Banding (geology)

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Handpiece of a ribbon gneiss - the prime example of a banded metamorphic rock

As banding or streaking (engl. Banding , layering ) is one in geology usually the structure of metamorphic rock from changing, centimeter to decimeter powerful referred to more or less sharply demarcated from each other layers of different composition. As a sub-form of foliation , it therefore describes the texture of a rock. Depending on the point of view, the term can exclude or include the metamorphic overprinting of a primary stratification or an igneous layer structure (for example in lava flows ). In the latter case, a harness, which goes back to a primary layer structure, as reliktisch designated.

Emergence

A non-relictic banding occurs when, as a result of tectonic processes, a relatively high directional pressure (approx. 1 kilobar) acts on a compositionally heterogeneous, but not primarily layered rock body. In conjunction with temperatures of at least 500 ° C, this pressure leads to a ductile deformation of the rock and therefore to at least moderate regional metamorphosis . In this case, the Felsic mineral phases can preferentially dissolve and migrate in the intergranular space into areas with an already slightly increased proportion of felsic minerals ( quartz , feldspars ), where they finally crystallize again, while the mafic mineral phases (mostly hornblende ) accumulate in the areas where they migrate . This process is known as metamorphic segregation or differentiation . If the directed pressure creates a shear stress , this ensures that the enrichment and depletion takes place in layers oriented parallel to the shear direction. Such banding usually occurs in gneiss , which are then appropriately referred to as banded gneiss e. Another possible cause for the formation of a non-relictic banding is the extreme elongation, parallel to the shear direction, of larger mineral or rock inclusions that were previously in the rock. In both cases, an increase in the shear deformation is accompanied by a tighter banding, i.e. there is a smooth transition to the mylonitic structure.

Banding in the broader sense

A significantly more inclusive definition of the term "banding" is sufficient if the color of the rock changes in layers as a criterion, and in addition to the "primary differentiation during formation" it also includes metasomatic striations caused by chemical weathering processes , in which the bands mark former chemical reaction fronts (e.g. B. with banded skarns or Liesegang's bands ).

Individual evidence

  1. ↑ a b c Roland Vinx: Rock determination in the field . 4th edition. Springer Spectrum, 2015, ISBN 978-3-642-55417-9 , pp. 367 ff .
  2. ↑ Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
  3. ^ A b c Andrew J. Barker: Introduction to Metamorphic Textures and Microstructures. 2nd Edition. Stanley Thornes, Cheltenham (UK) 1998, ISBN 0-7487-3985-8 , p. 41 f.
  4. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary. 12th edition. Spektrum Akademischer Verlag, 2010, ISBN 978-3-8274-1810-4 , p. 14.