PETROCHEMISTRY OF AN UPPER PRECAMBRIAN METASEDIMENTARY BELT IN SOUTHERN BAHIA - THE SIGNIFICANCE OF CORDIERITE

Abstract

Petrology and chemistry of an Upper Precambrian metasedimentary belt in Southern Bahia (Brazil) have been studied. The Belt consists essentially of muscovite-biotite gneisses and of sillimanite-grade rocks (kinzigites) containing lenses of acid-and sub-acid granulites, and pluton-like bodies of migmatitic-granitic rocks. Cordierite-bearing rocks are widely spread within the sillimanite-grade rocks. Cordierite appears always as the last paragenetic mineral and probably is formed through the reaction sillimanite+garnet+quartz = cordierite. Chemical data indicate that the original material for mica gneisses and sillimanite-grade rocks consisted of a misture of graywaekes and pelitic rocks. Comparison with modern pelagic sediments reveals low contents of metals (Fe, Mn, Ni, etc.), relatively mobile in marine environments, thus suggesting possible submarine leaching. Bulk chemistry (showing low alkali and silica contets) was found to control closely cordierite formation which is considered related to the removal of fractions of anatectic silicate liquids. The more general significance of cordierite in metasediments is discussed stressing the role of the partial H 2 0 pressure in the metamorphic fluids. The presence of even small amounts of H 2 0-undersaturated melts strongly reduces the molar H 2 0 fraction and favours the stabilization of cordierite. Preservation of cordierite under more hydrous conditions depends on the further development of the P-T metamorphic regime.