Geochemistry, zircon ages and whole-rock Nd isotopic systematics for Palaeoproterozoic A-type granitoids in the northern part of the Delhi belt, Rajasthan, NW India: implications for late Palaeoproterozoic crustal evolution of the Aravalli craton

摘要

Determination of zircon ages as well as geochemical and Sm-Nd isotope systematics of granitoids in the Khetri Copper Belt of the Aravalli mountains, NW India, constrain the late Palaeoproterozoic crustal evolution of the Aravalli craton. The plutons are typical A-type within-plate granites, derived from melts generated in an extensional tectonic environment. They display REE and multi-element patterns characterized by steep LREE-enriched and almost flat HREE profiles and distinct negative anomalies for Sr, P and Ti. Initial epsilon(Nd) values range from - 1.3 to - 6.2 and correspond to crustal sources with mean crustal residence ages of 2.5 to 2.1 Ga. A lower malic crustal anatectic origin is envisaged for these granitoids, and the heterogeneous epsilon(Nd(t)) values are inferred to have been acquired from the magma source region. Zircon Pb-Pb evaporation and U-Pb ages indicate widespread rift-related A-type magmatism at 1711-1660 Ma in the northern Delhi belt and also suggest a discrete older magmatic event at around 1800 Ma. The emplacement ages of the compositionally distinct A-type granitoid plutons, and virtually coeval granulite metamorphism and exhumation in another segment of the Aravalli mountains, further signify that part of the Aravalli crust evolved during a widespread extensional event in late Palaeoproterozoic time.