Metamorphic Textures

  • 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
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.