Bimodal magmatism in the Eastern Dharwar Craton, southern India: Implications for Neoarchean crustal evolution

Abstract

Archean cratons provide windows to the crustal evolution history in the early Earth. The Dharwar Craton is one of the major cratonic blocks in Peninsular India and is composed of several ancient microcontinents where multiple continental growth and recycling occurred from 3.8 to 2.5 Ga. Here we investigate a suite of magmatic rocks (including granitoids, mafic magmatic enclaves, ultramafic inclusions and synplutonic mafic dyke) from the western margin of the Eastern Dharwar Craton which provide insights on bimodal magmatism in a subduction-related arc setting and crustal growth during Neoarchean. Field evidence and geochemical features suggest mixing and mingling of mafic and felsic magmas. Geochemical features including incompatible and high field strength element features of the enclaves indicate heterogeneous sources involving dominantly enriched mantle reservoirs with minor depleted mantle whereas the host granitoids display depleted to enriched source. We present zircon U[sbnd]Pb age data from the various rock types and the combined results display unimodal distribution with a single peak of 207Pb/206Pb age at 2531 Ma (n = 159) suggesting a major Neoarchean bimodal magmatic pulse. The zircon U[sbnd]Pb ages of the mafic enclaves indicate either similar ages or slightly younger ages as compared to their host granitoids implying that enclaves represent late stage injections into crystallizing host granitoid magmas. The trace and rare earth element data on zircon from these rocks are consistent with magmatic crystallization. Zircons from most of the rocks in our study display uniformly positive εHf(t) (0.3–8.6), suggesting depleted mantle (juvenile) source for the Neoarchean arc building, and formation of new crust. Some of the mafic magmatic and ultramafic enclaves, as well as the mafic dyke and charnockite display mixed positive and negative εHf(t) values (−7.0 to 3.8), implying mixed depleted mantle and reworked crustal components of Neoarchean to Mesoarchean age. Most of the Hf depleted mantle model ages (TDM; 2520–3115 Ma) and crustal residence ages (TDMC; 2507 Ma to 3452 Ma) fall within the region bounding CHUR and depleted mantle lines, suggesting that the dominant magma source involved juvenile (depleted mantle) components. Thus, latest Neoarchean marks a major phase of continent building in the Eastern Dharwar Craton, and the results presented in our study suggest that bimodal magmatism in a subduction-related setting identical to those in modern convergent margins contributed to crustal growth.