Paleoproterozoic evolution of the arc–back-arc system in the east Sarmatian orogen (East European Craton): zircon shrimp geochronology and geochemistry of the Losevo volcanic suite

The East European Craton has a thick sequence of volcano-sedimentary rocks preserved in the Losevo belt that developed along the junction between Sarmatian and Volgo-Uralian microcontinents. The major lithologies of the Losevo terrain (LT) are a dominant bimodal volcanic suite and a basalt–andesite–dacite–rhyolite assemblages (BADR). The LT rocks have been divided from lower to upper sequences into the Terrigene, Strelitsa and Podgornoye Formations, but the stratigraphic subdivisions have not been geochronologically tested. Here we present geochemistry and SHRIMP zircon geochronology of volcanic rocks from the LT. The volcanic suite from the Terrigene Formation has tholeiitic and calc-alkaline affinites, significant enrichment in LILE and LREE and strong depletion in HFSE with εNd(t)=+ 2.6, whereas the felsic dikes display an A-type affinity, with typical enrichment in Zr, Nb, Y, and depletion in Sr and Ti, fractionated REE patterns, and strong negative Eu anomalies with εNd(t) in the range of -0.5 to 2.6. The bimodal volcanic suite of the Strelitsa Formation is composed of tholeiites displaying minor depletion in LREE, slight enrichment of LILE, no or weak depletion of Nb resembling transition MORB with εNd(t)=+3.0 to +3.6) and rhyolites with high LREE/HREE, high Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (εNd(t)=+1.8 to +2.9) resembling slab-derived high pressure adakitic melts. The volcanic rocks of the Podgornoye Formation are bimodal with tholeiitic chemistry, lack enrichment in LILE and LREE and have a slight depletion in HFSE (εNd(t)=+3.7) together with rhyolites having high LREE/HREE, moderate Sr/Y, no Eu anomaly, and strong depletion in Nb and Ti (εNd(t)=+2.1 to +2.6) resembling slab derived relatively low-pressure adakite-like melts. The BADR assemblage has significant enrichment in LILE and LREE and strong depletion in HFSE, similar to arc-like volcanics. Geochronological data indicate that the early LT volcanic rocks were formed during the early (Terrigene Formation) stage of intra-continental arc with a continental basement whereas the Strelitsa bimodal volcanic rocks were formed during a middle stage of back-arc extension and the Podgornoye bimodal volcanic rocks and BADR were formed during a later stage intra-oceanic arc. The identification of a 2170 to 2120 Ma back-arc basin in the East Sarmatian Orogen together with broadly coeval arcs indicate that the eastern margin of the Sarmatia was active with an arc–back-arc environment. Our new data suggest that the initial melts of the bimodal suite were adakitic derived by slab melting, followed by mantle metasomatism, whereas the basaltic magmas formed in an island arc setting. The LT and similar-aged volcanic belts in other terrains are considered to represent the initial (2.1–2.0 Ga), subduction-related growth of the Paleoproterozoic Columbia supercontinent.Roman A. Terentiev, Konstantin A. Savko and M. Santos