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06Metamorphs.Pdf Clicker • The transformation of one rock into another by solid-state Metamorphism and recrystallization is known as Metamorphic Rocks – a) Sedimentation – b) Metamorphism – c) Melting – d) Metasomatism – e) Hydrothermal alteration Metamorphism • Metamorphism is the solid- state transformation of pre- existing rock into texturally or mineralogically distinct new rock as the result of high temperature, high pressure, or both. Diamond is Metamorphic Metamorphism • The mineralogy of a metamorphic rock changes with temperature and pressure. • In general, the highest temperature mineral assemblage is preserved. • It is possible to infer the highest P- T conditions from the mineralogy. 1 Metamorphic Environments Metamorphic Environments • Regional metamorphism • Where do you find metamorphic involves the burial and rocks? metamorphism of entire regions • In the mountains and in continental (hundreds of km2) shield areas. • Contact metamorphism results from local heating adjacent to • Why? igneous intrusions. (several meters) • Because elsewhere they are covered by sediments. Metamorphic Environments Regional Metamorphism • Because the tectonic forces required to bury, metamorphose, and re- exhume entire regions are slow, • most regionally metamorphosed terranes are old (> 500 MY), and • most Precambrian (> 500 MY) terranes are metamorphosed. Regional Metamorphism Regional Metamorphism • Regional metamorphism is typically • Regional metamorphism is typically isochemical (composition of rock isochemical (composition of rock does not change), although water does not change), although water may be lost. may be lost. • Metasomatism is a term for non- • Metasomatism is a term for non- isochemical metamorphism. isochemical metamorphism. • Contact metamorphism is typically • Contact metamorphism is typically metasomatic. metasomatic. 2 Effect of Increasing Conditions of Metamorphism Temperature • Changing the mineralogy of a sediment requires temperature > • Increases the atomic vibrations 300ºC and pressures > 2000 • Decreases the density (thermal atmospheres (~6 km deep). expansion) • Temperature increases downward at • Increases the speed of reactions ~20ºC/km in crust. • Pressure increases downward at ~1000 atm (1kbar) per 3 km (~330 bar/km). Effect of Increasing Pressure • Increases the density (compression) • Pressure can be hydrostatic (equal in all directions), or • Pressure can be directed as a shear which can cause – Foliation (alignment of platy minerals) – Lineation (alignment of needle-like minerals). Temperature and Pressure • Continental temperatures increases by ~30°C / km (90°C = 3 km). • Pressure increases by 330 atm / km (1kbar = 3km). 3 Schist: Metamorphosed Shale Metamorphosed Sediments • Conglomerate Metaconglomerate • Sandstone Quartzite • Shale Phyllite / Slate • Shale Schist • Shale Gneiss • Limestone Marble • Gypsum (Anhydrite) Blueschist Marble & Blueschist: Metamorphosed Shale + Limestone Marble Metamorphosed Igneous Rock • Granite Gneiss • Diorite Gneiss • Basalt/Gabbro Eclogite • All are high-grade • Partial melting Migmatite 4 Garnet in Schist Gneiss Migmatite (Partial Melting) Migmatite (Partial Melting) Garnet Amphibolite Gneiss Inner Gorge, Grand Canyon 5 Garnet Amphibolite Gneiss Garnet Chlorite Schist Diamond in Eclogite Blueschist Aosta Valley, Italy Metamorphism Terms Black Canyon • Regional Metamorphism • Quartzite of the • Contact Metamorphism • Phyllite • Isochemical • Slate Gunnison • Metasomatism • Schist • Hydrostatic • Gneiss • Foliation • Marble • Lineation • Eclogite • Terrane • Granulite • Migmatite 6 Clicker Question Clicker Question • Which is a term for • Which is a term for metamorphism that involves metamorphism that involves significant chemical change? significant chemical change? – A. Regional metamorphism – A. Regional metamorphism – B. Contact metamorphism – B. Contact metamorphism – C. Metasomatism –C. Metasomatism – D. Diapirism – D. Diapirism – E. Lithification – E. Lithification Clicker Question Clicker Question • The high grade metamorphic • The high grade metamorphic equivalent of limestone is? equivalent of limestone is? – A. Eclogite – A. Eclogite – B. Quartzite – B. Quartzite – C. Marble –C. Marble – D. Slate – D. Slate – E. Gneiss – E. Gneiss Clicker Question Clicker Question • A shale caught in a • A shale caught in a subduction zone and carried subduction zone and carried to high pressures at low to high pressures at low temperatures would be a? temperatures would be a? – A. Blueschist –A. Blueschist – B. Quartzite – B. Quartzite – C. Marble – C. Marble – D. Slate – D. Slate – E. Anhydrite – E. Anhydrite 7.
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