白亜紀〜古第三紀酸性火成活動

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Intense acid magmatism occurred in the Japanese Islands during Cretaceous to Paleogene age, resulting in the large quantities of granitic rocks and pyroclastic flow deposits, mainly welded tuffs. Descriptive studies on the products in the 1970s, which are further advanced by many concrete and detailed surveys, analyses, and discussions, are reviewed. Plutonism in the Ryoke belt was believed to be a synorogenic one associated with the Honshu Orogeny. In the last decade, however, it has revealed by geological, geochronological, and geochemical investigations that a sequence of volcano-Plutonism including the Ryoke plutonism began in the late Early Cretaceous age (about 110-120 Ma), with some intermittences, continued through out Late Cretaceous to Paleogene, partly into Miocene (about 20-25Ma) in Southwest Japan. The igneous rocks of the Inner Zone of Southwest Japan constitute three petrographic provinces, the Ryoke, Sanyo, and San-in from south to north, which are arranged parallel to the Median Tectonic Line, and the front of the magmatisms migrated northwards with the time. The metallogenic provinces and the distribution of magnetite-series and ilmenite-series granitoids (ISHIHARA, 1977) are harmonic with these zonal arrangements, i.e. the magnetite-series granitoids are predominant in the San-in belt, in the continental side, and younger in age than the ilmenite-series granitoids. Cretaceous granitoids of the northern part of Kyushu belong to the magnetite-series, though these radiometric ages coincide with the Ryoke and San-yo granitoids. The eastern extention of the zonal distribution of the seigneous rocks is ascertained to the Tanakura Tectonic Line beyond the Fossa Magna. In Northeast Japan, zonal distribution of granitoids is also recognized, radiometric ages of the granitoids are older than those of Southwest Japan (about 95-135 Ma). The magnetite-series granitoids, however, are predominant in the Kitakami mountains and the ilmenite-series granitoids are only dominant in the western part of the Abukuma Highlands. The metallogenic and other geochemical characters tend to vary from the Tanakura Tectonic Line to the east (oceanwards).
日本地質学会の論文
1985-03-30

白亜紀〜古第三紀酸性火成活動

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