Northern Kyushu, characterized by the subduction of Philippine Sea plate slab with differentages (a young Shikoku basin and a old West Philippine basin) beneath Eurasian plate, forms acomplex portion of Southwestern Japan arc. In order to evaluate the spatial and temporalinfluences of slab-derived fluids from these two contrasting oceanic plates on the subarcmantle, the author addresses the following two topics for discussion. 1. Boron contents in basaltic rocks from northern Kyushu.2. Temporal changes of the composition of magma source beneath Aso area
