous rocks F. Hatch et al. Petrology of the metamorphic rocks R. Mason Planetary landscapes R. Greeley A practical introduction to optical mineralogy C. D. Gribble & A. J. Hall Rheology of the Earth G. Ranalli Rutley's elements of mineralogy C. D. Gribble Simulating the Earth J. Holloway & B. Wood Statistical methods in geology R. F. Cheeney Volcanic successions R. Cas&J. V. Wright The young Earth E. G. Nisbet

3 THE INTERPRETATION OF IGNEOUS ROCKS K. G. Cox, 1. D. Bell and R. 1. Pankhurst [nnl Springer-Science+Business Media, B.v.

4 First edition 1979 Reprinted K.G. Cox, J.D.Bell and R.J. Pankhurst Originally published by Chapman & Hall in 1979 ISBN ISBN (ebook) DOI / Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright Designs and Patents Act, 1988, this publication may not be reproduced, stored, or transmitted, in any form or by any means, without the prior permission in writing of the publishers, or in the case of reprographic reproduction only in accordance with the terms of the licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licences issued by the appropriate Reproduction Rights Organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to the publishers at the London address printed on this page. 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 or liability for any errors or omissions that may be made. A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data available

5 Now how Elephants should come to be buried in Churches, is a Question not easily answered... Robert Plot (1677)

6 Preface Our aim in writing this book is to try to show how igneous rocks can be persuaded to reveal some ofthe secrets of their origins. The data of igneous rocks consist of field relations, texture, mineralogy, and geochemistry. Additionally, experimental petrology tells us how igneous systems might be expected to behave. Working on this material we attempt to show how hypotheses concerning the origins and evolution of magmas are proposed and tested, and thus illuminate the interesting and fundamental problems of petrogenesis. The book assumes a modest knowledge of basic petrography, mineralogy, classification, and regional igneous geology. It has a role complementary to various established texts, several of which are descriptively good and give wide coverage and evaluation of petrogenetic ideas in various degrees of detail. Existing texts do not on the whole, however, deal with methodology, though this is one of the more important aspects of the subject. At first sight it may appear that the current work is a guidebook for the prospective research worker and thus has little relevance for the non-specialist student of geology. We hope this will prove to be far from the case. The methodological approach has an inherent interest because it can provide the reader with problems he can solve for himself, and as an almost incidental consequence he will acquire a satisfying understanding. Moreover, if the study of igneous rocks has any value at all for the non-specialist, it is at least as likely to lie in the scientific discipline of its methods as in any other aspect of the subject. This book is not built round any specific course. There is much in it that would be of benefit to and accessible to any undergraduate student of geology; but at the same time postgraduate students and even teachers of petrology may find it useful. In short, it is offered to petrologists of all ages. The authors are particularly grateful to R. W. Cleverly, P. J. Betton and D. Kiernan-Walker for their attempts to minimise the number of mistakes in the manuscript, and to Mrs P. Jackson for her excellent typing. Oxford 1978 K. G. Cox J. D. Bell R. J. Pankhurst

7 Contents Preface List of tables 1. Fractionation in igneous processes Introduction The concept of fractionation Classification of fractionation processes Open systems Summary page ix xiv Compositional variation in magmas Inter-element correlations Summary of progress Variation diagrams - introduction Triangular variation diagrams The liquid line-of-descent Phase diagrams - introduction Classification of systems Experimental method Representation of results The phase rule Construction of a phase diagram by the quenching method The reading of phase diagrams Binary diagrams Ternary systems - I Representation of composition Liquidus projections The solidus Primary phase fields Isothermal sections Equilibria involving two solids + liquid Equilibria involving three solids + liquid Equilibrium crystallisation and melting paths in ternary systems Fractional crystallisation paths Pseudo binary systems Some ternary systems of geological interest

