METAMORPHIC TEXTURES by ALAN SPRY Department of Physics^ Monash University, Melbourne, Australia PERGAMON PRESS OXFORD · NEW YORK · TORONTO · SYDNEY Pergamon Press Ltd., Headington Hill Hall, Oxford Pergamon Press Inc., Maxwell House, Fairview Park, Elmsford, New York 10523 Pergamon of Canada Ltd., 207 Queen's Quay West, Toronto 1 Pergamon Press (Aust.) Pty. Ltd., 19a Boundary Street, Rushcutters Bay, N.S.W. 2011, Australia Copyright © 1969 Pergamon Press Ltd. First edition 1969 Reprinted 1974 Library of Congress Catalog Card No. 68-59126 Printed in Great Britain by A. Wheaton & Co,, Exeter This book is sold subject to the condition that it shall not, by way of trade, be lent, resold, hired out, or otherwise disposed of without the publisher's consent, in any form of binding or cover other than that in which it is published. 08 013315 0 (flexicover) 08 013316 9 (hard cover) Preface METAMORPHIC petrology is at present undergoing a change in emphasis. For the past two decades, metamorphic processes have been considered largely in terms of mineralogical and chemical changes and many petrologists have been so preoccupied with experimental synthesis and the determination of the physical conditions under which minerals or mineral assemblages are stable that the significance of rock textures has been overlooked. There has been an upsurge of interest in textures in the past few years but no compre­ hensive evaluation of texture-forming processes has been made since Harker's great work of more than thirty years ago. It is abundantly clear that a study of both texture and mineral assemblage is essential to an understanding of metamorphic rocks and that a great deal of information pertinent to this study is available in the literature on metal­ lurgy, ceramic studies and solid state chemistry or physics. The purpose of this book is first, to provide definitions, descriptions and illustrations of metamorphic textures for the senior undergraduate student and second, to discuss the fundamental processes involved in textural develop­ ment at a level appropriate to the graduate student and practising petrologist. The fairly extensive (but by no means exhaustive) bibliography attempts to include references to original definitions, to important examples in geological literature and to the most relevant papers in adjacent fields. If this book hsis any underlying theme, it is that an understanding of tex­ tures requires that metamorphism be regarded as a series of structural trans­ formations rather than as chemical reactions. These transformations affect real, not ideal crystals, and to regard minerals as ideal crystals rather than imperfect solids containing structural defects is no more satisfactory in meta­ morphic petrology than in metallurgy where many processes are considered quantitatively in terms of dislocations. An elementary knowledge of the part that dislocations play in gliding, twinning, nucleation and growth is essential to the petrologist. Preparation of this book W2is made possible by the generosity of the Nuffield Foundation in awarding a Dominions Travelling Fellowship at Imperial College, London, in 1965, and in providing further funds in 1966 for preparation costs. viii Preface I am very grateful to the following friends who criticized various parts of the manuscript: F. C. Beavis, R. A. Binns, K. A. Crook, D. Flinn, A. W. Kleeman, R. Kretz, C. B. Raleigh, R. L. Stanton, J. Sutton, and R. H. Vernon. Excellent photomicrographs provided by S. Amelinckx, R. A. Binns, J. Christie, A. C. McLaren, P. A. Sabine, W. S. Treffner and R. H. Vernon are acknowledged in appropriate places. My thanks are due to the following bodies for permission to use diagrams for which they hold copyright: American Ceramic Society, American Mineralogist, Butterworths, Geological Society of America, Geological Survey of Great Britain, Journal of Applied Physics, McGraw-Hill Book Co., M.I.T. Press, Oxford University Press and John Wiley & Sons. CHAPTER 1 Metamorphism and Metamorphic Processes METAMORPHISM is the mineralogical and structural (textural) adjustment of (dominantly) solid rocks to physical and chemical conditions which differ from those under which the rocks originated. Weathering and similar pro­ cesses are conventionally excluded. The type of metamorphism depends on the relative values of temperature (T), confining pressure (/*con)> pressure or chemical activity or fugacity of water (expressed generally as PHJO)* deforma­ tion or directed pressures (P^ir) their variation with time {t). There is only a limited degree of interdependence of these controls and the meta­ morphic history of a rock is an expression of their mutual interaction with time (Fyfe, Turner and Verhoogen, 1958; Pitcher and Flinn, 1965). In the simplest possible terms, the whole field of metamorphism may be divided into Thermal (Contact) Metamorphism due to heat, Dynamic Metamorphism due to directed pressures at rather low temperatures, and Regional (also Plutonic) Metamorphism due to heat plus directed pressures. Shock Metamorphism may be regarded as a special type of Dynamic Meta­ morphism in which the