The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 122, Issue 6
Displaying 1-5 of 5 articles from this issue
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Article
  • Revised stratigraphy and zircon U-Pb age data of the Hitachi metamorphic formations in the southern Abukuma Mountains, and reconstruction of the basement of the Northeast Japan Arc before the opening of the Japan Sea
    Michio Tagiri, Kenji Horie, Tatsuro Adachi
    2016 Volume 122 Issue 6 Pages 231-247
    Published: June 15, 2016
    Released on J-STAGE: August 01, 2016
    DOIhttps://doi.org/10.5575/geosoc.2016.0012
    JOURNAL FREE ACCESS
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    We revised the stratigraphy of the Hitachi metamorphic district, based on supplementary U-Pb SHRIMP zircon ages and a review of the geochronological data available for this district. Early to late Cambrian formations and intrusives are grouped into the Cambrian Hitachi volcano-plutonic complex, which consists of the Akazawa Formation, the Tamadare Formation, meta-granites and meta-porphyry. Post-Silurian mica gneiss that occurs near the Tamadare Formation is newly defined as the Takizawa Formation, distinct from the Tamadare Formation. The Daioin and Ayukawa formations are early Carboniferous and early Permian in age, respectively. The contact between the Hitachi volcano-plutonic complex and the Daioin Formation is marked by a prolonged hiatus. There are no geochronological data available for the Omika Formation.
    Zircon U-Pb age data of the Nishidohira Formation indicate deposition during the Early Cretaceous. The Nishidohira Formation was intruded by 119 Ma granitoids and was metamorphosed during 118-96 Ma under conditions of the amphibolite facies, kyanite-sillimanite zone. Sedimentation, metamorphism and exhumation, and intrusion of granitic magma in the terrain of the Nishidohira Formation occurred over a short period of time during the Cretaceous. A similar rapid metamorphism and exhumation is also recorded in the Takanuki metamorphic rocks of the Abukuma Belt.
    We reconstructed the basement of the Northeast Japan Arc for the period before the opening of the Japan Sea, assuming that the Median Tectonic Line had a linear trend from the Southwest Japan Arc to the Northeast Japan Arc during the Paleogene. After the reconstruction, the Paleozoic Hitachi Formations and the Abukuma Belt move closer to the Jiamusi and Khanka blocks of Northeast China. This result indicates that the Cambrian Hitachi volcano-plutonic complex was originally marginal part of the North China Block.
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  • Middle and Late Permian radiolarians from chert blocks within conglomerates of the Kamiaso Unit of the Mino terrane in Gifu Prefecture, central Japan
    Tsuyoshi Ito, Yusuke Kitagawa, Atsushi Matsuoka
    2016 Volume 122 Issue 6 Pages 249-259
    Published: June 15, 2016
    Released on J-STAGE: August 01, 2016
    DOIhttps://doi.org/10.5575/geosoc.2016.0013
    JOURNAL FREE ACCESS
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    Middle and Late Permian (Capitanian and Changhsingian) radiolarians were obtained from chert blocks located within conglomerates of the Kamiaso Unit of the Mino terrane along Tsubogawa River, Gifu Prefecture, Central Japan. Previously, the Capitanian and Changhsingian cherts have not been reported from the sequences of the Akiyoshi terrane. Therefore, the collected chert blocks were not derived from the Akiyoshi terrane. The Capitanian and Changhsingian cherts have been recovered from the Permian sequences of Jurassic accretionary complexes in Southwest Japan; however, the characteristics of the Permian radiolarian fauna of the chert blocks are inconsistent with those from previously recovered Permian cherts. This indicates that the chert blocks were not derived from previously known units including Permian sequences. A previous study suggested that Triassic-Jurassic chert clasts within the chert breccias of the Kamiaso Unit were derived from the accreted Kamiaso Unit on the landward trench slope. As the Permian chert has not been identified in the coherent facies of the Kamiaso Unit, the Permian sequences must have been subducted during accretion. Our results suggest that the chert blocks were derived from the pre-accretion Kamiaso Unit located on the outer trench slope.
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  • Latest Jurassic to Cretaceous radiolarians from the Miocene Chichibumachi Formation, Akahira Group, central Japan
    Kumiko Matsui, Katsuo Sashida, Sachiko Agematsu
    2016 Volume 122 Issue 6 Pages 261-266
    Published: June 15, 2016
    Released on J-STAGE: August 01, 2016
    DOIhttps://doi.org/10.5575/geosoc.2016.0016
    JOURNAL FREE ACCESS
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    Latest Jurassic to Cretaceous radiolarians have been identified in coarse-grained sandstones from the Middle Miocene Chichibumachi Formation of the Akahira Group, which occurs in the Chichibu Basin, Saitama Prefecture, central Japan. The following radiolarians were identified: Pseudodictyomitra lanceloti Schaaf, Holocryptocanium barbui Dumitrica, Diacanthocapsa betica O'Dogherty, Thanarla brouweri (Tan), Archaeodictyomitra sp. A, Archaeodictyomitra sp. B, Archaeodictyomitra? sp., Sethocapsa sp. A, Sethocapsa sp. B, Sethocapsa? sp., and Diacanthocapsa? sp. Radiolarian-bearing sandstone is present at ca. 22 m above the level containing Middle Miocene Paleoparadoxia tabatai, which is a marine mammalian fossil. Radiolarians occur within the sandstone as isolated specimens, along with fragments of bivalves, foraminifers, plant fossils, and quartz grains, some of the radiolarian specimens appear to have been reworked from the Ishido Formation or younger formations of the Sanchu Cretaceous System that crops out to the northwest of the Chichibu Basin.
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  • Stratigraphy and radiolarian ages of stratiform manganese deposits from the Chichibu Belt in the Saiki area, eastern Oita Prefecture, Japan
    Yuki Tomimatsu, Tetsuji Onoue, Tatsuo Nozaki
    2016 Volume 122 Issue 6 Pages 267-273
    Published: June 15, 2016
    Released on J-STAGE: August 01, 2016
    DOIhttps://doi.org/10.5575/geosoc.2016.0015
    JOURNAL FREE ACCESS
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    Stratiform manganese deposits are present in Triassic to Jurassic bedded chert sequences of the Chichibu Belt in southwest Japan, which are considered to have accumulated in a mid-ocean basin of the Panthalassa Ocean. To constrain the age and depositional setting of these deposits, we have investigated the field geology, stratigraphy, and age of chert-hosted manganese deposits (Takahira, Takahama, and Kubodomari) from the Chichibu Belt in the Saiki area, Oita Prefecture. The stratiform manganese ores range in thickness from 80 to 150 cm and are intercalated with bedded chert. The age of the deposits is constrained by the presence of radiolarian fossils in the associated bedded chert. The red-bedded chert above the Takahira manganese ores contains Late Triassic (Carnian) radiolarian fossils, including Trialatus longicornutus, Trialatus megacornutus, and Enoplocampe yehae. In contrast, radiolarian fossils from the Takahama and Kubodomari deposits occur in stratiform manganese ores of Early Jurassic age (possibly Toarcian), based on the occurrence of Parvicingula nanoconica with Parahsuum and Trillus species. These results suggest that two manganese ore forming events occurred in the pelagic Panthalassa Ocean during the Late Triassic and Early Jurassic.
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