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 subalkaline 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, intra-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.