濃飛岩体東縁部における流紋岩類の層序と形成史

抄録

Nohi rhyolite, a representative product of the Cretaceous silicic volcanism in Central Japan, was studied in its eastern marginal part, both stratigraphically and volcanologically. Stratigraphic succession of the rhyolite in the studied area is shown in the following table. Nohi rhyolite in this area are roughly divided, stratigraphically, into the lower group (stages III and IV) and the upper group (stage V) by a marked structural disharmony, although the latter is subordinate in amount. Volcanic products of stages I and II are distributed in the westerly neighboring area. The lower group constitutes a large-scale volcanic pile of two or three thousands meters thick and is composed of densely welded rhyolite to rhyodacite tuffs, intercalating some lacustrine deposits. It shows rather complicated structure formed by the basin-forming movement and the block movement. It is in contact with the fractured zone of the Permian geosynclinal strata in the east by a NW-SE trending fault. The upper group is distributed mainly in the above-mentioned fracutured zone, overlying the Permian strate with a marked unconformity. Welded tuff sheets composing the upper group are rather thinner than the ones of the lower group and show nearly horizontal structure. Senzawa welded tuff sheet, the lowermost one of the group, is characterized by the abundance of the accidental rock fragments and also by the intercalation of three or more beds of breccias mostly composed of angular, unsorted Paleozoic rock fragments. The mechanism of emplacement of these breccias is discussed in some detail and they are inferred to have been formed by singular pyroclastic flow deposits due to a kind of explosive eruption. The welded tuffs of the upper group is intruded by the adamellite porphyries with a NW-SE trending elongation. The last-stage volcanism in this area can be pictured in such a scheme that it is initiated by the explosive phreatic eruption, followed by the intermittently repeated eruption of the pyroclastic flows and terminated by the intrusion of adamellite porphyries of co-magmatic nature with the erupted materials.