Cenozoic Volcanism of Sakhalin and Back-arc Spreading

Abstract

The Cenozoic volcanic rocks in Sakhalin Island record a tectonic transition from convergence to divergence in connection with the formation of the Japan Basin and the Kuril Basin. The magmatism of this region comprises the following distinct stages of tectonomagmatic evolution: 1) subduction-related calc-alkaline volcanism in the Oligocene along the northeastern edge of Eurasian continent. 2) early Miocene volcanism characterized by island arc tholeiites surrounding the developing back-arc basins. 3) late Miocene to Pliocene volcanism, yielding within-plate geochemical signatures. The temporal geochemical trends over 55 m. y. in volcanic rocks from Sakhalin and nearby Sikhote-Alin suggest that there was a change in magma source from the lithosphere to the depleted asthenosphere as both the Japan Sea and the Okhotsk Sea opening progressed.
The early Miocene volcanism along the western edge of central Hokkaido from Rebun Island to off the coast of Tomakomai is characterized by bimodal volcanic rocks. The lack of correlation between relatively enriched basalts from central Hokkaido and depleted basalts from Sakhalin and Sikhote-Alin suggests that the source of the former basalts does not have a significant contribution from the asthenosphere.