The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 113, Issue 5
Displaying 1-5 of 5 articles from this issue
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Articles
  • Tectonic boundary between the Sanbagawa belt and the Shimanto belt in central Shikoku, Japan
    Kazumasa Aoki, Tsuyoshi Iizuka, Takafumi Hirata, Shigenori Maruyama, M ...
    2007 Volume 113 Issue 5 Pages 171-183
    Published: 2007
    Released on J-STAGE: December 28, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.171
    JOURNAL FREE ACCESS
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    In order to make it clear the mode of occurrence of the Sanbagawa belt, we carried out in situ U-Pb isotope analyses of igneous zircon grains from the Oboke area that was a type area of the Sanbagawa belt in central Shikoku, Japan. Analyzed igneous zircons were separated from psammitic schist in the Minawa and Kawaguchi Formations and from igneous cobbles in the Koboke Formation. Spot analyses were performed on the laser ablation-inductively coupled plasma mass spectrometer (LA-ICP-MS). The youngest U-Pb ages of zircon grains from the Koboke Formation and the Kawaguchi Formation showed 92±4 Ma and 82±11 Ma, respectively. On the other hand, zircons from the Minawa Formation yielded remarkably older ages clustered around 1900-1800 Ma. There is a large chronological gap between protolith sedimentary clasts of the Minawa and those of the other two formations. The protolith sedimentary ages of the Sanbagawa belt have been well constrained as older than 130 Ma based on fossil and U-Pb isotopic ages. The peak metamorphism occurred in 120-110 Ma. Therefore, both Koboke and Kawaguchi Formations must not belong to the Sanbagawa belt, because the timing of formation of accretionary complex must be later than 92±4 Ma for the Koboke Formation and 82±11 Ma for the Kawaguchi Formation. Both the Koboke and Kawaguchi Formations correspond to the late Cretaceous accretionary complex, and they are equivalent to the Northern Shimanto belt. The tectonic boundary between the Sanbagawa and the Northern Shimanto belts is reverse fault and the Northern Shimanto belt appears as a tectonic window in the Sanbagawa belt, central Shikoku. The whole package of the Sanbagawa and underlying Shimanto belts are deformed by the secondary fault movement and doming after the tectonic juxtaposition at the mid-crustal levels.
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  • Abrupt change of sedimentary environment in ca. 7300 years ago recorded in the piston cores from the central Okinawa Trough floor
    Kiichiro Kawamura, Ken Ikehara, Kantaro Fujioka
    2007 Volume 113 Issue 5 Pages 184-192
    Published: 2007
    Released on J-STAGE: December 28, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.184
    JOURNAL FREE ACCESS
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    An abrupt change of the depositional system is recorded in two piston cores in the central Okinawa Trough. The cores are approximately 3 and 5 meters long, respectively, and contain the K-Ah tephra layer of the age of 7325 years B.P. The median of the grain size of the constituent quartz grains in the cores decreases suddenly upwards at the K-Ah horizon. This decrease results from the removal of siliciclastic grains coarser than ca. 20 μm in diameter from the sediments. This change of sedimentary environment could be explained in the following ways: 1) the distance between the Okinawa Trough and the Chinese continent of the sediment source has increased due to sea level rising at this time, 2) a barrier of the Kuroshio Current was formed along the edge of the continental shelf during 8000-7000 years B.P., and most of the sediment supplies from the continental shelf was swept away by the strong current such as the Kuroshio. The sediment microfabric also changed at this time from horizontal to random, as revealed by the change of the anisotropy of magnetic susceptibility. This horizontal fabric is considered to have been formed under the high sedimentation rate conditions without bioturbation. In conclusion, it is suggested that the present depositional system in the central Okinawa Trough was established at the age of ca. 7300 years B.P. and persists until now.
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  • Reexamination of volcanic activity of Yatsugatake area, central Japan
    Kuniaki Nishiki, Akikazu Matsumoto, Kozo Uto, Kou Takahashi, Yasuyuki ...
    2007 Volume 113 Issue 5 Pages 193-211
    Published: 2007
    Released on J-STAGE: December 28, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.193
    JOURNAL FREE ACCESS
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    We revised the volcanic history of the Yatsugatake area, located in the eastern area of Nagano Prefecture, central Japan, by means of volcanostratigraphy, K-Ar age determination and magnetostratigraphy. As a result, it was clarified that the volcanic activities occurred at around 1.2-0.8 Ma and 0.5-0 Ma in the Northern Yatsugatake area, and at around 0.5-0.1 Ma in the Southern Yatsugatake area. As the volcanic activities older than 0.8 Ma are restricted only in the northern area, the volcanoes in this area are newly defined as “Yabashira Volcanoes”. We also redefined “Yatsugatake Volcanoes” as those which were formed in both northern and southern areas after 0.5 Ma. The Yabashira Volcanoes have more than four eruptive centers, which are lined up in the NW-SE direction. The arrangement of these volcanic centers is different from the N-S arrangement of the volcanic centers of the Yatsugatake Volcanoes. To the west of the Yabashira Volcanoes, the volcanic ejecta of the same age including those in Yashigamine, Kirigamine and Wadatoge areas are distributed, forming a volcanic field spreading in the E-W direction within the area of Yatsugatake-Chushin Highland area.
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  • Genesis of the Poroshiri ophiolite, Hokkaido, Japan; Inference from geochemical evidence
    Sumio Miyashita, Yoshiko Adachi, Shinji Tanaka, Mitsuhiro Nakagawa, Ju ...
    2007 Volume 113 Issue 5 Pages 212-221
    Published: 2007
    Released on J-STAGE: December 28, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.212
    JOURNAL FREE ACCESS
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    Seven samples (four metabasaltic rocks and three meta sheeted-dike samples) and two samples of later metadolerite dikes of the Poroshiri ophiolite were analysed by ICP-MS. Chondrite normalized REE values of the metabasaltic rocks exhibit N-MORB pattern. Multi-element spider diagram shows a N-MORB signature except for variable concentrations in LIL elements that are ascribed to secondary modification. These metabasaltic rocks plot in N-MORB fields in discrimination diagrams using immobile elements and are distinct from such as IAT, BABB and E-MORB. Later dolerite dikes, which intruded after considerable cooling of the ophiolite, have also N-MORB signatures. However, the later dikes are slightly depleted in HREE and enriched in LREE concentrations, suggesting more fertile source than the Poroshiri ophiolite. These geochemical signatures and wide compositional range of the metabasaltic rocks with evolved rocks (e.g., ~3 wt % in TiO2 and ~3.0 in FeO*/MgO) suggest that the Poroshiri ophiolite was generated at fast-spreading ridge. Zr/Y and Zr/TiO2 ratios from the EPR basalts tend to decrease with increasing spreading rates. The Zr/Y and Zr/TiO2 ratios of the metabasaltic rocks are as low as those from the ODP Hole 1256D basalts generated at the EPR with an ultra-fast spreading rate. Assuming that the mantle source of the ophiolite was similar to that of the East Pacific Rise (EPR), this fact suggests that the spreading ridge produced the Poroshiri ophiolite spread at an ultra-fast rate more than 20 cm/year.
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