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Gregory G. Dimijian/Photo Researchers

Metamorphic Rocks. cd/EM - F. Gregory G. Dimijian/Photo Researchers. Metamorphism : to change form Metamorphic rock : any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in mineralogical or texture composition in the solid state. Metamorphic Rocks.

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Gregory G. Dimijian/Photo Researchers

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  1. Metamorphic Rocks cd/EM - F Gregory G. Dimijian/Photo Researchers

  2. Metamorphism: to change form Metamorphic rock: any rock (sedimentary, volcanic, plutonic, metamorphic) that has undergone changes in mineralogical or texture composition in the solid state. Metamorphic Rocks

  3. Metamorphic rocks result from the partial or complete recrystallization of minerals in rocks over long periods of time. Rocks remain essentially solid during metamorphism. No single mode of origin

  4. HEAT: stability region of mineral sensitive to T. With increased T, pore fluid decreases. PRESSURE: greater pressure tends to decrease space available; metamorphic minerals tend to be dense. Increased P can come from any stress (burial). Directed stress will bring about a preferred orientation of minerals. FLUIDS (H2O): acts as a catalyst during metamorphism; aids the exchange of ions between growing crystals. Metamorphic processes

  5. Refers to the intensity of metamorphism. High grade: high T (>5500C), P Intermediate grade: T (320-5500C), P Low grade: low T (200-3200C), P Metamorphic grade

  6. Metamorphic Grade

  7. Pressure (depth) and Temperature vs. Rock Type

  8. Regional: Widespread changes in temperature and pressure bring about changes in rocks due to tectonic forces. Contact: Colder country rocks “cooked” by intrusion of magma (affected area is proportional to the size and temperature of the intrusion but always only a local phenomenon). Deformational: Changes in rocks associated with faulting and folding (regional or local). Types of Metamorphism

  9. Burial: Changes in a rock due to the gradual changes in T and P due to successive burial (regional). Impact: Changes due to VERY rapid increase in pressure (only localized). Hydrothermal: heated fluids leach surrounding rocks (local) Types of metamorphism (cont.)

  10. Plate Tectonics & Metamorphism Fig. 9.3

  11. Depends on: grade of metamorphism duration of metamorphism composition of the rock How much can a rock change?

  12. Mineralogical changes (e.g., clay to mica): Many complicated reactions in which new assemblages form — depend on pressure, temperature, composition. Textural changes: recrystallization (grain boundaries more compact) and foliation (preferred orientation of minerals). Metamorphic reactions

  13. Most minerals are stable over a relatively narrow range of P and T (e.g., ice unstable above 0°C). The stability range of different minerals overlap and provide constraints on the metamorphic history of rocks -> METAMORPHIC ZONES. Mineralogical Changes(Mineral Stability)

  14. Mineralogic Changes in Metamorphosed Shales (new assemblages develop -> zones)

  15. Mineralogic Changes in Metamorphosed Shales

  16. Contact Metamorphism adjacentto a Granite Intrusion

  17. Grain size Recrystallization - Granular Texture Grain size can either decrease or increase. Orientation of minerals Directed stress will orient minerals perpendicular to stress: Lineation Foliation Changes in Texture

  18. Fig. Story 9.4

  19. Textural Changes:Non Foliated or Granular • Granular texture • Coarse to fine interlocking grains • Little or no preferred orientation

  20. Quartzite (from sandstone) Breck P. Kent

  21. Quartzite in Thin Section

  22. Marble (from limestone) Breck P. Kent

  23. Marble in Thin Section

  24. FOLIATED ROCKS • Mineral grains show preferred orientation • See different kinds of foliation: • Slaty cleavage • Schistosity • Gneissic Foliation

  25. (sheen) (sparkles)

  26. Slate with Foliation and Relict Bedding

  27. Slaty Cleavage Martin Miller

  28. Metamorphic Foliation

  29. Metamorphic Foliation Direction of Compressive Forces S. Dobos

  30. Based on the texture and composition of the rock: Low grade Slate Phyllite Intermediate grade Schist Gneiss High grade Migmatite Classification of metamorphic shale

  31. Slate Andrew J. Martinez/Photo Researchers

  32. Schist BiophotoAssociates/Photo Researchers

  33. Schist in Thin Section

  34. Gneiss Breck P. Kent

  35. Metamorphism In Hawai’i • Not important except for CONTACT METAMORPHISM (of course) at dike margins • Only see Low Grade metamorphic rocks in rock quarries with lots of Zeolites • (The following slides are for your edification only, so you can say that you have seen these figures!)

  36. A given set of metamorphic conditions Each facies is characteristic of particular tectonic environments and will have certain rocks that are diagnostic of those conditions. Therefore, certain rocks can be clues to the (P,T) history of the region. Facies are: zeolite*, greenschist, amphibolite, granulite, blueschist, eclogite Metamorphic facies

  37. Metamorphic Facies *

  38. Plate Setting and Metamorphism The end!

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