Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
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Volume 77, Issue 4
Displaying 1-4 of 4 articles from this issue
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Review
  • Generative process of the Japan Sea -the current state of research and a trial hypothesis-
    Takao YANO
    Article type: Review
    2023 Volume 77 Issue 4 Pages 127-145
    Published: October 25, 2023
    Released on J-STAGE: December 23, 2023
    DOIhttps://doi.org/10.15080/agcjchikyukagaku.77.4_127
    JOURNAL FREE ACCESS
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    The opinions on the generative process of the Japan Sea proposed since 1903 are classified into (1) block tilting, (2) opening and (3) vertical movement, being still controversial. The marginal sea was formed in some period during Paleogene to Miocene, so that its generative process should be recorded in the pre-Cenozoic basements in and around. The basements consist of the parallel-arranged three zones of volcano-plutonic, high-pressure metamorphic and “accretionary”, with no records of large-scale extension due to opening. A trial hypothesis proposed here attributes the Japan Sea genesis to crustal thinning/densification due to the upwelling of a DMM plume closest to upper continental crust.

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Research Article
  • Field occurrence and chemical composition of the melanocratic porphyritic facies of the Yusubaru Granite, northern Kyushu, Southwest Japan
    Masaki YUHARA, Eriko NISHI, Kairi KIYOURA, Atsushi KAMEI, Yoshinobu KA ...
    Article type: Research Article
    2023 Volume 77 Issue 4 Pages 147-163
    Published: October 25, 2023
    Released on J-STAGE: December 23, 2023
    DOIhttps://doi.org/10.15080/agcjchikyukagaku.77.4_147
    JOURNAL FREE ACCESS
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    We conducted geochemical analyses of whole-rock major and trace elements including rare earth elements, and Sr-Nd isotopic compositions for the melanocratic porphyritic facies of the Yusubaru Granite. The facies were found by Yuhara et al. (2022), and were composed of hornblende - biotite granodiorite to granite. The facies are distributed at the boundary of the Aka body of the Asakura Granodiorite and the Yusubaru Granite, and are scattered in the Yusubaru Granite. Mineral textures of the melanocratic porphyritic facies indicate magma mixing. Whole-rock chemical variations and modal compositions of the melanocratic porphyritic facies are between the main facies and Aka body of the Asakura Granodiorite. U-Pb zircon dating separated from the Aka body of the Asakura Granodiorite gives an age of 98.7±0.6 Ma, which is considered to be a timing of the activity of this body, and the same timing of the Yusubaru Granite. The melanocratic porphyritic facies of the Yusubaru Granite was formed by mixing of the Yusubaru Granite magma and granodioritic magma, similar to the Aka body of the Asakura Granodiorite.

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