Metamorphism and Metamorphic Rocks

1. Metamorphism and Metamorphic Rocks • Metamorphism is the transformation of rock by temperature and pressure • Metamorphic rocks are produced by transformation of: • Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocks Kyanite, Sillimanite, and Andalucite

2. Sedimentary rock 0 km Metamorphic rock Igneous Sediment rock 10 km Metamor-phism occurs between about 10 and 50 km of depth ~200ºC Sedimentary rock The rocks don’t melt Metamorphism Increasing depth and temperature 50 km Melting ~800ºC

3. Glaciers exposed the Rocky Mountains Canadian Shield North Cascades Black Hills Best US exposures in New England and the South Appalachian Mountains Grand Canyon Llano Uplift Usually buried deep, metamorphic rocks are seen when erosion removes covering rocks, and in the cores of mountains

4. Metamorphism • Metamorphism progresses from low to high grades • Rocks remain solid during metamorphism • Metamorphism occurs above the 50km melting depth for felsic minerals

5. What causes metamorphism? 1. Heat • Most important agent • Heat drives recrystallization - creates new, stable minerals • Increasing Heat with Depth

6. Temperature Increase with Depth • “Geothermal Gradient” due to: • Radioactive Isotopes • Intruding Magma • Friction Between Moving Bodies of Rock

7. What causes metamorphism? • 2. Pressure (stress) =Force/meters2 • Increases with depth • Pressure can be applied equally in all directions or differentially • All Directions = “Confining Pressure” • also called “lithostatic pressure” • Differential = “Directed Pressure”

8. Origin of pressure in metamorphism Confining pressure aka “lithostatic” (due to burial) (Convergent Margin)

9. Confining Pressure

10. Directed Pressure

11. Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation” Original bedding is obliterated Source:Kenneth Murray/Photo Researchers Inc.

12. Foliation • Minerals Recrystallize Perpendicular to the Directed Pressure • If the minerals are flat, such as sheet-like Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock.

13. Main factors affecting metamorphism 3. Parent rock • Metamorphic rocks typically have the same chemical composition as the rock they were formed from. • Different minerals, but made of the same atoms. • Exception: when hot water is involved.

14. Metamorphic Settings • Three types of metamorphic settings: • 1. Contact metamorphism – due heat from adjacent rocks • 2. Hydrothermal metamorphism – chemical alterations from hot, ion-rich water • 3. Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes: • a. Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliated • b. Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes - foliated Most is Dynamothermal

15. 1.Contact Metamorphism • Baking due to nearby Magma • Effect strongest in rocks in immediate contact

16. Contact metamorphism Produced mostly by local heat source

17. Contact Metamorphism Metamorphic Aureole

18. 2.Hydrothermal Metamorphism • Due circulation of water near Magma • Important at mid-ocean ridge

19. Hydrothermal Metamorphism

20. 3. Regional Metamorhism • Most Dynamothermal metamorphism occurs along convergent plate boundaries Example 1: Continent-Continent Collisions • Compressional stresses deforms plate edge • Continents Collide • Major Folded Mountain Belts: Alps, Himalayas, and Appalachian Mts.

21. Dynamothermal Metamorphism, Before collision Sediments are “unconsolidated”. They will fold if pushed.

22. Dynamothermal Metamorphism, After continental collision Felsic continental materials and sediments are buoyant, they have low density They float, cannot be subducted, so they get squashed.

23. 2. Regional Metamorphism (continued) • Most Dynamothermal metamorphism occurs along convergent plate boundaries • Example 2: In Subduction Zones

24. Metamorphism in a Subduction Zone

25. Metamorphic Grade and Index Minerals • Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form

26. Note Temperature gradient Index Minerals in metamorphic rocks 580oC 220oC 460oC 690oC Note Quartz and Feldspar are not index minerals: Why?

27. Here is an internally heated pressure vessel at the AMNH With these you can study, for example: 1. the temperature and pressure conditions at which polymorphs change from one form to another. (next slide) 2. the reactions of minerals with fluids (for example salty or alkaline water) at high temperatures and pressures. 3. the conditions necessary to change one assemblage of minerals to another http://research.amnh.org/earthplan/research/Equipment/Petrology

28. Thermometers and Pressure Gauges Sillimanite Kyanite Polymorphs of Al2SiO5 Andalusite

29. CANADA 7_21 New England Dynamothermal Metamorphism  MAINE Augusta CANADA  Caused by C-C collision M-P Most of Appalachians U.S.A. Montpelier NEW HAMPSHIRE VERMONT  Concord ATLANTIC OCEAN  Boston  Albany MASSACHUSETTS  NEW YORK  R.I. Hartford  Providence Binghamton CONNECTICUT Unmetamorphosed y e  Chlorite/muscovite zone Low grade l a l PENNSYLVANIA v  Biotite zone Scranton Long Island Garnet zone Medium grade t f Staurolite zone i r NEW Newark High grade Sillimanite zone JERSEY Increasing pressure and temperature LOW GRADE DIAGENESIS HIGH GRADE MELTING INTERMEDIATE GRADE Chlorite and muscovite Biotite Garnet Staurolite Sillimanite

30. Metamorphic Environments • Metamorphic grade or Facies • A group of minerals that form in a particular P-T environment • Can be used to deduce T-P conditions of formation

31. Metamorphic Environments in Subduction Zones We can look at minerals in Metamorphic Rocks and determine where they formed. Water facilitates metamorphic reactions by allowing movement of atoms and ions

32. Greenschist Hand Sample Greenschist Thin Section Chl-Ep

33. Blueschist glaucophane Amphibolite

34. Common metamorphic rocks • Nonfoliated rocks: the Field Geologist’s friend • Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together • Forms in intermediate T, P conditions

35. Field Geologists are grateful for quartzites. They don’t foliate, so you can see the folds. Mudrocks foliate; much harder to map. Sample of quartzite Thin section of quartzite

36. Flattening of quartz grains in quartzite

37. 7_18 Fracture Sandstone: grains and cement Fracture Quartzite: grains interlock

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

39. Marble (nonfoliated)

40. Common metamorphic rocks • Foliated rocks • Type formed depends on metamorphic grade • Grade depends on depth

41. Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions Change in metamorphic grade with depth Mudstone is most common sedimentary rock. When metamorphosed, rocks reveal grade: Increasing Directed Pressure and increasing Temps =>

42. Common metamorphic rocks • Foliated rocks • Slate • Very fine-grained • Excellent rock cleavage, often perp. to original • Made by low-grade metamorphism of shale

43. Example of slate

44. 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

45. Phyllite (l) and Slate (r) lack visible mineral grains

46. 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)

47. A mica garnet schist

48. 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

49. Gneiss displays bands of light and dark minerals

50. 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, perpendicular to the directed pressure • Common near convergent margins