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Metamorphism

Metamorphism

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Metamorphism

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  1. Metamorphism • The transformation of rock by temperature and pressure • Metamorphic rocks are produced by transformation of: • Igneous, sedimentary and igneous rxs Thanks to CU Boulder Geology Dept for use of some of these slides

  2. Metamorphism • Metamorphism progresses from low to high grades • Rocks remain solid during metamorphism

  3. What causes metamorphism? • Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Pressure (stress) • Increases with depth • Pressure can be applied equally in all directions or differentially

  4. Origin of pressure in metamorphism Confining or hydrostatic pressure: equal in all directions Directed pressure: largely in one direction or along a particular axis

  5. Main factor affecting metamorphism • Parent rock • Metamorphic rocks typically have the same chemical composition as the rock they were formed from • Different minerals, but made of the same stuff. • Exception: gases (carbon dioxide, CO2) and water (H2O) may be released

  6. Progressive metamorphism of a shale Shale

  7. Progressive metamorphism of a shale Slate

  8. Progressive metamorphism of a shale Phyllite

  9. Progressive metamorphism of a shale Schist

  10. Progressive metamorphism of a shale Gneiss

  11. Metamorphism • Three types of metamorphic settings: • Contact metamorphism– from a rise in temperature within host rock • Hydrothermal metamorphism– chemical alterations from hot, ion-rich water • Regional metamorphism -- Occurs in the cores of mountain belts and makes great volumes of metamorphic rock

  12. Contact metamorphism Produced mostly by local heat source

  13. Hydrothermal metamorphism Requires hot circulation of fluids

  14. Regional metamorphismin Mountain Belts Requires crustal thickening and shortening

  15. Metamorphism and plate tectonics • Most regional metamorphism occurs along convergent plate boundaries • Compressional stresses deform plate edge • Occurs in major mountain belts: Alps, Himalayas, and Appalachians • Not necessarily in all mountains, depends on orogenic forces, i.e., uplift versus compression

  16. Metamorphism and plate tectonics • Metamorphism at subduction zones • Cores of subduction zones contain linear belts ofmetamorphic rocks • High-P, low-T zones near trench • High-T, low-P zones in region of igneous activity (arc)

  17. Location of metamorphic zones in a subduction zone

  18. Metamorphic Environments • Index minerals and metamorphic grade • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

  19. Index Minerals in metamorphic rocks

  20. Metamorphic Environments • Metamorphic grade • A group of minerals that form in a particular P-T environment Zeolite (really low T,P; <200C) Greenschist (low T, P; 200-450C, 10-15 km) Blueschist (low T, high P - subduction zones) Amphibolite (high T, P; 450-650C, 15-20 km) Granulite (super high T, P; >700C, >25km)

  21. Metamorphic Environments in Subduction Zones

  22. Greenschist Hand Sample Greenschist Thin Section

  23. Mica Schist

  24. Blueschist Amphibolite

  25. Common metamorphic rocks • Nonfoliated rocks • Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together • Forms in intermediate T, P conditions

  26. Sample of quartzite Thin section of quartzite

  27. Flattening of quartz grains in quartzite

  28. Common metamorphic rocks • Nonfoliated rocks • Marble • Coarse, crystalline • Parent rock usually limestone • Composed of calcite crystals • Fabric can be random or oriented

  29. Marble (Random fabric = annealing; nonfoliated)

  30. Change in metamorphic grade with depth

  31. Common metamorphic rocks • Foliated rocks • Slate • Very fine-grained • Excellent rock cleavage • Made by low-grade metamorphism of shale

  32. Example of slate

  33. Slate roof

  34. Common metamorphic rocks • Foliated rocks • Phyllite • Grade of metamorphism between slate and schist • Made of small platy minerals • Glossy sheen with rock cleavage • Composed mainly of muscovite and/or chlorite

  35. Phyllite (left) and Slate (right) lack visible mineral grains

  36. Common metamorphic rocks • Foliated rocks • Schist • Medium- to coarse-grained • Comprised of platy minerals (micas) • The term schist describes the texture • To indicate composition, mineral names are used (such as mica schist)

  37. Mica Schist - note well developed foliation

  38. A mica garnet schist

  39. Common metamorphic rocks • Foliated rocks • Gneiss • Medium- to coarse-grained • Banded appearance • High-grade metamorphism • Composed of light-colored feldspar layers with bands of dark mafic minerals

  40. Gneiss displays bands of light and dark minerals

  41. Diorite to Gneiss Morph (orthogneiss - from igneous protolith)

  42. What are metamorphic textures? • Texture refers to the size, shape, and arrangement of mineral grains within a rock • Foliation – planar arrangement of mineral grains within a rock

  43. Outcrop of foliated gneiss

  44. Metamorphic textures • Foliation • Foliation can form in various ways: • Rotation of platy or elongated minerals • Recrystallization of minerals in a preferred orientation • Changing the shape of equidimensional grains into elongated and aligned shapes

  45. Flattened Pebble Conglomerate = flattening

  46. Development of foliation due to directed pressure