抄録
High magnesian tholeiitic basalts of late Miocene to Pliocene age are found in the Yabakei district, northern part of Oita Prefecture, southwest Japan. These basalts (Yabakei basalts) are ± orthopyroxene clinopyroxene olivine basalt and are characterized by high MgO (11.4-9.1 wt %), Cr (859-528 ppm), Ni (206-156 ppm) contents and low FeO*/MgO ratio (0.69-0.78). The most primitive basalt has olivine phenocrysts with narrow range of Fo composition (Fo90-85) and high NiO content (0.35 wt %). Yabakei basalts are plotted into P-type MORB field of 2 Nb-Zr/4-Y diagram. The primordial mantle normalized spidergram for Yabakei basalts have simi-lar configurations to alkali basalts from northen Kyushu, with significant trough at K and peaks at Ba. This suggests that the sources mantle of Yabakei basalts have geochemical characteristics similar to those of northern Kyushu basalts.
