Arc–Back Arc Cohabitation and Associated Bimodal Volcanism: Evidence from Neoarchean Raichur Greenstone Belt, Eastern Dharwar Craton, India

Abstract

AbstractThe Raichur greenstone belt of eastern Dharwar craton preserves the mafic and felsic volcanic rocks that are characterized as bimodal suite in which the mafic rocks are composed of amphibole and plagioclase with minor amount of quartz and pyroxene, whereas the felsic rocks consist of K-feldspars, quartz and plagioclase. These are geochemically distinguished as basalts and dacites in which the silica content of the mafic volcanic rocks ranges from 49.38 to 52.76 wt.%, whereas the felsic counterparts show a variation of 64.20–65.91 wt.%. The mafic volcanic rocks exhibit flat chondrite normalized rare earth element patterns with feeble negative Eu anomalies, whereas the felsic volcanic rocks show differentiated pattern with steep Eu trough and flat HREE. The low Th/Nb (avg.0.17) and La/Nb (avg. 1.63) indicate the retention of pristine mantle character in the mafic rocks. However, low Nb/Th (<8) with high Al2O3, K2O, Rb and Th confirm the crustal contamination coupled with the fractional crystallization process in the generation of the felsic volcanic rocks. The mafic volcanic rocks of this belt correspond to back arc regime with a MORB composition, whereas active continental margin is corroborated for the genesis of felsic volcanic rocks. The Dy/Yb ratio (1.49–1.61) of these mafic volcanic rocks reflects their melting in spinel lherzolite melting regime. The geochemical characteristics of these volcanic rocks indicate the cohabitation of Neoarchean arc and back arc processes in the Raichur belt which is responsible for the crustal growth and gold mineralization in the eastern Dharwar craton.KeywordsRaichur beltBimodal volcanismArcBack arcNeoarchean