Metamorphic rocks

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Metamorphic Rocks Metamorphism • Literally translates to “change of form” • In geology it refers to solid-state changes in mineral assemblages of a rock, and/or the texture of these minerals • Due to changes in temperature and/or pressure Sources of Heat for Metamorphism • Heat from Earth’s interior • Geothermal gradient is the increase in temperature with depth – Typical continental geothermal gradient is 25-30°C/km – Volcanically active areas have geothermal gradients of 3050°C/km – Oceanic trenches have geothermal gradients as low as 5-10°C/km Sources of Heat for Metamorphism • Heat from magma • Emplacement of magma chambers will add heat to the immediately surrounding rock – Gabbroic magma ~1300°C – Granitic magma ~700°C Pressure Associated with Metamorphism • Lithostatic pressure: the confining pressure created by the material that sits above a particular location Lithostatic pressure is equal in all directions and compresses the volume of rock – Basalt: g/cm3 (3000 kg/m3) – Granite: 2.7 g/cm3 (2700 kg/m3) – The lithostatic pressure at a 10 km depth is ≈ kbar = 0.3 GPa Pressure Associated with Metamorphism • Directed pressure: pressure is imposed in a particular direction due to a regional stress field • Directed pressure affects the shape and arrangement of the minerals • Directed pressure varies with tectonic environment – Compressional environments: Horz > Vert Pressure – Extensional environments: Vert > Horz Pressure Types of Metamorphism • Contact Metamorphism – Thermal variation controls processes • Regional Metamorphism – Orogenic Metamorphism • Combination of temperature and directed pressure – Burial Metamorphism • Combination of temperature and lithostatic pressure • Fault-Zone Metamorphism – Directed pressure controls processes (GEOL 41.1) Contact Metamorphism • Occurs adjacent to igneous intrusions • Temperature contrast between magma chamber and host rock • Most evident in lowpressure (near-surface) environments Regional Metamorphism • Increase in temperature is accompanied by an increase in pressure • Usually there is directed pressure, so rock deformation increases with metamorphic grade Development of Foliations Recrystallization Pressure Solution Remobilization Rotation Metamorphic Zones and Facies Mineral Zones of G.W Barrow (1893) • Barrow noted that pelitic rocks of the Scottish Highlands had distinct mineral zones (Gt, Ky, Sil) • He concluded that this was the result of increasing metamorphic grade (T) • Tilley (1925) added the lowgrade Biotite and Chlorite zones Excerpted From Gillen (1982) Metamorphic Geology An Introduction to Tectonic and Metamorphic Processes • Bt, Gt, St, Ky, and Sil are Index Minerals in metapelitic rocks Index Mineral Isograds • The line that defines the first appearance of an index mineral corresponds to a line of equal metamorphic grade • Introduction of the concept of an isograd Eskola and Metamorphic Facies • Eskola (1914, 1915) noted that metapelitic rocks in southern Finland (Orijärvi) contained the assemblage Bt-Ms whereas near Oslo, rocks contained the compositionally equivalent mineral assemblage Kf-Cd • If rocks are the same composition, then the KMg3AlSi3O10(OH)2 + KAl2AlSi3O10(OH)2 + 15 SiO2 mineralogical difference ↔ must be due to a difference Mg2Al4Si5O18 + KAlSi3O8 + H2O in physical conditions Eskola and Metamorphic Facies • Eskola (1915) introduced the concept of metamorphic facies: – “In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, the mineral composition is controlled only by the chemical composition.” • A metamorphic facies is a set of repeatedly associated metamorphic mineral assemblages • If you find a specified mineral assemblage, then you can assign a metamorphic facies to the area, and thereby assign a range of pressure and temperature conditions Eskola and Metamorphic Facies • In 1920, Eskola introduced five metamorphic facies that were defined by mineral assemblages in metabasites: – Greenschist – Amphibolite – Hornfels – Sanidinite – Eclogite • In 1939, Eskola added an additional metamorphic facies: – Granulite – Epidote-amphibolite – Glaucophane-schist (now called Blueschist) • In 1959 and 1960, Coombs added two additional metamorphic facies: – Zeolite – Prehnite-Pumpellyite (now called Subgreenschist) Progressive Metamorphism of Shales Increasing Temperature Slate Phyllite Schist Gneiss Non-Foliated Metamorphic Rocks [...].. .Metamorphic Zones and Facies Mineral Zones of G.W Barrow (1893) • Barrow noted that pelitic rocks of the Scottish Highlands had distinct mineral zones (Gt, Ky, Sil) • He concluded that this was the result of increasing metamorphic grade (T) • Tilley (1925) added the lowgrade Biotite and Chlorite zones Excerpted From Gillen (1982) Metamorphic Geology An Introduction to Tectonic and Metamorphic. .. Index Minerals in metapelitic rocks Index Mineral Isograds • The line that defines the first appearance of an index mineral corresponds to a line of equal metamorphic grade • Introduction of the concept of an isograd Eskola and Metamorphic Facies • Eskola (1914, 1915) noted that metapelitic rocks in southern Finland (Orijärvi) contained the assemblage Bt-Ms whereas near Oslo, rocks contained the compositionally... controlled only by the chemical composition.” • A metamorphic facies is a set of repeatedly associated metamorphic mineral assemblages • If you find a specified mineral assemblage, then you can assign a metamorphic facies to the area, and thereby assign a range of pressure and temperature conditions Eskola and Metamorphic Facies • In 1920, Eskola introduced five metamorphic facies that were defined by mineral... compositionally equivalent mineral assemblage Kf-Cd • If rocks are the same composition, then the 2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH)2 + 15 SiO2 mineralogical difference ↔ must be due to a difference 3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O in physical conditions Eskola and Metamorphic Facies • Eskola (1915) introduced the concept of metamorphic facies: – “In any rock or metamorphic formation which has arrived at... 1939, Eskola added an additional 3 metamorphic facies: – Granulite – Epidote-amphibolite – Glaucophane-schist (now called Blueschist) • In 1959 and 1960, Coombs added two additional metamorphic facies: – Zeolite – Prehnite-Pumpellyite (now called Subgreenschist) Progressive Metamorphism of Shales Increasing Temperature Slate Phyllite Schist Gneiss Non-Foliated Metamorphic Rocks
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