The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 126, Issue 12
Displaying 1-4 of 4 articles from this issue
Articles
  • Yukimasa Oho, Yasuyuki Hirayama, Naomi Kawamoto, Chiho Ishikawa
    2020 Volume 126 Issue 12 Pages 663-678
    Published: December 15, 2020
    Released on J-STAGE: March 26, 2021
    JOURNAL FREE ACCESS

    This study examined meso- and microscale structures in tectonic mélange of the Jurassic Chizu Complex in the Kuse-Kagamino area, Okayama Prefecture, Southwest Japan. The Chizu Complex consists of coherent clastic rocks (mudstone, sandstone, and alternating beds of mudstone and sandstone), and chaotic rocks of sedimentary origin. They have been deformed by shearing to form tectonic mélange. Four stages of deformation are recognized: (1) layer-parallel elongation and extensional fractures; (2) layer-parallel shearing and disaggregation; (3) formation of slaty cleavage subparallel to bedding; and (4) formation of crenulation cleavage, overprinting the slaty cleavage. The shearing of the second stage formed asymmetric structures such as a Y-P composite planar fabric and disaggregation of sandstone clasts. It is likely that disaggregation enhanced the deformation of the sandstone clasts by grain boundary sliding. The slaty cleavage of the third stage is defined by the preferred orientation of platy minerals and formed by layer-perpendicular flattening that resulted in the development of symmetrical pressure shadow around pyrite grains. The crenulation cleavage of the fourth stage indicates layer-parallel shorting. The stage one deformation affected poorly consolidated sediments at shallow depths, whereas the following three deformation stages affected consolidated sediments at greater depths. The asymmetric geometric relationship between Y shears and the P foliation of the second stage indicates top-to-the-NNE shear, after correcting for the rotation of Southwest Japan related to the opening of the Japan Sea.

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  • Yayoi Muraoka, Masaaki Owada, Teruyoshi Imaoka, Atsushi Kamei, Kazuhir ...
    2020 Volume 126 Issue 12 Pages 679-695
    Published: December 15, 2020
    Released on J-STAGE: March 26, 2021
    JOURNAL FREE ACCESS

    Widespread Cretaceous granitic rocks in north Kyushu have been geochemically classified into low-Sr and high-Sr granites by Izawa et al. (1990, 1994). The Kokura-Tagawa Tectonic Line (KTT) in the region separates high-Sr granite to the west from low-Sr granite to the east. The Hirao granodiorite and a related granite dike are exposed on the east side of the KTT and have low Sr contents of 213-291 ppm. In this study we consider the petrogenesis of the Hirao granodiorite and other low-Sr granites, in comparison with the nearby high-Sr Ushikiri-yama granodiorite. The Hirao granodiorite shows similar rare earth element(REE) patterns and Sr-Nd isotopic values to the granite dike. Mass balance calculations reveal that the granite dike magma could have been formed by the removal of plagioclase and hornblende from the Hirao granodiorite magma. The geochemical characteristics of the Hirao granodiorite, combined with the results of the melting experiments, confirm that the low-Sr granite magma could have been produced by relatively low-pressure melting, although the composition of the source of the Hirao granodiorite is similar to that of the high-Sr Ushikiri-yama granodiorite.

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  • Kuniyuki Furukawa, Shoji Nishimoto, Tatsuo Kanamaru, Yutaka Wada, Hiro ...
    2020 Volume 126 Issue 12 Pages 697-712
    Published: December 15, 2020
    Released on J-STAGE: March 26, 2021
    JOURNAL FREE ACCESS

    This paper discusses the formation of veins less than 20 cm in width which are abundant in the Nakamura Formation of the Miocene Mizunami Group. These veins consist of a central clastic and adjacent dense portions. Field observations and chemical and X-ray diffraction analyses show that the clastic part was formed by hydraulic fracturing under hydrothermal conditions, and that light (Lm) and dark colored parts (Dm) of the veins were formed by precipitation of opal-CT from hydrothermal fluids. The formation of Lm and Dm are probably related to fluids that dissolved pumice fragments and acidic fluids formed in a reducing environment, respectively. Fluid penetration of a host rock with intergranular porosity adjacent to the clastic part of a vein and subsequent precipitation of Lm formed the dense part of the vein. Based on microscopic observation and XRD analysis, hydrothermal alteration did not affect the dense parts of the veins, unlike the host rocks. Progression of water-rock interaction occurred after the veins solidified.

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