Petrology Session 2. Metamorphism: Alteration of Rocks by Temperature and Pressure

Transcription

1 Petrology Session 2 Metamorphism: Alteration of Rocks by Temperature and Pressure

2 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.

3 Causes of Metamorphism internal heat of Earth internal pressure of Earth fluid composition inside Earth

4 Causes of Metamorphism temperature increases with depth rate = 20º to 60ºC per km at 15 km depth: 450ºC

5 Causes of Metamorphism pressure and temperature increase with depth in all regions

6

7 Causes of Metamorphism

8 Causes of Metamorphism the role of temperature geothermal gradient shallow (20ºC / km) steep (50ºC / km)

9 Causes of Metamorphism the role of pressure (stress) confining pressure directed pressure

10 Causes of Metamorphism the role of pressure (stress) rate of increase = 0.3 to 0.4 kbar / km minerals are geobarometers

11 Types of Metamorphism the role of fluids metasomatism accelerated chemical reactions

12 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

13 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

14 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

15 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

16 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

17 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

18 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

19 3. Metamorphic Textures

20 3. Metamorphic Textures shale sandstone layers 5 cm

21 The original bedding can be seen in the thin sandy layers. shale sandstone layers 5 cm

22 The original bedding can be seen in the thin sandy layers. foliation plane shale sandstone layers original bedding 5 cm

23 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

24 3. Metamorphic Textures Low grade Intermediate grade

25 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

26 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

27 Metamorphic Textures classification of foliated rocks metamorphic grade crystal size type of foliation banding

28 Metamorphic Textures Nomenclature of foliated rocks slate phyllite schist gneiss migmatite

29 Foliated texture: schist with garnet porphroblasts

30 Metamorphic Textures Nomenclature of granoblastic (non-foliated) metamorphic rocks hornfels quartzites marbles greenstones amphibolites granulites (granofels)

31 Granoblastic Texture

32

33 Regional Metamorphism and Metamorphic Grade Grades of metamorphism low intermediate high

34 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

35 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

36 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

37 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

38 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

39

40

41

42 Depth (km) 10 Granulite 0 zone ction 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 Temperature ( C) Metamorphic facies correspond to particular combinations of pressure and temperature... and can be used to indicate specific tectonic environments.

43 Plate Tectonics and Metamorphism Metamorphism occurs in or near plate interiors divergent plate margins convergent plate margins transform plate margins

44 Tectonic transport moves rocks through different pressure-temperature zones, Low P, Low T High P, High T

45 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.

46 Plate Tectonics and Metamorphism Metamorphic pressure-temperature paths history of burial and exhumation prograde and retrograde paths

47 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