Abstract
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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