Illite crystallinity index from the Mesoproterozoic sedimentary cover of the Kaladgi basin, southwestern India: Implications on crustal depths of subsidence and deformation

Abstract

The grade of metamorphism and thermal maturity of the Mesoproterozoic Bagalkot Group in the Kaladgi basin of southwestern India has been determined using the illite crystallinity (IC) index. IC index was determined from the argillite samples of four stratigraphic levels viz., Ramdurg Formation (basal unit), Yargatti Formation (intermediate lower unit), Yadahalli Formation (intermediate upper unit) and Hoskatti formation (upper unit). IC index (Kubler equivalent) values range between 0.54 ∘Δ 2 θ and 0.24 ∘Δ 2 θ (in a set of 37 samples) and indicate a deep diagenetic to high anchizone metamorphic grade within a temperature range of ∼ 180 to 300∘C. The Mesoproterozoic sedimentary cover of the Kaladgi basin is deformed due to southerly directed gravity gliding of the cover over the basement. The general variation of the IC index along and across the basin as indicated by the distribution of IC index values and isocryst contour patterns is due to the combined effect of deformation and variable subsidence of the Mesoproterozoic cover of the basin. Considering an average Mesoproterozoic geothermal gradient of 35 ∘C/km , the crustal depth of deformation and/or subsidence of the Mesoproterozoic cover up to the sample point is estimated to vary between 5.14 and 8.57 km.