日本の古生代・初期中生代地向斜火山活動史

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The volcanicity of Palaeozoic (middle Silurian to Permian) and early Mesozoic geosynclines of Japan is summarized on stratigraphic positions, spatial distributions and tectonic relations to the development of the geosynclines. Important manifestations are as follows: 1) The volcanicity of the Palaeozoic geosyncline took place in many ages as shown in Fig.1, and is not confined to such a limited age as has been known in European and American geosynclines as the initial volcanism in the early sedimentary stage. 2) As for the fundamental sedimentary and tectonic framework of the Palaeozoic geosyncline, three domains, arranged in echelon, are recognized, the eastern, the inner, and the outer, each of which comprises a ridge (South Kitakami-Abukuma, Hida, and Kurosegawa, respectively for each) and a surrounding sedimentary basin. Volcanic manifestations are fairly different among the three domains, although major common features are recognized with respect to general volcanic history as well as rock suites. 3) Distribution of Volcanic sediments in space and time indicates that, in each domain, the site of volcanicity has shifted with age from a ridge to the outer side (e.g. Fig. 5). 4) In each domain there are some thick piles of volcanic sediments of a limited age which are distributed narrowly but extensively (e.g. Figs. 2 and 4). The geotectonic position of such volcanic zones can be deduced to be a transitional zone in lithofacies and/or thickness of the sedimentary series, which could be a line of crustal weakness by flexure or tension. 5) The Silurian and Devonian volcanics are characterized by acidic rocks (mainly rhyolite) with some basic ones. They are associated with coralgal reef limestones or shales yielding plant remains, and occur adjacent to or near the squeezed out masses of primarily deep-seated high-grade metamorphic rocks. These features suggest that the volcanicity took place on and along a shallow geanticlinal ridge of each domain which was generated by older orogenic cycles. 6) The Carboniferous and Permian rocks are of eugeosynclinal lithologic assemblage, expect the Permian sediments on the South Kitakami insular shelf which was stabilized by the Abean movements of late Carboniferous times. The volcanics are mostly of basic composition, occurring as basaltic and diabasic lavas and pyroclastics. Gabbroic rocks are also associated with them usually in the case of thick accumulations. The greatest volcanic emanations took place in early Permian (Pseudoschwagerina zone to Misellina zone) in all of the three domains and are as thick as 500-1400 m. Another enormous eruptions are in the Tournaisian of South Kitakami and are accompanied by kelatophyre and trachyandesite. No volcanism has been known in the Visean of the eastern and outer domains, but isolated, short-lived but violent volcanism of that age took place in the inner domain. Late Carboniferous volcanicity is generally weak, but is most prominent in the outer domain than the other two. While middle Permian volcanicity is most remarkable in the inner domain and emanated, besides basic rocks, rhyolitic lava and pyroclastics. This probably means that the orgenic movements of the inner domain anteceded those of the outer one. 7) The Palaeozoic geosyncline was subject to orogenic movements in late Permian to Triassic times, which gave rise to southward shifting of the main axis of depositional basin to the outer side. The site of volcanicity was shifted back in late Permian to the inner side of the inner domain and there occurred a fairly large ammout of rhyolitic lava and tuffs with subsidiary extrusions of dioritic and granitic rocks. The Triassic volcanicity is found only in the newly generated geosyncline in the outer side of the outer domain, where the thick piles of basaltic and diabasic lavas and pyroclastics were accumulated together with many beds of chert and micritic limestone containing pelagic faunas.
日本地質学会の論文
1971-07-31

日本の古生代・初期中生代地向斜火山活動史

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