Metamorphism. Metamorphic Rocks. Sources of Heat for Metamorphism. Sources of Heat for Metamorphism. in mineral assemblages of a rock, and/or

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1 Metamorphic Rocks Sources of Heat for Metamorphism Heat from Earth s interior Geothermal gradient is the increase in temperature with depth Typical continental geothermal gradient is C/km Volcanically active areas have geothermal gradients of C/km Oceanic trenches have geothermal gradients as low as 5-10 C/km 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 magma Emplacement of magma chambers will add heat to the immediately surrounding rock Gabbroic magma ~1300 C Granitic magma ~700 C 1

2 Fault-Zone Metamorphism Directed pressure controls processes (GEOL 41.1) Regional Metamorphism Orogenic Metamorphism Combination of temperature and directed pressure Burial Metamorphism Combination of temperature and lithostatic pressure Contact Metamorphism Thermal variation controls processes Types of Metamorphism The lithostatic pressure at a 10 km depth is 3 kbar = 0.3 GPa Granite: 2.7 g/cm3 (2700 kg/m3) Most evident in lowpressure (near-surface) environments Temperature contrast between magma chamber and host rock Occurs adjacent to igneous intrusions Contact Metamorphism Compressional environments: Horz > Vert Pressure Extensional environments: Vert > Horz Pressure Directed pressure varies with tectonic environment Directed pressure affects the shape and arrangement of the minerals Directed pressure: pressure is imposed in a particular direction due to a regional stress field. 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: 3 g/cm3 (3000 kg/m3) Pressure Associated with Metamorphism Pressure Associated with Metamorphism 2

3 Regional Metamorphism Increase in temperature is accompanied by an increase in pressure Usually there is directed pressure, so rock deformation increases with metamorphic grade Metamorphic Zones and Facies Development of Foliations Recrystallization Pressure Solution Remobilization Rotation 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 3

4 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 (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 (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 mineralogical difference must be due to a difference in physical conditions 2 KMg 3 AlSi 3 O 10 (OH) KAl 2 AlSi 3 O 10 (OH) SiO 2 3 Mg 2 Al 4 Si 5 O KAlSi 3 O H 2 O 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 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) 4

5 Non-Foliated Metamorphic Rocks Progressive Metamorphism of Shales Increasing Temperature Slate Phyllite Schist Gneiss 5