0016-7622/2008-71-1-79/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.71, January 2008, pp.79-88 Widespread Arkose along the Northern Margin of the Proterozoic Kaladgi Basin, Karnataka: Product of Uplifted Granitic Source or K-metasomatism? SUKANTA DEY 1 , A.K. RAI 1 and ANJAN CHAKI 2 Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy 1 Nagarabhavi, Bangalore - 560 072, 2 Begumpet, Hyderabad - 500 016 Email: sukantadeygeo@yahoo.co.in Abstract: The basal conglomerate and sandstone all along the northern margin of the Proterozoic Kaladgi basin are highly enriched in K-feldspar ranging in size from sand to pebble. They are believed to have undergone K-metasomatism by earlier workers. However, the present study shows that K-feldspar grains present in these rocks are predominantly terrigenous detrital. This is indicated by sympathetic relation between constituent quartz and feldspar in size, presence of fresh rounded microcline with or without authigenic overgrowth, typical arkosic texture and geochemistry of the rocks, and their sharp erosional contact with the underlying granitoids. An uplifted, restricted source, dominantly consisting of K-rich granitoids and pegmatites to the north of the basin supplied the immature arkosic detritus possibly under humid and warm climatic condition. The testimony for the presence of a highly K-rich source weakens the theory of K- metasomatism in the Kaladgi basin as proposed by some earlier authors. Keywords: Arkose, Petrography, Geochemistry, Provenance composition and tectonism, Kaladgi Basin, Karnataka. margin of the Kaladgi Basin were affected by granitization and attendant K-metasomatism. The fact, if true, implies that the northern margin of the basin forms a highly favourable target for uranium. However, any exploration assignment is costly in terms of money and manpower. In this paper we present a different hypothesis for the origin of K-feldspar enrichment in these sandstone and conglomerate with new field, petrographic and geochemical data. We will also discuss the provenance composition and tectonics. GEOLOGICAL SETTING The Kaladgi Basin extends over an east-west trending irregular area in the northern part of the Dharwar craton, Karnataka (inset of Fig. 1). The Kaladgi Supergroup, occupying the basin, is divided into an older Bagalkot Group and a younger Badami Group (Jayaprakash et al. 1987). The Bagalkot Group is subdivided into lower Lokapur Subgroup and upper Simikeri Subgroup. To the north and west, basalts of the late Cretaceous Deccan Traps cover parts of the Kaladgi Supergroup. Detailed description of stratigraphy of the Supergroup is given by Vishwanathaiah (1977) and Jayaprakash et al. (1987). The northern margin of the Kaladgi basin exposes Salgundi INTRODUCTION The middle to late Proterozoic ‘Purana’ basins of Peninsular India are characterized by quartz arenite- carbonate-shale association deposited in shallow platformal setting (Singh, 1980; Radhakrishna, 1987; Kale and Phansalkar, 1991). Sandstones deposited in these basins are generally mature and quartz arenite in composition indicating tectonically stable, denuded source (Radhakrishna, 1987). However, basal conglomerate and sandstone exposed along the northern margin of the Kaladgi Basin in southern India are notable exceptions which contain substantial amount of K-feldspar over a 100 km stretch from Terdal to Almatti (Fig. 1). Geological information on these conglomerate and sandstone, nevertheless, is very scanty (Govindarajulu and Nagaraja, 1967; Vishwanathaiah and Sreedhara Murthy, 1977). Among the Proterozoic basins of India, the Kaladgi Basin forms one of the most favourable targets for uranium, especially after the location of arenite-hosted uranium mineralizations at Tugunshi and Deshnur in the eastern and western parts of the basin respectively (Nagaraju and Gorikhan, 2002; Nanda et al. 2006) (inset of Fig.1). According to Vishwanathaiah and Sreedhara Murthy (1977), the conglomerate and sandstone exposed along the northern