奥尻島漸新世火山岩の岩石学 東北日本漸新世，陸弧火山フロントにおける火山活動

Abstract

The Oligocene volcanism of the Okushiri Island, situated on the northeastern border of the Japan Sea, may have taken place as the volcanic front of the Eurasian continental margin prior to the formation of the Japan Sea basin. The Oligocene volcanic rocks from the Okushiri Island consist of High-Mg andesite (HMA), High-Ti basalt to andesite (HTV), and Low-Ti andesite (LTV). The rocks of HMA have high contents of MgO, Cr and Ni, and have low Sr and high Nd isotopic ratios. The patterns of primordial mantle-normalized incompatible elements indicate that the HMA rocks are enriched in Rb, Ba, K and Sr and depleted in Nb, and show patterns which resemble those of calc-alkaline rocks from back arc side of the NE Japan arc. The HTV rocks, comparable with the high alumina basalt series, are characterized by high concentrations of both LIL and HFS elements, high Sr isotopic ratio. The primordial mantle-normalized geochemical patterns and the Sr and Nd isotopic ratios of the HTV are similar to those of an enriched island arc tholeiite in the NE Japan arc. By reference to melting experiments at high pressure, the HTV magmas were inferred to have been segregated at 50-60km depth. The LTV rocks are locally exposed, in which contamination phenomena are apparent from the petrographical and geochemical point of views. In comparison with the NE Japan arc magma, we discussed the chemical heterogeneity of the wedge mantle, the characteristics of the Oligocene frontal volcanism at the Eurasian continental margin, and the temporal variation of the Sr isotopic ratios of the Cenozoic volcanic rocks from SW Hokkaido, Japan.