Mica fish

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Mica fish are diamond-shaped single crystals (asymmetrical porphyroclasts ) mainly made of mica (e.g. muscovite ), which, due to their preferred orientation, are used in tectonics to determine the sense of shear . Its long axis forms a small angle with the mylonitic foliation plane .

Initial description

Thin section with a group 2 mica fish from a quartz mylonite from the Italian Alps. Right-shifting sense of shear.

Mica fish from Nova Scotia were first scientifically described by GH Eisbacher in 1970.

introduction

The term mica fish, English mica fish , refers to the Streamlined, fish-shaped appearance of the mica crystals. They can also take the form of elongated diamonds, parallelograms, or lenses. Long-drawn trails made of small pieces of mica often emanate from their endpoints . These tracks run parallel to each other, but are offset by the mica fish in a stair -stepping manner .

The angle of the mica fish long axes with the mylonitic foliation can vary by up to 40 °, but averages only 13 °. The angle of the basal (001) planes to the foliation reaches an average of only 11 °, but in very rare cases can vary by almost 50 °.

Occurrence

Mica fish are quite common in mylonites , ultramylonites, and mylonitized gneisses . The mylonites are usually derived from mica-rich quartzites . Quartzites containing mica fish can be classified as a special form under SC-Mylonite. SC structures are characterized by two levels of foliation, the S and C surfaces. The S-surfaces (from French schistosité ) represent an oblique foliation , created by accumulated deformations of the individual grains, whereas on the C-surfaces (from French cisaillement ) there was an actual sliding offset.

Mica fish structures not only appear in mica such as muscovite , biotite , phengite and chlorite , but also appear in other minerals such as tourmaline , allanite , leukoxene , feldspar ( alkali feldspar and plagioclase ), amphibole ( hornblende ), diopside , garnet , pyroxene ( hypersthen ) , Thistle , sillimanite , staurolite , epidote , apatite , rutile , hematite , pyrite , graphite , calcite and quartz .

Due to the variety of fish structures, this can be specified with its mineral name now, for example, as leucoxene fish , or are now available as since 2000 Mineral fish (English mineral fish called).

Emergence

The origin of mica fish is still not fully understood, but is likely to be based on the following processes or a combination of the same:

  • intracrystalline deformation such as sliding parallel to the base (Treagus and Lan, 2003)
  • Solid rotation
  • Bending and folding of mica crystals (recognizable by the indolent extinction )
  • Grain size reduction by recrystallization at the edges or by tearing away fragments
  • Printing solution (English pressure solution ) accompanied by local growth takes place particularly at the edges precipitate dissolved material

Microboudination in the grinding process of larger fish may also be an option . However, the conflict with small folds and creases (English kinks ) along the longitudinal axes - signs that the mica fish were shortened and not stretched. Rather, it appears that the tips of the mica fish were folded isoclinally and then broke off at their hinge.

Typology

On thin sections ten Grotenhuis and colleagues (2003) were able to distinguish a total of 6 different mica fish groups. For groups 1, 2, 3 and 6 the mica base plane (001) is parallel to the shear plane (C), for groups 4 and 5 it is perpendicular to this. The simplest type is found in group 3 (frequency 25%), in which the final diamond shape is achieved by shearing along the individual (001) planes. The development of group 1 is very similar (frequency 33%), but it also experiences an additional reverse rotation, so that the base levels are now inclined. Group 2 (frequency 19%) develops like group 1, but is still beveled by concentrated shearing at the bottom and top of the fish. In group 4 (8% frequency) are rotated, the steeply standing base planes by simple shear in the shear direction, the diamond shape is then obtained by sliding antithetical to the individual base planes (after the bookshelf mechanism , English bookshelfing ). Group 5 (frequency 5%) has a similar development to group 4, but its lower and upper margins show a rounded shape, which was probably achieved by shearing off using synthetic shear bands of the C 'type. The somewhat out of line group 6 (frequency 5%) was probably bent and elongated under drag folding using shear bands of the C 'type. Around 5% of the mica fish observed do not belong to any of the 6 groups listed.

use

Despite their very different types, mica fish are generally a reliable indicator of shear sense. Their inclination (S) compared to the Mylonitic foliation or shear bands (C) points in the direction of shear. The sense of shear can also be read from the stair-like tracks. Its monoclinic exterior with one curved and one straight side also reveals the direction of shear. An exception is group 6, which suggests an opposite sense of shear.

literature

  • Saskia Martine ten Grotenhuis: Mica fish in mylonites - deformation mechanisms and implications for rheology . In: PhD thesis . Johannes Gutenberg University, Mainz 2000. [1]
  • Soumyajit Mukherjee: Mineral fish: their morphological classification, usefulness as shear sense indicators and genesis . In: International Journal of Earth Sciences (Geologische Rundschau) . tape 100 , 2011, p. 1303-1314 , doi : 10.1007 / s00531-010-0535-0 .

Individual evidence

  1. ^ GH Eisbacher: Deformation mechanisms of mylonitic rocks and fractured granulites in Cobequid Mountains, Nova Scotia, Canada . In: Geological Society of America Bulletin . tape 81 , 1970, pp. 2009-2020 .
  2. ↑ D. Berthé, P. Choukroune and P. Jegouzo: Orthogneiss, mylonite and non-coaxial deformation of granites: the example of the South Armorican shear zone . In: Journal of Structural Geology . tape 1 , 1979, p. 31-42 .
  3. ↑ DH Oliver and JW Goodge: Leucoxene fish as a micro-kinematic indicator . In: Journal of Structural Geology . tape 18 , 1996, p. 1493-1497 .
  4. ↑ A. Azor, J. Ferando Simancas, I. Exposito, F. Gonzalez Lodeiro and DJ Martinez Poyatos, DJ: deformation of garnets in a low-grade shear zone . In: Journal of Structural Geology . tape 19 , 1997, pp. 1137-1148 .
  5. ↑ Cornelis W. Passchier: Mylonitic deformation in the Saint-Barthélemy Massif, French Pyrenees, with emphasis on the genetic relationship between ultramylonite and pseudotachylyte . In: GUA Pap Geol Ser . tape 1 16 , 1982, pp. 1-173 .
  6. ↑ M. Bestmann: Lattice diffusion creep as a possible deformation mechanism for quartz porphyroclasts within a calcite marble shear zone . In: Abstract volume deformation mechanisms, rheology, microstructures . 1999, p. 69 .
  7. ^ S. Mukherjee and P. Pal: Tectonic structures of the Karakoram metamorphic belt, its significance in the geodynamic evolution. Unpublished Report. In: Summer Undergraduate Research Award . University of Roorkee, 2000.
  8. ↑ GS Lister and AW Snoke: SC-Mylonites . In: Journal of Structural Geology . tape 6 , 1984, pp. 617-638 .
  9. ↑ SH Treagus and L. Lan L: Simple shear of deformable objects square . In: Journal of Structural Geology . tape 25 , 2003, p. 1993-2003 .
  10. ↑ SM ten Grotenhuis, Rudolf AJ Trouw and Cornelis W. Passchier: Evolution of mica fish in mylonitic rocks . In: Tectonophysics . tape 372 , 2003, p. 1-21 .