Quaternary Bishamon-dake volcano, central Japan, is composed of calc-alkalic clinopyroxene-orthopyroxene andesite and dacite with subordinate hornblende andesite having Sr/Y>40, SiO₂ ∼63%, and Al₂O₃ ∼18%. The volcanic rocks are characteristically poorer in incompatible elements such as K₂O and Rb than rocks from the adjacent volcanoes (Dainichi-dake, Eboshi and Washiga-take, Gankyoji, Haku-san, Kyoga-take, and Tomuro). They are essentially mixing products between two magmas; an andesite having SiO₂ ∼60% and higher in FeO.t/MgO (FeO.t denotes total iron as FeO), and an adakite richer in SiO₂ and lower in FeO.t/MgO carrying phenocrysts of orthopyroxene with Mg/(Mg+Fe) ∼0.80 and clinopyroxene with Mg/(Mg+Fe) ∼0.82 and presumed mantle-derived olivine (Fo ∼90) xenocrysts. The end-member andesite magma may have been formed by regional volcanism or lower crustal anatexis, while the end-member adakite magma is likely to have been locally generated beneath Bishamon-dake. The end-member adakite magma had SiO₂>65%, Na₂O≤3.2%, K₂O≤1.2%, Rb≤20 ppm, Y<10 ppm, Nb≤6 ppm, FeO.t/MgO≤1.6 and Sr/Y>60, and was presumably generated by melting of the subducting Philippine Sea plate.

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Homogeneity of neutron fluxes during irradiation experiments done in Research Reactor Institute, Kyoto University, was evaluated using J-values collected in ⁴⁰Ar/³⁹Ar age determinations. The laser fusion results of approximately 0.3 mm-size individual mineral grains (Hb3gr hornblendes) indicate insignificant change in J-values both in horizontal and vertical directions within a size of the cylindrical sample holder (φ16×18). The reactor seems to have a relatively uniform fast neutron density distribution within the dimensions for ⁴⁰Ar/³⁹Ar dating. Correction factors obtained from potassium and calcium salts were also reviewed. The J-values of Hb3gr have minor effects from these factors because of little contribution of the interfering isotopes.

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The K-Ar ages and paleomagnetic stratigraphy of the Plio-Pleistocene volcanic rocks from the Ariake Sea region have been studied to clarify the evolution of volcanism in the western part of Beppu-Shimabara graben, west Kyushu, Japan. This region has seven domains of volcanic rocks. We determined K-Ar ages of ten volcanic rocks from five domains: the Misumi volcanic rocks=4.51 Ma and 3.97 Ma; the Kunisaki andesite=1.45 Ma; the Ibinoodake andesite=3.97 Ma and 4.52 Ma; the Uki volcanic rocks=1.63 Ma; the Yushima basalt=0.82 Ma; the Otake volcanic rocks=1.82, 1.77, and 1.72 Ma. Paleomagnetic measurements were made on volcanic rocks collected from all seven domains. Magnetic property of lavas from these domains can be classified into four magnetic polarity epochs: Brunhes Chron (Ninotake volcano, Takesaki basalt and Yushima basalt), Gauss Chron (Nogamajima volcano), Gilbert Chron (Ibinoodake andesite and Hidake lava dome), and Matsuyama Chron (the other volcano). The age of Ohya tuff, which belongs to the Kuchinotsu Formation, corresponds to Gilsa event. We synthesized our new data together with previously reported ages and stratigraphic data, and examined the volcanic activity in this region in terms of a spatial and temporal distribution. There is no correlation between spatial distribution of the volcanic rocks and eruption ages, but we revealed that the volcanic activity in this region is episodic during the last 5 million years. We can identify the magmatic activities as five episodes: 5-4 Ma, 3.2 Ma, 2.0-1.4 Ma, 1.0 Ma, and <0.9 Ma. The intensity of volcanic activity in the Ariake Sea region increased with time.

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