中央アルプスとその西域の地質 : その2:濃飛流紋岩類(総会特集)

概要

論文の詳細を見る
The Nohi rhyolites with subordinate sediments (Adera formation and Schirakawaguchi formation) are widely developed in the non-metamorphosed Paleozoic terrain, and form a large elongated mass which intersects with a trend of the Paleozoic formations at nearly right angle. They occupy an area of about 5,000 km^2 and seem to have a volume of about 3,000 km^3. The Nohi rhyolites were called, hitherto, "quartz porphyries", but now they are revealed to be the horizontal piles of rhyolite welded tuff as a whole, so we rename them "Nohi rhyolites". The rocks are compact, mostly massive and dark gray in color, and they have highly porphyritic appearance due to phenocrystic quartz, plagioclase (oligoclase-andesine), potash feldspar and mafic minerals in order of abundance. These phenocrysts show, characteristically, angular or corroded forms. Mafic minerals are brown biotite, green hornblende, brown hornblende, augite, hypersthene, etc., being altered to secondary minerals in almost all cases. The amount of each mafic mineral is quite variable. Matrix of the rocks is composed chiefly of volcanic glass and pumice fragments which are tightly welded and remarkably devitrified to fine-grained silica minerals. Lenticular aggregates of rather well-formed quartz and potash feldspar are sometimes included in the matrix. In addition to the above-mentioned minerals, the rocks bear, abundantly in some parts, angular lithic fragments such as Paleozoic rocks and rhyolites, the latter being assumed as congnate inclusion. Besides such porphyritic rhyolite welded tuff, several layers of non-porphyritic glassy welded tuff and andesite welded tuff occur as minor members. At the eastern border of the mass, the Nohi rhyolites accompanied with the Adera formation overlie the Paleozoic rocks unconformably. The formation mainly composed of conglomerate, mudstone and tuff is covered by or intercalated in the rhyolites. This fact proves the presence of hiatus during the deposition of the welded tuff. On the other hand, along the western border of the mass, the rhyolites are often in contact with the Paleozoic rocks by the intrusive-like relation. There, the Paleozoic rocks are strongly disturbed and crushed by faults of NW-SE trend. At the neighbouring area of Schirakawaguchi, peculiar deposits (the Schirakawaguchi formation) chiefly composed of unsorted breccias of the Palezoics rocks which seem to be fault-breccias lie between the rhyolites and Paleozoic rocks. Moreover, such deposits are also found in upper horizon of the rhyolites at some places along the western border. Judging from the modes of occurrences of the formation, it is assumed that faulting and succeeding eruption of the rhyolites occurred repeatedly. The rhyolites are succeeded, soon after their consolidation, by the intrusion of granite porphyry which mostly dyke-formed and sometimes stock-formed, and lastly by the batholithic intrusion of biotite granite. Such a series of magmatism (rhyolite-granite prophyry-granite) seems to be one of the most characteristic igneous events in the late Mesozoic age, not only in Japan, but also in the wide area, so-called Circum Pacific zone.
地学団体研究会の論文
1961-05-03

中央アルプスとその西域の地質 : その2:濃飛流紋岩類(総会特集)

スポンサーリンク

概要

著者

関連論文

スポンサーリンク