Cenozoic magmatism of the north-eastern Eurasian margin: The role of lithosphere versus asthenosphere

Sikhote-Alin and Sakhalin are located in the Russian Far East-flank of the northernmost part of the Sea of Japan. Magmatism in this region preceded, was concurrent with, and continued after the extension and sea-floor spreading (25-18 Ma) that formed the Sea of Japan. Among the Sikhote-Alin and Sakhalin volcanic suites, Eocene-Oligocene (55-24 Ma) lavas are characterized by greater large ion lithophile element and rare earth element enrichments compared with Early-Mid-Miocene (23-15 Ma) tholeiites, and also show a depletion in high field strength elements (HFSE). The geochemical characteristics of the Eocene-Oligocene and Early-Mid-Miocene basalts are consistent with migration of the locus of magma generation beneath the Sikhote-Alin and Sakhalin areas from subduction-modified lithospheric mantle into mid-ocean ridge basalt (MORB)-source asthenosphere as spreading in the Sea of Japan progressed. Mid-Miocene-Pliocene (14-5 Ma) lavas, erupted following the opening of the Sea of Japan, include alkaline and sub-alkline basalts with wide ranges in trace-element abundances, varying between two distinct end-members: (1) volumetrically minor alkaline basalts with Zr-Nb and Sr-Nb-Pb isotope compositions similar to asthenosphere-derived, int-ra-plate-hotspot basalts from eastern China; (2) more abundant, lithosphere-derived, low-alkali tholeiites depleted in HFSE. The similarity of isotopic signatures coupled with systematically different rare earth element (REE) abundances in the Mid-Miocene-Pliocene and Chinese basalts are best modeled by similar extents of melting of spinel lherzolite and garnet lherzolite, respectively. The Mid-Miocene-Pliocene alkali basalts were generated by small degrees of partial melting of hot asthenosphere beneath a thin lithospheric lid; the thin lithospheric mantle beneath the Sikhote-Alin and Sakhalin region resulted from heating and extension associated with the opening of the Sea of Japan.