Clastic rock geochemistry of Punagarh basin, trans-Aravalli region, NW Indian shield: implications for paleoweathering, provenance, and tectonic setting

Abstract

The trans-Aravalli region of the northwestern Indian shield is characterized by the presence of Meso–Neoproterozoic linear sedimentary Punagarh basin comprising shales and sandstones and mafic volcanic flows in the upper formations. It is constituted by four formations (Sojat, Bambolai, Khamal, and Sowania), among which the Sojat Formation, in unconformable contact with overlying formations, is significantly older. Major and trace element compositions of sandstones and shales of this basin have been investigated to infer the nature of provenance, weathering processes, and tectonic setting. Provenance analysis using trace elements and petrography suggests that this basin initially received detritus from Archean Banded Gneissic Complex (BGC) and later Delhi arc of the Aravalli craton. The shale and sandstone constituting the lower part of the Punagarh basin, namely, Sojat Formation, are distinctive in age, field, and geochemical characters in comparison to succeeding sedimentary units comprising the upper formations of the Punagarh basin. The significantly different geochemical characteristics of Sojat Formation (i.e., highest SiO2/Al2O3, K2O/Na2O, Th/U, and Cr/Th ratios coupled with lowest Zr content, least fractionated light-group rare-earth element (LREE), more fractionated heavy-group rare-earth element (HREE), and largest Eu* anomaly) suggest a change in provenance with sediments in the lower formation being derived from distal source and those of the upper formations from proximal source. Based on REE patterns and Eu/Eu* values, the clastic sediments of Sojat Formation can be modeled by a mixture of 0.24:0.42:0.34 (granite/tonalite–trondhjemite–granodiorite/mafic enclaves). On the other hand, the upper formations are best modeled with a Delhi arc in the proportion 0.25:0.40:0.35 (tholeiites/calc-alkalines/felsites). Weathering indices like chemical index of alteration (CIA), plagioclase index of alteration (PIA), A–CN–K plot, and trace element ratios suggest that the source areas experienced low to high degree of chemical weathering under warm and humid conditions. The shift of sediment source from distal in the beginning to local during later period suggests sedimentation in a tectonically active basin. The depositional history, tectonic setting, and present position of the Punagarh basin provide inputs about the role of assembly and breaking of supercontinents in the buildup of Greater India during the Proterozoic period.