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