Re-Os isotope systematics of Archean chromitites from the Chimalpahad Anorthosite Complex, south-east India: implications for mantle extraction processes

The Chimalpahad Anorthosite Complex, which contains layers of chromitites, is located within the Archean Peninsular Gneissic Complex in the SE Indian shield. Here we present Re–Os isotope systematics of the chromitites and associated websterites and anorthosites in tracing the petrogenetic history and heterogeneity of the source mantle. The average initial ¹⁸⁷Os/¹⁸⁸Os compositions of chromitites are 0.1045 with negative γOs values of 17.67, whereas the websterites and anorthosites have average initial ¹⁸⁷Os/¹⁸⁸Os compositions of 14.86 and 11.79 with positive γOs values of + 467 and + 215, respectively. Mantle extraction ages (TMA) of the chromites indicate depletion of primitive mantle beginning as early as ~ 3.5 Ga. The subchondritic signature of Os in the chromitite is comparable with those of the chromites from ultramafic intrusions in the Zimbabwe Craton interpreted to reflect sequential extraction from evolving Archean subcontinental lithospheric mantle. The younger (TMA) ages of 2.77–1.41 Ga of the websterites and anorthosites indicate Proterozoic melt depletion or complex isotopic systematics resulting from overprinting of tectonic events. The geochemical data and Re-Os isotope systematics presented in this study suggest that the Chimalpahad Complex formed from protracted evolution of ultramafic–mafic magmas in a deep-seated chamber of a magmatic arc along the SE margin of the Indian plate.C.V. Dharma Rao, M. Santosh, Yan-Jie Tan