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Metamorphic Rocks

Metamorphic Rocks. Metamorphic Rocks. Recrystallization in the solid state Caused by changes in T, P or fluids New environment = new minerals Growing minerals create a new texture. Sources of metamorphic change. Metamorphism. Recrystallization

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Metamorphic Rocks

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  1. Metamorphic Rocks

  2. Metamorphic Rocks • Recrystallization in the solid state • Caused by changes in T, P or fluids • New environment = new minerals • Growing minerals create a new texture

  3. Sources of metamorphic change

  4. Metamorphism • Recrystallization • T, P or change in pore fluids initiate change in the mineral assemblage • Reaction occur entirely in the solid state Shale Schist

  5. Textural changes occur during metamorphism • New minerals grow during metamorphism • Precursor rock textures are modified

  6. Protolith The parent rock subjected to Metamorphism -can be any rock type: igneous, sedimentary, or metamorphic Shale Schist

  7. Protolith

  8. Origin of Metamorphic Rocks • Metamorphism Temperature exceeds 200OC Pressure exceeds 300 M Pa (approx. 10,000 ft deep) • Metamorphism ends when melting begins Melting begins at ~700oC

  9. Stability ranges for Al2 SiO5 minerals

  10. Source of heat • Proximity to igneous intrusions • Contact metamorphism • Different metamorphic grade ring the intrusion • Depth of burial • 15-30oC increase per km • Geothermal Gradient

  11. Zone of Contact Metamorphism Magma Body -source of heat

  12. Pressure and Stress • Lithostatic Pressure-pressure increases with depth due to the weight of the overlying rocks. • -equal in all directions: uniform stress • -a cubic foot of granite weighs 167.9 pounds.

  13. Tectonic Pressure/Stress -directional/non-uniform -referred to as deviatoric stress

  14. Folded Metamorphic Rocks -subjected to non-uniform deviatoric stress

  15. T & P determine degree of metamorphism Low-grade metamorphism- 200 to 350 OC Intermediate-grade metamorphism-350 to 550 OC High-grade metamorphism - very high temperatures, above 550OC

  16. Progressive metamorphism of shale

  17. Fig. 6.5. Stability ranges for Al2O5 minerals

  18. Index Minerals

  19. Metamorphic Rocks • Classified by texture and composition • Texture is divided into foliated & nonfoliated rocks • Mineral composition qualifies textural name

  20. Foliated Nonfoliated textures

  21. Foliation: -preferred orientation or location of minerals

  22. Slate

  23. Phyllite • Similar to slate • More Recrystallization • Crystals are larger • Common Protolith: • shales

  24. Schist • Strongly foliated rock • Medium to coarse grained -dominated by platy minerals • Protoliths: multiple??

  25. [Garnet] Schist w/porphyroblasts

  26. Gneiss • Coarse grained granular rock • Foliation: alternating bands of light & dark minerals • Quartz, feldspar, amphibole, • biotite • Protoliths: ??

  27. Nonfoliated Rocks • Marble • Interlocking, coarse grained calcite • Recrystallization of limestone or dolostone • Sedimentary features destroyed

  28. Quartzite • Metamorphism of quartz sandstone • Entire rock is recrystallized

  29. Amphibolite [pillow basalt] • Amphibolites & Eclogites • Metamorphism of mafic rocks • Mineral assemblage gives green color Eclogite from Norway

  30. Regional Metamorphic Zones • Index minerals • A mineral that forms within a specific,often narrow range of conditions • Identifies a specific grade of metamorphism • Allows further subdivision of rock types

  31. Metamorphic Index Minerals

  32. Protolith controls final mineral assemblage • Different mineral assemblages form at a given P & T for various parent rocks • Metamorphic facies describe a group of minerals formed under similar conditions • May be found in different rocks

  33. Regional Metamorphic Gradients

  34. Metamorphic facies

  35. Metamorphic Rocks & Tectonics • Most metamorphic rocks develop due to plate collisions • Processes are not observable in the field • Conditions at various plate boundaries define possible P & T range • Parent rocks present control mineral assemblages

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