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Reinvestigation of the historical eruption and fumarolic activity records at Ontake Volcano, central Japan. -Misunderstanding reports about the 774 AD and 1892 AD eruptions- Teruki Oikawa The historical eruption records of Ontake Volcano were reexamined by this study. Previous Stratigraphic revision of the Monzen Formation, Oga Peninsula, NE Japan. Norihiko Kobayashi, Takeshi Ohguchi and Kazuhiko Kano The Monzen Formation is defined to occur between the Akashima Formation and the Nomuragawa and Daijima Formations and the type locality is specified along the coast from Monzen to Akashima, western Oga Peninsula. The lower contact with the Akashima Formation is faulted and the upper contact with the Nomuragawa and Daijima Formations is unconformable. The Monzen Formation is newly divided into the following members in ascending stratigraphic order: Butaijima Basalt, Ryugashima Dacite, Chorakuji Sandstone and Conglomerate, Chorakuji Basalt, Nagasaki Dacite, Kenashiyama Andesite, and Shinzan Rhyolite. The Chorakuji Basalt is locally interbedded with Chorakuji Sandstone and Conglomerate. The volcanic rocks dominated in the Monzen Formation erupted in a short period of time most likely from 36 Ma to 34 Ma to constitute volcanic edifices in a terrestrial to shallowwater area slowly rifting under extensional stresses. The geologic processes of the Monzen Formation are likely to represent precursors of the Early Miocene rapid opening of the Japan Sea. However, there is a large time gap over 107 years between the Monzen Formation and the overlying Daijima Formation, and it is necessary to extend the survey area to find the missing link between the two units. Chemical compositions of the Paleogene granitoids of eastern Shimane Prefecture, Sanin District, Southwest Japan Shunso Ishihara and Bruce W. Chappell Magnetite-series Paleogene granitoids of Mo-mineralized region of eastern Shimane Prefecture were studied chemically and compared with ilmenite-series granitoids of the Ryoke belt of the Chubu district. The eastern Shimane granitoids are divided into coarse-grained granodiorite and granite of batholithic bodies, and fine-grained granitoids occurring close to the roof-pendant. The fine-grained ones, varying in composition from quartz diorite to aplite are further subdivided into: Zakka and Kawai types, Rengeji type, leucogranites in the ore horizon, Yamasa type, Shimokuno type, and Ouchidani type. These granitoids are poor in A/CNK, Ga, Ga x 10000/A, K2O, Rb, Ba, Pb, CaO, Fe2O3, Zn, Y, La and Ce, and rich in Na2O, MgO, V and U, as compared with those of the Ryoke granitoids in the Chubu district. The chemical data indicate that the eastern Shimane granitoids are originated in source rocks with poor continental components, such as Al2O3, K2O, REE, and organic carbon. Some gabbroids which are high in Sr content and Sr/Y ratio may have been derived from slab-melting. Rengeji Older Granite is potassic being K2O>Na2O, and rich in La, Ce, Y, Nb, Th, U; thus continental in the source rocks. There occur small metamorphic bodies containing locally spinel and andalusite along the northwestern margin of the Rengeji Granodiorite. Kanenari hornfels is considered mainly psammitic with peraluminous layers. Togiishiyama hornfels has high A/CNK ratio but the K2O contents are normal as to the SiO2 contents. This rock contains rock-forming mineral of magnetite and has unusually high amounts of S, Cu, Pb, Zn and MnO. Therefore, the original rocks are considered intermediate to felsic tuffaceous sediments containing very little organic carbon. Mo-contents are high as 2.0 to 6.3 ppm Mo in average of the granitoids that host major molybdenite deposits, such as Kawai mingled rocks (Daito mine), Rengejileucogranites (Seikyu and Higashiyama mines) and Yamasa leucogranite (Yamasa mine). Therefore, trace amounts of Mo of fresh granitoids can be used as an exploration indicator. Late Eocene shoreline volcanism along the continental margin: the volcanic succession at Kabuki Iwa, Oga Peninsula, NE Japan Takeshi Ohguchi, Hiromitsu Yamagishi, Norihiko Kobayashi and Kazuhiko Kano Kabuki Iwa (Rock) at the northwestern shore of Oga Peninsula is composed of Late Eocene basaltic andesite aa lava flows and pillowed lava flows, dacitic pyroclastic flows, debris flows and other epiclastic rocks. This close association of the subaerial and subaqueous volcanic products demonstrates a transitional environment between land and shallow water. NE-SW-trending parallel dikes and normal faults are also associated with these rocks in the surrounding areas, and the volcanic succession at Kabuki Iwa is interpreted to have accumulated in an extensional basin which slowly subsided with volcanism before the rapid opening of the Japan Sea. Changes of geochemical characteristics of water in hydrological cycle in Koichi Okuzawa, Yoji Seki, Kazuki Naito, Atsushi Kamei, Masaya Suzuki, In order to clarify the change in the geochemical characteristics of water during hydrological Bulletin of the Geological Survey of Japan Top Page |