Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
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Research Article
  • Adil KASYMBEKOV, Akira TAKASU, Md Fazle KABIR, Shunsuke ENDO, Apas B. ...
    Type: research-article
    2020 Volume 74 Issue 2 Pages 47-64
    Published: May 25, 2020
    Released: July 05, 2020
    JOURNALS FREE ACCESS

    Garnet-free pelitic schist (KG1251) and garnet and chloritoid-bearing pelitic schist (KG1244) of the Neldy Formation in the Makbal HP-UHP Metamorphic Complex, Kyrgyz Northern Tien-Shan are petrologically and geochronologically described. The pelitic schist (KG1251) consists mainly of phengite, chlorite and quartz, with small amounts of albite, titanite, calcite, rutile and carbonaceous matter. The peak metamorphic conditions are roughly constrained as T <630 °C and P = 0.9–1.7 GPa. A K-Ar age for schistosity-forming phengite is obtained as 524 ± 13 Ma, and it is almost identical to the previously reported peak metamorphic ages (ca. 500 Ma) of eclogites and garnet-chloritoid-talc schists in the Makbal Complex. The pelitic schist (KG1244) consists mainly of white mica (phengite-core and muscovite-rim), chlorite and quartz, with minor amounts of garnet, chloritoid, albite, tourmaline, zircon, monazite, titanite, rutile, calcite and carbonaceous matter. The peak metamorphic conditions are estimated as T = 485–545 °C and P = 1.2–1.5 GPa (high-P/T metamorphism), followed by a low-P/T metamorphism of T = ca. 500 °C and P >0.3 GPa, which is probably caused by contact metamorphism of granitic intrusions. The K-Ar white mica age of 474 ± 12 Ma obtained by the present study is similar to the ages of the Ordovician granitic intrusions. The peak metamorphic conditions of the Neldy pelitic schists (KG1251 and KG1244) are significantly lower in pressure than those of the previously reported eclogites and garnet-chloritoid-talc schists located in the lower tectono-structural levels of the Makbal Complex.

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  • Noboru HATTORI, Fujio MASUDA, Yu SAITOH, Shiro ISHIDA
    Type: research-article
    2020 Volume 74 Issue 2 Pages 65-76
    Published: May 25, 2020
    Released: July 05, 2020
    JOURNALS FREE ACCESS

    The Pleistocene Katata Formation, Kobiwako Group distributed in the western littoral area of Lake Biwa is constructed by lacustrine delta system. By sedimentary facies analysis of the Katata formation, we identified several depositional environments including delta plane, shore slope, delta front, prodelta, and lake floor with the occurrence of subaqueous sediment-gravity flow deposits, such as hyperpycnites. The succession of the facies indicates that the paleo-lake levels changed asymmetrically: the depositional environment shallowed gradually and deepened rapidly. The asymmetric changes formed thick accommodations and thick deposition of stratum. Notably, every horizon of rapid rise of lake level is accompanied with the deformation structures, such as convolute and slump structures. This suggests that the increase of the lake level was caused by tectonic subsidence of the lake basin. The formative mechanism of the Katata Formation in the Kobiwako Group was firstly reported in this study: repetitions of lake-level rising by seismogenic faulting accumulated a thick sediment succession.

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