Neoarchean arc‐back arc subduction system in the Indian Peninsula: Evidence from mafic magmatism in the Shimoga greenstone belt, western Dharwar Craton

Abstract

The Neoarchean Shimoga greenstone belt (SGB) of western Dharwar Craton is dominated by ultramafic‐mafic‐felsic magmatic suite and sedimentary rocks. Here, we investigate the mineral chemistry and whole‐rock geochemistry of the Koppa–Herambapura mafic magmatic rocks (tholeiitic metagabbro and metabasalt) from the southern SGB in order to understand their petrogenesis and geodynamic implication. The geochemical features classify the Shimoga rocks as basaltic‐basaltic andesite with a sub‐alkaline tholeiitic signature. Both the rock types display uniform depletion in HFSE relative to LILE‐LREE, higher LILE/HFSE, and LREE/HFSE ratios (La/Nb = 1.30–8.07, Nb/Ta = 11.58–16.19, Zr/Sm = 10.24–29.77) and negative Nb–Ta, Zr–Hf, and Ti anomalies in multi‐element patterns, corresponding to magmatic arc rocks generated within intraoceanic subduction environment. Their Nb/Th (2–15), Zr/Sm (10–29), and Zr/Hf (44–233) ratios indicate a depleted to enriched mantle source, and the parental magma of the mafic rock suite was generated by low to medium degrees (~8–30%) of mantle melting at depths corresponding to spinel to garnet peridotite fields of arc‐back arc setting. Our field and analytical data suggest that the tholeiitic metagabbros were derived from depleted mantle source generated in an arc environment, whereas the metabasalts had a mantle source attributed to (a) development of juvenile back‐arc rift; (b) upwelling of MORB‐like mantle; and (c) flux‐induced melting in the compositional domain of spinel‐garnet peridotite. Based on these results, we conclude that the mafic rock suite from the southern SGB possibly marks one of the earliest Neoarchean arc‐back arc subduction in the western Dharwar Craton.