火砕流堆積物としての濃飛流紋岩(特集濃飛流紋岩およびそれに関連する酸性火成作用-地団研第24回総会シンポジウム-)

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A group of silicic volcanic rocks hitherto called "quartz porphyries" with subordinate intercalation of clastic sediments in Central Japan was named Nohi rhyolite in 1961 by the first two writers and their co-workers. Now it is revealed that it is mostly consisted of a large number of pyroclastic flow deposits, densely welded and wholly devitrified. These welded tuffs range from rhyolite to rhyodacite in chemical composition. They include the abundant crystal fragments (30〜50 volume percent of the rocks in the most parts), in the compact cryptocrystalline to holocrystalline matrix, such as quartz, plagioclase, orthoclase, biotite, hornblende, rarely ortho- and clinopyroxenes and, very rarely, Fe-rich olivine. Commonly, they have essential lenticluar fragments which are nearly equal to their enclosing rocks in chemical and mineralogical composition, and, also have lithic fragments, in a more or less degree of abundance, mainly derived from the underlying Paleozoic rocks. Emplacement of the rhyolite is inferred to have been in Late Cretaceous period from geolgical evidences and, more definitely, in a certain period between 100 m.y. and 80m.y. ago according to the isotopic radiometry of the granites both prior and posterior to it. The Nohi rhyolite, accopanied with hypabyssal intrusives of co-magmatic origin, constitutes an enormous mass whose total area of distribution attain to about 5,000km^2, mean thickness is 2,000 m or more and, consequently, original volume is about 10,000km^3 or more. It is extending with NW-SE trend mainly in the non-metamorphic Upper Paleozoic terrain (so-called Mino belt) and, partly, in the northern-neighboring metamorphic terrain (Hida metamorphic belt), and evidently truncate such the basement structure as a whole. The rhyolite seems to have effused out of the fissures of NW-SE trend which took place in the sheared zones of the basement and deposited in a large graben with the same trend of elongation. In fact, vent-breccias found in the western margin of the rhyolite mass, paleo-talus deposits covered by some welded tuffs on the fault scarp in the above area and frequent alternation of welded tuffs and nonsorted very coarse-grained sediments express eloquently the repeating of the eruption and the volcano-tectonic depression, at least in its western marginal part of the mass. On the basis of detailed stratigraphic survey, the volcanic activities resulted in the deposition of pyroclastic flow can divided into five stages, between which existed some time intervals indicated by deposition of the lacustrine sediments (Atera formation, Shirakawa formation and others). Each stage is consisted of two or more welded tuff sheets each of which is generally 200 m or more in thickness, nearly homogeneous, in the most cases, both vertically and horizontally in their lithological, petrographical and petrochemical features and fairly different to others in the same features. The first stage is represented by the highly silicic and relatively crystal-poor welded tuffs, in general, and the second stage, on the contrary, by rather mafic (rhyodacite) and crystal-rich welded tuffs. Soon after the second stage, granodiorite porphyries, chemically and mineralogically similar to the welded tuffs of the second stage, were intruded into the welded tuffs of both the first and the second stage as several stocks now occupying the central and southern parts of the Nohi rhyolite mass. The third and the fourth stages are represented by the welded tuffs which have intermediate composition between the first and the second ones, and their areal distribution area was rather shifted to the east compared to the preceding stages. Products of the last stage volcanism are only poorly developed in the western marginal part of the mass, and mainly consisted of the explosion breccia deposits with several thin layers of rhyolite welded tuffs. Soon after the last stage volcanism, numerous small dikes of granite porphyry having adamellitic composition were intruded along the weak lines (faults ?) in the Nohi rhyolite mass with both the NW-SE and NE-SW trend and also intruded into the adjacent Paleozoic strata, although they are most abundant in the western and the eastern marginal parts of the mass. It is sug gested that "the Nohi rhyolite magma" has ceased to be active after such the minor intrusion. At the end of Cretaceous period, granitic intrusion occurred in the extensive area of the inner side of the Southwest Japan including Central Japan, and, consequently, the southern part of the rhyolite mass were invaded and thermally metamorphosed by the granites. During almost all the Cenozoic era, this area continued to be uplifted and eroded, and now the deeper parts of the rhyolite mass are exposed on the land surface together with the granitic rocks intruding into them.
地学団体研究会の論文
1971-03-25

火砕流堆積物としての濃飛流紋岩(特集濃飛流紋岩およびそれに関連する酸性火成作用-地団研第24回総会シンポジウム-)

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火砕流堆積物としての濃飛流紋岩(特集 濃飛流紋岩およびそれに関連する酸性火成作用-地団研第24回総会シンポジウム-)

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火砕流堆積物としての濃飛流紋岩(特集濃飛流紋岩およびそれに関連する酸性火成作用-地団研第24回総会シンポジウム-)