日本列島における中新世末期～洪積世前期の火成活動の変遷

Igneous activity of the Japanese islands from late Miocene to Pleistocene is divided into the activities of the following four stages; late Miocene, Pleiocene, early Pleistocene and middle to late Pleistocene. One of the activity of late Miocene is effusion of dacite and/or aridesite in several collapse basins generated at the upheaval parts of the Green tuff region and volcanic rocks of this time are mostly pyroclastic flow sediments of calc-alkali rock series. Another activity is effusion of andesite or basalt in the subsiding basins on the Japan Sea side. These activities are those of the last stage of the Green tuff movement. Igneous activity of Pleiocene has occurred in the limited areas of the Japanese islands, namely the northern Fossa Magna, southwestern Hokkaido, San'in areas and others. The activity of the northern Fossa magna area is most typical and has occured in the subsiding basins of the so-called Shin'etsu geosyncline. Volcanic rocks of the area are mostly calc-alkali andesites and some of them have gabbroic xenolith and hornblende and/or diopside megacrysts. Igneous activity of San'in area of this time is those of the Misasa and Matsue formations. The activity of the former has occured at the upheaval part and volcanic rocks are alkali basalt and calc-alkali andesite, on the other hand, the activity of the latter has occurred in the subsiding basin and is the activity of alkali basalt. Although both activities have occurred at different geological environment, they are the same with respect to the beginning of the activity in the Circum-Japan Sea alkaline province. Igneous activity of early Pleistocene has also occured at limited area, which mainly are the northern Fassa Magna, Hokkaido, central Kyushu areas and has two characteristic modes of effusion except basalt lava flow of the San'in area. One is effusion of the so-called flat lava of calc-alkali andesite at the upheaval part and another is eruption of pyroclastic flow of dacite which has occurred at the central Hokkaido area and the Shirakawa-Aizu area of the northeastern Japan. Quaternary volcanoes having several volcanic chains along the present arc-trench systems have been formed from middle Pleistocene to recent. On the igneous activity from late Miocene to Pleistocene, that of early Pleistocene is most characteristic on the points of spacial distribution and mode of effusion. This activity is not related to the Green tuff movement, but may be related to the Island Arc movement proposed by FUJITA (1970). Main field of the activity of this time may be situated in the area of the arc-junction of the Japanese islands.