Petrographic, Geochemical and Sr–Nd Isotopic Studies of the Shales From the Lower Cuddapah Basin: Implications for Palaeoweathering, Provenance and Proterozoic Crustal Evolution

Abstract

The Proterozoic to Neoproterozoic Cuddapah Basin (CB) contains a thick pile (i.e., 6–12 km) of arenaceous, argillaceous and carbonate rocks with substantial igneous rocks. The evolution of the CB is mainly defined based on igneous rocks with a limited study on siliciclastic rocks. Here we report geochemical and isotopic data from shale samples of the Lower Cuddapah formations for the first time to comprehend their provenance, palaeoweathering, and tectonic setting. The shale samples were collected from Gulcheru, Vempalle and Tadpatri formations of the Lower CB. The geochemical and isotopic data divide Gulcheru Formation shales into two groups: Gulcheru shale I (GS I) and Gulcheru shale II (GS II). The CIA (chemical index of alteration), CIW (chemical index of weathering) and Al₂O₃–CaO + Na₂O–K₂O (ACNK) diagram of the shales decipher a low chemical weathering for the GS I. In contrast, GS II, Vempalle shale (VS) and Tadpatri shale (TS) exhibit a medium to high degree of chemical weathering. The major and trace element ratios, provenance diagrams, and higher sub-chondritic εNd(t) values of GS I (−8.18 to −7.62) compared to GS II (−5.34 to −3.63), VS (−6.25 to −4.41) and TS (−5.2 to −3.78) along with f_Sm/Nd versus ɛNd(t) diagram suggest the decrease of sediment input from older crustal provenance, that is, Eastern Dharwar Craton (EDC) with increase of ‘more juvenile’ source. The tectonic discrimination diagrams, as well as the primitive mantle (PM) normalised multi-element diagram, characterise a back-arc origin of the CB. The more evolved εNd(t) values and younger detrital zircon grains in the Pulivendla Formation indicate that the basin was opened by westerly subduction along the east coast of India at ~2 Ga. The continuous collision of the Dharwar Craton with Antarctica at ~1.6 Ga suggests the last phase of the Columbia Supercontinent formation.