1. Metamorphic Textures

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

3. 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

4. Recrystallization of Quartz-Mica Rock
Interfacial free energy of Quartz-Mica >> Quartz-Quartz
Interfacial angle increases from 120o to 180o

5. 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.

6. Granoblastic/Decussate Textures (Characteristic of Contact or Thermal Metamorphism)

7. 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

8. 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

9. Textural Effects of Differential Stress
Stressed grains store strain energy released during metamorphism by clearing defects and dislocations (polygonization)
Undulose extinction
Subgrain development

10. Defects Dislocations and Deformation

11. Deformation by Dislocation

12. Pressure Solution
Differential stress produces serated grain boundaries

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

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

15. Pressure Fringes
Staurolite
Pyrite
Sillimanite
Muscovite

16. Metamorphic Mineral Growth Relative to Deformation

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

18. Syn-tectonic Texture

19. Snowball Garnet with Pressure Shadows

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

21. 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

22. Stages in Polymetamorphism (S0, S1)

23. Stages in Polymetamorphism (S2)

24. Stages in Polymetamorphism (S3)

25. Corona Textures