Characteristic Geology and Petrology of Non-arc Type Volcanism at Oninomi Monogenetic Volcano, Yufu-Tsurumi Graben

Abstract

Ominomi monogenetic volcano erupted at the Yufu-Tsurumi Graben in central Kyushu, after the deposition of Aira Tn ash (22,000 y.B.P.) and before that of Kikai-Akahoya ash (6,000-6,500 y.B.P.). This volcano consists of a scoria cone and two lava flows. The height of scoria cone is about 30 m, and the diameter is 250-300 m. The crater opens to the west and two lava flows extend about 1 km from the cone. The rocks of Oninomi monogenetic volcano are pargasite bearing clinopyroxene olivine basalt, pargasite clinopyroxene olivine basaltic andesite and orthopyroxene bearing clinopyroxene pargasite olivine andesite. The analyzed samples from this volcano show a SiO₂ range of 51.0-54.5 wt.%, and higher K₂O, TiO₂ and P₂O₅ contents than the volcanic rocks in central Kyushu. The relation between Fo % in olivine and An % in plagioclase in the volcanic rocks from this volcano is similar to that found in the cumulates from ocean island or MORB settings. Ti, Cr, Ca and Na contents in clinopyroxene of this volcano indicate that the magma is non-orogenic type. Moreover, the feature of bulk chemistry especially shown by Ti-Mn-P diagram and Ti-Zr-Y diagram, reveals that the magma of this volcano is not an island arc type but a within-plate type. These data suggest that Oninomi monogenetic volcano represent a non-arc type volcanism.