Metamorphic Rocks Lect 15QuestionsMeta rocks, continued2. Contact Metamorphism Non-foliated RocksPowerPoint PresentationContact metamorphismProtoliths control meta rock compositionMarbleSlide 9QuartziteSlide 11HornfelsSlide 13Slide 14Slide 15Slide 16Slide 17Index MineralsSlide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Boundaries of MetamorphismWhy the boundary at 200˚CSlide 29Example of diagenesisMetamorphsim and tectonicsSlide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Continents (felsic crust) have grown over timeShields and platformsSlide 43Slide 44Metamorphic RocksMetamorphic RocksLect 15Lect 15Questions•Why do index minerals form?•How is metamorphism a differentiation process?•How does the distribution and age of metamorphic rocks tell us about development of continents over time?Meta rocks, continued1. RegionalSlate > phyllite > schist > gneissLow grade to high grade2. Contact MetamorphismNon-foliated RocksLava FlowBaked ZoneAsh LayerContact metamorphism•From Med to High Temp. conditions•Associated with Intrusions, Lava flows•Smaller affected area than with regional•Usually recrystallize but retain original mineralogyProtoliths control meta rock composition•Carbonates > Marble CaCO3CaCO3Protoliths of marble include:Limestone & DolostoneMarbleThere is no bedding structureand mineral grains haverecrystallized and grownMarble = CaCO3 (Calcite)Hardness of 3 means it is soft and easily polishedQuartzite•Recrystallized quartz•Sandstone is the ProtolithHornfels•“Baked” ash or other fine grained or clay-rich rocksLava FlowBaked ZoneAsh LayerOther aspects of metamorphism3. Hydrothermal fluids (High T fluid)a. Circulating fluids near magma bodies may chemically alter rocksb. Many ore reserves and special cases of minerals occur from hydrothermal fluids, especially metal + sulfide depositsHydrothermal fluidsHydrothermalVein DepositsIndex Minerals•Represent their environmentRocksRocksPorphyroblastic textureIndex minerals indicate degree of metamorphismBoundaries of MetamorphismWhy the boundary at 200˚C•Changes in the rocks are a continuum of processes•Changes begin to occur during burial and compaction of sediments•DIAGENESIS – all the changes in the rock after Burial of sediments to Lithification, but, where does diagenesis stop and metamorphism begin?Example of diagenesis•Limestone > Dolostone CaCO3 > (Ca, Mg) CO3Dolostone does NOT naturally form as a sedimentary rock, but is altered limestone by fluids, pressure etc. at low Temps.Converting from Limestone to dolostone does result in a 10 % reduction in rock volume!Metamorphsim and tectonics•Metamorphism is yet another way in which earth materials are differentiated•In every aspect, metamorphism is tied to tectonic processes–Weathering & deposition of sediments–The burial of sediments–Collisional and shearing forcesWeathering and erosion produceSilica dominatedmaterialsSediments areMetamorphosed and added tocontinental crustContinents (felsic crust) have grown over time•Accretion of crust adds to volume•Differentiation of material produces “granitic” continental crust (silica-dominated)–Igneous differentiation–Weathering, erosion, deposition–Metamorphism & AccretionShields and platforms•Shields - ancient “cores” of continents/ (find older rocks here)–Highly metamorphosed•Platforms - sedimentary and meta sedimentary packages flanking Shields•Cratons – Shields +
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