九州北部,英彦山地域の後期新生代火山層序および地質構造

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Vast volumes of Neogene calc-alkalic volcanic rocks are widely distributed at the vicinity of the boundary between Fukuoka and Ooita Prefectures, northern Kyushu. In this paper, the authors describe geology, geological structures, petrography of the late Cenozoic formations in the area and discuss its geologic development. 1. Basement rocks of the Neogene formations are the Tagawa metamorphics and Irahara granodiorite. The former is composed of pelitic schist and biotite schist, and the latter intruded into the former with the trend of NE-SW-NNE-SSW. 2. The Neogene formations are divided into three formations in ascending order, Yamakuni Formation, Kita-sakamoto Formation and Hikosan volcanics. The Yamakuni Formation and Kitasakamoto Formation correspond to the green tuff formation of the Northeast Japan, and date the early to middle Miocene and late Miocene respectively. The age of the Hikosan volcanics is about 4.6〜4.8 Ma, based on the fission track dating of zircon in lavas. So the volcanism related to so-called "Island -arc disturbance" in this area began in the early Pliocene 3. Each volcanic activity of the above-mentioned three formations made progressed from felsic volcanism through accumulation of enormous pyroclastics, frequently associating volcanic conglomerates, to eruption of pyroxene andesite lava flows of plateau-forming type. Such volcanic activity was rhythmically repeated three times through the Green tuff movement and the Island-arc disturbance. 4. Geological structure of the green tuff formations is characterized by predominance of E-W trend, which is given by directions of elongations of the sedimentary basins, fault systems, and by longitudinal axis of many intrusive masses. Such trends indicate that the area is situated in the inner belt of the Southwest Japan arc. The sedimentary basin where the Formation attained to 1400 meters in thickness of the Kitasakamoto Formation was formed by southeastward shift of the center of maximum sinking, and subsequently sites of volcanism also shifted southeastward. It is very interesting that the basin was kept under non-marine condition without any episode of marine invasion. The Hikosan lava piles are topography arranged in echelon showing WSW-ENE trend, with an extension of E-W trend respectively. 5. The following three types of alteration are distinguished among the Miocene volcanics; firstly by diagenetic alteration, most widely recognized, secondly by contact hydrothermal alteration by intrusion of porphyrites, and thirdly by mineralization associating gold-silver quartz veins. The gold-silver mineralization finished before deposition of the Kitasakamoto Formation. The Hikosan volcanics are free from these alterations.
日本地質学会の論文
1984-11-30

九州北部,英彦山地域の後期新生代火山層序および地質構造

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