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Metamorphic Rocks Chapter 7

Metamorphic Rocks Chapter 7. Metamorphism means "changed form". . Uplifted and metamorphosed rocks make up the Circo de Pineta in France's Pyrenees Mountains. Metamorphism. The transition of one rock into another by temperatures and/or pressures unlike those in which it formed

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Metamorphic Rocks Chapter 7

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  1. Metamorphic RocksChapter 7 Metamorphism means "changed form".

  2. Uplifted and metamorphosed rocks make up the Circo de Pineta in France's Pyrenees Mountains.

  3. Metamorphism • The transition of one rock into another by temperatures and/or pressures unlike those in which it formed • Metamorphic rocks are produced from • Igneous rocks • Sedimentary rocks • Other metamorphic rocks

  4. Metamorphic Environments Agents of metamorphism • Heat • Pressure • Chemically active fluids Changes are both textural and mineralogical

  5. Agents of Metamorphism Heat • Provides energy to drive chemical changes • Comes from: 1. Igneous bodies 2. Increasing depth (geothermal gradient, 20o –30o C/km)

  6. Geothermal gradient and its role in metamorphism Notice how the geothermal gradient is lowered by the subduction of comparatively cool oceanic lithosphere. By contrast, thermal heating is evident where magma intrudes the upper crust.

  7. Pressure and Stress • Confining pressure – increases with depth due to load from rocks above • Differential stress (or directed stress) – due to tectonic forces during mountain building

  8. Confining pressure Pressure as metamorphic agent Directed pressure

  9. Chemical Metamorphism • Chemically active fluids – ions in solution Hydrothermal metamorphism occurs when hot, chemically active, mineral laden waters interact with a surrounding preexisting rock (called the country rock). Most hydrothermal metamorphism takes place at low pressures and relatively low temperature.

  10. Hydrothermal metamorphism along a mid-ocean ridge

  11. How metamorphism alters rocks Textural Changes • Foliation • Slaty Cleavage • Schistosity • Gneissic texture • Non-Foliated

  12. Effects of directed stress

  13. Foliation of Metamorphic Rock

  14. Foliation Processes

  15. Metaconglomerate

  16. Relationship between cleavage and bedding planes

  17. Slaty Cleavage • The parallel mineral alignment allows the rock to split easily into the flat plates visible in the photo

  18. How metamorphism alters rocks Mineralogic Changes • Clays alter to micas (muscovite, biotite, chlorite) • New minerals may grow during metamorphism (CaCO3 + SiO2 = CaSiO3 + CO2Wollastonite) • Hydrothermal solutions may add new elements ore deposits

  19. Common Metamorphic Rocks Foliated • Slate – Fine grained, composed of minute micas • Phyllite – larger micas, glossy sheen • Schist - >50% platy (muscovite, biotite) or elongate (amphibole) minerals • Gneiss – compositionally banded

  20. Slate

  21. Phyllite

  22. Schist

  23. Garnet-mica Schist

  24. Gneiss

  25. Gneissic foliation close-up

  26. Folded gneiss outcrop

  27. Common Metamorphic Rocks Non-foliated • Quartzite – metamorphosed sandstones, also called metaquartzites. • Marble – metamorphosed limestones and dolomites. • Hornfels – fine-grained (shales and siltstones) metamorphosed by contact metamorphism

  28. Marble

  29. Quartzite

  30. Contact Metamorphism • occurs when rocks undergo metamorphism because they come in contact with a heat source (usually a magma body).

  31. Contact Metamorphism

  32. Contact metamorphism • along a narrow (approx. 1 meter wide) diabase dike in the Deep River Basin of North Carolina. Diabase weathers tan. Contact metamorphic aureole rocks (hornfels) are gray. Host rocks are red siltstones

  33. Fault Zone Metamorphism • Fault breccias develop shallow in the faults where the rock is “ground up” • Mylonites occur deep in the fault zones where temperature and pressure cause the rocks to deform in a ductile manner.

  34. Fault Breccia

  35. Mylonite

  36. Classification of metamorphic rocks

  37. Regional Metamorphism • Regional metamorphism occurs where rocks are squeezed between converging plates during mountain building • Zones of metamorphic intensity may exist in areas of regional metamorphism • Index minerals are used to determine the intensity or grade of metamorphism • Index minerals include chlorite, muscovite, biotite, garnet, staurolite, and sillimanite

  38. Regional metamorphism

  39. Progressive regional metamorphism Increasing metamorphism Low grade High grade

  40. Typical transition in progressive metamorphism of shale

  41. Zones of metamorphic intensity

  42. Metamorphic environments(Plate Tectonics model)

  43. Crenulation Cleavage • Develops when rocks undergo more than one episode of regional metamorphism • Cleavage from first episode is folded and a second cleavage develops

  44. Crenulation Cleavage

  45. Migmatite • Temperatures are reached where some minerals begin to melt (Qtz., K-spar) • Transitional between metamorphic and igneous rocks.

  46. Migmatite – Near Great Falls, MD

  47. Folded metamorphic rocks near Black Pond, VA

  48. Deformed Rocks - CT

  49. Deformed Rocks – Death Valley

  50. Global occurrences of metamorphic rocks

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