Geochemistry and chemical (Th–U–Pb) monazite age of Pamuru Granite interface of Nellore Schist Belt and Nallamalai Fold Belt, Southern India: Implication for Columbia Break up and Pan-African assembly

The Proterozoic granitic magmatism along the interface of the Nallamalai Fold Belt (NFB) and the adjoining Nellore Schist Belt (NSB) in southern India exhibits a distinct arc-back arc tectonic setting. The granites emplaced along the interface of Cuddapah/ NFB and NSB exhibited distinct geochemical traits and geochronological ages ie., granites (e.g. Vinukonda) with age ∼1575 Ma display an arc-magmatic signature, while other granites (e.g. Kanigiri) show age of 1240 Ma with rift-related petrogenetic signature indicative of an extensional arc setting. This study presents a detailed petrogenetic analysis of the Pamuru granite (PG) and reports chemical Th–U–Pb monazite ages situated along the Terrain Boundary Tectonic Zone (TBTZ) along NSB and Cuddapah Basin. The PG predominantly consist of plagioclase feldspar, K-feldspar, and quartz, along with biotite and muscovite. It also hosts accessory minerals such as allanite, zircon, monazite, epidote, garnet and sphene. Geochemically, PG exhibits high calc-alkaline to shoshonitic affinity, a compositional trend ranging from peralkaline to peraluminous, and is typically ferroan in nature. These granites plot within the post-collisional to late-orogenic tectonic fields, suggesting emplacement in a within-plate setting. The trace element signatures of PG highlight the role of highly differentiated magma and indicate significant contribution from the lower crust in the evolution of this A2-type granite. Th–U–Pb chemical dating of rounded, zoned monazite grains from PG reveals two distinct age populations: 1651 ± 58 Ma and 513 ± 50 Ma, corresponding to the well-differentiated cores (ThO2: avg. 6.61–13.82 wt.%) and rims (ThO2: avg. 7.25–8.58 wt.%), respectively. Textural evidence and previously reported age data suggest that these monazites are xenocrystic in nature, likely derived from the 1615 ± 25 Ma to 1589 ± 5.7 Ma analogous granitic magma. However, PG geochemical characteristics exhibit a contrasting nature with VIN granites (1615 Ma). But, it shows similar geochemical signatures akin to TBTZ granite (1284 Ma). The upper intercept ages (1681 ± 55 Ma and 1567 ± 73 Ma) represent magmatic emplacement events, consistent with the final assembly of the Columbia supercontinent (1.8 to 1.6 Ga). In contrast, the lower intercept ages (511 ± 47 Ma and 515 ± 53 Ma) recorded in the monazite rims reflect a Pan-African thermal overprint on the PG. These ages represent the Pan-African incorporation of India into the Gondwana supercontinent.