Nd and Sr isotope systematics of Miocene to Holocene volcanic rocks from Southwest Japan: volcanism since the opening of the Japan Sea

Abstract

Isotope (Nd and Sr) and geochemical data for twelve volcanic rocks from the Shimane-Oki (San'in) region of Southwest Japan give some insight into the Miocene-Recent volcanic history of southern Japan since rifting from the Asian mainland. The oldest rocks examined (Dolerites and Ushikiri Formation) were erupted during the opening of the Japan Sea at about 15 Ma, and comprise arc tholeiites derived from a depleted source (ε Nd > 0; ε Sr < 0). 11 Ma old basalts of the nearby Matsue Formation have transitional arc—within-plate trace element patterns, and ε Nd < 0 and ε Sr > 0, interpreted in terms of a depleted mantle source modified by a slab-derived fluid. Volcanics of Oki Dozen Island in the Japan Sea are alkalic, derived from an enriched mantle plume, isotopically distinct from nearly Oki Dogo Island. Eruption of the calc-alkaline Wakurayama Andesite (early Pliocene) and both arc and within-plate-type basalts (< 1 Ma) show that magmas were tapped from a variety of sources. Taking into account plate tectonics and seismic data, the subduction influence until the Quaternary was the Pacific plate, with Philippines Sea plate subduction affecting Quaternary and younger volcanics.