Metamorphic Differentiation; a Mechanism Indicated by Zoned Kyanite Crystals in Some Rocks from the Lukmanier Region, Switzerland

M. G. Bramwell
Department of Geology, Queen's University, Belfast BT7 1NN

Abstract: Two samples of garnet-kyanite-staurolite schist from Lukmanier, Switzerland, each contain two chemical varieties of kyanite which occur in texturally distinct areas of the rock. Type 1 form large idiomorphic crystals within an open crenulation cleavage S3. They exhibit a systematic zonation of F2O3, with core values of 0.8% decreasing to 0.3% at the crystal margin. Type 2 form small, much less abundant crystals in areas between S3 cleavage zones, and have a homogeneous distribution of 0.3% Fe2O3 throughout the crystal.

It is suggested that the first-nucleated crystals contain the highest core concentration of Fe2O3 and are the largest. A positive correlation between core Fe2O3 values and crystal size is interpreted as a nucleation and growth sequence. This indicates that the first crystals formed preferentially in S3 (Type 1), with Type 2 crystals growing later outside the S3 zones.

Concentration of kyanite in S3 zones produces a distinct mineral banding in the rock. A mechanism for the development of metamorphic differentiation by preferred nucleation of kyanite in S3 is proposed.

Keywords: metamorphic differentiation • kyanite • schist • Lukmanier region • Switzerland

Mineralogical Magazine; March 1985 v. 49; no. 350; p. 59-64; DOI: 10.1180/minmag.1985.049.350.07
© 1985, The Mineralogical Society
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