Abstract
The volcanic island of Oshima-Kojima, which as an area of ~1.5 km2, is located 30 km west of the Oshima Peninsula in southwestern Hokkaido. We surveyed a part of the island to reveal the geology of the volcano and collected volcanic rocks to determine K–Ar ages and whole-rock chemistry. The volcano started the activity under the shallow subaqueous condition and continued to effuse lava flows in the subaerial condition with the shift of vent position. The newly obtained K–Ar ages of two samples from subaqueous and subaerial lavas are 0.11 ± 0.02 and 0.16 ± 0.01 Ma, respectively. Thus, the volcanic edifice developed over a relatively short period during the MiddleLate Pleistocene. The rocks of the island range from basalt to dacite and contain plagioclase, olivine, clinopyroxene, orthopyroxene, amphibole, biotite, quartz, apatite, and opaque minerals as phenocrysts. The rocks are classified as calc-alkaline series on a FeO*/MgO–SiO2 diagram, and as transitional between high-K and medium-K on a SiO2–K2O diagram. They have a similar geochemical signature to rocks from volcanoes on the back-arc side of the northeastern Japan arc with respect to incompatible elements and Sr–Nd isotopes. The petrological features and whole-rock chemistry indicate that mixing between basaltic magmas and dacitic magma was the main magmatic process. The variations in Sr and Nd isotopic compositions and the existence of several different trends in some of the oxide–SiO2 diagrams indicate that mafic end-member magmas are composed of several distinct magma types.
