Precambrian Geology. The Dynamic Evolution Of The Continental Crust

Dedicated to Marion There are more things in Heaven and Earth, Horatio, Than are dreamed of in our Philosophy. Hamlet, William Shakespeare. Myself when young did eagerly frequent Doctor and Saint and heard great Argument About it and about: but evermore Came out by the same Door as in I went. XXVII, Rubaiyat of Omar Khayyam. Precambrian Geology The Dynamic Evolution of the Continental Crust Alan M Goodwin Department of Geology University of Toronto Toronto, Canada ACADEMIC PRESS Harcourt Brace Jovanovich, Publishers London San Diego New York Berkeley Boston Sydney Tokyo Toronto ACADEMIC PRESS LIMITED 24-28 Oval Road London NW1 7DX United States Edition published by ACADEMIC PRESS, INC. San Diego, CA 92101 This book is printed on acid-free paper Copyright © 1991 by ACADEMIC PRESS LIMITED All rights reserved No part of this book may be reproduced in any form by photostat, microfilm or any other means, without permission from the publishers. British Library Cataloguing in Publication Data Goodwin, A. M. Precambrian geology, 1. Pre-Cambrian strata I. Title 551.71 ISBN 0-12-289870-2 Typeset by Photo-graphics, Honiton, Devon Printed in Great Britain by Thomson Litho Ltd, East Kilbride, Scotland Preface During my professional career, beginning in the late 1940s, the methods of studying Earth's continental crust have changed dramatically. The new approaches have radically changed the conceptual framework of geology. Four revolutions have unalterably changed our view of the dynamic evolution of the continental crust. Three sprang from the study of recently formed rocks and, true to uniformitarianism, were quickly applied, with varying degrees of success, to the study of Earth's older, or Precambrian, rocks—the topic of this book. ( 1 ) The plate tectonic model. The modern paradigm encompasses the recognition of the presence today of some seven major lithospheric plates, each about 120 km thick, moving horizontally in response to subjacent mantle thermal convections; the relative plate motions, whether divergent, convergent or transform, control the development of ocean basins, mountain ranges and other first-order crustal phenomena. Of particular importance are subduction zones, products of plate convergence and collisional tectonics, with their distinctive crustal signatures including calc-alkalic magmatic rocks, linear fold belts, ophiolites (oceanic crust), high pressure ('blue schist') rocks, paired (high and low pressure) metamorphic belts, chemical gradients and thick clastic wedges. Considering the application of modern plate tectonic processes (actualism) to the Precambrian record, the older the terrain the less convincing the interpretation. Curiously, the oldest recorded Precambrian ophiolite (hallmark of the modern plate tectonic process) is 1.9 Ga old; next in age is —1.0 Ga old (Ma = 10 6 years; Ga = 10 9 years). The formative plate tectonic processes operating in these and still older Precambrian times remain obscure and controversial. (2) Chemical isotope studies. The possibility that the parent : daughter ratios of certain radioactive nuclides with very long half-lives could provide the basis of geological correlation in Precambrian terrains was first stated explicitl