Abstract
We identify four stages in the Miocene magmatic activity on Rebun Island, northern Hokkaido, Japan: Stage I (18 Ma), Stage II (13 Ma), Stage III (10 Ma), and Stage IV (<10 Ma). Stages I and II produced small intrusions of dolerite and dacite, respectively. During Stage III, basalt to andesite lavas erupted, while the magmatism of Stage IV formed large intrusions of dolerite, porphyrite, and andesite with dacitic veins. The large variations of major and trace element compositions of Stage III and IV rocks can be explained by considering the fractional crystallization of a basaltic melt; however, the proportion of phases fractionating is different for the two stages. Further, in comparison to Stage IV rocks, Stage III rocks have lower concentrations of incompatible elements and higher Zr/Y ratios with similar Zr/Nb ratios, suggesting higher degree of partial melting of the source material. The initial values of 87Sr/86Sr (SrI) and 143Nd/144Nd (NdI) indicate that the rocks of all stages were derived from a common source material, although Stage IV andesites were apparently formed by assimilation fractional crystallization (AFC) during their intrusion into sedimentary rock.
Most of the rocks have SrI and NdI values similar to those of basaltic rocks derived from the upwelling of depleted asthenospheric mantle during the spreading of the Japan Sea back-arc basin. In addition, incompatible element signatures obtained for Stage I rock are similar to those obtained for the basalts in the Japan Sea back-arc basin. In contrast, Stage II-IV rocks are similar to the basaltic rocks that originated from subduction-related magmatism in northern Hokkaido.
