Metamorphism

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

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

6. 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

7. Progressive metamorphism of a shale Shale

8. Progressive metamorphism of a shale Slate

9. Progressive metamorphism of a shale Phyllite

10. Progressive metamorphism of a shale Schist

11. Progressive metamorphism of a shale Gneiss

12. 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

13. Contact metamorphism Produced mostly by local heat source

14. Hydrothermal metamorphism Requires hot circulation of fluids

15. Regional metamorphismin Mountain Belts Requires crustal thickening and shortening

16. 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

17. 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)

18. Location of metamorphic zones in a subduction zone

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

20. Index Minerals in metamorphic rocks

21. 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)

22. Metamorphic Environments in Subduction Zones

23. Greenschist Hand Sample Greenschist Thin Section

24. Mica Schist

25. Blueschist Amphibolite

26. 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

27. Sample of quartzite Thin section of quartzite

28. Flattening of quartz grains in quartzite

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

30. Marble (Random fabric = annealing; nonfoliated)

31. Change in metamorphic grade with depth

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

33. Example of slate

34. Slate roof

35. 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

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

37. 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)

38. Mica Schist - note well developed foliation

39. A mica garnet schist

40. 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

41. Gneiss displays bands of light and dark minerals

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

43. 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

44. Outcrop of foliated gneiss

45. 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

46. Flattened Pebble Conglomerate = flattening

47. Development of foliation due to directed pressure