GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY
Lecture 21
Textures of Metamorphic Rocks
March 25, 2009
TEXTURE VS. STRUCTURES
Texture – Small-scale features that PENETRATE the entire rock and can be view on a thin section scale
Structures – Large-scale (hand-sample and larger) feature (folds, kink bands, gneissic banding)
Textures of metamorphic rocks reflect the combined processes of: • detachment and diffusion of matter IN THE SOLID STATE
(though often in the presence of a fluid phase)• crystal nucleation• crystal growth• deformation (strain development)• strain recovery-recrystallization
-blastic – of metamorphic origin (e.g. porphyroblastic, poikiloblastic)
relict – belonging to the original rock (e.g., relic bedding)
MECHANISMS OF DEFORMATION
1. Cataclasis Flow – Low T deformation by mechanical fragmentation, sliding, and rotation; Produces cataclasite, fault breccia, fault gouge
2. Pressure Solution – Dissolution at grain boundaries and reprecipitation in voids; requires the presence of a fluid.
MECHANISMS OF DEFORMATION
3. Intracrystalline Deformation - bending (elastic - recoverable)- crystal lattice defects (permanent dislocations) manifest as undulose extinction and deformation twinning
Undulose Extinction in Quartz Deformation Twinning in Calcite
MECHANISMS OF DEFORMATION
4. Strain Recovery – stored strain energy (by accumulated defects) can decrease the stability of a mineral; this energy can be lowered by migration of defects which occurs at elevated T
Migration of defects to a dislocation wall creates two of more subgrains of lower internal strain.
Subgrain domains of strain-recovered quartz
MECHANISMS OF DEFORMATION
5. Recrystallization – stored strain energy can also be released by the migration of grain boundaries, the rotation of subgrains, or the reduction of grain boundary area; all are best accomplished at high T in the presence of a fluid
Figure 23-6. Recrystallization by (a) grain-boundary migration (including nucleation) and (b) subgrain rotation. From Passchier and Trouw (1996) Microtectonics. Springer-Verlag. Berlin.
HigherStrained Grain
LowerStrained Grain
Grain boundary area reduction occurs as minerals strive to minimize their surface-area-to-volume ratio; this is often accomplished by coarsening and developing straighter boundaries.
Recrystallization of Quartz by grain boundary migration
TEXTURES FORMED BY CONTACT METAMORPHISM
Typically shallow pluton aureoles (low-P)
Crystallization/recrystallization is near-static Monomineralic with low
differential surface energy granoblastic polygonal texture
Larger differential surface energy decussate texture
Homogeneous textures (hornfels, granofels)
Relict textures are common
DEVELOPMENT OF DIHEDRAL ANGLESGrain boundaries of like minerals (A-A) have higher
surface energies that boundaries between different minerals (A-B). So as minerals recrystallize, the A-B boundaries will lengthen relative to A-A boundaries and thus decrease the dihedral angle - θ
Pl + Cpxnote low θ at Pl-Pl-Cpx jcts and 120 jcts at Pl-Pl-Pl jcts
Qtz + Micasurface E of (001) face is much lower than other faces – so maximizes size of that face. This restricts the abilitiy of quartz to coarsen.
TEXTURE DEVELOPMENT DURING PROGRESSIVE THERMAL
METAMORPHISM
Progressive thermal metamorphism of a diabase (coarse basalt). From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.
Altered Basalt
Mafic Hornfels
BASALT
TEXTURE DEVELOPMENT DURING PROGRESSIVE THERMAL
METAMORPHISM
Progressive thermal metamorphism of slate. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.
SLATE
PORPHYROBLASTIC TO SKELTAL TEXTURE
Increasing Crystallization Rate
Inclusions in poikiloblasts may form by: - being inert phases not used by the growing crystal - being a co-product of the poikilitic crystal-f orming reaction - a reactant that was not completely consumed
Porphyroblasts form by low rates of nucleation which then requires diffusion over large areas. This results in large, widely spaced crystals.
THE 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
Differences in development of crystal form among some metamorphic minerals. From Best (1982). Igneous and Metamorphic Petrology. W. H. Freeman. San Francisco.
TEXTURES FORMED IN HIGHLY STRAINED ROCKS
PROGRESSIVE DEVELOPMENT OF MYLONITE
FROM A GRANITE
Figure 23-15. Progressive mylonitization of a granite. From Shelton (1966). Geology Illustrated. Photos courtesy © John Shelton.
SHEAR SENSE INDICATORS________________________________________________________________________________________________
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Shear Plane Cleavage
Oblique Foliation
Shear Band (S-C) Cleavage in
micaceous rock
Acute Angle gives Sense of
Shear
Oblique Foliation in granular rock
Dextral Shear =Right Lateral
Sinistral Shear =Left Lateral
SENSE OF SHEAR INDICATORSMANTLED PORPHYROBLASTS
Rigid Porphyroblasts of K-feldspar in ductile mica+qtz matrix
Not useful shear indicators
OTHER SENSE OF
SHEAR INDICATORS
Concentration of Mica due to dissolution of porphyroblast
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