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 113, Issue 4
Displaying 1-3 of 3 articles from this issue
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Articles
  • Provenance of detrital garnets from the Yukawa Formation, Yanase district, Shimanto belt, Kii Peninsula, Southwest Japan
    Kyaw Soe Win, Makoto Takeuchi, Seiichiro Iwakiri, Tetsuya Tokiwa
    2007 Volume 113 Issue 4 Pages 133-145
    Published: 2007
    Released on J-STAGE: December 22, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.133
    JOURNAL FREE ACCESS
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    Detrital garnet provenance analysis was carried out on sandstones of the Cretaceous Yukawa Formation in the Shimanto belt of central Kii Peninsula, Southwest Japan. The Yukawa Formation is an upper continental slope deposit of late Albian to Turonian age. The sandstones of the Yukawa Formation in central Kii Peninsula are feldspathic-lithic sandstones. Spessartine-almandine garnets containing some pyrope and grossular contents and originating from low to intermediate grade metamorphic rocks and felsic igneous rocks are the main components of the Yukawa assemblage in the study area. Pyrope-rich almandine garnet derived from granulite-facies metamorphic rocks originating from psammitic or pelitic rocks also occur, along with grandite garnet derived from metamorphosed calcareous rocks, and minor amounts of high P/T garnet sourced from mafic gneiss and garnet-amphibolite.
    The source area for garnets derived from granitoids and low to intermediate grade metamorphic rocks, thermally metamorphosed noncalcareous and calcareous rocks is considered to be the Higo belt that may be a constituent of the Paleo-Ryoke belt. The source of granulite-facies metamorphic garnets was probably Precambrian continental basement in East Asia, possibly the Korean Peninsula or North China. Eclogite-origin and high P/T garnets may have been derived from the Kurosegawa belt.
    The presence or absence of eclogite-origin garnets, appearance and disappearance of grandite garnets and their different concentrations in coeval deposits indicate the detritus supplied to the Yukawa depocenter was heterogenous, due to varying proportions of lithotypes in the individual source regions.
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  • Incipient slaty cleavage in the Taisyu Group of north Tsushima Islands, southwest Japan
    Yukimasa Oho, Yuya Yamaguchi, Yasuyuki Hirayama
    2007 Volume 113 Issue 4 Pages 146-157
    Published: 2007
    Released on J-STAGE: December 22, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.146
    JOURNAL FREE ACCESS
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    Slaty cleavage and chlorite-mica stacks in the Tertiary Taisyu Group, Tsushima Island, northwest of Kyusyu, have been studied by XRD, SEM and petrographic techniques. The intensity of cleavage development is classified into 3 classes; 0-, I- and II-classes. In the 0-class, cleavage can not be observed. The I- class is characterized by non-penetrative cleavage domains and pencil structure, and II- class by penetrative cleavage domains. Chlorite-mica stacks have generally tabular form with a long axis nearly parallel to bedding plane in the 0-class and show deformational features such as folding and kinking especially in the II- class. The thickness of the stacks increases from the 0-class to II- class: 9.4 μm in the 0-class; 12.7 μm in the I- class; 21.5 μm in the II- class. These data indicate that the shape change of the stacks accomplished by the chlorite and mica intergrowth during strain-controlled deformation and formation of slaty cleavage. Illite crystallinity values obtained from the typical samples of each class is 0.39-0.43Δ° 2θ in the 0-class; 0.36-0.40Δ° 2θ in the I- class; 0.35Δ° 2θ in the II- class, corresponding to diagenetic zone to lower anchizone.
    Considering about the extensional setting of the Japan Sea, temperature of lower anchizone and thickness of the Taisyu Group, it is inferred that the geothermal gradient was relatively higher than the moderate one at the formation of slaty cleavage.
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  • The parentage of low-grade metasediments in the Sanbagawa Metamorphic Belt, southern area of Mima City, Tokushima Prefecture, Japan
    Kazuo Kiminami, Yuki Toda
    2007 Volume 113 Issue 4 Pages 158-167
    Published: 2007
    Released on J-STAGE: December 22, 2007
    DOIhttps://doi.org/10.5575/geosoc.113.158
    JOURNAL FREE ACCESS
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    Tectonostratigraphic relationships and paleontological evidence suggest a post-Middle Jurassic depositional age for the protoliths of the Sanbagawa psammitic/pelitic schists. In order to identify the parentage of low-grade metasediments in the Sanbagawa Metamorphic Belt in the southern area of Mima City, Tokushima Prefecure, we have studied the bulk chemistry of psammitic and pelitic schists in the Sanbagawa Belt. The Sanbagawa Belt in the study area can be divided into northern and southern parts on the basis of lithology and structure. Some geochemical signatures of psammitic schists differ from those of the Middle/Late Jurassic sandstones in the Southern Chichibu Belt. The geochemistry of the psammitic and pelitic schists suggests that the parents of the northern and southern parts are the KS-II Unit (Coniacian-Campanian) and KS-I Unit (Albian?-Early Coniacian) of the Northern Shimanto Belt, respectively. These conclusions imply that most of the low-grade metasediments of the Sanbagawa Belt in Shikoku are an underplated, deeper facies of the Cretaceous Shimanto accretionary complex.
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