Metamorphic Textures

See http://plate-tectonic.narod.ru/metpetrographylinks.html
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html

A. Definitions

-Texture: penetrative, small-scale features
-Structure: large-scale

B. Deformation

-Cataclastic flow = physical breakdown
-Pressure solution: mineral grains dissolve at pressure points; grow where pressure is low
-Crystal lattice deformation: can lead to undulose extinction
-Recovery = release of strain, lowering of energy

II. Contact metamorphism

A. Changes result from increase in T, metasomatism (action of fluid).

-Lack of P effects => no preferred orientation.
-Features of protolith may be preserved = relict textures

B. Textural terms

-Granoblastic texture: grains boundaries are straight, grains meet at 120o angles
-Porphyroblast = like a phenocryst, but metamorphic: may be euhedral/idioblastic; subhedral/hypidioblastic ; anhedral/xenoblastic
-Poikiloblastic = porphyroblasts with many inclusions
-Depletion haloes: growth of porphyroblast uses up some element(s) in a zone around crystal; spotted slates are common.

C. Increasing grade = higher temp. Leads to:

-Fewer relict textures, more extensive recrystallization
-Larger grains
-Straighter boundaries, less strain

III. High-strain metamorphism

A. Increasing strain

-Brittle deformation = cataclastic texture = broken fragments
-Ductile deformation = mylonite
-Incipient melting from shear heating = pseudotachylite

B. Sense of shear

-Direction is easy, || fault zone, but which side moved up (or left)?
-Various ways to tell: oblique foliation; shear bands; "tails" on porphyroclasts

IV. Regional metamorphism
A. Most common occurrence is associated with orogeny

-Metamorphic cycle = burial, heating, uplift, erosion - tectonite

B. Foliation

-Cleavage = very fine planar fabric
-Schistosity = coarser (platy grains large enough to see)
-Gneissose = very coarse or layered fabric
-Crenulation cleavage = combination - early cleavage or schistosity is later folded
-Formation of foliation: rotation; growth of new minerals aligned with pressure; recrystallization

C. Rotation

-S = planar feature, L = lineation, D = deformation, M = metamorphism: subscripts give order; S0, L0 = relict textures (bedding, igneous layering)

-Si = internal foliation in a poikiloblast