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Dr. James Wittke [email protected] Transforms one rock into another ◦ Change temperature ◦ Change pressure ◦ Interact with fluids Rocks remain solid during metamorphism Heat ◦ From burial (usually tectonic) ◦ From intrusion of magma Pressure (stress) Pressure Tectonic stress ◦ Confining pressure (weight of overlying rocks) ◦ Differential stress (tectonic stresses) Chemically active fluids Heat Fluids Provides energy for chemical reactions ◦ Existing minerals recrystallize larger crystals form ◦ Existing minerals become unstable new stable minerals grow ◦ Overall composition does not change Two heat sources ◦ Heat from interior (radioactivity, residual heat of accretion) temperature increases with depth (geothermal gradient or geotherm) ◦ Heat introduced by magma (intrusions) Confining pressure applies equal force in all directions ◦ Increases with depth ◦ Closes spaces between Minerals transform into denser forms Differential stress due to tectonic forces (unequal in different directions) ◦ Brittle vs. ductile deformation (breaking vs. bending/flowing) H2O and CO2 present in all rocks ◦ Pore spaces of sedimentary rocks ◦ Fractures in igneous rocks ◦ Breakdown of hydrous minerals (clay, mica, amphibole) Fluids enhance migration of ions ◦ Recrystallization of existing minerals ◦ Growth of new minerals Invading fluids may change bulk composition of rock metasomatism Quartz veins deposited by fluids Most metamorphic rocks retain chemical composition of original rock ◦ Exception: addition/loss of volatiles (water, CO2) Original mineral makeup determines what and how much metamorphism will occur ◦ Quartz and calcite do not react (no new minerals form) ◦ Feldspar, mica, amphibole (etc.) are reactive (new minerals form) Foliation ◦ Parallel alignment of platy minerals or flattened mineral grains and pebbles ◦ Compositional banding Formed by differential stress Rotation of platy or elongated minerals Recrystallization of minerals in preferred orientation Changing shape of grains (pressure solution) into elongated shapes that are aligned Changing shape of grains by movement (slip) along crystal planes Also termed rock cleavage Cannot see platy minerals with unaided eye Closely spaced planar surfaces along which rocks split Schistosity ◦ Growth of platy minerals to discernible sizes Gneissic texture ◦ Ion migration yields bands of light and dark minerals Non-foliated ◦ ◦ ◦ ◦ No platy minerals Minimal deformation Parental rocks with equidimensional grains Recrystallization yields larger grains Porphyoblastic ◦ Large grains surrounded by fine grained minerals Slate ◦ Very fine grained (<0.5 mm), excellent cleavage, tiny mica flakes Phyllite ◦ Larger crystals than slate, surface sheen ◦ Dominated by muscovite and chlorite (green platy mineral) Schist ◦ Medium- to coarse-grained, dominated by platy minerals, subtypes given names (i.e., garnet-mica schist) Gneiss ◦ Distinctive banded appearance Marble Quartzite Some metamorphic rocks nonfoliated Develop where differential stress is minimal Porphyroblasts ◦ Large grains in finegrained matrix of other minerals ◦ Typically, large crystals are garnet, staurolite, or andalusite Contact (thermal) metamorphism Hydrothermal metamorphism Regional metamorphism Burial metamorphism (diagenesis) Metamorphism along fault zones Impact metamorphism Magma intrudes host rock heats adjacent host rock Zone of alteration (aureole) forms in rock surrounding the magma Chemical alteration caused when hot, ionrich hydrothermal fluids circulate through fissures and cracks Common along mid-oceanic ridge systems Black Smokers Hydrothermal metamorphism in continental environment Burial & Subduction Metamorphism Burial metamorphism ◦ Occurs in thick piles of sediment ◦ Bottom of pile hotter and under more pressure (low-grade metamorphic conditions) ◦ Depth required depends upon geothermal gradient (typically 8 km) Subduction (blueschist) metamorphism ◦ Cold rocks carried deep into mantle (high pressures) ◦ Low-T/ high-P environment yields distinctive minerals (blue colors) Produces greatest quantity of metamorphic rock Associated with mountain building In shallow part of fault zone rock breaks (fault breccia) In deeper portions where temperature is higher minerals flow yielding mylonite Caused by meteorite impact Unique conditions ◦ Very high pressures and temperatures ◦ Short duration Features ◦ Crushed pulverized rock ◦ Impactites and tektites (melt) ◦ High pressure polymorphs (coesite, stishovite, diamond) Systematic variations mineralogy (and often texture) related to variations in degree of metamorphism Index minerals used to map metamorphic grade Increasing grade yields: ◦ ◦ ◦ ◦ ◦ Chlorite Biotite Garnet Staurolite Sillimanite Melting of metamorphic rocks begins at highest grades Melting assisted by breakdown of waterbearing minerals (dehydration melting) Rocks have light bands of melted material along with areas of unmelted rock Assemblages of minerals (metamorphic facies) metamorphic conditions (T, P)