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Download Metamorphism and Metamorphic Rocks Metamorphism
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Metamorphism Metamorphism and Metamorphic Rocks Metamorphic Environments • Regional metamorphism involves the burial and metamorphism of entire regions (hundreds of km 2) • Contact metamorphism results from local heating adjacent to igneous intrusions. (several meters) Metamorphic Environments • Metamorphism is the solid- state transformation of preexisting rock into texturally or mineralogically distinct new rock as the result of high temperature high pressure, or both. Metamorphic Environments • Where do you find metamorphic rocks? • In the mountains. • Why? • Because they are covered by sediments elsewhere. Regional Metamorphism • Because the tectonic forces required to bury, metamorphose, and reexhume entire regions are slow, • most regionally metamorphosed terranes are old (>500MY) and • most Precambrian (>500MY) terranes are metamorphosed. 1 Regional Metamorphism Conditions of Metamorphism • Regional metamorphism is typically isochemical (composition of rock does not change), although water may be lost. • Metasomatism is a term for nonisochemical metamorphism. • Contact metamorphism is typically metasomatic. metasomatic • Changing the mineralogy of a sediment requires temperature > 300ºC and pressures > 2000 atmospheres (6 km deep) • Temperature increases downward at ~20ºC/km in crust. • Pressure increases downward at ~1000 atm (1kbar) per 3 km. Effect of Increasing Pressure Effect of Increasing Temperature • Increases the density (compression) • Pressure can be Hydrostatic (equal in all directions), or • Pressure can be directed as a shear .which can cause • Increases the atomic vibrations • Decreases the density (thermal expansion) • Increases the speed of reactions – Foliation (alignment of platy mineral) – Lineation (alignment of needle-like minerals). Metamorphosed Sediments • • • • • • • Conglomerate Sandstone Shale Shale Shale Limestone Gypsum Metaconglomerate Quartzite Phyllite / Slate Schist Gneiss Marble (Anhydrite) 2 Schist: Metamorphosed Shale Blueschist Marble Marble & Blueschist: Metamorphosed Shale + Limestone Metamorphosed Igneous Rock • Granite • Diorite • Basalt/Gabbro Garnet in Schist Gneiss Gneiss Eclogite • All are high-grade • Partial melting Migmatite 3 Gneiss Migmatite (Partial Melting) Inner Gorge, Grand Canyon Garnet Amphibolite Gneiss Eclogite: Garnet + CPx Blueschist Aosta, Valley, Italy 4 Diamond in Eclogite Black Canyon of the Gunnison Terms • Regional Metamorphism • Contact Metamorphism • Quartzite • Phyllite • Isochemical • Metasomatism • Slate • Schist • Hydrostatic • Foliation • Gneiss • Marble • Lineation • Eclogite • Granulite • Migmatite 5
