intro - GLY 421: Igneous & Metamorphic Petrology El-Shazly,...

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El-Shazly, 2004 1 I- Introduction Definitions: Petrology : Is the study of rocks. Igneous petrology is that branch of petrology dealing with the study of rocks that were originally molten (i.e. which formed from magmas or lavas). Volcanology is a subdiscipline of igneous petrology which deals with studying all aspects of volcanoes and their products. Metamorphic Petrology is the study of all rocks that formed by recrystallization in the solid state, beyond the field of diagenesis. In general, all branches of petrology (whether igneous, sedimentary or metamorphic) consist of two main components: (a) Petrography: which is the descriptive component of petrology, and consists of describing rocks mainly under the microscope. (b) Petrogenesis: which is that part of petrology which aims at understanding the origin of rocks. The study of igneous rocks, which is essential for understanding the tectonic history of an area, therefore consists of several stages: (i) field work where the relations between the different rock types are documented (usually with the help of mapping), and representative samples are collected, (ii) petrographic description of important samples, where mineral assemblages and key textures are identified, and the sequence of crystallization of minerals is established, (iii) identification of the chemical composition of different minerals, particularly those participating in certain reactions, (iv) identification of the bulk rock chemical composition of samples, (v) identifying the chemical relations between the different samples in the study area, and the (possible) parental magmas to establish the origins of these different rocks (relies heavily on experimental petrology and theoretical calculations and modeling), (vi) determining the ages of crystallization of the various minerals by suitable radiometric methods, and (vii) making tectonic interpretations for the study area. Abundance of igneous rocks in the crust and mantle: Although igneous + metamorphic rocks constitute only 25% of the total rock outcrops on the continents, they actually constitute 95% of the entire crust. This should not come as a surprise, since the oceanic crust consists almost entirely of igneous rocks (with only a small sedimentary cover). On the other hand, the mantle can be considered to consist predominantly of metamorphic rocks (garnet or spinel peridotites + eclogites). Nevertheless, mantle rocks undergo partial melting to produce magmas of varying
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.

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intro - GLY 421: Igneous & Metamorphic Petrology El-Shazly,...

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