Transcript Slide 1
Metamorphic Textures
Recrystallization: Minimization of Interfacial Free Energy by Minimizing Surface Area Sandstone Quartzite Cement Crack Crack
Recrystallization of a Bi-minerallic Rock a) interfacial free energy of AA = AB; dihedral angle ( q ) is thus 120 o b) interfacial free energy of AA > AB; dihedral angle decreases to decrease surface area of AA Plagioclase Cpx
Recrystallization of Quartz-Mica Rock Interfacial free energy of Quartz-Mica >> Quartz-Quartz Interfacial angle increases from 120 o to 180 o
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.
Granoblastic/Decussate Textures (Characteristic of Contact or Thermal Metamorphism)
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
Crystalloblastic Series 1. Titanite, rutile, pyrite, spinel 2. Garnet, sillimanite, staurolite, tourmaline 3. Epidote, magnetite, ilmenite 4. Andalusite, pyroxene, amphibole 5. Mica, chlorite, dolomite, kyanite 6. Calcite, vesuvianite, scapolite 7. Feldspar, quartz, cordierite
Most Euhedral Least Euhedral
Textural Effects of Differential Stress Stressed grains store strain energy released during metamorphism by clearing defects and dislocations (polygonization) Undulose extinction Subgrain development
Defects Dislocations and Deformation Defects Dislocation
Deformation by Dislocation
Pressure Solution Differential stress produces serated grain boundaries
S 1 Foliation Development s 1 Schistocity S 1 flattened around garnet porphyroblasts S 2 S 1 Crenulation cleavage S 2 developed by folding S 1
Pressure Shadows Chlorite in shadow around garnet Quartz in shadow around staurolite
Pressure Fringes Pyrite Muscovite Staurolite Sillimanite
Metamorphic Mineral Growth Relative to Deformation
Pre-tectonic Textures Undulose extinction Pressure shadows Pressure fringes Crenulation cleavage Micro-boudinage Deformation twins
Syn-tectonic Texture
Snowball Garnet with Pressure Shadows
Helicitic folds Randomly oriented crystals Polygonal arc Chiastolite Overgrowth on snowball Pseudomorph
Polymetamorphism
Stages in Polymetamorphism (S 0 , S 1 )
Stages in Polymetamorphism (S 2 )
Stages in Polymetamorphism (S 3 )
Corona Textures