1Metamorphism and Metamorphic Rock ? Metamorphic Conditions ? Pressure ? Temperature ? Hot Fluids ? Where does metamorphism occur? ? Metamorphic Rock Classification ? Foliated and Non-foliated Metamorphism and Metamorphic Rocks Metamorphism occurs when preexisting rocks are changed when exposed to increased pressure and temperature conditions or come in contact with hot fluids. These changes result in new rock textures and formation of new minerals all in the solid state. This means no melting of the existing rock. METAMORPHISM: CONDITIONS IN P/T SPACE Laboratory experiments on metamorphic mineral help to limit the range of stability. This allows petrologists to determine the pressure & temperature range for a rock containing those minerals. garnet + chlorite + sillimanite is stable @ 600°C & >3-5kbar Kyanite Andalusite Sillimanite 0 5 10 P re ss u re ( k b a rs ) P re ss u re ( k b a rs ) 200 400 600 800 Temperature ( C) hi pressure hi temp Low pressure Ky An Si 0 5 10 P re ss u re ( k b a rs ) P re ss u re ( k b a rs ) 200 400 600 800 Temperature ( C) X 2HEAT AND METAMORPHISM ? During burial, rocks experience higher temperature with depth. This pattern is called the geothermal gradient. ? Temperature at depth will rise an average 3oC/100 m. At 10km = 300oC, 20km = 600 oC ? Radioactive elements like uranium decay within rocks in the crust also generates tremendous heat. ? Igneous intrusions like plutons, dikes and sills expose the surrounding rock to very high temperatures. High temperatures results in recrystallization and pronounced mineral growth. Reduced pore space and triple point grain boundaries. EFFECT OF HOT FLUIDS ? Fluids such as water and gases like CO2 also metamorphose existing rock. ? These fluids react with the existing minerals to form new minerals not present in the rock. ? Remember chemical reaction occur more easily in the presence of water and heat. PRESSURE EFFECTS The weight of overlying rock exerts pressure over time. When this pressure reaches a critical point, minerals in existing rock, recrystallize in areas of lower pressure. These mineral grains tend to be elongated in directions perpendicular (at right angles to) the pressure. Pore (empty) space around the grains is reduced. 3CONTACT METAMORPHISM As igneous intrusions are emplaced into cold country rock, the heat radiates into surrounding rock. This causes existing minerals to recrystallize. In addition, recall that magma also contains various mineral rich fluids that can also flow into surround rock promoting chemical reactions and growth of new minerals. REGIONAL METAMORPHISM Large regional-scale movement of lithospheric plates causes intense pressures and high temperatures. high Pressure high Pressure high Temperature high Temperature METAMORPHIC GRADE Regional Metamorphism produces a range metamorphic conditions low-grade, medium-grade and high-grade: These terms are used to describe the intensity of pressure and temperature affecting the region. Key minerals or index minerals form at particular P/T conditions. Biotite and chlorite form under low-grade (pressure and temperature), Garnet forms at medium-grade Staurolite at medium to higher grades Photomicrograph of thin sections of samples from Black Hills, South Dakota in 1994 4CLASSIFICATION OF METAMORPHIC ROCKS Classification is based on the texture of the rocks & mineral composition. Foliated Minerals show preferred alignment Non-foliated Minerals grains are random METAMORPHIC TEXTURE: SLATEY CLEAVAGE ? Fine-grained minerals are aligned along parallel planes making the rock easy to split along flat surfaces. ? Characteristic of low grade metamorphism. slate phyllite FOLIATED TEXTURE: SCHISTOCITY ? A texture describe by the alignment of coarser-grained platy minerals. ? Low-medium grade conditions. Mica schist 5FOLIATED TEXTURE: GNEISSIC LAYERING ? Results in the alternation of bands of light and dark minerals. ? Medium to high grade conditions. Gneiss NON-FOLIATED METAMORPHIC ROCKS This texture occurs in rocks that contain equal sized, randomly oriented minerals grains. Composition Rock Name Parent rock Calcite Marble limestone Quartz sand Quartzite sandstone Produced most commonly by contact metamorphism. Sue Clement-Smith Metamorphism and Metamorphic Rocks
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