The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 109 , Issue 5
Showing 1-6 articles out of 6 articles from the selected issue
  • Kenya Ono, Makoto Takeuchi, Kazuhiro Suzuki
    2003 Volume 109 Issue 5 Pages 257-266
    Published: May 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    CHIME (chemical Th-U-total Pb isochron method) dating of granodiorite clasts from the Motodo Formation of the Hida-Gaien Belt was carried out by precise microprobe analyses of zircon grains. The CHIME ages are 201±20 Ma and 202±30 Ma, indicating that the granodiorite intrusion in the provenance areas of the Motodo Formation occurred in the Early Jurassic time. The depositional age of the Motodo Formation was constrained by previous authors between the Late Permian and Early Cretaceous, but our data further restrict it between the Early Jurassic and Early Cretaceous. On the basis of age constraints and lithological similarities, the Motodo Formation correlates with the Kyongsang Supergroup in South Korea and the Kanmon Group in Southwest Japan. Early Jurassic granitoids are distributed in the Hida Belt in Japan and in the Korean Peninsula, which are considered as likely provenance areas for the sediments of the Motodo Formation.
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  • Hideo Ishizuka, Maki Miyake, Naoko Takeda
    2003 Volume 109 Issue 5 Pages 267-279
    Published: May 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    In the Sanbosan unit of the Southern Chichibu Belt, west to central east Shikoku, small but significant amounts of greenstones occur as various-sized blocks or lenses embedded within mudstone or siliceous mudstone. Some greenstones preserve pillowed structure with or without interpillow limestone, and related hyaloclastite and brecciated pillowed structures. They are rarely aphyric but largely phyric with phenocrysts of plagioclase, olivine and clinopyroxene. Bulk rock compositions divide the greenstones into high and low Nb/Zr types. The former has an affinity with oceanic island basalts and the latter with mid-oceanic ridge basalts or back-arc basin basalts. Phenocryst of clinopyroxene is restricted to the high Nb/Zr type. With increasing iron content, clinopyroxene compositions of phenocryst and/or groundmass indicate a Ca-enrichment in the high Nb/Zr type, but a Ca-depletion in the low Nb/Zr variety. Metamorphic mineral assemblages reveal that the greenstones have been metamorphosed under the medium- to high-grade zeolite facies in the high Nb/Zr type and the prehnite-pumpellyite (PP) to pumpellyite-actinolite (PA) facies in the low Nb/Zr type and a part of the high Nb/Zr variety. The high Nb/Zr type of the zeolite facies is widespread in the Sanbosan unit forming a narrow zone just north of the Butsuzo Tectonic Line, whereas the low Nb/Zr and high Nb/Zr types of the PP to PA facies are restricted to the Sanbosan unit called the Tosaka Window and Nakatsugawa Window (Matsuoka, 1998), respectively, in west Shikoku. In particular, the greenstones occurring in the Tosaka and Nakatsugawa Windows have some similarities in chemistry and metamorphic facies to those of the Northern Chichibu Belt. These results lead to the following two interpretations ; 1) all these greenstones occur forming several stratigraphic horizons in the Sanbosan unit of the Southern Chichibu Belt, or 2) the greenstones occurring only in the Tosaka and Nakatsugawa Windows belong to the Northern Chichibu Belt.
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  • Tokiyuki Sato, Takeshi Higuchi, Takaaki Ishii, Shiho Yuguchi, Kazutaka ...
    2003 Volume 109 Issue 5 Pages 280-292
    Published: May 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    The calcareous nannofossil biostratigraphy of the upper Pliocene to lowermost Pleistocene Tentokuji and Sasaoka Formations distributed in the Minehama, Tokiwa, and Masayamazawa areas located in the northern part of Akita Prefecture, Honshu, Japan are described in detail. The assemblages of the Tentokuji Formation in the Minehama and Masayamazawa areas, and of the Sasaoka and Tentokuji Formations in the Tokiwa area, are characterized by abundant Reticulofenestra spp.(small form) and Dictyococcites spp.(small form). The Sasaoka Formation in the Minehama and Masayamazawa areas is characterized by the abundant occurrence of the cold water nannofossil, Coccolithus pelagicus. Gephyrocapsa oceanica and G. caribbeanica, which indicate Pleistocene age, are found only in the uppermost Sasaoka Formation in the Minehama area. The Tentokuji and Sasaoka Formations, except the uppermost Sasaoka Formation in the Minehama area, correlate with the late Pliocene, between 3.85 and 1.73Ma. Datum plane A (2.75Ma) which is correlated with the start of heavy glaciation in the high northern latitudes in the early late Pliocene, is traceable to between the Sasaoka and Tentokuji Formations in the Minehama and Masayamazawa areas. We reconstructed the paleoenvironmental change during the late Pliocene to early Pleistocene in the Akita area based on the correlation among the uppermost Cenozoic formations distributed in the Akita areas. Calcareous nannofossil assemblages that are found in the upper Pliocene on the Japan Sea side are very similar to those in the high latitudes of the north Pacific and Arctic Ocean. The assemblages contrast with those on the Pacific side of Japan that are characterized by occurrences of warm water flora. These data indicate that the drastic increase of the ice sheet in the Arctic Ocean at 2.75Ma strongly influenced the nannofossil and Omma-Manganji molluscan fauna on the Japan Sea side.
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  • Makoto Saito, Ken'ichi Saiki, Seiichi Toshimitsu
    2003 Volume 109 Issue 5 Pages 293-298
    Published: May 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Late Devonian Leptophloeum has been obtained from the coherent strata in the Kurosegawa Belt in eastern Tomochi Town, central Kyushu. This reveals that the Upper Devonian formation extends in the E-W direction the south side of Usuki-Yatsushiro Tectonic line. This Upper Devonian is compared with the "Naidaijin Formation" which occupies the eastern part of the district. It is not connected with the "Naidaijin Formation" and they are arranged in two zones with E-W trend. The Upper Devonians in Japan which yields Leptophloeum are divided into two types of facies : one is mudstone facies, the other is acidic tuff facies. The Upper Devonian in the Tomochi district belongs to the former.
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  • Akira Ishiwatari
    2003 Volume 109 Issue 5 Pages 299-302
    Published: May 15, 2003
    Released: April 11, 2008
    JOURNALS FREE ACCESS
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  • Akihisa Motoki, Yuji Orihashi, Jose Antonio Naranjo, Daiji Hirata, Tak ...
    2003 Volume 109 Issue 5 Pages IX-X
    Published: 2003
    Released: December 14, 2010
    JOURNALS FREE ACCESS
    The Lautaro Volcano, Chilean Patagonia, is situated at 49° 01.4' S, 73° 32.5' W, being the closest volcano to the Chile Triple Junction, in which on-going subduction of active ridge takes place. The volcanoes of the Austral Volcanic Zone are characterised by adakitic composition (Stern and Killian, 1996), and their magma origin is attributed to slab-melting. Such typical ridge subduction volcanism can clarify juvenile continental crust formation process, as a modern analogue of Archaean magmatism.
    The access to the Lautaro Volcano, 3, 607 m above the sea level, is extremely difficult because of accidental morphology and unfavourable climate conditions. The volcano edifice stands out from the glacial plateau of 2, 400 m high, so-called South Ice Field, therefore, no research group had got rock samples in enough amount and variety. On February 28, 2002, with help of helicopter, the authors' research group accomplished a successful sampling of eruptive materials of this volcano from deposits of the Lautaro Glacier, first target, and the O'Higgins Glacier, second target.
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