confining pressiu-e and temperature may be high but which is characterized by a very high rate of deformation and very high directed pressures. Burial Metamorphism may be regarded as being tran­ sitional between diagenesis and metamorphism (either Thermal or Regional). Metamorphism is essentially a thermal phenomenon and heat is the most important source of energy allowing mineralogical and textural reconstruc­ tion during metamorphism. Energy is required to overcome initial activa­ tion energies and to allow sufficient thermal vibration for ion movement and structural change; most metamorphic reactions are endothermic and require energy. Thermal or contact metamorphism takes place with moderate to high 7, low Pcon> variable ΡΗ,Ο ^^'^ generally zero P^^, The textures (and minera­ logies) of rocks in different aureoles are controlled largely by the relative rates of heat transport and mineralogical transformation. 1 2 Metamorphic Textures A rock undergoing metamorphism is subjected to a complex variety of pressures consisting of the confining pressure, intergranular fluid or gas pressure and possibly directed tectonic pressures. The confining (or load) pressure is due to the weight of the overlying material and is a function of depth and the density of the load. A porous sandstone (with an open framework) whose pores are filled with ground water and which is located near the surface will have an intergranular PH^Q between \ and \ of the Ρ^οη· However, with increasing depth the grains are driven closer together and the intergranular water bears a proportion of the load, the proportion depending on how quickly the water can be driven out. As the permeability decreases due to compaction, a state is reached when the water cannot escape and here P^^Q = Peon 5 this is generally regarded as the most common metamorphic circumstance but is by no means universal and in a crystalline rock which is essentially dry, metamorphism takes place with PHJO < ^con- Where gas is released in considerable quantities PH^O exceed Ρ^ο^; in fact the gas pressure may be sufficiently in excess to lift the roof, disrupt the rock or lower the strength by introducing a fluid between the grains. The pressure in the pores is not only due to water; carbon dioxide will be present in metamorphosed limestones where Pco, more important than PHJO- The pore-fluid pressure is the sum of the partial pressures of the different gases, i.e. Pp^^e = Pu,o + ^co» etc. Non-directed pressures not only cause mechanical effects such as closer packing, reduction of pore space and increase in density but are important also in controlling mineralogical changes and melting. In general, higher confining pressures favour more closely packed atomic structures and thus more dense phases. Regional metamorphism of a basalt at low temperatures gives a greenschist in which sodium is in the feldspar (albite) and iron and magnesium are contained in ferromagnesian minerals such as chlorite and actinolite. Under higher pressures sodium may go into a ferromagnesian to give a glaucophane schist. The behaviour of sodium in eclogites is similarly pressure-controlled. However, the effects of directed pressures are almost entirely textural or structural rather than mineralogical. Harker's suggestion that metamorphic minerals could be divided into stress and anti-stress minerals and that the for­ mation ΟΪ stress minerals either required or w2is favoured by directed pressures, and that anti-stress minerals could only be formed in non-stress conditions, does not appear to have stood the test of time although there is a little evidence suggesting that directed pressures may control the stability fields of some minerals. There is considerable evidence that directed stresses act almost catalytically in assisting metamorphic changes, probably by lowering activation energies and assisting diffusion; thermodynamically, the directed pressure may be added to the non-directed pressure in calculating total Metamorphism and Metamorphic Processes 3 pressure. Directed stresses and movements enable large amounts of energy to be added quickly to a system and to be localized, e.g. along thrust zones. A rock at a certain depth in the crust is subjected to a confining pressure due to the load of the overlying rocks and if the region is subjected to unequal stresses, the rock will flow in response. While it is flowing it is sub­ jected to a tectonic overpressure (Rutland, 1965) so that it is, in effect, con­ fined by a greater pressure than that attributable to the load. Overpressures have been invoked in an attempt to explain discrepancies between the physical conditions for various kinds of metamorphism implied by experi­ ment, and those indicated by the geological evidence, particularly with respect to the occurrence of kyanite. The textures alone of kyanite-bearing rocks indicate that the tectonic overpressure explanation is invalid. Kyanite is most conmion as a post-tec­ tonic mineral and the kyanite of the most closely studied metamorphic areas occurs as an interkinematic mineral which was formed in the static period between metamorphic phases.