• Neoarchean Gingla granitoids include subduction-related TTGs, and high-K granitoids. • TTGs formed by variable-depth melting of hydrated mafic crust. • TTG melts assimilated pre-existing felsic crust to form transitional TTGs. • Potassic granites formed by anatexis of older transitional TTGs. • Back-arc metabasite dykes reflect slab-melt metasomatised mantle source. Despite the vast expanse of Neoarchean granitoids in the southern Aravalli–Banded Gneissic Complex (Aravalli-BGC) of northwestern India, their petrogenesis and geodynamic significance remain poorly constrained. This lack of integrated data limits our understanding of the region’s Neoarchean geodynamic evolution, particularly as the associated metabasites, essential for deciphering Archean geodynamics, have yet to be investigated. This study addresses these fundamental gaps through a comprehensive petrological and geochemical analysis of the Gingla granitoids and their associated metabasites. The granitoids comprise nearly coeval TTGs and high-K granitoids, including transitional TTGs and potassic granites. No cross-cutting relationships are observed among the granitoid types, whereas metabasites occur as relatively younger dykes. The TTGs were generated by partial melting of hydrated mafic crust in a subduction setting at variable depths, generating high-HREE/low-P Group-I TTGs (ΣHREE = 6.0–14.7 ppm, Nb = 3–11 ppm, Ta = 0.3–1.2 ppm, Sr/Y = 13.8–40.1, (La/Yb) N = 11.9–45.9 and Eu/Eu* = 0.56–0.89) at shallower levels (1.0–1.2 GPa) and medium-HREE/medium-P Group-II TTGs (ΣHREE = 4.2–5.9 ppm, Nb = 4 ppm, Ta = 0.4 ppm, Sr/Y = 96.0–124.6, (La/Yb) N = 51.1–127.4 and Eu/Eu* = 1.14–1.33) at moderate depths (ca. 1.5 GPa), leaving 10–15% garnet-rich residue. During ascent, TTG melts underwent assimilation of pre-existing felsic crust, resulting in the formation of transitional TTGs. Potassic granites likely formed by anatexis of an older generation of transitional TTGs or represent a slightly younger magmatic phase, separated by a time gap of 25–35 Myr. The coexistence of TTGs, transitional TTGs and potassic granites reflects Neoarchean magmatism under elevated geothermal gradients, estimated at 19–20 °C/km for medium-HREE/medium-P Group-II TTGs, and 23–30 °C/km for those of high-HREE/low-P Group-I TTGs, transitional TTGs and potassic granites, significantly higher than those of modern subduction zones (5–8 °C/km). The metabasite dykes were derived by the 15–20% partial melting of depleted lithospheric mantle in the spinel stability field, metasomatised by 1–2% input of subducted material in a back-arc basin setting. The spatial association of TTGs, high-K granitoids and back-arc metabasites under elevated geothermal regimes is best explained by the heat supplied through asthenospheric upwelling induced by slab break-off.
Sharma et al. (Fri,) studied this question.
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