Geochemistry of late Archaean metagreywackes from the Western Dharwar Craton,South India: Implications for provenance and nature of the Late Archaean crust

Late Archaean metagreywackes of the Ranibennur Formation, Dharwar Supergroup, in the Dharwar–Shimoga schist belt of the Western Dharwar Craton (WDC) are texturally and mineralogically immature of the quartz-intermediate type. The SiO₂ content in them ranges from 60.58 to 65.26 wt.%. Chemical Index of weathering (CIW) values varies between 50 and 65. 4 indicating a low degree of chemical alteration of the provenance rocks. A high degree of correlation between K₂O and Al₂O₃ (r = ? 0.73) and low Rb/Sr ratios also suggest a low degree of alteration of provenance rocks. Abundances of transition group elements (Cr = 118–221; N = 89–154; V = 89–192 and Sc = 11–16 ppm) as well Zr (132–191 ppm) suggest a mixed mafic–felsic provenance for the metagreywackes. Low HREE and Y content, and low Tb/Yb ratios (0.23–0.41) suggest the presence of tonalite as an important component in the provenance areas. Values of Eu/Eu?(0.78) and Th/Sc (0.55) suggest that the granodioritic upper crust had evolved prior to serving as the provenance. Mixing calculations suggest 50–55 vol.% tonalite, 20–25 vol.% granite, 18–20 vol.% basalt and ~ 5 vol.% komatiite composition for the provenance. Geochemical characteristics of the Ranibennur metagreywackes suggest that sedimentary basin formed in the vicinity of a magmatic arc in a continental island arc setting, and the detritus were shed from the arc rock.     

Geostatistical investigations of gabbro occurrences near Savantvadi, Ratnagiri district, India

This paper embodies petrographical and statistical investigations of three gabbro occurrences. The Mangaon and Kunkeri gabbros are similar in petrography and petrochemistry, whereas they are completely different from the Vajrat gabbro. Application of recently developed statistical tests help in classifying these gabbros by using three oxides, CaO, FeO, and K₂O.This study shows that the evolution of the Mangaon and Kunkeri gabbros probably proceeded along different lines as compared to the Vajrat gabbro.