北部フォッサ・マグナ,志賀高原～津南町周辺の鮮新世～更新世火山岩類

その1;鳥甲火山の火山岩類

抄録

Torikabuto volcano, an early Pleistocene composite volcano, is located on the upper Nakatsu River in North Fossa Magna. The geologic history of Torikabuto volcano is divided into four stages as follows; The activity of the oldest stage, stage I (Maekura Stage), is characterized by the pyroxene andesite pyroclastic flows and lava flows. The distribution of these flows is limited to Mt. Nunoiwa and its vicinity. The next stage II (Onizawa Stage) is characterized by the large amount of pyroclastic flows. They are composed of the rocks ranging from basic andesite to dacite. The formation of a great deal of lava flows is characteristic of Stage III (Torikabuto Stage). The lavas consist of olivine-pyroxene andesite, quartz-biotite bearing hornblende pyroxene andesite and quartz bearing pyroxene andesite. They contain gabbroic inclusions and sometimes show banded structure. They formed several cones. Only a small quantity of dacitic pyroclastic flows were formed during the last stage IV (Post Torikabuto Stage). Torikabuto volcano is mainly made up of calc-alkaline rocks. Chemically the rocks of Maekura Stage are poor in Na2O, Al2O3, Sr and are rich in K2O, Rb, compared with the rocks of the other stages. The rocks of stage II to III have clear linear correlations on SiO2-oxides variation diagram. On chondrite Leedy-normalized REE pattern daiagram, the rock of stage I shows Eu-depleted anomaly. The rocks of stage I and the gabbroic inclusions plot apart from the trend of the volcanic rocks of stage II to III on Sr/Ca-Ba/Ca diagram. On the same diagram, the gabbroic inclusions seem to be connected with the rocks of stage I . The gabbroic inclusions are likely to have some connections with the rocks of stage I chemically on the other diagrams. The rocks of stage II and III contain plagioclase with reverse zoning and bimodal compositions, suggesting the magma mixing process. The facts mentioned above show that the magma forming the rocks of Maekura Stage is different from the one forming those of Onizawa and Torikabuto Stages. The rocks after Onizawa Stage are formed by mixing between basic andesitic magma and dacitic magma. The gabbroic inclusions are cumlates formed from the already differenciated magma which has a correlation with the volcanic rocks of Maekura Stage.