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