Compositional restraints on the sillimanite paragenesis in metapelites from Kuanthal,district Udaipur,India

The rocks exposed in the area around Kuanthal represent one of the oldest Precambrian formations of India and constitute the Banded Gneissic Complex (B.G.C.) of Heron (1953). The main rock types are staurolite-sillimanite schists/gneisses, sillimanite gneisses with or without K-feldspar, migmatites and amphibolites, often intimately and inextricably mixed up with one another. The petrochemical studies of the metapelitic rocks show that the formation of sillimanite is controlled by parent rock chemistry. The critical controlling parameters area: alumina, alkalies (Na₂O + K₂O) and oxidation ratio of rocks. These criteria are equally valid for other well-known areas of high-grade metamorphism. The validity of this proposition has been tested by statistical methods such as multivariate analysis and Mahalonbis's generalized distance D². The linear discriminant function controlling sillimanite paragenesis in metapelites has been worked out and an equation has been evolved to delineate the ‘SB’ (sillimanite-bearing) and ‘SF’ (sillimanite-free) rocks.