Sm-Nd ages of two meta-anorthosite complexes around Holenarsipur: Constraints on the antiquity of Archean supracrustal rocks of the Dharwar craton

Whole-rock Sm-Nd isochron ages are reported for two stratiform meta-anortho site complexes emplaced into the Archean supracrustal-gneiss association in the amphibolite facies terrain around Holenarsipur, in the Dharwar craton, South India. While these metaperidotite-pyroxenite-gabbro-anorthosite comp lexes are petrologically and geochemically similar, they differ in the inte nsity of tectonic fabric developed during the late Archean (c. 2.5 Ga) defo rmation. They also differ in their whole-rock Sm-Nd isochron ages and initi al Nd isotopic compositions: 3.285 +/- 0.17 Ga, epsilon(Nd) = 0.82 +/- 0.78 for the Honnavalli meta-anorthosite complex from a supracrustal enclave in the low-strain zone, and 2.495 +/- 0.033 Ga, epsilon(Nd) = -2.2 +/- 0.3 fo r the Dodkadnur meta-anorthosites from the high-strain southern arm of the Holenarsipur Supracrustal Belt (HSB). We interpret these results as indicat ing that the magmatic protoliths of both meta-anorthosite complexes were de rived from a marginally depleted mantle at c. 3.29 Ga but only the Dodkadnu r rocks were isotopically reequilibrated on a cm-scale about 800 Ma later p resumably due to the development of strong penetrative fabrics in them duri ng Late Archean thermotectonic event around 2.5 Ga. Our results set a young er age limit at c. 3.29 Ga for the supracrustal rocks of the HSB in the Dha rwar craton.