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4.3) Examples 4.4) Veins formation  

The crystallisation in these gashes, the pegmatitic texture and the presence sometimes important of tourmaline are indicator of a crystallisation from an aqueous fluid (Kerrick 1990 [reference]). This aqueous fluid has to be rich in aluminium to allow crystallisation of aluminosilicate and plagioclase in such abundance. Fluids seem to come from the near surrounding rocks and aluminium is probably transported as alcali-aluminous complex in fluids.

Biotite sometimes forms concentrations along vein border. These concentrations are interpreted as a rest of surrounding rocks after impoverishment of alkalis and aluminous phases by metamorphic segregation (Keller 1968 [reference], Klein 1976 [reference] et Kerrick 1988 [reference]).

To start mechanism of vein opening, a weakness zone is necessary, such as a fracture, shear plane, or pre-existing quartz veins. Release of local metamorphic fluids generates a fluid overpressure which will help to open the fracture perpendicular to the local main stress. Release of pressure in the veins will allow the precipitation of cations dissolve in metamorphic fluids (Si, Al, K, Na,...) and the crystallisation of quartz, plagioclase, aluminosilicates and white micas. The cycle can start again with a next peak of fluid pressure.

Border of Knauern
Enriched border with biotite in a kyanite-, andalusite- and plagioclase-bearing vein

Cristallisation of Ky parallel to gashes opening
Sample showing kyanite crystallisation, parallel to the opening (here N-S) of the vein

1) Introduction | 2) Structural | 3) Thermobarometry | 4) TENSION GASHES | 5) Cartography | Bibliography | Thanks
Aluminosilicate-bearing veines: 4.1) Mineralogy | 4.2) Structure | 4.3) Examples | 4.4) FORMATION

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Photo - serie 2

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Created: 10th May 2003
Last update: 3rd March 2012