Geochemistry of basalts in unravelling the mantle processes and crustal evolution: Insights from the greenstone belts of western Dharwar Craton

Basalts are an integral part of all the greenstone belts of Dharwar Craton which played a vital role in the thermo-tectonic evolution and mantle processes during the Archean time. Here, we evaluate the basalt geochemistry from the Holenarsipur, Shimoga and Chitradurga greenstone belts of western Dharwar Craton wherein both Sargur and Dharwar Supergroups are present reflecting a gradual change in the geodynamic processes. They have overlapping petrographic and distinct geochemical characteristics with slightly fractionated chondrite normalized rare earth element patterns for the Shimoga basalts, low order negative Eu anomalies and negative Nb and Ti anomalies with a hump at Zr-Hf. The Holenarsipur basalts display nearly flat REE patterns with low LREE/HREE [(La/Yb)N = 0.26-0.97] and negative to positive Eu (Eu/Eu* = 0.93-1.05) anomalies whereas the Chitradurga basalts show flat REE patterns [(La/Yb)N = 0.92-1.28] analogous to MORB. The U-Pb zircon age of the Holenarsipur basalts indicate 3235 ± 54 Ma with multiple thermal overprints after their deposition due to Paleoproterozoic to Mesozoic dyke emplacement. The Holenarsipur and Chitradurga basalts show partial melting trend whereas the Shimoga basalts follow fractional crystallization trajectory. Nb/La (0.13-1.33) and La/Yb (0.38-6.76) ratios of all these basalts represent the derivation of their primary magma from the lithospheric mantle with a significant asthenospheric contribution reflecting on mantle recycling. Furthermore, Nb/Yb and La/Yb relationship indicates that the Holenarsipur and Chitradurga basalts are derived from N-MORB type mantle whereas Shimoga basalts are derived from slightly enriched mantle. Nb/Yb and Th/Yb relationship show back arc basin affinity for the Chitradurga basalts whereas Holenarsipur basalts endorse oceanic arc characteristics and the Shimoga basalts display continental arc affinity. These basalts follow the spinel to spinel-garnet lherzolite melting regime and the degree of melting ranges from 10-30% resembling the global counterparts. Considering the regional geology, geochemistry and geochronology of these three greenstone belts, it is proposed that the WDC experienced three different tectono-magmatic events viz. the oceanic arc, back arc basin and continental arc during the Meso-Neoarchean which played a key role in the crustal evolution of Dharwar Craton.