Roots of a Collapsed Paleoproterozoic Island Arc in the Volga−Don Ocean, East Sarmatian Composite Terrane: Geochemistry, Age, and Petrogenesis

The Archean Varvarinsky block was distinguished in the southern Vorontsovka terrane of Eastern Sarmatia in the late 1960s. It was proved to consist of Paleoproterozoic I-type sodic diorites and granodiorites dated at 2.09 Ga. Isotopic and geochemical characteristics of its rocks (highly radiogenic Nd composition, enrichment in Cr and Ni, depletion in LILE, HFSE, and LREE, as well as low Sr/Y values) suggest their island-arc nature. The sources were aqueous melts from the mantle wedge, metasomatized by fluids during the melting of a subducted oceanic slab, followed by fractionation in intermediate chambers. The finds of the most ancient precollisional granitoid intrusions provide direct evidence that island arc structures occurred in the Volga−Don Ocean and were composed of calc-alkaline volcanics of basalt−andesite−dacite−rhyolite (BADR) composition. They are the intrusive roots of the destroyed volcanic island arcs produced during the subduction of oceanic slabs in the Volga−Don Ocean.