The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 123, Issue 2
Displaying 1-5 of 5 articles from this issue
Article
  • Akira Miyake, Yukari Igarashi, Takumi Inaishi, Tomoki Taguchi
    2017 Volume 123 Issue 2 Pages 59-72
    Published: February 15, 2017
    Released on J-STAGE: May 15, 2017
    JOURNAL FREE ACCESS

    Schistose staurolite-bearing pelites occur in the Ryoke Metamorphic Belt, Dando-san area, Aichi Prefecture, Japan. These rocks contain andalusite porphyroblasts and coarse-grained elongate mica-quartz aggregates composed of muscovite, biotite, and quartz in a matrix of muscovite, biotite, quartz, and fibrolite. Staurolite occurs as inclusions in andalusite porphyroblasts and in the minerals of the micaquartz aggregates. Detailed microscopy indicates that a staurolite-consuming and andalusite-producing reaction occurred before the deformation causing the schistosity, such that staurolite grains apart from andalusite porphyroblasts have been replaced by mica-quartz aggregates, and those adjacent to the porphyroblasts have been replaced by andalusite. It is possible that the prograde P-T paths of the staurolite-bearing pelites of the Dando-san and Hazu-Hongu-san areas lie on the high-pressure side of those of staurolite-free pelitic rocks with similar metamorphic grades in other areas of the Ryoke Belt.

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  • Kosuke Ishige, Mitsuhiro Nakagawa
    2017 Volume 123 Issue 2 Pages 73-91
    Published: February 15, 2017
    Released on J-STAGE: May 15, 2017
    JOURNAL FREE ACCESS

    The Asahidake subgroup is the youngest subgroup of the Taisetsu volcano group, which has been active since ca. 1 Ma. The subgroup became active after the formation of the Ohachidaira caldera (ca. 34 ka), and erupted from a source on the southwestern flank of the caldera. The subgroup is composed of three volcanic edifices: Kumagatake, Ushiro-Asahidake, and Asahidake in ascending order. Kumagatake is a cinder cone (0.4 km3 dense-rock equivalent, DRE) with three craters, and Ushiro-Asahidake consists of lava flows and a dome (combined volume of 0.5 km3 DRE). The activity of Asahidake comprised two eruptive stages: early and late. During the early stage (until ca. 5 ka), repeated magmatic eruptions formed a stratovolcano, while the late stage (since 2-3 ka) is characterized by phreatic eruptions. Temporal changes in eruptive scale indicate the early stage can be further divided into two sub-stages: E-1 and E-2. The E-1 sub-stage is characterized by relatively large pyroclastic eruptions and lava effusions (7.1 km3 DRE), while the E-2 sub-stage is characterized by a smaller eruptive scale, and pyroclastic materials and lava flows are restricted to near the summit area (1.7 km3 DRE). This later stage started with edifice collapse, forming a horseshoe-shaped crater (Jigokudani crater). Since then, phreatic eruptions have frequently occurred in the crater and fumarole activity has continued until the present day. The Asahidake rocks range from basaltic andesite to dacite, and usually contain mafic inclusions. Silicic andesite was dominant until the E-1 sub-stage, and mafic andesite was commonly erupted during the E-2 sub-stage.

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  • Hirohisa Matsuura, Mamoru Seno
    2017 Volume 123 Issue 2 Pages 93-99
    Published: February 15, 2017
    Released on J-STAGE: May 15, 2017
    JOURNAL FREE ACCESS

    The recently discovered middle Eocene Banda (41.2±1.0 Ma) and Kushinoyama (46.1±1.1 Ma) basalts occur in the southern Okayama district, near the Seto Inland Sea, Chugoku region, southwest Japan. Middle Eocene to Oligocene igneous rocks were known only from a narrow belt along the Japan Sea in the Chugoku region; thus, this discovery shows that Paleogene igneous activity also occurred in the southern parts of the region.

    The Banda Basalt unconformably overlies the sandstones and conglomerates of the Kitagata Formation, placing the deposition of the formation in the middle Eocene or earlier.

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