Origin, petrogenesis, and tectonic implications of post-collisional A-type peraluminous granite-gneisses from the Aravalli–Delhi Fold Belt, NW India: Constraints from geochemistry, Sr–Nd isotopes, and zircon U–Pb geochronology

Within the Aravalli-Delhi Fold Belt, NW India, sporadic granitic-gneisses outcrop are exposed in the Anasagar–Beawar region, whose age, origin, petrogenesis, tectonic setting, and stratigraphic status remain poorly constrained. Our zircon U–Pb geochronological data indicate that the protoliths of the Anasagar granite-gneiss (AGG) and Beawar granitic-gneiss (BGG) were emplaced at 1851 ± 20 Ma and 1130 ± 69 Ma, respectively. Field observations reveal that the AGG occurs as concordant, sheet-like bodies within the surrounding supracrustal rocks, giving a deceptive appearance of being the basement, whereas the BGG is exposed as isolated plutons surrounded by extensive soil cover. These relationships indicate that the Anasagar supracrustal sequence is older than 1.85 Ga—the crystallisation age of the AGG. Consequently, the AGG and its associated supracrustal rocks predate the majority of lithologies of the Delhi Supergroup. Therefore, these rocks are not part of the Delhi Supergroup but instead represent a small, isolated basin that developed during the closure of the Aravalli basin. Both AGG and BGG are megacrystic granitoids composed predominantly of quartz, K-feldspar, and plagioclase. They are calc-alkaline to shoshonitic, magnesian to ferroan, alkali-calcic to calcic, and are strongly peraluminous. The granitoids are characterised by high SiO₂, Na₂O + K₂O, low CaO and MgO, high FeO^t/(FeO^t + MgO), high Ga/Al ratios, and high average zircon saturation temperatures (~900°C for AGG and ~880°C for BGG), typical of A-type granitoids. Their enriched LREEs and LILEs, low Nb/Ta, and negative Eu anomalies indicate crustal sources. Both the granite-gneisses are classified as peraluminous A-type granitoids emplaced in a post-collisional extensional setting. The AGG exhibits εNd(t) values ranging from –6.4 to –4.6 and T_DM ages between 2.57 and 2.78 Ga, whereas the BGG shows εNd(t) values from –11.2 to –10.4 and T_DM ages between 2.53 and 2.81 Ga. Petrogenetic evidence suggests that both the granite-gneisses originated from the dehydration partial melting of Neoarchean crustal rocks, likely Banded Gneissic Complex (BGC) granitoids and Tonalite–Trondhjemite–Granodiorite (TTG) gneisses, under high-temperature and low-pressure conditions. The required heat flux was likely provided by large-scale mafic underplating or asthenosphere upwelling. Our findings suggest that the late Paleoproterozoic to early Mesoproterozoic tectonic evolution of the study area reflects two major events. The emplacement of the AGG represents the assembly of the Columbia supercontinent, whereas the BGG is not related to the assembly of Rodinia, as previously considered; instead, it more likely represents the pre-Rodinia assembly extension phase.

Keywords: NW India, Aravalli-Delhi Fold Belt, zircon U-Pb geochronology, A-type peraluminous granitoids, Sr-Nd isotopes, Crustal evolution