Metamorphism and Metamorphic Rocks Physical Geology

Metamorphism  The solid state transformation of pre- existing 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 Increasing  Biotite Metamorphic  Garnet Grade  Staurolite  Kyarite  Sillminite

Zones vs. Facies  Metamorphic Facies  metamorphic rocks characterized by particular assemblages  Green Schist facies (low temp, low pressure)  Amphibolite facies (high temp, high pressure)  Granulite facies (higher temp and pressure)  Blue Schist facies (low temp, high pressure)

Metamorphism and Plate Tectonics  Divergent Boundary  Low to medium metamorphism  Transform 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 Increasing  Phyllite Metamophic  Schist Grade  Gneiss and grain size!

Classification  Nonfoliated  Rely on composition to determine type  CaCO 3 (limestone)  marble  SiO 2 (quartz/chert)  quartzite  Compact, dark green mafic rocks  greenstone  Fine grained, contact metamorphism  hornfels  Hard coal  anthracite