Geochemistry of Archaean Arenites Formed by Anoxic Exogenic Processes-An Example from Bababudan Schist Belt, India

Sandy matrix of QPC of Archaean Bababudan schist belt is distinguished, by the abundance of well rounded detrital pyrite, uraninite, chromite, zircon, rutile and micas, and clasts of fuchsite quartzite. Presence of detrital pyrite and uraninite in mature arenites deposited during the initial stage of shallow stable basin, indicate the anoxic nature of ambient atmosphere. Compositional characteristics such as the abundance of Al₂O₃, K₂O, MgO, Cr, Ni, Co, Zr, V, Sc, Rb, Sr and ∑REE reveal that only restites could survive intense chemical weathering and strong hydrodynamic action. High to moderately fractionated REE patterns (LaN/LuN-10) with essentially negative Eu anomalies of the matrix of QPC, typically resemble the Archaean arenitic and pelitic rocks. These characlcristics of the QPC indicate removal of mafic-ultramafic and other unstable constituents of provenance to the deeper part of the basin. Silicic and other resistive part of provenance were deposited on the margin of the basin. Geochemical parameters of these rocks show that initial anoxic stage was changed to rhythmically oxidizing environment, in which very thick oxide facies of BIF got precipitated. Stable tectonic condition was changed to active margin setting, in which immature Kaldurga polymictic conglomerate (KCM) with greywacke matrix was deposited. Available data on various rock types of the Bababudan basin, demonstrate that the composition of atmosphere changes in tectonic setting and depositional environment resulted in the deposition of sediments of divergent type in the basin at various stratigraphic levels.