Geochemistry and Sr-Nd isotopic studies of Precambrian gneisses from central Aravalli Craton, NW India: Implications for crustal evolution and reworking

Abstract

The Aravalli Craton of the Indian shield constitutes heterogeneous basement lithologies (Banded Gneissic Complex; BGC), and among them, the granitoids are the most voluminous lithology. The BGC comprises two lithotectonic units, viz., BGC-I and BGC-II. The BGC-II has been further classified as amphibolite facies Mangalwar and granulite facies Sandmata Complexes. In the present study, the gneisses of the Mangalwar Complex are geochemically categorized into (i) low-and high-pressure sodic gneisses and (ii) potassic gneisses. The sodic gneisses are metaluminous and characterized by high Sr/Y and LaN/YbN ratios; and exhibit subduction-related negative anomalies of Nb and Ti. The εNd (t = 2992 Ma) ranges from +2.3 to +3.1, with an average Nd-depleted mantle model age (TDM) of 3.06 Ga. The whole-rock Sm-Nd isochron age is ∼3.0 Ga (2992 ± 340 Ma). Genetically, the sodic gneisses originated from the melting of an enriched precursor (oceanic plateau) in an arc environment. These gneisses show strong correlations with the gneisses from BGC-I depicting similar geochemical signatures. In contrast, the potassic gneisses are characterized by slightly higher SiO2 along with high K2O and high large-ion lithophile elements and negative Eu anomalies along with negative εNd (t = 1.7 Ga) (−13.2 to −3.9), higher initial 87Sr/86Sr isotopic ratios and average TDM = 2.87 Ga. These geochemical features of the potassic gneisses indicate that they were derived from the reworking of the pre-existing TTG-like (sodic gneisses) crust during the Paleoproterozoic Era.