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

摘要

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 Pb-207/Pb-206 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 Pb-207/Pb-206 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 epsilon Hf(t) values of -4.9--0.7 with T-DM(C) (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' epsilon