Petrology and geochemistry of paleoproterozoic A-type granite at Kanigiri in the Nellore–Khammam schist belt, Andhra Pradesh, India

Abstract

The Kanigiri Pluton (6.2 × 2.1 km 2; 995 Ma) occurs close to a major fault and is intrusive into the meta-volcanics in the western margin of the Nellore–Khammam schist belt in Andhra Pradesh, India. The dominantly metaluminous pluton displays geochemical characteristics of A-type granites, including high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu, displaying distinct variation trends with increasing silica content. It has less fractionated REE patterns and large negative Eu anomalies. The cluster analyses of major and trace elements indicate three main associations: (a) A primary magmatic association characterized by the presence of La, Ce, Fe, Ti, Mn, K, Ba, and Sr, (b) Ore related association consisting of Zr, Hf, Nb, Y, and (c) a volatile association with Rb, Ta, Li, and Cs. Systematic variation in Sr, Rb/Sr, Rb/Ba ratios along with and low K/Rb ratios, reflects fractionation of feldspars. Variations in Sc, and Ta result from fractionation of ferromagnesian silicates. Removal of zircon and allanite affected the Hf/Ta and Ce/Yb ratios. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with the results of melting experiments and indicates that metaluminous granitoids of this pluton were probably derived through melting of lower crustal sources. Geochemical, field and petrological data for the Kanigiri granite suggest that the granite is a partial melting product of mafic rocks like gabbros that are also present in the study region.