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Metamorphism and Metamorphic Rocks Physical Geology Metamorphism The solid state transformation of preexisting rock due to changes in the environment Agents of Metamorphism: Heat Pressure Chemically active fluids Metamorphism Limits of metamorphism Needs to be < 200˚C, otherwise melting occurs (back to magma!) Needs to be < 300 Mpa (300,000 atm) To low and you have sedimentary rocks To high and you have igneous rocks Metamorphism Pressure at depth is confining, so the pressure is felt in all directions Hydrostatic pressure is confining pressure from water Lithostatic pressure is confining pressure from rocks Metamorphism Pressure can be differential: Acts unequally on different sides of the rock Rocks become distorted Directional compression (flattening of rock) Shearing Types of Metamorphism Contact Metamorphism Dynamic Metamorphism Regional Metamorphism Contact Metamorphism This occurs at or near a contact with igneous rocks Agents fluids and heat Aureole is the area of metamorphism surrounding an intrusion or a igneous body Zones of decreasing intensity (i.e. grade of metamorphism) occur as you move outward from the contact zone Contact Metamorphism Metasomatism Fluids in veins and dikes dissolve some of the rock constituents and precipitate others from magma chamber These are very valuable ore deposits! Dynamic Metamorphism Occurs along a fault Agent pressure Both differential and shear pressure are applied to the rocks Results in mylonites Regional Metamorphism Most Common Occurs most often in convergent plate margins Can occur in divergent margins but only at very shallow depths Agents: High pressure and High Temperature Regional Metamorphism Occurs in large areas and at great depths Gradation in metamorphism We recognize it by the minerals that are present Regional Metamorphism Different zones or grades of metamorphism Zone the first appearance of new metamorphic minerals Index Minerals: Chlorite Biotite Garnet Staurolite Kyarite Sillminite Increasing Metamorphic Grade Zones vs. Facies Metamorphic Facies metamorphic rocks characterized by particular assemblages Green Schist facies Amphibolite facies Granulite facies Blue Schist facies (low temp, low pressure) (high temp, high pressure) (higher temp and pressure) (low temp, high pressure) Metamorphism and Plate Tectonics Divergent Low to medium metamorphism Transform Boundary Boundary Dynamic Metamorphism Oceanic/Continental Convergent Boundary Paired metamorphic belts on both plates Classification We classify metamorphic rocks my texture and composition Texture is used first though! Foliated minerals arranged in parallel layers Nonfoliated mosaics of minerals Classification Foliated: Slate Phyllite Schist Gneiss Increasing Metamophic Grade and grain size! Classification Nonfoliated Rely on composition to determine type CaCO3 (limestone) marble SiO2 (quartz/chert) quartzite Compact, dark green mafic rocks greenstone Fine grained, contact metamorphism hornfels Hard coal anthracite
