Topic 3 ( metamorphic process )

• Cataclasis

  Low-T, brittle grain-fracturing processes that reduces grain size by mechanical grinding, rotation, and crushing. 

  This process produces small, angular grains that infill the larger grains, resulting to a Mortar Texture. 

  Occur in fault zones
• Mylonitization

  Ductile grain reduction processes that produces oriented small-sized grains. This process combines grain fracturing, plastic bending, and internal deformation, resulting to visible foliations. 

  Occur in high-T, high strain shear zones of the lower crust and upper mantle.
• Diffusion

  Atoms or molecules migrate to a new location in the rock body. 

  Solid-state diffusion is facilitated by crystal dislocation, and is important in high-T metamorphic reactions.

  Most efficient in gaseous state, and variable in liquid state
• Pressure Solution

  Dissolution of solid grains under high compressive stress, altering the crystal lattice structure and producing an aqueous phase at high stress sites. One grain impinges another grain, producing a sutured contact. Insoluble minerals accumulate and form a Stylolite.
• Recrystallization - Existing minerals are transformed under higher P-T without significantly changing the composition. The state of stress within the crystal lattice is reduced due to higher P-T, either by diffusion or migration. 
• Neocrystallization

  Nucleation and growth of new minerals as pre-existing minerals become unstable due to changes in P-T. These new minerals are larger than the minerals in the matrix; termed as Porphyroblast. 
• Porphyroclasts -

  are the original rocks that are coherent or strong enough to resist deformation while the rest of the rock mass formed the finer groundmass. They are OLDER than the groundmass.
• Differentiation

  Segregation of minerals from an initially homogenous rock due to different physical or chemical characteristics. This forms the distinct metamorphic banding of gneiss and migmatites, as well as enhance the growth and elongation of porphyroblasts.