抄録
The geology and K-Ar geochronology of the Gassan volcano in Northeast Japan were carried out to establish the volcanic history.
This volcano is belonging to the Chokai volcanic zone, and composed of several volcanic edifices. On the basis of the field geology and K-Ar dating of the ejecta, the volcanic history is divided into five stages;
Stage I (0.88 Ma): the Amamoriyama was formed by dacite volcanism. The dacite lava is composed with hypersthene, augite, hornblende, biotite and quartz phenocrysts.
Stage II (0.7 Ma): The Yudonosan, which is composed of a stratovolcano and two lava domes of mainly dacite lava and pyroclastics.
Stage III: The volcanism occurred in the north of the Yudonosan. The dacite lava and pyroclastics covered on the basements in the caldera and the lower Yudonosan lava in the Ishihane river.
Stage IV (younger than 0.6 Ma): The volcanism of the major stratovolcanoes of central Gassan, the Waratahageyama and the Ubagatake, which are composed of lavas and pyroclastics. The rocks are calc-alkali andesite containing hypersthene, augite, sometimes a small amount of olivine and rarely hornblende.
Stage V: The collapse of the northwestern half of Gassan stratovolcano, caused the formation of a horseshoe-shaped caldera and dry avalanche deposits.
The rocks of the Gassan volcano are divided into two groups, dacite and andesite in their temporal as spatial distributions.
