Small-scale features that occur in virtually all of the rock body at the microscopic scale
The term ‘Fabric’ is also used synonymously.
As the rock-forming processes differ among types of metamorphism, the resulting texture is also different.
Textures retained from parent rock are called “Relict” texture
Suffix ‘blust’ is used for textures that are solely metamorphic in origin
Textures of Contact Metamorphism
Recrystallization is the primary process without any deformation.
The resulting rock is granular, non-foliated
Depending on grain size, shape, grain to grain contact, it can be named differently.
As metamorphic grade (mostly temperature) increases, recrystallization becomes more dominant.
Following are the most pronounced textural effects of increasing metamorphic grade in contact aureoles:
Fewer relict textures and a more fully recrystallized metamorphic texture
Increased grain size
Straighter grain boundaries and less evidence of strain
Textures of Contact Metamorphism
1) Granoblastic Polygonal
Grains are more or less of the same size and equidimensional.
In thin section appears as a polygonal mosaic.
Grains meet at the triple junction with approximately 120-degree angles between them.
Common in Quartzites and Marble
It can also be polymineralic.
2) Decussate
Grains are of more or less equal size but not equidimensional, rather plate-like, e.g., mica and amphiboles.
In thin section appears to have overlapping grain boundaries.
3) Porphyroblastic
Just like the porphyritic texture of igneous rocks- larger crystals surrounded by finer crystals.
When the porphyroblasts (larger grains) are of nodular shape, the texture is commonly referred to as nodular texture.
Not limited to contact metamorphism only.
4) Poikiloblastic
Just like the poikilitic texture of igneous rocks – when the porphyroblasts (larger crystal) contains numerous inclusions
This texture is common in garnet, staurolite, cordierite, and hornblende.
5) Skeletal (also called Web or Spongy)
An extreme case of poikiloblastic texture
Inclusion occurs almost as an intergranular, crystallographically continuous network.
High-strain Metamorphic Textures
The deformation of crystalline solids involves several processes
Cataclastic flow
mechanical fragmentation of a rock and the sliding and rotation of the fragments
Solution transfer
Dissolution at grain contact
Produced sutured grain boundary
Intracrystalline deformation of a plastic-type
Permanent deformation of a rock body
Permanent deformation requires more significant changes in the position of atoms/ions, typically involving the breaking of chemical bonds.
Example: horizontal shortening of crystal by vacancy migration
As dislocations form and migrate, portions of a crystal’s lattice become reoriented. An easily observed result of this is undulose extinction.
Recovery
It is a process of orderly migration of defects.
During deformation, the formation and disorderly migration of defects compete with the recovery processes.
Recovery may transform a highly strained grain into two unstrained subgrains.
Recrystallization
is another way to reduce stored lattice strain energy
that involves the movement of grain boundaries or the development of new boundaries, both of which produce a different configuration of grains, not subgrains.
Depending on which process was dominant, various textures can be developed.
Porphyroclasts
Larger relict grains (sedimentary grains or phenocrysts) in a matrix of crushed material.
Mortar texture:
Matrix is the crushed portion of the Porphyroclast itself.
Mylonitic
Foliated cataclastic/plastic texture.
Pseudotachylite
irregular dendritic veins containing deformed grains suspended in a glassy matrix
the name comes from tachylite, a basaltic glass
Polygonized:
Incipient recrystallization is where larger deformed crystals break down into smaller, undeformed subgrains. The outlines of the larger crystals are still distinguishable.
Ribbon Texture
Ribbons are highly elongated disc- or lens-shaped crystals or aggregates of quartz, common in mylonites and high-grade rocks.
In the thin sections, the crystals are ribbon-shaped, hence the name.
Quartz ribbons form by flattening originally equidimensional quartz grains or possibly by migration of grain boundaries to form single large grains from more fine-grained parent aggregates.
They may exhibit undulose extinction or be recrystallized into polycrystalline ribbons.
Mantled porphyroclasts and Augen
Mantled porphyroclast: Resistant porphyroclast with a recrystallized rim with the same mineralogy as the porphyroclast. The mantles are assumed to be derived from the porphyroclast by grinding.
Augen, flaser: Eye-shaped mantled porphyroclasts.
Mica-fish are elongated lozenges or lens-shaped mica crystals.
Quarter structure:
A structure in which the four quadrants defined by the foliation and its normal are not symmetric.
Quarter fold:
Small fold in the foliation around porphyroclasts due to drag.
Quarter mat:
The concentration of mica resulting from the dissolution of quartz in the shortened quadrants of porphyroclast.
Oblique foliation:
Foliations that cut across the foliation developed due to shear offsets.
Shear band cleavage or S-C texture:
A texture that has shear bands (or C foliations), which are spaced cleavages that transect well-developed mineral foliation (S foliation) at a small angle.
Shear band cleavage or S-C texture
Cracked:
Crystals showing apparent cracks.
Crushed:
Crystals are ground by shear and exhibit fine-grained margins derived from the original larger crystals. The proportion of crushed material can vary considerably and even comprise 90% of the rock.
Shredded:
The intense breakup of minerals (usually phyllosilicates) along cleavages produces very fine locally crushed margins.
Kink band:
Zones bounded by parallel planes in which some features, usual cleavages, have a different orientation.
Deformation twin:
Twinning or lamellae are produced by deformation. Common in carbonates and recognized in feldspars when the twins/lamellae are bent or dominantly wedge-shaped
Deformation band/lamellae:
Dimensionally elongated bands within a crystal resulting from cracking or slip.
Sutured:
Incipient recrystallization in which the larger crystals differentially incorporate the marginal crush matrix to produce an interdigitation of grain boundaries.
Symplectic:
An intimate intergrowth of two or more minerals that grew simultaneously.
Corona:
A rim of one or more minerals surrounding another mineral.
Moat:
A coronitic rim surrounds a mineral.
Kelyphite
is a petrographical textural term referring to a fine-grained, fibrous intergrowth of multiple phases such as pyroxene, spinel, and amphibole, typically developed forming a rim (or a corona) surrounding garnet
Texture in Regional Orogenic Metamorphic Rocks
Foliation: Any planar fabric element.
Lineation: Any linear fabric element.
Spaced foliation: A foliation developed in zones separated by non-foliated “microlithons.”
Continuous foliation: A foliation that is not spaced but occurs continuously.
Cleavage: Any foliation in which the aligned platy phyllosilicates are too fine to see with the unaided eye.
Slaty cleavage: Fine-grained continuous cleavage.
Crenulation cleavage: A cleavage or schistosity that becomes micro-folded.
Schistosity: A planar orientation of elongated mineral grains or grain aggregates produced by metamorphic processes. Aligned minerals are coarse enough to see with the unaided eye.
