[SVC50-P12] Petrology of Takikawa monogenetic volcano group and Shokanbetsu volcano: Temporal and spatial variation of magma at the arc-arc junction
Keywords:Monogenetic volcano group, Shokanbetsu volcano, Arc-arc junction, Basalt, Mantle plume
Monogenetic volcanoes at this district could have been formed under extensional stress field accompanied with spreading of Takikawa structural basin (Nakagawa et al.,1993). Therefore, the geochemical features of Takikawa volcanic rocks are expected to reflect temporal and spatial variations of magmatism around the junction of the Kuril and the NE Japan arcs during late Miocene to early Pleistocene. These features can provide us the important constraint for understanding the tectonics at the arc-arc junction. In order to reveal the magmatic process at arc-arc junction, we have been carried out the petrological and geochemical study about Takikawa volcanic field. We investigated 17 rock bodies in TMV, three volcanoes of SHV: Minamishokan, Etai and Ofuyu. In this presentation, we report the petrographical features, and major and trace elements of whole-rock chemistry of these volcanic rocks.
Most of rocks in TMV are augite-olivine basalt. They are absent from plagioclase.. Andesite including plagioclase, hypersthene, and resorbed quartz are rarely occurred. In Minamishokan, the rocks in upper part are hypersthenes-augite andesite with plagioclase and mafic inclusion. In contrast, the lower rocks are quartz-bearing olivine basalt, including plagioclase with dusty zone and resorbed quartz with augite reaction rim. The rocks of Etai are clinopyroxene-olivine basalt, having plagioclase with dusty zone and inclusions with glomeroporphyritic texture. Ofuyu is composed of quartz-bearing augite-olivine basalt, similar to those of Etai, except for the existence of resorbed quartz.
Focusing on incompatible elements of the primitive basalt, SHV has relatively narrow range showing relatively higher Rb/Zr and Ba/Zr ratios, and lower Nb/Zr ratio. In contrast, TMV shows relatively wide range with lower Rb/Zr and Ba/Zr ratios, higher Nb/Zr ratio. The various ratios of incompatible elements of primitive basalts in this district cannot be produced by crystallization of a single primary magma, but be derived from multiple various primary magmas. Considering the difference in ratios of incompatible elements primitive basalts, SHV has the feature of island arc basalt (low-Nb/Zr and high-Ba/Zr ratios). TMV exhibits the feature of back arc basin basalt with.high-Nb/Zr and low-Ba/Zr ratios. These spatial distribution of ratios of incompatible elements shows ellipse zonation, similar to the zonation of SiO2 wt.% reported by Nakagawa et al. (1993). This feature might reflect the difference in degree of partial melting at producing primary magma or the compositional variation in mantle source, corresponding to the mantle plume model under this district, of which SHV as a center, proposed by Nakagawa et al. (1993). In order to examine this possibility, we are going to carry out the additional geochemical study using rare earth elements and isotopic compositions.