A Lower Proterozoic Palaeosol at BGC-Aravalli Boundary in South-Central Rajasthan, India

A metamorphosed palaeosol surface separates the upper part of the Archaean Banded Gneissic complex from the basal Proterozoic Debari Formation of the Aravalli Supergroup. Mineralogical and petrological studies reflect presence of two mineralogical assemblages namely, quartz-feldspar-white mica-sericite-biotite-chlorite-calcite and quartz-kyanite-white mica-garnet. Geochemistry of the rocks collected along vertical profiles through the palaeosol units show high CIA indices in comparison to low CIA values of less altered basement gneisses. From bottom to the top, in the basement rocks as well as the palaeosol unit, a perceptible increase in Al₂O₃, SiO₂, and TiO₂, contents could be seen. Iron shows gradual upward increment in the assemblage-I while assemblage-2 contains very little iron. Na₂O shows upward decrease while K₂O shows quantum jump in abundance. K-Al correlation is strong and significant. Another iron-rich palaeosol unit is described between the basement gneisses and the Matoon dolomites. Based on the identification and characterization of the palaeosol units, the geological conditions prevailing before the onset of Aravalli sedimentation has been discussed. Diagenetic and metamorphic imprints on the palaeosols, now occuring as metamorphosed rock units have been defined and interpreted. This study has been utilised for reflecting the atmospheric conditions dominating during the early Proterozoic time.