Igneous Petrogenesis

Abstract

My main objective in writing this book has been to review the processes involved in present-day magma generation and their relationship to global tectonic processes. Clearly, these are fundamental to our understanding of the petrogenesis of ancient volcanic and plutonic sequences, the original tectonic setting of which may have been obscured by subsequent deformation and metamorphism. Until fairly recently, undergraduate courses in igneous petrology tended to follow rather classical lines, based on the classification of igneous rocks, descriptive petrography, volcanic landforms, types of igneous intrusions and regional petrology . However, the geologist of the late 1980s requires, in addition, an insight into the fundamental processes which operate within the Earth, and are responsible for the diversity of igneous rocks presently exposed. Unfortunately, attempts to make petrology PREFACE courses more petrogenesis-orientated are immediately confronted with a basic problem; the average student does not have a strong enough background in geochemistry to understand the finer points of most of the relevant publications in scientific journals. It is virtually impossible to fmd suitable reading material for such students, as most authors of igneous petrology textbooks have deliberately steered clear of potentially controversial petrogenetic models. Even the most recent texts place very little emphasis on the geochemistry of magmas erupted in different tectonic settings, despite extensive discussions of the processes responsible for the chemical diversity of magmas. Perhaps even more surprisingly, few texts have attempted to relate igneous activity to global tectonic processes, except at the most elementary levels. This book has arisen out of ten years of teaching igneous petrology to undergraduate and graduate students in the Department of Earth Sciences, University of Leeds, UK, and the resultant frustration at having no appropriate text which discusses the geochemical characteristics of magmas in relationship to their tectonic setting. It should appeal to the more advanced undergraduate and beginning postgraduate students, and their teachers, who already have a basic foundation in petrological principles, including elementary geochemistry, phase diagrams, mineralogy, regional geology and global tectonics. The text has been deliberately put together in such a way that certain sections can be omitted at first reading if the background of the student is not appropriate. Thus, for example, a beginning student could omit the more detailed sections on the trace element and isotope geochemistry of magmas generated in a particular tectonic setting. As far as possible, continuity in the presentation of data for individual provinces has been maintained for ease of inter-province com parisions. Thus, in each chapter, sections will be found covering, for example, major element, trace element and radiogenic isotope geochemistry. Clearly, a text of this length cannot hope to cover all examples of a particular type of igneous activity. Consequently, I have tended to focus on volcanic rocks from those recently active provinces for which we have the most comprehensive geochemical data sets. Obviously, new data are appearing all the time and therefore the shape of many of the fields depicted in the geochemical variation diagrams may change with time. Nevertheless, I believe that we have reached a stage in our understanding of the petrogenesis of igneous rocks at which most of the fundamental principles are well established. Consequently, the time is right for a text ofthis nature. I have deliberately not devoted much space to the discussion oftopics such as phase diagrams and the fluid properties of magmas, as these have been extensively covered in several recent texts. I would like to thank the following for their help and encouragement throughout the completion of this project: K. G. Cox for his much valued comments on the original manuscript; P.J. Wyllie and J.B. Gill for useful discussion of the format of the book and the early chapters; my colleagues in the Department of Earth Sciences, G.R. Davies, P.H. Nixon, E.H. Francis and R.A. Cliff, for their enthusiastic approach to all aspects of igneous petrology which has led to many fruitful discussions; S. Caunt for word processing the large number of references, and for much needed moral support during the final stages. I have been fortunate to have had a particularly lively group of research students over the past ten years, including I. Luff, Z. Palacz, H. Downes, J.P. Davidson, I. Wilson, D. Chaffey, S. Amini and S. Caunt, who have kept me on my toes. Many people, including the above, have contributed to my ideas on igneous petrogenesis over the years, in particular R. Powell, J.D. Bell, S.W. Richardson, K.G. Cox, M.J. O'Hara, P.E. Baker, C.J. Hawkesworth, M.J. Norry, the late P.J. Betton, I.S.E. Carmichael, A. Ewart, W. Hildreth, B.D. Marsh, C. Bacon, H. Helgeson, G.M. Brown and C.H. Emeleus. Thanks must go finally to my family and friends for putting up with my reclusive habits over the past few months, especially to my mother, my sister Mary and her husband Andrew, my step-children Jane and Gary (and step son-in-law David), the latest addition to the family, my step-grandson Anthony, and to Anne and Brian Frost. Thanks also to Roger Jones of Unwin Hyman for being such an understanding editor.