濃飛岩体西部地域の流紋岩類 : とくに,陥没運動と火山活動のステージについて

The Nohi Rhyolite is a large volcanic pile of the Cretaceous age, extensively distributed in central Japan and consisting chiefly of silicic pyroclastic flow deposits, mainly welded tuffs. A series of geological survey has been carried out by the authors in the western part of the rhyolite mass who recognized nine welded tuffs and two sedimentary units as shown in Table 1 and correlated these units with previously established succession in other parts of the mass. As a result, it is suggested that the succession of the main part of the Nohi Rhyolite is grouped into five volcanic stages (Stage I-V). In the western marginal part of the mass, the Ako Welded Tuff belonging to the Stage Ib filled up the depression (graben) which elongates in a NW-SE direction. Volcanism of the Stage II began with the subsidence in the western part of the mass evidently apart from the above-mentioned depression of the Stage Ib as well as in the northern part, which was followed by the sedimentation of coarse-grained lacustrine deposits (Futatsumoriyama Conglomerate). Successively, enormous pyroclastic flow deposits, ranging from rhyolite to rhyodacite in an ascending order, filled up these depressions and a part of them overflowed these areas. Finally, several stocks of granodiorite porphyry which are supposed to be co-magmatic with the main part of the pyroclastic flow deposits of the Stage II, especially with the Higashimata Welded Tuff, intruded the center and the periphery of the depression. Volcanism of the Stage III is also inferred to have begun with subsidence of the central part of the mass, judging from the abrupt lateral change in thickness of the lacustrine Atera Formation which are supposed to have filled up this depression, and have been followed by successive deposition of the pyroclastic flows (Takadaru Welded Tuff and others), mainly of rhyolitic composition. Consequently, it is suggested that a series of events as shown in a following scheme has occurred in each stage of the Nohi Rhyolite: subsidence→sedimentation of lacustrine deposits→deposition of a thick pyroclastic flow→intrusion of granodiorite porphyry (or adamellite porphyry in some cases). It is also noticed that the above-mentioned depression of each stage migrated in a following direction in the Nohi Rhyolite mass: (southern marginal part)→western marginal part→western part (and northern part)→central part→eastern part→eastern marginal part.