2001 Volume 107 Issue 6 Pages 392-405
The Shinji Rift System forms a half graben more than 160 km long at least reaching Tottori offshore. A hitherto unknown Pleistocene andesitic volcano has been discovered recently in Miho Bay, along the axial zone of the Shinji Rift System. The volcanic edifice has a dome-like shape approximately 500 m in diameter, and the summit lies 26 m below the present sea level. K-Ar whole rock and zircon fission track ages are 0.9±0.3 Ma and 0.16±0.04 Ma, respectively.The andesite is extremely peraluminous with Al2O3=22.8-18.6 wt% and SiO2=60-61 wt%, and up to 13 wt% normative corundum. It has a diverse mineral assemblage. Phenocrysts are Cr-bronzite-hypersthene, Cr-diopside-augite and plagioclase with a wide compositional range. Orthopyroxene, clinopyroxene, pigeonite, hornblende, plagioclase, quartz, alkali feldspar, Cr-spinel, magnetite, ilmenite, hemoilmenite, zircon, apatite, silica minerals, an unknown Al-Mg material and glass occur as groundmass constituents and inclusions in phenocrysts. There are two kinds of opacite phenocrysts consisting of F-phlogopite (up to 6.6 wt% F), hypersthene, augite, plagioclase, quartz, spinel, Fe-Ti oxides, cummingtonite and actinolite.We infer that the volcano originally formed on land in the glacial stage, and was then eroded and drowned by transgression. This new edifice and several other Pliocene and Pleistocene eruptive centers are aligned along the axial zone of the Shinji Rift System, indicating presence of a deep fracture zone originally formed by rifting related to opening of the Japan Sea.
