Study of archean chromites from mysore district, western dharwar craton - compositional variations, thermometry and possible parental magmas

Abstract

The Sargur greenstone belts occur as enclaves of metavolcanics, metasediments, basic granulites and serpentinized ultramafics embedded in a granodioritic continental crust. Talur, Sindhuvalli and Doddakanya is in this high-grade terrain. In Talur, chromite deposits are associated with magnesite deposits. In Sindhuvalli and Doddakanya, there are chromite deposits. Detailed studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (≈50% modal chromite) and serpentinite (≈2% modal chromite) throughout the entire ultramafic sequence. However, the primary composition of chromite is preserved in the massive chromitites (≈60-75% modal chromite) from Talur and Sindhuvalli deposits in the Sargur schist belt. In Talur, the chromites in massive chromitite is characterized by relatively lower Mg# (0.28-0.39) compared to chromites from massive chromitite in Sindhuvalli (0.63-0.69). This might signify that the parental melt associated with chromites in Talur was more fractionated compared to the melt associated with chromites from massive chromitite in Sindhuvalli. The primary Fo content of olivine in dunites from Talur (Fo91-92) showed lower values compared to olivine in dunites from Doddakanya (Fo92-93). This signifies that the parental melt in Talur was also more evolved compared to the parent melt in Doddakanya. The accessory chromites in dunite from Talur show higher Cr#, Fe3+#, TiO2 and MnO compared to other samples. This is due to sub-solidus re-equilibration associated with alteration and metamorphism. From olivine-chromite thermometry, the re-equilibrated temperatures obtained from olivine-chromite pairs in dunite from Doddakanya is 674-884 °C and olivine-chromite pairs in net-textured chromitite in Sindhuvalli is 708-750 °C. This temperature corresponds to the stability field of amphibolite facies metamorphism. In Talur-dunite, the chromite-olivine pairs have re-equilibrated to temperature of 477-537 °C which coincides with the stability field of talc and magnesite. The calculated Al2O3, FeO/MgO ratio and TiO2 content of the parental melt shows close resemblance with the komatiitic parent melt of chromites in the Nuggihalli greenstone belt, Western Dhawar craton. The compositions of chromite from massive chromitite in Cr#-Mg# plot as well as in the Fe3+ -Cr-Al ternary plot the samples from Sargur greenstone belt fall in the field of chromites from komatiites metamorphosed to greenschist and amphibolite facies.