GEOCHEMISTRY, MINERALOGY, AND PETROGENESIS OF THE MIOCENE-QUATERNARY PLATEAU VOLCANICS OF THE EASTERN SIKHOTE-ALIN

Detailed petrological studies of the flood basalts of the Sovgavan' an d Nel'ma plateaus of the eastern Sikhote Alin, including their Sr and Nd isotope composition and REE distributions, demonstrate that they co mpose two discrete genetic types. The low-potassium tholeiites (<0.5 w t % K2O) and alkalic rocks (>2 wt % K2O) were probably formed by the m elting of continental lithosphere that had been reworked metasomatical ly by old (sodium metasomatism) and relatively young (subduction of an oceanic plate) processes. The magmatic source of the moderately potas sic basalts (0.8 - 1.8 wt % K2O) was probably also restricted to the c ontinental lithosphere and was chemically similar to material of the p rimitive mantle that had been slightly altered by metasomatic processe s, which were related to the subduction of an oceanic plate. On the ba sis of combined geological and petrological data, it is suggested that the formation of the plateau volcanics of the eastern Sikhote Alin wa s not related to the evolution of a plume source but resulted from the melting of the continental lithosphere due to back-are spreading in t he rear part of the modern Japanese island-are system. The geological and geochemical characteristics of these rocks distinguish them from t he Late Cenozoic plateau volcanics of China (large volumes of low-pota ssium lavas, enrichment in LILE, and occasional Ta depletion) and refl ect their specific geotectonic position: they occur in the direct vici nity of the rift basin of the Sea of Japan and above a fossil subducti on zone.