We determined K-Ar ages for four andesites of the Hisatsu volcanic rocks from Minamata and for rhyolite of the Gesujima welded tuff from Amakusa Shimoshima, southwest Kyushu Japan. The andesites include three Yahazudake volcanic rocks and one Ontake volcanic rocks. The obtained K-Ar ages for the Yahazudake type andesite range from 1.98 to 2.08 Ma. The ages of Ontake type andesite and Gesujima welded tuff are 2.15 Ma and 2.89 Ma respectively.
     From our new K-Ar age data combined with previously reported geological, petrological and chronological data of the Hisatsu volcanic rocks, we discussed the space and time distribution of these volcanic rocks. Andesites from Hisatsu volcanic rocks are divided into two categories, “Ontake type andesite (K₂O>1.7 wt. %)” and “Yahazudake type andesite (K₂O<1.2 wt. %)”, based on potassium content in andesite at given SiO₂ range (57-63 wt.%). In terms of this criterion, most of the Hisatsu volcanic rocks and andesites from Kirishima volcano are defined as the “Ontake type andesite”. Consequently, the Ontake type andesite distributes whole area of the Hisatsu volcanic province extending from Nagashima area to Kirishima volcano. Andesites of this type, as a whole, have a positive correlation between latest eruption age of each volcano and the distance from the present volcanic front. This spatial arrangement of the Ontake type andesite implies that the width of volcanic zone has narrowed from Nagashima to Kirishima volcano during last three million years. Although the distribution area of Yahazudake type andesites overlaps the Ontake type andesites, the volcanism of the Yahazudake type andesites is quite limited in age ranging from 1.6 to 2.0 Ma. The production of each magma types may belong to different tectonic events.

抄録全体を表示

A large volume of lavas and pyroclastic rocks with varying composition erupted in the latest Miocene in the Oki-Dogo island. In this paper, the origin of shoshonite, trachyte, and low and high-SiO₂ rhyolites of the Oki Alkaline Volcanic Group is examined.
     On the basis of model calculation, trachyte magma is inferred to be generated by fractional crystallization of plagioclase, clinopyroxene, magnetite and apatite from shoshonite magma, or by partial melting of crustal material or solidified shoshonite magma with complete melting of K-feldspar and/or biotite. From consideration of variation diagrams, low-SiO₂ rhyolites magma was not generated by fractional crystallization from trachyte magma or by magma mixing between trachyte and high-SiO₂ rhyolites magmas. High-SiO₂ rhyolites magmas were not generated by fractional crystallization from low-SiO₂ rhyolites magma.
     Aphyric low- and high-SiO₂ rhyolites plot in restricted, but separate areas on the normative Qz-Ab-Or diagram. This suggests that these magmas were generated by partial melting of crustal materials under different P_H2O conditions: that is, low-SiO₂ rhyolites were generated under higher P_H2O than were the high-SiO₂ rhyolites.
     Fe, Ce, Pb, Ba, Nb, Rb, Zr, Y and Th contents of both low- and high-SiO₂ rhyolites vary between factors of 1.1 to 6 times, compared with restricted SiO₂ variation of only 71.4 to 72.7 wt% for the low-SiO₂ rhyolites, and 75.0 to 75.3 wt% for high-SiO₂ rhyolites. The variations in element concentration are interpreted to be due to the following process: during partial melting at the cotectic point, in the some source areas following minerals such as apatite (affecting Y, Ce due to high partition coefficients), zircon (Zr, Th, Ce), sphene (Nb), allanite (Ce, Th), garnet (Y), magnetite (Fe) and biotite (Rb, Ba, Nb) remained as residual phases, in others they did not. This produced separate magmas with low or high concentrations of the elements concerned. This interpretation is supported by the observation that partial melt glasses in crustal xenoliths from Cenozoic basalts in Oki-Dogo Island also show similar compositional trends to the bulk composition of the low- and high-SiO₂ rhyolites. Similar compositional variation is recognized in one high viscosity rhyolite lava flow unit, suggesting that incomplete magma mixing took place between separate magmas generated by different degrees of partial melting.

抄録全体を表示

Two K-Ar age determinations were carried out on the Mimuro and Mitsumori Granites, which are located in the chain of Eocene fine-grained plutons. These yield Late Cretaceous ages of 77.1±3.9 Ma and 81.8±4.1 Ma, respectivery.

抄録全体を表示