Metasomatic origin of whiteschist under HP metamorphic conditions

Abstract

Whiteschist is a metamorphic rock type characterized by the occurrence of kyanite-talc assemblage, and extremely high MgO and Al2O3 bulk composition (Schreyer, 1977). Alpine geologists suggested two main genetic hypotheses; metasomatic or sedimentary. The metasomatic interpretation was mainly derived from the close field association of orthogneiss and whiteschist (Bearth, 1952).According to the sedimentary hypothesis, the whiteschist protolith is a highly magnesian meta-evaporite or an Mg-rich metapelite (Schreyer, 1977).We present recently obtained several lines of evidence suggesting the metasomatic origin of the whiteschist, which was formed by the influx of Mg-rich fluid into shear zones developed in granitoids under Alpine HP conditions.