Neoarchean crustal reworking inferred from granitoids in the western Dharwar Craton: Constraints from Nd isotopic composition, trace elements, and phase equilibrium modelling

Abstract

The granitoid magmatism in the Chitradurga greenstone belt implies crustal reworking in the western Dharwar Craton during the Neoarchean. High-K granites and low-K trondhjemites are studied for their petrogenesis using the whole-rock elemental and Nd isotopic composition and compared with the earlier works. These granitoids were formed by partial reworking of older crust, which includes TTG gneisses and metabasites. Low Sr/Y ratios (0.1–7.6) and zircon saturation temperatures (< 850 °C) suggest that crustal anatexis occurred at the shallower crustal levels by low-T fluid-present melting. Phase equilibrium and trace element modelling show that the potassic granites were formed by partial melting of TTG gneisses at shallower depths corresponding to 4–6 kb pressures and melt extraction in batches leaving a final residue comprising ∼66% plagioclase, ∼ 30% quartz, ∼ 3% orthopyroxene, and ∼ 1% ilmenite. Monzogranite was formed from the melt extracted at a higher temperature with some entrained minerals, whereas the early batches of water saturated melt modified by fractional crystallization during the ascent formed syenogranites. The low-K trondhjemites were formed by 10–12% partial melting of metabasites at 4–5 kb pressures, leaving residual assemblages with plagioclase (42–46%), amphibole (24–26%), orthopyroxene (20–21%), ilmenite (4–5%), and garnet (2–9%). The partial melting of various crustal source rocks at different crustal levels indicates a reworking event in the terrane, which was likely triggered by mafic underplating and H2O dominated fluid flux.