NEOPROTEROZOIC GARNETIFEROUS TUFF BEDS IN THE CHATTISGARH BASIN OF CENTRAL INDIA: FIRST REPORT

We report the discovery of thick (" ca 300m) unmetamorphosed mostly massive, normally graded volcaniclastic sandstones, minor conglomerates, and garnetiferous rhyolitic welded tuff interstratified with offshore calcareous Neoproterozoic sediments of the Chattisgarh Basin in central India. The sandstone beds, containing rip-off mudstone clasts, are interpreted as mass-flow deposits associated with the volcanism. Eutexitic texture of the welded tuffs indicates emplacement by high-density particulate flows that were deposited near the source and that they were likely of intrabasinal origin. Their high-silica peraluminous whole rock composition (SiO2 78-89%; Al2O3 3-11%; CaO 0.6-2%; Na2O 0.5-6%; K2O 0.2-0.7%) is consistent with magma derivation from partial melting of pelitic source rocks. Inverse variation of SiO2 with Na2O suggests possible albite fractionation. Chondrite-normalized REE patterns of the tuffs are very similar to each other and form a coherent cluster suggesting a common magma source. Principal phenocrysts are fractured almandine garnet (SiO2 37%; Al2O3 21%; MgO 3%; FeO 35%; MnO 0.8%; CaO 2.5%) and embayed (resorbed) clear quartz. Albite, ilmenite, apatite and zircon are minor phases. All of these phases occur together with sharp grain contacts. Color SEM-CL shows quartz grains to be bluish with textural attributes that indicate a volcanic origin. Electron probe traverses of individual garnet phenocrysts, BSE images and elemental x-ray mapping show that they are unzoned and free of inclusions. We interpret the mineral assemblage as primary igneous. We propose that partial melting of early- to mid-Proterozoic pelitic schists below this basin generated the rhyolitic magma.