The eruptive history of post-caldera volcanoes of Kutcharo caldera Atosanupuri, Nakajima, and Mashu , eastern Hokkaido, Japan, was re-examined based on tephrostratigraphy above the layer KpI ka , which is the youngest caldera-forming ignimbrite produced from the caldera. The studied areas are located to the east Konsen and north Shari of Kutcharo caldera. Between KpI and Ma-l ka; the first eruption of the caldera-forming stage of Mashu volcano , at least explosive eruptionsMl-a to M -d, Ch-a to Ch-d, and Nu-a to Nu-r, in descending order are recognized in the Konsen area, and KoP, NaP, WTfl-, KgSc, WTfl-, KuSc, TyP, HkP, Ds-Oh, and YmP are recognized in the Shari area. In addition, more than thin scoria layers were found. Eight tephra layers in the Shari area can be correlated with those in the Konsen area, based on stratigraphy and petrology e.g., glass chemistry and the mineralogy of juvenile materials . The main phenocryst phases in these tephras are plagioclase, two pyroxenes, and Fe-Ti oxides, although some layers contain olivine and Nu-q includes hornblende. These tephras are divided into the following three groups based on the K O content of glass: Low-K K O . -. wt. , Medium-K . -. wt. , and High-K . -. wt. . Nu-q, characterized by the presence of hornblende and a High-K composition, was possibly produced by the Daisetsu Ohachidaira caldera of central Hokkaido. The Medium-K group consists of porphyritic phenocrysts , white-colored tephra layers. All other tephras, including the thin scoria layers, are assigned to the Low-K group. Based on geological and petrological data, and correlations with proximal deposits and edifices of each post-caldera volcano, we infer that the source volcanoes of the Medium-K and Low-K groups were the Atosanupuri/ Nakajima volcanoes and Mashu volcano, respectively. These results indicate that explosive eruptions occurred at the Atosanupuri/Nakajima volcanoes over the period from to ka, with a total tephra volume of km . Mashu volcano produced more than plinian eruptions total tephra volume km over the period from to ka, with no dormant periods exceeding several thousand years in duration.
Abstract