Understanding Earth Fifth Edition Grotzinger Jordan Press Siever Chapter 6: METAMORPHISM Modification of Rocks by Temperature and Pressure Lecturer: H Mohammadzadeh Assistant professors, Department of Geology, FUM Copyright 2007 by W. H. Freeman and Company Chapter 6 Metamorphism: Modification of Rocks by Temperature and Pressure About Metamorphism Changes in heat, pressure, and the chemical environment of rocks can alter mineral compositions and crystalline textures, making them metamorphic. Metamorphic changes occur in the solid state, so there is no melting. 1
Lecture Outline 1. Metamorphism and the Earth system 2. Causes of 3. Types of 4. Metamorphic textures 5. Regional and metamorphic grade 6. Plate tectonics and 1. Metamorphism and the Earth system driven by Earth s internal heat closely related to plate tectonics releases gasses into atmosphere 2. Causes of internal heat of Earth temperature increases with depth rate = 20º to 60º C per km at 15 km depth: 450º C internal pressure of Earth fluid composition inside Earth 2
2. Causes of pressure and temperature increase with depth in all regions 2. Causes of grades of (low, intermediate and high) 3
2. Causes of the role of temperature geothermal gradient shallow (20º C / km) steep (50º C / km) the role of pressure (stress) confining pressure directed pressure rate of increase = 0.3 to 0.4 kbar / km minerals are geobarometers the role of fluids metasomatism accelerated chemical reactions 3. Types of Shock Regional Regional high-pressure Contact Depth, km 0 35 75 Regional Oceanic crust Oceanic lithosphere Burial Water Seafloor 4
4. Metamorphic textures Metamorphism changes sedimentary rocks, thus forming slaty cleavage planes. slaty cleavage planes bedding planes 4. Metamorphic textures The original bedding can be seen in the thin sandy layers. Regional causes cleavage planes to develop. foliation plane shale sandstone layers original bedding 5 cm 5
4. Metamorphic textures Mineral crystals become elongated perpendicular to the compressive force. Feldspar Foliated rocks contain platy minerals that are aligned along a preferred orientation. Mica Pyrite Staurolite Quartz Staurolite crystal Mica Foliation is the result of compressive forces. 4. Metamorphic textures Increasing intensity of Low grade Intermediate grade High grade Increasing crystal size Increasing coarseness of foliation Foliated rocks are classified by the degree of cleavage, schistosity, and banding. Diagenesis Low grade Intermediate grade High grade Slate Phyllite Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Slaty cleavage Schistosity Banding Banding 6
Foliated texture: Schist with garnet porphyro-blasts Granoblastic textures 4. Metamorphic textures 7
5. Regional and metamorphic grade mineral isograds (zones of change) index minerals reflect pressure and temperature conditions (or define metamorphic zones), groups of 2 to 3 index minerals form an isograd Isograds can be used to plot the Canada level or degree of ME. NY Isograds VT NH MA Key: Not metamorphosed CT RI Low grade Chlorite zone Biotite zone Medium grade Garnet zone Staurolite zone High grade Sillimanite zone Low Pressure (kilobars kilobars) Slate Phyllite Blueschist Intermediate Schist Gneiss High Migmatite Depth (km) Temperature ( C) 8
Degree of Diagenesis Low Intermediate High Greenschists Amphibolites Granulites Chlorite White mica (mainly muscovite) Biotite Garnet Staurolite Kyanite Sillimanite Albite (sodium plagioclase) With increasing metamorphic grade, mineral composition changes. Mineral suites define metamorphic facies. 0 Hornfels 0 5 Pressure (kilobars) 5 10 15 13.5 Greenschist Amphibolite Granulite 10 15 20 25 30 Depth (km) Eclogite 35 20 40 0 200 400 600 800 1000 Temperature ( C) Metamorphic facies correspond to particular combinations of pressure and temperature... and can be used to indicate specific tectonic environments. 9
5. Regional and metamorphic grade 6. Plate tectonics and occurs in or near plate interiors divergent plate margins convergent plate margins transform plate margins Tectonic transport moves rocks through different pressure-temperature zones, and then transports them back to the shallow crust or the surface. Low P, Low T High P, High T 10
6. Plate tectonics and metamorphic pressure and temperature paths During, Composition of crystal a garnet crystal grows, can be plotted on the and its composition P-T path as it grows. changes. thin section of garnet gneiss growth zone in garnet The garnet crystal initially grows in a schist but ends up growing in a gneiss. Low Pressure (kilobars) Intermediate High Depth (km) Temperature ( C) 11
Pressure (kilobars) Low High Depth (km) Peak Temperature ( C) Continental crust Deep-ocean Shelf sediment sediment Oceanic crust Subduction mélange, low temperature high pressure Trench Mélange ophiolites Continental crust High temperature high pressure within mountain belts Suture Continental crust Multiple thrusts Deformed and metamorphosed shallow- and deepocean sediments Continental crust Prograde path Peak Retrograde path Prograde path Peak Retrograde path Key terms and concepts Amphibolite Blueschist Contact Eclogite Foliated rock Foliation Gneiss Granoblastic rock Granulite High-pressure Hornfels Low-grade Marble Metamorphic facies Metamorphic rock Metasomatism Migmatite Phyllite Porphyroblast Quartzite Regional Schist Seafloor Shock Slate Stress Ultra-high pressure Zeolite 12