Rodney Grapes Pyrometamorphism
With 192 Figures
Author Professor Dr. Rodney H. Grapes
Mineralogisch-Geochemisches Institut Albert-Ludwigs-Universität Freiburg Albertstr. 23 b 79104 Freiburg Germany Now at: Department of Geosciences Sun Yat-sen (Zhongshan) University Guangzhou (Canton) 510275 China
Library of Congress Control Number: 2005934782ISBN-10 ISBN-13 3-540-29453-8 Springer Berlin Heidelberg New York 978-3-540-29453-5 Springer Berlin Heidelberg New York
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My interest in pyrometamorphosed rocks began in 1981–82 as a Humboldt Fellow at the Institute for Mineralogy, Ruhr University, Bochum, Germany, with Professor Werner Schreyer, where I began a study of basement xenoliths erupted from theWehr Volcano, east Eifel. A comprehensive suite of xenoliths had already been obtained by Gerhard Wörner (now Professor of Geochemistry at Gottingen University) and I was fortunate to be able to select samples from his collection for further detailed study. The xenoliths, mainly mica schists, had all undergone various stages of reaction and fusion. Because the high temperature reaction productsand textures were fine grained they were difficult to study petrographically and for me they presented an entirely new group of rocks that I had had almost no experience with. However, by extensive use of the backscattered electron image technique of the electron microprobe the “wonderful world” of mineral reaction/melting textures arrested in various stages of up-temperature transformation byquenching was revealed. Almost every image showed something new. I had entered the seemingly complex and at times contradictory realm of disequilibrium where metastable nucleation and crystal growth is the norm. Pyrometamorphism is a type of thermal metamorphism involving very high temperatures often to the point of causing fusion in suitable lithologies at very low pressures. The high temperaturesare provided by flow of mafic magma through conduits, by way of spontaneous combustion of coal, carbonaceous sediments, oil and gas, and through the action of lightning strikes. These conditions characterise the sanidinite facies of contact metamorphism. Although pyrometamorphic effects related to igneous activity are usually restricted to very narrow aureoles and xenoliths and to the point ofimpact in lightning strikes, pyrometamorphic rocks may be exposed over a surface area of hundreds to thousands of square kilometres in the case of combustion of gently dipping coal seams. In all these instances, temperature gradients are extreme, varying by several hundred degrees over a few metres or even centimetres. Relatively short periods of heating create an environment dominated by metastable...
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