グリーン・タフ時代の火成活動に関する一試論(<特集>日本列島の構造発達史-とくにグリン・タフ時代を中心として(その1)-)

抄録

It seems to be reasonable to regard the "green tuff movement" as a result of the activation of the zone of dislocation in the boundary area between the continent and the ocean floor. And it is most probable that this zone of dislocation has resulted from the differential movement of the Asiatic continent and the Pacific ocean floor,・the "Pacific Movement" of S. IJIRI. The activation took place for four stages, i.e. the inital, middle, late and the final stages. In the initial stage, the green tuff basins appeared, and the tholeiitic magmas were generated in the mantle. In the middle stage, the non-green tuff area such as the Setouchi province in southwestern Japan and the Sendai-Abukuma province in northeastern Japan were also activated, and the shallow basins with lacustrine circumstances were formed. The typical clastic sediments of such basins are found in the First Setouchi Supergroup. But in the green tuff area, formations characterized by the Miogypsina-Operculina zone certainly represent the middle stage. In this stage, the acidic magmas were generated in the deep crust, resulting in the activities of plagio-liparite and pitchstone. In the non-green tuff area, besides the rocks mentioned above, sanukitoid rocks are characteristic. The late stage is characterized by the folding and faulting, the green tuff area being localized upon areas of upheaval and subsidence. The non-green tuff areas were changed from subsiding areas into errosional areas. Calcalkaline, intermediate and acidic magmatisms were characteristic for this stage. It is noticed that in southwestern Japan the activities of tholeiitic magmas are recognized only in the initial stage, while, in northeastern Japan, activities of this kind were succeeded to the later stages. In the final stage, alkaline magmas and or silica poor olivine basalt magmas were generated in the inner side of southwestern Japan, representing the generation of the Circum Japan Sea Petrographic Province. The genesis of such alkaline and the related magmas might have been related intimately to the fracturing movement of relatively upper layer of the mantle in the post green tuff movement. The "propyritization" of rocks found frequently in the green tuff area can be regarded as one kind of low-grade regional metamorphism. This metamorphism is more intensive in northeastern Japan. It would mean that the activities of the zone of dislocation were more intensive in northeastern Japan than in southwestern Japan.