Volacanic rocks from Oze-Hiuchigatake, a small (8km³) stratovolcano located in central Honshu, show petrographical and geochemical evidences of magma mixing. Among the four main stages of activity, samples of stages I and II are mainly composed of hypersthene-augite andesite, whereas those of stages III and IV are composed mainly of dacitic andesite. The latter samples have complicated phenocryst assemblages (Ol+Di+Aug+Hyp+Pl+Qz+Mt±Hb±Bi) and show disequilibrium features such as quartz with pyroxene jacket, plagioclase with dusty zone, overgrowth of augite on orthoproxene and coexistence of magnesian olivine and quartz. They frequently contain basic inclusions (52.7%-56.9% SiO₂), which result from the the mixing of dacitic magma and olivine augite basaltic magma.
Chemical analyses of 48 volcanic samples revealed a wide compositional variation of 52.7% to 68.5% SiO₂. They are plotted in the field of calc-alkaline rock series in AFM diagram, but in the upper-most region of the tholeiitic rock series in the alkali vs. silica diagram. They are richer in K₂O and poorer in Na₂O than calc-alkaline rocks of nor-them subzone of Nasu volcanic zone, and are similar to those of the southern subzone. In the plots, samples of stage III form a nearly straight line indicating a mixing between dacitic lavas and basic inclusions. Parental liquid for these basic inclusions is richer in MgO, CaO and poorer in Al₂O₃ than the parental magma of stages I and II: thus basic mixing endmember of stage III was not generated from the latter.

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High-pressure experimental study has been made on the melting relations in a primitive midoceanic basalt composition at 3-16 GPa and 1400-2100°C. The two phases of clinopyroxene and garnet (eclogite facies) are coexisting with partial melt up to 10 GPa, whereas the single phase of garnet is coexisting at higher pressures (garnetite facies). The compositions of the partial melts are observed to show pressure variations according to the coexisting mineralogical assemblage and the compositions of their constituent phases. The Ca fractionation between bulk solid and partial melt is more prominent in eclogite facies, due to the enhanced solution of the Ca component in garnetite facies. The crystal/liquid partition coefficients of K and Sr increase with pressure both for clinopyoxene and garnet (D^CPX_K=0.02-0.4, D^CPX_Sr=0.08-0.2, 3-10 GPa; D^Gt_K=0.008-0.05, D^Gt_Sr=0.03-0.1, 5.5-16 GPa). The partition coefficient of Pb (-0.05) is higher than those of Th and U (-0.01) for both clinopyroxene and garnet, and these values are only weakly dependent on pressure.

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高温高圧下で合成された斜方輝石における結晶内陽イオン分布は,主に有効イオン半径の比によって支配されている。ニッケル及び亜鉛を含有する場合にみられる分配係数の多様性は,それらのイオンが,有効イオン半径に多様性をもつためである。斜方輝石中の6配位Niイオン及びZnイオンの有効イオソ半径は,圧力条件を反映してそれぞれ2種類ある。 Niでは0.710 Åと0.690 Å, Znでは0.79₆ Åと0.750 Åである。

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Two mugearites from Dogo, Oki islands, southwestern Japan were dated by the K-Ar method with a basalt from Omine basalt group to provide stratigraphic anchors of the geological succession of the volcanic rocks in the island. The obtained K-Ar ages of the mugearites are 5.4 Ma, and identical within analytical error. The K-Ar age of the Omine basalt from Daimanji-yama in the eastern area of the island is 4.6 Ma. The mugearite group seems to be not accompanied by a contemporary basalt suit. The Omine basalt group can be divided into 4 groups or more by means of ages, petrochemistry, and distribution area.

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