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Part 2

Metamorphic Rocks. Part 2. OBJECTIVES. Metamorphic Grade or Facies Metamorphic Rock Examples. METAMORPHIC GRADE “FACES”. Metamorphic grade or Facies. A group of minerals that form in a particular P-T environment Can be used as Index Minerals to deduce T-P conditions of formation.

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Part 2

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  1. Metamorphic Rocks Part 2

  2. OBJECTIVES • Metamorphic Grade or Facies • Metamorphic Rock Examples

  3. METAMORPHIC GRADE“FACES”

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

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

  6. 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

  7. Metamorphic Facies

  8. COMMON METAMORPHIC ROCK EXAMPLES

  9. Metamorphic Rocks • Nonfoliated • Quartzite • Marble • Foliated • Slate • Phyllite • Schist • Gneiss • Migmatites

  10. NONFOLLIATED“Granoblastic”

  11. Nonfoliated rocks Quartzite • Formed from a parent rock of quartz-rich sandstone • Quartz grains are fused together • Forms in intermediate T, P conditions

  12. 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

  13. Nonfoliated rocks Marble • Coarse, crystalline • Parent rock usually limestone • Composed of calcite crystals • Fabric can be random or oriented

  14. Marble (nonfoliated)

  15. FOLLIATED

  16. Common metamorphic rocks • Foliated rocks • Type formed depends on Metamorphic Grade • Grade: Amount of heat and pressure • High Grade: High heat and pressure • Grade depends on depth

  17. Mudstones are sediments, can be squashed by burial and/or in continent-continent collisions Change in metamorphic grade with depth Increasing Directed Pressure and increasing Temps

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

  19. Example of slate

  20. 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

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

  22. 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)

  23. A mica garnet schist

  24. Foliated rocks Gneiss • Medium- to coarse-grained • Banded appearance • High-grade metamorphism • Composed of light-colored feldspar layers with bands of dark mafic minerals

  25. Gneiss displays bands of light and dark minerals

  26. Flattened Pebble Conglomerate = flattening

  27. Foliated rocks Migmatites • Minerals with the lowest melting points (Qtz, Feldspar) melt then recrystallize, • Creates separate bands of Metamorphic and Igneous rock.

  28. Migmatites

  29. THE END

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