Metamorphic Textures

Recrystallization: Minimization of Interfacial Free Energy by Minimizing Surface Area Crack Sandstone Quartzite Recrystallization Crack Crack Cement

Recrystallization of a Bi-minerallic Rock a) interfacial free energy of AA = AB; dihedral angle (q) is thus 120o b) interfacial free energy of AA > AB; dihedral angle decreases to decrease surface area of AA a) A-A GBE > A-B -> larger dihedral angle b) A-A GBE < A-B -> smaller dihedral c) Sketch of plag-cpx in which cpx has lower angle 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 Euhedral Titanite, rutile, pyrite, spinel Garnet, sillimanite, staurolite, tourmaline Epidote, magnetite, ilmenite Andalusite, pyroxene, amphibole Mica, chlorite, dolomite, kyanite Calcite, vesuvianite, scapolite Feldspar, quartz, cordierite Least Euhedral Crystalloblastic Series

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

Deformation by Dislocation

Pressure Solution Differential stress produces serated grain boundaries

s1 Foliation Development S1 S2 S1 Schistocity S1 flattened around garnet porphyroblasts S2 S1 Crenulation cleavage S2 developed by folding S1

Pressure Shadows Chlorite in shadow around garnet Quartz in shadow around staurolite

Pressure Fringes Staurolite Pyrite Sillimanite Muscovite

Metamorphic Mineral Growth Relative to Deformation

Pre-tectonic Textures Undulose extinction Pressure shadows Crenulation cleavage Pressure fringes Micro-boudinage Deformation twins

Syn-tectonic Texture

Snowball Garnet with Pressure Shadows

Post-tectonic Textures Post-tectonic Textures Randomly oriented crystals Helicitic folds Polygonal arc Pseudomorph Chiastolite Overgrowth on snowball

Polymetamorphism Layers may be bedding (So) or metamorphic differentiation (S1) continuous or lensoidal? Orientation of micas is S1 Good spiral Si in garnets and “Si = Se” Layering slightly compressed about garnets Good granoblastic polygonal texture in quartz layers where no mica: xlliz outlasted defm

Stages in Polymetamorphism (S0, S1)

Stages in Polymetamorphism (S2)

Stages in Polymetamorphism (S3)

Corona Textures