The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Articles
  • A geochronological approach to magma chamber process
    Takashi Nakajima, Hiroyuki Kamiyama, Ian S. Williams
    2021 Volume 127 Issue 2 Pages 69-78
    Published: February 15, 2021
    Released: May 15, 2021
    JOURNALS FREE ACCESS

    Syn-plutonic dikes show the intermingling process of partially molten silicic and mafic magmas. We found some syn-plutonic dikes with different ages between their silicic and mafic rocks from the Chubu and Kinki districts. The zircon U-Pb ages of the silicic rocks are 90-94 Ma while those of the mafic rocks are 70-75 Ma. It is unlikely that the granitic magma of 90-94 Ma survived in partially molten state until the injection of the mafic magma at 70-75 Ma because the time discrepancy of 14-20 Ma seems too long for a lifetime of a single magma chamber.

    In a silicic rock of the syn-plutonic dike we found a zircon grain which show Cathodoluminescence image of an irregular zoning with partly dissolved and re-grown texture. The U-Pb age of the re-grown part of it is ca. 70 Ma and the that of the undissolved core is ca. 90 Ma. Considering that the mafic rocks nearby is of 70 Ma and the host rock of the syn-plutonic dike is Older Ryoke granitoid of 90-94 Ma, it could be reasonably interpreted that the syn-plutonic dike was formed by injection of mafic magma at ca. 70 Ma into the already solidified 90-94 Ma granitoids and partially melted them, leading to the physical interaction of the local silicic crystal mush and the mafic magma, unlike the case where mafic magma was injected into pre-existing mushy granitic magma chamber.

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  • Jun-ichi Tazawa, Keisuke Suzuki, Yukio Miyake
    2021 Volume 127 Issue 2 Pages 79-90
    Published: February 15, 2021
    Released: May 15, 2021
    JOURNALS FREE ACCESS

    Here we describe an early Carboniferous brachiopod fauna from the middle part of the Arakigawa Formation in the Hongo area, Hida Gaien Belt, central Japan. The Hongo fauna consists of six species in five genera: Pugilis sp., Marginatia sp., Fluctuaria undata (Defrance), Fluctuaria sp., Actinoconchus sp., and Imbrexia sp. The fauna is identified as late Visean in age, thus correlating the middle part of the Arakigawa Formation with the upper Visean. Palaeo-biogeographically, the Hongo fauna exhibits affinities with early Carboniferous brachiopod faunas of northwestern China (Xinjiang) and northeastern Japan (South Kitakami Belt). The fauna probably belonged to the North China Province, which covered a broad area between the North China Block and the Siberian Block (i.e., the Central Asian Orogenic Belt, CAOB) during the late Visean. The Hida Gaien Belt, including the Hongo area, was likely located in the CAOB during the late Visean. Similarities between the lithology and fauna of the Arakigawa Formation and those of the Carboniferous formations of the South Kitakami Belt provide positive evidence of a tectonic connection between the Hida Gaien and South Kitakami belts.

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  • Akira Furusawa, Toshinori Sasaki, Norihisa Goto
    2021 Volume 127 Issue 2 Pages 91-103
    Published: February 15, 2021
    Released: May 15, 2021
    JOURNALS FREE ACCESS
    Supplementary material

    The major and trace element compositions of hornblende from the Mihama tephra, sampled near its type locality, were compared with those of tephra erupted from Kyushu volcanoes and preserved in the MD012422 piston core from the continental slope basin, off Shikoku. These comparisons show that the Mihama tephra may be correlated to the GHo-2 cryptotephra in the MD012422 core (sample No. 26.10 from 26.00-26.10 mbsf), because the petrological characteristics of hornblende and the stratigraphic position of the GHo-2 cryptotephra are consistent with those of the Mihama tephra. The Mihama tephra was likely erupted from Kyushu volcanoes during the early stage of MIS5.

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  • Yuji Kato, Yukio Yanagisawa
    2021 Volume 127 Issue 2 Pages 105-120
    Published: February 15, 2021
    Released: May 15, 2021
    JOURNALS FREE ACCESS
    Supplementary material

    This paper describes the geology and chronostratigraphy of the Neogene marine sedimentary sequence in the northeastern part of the Sasamori Hills of the Dewa Mountains, Akita Prefecture, Japan. The detailed geological study included reexamination of the lithostratigraphy and establishment of a marine diatom biostratigraphy. The Neogene marine sediments in this area are composed of the Sugota, Onnagawa, Funakawa, and Tentokuji formations, from oldest to youngest. The boundary between the Onnagawa and Funakawa formations, previously considered to be conformable, is now identified as a hiatus with discontinuity from 9.6 to 9.3 Ma. Four glauconite beds have been recognized: three in the Sugota Formation, and one in the Onnagawa Formation. Integrating the diatom biostratigraphy with reported calcareous microfossil biostratigraphy provides the first reliable age estimates for these glauconite beds, which are mostly consistent with ages previously reported for glauconite beds in Japan. Extremely low sedimentation rates are confirmed for the glauconite beds.

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Letter
  • Kenichiro Iwamizu, Yasutaka Hayasaka, Ji-Hoon Kang, Kosuke Kimura, Tom ...
    2021 Volume 127 Issue 2 Pages 121-129
    Published: February 15, 2021
    Released: May 15, 2021
    JOURNALS FREE ACCESS

    A Paleoproterozoic granite gneiss body was discovered in the Jurassic Daebo granite of the Ogcheon Belt, Geumsan area, South Korea. The granite gneiss is a two-mica variety with a small amount of garnet, and has a peraluminous bulk-rock composition. U–Pb isotope analysis of zircon grain cores by LA-ICP-MS yields a 207Pb/206Pb magma crystallization age of ca. 2.5 Ga. The granite gneiss body is therefore one of the second oldest rocks in South Korea. Zircon grain mantle and rim data yield an upper intercept age of ca. 1.89 Ga and a lower intercept age of 174±74 Ma along the discordia line, which are interpreted to be metamorphic ages related to Paleoproterozoic and Jurassic magmatism, respectively.

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