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Metamorphic Textures
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Crack
Recrystallization: Minimization of Interfacial Free Energy by Minimizing Surface Area
Cement
Sandstone Quartzite
Crack Crack
Recrystallization
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Recrystallization of a Bi-minerallic Rock
a) interfacial free energy of AA = AB; dihedral angle () is thus 120o
b) interfacial free energy of AA > AB; dihedral angle decreases to decrease surface area of AA
Plagioclase
Cpx
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Recrystallization of Quartz-Mica Rock
Interfacial free energy of Quartz-Mica >> Quartz-Quartz Interfacial angle increases from 120o to 180o
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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.
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Granoblastic/Decussate Textures (Characteristic of Contact or Thermal Metamorphism)
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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
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Crystalloblastic Series
Most Euhedral1. Titanite, rutile, pyrite, spinel2. Garnet, sillimanite, staurolite, tourmaline3. Epidote, magnetite, ilmenite4. Andalusite, pyroxene, amphibole5. Mica, chlorite, dolomite, kyanite6. Calcite, vesuvianite, scapolite7. Feldspar, quartz, cordierite Least Euhedral
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Textural Effects of Differential StressStressed grains store strain energy released during metamorphism by clearing defects and dislocations (polygonization)
Undulose extinction
Subgrain development
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Defects Dislocations and Deformation
Defects
Dislocation
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Deformation by Dislocation
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Pressure Solution
Differential stress produces serated grain boundaries
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Foliation Development
S1
S1
S2
1
Schistocity S1 flattened around garnet porphyroblasts
Crenulation cleavage S2 developed by folding S1
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Pressure Shadows
Chlorite in shadow around garnet
Quartz in shadow around staurolite
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Pressure Fringes
Staurolite
SillimanitePyrite
Muscovite
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Metamorphic Mineral Growth Relative to Deformation
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Pre-tectonic Textures
Undulose extinctionPressure shadows
Pressure fringes Crenulation cleavage
Micro-boudinage Deformation twins
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Syn-tectonic Texture
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Snowball Garnet with Pressure Shadows
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Post-tectonic Textures
Helicitic folds
Post-tectonic Textures
Randomly oriented crystals Polygonal arc
Chiastolite Overgrowth on snowball Pseudomorph
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Polymetamorphism
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Stages in Polymetamorphism (S0, S1)
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Stages in Polymetamorphism (S2)
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Stages in Polymetamorphism (S3)
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Corona Textures