The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 111, Issue 6
Displaying 1-7 of 7 articles from this issue
Articles
  • Kei Odawara, Shigeo Kudo, Yasufumi Iryu, Tokiyuki Sato
    2005 Volume 111 Issue 6 Pages 313-331
    Published: 2005
    Released on J-STAGE: October 01, 2005
    JOURNAL FREE ACCESS
    The Pleistocene Ryukyu Group composed of reef complex deposits crops out in Yomitan area, Okinawa-jima, Ryukyu Islands, southwestern Japan. The group rests unconformably on the upper Cenozoic Zakimi Formation and the pre-Tertiary Nago Formation. We propose a major revision of the previous stratigraphic scheme for the Zakimi Formation and the Ryukyu Group, based on new investigations, and provide a formal stratigraphic description. The Zakimi Formation (>40 m thick) consists of conglomerate comprising poorly sorted, up to cobble-sized, subrounded gravel derived from the Nago Formation, displaying a clast-supported texture. The Ryukyu Group comprises the Iramina and Sobe Formations in study area. The Iramina Formation, observed in drill cores, is thin (>11 m thick) and consists mainly of conglomerate, coral limestone, and detrital limestone. The Sobe Formation unconformably overlies the Iramina Formation, reaches 70 m in thickness, and crops out at elevations of up to 80 m. It is divisible into five units. Each unit is composed of coral limestone overlain by rhodolith, Cycloclypeus-Operculina, and detrital limestones and shows a deepening-upward sequence. Of these units, Units 4 is most extensively exposed in this area. This unit rests unconformably on Units 2 and 3 at elevations greater than 20 m and conformably overlies Unit 3 below the elevation. It is equivalent to the Yontan Limestone in previous stratigraphic literatures. Calcareous nannofossil biostratigraphy indicates that the lower part of the Sobe Formation was deposited in early Pleistocene time, ranging from 0.41-0.85 Ma. Stratigraphic position and age-diagnostic nannofossils indicate that the Sobe Formation may be partly correlative to the main body of the Ryukyu Group on Toku-no-shima, Okierabu-jima, Yoron-jima, southern Okinawa-jima, and Miyako-jima.
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  • Kiyokazu Kawajiri
    2005 Volume 111 Issue 6 Pages 332-349
    Published: 2005
    Released on J-STAGE: October 01, 2005
    JOURNAL FREE ACCESS
    Gabbroic dikes intruding into Carboniferous-Permian sedimentary rocks of the Hida Gaien belt in the northern part of Takayama City consist of hornblende gabbro and hornblende-clinopyroxene gabbro. On the diagrams SiO2 versus FeO* and V, and FeO*/MgO versus SiO2, the hornblende gabbros show a trend of calc-alkaline magma suite, whereas they have TiO2 concentration as high as mid-ocean ridge basalt (MORB). MORB-normalized high-field-strength element patterns for most hornblende gabbros are similar to those for island arc basalt at the back-arc side and back-arc basin basalt. In addition, one hornblende gabbro with high-K hornblende probably originated from shoshonitic to high-K calk-alkaline magma. These facts suggest that the hornblende gabbro originated from back-arc side of island arc to back-arc basin basaltic magmas. Because the hornblende-clinopyroxene gabbro contains anorthite-rich plagioclase (An=ca. 92) coexisting with moderately evolved clinopyroxene (Mg/(Mg+Fe)=ca. 80), it probably originated from an island arc basaltic magma. Such a diversity of magma types suggests that the Hida Gaien belt was situated in an island arc to a back-arc basin setting in latest Permian time.
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  • Junko Komatsubara, Hiroaki Ugai, Tohru Danhara, Hideki Iwano, Tetsu Yo ...
    2005 Volume 111 Issue 6 Pages 350-360
    Published: 2005
    Released on J-STAGE: October 01, 2005
    JOURNAL FREE ACCESS
    The Lower to Middle Miocene Nojima Group, NW Kyushu, Japan is divided into the Oya, Fukazuki, and Minamitabira Formations in ascending order. Seven samples were collected for the fission-track dating: two samples from the Oya Formation, two samples from the basal Kojimazaki Tuff Breccia of the Fukazuki Formation, and two samples from the lower Fukazuki Formation, and one sample from the Minamitabira Formation. Fission-track dating was carried out by the external detector method, which was applied to the internal (ED1) and external surfaces (ED2) of zircons. Obtained ages ranges from 21 to 15 Ma and are partly inconsistent with the stratigraphic levels, perhaps due to contamination of exotic zircons. Statistically significant ages obtained are 17.5±1.4 Ma and 17.7±0.9 Ma for the Oya Formation, and 15.3±0.5 Ma (1σ) for the lower Minamitabira Formation. The latter age is consistent with the occurrence of the Kadonosawa Fauna (Its known age is 16.5-15 Ma). These reliable ages allow calculation of the accumulation rate for the Nojima Group to be at least 500 m/m.y., which suggests very rapid rifting of the basin during the climactic stage of the Japan Sea opening.
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  • Yukiyasu Saka, Fumiyo Saruwatari, Kazuhiro Suzuki
    2005 Volume 111 Issue 6 Pages 361-368
    Published: 2005
    Released on J-STAGE: October 01, 2005
    JOURNAL FREE ACCESS
    CHIME (chemical Th-U-total Pb isochron method) dating of garnet-granite, which is exposed in the southernmost part of the Naguri Fault Zone in the Kanto Mountains, was carried out by precise microprobe analysis of zircon grains. The CHIME age is 432±40 Ma. This age is concordant with that of granitoids (378-447 Ma) of the Kurosegawa Terrane in the other areas of Southwest Japan. It is a conclusive evidence indicating that the Naguri Fault Zone represents an eastern extension of the Kurosegawa Terrane, in addition to the other characteristics such as the tectonic situation of the Naguri Fault Zone, mode of occurrence as well as petrography and petrochemistry of rocks occurring in this fault zone.
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