Clay mineralogy and provenance modeling of the Paleoproterozoic Kaladgi shales, Dharwar Craton, Southern India: Implications on paleoweathering and source rock compositions

Abstract

Proterozoic clastic sediments reveal vast information regarding provenance, depositional conditions, and environmental evolutions. Peninsular India comprising Archean cratons, also have numerous intracratonic Proterozoic sedimentary basins along their margins. The Archean Dharwar craton in southern India has many Proterozoic successions, namely Cuddapah, Kurnool towards the East, and Kaladgi, Badami, and Bhima towards the northern margin. The Paleoproterozoic Kaladgi Basin (∼1.85 Ga) consists of siliciclastic sedimentary rocks with stromatolitic carbonate formations. XRD analysis of shale layers of the Lower Lokapur and Upper Simikere Subgroups have been carried out to understand the primary clay mineral assemblages, weathering history, and provenance. The Lower and Upper shale layers of Lokapur and Simikere Subgroups show a dominance of montmorillonite and kaolinite, respectively. The geochemical affinities and the clay mineral assemblages indicate a more mafic source to the lower shales (Manoli and Hebbal Formations) and increased felsic contribution to the upper shales (Govindakoppa and Daddanhatti Formations). Illite is ubiquitous in all the shales of the Kaladgi Supergroup possibly representing the diagenetic transformation of montmorillonite and kaolinite to illite. Geochemical modeling of provenance has been carried out using (Eu/Eu*)N, (La/Yb)N, (Gd/Yb)N, and (La/Sm)N of all the plausible source rocks and the average compositions of lower and upper shales. The modeling results suggest that the lower shales are derived from a source of mafic rocks – 45 %, K-rich granite – 35 %, and TTG – 20 %. While the upper shales are derived from source characterized by K-rich granites – 61 % and intermediate volcanic rocks – 39 %. These results signify the classical unroofing of TTG-greenstone belts exposing K-rich granites with the progression of sedimentation. Further, a good correlation between K-enrichment (a measure of % difference between CIA and pre-metasomatic CIA) and ƩLREE is attributed to the abundance of kaolinites that fractionate more LREE.