Metamorphic Textures

Crack

Recrystallization: Minimization of Interfacial Free Energy by Minimizing Surface Area

Cement

Sandstone Quartzite

Crack Crack

Recrystallization

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

Recrystallization of Quartz-Mica Rock

Interfacial free energy of Quartz-Mica >> Quartz-Quartz Interfacial angle increases from 120o to 180o

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

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

Textural Effects of Differential StressStressed 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

Foliation Development

S1

S1

S2

1

Schistocity S1 flattened around garnet porphyroblasts

Crenulation cleavage S2 developed by folding S1

Pressure Shadows

Chlorite in shadow around garnet

Quartz in shadow around staurolite

Pressure Fringes

Staurolite

SillimanitePyrite

Muscovite

Metamorphic Mineral Growth Relative to Deformation

Pre-tectonic Textures

Undulose extinctionPressure shadows

Pressure fringes Crenulation cleavage

Micro-boudinage Deformation twins

Syn-tectonic Texture

Snowball Garnet with Pressure Shadows

Post-tectonic Textures

Helicitic folds

Post-tectonic Textures

Randomly oriented crystals Polygonal arc

Chiastolite Overgrowth on snowball Pseudomorph

Polymetamorphism

Stages in Polymetamorphism (S0, S1)

Stages in Polymetamorphism (S2)

Stages in Polymetamorphism (S3)

Corona Textures