The eruption of deep-seated xenoliths in basaltic, alnoitic, kimberlitic, etc volcanoes provides the geologist with an important direct means of examining the fragments of the earth's mantle and lower crust.

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courses more petrogenesis-orientated are im My main objective in writing this book has been to mediately confronted with a basic problem; the review the processes involved in present-day mag ma generation and their relationship to global average student does not have a strong enough tectonic processes. Clearly, these are fundamental background in geochemistry to understand the to our understanding of the petrogenesis of ancient finer points of most of the relevant publications in volcanic and plutonic sequences, the original tec scientific journals. It is virtually impossible to fmd tonic setting of which may have been obscured by suitable reading material for such students, as most subsequent deformation and metamorphism. authors of igneous petrology textbooks have de Until fairly recently, undergraduate courses in liberately steered clear of potentially controversial igneous petrology tended to follow rather classical petrogenetic models. Even the most recent texts lines, based on the classification of igneous rocks, place very little emphasis on the geochemistry of descriptive petrography, volcanic landforms, types magmas erupted in different tectonic settings, of igneous intrusions and regional petrology . despite extensive discussions of the processes re However, the geologist of the late 1980s requires, in sponsible for the chemical diversity of magmas.