Zircon U-Pb geochronology, Lu-Hf isotope systematics, and geochemistry of bimodal volcanic rocks and associated granitoids from Kotri Belt, Central India: Implications for Neoarchean–Paleoproterozoic crustal growth

Abstract

The Bastar Craton of Central India has a thick sequence of volcano–sedimentary rocks preserved in Kotri–Dongargarh belt that developed on a tonalite-trondhjemite-granodiorite (TTG) basement followed upwards by the Amgaon, Bengpal, Bailadila, and Nandgaon Groups of rocks. Here, we report the U-Pb geochronology and Lu-Hf isotope systematics and whole rock geochemistry of volcanic rocks and associated granitoids belonging to the Pitepani basalts, Bijli rhyolites, and Dongargarh granite in the Nandgaon Group of the Kotri belt. The volcanic rocks of the Nandgaon Group are bimodal in nature in which the basalts exhibit intergranular, porphyritic to spherulitic texture composed of pyroxenes, plagioclase, tremolite, actinolite, and chlorite±Fe oxides. The rhyolites display porphyritic texture consisting of K-feldspar, quartz, and plagioclase as phenocrysts. The associated porphyritic granitoids have K-feldspar, microcline, plagioclase, and biotite phenocrysts within a groundmass of similar composition. The bimodal suite displays LILE, LREE enrichment, and HFSE depletion with significant negative Nb-Ta anomalies combined with slightly fractionated REE patterns in the basalts and highly fractionated patterns and prominent negative Eu anomalies in the rhyolites endorsing their generation in an island-arc/back-arc tectonic setting. The geochemical features of the associated granitoids indicate that these are potassic and classify as within-plate A-type granites. Zircons from the basalts show clear oscillatory zoning in their CL images. They cluster as a coherent group with 207Pb/206Pb spot ages ranging from 2446 to 2522Ma and weighted mean age of 2471±7Ma. Zircons from the rhyolite samples are subhedral to euhedral and show simple oscillatory zoning with some heterogeneous fractured domains. The data from two samples define upper intercept ages of 2479±13Ma and 2463±14Ma. Zircon grains in the granite show clear oscillatory zoning and their U-Pb data define an upper intercept age of 2506±50Ma. The Lu-Hf isotopic data on the zircons from the basalts show initial 176Hf/177Hf ratios from 0.280925 to 0.281018. Their εHf(t) values are in the range of −10.0 to −6.7. The Hf-depleted model ages (TDM) are between 3038Ma and 3171Ma, and Hf crustal model ages (TDMC) vary from 3387–3589Ma. The zircons from the rhyolites show initial 176Hf/177Hf ratios from 0.280919 to 0.281020 and from 0.281000 to 0.281103, respectively, with εHf(t) values varying from −10 to −6.4 and from −7.5 to −3.9. Among these, one sample shows TDM between 3038Ma and 3182Ma, and TDMC varies from 3377 to 3596Ma, whereas the other sample shows ages of 2925Ma and 3072Ma with TDMC varying from 3208 to 3432Ma. The initial 176Hf/177Hf ratios of the granites range from 0.280937 to 0.281062 with εHf(t) values of −8.8 to −4.3. The TDM shows a range of 2979Ma and 3170Ma, and TDMC varies from 3269 to 3541Ma. The predominant negative εHf(t) values of zircons from these rocks suggest that the source material was evolved from the Paleoarchean crust. The geological, geochemical, and geochronological evidence suggests coeval tectonic and magmatic episodes of volcanic and plutonic activity in an island-arc setting where the arc migrated toward the continental margin and played a significant role in the Neoarchean–Paleoproterozoic crustal growth of the Kotri belt of Central India.