classification - GLY 421: Igneous & Metamorphic Petrology...

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El-Shazly, 2004 1 Classification of Igneous Rocks The classification of igneous rocks is not easy. This is simply because igneous rocks represent a continuum in nature (whether mineralogically or chemically), making their classification rather artificial. Nevertheless, classifications are needed to improve communication between scientists, and are generally of five different types: Mineralogical classifications: Are based on the mineralogical composition of the igneous rock, and the percentage by volume of essential minerals (minerals influencing its classification and constituting more than 5% of the rock). The volume % of any mineral is known as its mode . Obviously, in order to understand mineralogical classifications, we must know what kind of minerals occur in igneous rocks. Table 1 summarizes the most common igneous minerals and their chemical composition. Only rocks in which minerals can be identified in hand specimen and/or thin section can be classified mineralogically. Textural classifications: Textures are used as a secondary criterion for the classification of igneous rocks. For example, in the most commonly used mineralogical classifications of igneous rocks, textures are used to tell plutonic and hypabyssal rocks from volcanic ones. Each mineral assemblage will therefore correspond to two rock names: a plutonic one and its volcanic equivalent (see Streckeisen's classification below). On the other hand, textures are used as the only criterion for the classification of tuffs, . .. but then again, these are sedimentary (pyroclastic) rocks! Chemical classifications: Are based on the chemical composition of the igneous rocks, and therefore require the availability of bulk rock chemical analyses (obtained by XRF, ICP or wet chemical techniques). These classifications are needed for many volcanic rocks that are either too fine-grained to allow for proper identification of their mineralogies, or which contain considerable amounts of glass. Genetic classifications: Although the ultimate aim of the petrologist is to understand how the rocks formed, genetic classifications are not very useful for the field geologist or petrographer who is just starting a study! Tectonic classifications: These are useful for understanding the relationship between plate tectonics and igneous activity, and rely heavily on studying present day igneous activity at different types of plate boundaries/ interiors. Making use of these classifications to understand the tectonic setting of "older" igneous rocks still requires chemical, mineralogical and textural data. Among these five different types, it is clear that the easiest are the mineralogical and
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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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classification - GLY 421: Igneous & Metamorphic Petrology...

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