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Metamorphism and Metamorphic Rocks

Metamorphism and Metamorphic Rocks. Limestone Marble Sandstone Quartzite (meta-quartzite) Shale Slate Shale/sandstone Schist Shale Hornfels

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Metamorphism and Metamorphic Rocks

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  1. Metamorphism and Metamorphic Rocks

  2. Limestone Marble Sandstone Quartzite (meta-quartzite) Shale Slate Shale/sandstone Schist Shale Hornfels Shale/sandstone Gneiss What are metamorphic rocks? Increases in heat and pressure and changes in chemical environment can alter the mineral composition and crystalline textures of sedimentary and igneous rocks Metamorphism is the solid-state (without melting) transformation of pre-existing rock into texturally or mineralogically distinct new rock as the result of high temperature,high pressure, or both.

  3. Rock Cycle Sedimentary Erosion + Deposition Igneous Melting Erosion + Deposition Burial + Heating Burial + Heating Melting Metamorphic

  4. What are metamorphic rocks? Causes of metamorphism Heat & Pressure

  5. Occurs at depths greater than that of lithification (Diagenesis -> Sedimentary Process) Sedimentary Surface 15o C 5 km 200o C Metamorphic 30 km Igneous 600o C

  6. The 3 “R”s of Metamorphism Regrowth -crystals grow larger and more uniform in shape. Recrystallization -new minerals form from existing minerals. Reorientation - platy or elongated mineral crystals become layered and aligned.

  7. Factors Controlling Metamorphism Metamorphic changes bring a pre-existing rock into equilibrium with new surroundings • Temperature • Re-crystalisation – new mineral assemblages • Segregation of new minerals – banding • Pressure • Regrowth - Confining pressure – acts in all directions – alters mineralogy by squeezing atoms into a denser state • Reorientation - Directed pressure – acting in a particular direction – convergent plate boundaries – causes preferred orientation - foliation

  8. Kinds of Metamorphism Regional Regional Contact Contact Burial Cataclastic Hydrothermal

  9. Kinds of Metamorphism Regional Metamorphism • Occurs where both high temperature and high pressure are imposed over large belts of the crust • Destroys all original sedimentary or igneous textures through growth of new minerals • Occurs in deeper levels of the crust along convergent plate boundaries particularly continental-continental boundaries where active mountain building is taking place.

  10. Regional Metamorphism Occurs over a large area as a result of intense heat and pressure • Most metamorphic rocks Most obvious at convergent plate boundaries where rocks are intensely deformed and compacted during subduction and compression Metamorphic grades seen through index minerals

  11. Taconic Mountain Building Event (450 million years ago)

  12. Taconic Mountain Building Event (450 million years ago)

  13. Regional Metamorphism Taconic Mountain Building Event (450 million years ago)

  14. Regional Metamorphism

  15. Metamorphic Grade

  16. Progression of metamorphism Start with a shale and then hit it with pressure and heat. Slate Phyllite Schist

  17. You end up with something that is really Gneiss!

  18. Sedimentary Rock 3000 ATM 200° C Low Grade Metamorphic Rock High Grade Metamorphic Rock Migmatite 600° C Magma

  19. Migmatite • gneiss that begins to partially melt into granite.

  20. Kinds of Metamorphism Contact Metamorphism • Heat and pressure generated by igneous intrusions • Affects only a thin region of intruded rock. • At shallow depths the mineral transformations generally associated with heat – pressure becomes important for intrusion at great depth • A zone of alteration called an aureole forms in the rock surrounding the magma

  21. Types of Metamorphism Contact Metamorphism: The main metamorphic agent is heat.

  22. Contact Metamorphism

  23. Contact metamorphism

  24. Kinds of Metamorphism Dynamic / Cataclastic Metamorphism • Found along faults • Fault movement causes rocks on either side to fragment • The product is a rock with a broken and pulverised texture • Generally found in strongly deformed mountain belts – often found with regionally metamorphosed rocks

  25. Dynamic Metamorphism Associated with Fault Zones • San Andreas Fault, CA and Northern Scotland Low temp/high pressure Mylonites – hard, dense, fine-grained rocks, with thin laminations (less than 1 cm thick)

  26. Granite away from active fault

  27. Granite next to active fault

  28. Kinds of Metamorphism Hydrothermal Metamorphism • Often associated with mid-ocean ridges • Seawater heated by upwelling magma promotes chemical reactions with basalt causing alteration • Alteration is also common in other igneous rocks such as granite. • Alteration can result in the rock becoming weaker, more compressible and more permeable – the end product is very similar to that produced by chemical weathering

  29. Kinds of Metamorphism Burial Metamorphism (precursor to Regional) • Diagenisis grades into burial metamorphism • Low grade metamorphisms resulting from heat and pressure exerted by overlying sediments and sedimentary rocks • Bedding and other sedimentary structures are preserved • Burial metamorphism can grade into regional metamorphism

  30. Burial Metamorphism

  31. Classification of Metamorphic Rocks Foliated Rocks A set of flat or wavy parallel planes produced by the preferred orientation of minerals, particularly platy minerals like mica

  32. Foliated Rocks A foliation is any planar fabric in a metamorphic rock. In this case, the foliation is defined by aligned sheets of muscovite sandwiched between quartz grains. This slide is indicative of a phyllite. The foliation in this rock is a crenulation cleavage, and is developed after the horizontal foliation. This slide is indicative of a schist.

  33. Classification of Metamorphic Rocks Criteria Nature of foliation Grain size Degree of banding Metamorphic grade Foliated Rocks

  34. Common metamorphic rocks • Foliated rocks • Slate • Very fine-grained • Excellent rock cleavage • Most often generated from low-grade metamorphism of shale, mudstone, or siltstone

  35. Low Grade - Slate

  36. Mountain Building Burial

  37. Common metamorphic rocks • Foliated rocks • Phyllite • Gradation in the degree of metamorphism between slate and schist • Platy minerals not large enough to be identified with the unaided eye • Glossy sheen and wavy surfaces • Exhibits rock cleavage • Composed mainly of fine crystals of muscovite and/or chlorite

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

  39. Intermediate Grade – Phyllite

  40. Common metamorphic rocks • Foliated rocks • Schist • Medium- to coarse-grained • Platy minerals predominate • Commonly include the micas • The term schist describes the texture • To indicate composition, mineral names are used (such as mica schist)

  41. A mica garnet schist

  42. High Grade - Schist

  43. Common metamorphic rocks • Foliated rocks • Gneiss • Medium- to coarse-grained • Banded appearance • High-grade metamorphism • Often composed of white or light-colored feldspar-rich layers with bands of dark ferromagnesian minerals

  44. Gneiss typically displays a banded appearance

  45. High Grade - Gneiss

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