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Metamorphic Textures

Metamorphic Textures. Recrystallization: Minimization of Interfacial Free Energy by Minimizing Surface Area. Crack. Sandstone. Quartzite. Recrystallization. Crack. Crack. Cement. Recrystallization of a Bi-minerallic Rock.

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Metamorphic Textures

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  1. Metamorphic Textures

  2. Recrystallization: Minimization of Interfacial Free Energy by Minimizing Surface Area Crack Sandstone Quartzite Recrystallization Crack Crack Cement

  3. Recrystallization of a Bi-minerallic Rock • a) interfacial free energy of AA = AB; dihedral angle (q) is thus 120o • b) interfacial free energy of AA > AB; dihedral angle decreases to decrease surface area of AA Plagioclase Cpx

  4. Recrystallization of Quartz-Mica Rock • Interfacial free energy of Quartz-Mica >> Quartz-Quartz • Interfacial angle increases from 120o to 180o

  5. Decussate Texture Aggregate of mica grains in which most interfaces are parallel to {001}. This occurs because the surface energy of the {001} face is substantially lower than that of other faces, e.g. {110}. The texture may also be developed by other minerals.

  6. Granoblastic/Decussate Textures (Characteristic of Contact or Thermal Metamorphism)

  7. Porphyroblast development • Nucleation of new crystals requires excess free energy • If the nucleation energy is high, growth is favoured and porphyroblasts form • Porphyroblasts will be euhedral if their faces have high energy

  8. Crystalloblastic Series Most Euhedral • Titanite, rutile, pyrite, spinel • Garnet, sillimanite, staurolite, tourmaline • Epidote, magnetite, ilmenite • Andalusite, pyroxene, amphibole • Mica, chlorite, dolomite, kyanite • Calcite, vesuvianite, scapolite • Feldspar, quartz, cordierite Least Euhedral

  9. Textural Effects of Differential Stress Stressed grains store strain energy released during metamorphism by clearing defects and dislocations (polygonization) Undulose extinction Subgrain development

  10. Defects Dislocations and Deformation Defects Dislocation

  11. Deformation by Dislocation

  12. Pressure Solution Differential stress produces serated grain boundaries

  13. Foliation Development s1 S1 Schistocity S1 flattened around garnet porphyroblasts S2 S1 Crenulation cleavage S2 developed by folding S1

  14. Pressure Shadows Chlorite in shadow around garnet Quartz in shadow around staurolite

  15. Pressure Fringes Staurolite Pyrite Sillimanite Muscovite

  16. Metamorphic Mineral Growth Relative to Deformation

  17. Pre-tectonic Textures Undulose extinction Pressure shadows Crenulation cleavage Pressure fringes Micro-boudinage Deformation twins

  18. Syn-tectonic Texture

  19. Snowball Garnet with Pressure Shadows

  20. Post-tectonic Textures Post-tectonic Textures Randomly oriented crystals Polygonal arc Helicitic folds Pseudomorph Overgrowth on snowball Chiastolite

  21. Polymetamorphism

  22. Stages in Polymetamorphism (S0, S1)

  23. Stages in Polymetamorphism (S2)

  24. Stages in Polymetamorphism (S3)

  25. Corona Textures

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