Ca. 2.1 Ga Mahakoshal Supracrustal Belt: An allochthonous terrain in Central India Tectonic Zone

Abstract

Back-arc magmatism is perhaps extremely rare in a Paleoproterozoic mobile tectonic regime. The present study provides a detailed account on the petrogenesis of basalts that are interleaved with marine sedimentary rocks in the Agori Formation, which overlies the Chitrangi Formation in the ~2.1 Ga Mahakoshal supracrustal belt within the Central India Tectonic Zone. The basalts are sub alkaline, tholeiitic in nature. They have moderate concentrations of TiO2 (~0.9 wt%), Nb (2.6–5.2 ppm), and Zr (59–105 ppm), Zr/Nb (18–24) and Zr/Y (3.1–3.6) ratios, mildly enriched chondrite normalized rare earth element patterns (LaN/YbN = 1.6–2.8), and negative Nb and Ti anomalies on a primitive mantle normalized trace element variation diagram. Post-magmatic low grade metamorphism, alteration and trace element mobility, and/or contamination by Archean continental crust, if any, cannot relate to these patterns. The positive initial εHf (2.1 Ga) + 2.24 to +5.15 and εNd(2.1 Ga) + 1.02 to +1.54 isotopic compositions in these basalts are variable, yet consistent with a juvenile mantle source. We ascribe the isotopic variability in the Agori basalts to reflect the general isotopic variability in the depleted mantle at 2.1 Ga. Consequently, the basalts were produced by decompression partial melting in the shallow upper mantle in a back-arc setting. Subduction zone magmatic processes produced the arc volcanics in the upper stratigraphic unit (Agori), whereas, a plume source generated the ferropicrites in the underlying Chitrangi Formation. The evolution of the Mahakoshal supracrustal belt predates the collision and interlocking of North and South India cratonic blocks, which resulted in the formation of Central India Tectonic Zone. Hence, it represents an allochthonous plume-arc accreted terrain in the CITZ.