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

摘要

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 (similar to 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.