Petrology Session 2 Metamorphism: Alteration 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.
Causes of Metamorphism internal heat of Earth internal pressure of Earth fluid composition inside Earth
Causes of Metamorphism temperature increases with depth rate = 20º to 60ºC per km at 15 km depth: 450ºC
Causes of Metamorphism pressure and temperature increase with depth in all regions
Causes of Metamorphism
Causes of Metamorphism the role of temperature geothermal gradient shallow (20ºC / km) steep (50ºC / km)
Causes of Metamorphism the role of pressure (stress) confining pressure directed pressure
Causes of Metamorphism the role of pressure (stress) rate of increase = 0.3 to 0.4 kbar / km minerals are geobarometers
Types of Metamorphism the role of fluids metasomatism accelerated chemical reactions
Types of Metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Oceanic crust Oceanic lithosphere
Shock metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Oceanic crust Oceanic lithosphere
Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Oceanic crust Oceanic lithosphere
Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re High-pressure metamorphism Oceanic crust Oceanic lithosphere
Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re High-pressure metamorphism Contact metamorphism Oceanic crust Oceanic lithosphere
Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re Burial metamorphism High-pressure metamorphism Contact metamorphism Oceanic crust Oceanic lithosphere
Shock metamorphism Regional metamorphism Depth, km 0 Con tine nta 35 l cr Con ust tin ent al m 75 ant le l ith osp As her the e no sp he re High-pressure metamorphism Contact metamorphism Oceanic crust Oceanic lithosphere Water Burial metamorphism Seafloor metamorphism
3. Metamorphic Textures
3. Metamorphic Textures shale sandstone layers 5 cm
The original bedding can be seen in the thin sandy layers. shale sandstone layers 5 cm
The original bedding can be seen in the thin sandy layers. foliation plane shale sandstone layers original bedding 5 cm
The original bedding can be seen in the thin sandy layers. Regional metamorphism causes cleavage planes to develop. foliation plane shale sandstone layers original bedding 5 cm
3. Metamorphic Textures Low grade Intermediate grade
3. Metamorphic Textures Diagenesis Slate Slaty cleavage Low grade Intermediate grade Phyllite High grade Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Schistosity Banding Banding
Foliated rocks are classified by the degree of cleavage, schistosity, and banding. Diagenesis Slate Slaty cleavage Low grade Intermediate grade Phyllite High grade Schist (abundant micaceous minerals) Gneiss (fewer micaceous minerals) Migmatite Schistosity Banding Banding
Metamorphic Textures classification of foliated rocks metamorphic grade crystal size type of foliation banding
Metamorphic Textures Nomenclature of foliated rocks slate phyllite schist gneiss migmatite
Foliated texture: schist with garnet porphroblasts
Metamorphic Textures Nomenclature of granoblastic (non-foliated) metamorphic rocks hornfels quartzites marbles greenstones amphibolites granulites (granofels)
Granoblastic Texture
Regional Metamorphism and Metamorphic Grade Grades of metamorphism low intermediate high
Regional Metamorphism and Metamorphic Grade Mineral isograds (zones of change) index minerals reflect pressure and temperature conditions groups of 2 to 3 index minerals form an isograd
4. Regional Metamorphism Canada ME NY Isograds VT NH Key: MA CT RI Low grade Medium grade High grade Not metamorphosed Chlorite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone
Index minerals define metamorphic zones. Canada ME NY Isograds VT NH Key: MA CT RI Low grade Medium grade High grade Not metamorphosed Chlorite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone
Index minerals define metamorphic zones. Isograds can be used to plot the level or degree of metamorphism. Canada ME NY Isograds VT NH Key: MA CT RI Low grade Medium grade High grade Not metamorphosed Chlorite zone Biotite zone Garnet zone Staurolite zone Sillimanite zone
Inc rea Pressure (kilobars) Slate sing me t amo rp Intermediate Grade Phyllite hic gr a de High Grade Schist Blueschist Gneiss Migmatite Temperature ( C) Depth (km) Low Grade
200 400 15 20 25 Depth (km) 10 Granulite 0 zone ction 15 20 u Subd 13.5 Greenschist 10 5 Amphibolite 5 Contac t meta Con morph ism mo ditio unt ns ain ben bel eat h ts st hi sc ue Bl Pressure (kilobars) Ze ol it e 0 Hornfels Partial melt ing begins 0 30 Eclogite 35 600 40 1000 800 Temperature ( C) Metamorphic facies correspond to particular combinations of pressure and temperature... and can be used to indicate specific tectonic environments.
Plate Tectonics and Metamorphism Metamorphism occurs in or near plate interiors divergent plate margins convergent plate margins transform plate margins
Tectonic transport moves rocks through different pressure-temperature zones, Low P, Low T High P, High T
Tectonic transport moves rocks through different pressure-temperature zones, Low P, Low T High P, High T and then transports them back to the shallow crust or the surface.
Plate Tectonics and Metamorphism Metamorphic pressure-temperature paths history of burial and exhumation prograde and retrograde paths
Plate Tectonics and Metamorphism rapid erosion (exhumation) rates of mountain ranges show a relationship between tectonics (orogeny) climate controls the flow of metamorphic rocks to the surface