Petrography, Origin and Geochemical Characters of the Granitoids of the Southern Part of the Nellore Schist Belt, Andhra Pradesh, India

Abstract

The Archaean to Paleoproterozoic migmatitic gneisses and granitoids, surrounding Rapur, in the southern part of Nellore Schist Belt, Andhra Pradesh, include low-K Tonalite-Trondhjemite-Granodiorite Suite (TTG), Tonalite-Granodiorite-Monzogranite Suite (TGM) and Monzogranite-Syenogranite Suite (MS). The TTG suite shows calcic to calc-alkaline nature and compositionally straddle the boundaries between metaluminous and peraluminous as well as ferroan and magnesian characters. They display highly fractionated REE pattern in chondrite-normalized diagram along with a low HREE content, with moderate to small positive Eu anomalies. The TGM suite’s composition is transitional between the TTG and MS suites. The latter are comparable with continental margin calc-alkaline rocks, characterized with higher alkali content and lower CaO than the TTG suite, with a rather fractionated REE pattern highlighting a typically increased LREE rate, with slightly negative and positive Eu anomalies and flat to depleting HREE. The MS granites display a chemical homogeneity (especially in major elements), characterized with a K-rich nature bearing pink and grey feldspar and trace element percentage. The MS suite of granites exhibit increased LREE rates, with negative Eu anomaly, and flat HREE with a predominant proportion of LREE over HREE. The relating chemical compositions suggest well that the TGM rocky suites have been recycled through crustal re-melting processes, culminating in the MS suite associated granites. The mineralogical and geochemical shifts characterizing the granitoids composition are related to changes in magma source. The TGM and MS suites’ transition from TTG to more potassic granitoids proves to be gradual, with the more recently formed granitoids having evolved over a substantial period of time, ranging from the Archaean to the late Archaean age. The MME, as occurring within TGM suite, are fine grained, slightly dark and enriched in mafic minerals compared to the host granitoids, displaying a wide variation in REE contents, as judged by their compositional heterogenity, but generally characterized with high LREE (ΣLREE = 315.27) and low HREE (ΣHREE = 29.77) contents. The MME display fractionated LREE and flat HREE patterns, along with slightly negative (Eu/Eu* = 0.83), or even no Eu-anomalies. Most MMEs prove to demonstrate noticeable enrichment in Sr, Th, Zr, Hf, and REE relative to the mantle values.