Mafic-ultramafic suite from the Karwar Block, SW India: Implications for Mesoarchean geodynamics

Abstract

Archean mafic–ultramafic rocks provide important clues on the tectonic and crust building processes in the early Earth. The Western Dharwar Craton (WDC) and the Karwar Block (KB) in southern Peninsular India preserve some of the oldest records of craton building processes in the Archean Earth. The tectonic affinity of the Karwar Block that lies at the western periphery of the WDC remains uncertain. Here we present the results from an integrated study of a mafic–ultramafic rock suite from this block using petrology, whole-rock geochemistry, and zircon U–Pb and Lu-Hf data. These rocks occur as deformed and dismembered units within basement granitic gneiss. The primary igneous minerals such as pyroxene and plagioclase show variable degrees of alteration. The rocks show negative Nb anomalies and prominent Pb enrichment. The uniform enrichment of Th and U over Nb-Ta indicates the influence of metasomatic fluids in the mantle wedge. The geochemical data indicate that these rocks were derived from basaltic magmas generated within a subduction metasomatised mantle wedge in an intra-oceanic arc setting without any significant crustal contamination. U-Pb isotope analysis of zircon grains from the metapyroxenite shows upper intercept ages of 3172 to 3213 Ma and weighted mean ages of 3170 to 3203 Ma. A metagabbro sample yielded upper intercept age of 2792 Ma and the 207Pb/206Pb spot ages in another sample range up to 3202 Ma. Zircon U-Pb data suggest that the mafic–ultramafic suite formed at ca. 3.2 Ga and were overprinted by 2.7 Ga thermal event. The εHf(t) values are all positive and range between 0.73 and 6.02, suggesting that the magma was derived dominantly from Mesoarchean juvenile (depleted mantle) sources. We propose a tectonic model that involves a subduction-accretion stage followed by suturing of the ancient continental nuclei.