瀬戸内海大崎下島の変成古生層と白亜紀火成岩類

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In the studies of the Ryoke metamorphic belt of central Japan, we classified the granitic rocks into two groups, the granitic rocks intruded into the Nohi rhyolite probably of the Late Cretaceous and the older ones, and clarified that the formation of the Ryoke metamorphic rocks is closely associated with the older granitic activities. However, we could not resolve the problem of the age relation between the Nohi rhyolite and the oldei granitic rocks. In the Osaki-shimojima Island, western Japan, it has been known that the metamorphosed Paleozoic sediments are unconformably covered by the rhyolitic pyroclastic rocks called the Kubi formation which roughly corresponds to the Nohi rhyolite in age. The main purpose of our present study is to make clear whether the metamorphosed Paleozoic sediments are truely the Ryoke metamorphic rocks or not. In this island, the Kubi formation consists of, in ascending order, the basal conglomerate member which unconformably overlies the Paleozoic rocks, the rhyolitic tuff member, the andesitic welded tuff member and the dacitic welded tuff member. Both the Paleozoic sediments and the Kubi formation are intruded by the granites in the northern and the southen parts of the island, and, consequently, they are thermally metamorphosed in the most part. The metamorphosed Paleozoic sediments of this island are, mainly, biotite hornfels and cordierite hornfels. They are not the Ryoke metamorphic rocks, at least, in their present metamorphic features. On the other hand, the development of schistose structure and ptygmatic quartz vein is frequently conspicuous in these hornfelses, and their distribution has no relationship to the form of granitic bodies. Moreover, in the basal conglomerate of the Kubi formation, the metamorphic rocks with schistose structure and ptygmatic quartz vein are contained as pebbles. These facts suggest that the regional metamorphism took place before the sedimentation of the Kubi formation. This regional metamorphism is considered to be the Ryoke metamorphism, judging from the geology of surrounding areas, especially of the Itsukishima Island, belonging to the Ryoke metamorphic belt, 4 km south of the Osaki-shimojima Island. The granitic rocks of the Osaki-shimojima Island are not a member of the Ryoke granites, judging from the conspicuous lattice structure of microcline, the common existence of fluorite and bluish color of hornblende in addition to their general rock feature. They may belong to the Hiroshima granites possibly younger than the Ryoke granites.
地学団体研究会の論文
1975-01-20

瀬戸内海大崎下島の変成古生層と白亜紀火成岩類

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