Petrography, whole rock chemistry and phenocryst mineralogy revealed that compositional variation among Hime-shima volcanic rocks are mainly explained by magma mixing of aphyric rhyolite and plagioclase-, hornblende-bearing dacite.
All the major and trace element data plot near straight lines on the Harker diagrams. Least-squares calculations using major element contents show that some volcanic rocks are products of fractional crystallization from less silicic magma. But the variation of the trace elements cannot be explained by the same process if available distribution coefficients are used in the calculation of hornblende and plagioclase fractionation. Conspicuous resorption texture is observed among the plagioclase in the lavas of intermediate compositions, whereas hornblende is euhedral in all lavas. Positive correlation between the whole rock chemistry and the volume ratio of hornblende phenocrysts suggests that aphyric rhyolite magmas (75 wt. % SiO₂) mixed with a plagioclase-, hornblende-bearing dacite magma (66 wt. % SiO₂), and produced biotite-bearing hornblende dacite (68-73 wt. % SiO₂). The contrasting textures of plagioclase and hornblende will be ex-plained by their different liquidus temperatures: With increasing H₂O content (above 1 wt. % H₂0), the plagioclase liquidus drops sharply, whereas hornblende liquidus stays nearly constant.
Hime-shima volcanic group is composed of seven volcanoes which represent individual magma reservoirs, because volcanic rocks erupted from each volcano are distinguished by petro-graphy and chemistry. Field evidences show that the least silicic magma was the first to ascend and emplace. Several batches of silicic magmas ascended through this least-silicic magma reservoir and mixed with it in varying mixing ratios to form intermediate magmas.

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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 (Fo_90-85) 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.

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Major periods of volcanic activity in the Lake Toya district are marked by the formation of a Crater Lake-type caldera after the eruption of voluminous rhyolitic pyroclastic flows (Toya pyroclastic flow) and two post-caldera volcanoes: Nakajima Volcano (lava dome) and Usu Volcano (somma lava and lava dome). Major element and REE concentrations and Sr isotope ratios were determined for volcanic rocks from the Toya Caldera and Nakajima Volcano. In combination with the published results, the data indicate that the generation of magmas involved in caldera-formation and post-caldera volcanoes are independent of each other. Magmas as-sociated with each stage were probably tapped from short-lived reservoirs, and thus a single and compositionally zonated magma body as generalized for many Crater Lake-type calderas is not required. The geochemical data supports the funnel-shaped caldera structure model as deduced from geophysical investigations of Japanese Crater Lake-type calderas rather than a caldera subsidence model.

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