抄録
The volcanic rocks in the Nayoro-Asahikawa region, northern Hokkaido, can be divided petrographically into three districts from west to east as follows: Nakagawa-Horokanai, Utanobori-Toma and Omu-Kamikawa districts. In the Nakagawa-Horokanai district, hornblende andesite associated with pyroxene andesite is abundant, while pyroxene andesite is uniquely exposed in the Utanobori-Toma district. In the Omu-Kamikawa distict, pyroxene andesite occurs associated with rhyolite, dacite and basalt. K-Ar ages of these volcanic rocks indicate that the volcanism in the Omu-Kamikawa and the Utanobori-Toma districts took place 9.8-12.1 Ma and 9.3-13.8 Ma, respectively. Thus, it is considered that the volcanism in these three districts occurred simultaneously during the period from late Middle Miocene to early Late Miocene. Moreover, the ages of the volcanic rocks tend to become younger toward the south in the Nayoro-Asahikawa region. The sedimentary environment inferred from the sediments intercalated with these volcanic rocks shows that the volcanism was mostly terrestrial. From the viewpoints of the age, activity trend and distribution of volcanic rocks, the Middle Miocene to Late Miocene volcanism in the Nayoro-Asahikawa region is clearly distinguished from the Paleogene to Early Miocene plutonism in central Hokkaido and also from the volcanism having occurred along the Kulile Arc since Late Miocene.
