Anorthosites from an Archean continental arc in the Dharwar Craton, southern India: implications for terrane assembly and cratonization

Abstract

Anorthositic rocks have attracted attention in terms of their possible role as the primordial crust of our planet, and also as markers of Archean-Paleoproterozoic plate tectonic regimes. Here we investigate the Konkanhundi gabbro-anorthosite suite from Peninsular India, adjacent to the major collisional suture between the Western and Eastern Dharwar cratonic blocks. The anorthosites display high Al2O3 contents with negative correlation against MgO, high CaO and Sr contents, and positive Eu anomalies indicating plagioclase flotation in the magma chamber. The pyroxenite and gabbroic rocks of the suite show high Ni, Cr, Co contents and geochemical features typical of pyroxene cumulation. LA-ICPMS U-Pb analyses of magmatic zircon grains yield weighted mean 207Pb/206Pb ages of 2601 ± 12 Ma for pyroxenite, 2616 ± 12 Ma for gabbro, 2615 ± 13 Ma and 2627 ± 14 Ma for anorthositic gabbro, 2594 ± 16 Ma for anorthosite, and 2605 ± 27 Ma for microgabbro. The age data from the different rock types in the anorthosite suite are broadly consistent, marking the emplacement time as ca. 2.6 Ga, and providing insights on one of the oldest anorthosite complexes in Peninsular India. Zircon grains in the surrounding TTG gneisses into which the gabbro-anorthosite suite was emplaced show weighted mean 207Pb/206Pb age of 3321 ± 11 Ma suggesting Mesoarchean basement. The zircon grains display high Th/U values and REE patterns typical of magmatic crystallization with enriched HREE, positive Ce and Sm anomalies, and negative Pr, Nd, Eu anomalies. Zircon Lu-Hf analysis yield negative εHf(t) values of −4.9–−0.7 with (two stage model ages) of 3123–3376 Ma for pyroxenite, −4.5–−1.7 and 3192–3366 Ma for gabbro, −5.3–0.9 and 3034–3410 Ma for anorthositic gabbro, and −3.9–-2.4 and 3221–3311 Ma respectively, suggesting the incorporation of Mesoarchean components within the Neoarchean magmatic suite. However, zircon grains in the TTG gneiss possess more ‘juvenile’ εHf(t) values in the range of −0.2–1.3 with in the range of 3554–3646 Ma. The zircon Hf isotopes and trace element data, together with the whole rock geochemical features suggest that the parent magma of the Konkanhundi gabbro-anorthosite suite was derived from subduction-related depleted mantle source that also incorporated continental crustal components. The time of emplacement of the gabbro-anorthosite complex broadly correlates with widespread arc magmatism and crust production as well recycling in other domains of the Dharwar Craton. We envisage multiple convergence of microblocks during craton assembly at the end of Archean in Dharwar, with the greenstone belts marking zones of paleo-ocean closure.