神居古潭帯額平川流域の緑色岩類

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The Late Jurassic Sarugawa Formation, which is composed of pillow lava, hyaloclastite, volcaniclastic conglomerate to mudstone and a small amount of chert, is widely exposed along the Nukabiragawa, Shudonigawa, and Shukushubetsugawa Rivers in the southern part of the Kamuikotan Belt. The Pillow lavas and hyaloclastites are petrographically classified into four types; 1) clinopyroxene-plagioclase phyric basalt, 2) aphyric basalt, 3) dolerite, and 4) aphyric trachyte. Considerable numbers of dykes are also observed in this area and are classified into three types; 5) amphibole-free dolerite, 6) kaersutite dolerite and 7) hornblende dolerite. Major elements of 15 representative rock samples were chemically analyzed. The pillow lavas of clinopyroxene-plagioclase phyric basalt, dolerite and aphyric trachyte and the dykes of kaersutite dolerite from the Shukushubetsugawa and the Nukabirgawa areas are characteristically alkaline, whereas the pillow lavas of aphyric basalt from the Shudonigawa-Osoushizawa area are tholeiitic. The hornblende dolerite dykes have a high FeO^*/MgO ratio showing more evolved tholeiitic nature. Volcanic piles along the Shukushubetsugawa River and on the southern side of the Nukabiragawa River are considered to be detached slices of Jurassic seamounts and/or oceanic islands, taking the following geologic and petrologic aspects into account. 1) lithologic constitution of the volcanic piles are comparable to that of the present seamounts and oceanic islands. 2) Basaltic to trachytic pillow lavas and dykes have petrochemically alkalic nature. 3) Highly vesiculated pillow lavas and a large amount of hyaloclastites suggest submarine eruptions at a shallow level. 4) Structural features of the volcanic piles are similar to those from foot of submarine volcanic body. The pillow lavas and dykes of tholeiitic basalt and dolerite were probably derived from a Jurassic ocean floor. The petrochemical nature, however, is characterized by high FeO^*/MgO ratios and high MnO contents and low TiO_2 contents, which are slightly different from those of mid-oceanic ridge basalts.
地学団体研究会の論文
1987-05-25

神居古潭帯額平川流域の緑色岩類

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