a New minerals grow at expense of others I Introduction a Metamorphic changed b Protoliths i Parent rocks ii Undergo changes 1 Texture 2 Mineralogy II Metamorphism a Change in environment i Burial ii Heating magma iii Tectonic stresses iv Fluid alteration b Metamorphic character i Distinctive properties 1 Unique texture a Intergrown grains 2 Unique minerals a Staurolite b Kyanite c Sillimanite 3 Unique foliation a Planar fabric ii Unlike protoliths c Metamorphic processes i Slowly thousands of years 1 Recrystallization a Changes size shape b No change in mineralogy 2 Phase change a New minerals form i Same chemical formula ii Different crystal structure 3 Neocrystallization a New minerals due to chemical reactions 4 Pressure solution grains soften deform a High temperatures III Causes of Metamorphism a Agents i Heat ii Pressure iii Differential stress iv Hydrothermal fluids b Rocks overprinted by multiple events c Heat i 200 850oC 1 Rock composition 1 Foliation layered appearance perpendicular to compression a Minerals flatten dissolve and recrystallize 2 Inequant grains rotate into plane 2 Water content ii Heat breaks reforms bonds 1 Geotherm 2 Magma d Pressure Increases with depth i ii Stability depends on P T iii Conditions known for many minerals 1 Phase diagram iv Differential stress IV Classifying Metamorphic Rocks a Foliated v Hydrothermal fluids 1 Accelerate chemical reactions 2 Change rock composition Leaching a b Fracture fill 3 Called metasomatism i Differential stress ii Inequant minerals iii Slate fine grained clay rich Low grade metamorphism 1 Shale protolith 2 3 Slaty cleavage 4 Chalkboards a Break perpendicular to stress iv Phyllite fine grained mica rich Low medium grade alteration of slate 1 2 Clay minerals neocrystallize into tiny micas v Schist 1 Medium high grade 2 Distinct foliation a Alignment of large mica crystals 3 Other minerals due to neocrystallization a Quartz feldspar kyanite garnet staurolite Large non mica minerals are called porphyroblasts 4 vi Gneiss 1 Light bands silicic 2 Dark bands mafic 3 Develops Layering in protolith a b High temperature shearing vii Migmatite 1 Gneiss heated to partial melting 2 Yields a mixed igneous metamorphic rock 3 Mineralogy controls behavior a Light minerals melt at lower temperatures b Dark minerals melt at higher temperatures b Non foliated i No differential stress ii Equant minerals iii Amphibolite 1 Metamorphosed basalt or gabbro iv Hornfels heating without differential stress 1 Finely crystalline 2 Common near plutonic intrusions v Quartzite 1 Sandstone protolith a Grains recrystallize and fuse 2 Hard glassy and resistant vi Marble calcite or dolomite Limestone or dolostone protolith 1 2 Extensive crystallization 3 Textures and fossils obliterated vii Composition matters 1 Rock type dependent on protolith 2 Protolith minerals contribute elements 3 Specific protoliths yield specific rocks V Metamorphic Intensity a Temperature more important and pressure range i Produce different minerals b Grade degree of alteration i Low grade slight ii High grade intense VI Metamorphic Environments a Different geologic settings b Yield different effects due to variable i Geothermal gradient ii Differential stress iii Hydrothermal fluids c Settings i Contact metamorphism 1 Magma invades host rock 2 Banded zone of alteration in country rock a Contact aureole i Surrounds plutonic intrusion ii Zoned from high to low grade ii Burial metamorphism 1 As sediments are buried in basin a Pressure increases overburden b Temperature increases geothermal gradient 2 Burial is below 8 15 km a Depends on geothermal gradient iii Dynamic metamorphism 1 Breakage of rock by shearing at a fault zone a Shallow crust upper 10 15 km b Deeper crust below 10 15 km i Brittle ii Grains crush i Ductile ii Grains smear iii Mylonite iv Regional metamorphism 1 Mountains compression and thickening 2 Creates foliated rocks 3 Most important in terms of the amount of rock altered a Collisional belts i Thousands km long ii Hundreds of km wide v Hydrothermal metamorphism 1 Hot chemically aggressive water 2 Dominant near mid ocean ridge a Water seeps into fractures heats up i Reacts with mafic rock b Rises ejected via black smokers vi Subduction metamorphism 1 Blueschist facies 2 Trenches accretionary prisms a Low temperature high pressure vii Shock metamorphism 1 Bolide impact comets asteroids 2 Compressional shockwave a Extremely high pressure b Heat vaporizes large masses of rock i Coesite stishovite viii Exhumation 1 Exposure of deep rocks a Uplift b Extensional collapse c Weathering erosion VII Finding Metamorphics a Shields large regions of ancient high grade metamorphic rocks i Continental interiors ii Eroded remnants of mountains iii Basement under sedimentary cover