新潟,長野県境付近の津南-志賀地域の鮮新-更新世の火山岩類

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The Plio-Pleistocene volcanic rocks in the Tsunan-Shiga area are divided into five groups as follows: i) Late Pliocene to earliest Pleistocne time; the Nonomigawa, Kuwanagawa, and Shikumigawa volcanic formations, ii) Late early Pleistocene time; the Torikabuto volcano, iii) Early middle Pleistocene time; the older Kenashi volcano and the Hirao volcanic rocks; iv) Late middle Pleistocene time; the Naeba, younger Kenashi and Takapyo volcanoes; v) Late Pleistocene time; the Shiga volcano. The Nonomigawa and Kuwanagawa volcanic formations and some volcanic rocks of the Shikumigawa volcanic formation are the products of eruption at the bottom of fresh water. Other volcanic rocks, however, are almostly the products of eruption on land. The Torikabuto volcano formed a small caldera after the eruption of a large volume of lava flows and pyroclastic flows, and then a volcanic cone of lava flows was formed in the caldera. The Naeba volcano is a stratovolcano, composed mainly of lava flows and showing the flat topography. The Kenashi volcano is a kind of stratovolcano composed of younger two lava flows overlying the older stratovolcanoe roded partially. Volcanic rocks in this area are divided into pigeonitic and hyperthenic rock series on MFA diagram, though they contain a large amount of plagioclase phenocrysts. Volcanic rocks of the Nonomigawa, Kuwanagawa and Shikumigawa volcanic formation, the Ikenosawa volcanic rocks and some rocks of the Kenashi volcano belong to the former. On the other hand, those of the Torikabuto, Naeba and Shiga volcanoes and some rocks of the Kenashi volcano belong to the latter. The rock series determined by chemical composition coincide with those by groundmass mineral assemblage. Chemical compositions of many rocks in this area are plotted in the field of low alkali tholeiite series on Na_20+K_20-Si0_2 diagram, and in the field of medium-K andesite proposed by GILL (1981). These characters resemble to those of volcanic rocks in the southern subzone of the Nasu volcanic zone. Many of the Quaternary volcanic rocks in the North Fossa Magna contain characteristically hydrous mafic phenocrysts. In this area, however, only a few andesites in the Torikabuto volcano and the Takapyo summit lava contain hornblende or hornblende and biotite phenocrysts. Quartz-bearing andesite occurs in volcanic rocks of the Torikabuto and Naeba volcanoes and the Higashidateyama lava situated in the margin of so-called "Central Rift belt". Therefore, they may have been formed in relation to tectonic of the belt.
日本地質学会の論文
1984-11-30

新潟,長野県境付近の津南-志賀地域の鮮新-更新世の火山岩類

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