Formation and evolution of a Proterozoic magmatic arc: geochemical and geochronological constraints from meta-igneous rocks of the Ongole domain, Eastern Ghats Belt, India

Geochemical data and U-Pb zircon results are presented for the intrusive meta-igneous rocks of the Ongole domain, a granulite-facies terrain of the Eastern Ghats Belt in India, with the aim of inferring the tectonic setting and the timing of their formation. Geochemical data suggest that the intrusive meta-igneous rocks (mafic granulites and charnoenderbites) possess trace and rare earth element composition that are typical of magmatic arcs. They are subalkaline, enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high field strength elements like Nb, Ta and Ti. These characteristics indicate that the primary magmas of these rocks were derived by partial melting of a depleted mantle wedge that had been metasomatized by a slab component. Zircon grains collected from five charnoenderbites are large and euhedral to subhedral and display fine-scale oscillatory growth zoning in CL images, implying a magmatic origin. The grains frequently show narrow-to-broad unzoned overgrowths, implying a metamorphic origin. The oscillatory-zoned cores yield Paleoproterozoic concordia ages of ca. 1,750-1,710 Ma, interpreted as the time of magma emplacement. The unzoned overgrowths yield very late Paleoproterozoic ages of ca. 1,630-1,600 Ma, interpreted as the timing of metamorphism. An enderbite showing both magmatic and metamorphic concordia ages of ca. 1,605 Ma points to the existence of syn-metamorphic intrusions. Together, the presented geochemical and geochronological evidence suggests that the Ongole domain was a magmatic arc near the Indian continent during the Paleoproterozoic. Subsequently, the rocks were metamorphosed during the late Paleoproterozoic, and the terrain was accreted to the Indian craton during the early Mesoproterozoic. The formation and growth of the Ongole domain magmatic arc through subduction-related accretion can be correlated with the growth of Columbia (1.8-1.2 Ga), but its accretion to the Indian craton is apparently unrelated to the formation of any super-ontinent on a global scale.