8 xii Contents 5. Ternary systems with solid solutions page 120 Ternary solid solutions 130 The system diopside-albite-anorthite 131 The system nepheline-kalsilite-silica 133 Ternary feldspars The interpretation of two-element variation diagrams 145 Principles of the mixing calculation 146 Mixing calculations and the formulation of crystal fractionation hypotheses 147 Application to series of volcanic rocks 154 EXercises Petrographic aspects of volcanic rocks 176 Introduction 176 The phenocryst assemblage 176 Modal analysis of phenocrysts 181 Altered phenocrysts 181 Resorbed and mantled phenocrysts 183 Xenocrysts 187 Order of crystallisation 188 Quenching textures 189 Final products of solidification 192 Other groundmass textures Quaternary systems 197 Representation of composition 197 Sub-solidus joins 198 Liquidus diagrams 201 Crystallisation paths 202 Projection methods 205 The system forsterite-diopside-anorthite-silica Experimental work on natural basaltic and allied rocks 222 Introduction 222 Atmospheric pressure experiments 223 Computer simulation of crystallisation paths 238 High-pressure experiments on dry materials Water-bearing basic rock systems 258 Experiments on basaltic compositions 258 Experiments on synthetic peridotitic compositions 263 Experiments on natural peridotites 265 Phlogopite stability 267 The experiments of Nicholls and Ringwood (1973) Compositionally zoned magma bodies and their bearing on crystal settling 272 An ash-flow sheet from Aso caldera, south-western Japan 273 The Shamsan caldera sequence of Aden 276

9 Contents xiii A model for crystal settling in a non-convecting magma chamber Compensated crystal settling 12. Petrographic aspects of plutonic rocks Introduction Chilled margins Cumulates Sub-solidus textures Partial melting of plutonic rocks 13. The interpretation of data for plutonic rocks Volume of intrusions Time for emplacement of plutons Flow of magma Some thermal features of intrusions Convection in magmas Crystal settling Layered intrusions Some chemical features of layered intrusions 14. Trace elements in igneous processes Introduction Melting and crystallisation models More complex models Application of trace elements to petrogenesis 15. The use of isotopes in petrology Radiogenic isotope variations Strontium isotopes Neodymium isotopes Lead isotopes Stable isotope variations Further reading page Appendices 1. Nomenclature of igneous rocks Average major element compositions and CIPW norms of common igneous rock types Norm calculations 407 Calculation of the CIPW norm 408 Molecular norms Calculation of plotting parameters for O'Hara (1968) polybaric phase diagram Some representative mineral analyses 416 Answers to exercises 419 References 432 Index 446

10 List of Tables 1.1 States of matter in geological systems page Approximate equivalents of some K-rich and normal rock types Analyses of volcanic rocks from the Hanish-Zukur islands, Red Sea Total alkalis and silica contents of rocks termed trachyte and hawaiite in compilation of Carmichael et al. (1974) 40 2A Partial analyses of volcanic rocks Partial analyses of volcanic rocks Run data at 1000 C for the system A-B-C Analyses of compositions used in extract calculation Settling velocities of crystals in basic magma Analytical data for Exercise A Analytical data for Exercise Highest temperatures of crystallisation of major phases inkilauean lavas Calculation of plotting parameters for normative basalt system Rocks used to test normative basalt phase diagram 234 9A Chemical compositions and CIPW norms oftwo rocks investigated experimentally by Green and Ringwood (1967) Melting relations of AL and OT at various pressures Analyses of peridotites Melting relations of rocks at 1 atmosphere and 9 kbar Analyses for use in exercises Anhydrous melt compositions for Picture Gorge tholeiite Compositions and CIPW norms of experimentally melted rocks Glass representing liquid in equilibrium with Fo + Opx + Cpx + V at 1050 C and 20 kbar 265 loa Composition of rocks used by Mysen and Boettcher (1975a, b, ) 10.5 Compositions of partial melts formed from rocks Band E of Table 10.4 at various pressures, temperatures and values ofx H Details of phenocrysts in pumice blocks of Aso III ash-flow sheet Analyses of glasses (recalculated free of H 2 0 and CO 2) from Aso III pumice blocks Compositions of phenocrysts in Shamsan caldera sequence lavas Typical trace element distribution coefficients Ionic radius and charge of some important major and trace elements Radioactive decay schemes used in petrology Generalised initial 87Sr /86Sr ratios of igneous rocks Stable isotopes of light elements A 396 AU Nomenclature of rocks in the Q-A-P and A-P-F triangles 401