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METAMORPHISM

METAMORPHISM. Fig. Ta 7.1. Definition of Metamorphism. The alteration of a pre-existing igneous, sedimentary, or metamorphic rock (called the “parent rock” or “ protolith ”) by the application of heat and/or pressure. Complete melting does not occur. Metamorphism depends on:. Parent Rock.

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METAMORPHISM

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  1. METAMORPHISM

  2. Fig. Ta 7.1

  3. Definition of Metamorphism The alteration of a pre-existing igneous, sedimentary, or metamorphic rock (called the “parent rock” or “protolith”) by the application of heat and/or pressure. Complete melting does not occur.

  4. Metamorphism depends on: Parent Rock Geothermal Gradient Equilibrium Exchange Change in P-T-X Confining/lithostatic pressure Differential Stress: Shearing & Compression TIME

  5. Confining Pressure

  6. Compressive Stress Shearing

  7. Metamorphosed Conglomerate

  8. Effects of Metamorphism Mineralogical: - change minerals to reflect new P-T (equilibrium) conditions. Recrystallization: change of existing crystal size and shape to form layers (e.g., shale to schist), interlocking crystals (e.g., sandstone to quartzite). Recombination: Recombination of elements in pre-existing minerals to form stable new ones. Addition/subtraction of ions common when fluid is present.

  9. Effects of Metamorphism Textural: Re-orientation of mineral grains, especially platy (micaceous) grains in response to differential stress. Non-foliated: random orientation – either no platy minerals or no directed stress (isotropic); Granoblastic (equigranular or isotropic) or Porphyroblastic (metamorphic porphyritic texture). Foliated: fabric developed through directed pressure and/or shearing.

  10. Development of Foliation

  11. Non-Foliated

  12. Foliated

  13. Foliation Oriented Minerals: platy or needle-like minerals growing in a plane either re-orientation or existing or growing of new. Gives cleavage or planes of weakness. Slate: oriented clay minerals allows the rock to be easily cleaved. Phyllite: growth of microscopic micas to give strong foliation – rock is “shiny”. Schist: strongly foliated with visible micas and feldspars. Gneiss: strongly banded rock with dark bands of micas & pyroxenes, light bands of feldspars & quartz. Migmatite: strongly veined rock as it has partially melted.

  14. Table 07.01

  15. Typical Metamorphic Minerals Chlorite: Hydrous Fe-Mg-Al silicate Talc: Hydrous Mg-silicate Serpentine: Hydrous Mg silicate Graphite: Carbon Garnet: Ca-Mg-Fe-Al silicate Staurolite: Fe-Al silicate Also: Quartz, muscovite, biotite, feldspar, amphibole, calcite.

  16. Metamorphic Grade Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade. Prograde: slate-phyllite-schist-gneiss-migmatite (melting). Prograde metamorphic Reaction: CaCO3 + SiO2 CaSiO3 + CO2 Calcite Quartz Wollastonite Fluid

  17. Classification Mineralogical and textural if foliated: “garnet-mica schist” “quartz-feldspar gneiss” If non-foliated, named on compositional basis (and often color): “(white) quartzite” or “(pink) marble”

  18. Types of Metamorphism: Contact Thermal, local, around intrusions. Size of aureole depends on: Size of intrusion Heat (composition) Fluid content of magma Fluid content of country rock Country rock type

  19. Types of Metamorphism: Contact Produces non-foliated, granoblastic rocks: Hornfels (if clastic - shale) Quartzite (if sandstone) Marble (if carbonate)

  20. Types of Metamorphism: Regional Also known as dynamothermal- produces both foliated and non-foliated metamorphic rocks. Associated with mountain belts - affects very large areas. Heat & directed pressure on rocks buried deep within the Earth - Prograde or Retrograde. Prograde metamorphic reactions liberate a fluid. Retrograde is difficult – pore spaces decrease during prograde so getting fluid back in is not easy.

  21. Regional Metamorphism

  22. Types of Metamorphism: Shock Meteorite impact – sudden and intense deformation. See www.meteorcrater.com

  23. Types of Metamorphism: Shock High-pressure polymorph of quartz – coesite – can form. Impact melt can form. Lots of rock fragmentation & mineral deformation. Moon – no atmosphere so lots of meteorite impacts (micro and macro!). Produces regolith, rock flour, impact melt, breccias.

  24. Water in Metamorphism Provides transport mechanism and can promote reactions. Hydrothermal metamorphism: hot water streams add/remove ions. May promote ore formation.

  25. Water in Metamorphism Metasomatism: addition of ions from external source.

  26. Water in Metamorphism Metamorphic aureole is greater around granitic plutons than around gabbroic plutons, even though the magma temperature is lower.

  27. Ore-bearing veins in a mine

  28. Metamorphic Grade: Degree of parent rock alteration, mostly dependent on increasing temperature for increasing grade Prograde: SLATE -phyllite-schist-gneiss-migmatite (melting). SLATE: oriented clay minerals allows the rock to be easily cleaved.

  29. Foliation can be deformed

  30. Prograde: slate-PHYLLITE-schist-gneiss-migmatite (melting). PHYLLITE: growth of microscopic micas to give strong foliation – rock is “shiny”.

  31. SCHIST: strongly foliated with visible micas & feldspars. Prograde: slate-phyllite-SCHIST-gneiss-migmatite.

  32. Garnet Schist

  33. Fig. 07.14 GNEISS: strongly banded rock with dark bands of micas & pyroxenes and light bands of feldspars and quartz. Prograde: slate-phyllite-schist-GNEISS-migmatite.

  34. Gneissose Banding

  35. Prograde: slate-phyllite-schist-gneiss-MIGMATITE. MIGMATITE: strongly veined rock as it has partially melted.

  36. 0065

  37. Metamorphic Facies: Rocks having broadly similar mineral assemblages = same metamorphic facies – analogous to climatic zones. The metamorphic facies is determined by the specific mineral assemblage present. The facies is defined as a mineral assemblage that may only occur over a relatively narrow range of P-T conditions. Formed under broadly similar P-T conditions. Mineralogy dependent upon protolith composition.

  38. Metamorphic Facies:

  39. Plate Tectonics and Metamorphism

  40. Plate Tectonics and Metamorphism Gravitational collapse & spreading – central part of mountain becomes too high & collapses. Rock forced downward. Foliation developed parallel to collapse & spreading.

  41. Index Minerals Form over a restricted range of pressure & temperatures (determined in the laboratory). When found in rocks, the P-T of formation can be reasonably estimated.

  42. Index Minerals Kyanite, Andalusite, Sillimanite: all Al2SiO5, different crystal structures - polymorphs. Andalusite - low P & T; Kyanite - high P; Sillimanite - high T.

  43. 0062

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