Geochemistry and Sr and Pb isotope systematics of basement granitoids from north and west of palnad sub-basin, guntur and nalgonda districts, Andhra Pradesh

Basement granitoids exposed around Palnad sub-basin exhibit metaluminous, calc-alkaline nature and belong to adamellite– granodiorite–tonalite suite. They show wide variation in FeOt, CaO, TiO2 and P2O5 contentswith an overall predominance of sodium over potassium. Besides, predominance of LREE over HREE, fractionation among LREE and strong negative Eu anomalies point towards evolved nature of granitoids. In Zr+Nb vs. Rb plot, they fall in VAG field while R1–R2 diagram exhibits their development in late orogenic–post collisional geotectonic environment in active continental margin setup. A radioactive granite phase could also be distinguished exhibiting potash rich and peraluminous nature with higher degree of differentiation, high Rb/Sr and U/Th ratios. Positive correlation of U with K and Rb suggest that uranium is concentrated in the potash-rich phases. These basement granitoids have indicated comparable Rb–Sr (2659±120 Ma) and Pb–Pb (2638±94 Ma) isochron ages probably indicating the time of their emplacements while the initial 87Sr/86Sr ratio of 0.7012 suggests depleted upper mantle source. However, the radioactive granite sample has shown Pb–Pb age of ca. 1545±140 Ma with model μ1=8.3±1.4 and MSWD= 55. This younger Pb–Pb age with high MSWD value is presumably due to the local redistribution of U and Pb and is indicative of a younger reactivation event in the study area.