The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 127 , Issue 5
Showing 1-4 articles out of 4 articles from the selected issue
Article
  • Mitsuhiro Aikawa, Masataka Nakata, Kosei Komuro
    2021 Volume 127 Issue 5 Pages 253-267
    Published: May 15, 2021
    Released: September 29, 2021
    JOURNAL FREE ACCESS

    Surface precipitates are found on the surfaces of tuffaceous rocks near Susobana dam, Nagano City, Nagano Prefecture, Japan. These precipitates were identified, using X-ray powder diffraction with energy dispersive X-ray spectroscopic analysis, as alunogen, gypsum, epsomite, hexahydrite,pickeringite, and tamarugite. Thermodynamic phase diagrams suggest that epsomite and gypsum precipitate under low to high pH conditions, whereas alunogen and pickeringite form under strongly acidic conditions. Model calculations from evaporitic reactions of sulfate-bearing solutions, under equilibrium with wallrock minerals, demonstrates that sulfuric acid formed during oxidation is neutralized by calcium, magnesium, and aluminium in the wallrock minerals. This process leads to the formation of gypsum, epsomite and jurbanite as neutralization products, at pH conditions of 11.2, 7.0, and 3.5, respectively. It is also noted that neither gypsum nor epsomite necessarily form under acidic conditions, and that alunogen does not form under equilibrium with wallrock minerals. Model calculations for evaporitic reactions of sulfate-bearing solutions, under no equilibrium reaction with wallrock Al-bearing phases and precipitated minerals, indicate that the solutions formed are strongly acidified with no formation of alunogen. Model calculations for evaporitic reactions of sulfate-bearing solutions with SO4/Al ratios between 1 and 5, under equilibrium with precipitated minerals but also under no equilibrium reaction with wallrock minerals, suggest that alunogen would form with SO4/Al ratios between 1.5 and 4.7 and an evaporitic enrichment factor of over 1010. The sulfate mineral species of surface precipitates are hence inferred to be related to the leaching of wallrock minerals. For example, gypsum and epsomite form during alkali-earth leaching, pickeringite at the end-stage of alkali-earth leaching, and alunogen when alkali and alkali-earth elements are depleted.

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  • Critical review on the Horoman peridotite complex and new proposals
    Norikatsu Akizawa, Kazuhito Ozawa, Yoshikawa Masako
    2021 Volume 127 Issue 5 Pages 269-291
    Published: May 15, 2021
    Released: September 29, 2021
    JOURNAL FREE ACCESS

    Mantle peridotites exposed on the Earth's surface are “fossilized” mantle materials. Records of pressure-temperature-deformation histories (P-T-d-t paths) preserved in mantle peridotites reveal dynamic motion within the mantle with association of deformation, changes in pressure and temperature, and magma-related processes. Reconstruction of such P-T-d-t paths for mantle peridotites backward from the moment of “fossilization” through exhumation as far back in time as possible, will expand the current understanding of the long-term dynamic behavior of the mantle. The Horoman peridotite complex in the southern Hidaka metamorphic belt of Hokkaido, Japan, has long been examined using various approaches to decode past P-T-d-t paths recorded in the peridotite and mafic rocks. Here we critically review previous work related to reconstructions of P-T-d-t paths for the Horoman peridotite complex, and use the data to outline the dynamic history of the complex from the oldest events until their final ascent leading to emplacement within the crust.

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Letter
  • Kenta Kawaguchi, Kosuke Kimura, Yasutaka Hayasaka, Kenichi Hoshino, Ik ...
    2021 Volume 127 Issue 5 Pages 293-304
    Published: May 15, 2021
    Released: September 29, 2021
    JOURNAL FREE ACCESS
    Supplementary material

    The Suo Metamorphic Complex is characterized as subduction-related Triassic high P/T type metamorphic rocks occurring in southwest Japan. Recently, we have discovered tonalite mylonite (“Otabara tonalite”) from the Tsuno Group, Suo Metamorphic Complex in the western Chugoku province. LA-ICP-MS zircon U-Pb dating of the “Otabara tonalite” in its type locality yields the magmatic age of 285.4±1.9 Ma (MSWD = 0.72) and 278.8±2.0 Ma (MSWD = 0.92). Whole-rock chemical compositions from the “Otabara tonalite” show a volcanic arc signature. LA-ICP-MS detrital zircon U-Pb dating of psammitic schist from the Tsuno Group yields the approximated maximum depositional age of 242 Ma with the single peak cluster at ca. 262 Ma and the conspicuous lack of Precambrian zircons. The depositional age of psammitic schist is younger than the crystallization age of the “Otabara tonalite.” Possible origins of the “Otabara tonalite” are considered as 1) an autochthonous body and a member of the Suo Metamorphic Complex, or 2) an exotic block derived from another geotectonic unit. In the case of the possibility 2), the arc-type granitoid in the Southern Maizuru Belt is the potential origin of the “Otabara tonalite” as they share similar ages and geochemical compositions.

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Report
  • Koji Uno, Kuniyuki Furukawa
    2021 Volume 127 Issue 5 Pages 305-311
    Published: May 15, 2021
    Released: September 29, 2021
    JOURNAL FREE ACCESS

    In order to illustrate vertical and horizontal variations in the anisotropy of rock fabrics, particularly in highly viscous silicic lavas, we used anisotropy of magnetic susceptibility (AMS) measurements to investigate a thick rhyolitic lava flow with clearly marked flow structures. We describe the relationships between the geometry of the flow structures and the evolution of magnetic fabrics. Material was taken from three drill cores, located at different distances from the eruptive source. The dips of the AMS foliation planes agree with structural measurements taken throughout the section and with increasing distance from the source. This evolution corresponds to the transition flow structure geometries from stubby to lenticular and further to tabular. All of the flow structure geometries present oblate AMS fabrics, which is a characteristic deformation feature of rhyolitic lava flows.

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