Earth Science (Chikyu Kagaku)
Online ISSN : 2189-7212
Print ISSN : 0366-6611
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Volume 75, Issue 4
Displaying 1-6 of 6 articles from this issue
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Research Article
  • Metamorphism of garnet amphibolite and pelitic schist from the Upper Unit of theMakbal Complex, Kyrgyz Northern Tian-Shan
    Adil KASYMBEKOV, Akira TAKASU, Md Fazle KABIR, Shunsuke ENDO, Apas B B ...
    Article type: Research Article
    2021 Volume 75 Issue 4 Pages 271-288
    Published: October 25, 2021
    Released on J-STAGE: December 07, 2021
    DOIhttps://doi.org/10.15080/agcjchikyukagaku.75.4_271
    JOURNAL FREE ACCESS
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    In this study, garnet amphibolite and pelitic schist in the Upper Unit of the high-pressure–ultrahigh-pressure (HP–UHP) Makbal Complex in the Kyrgyz Northern Tian-Shan are petrologically described. Amphibolite and garnet amphibolite occur as lens-shaped bodies or thin layers within the pelitic schist matrix. The garnet amphibolite (KG1252A) consists mainly of amphibole, garnet, epidote, plagioclase, chlorite, and quartz. The peak metamorphic conditions are estimated as T = 575 ± 29 °C and P = 1.4 ± 0.3 GPa. The pelitic schist (KG1252B) collected from the same locality as that of the garnet amphibolite is composed mainly of white mica (comprising a phengite core and a muscovite rim), chlorite, and quartz. The peak metamorphic conditions of the pelitic schist are estimated as T = 500–600 °C and P = 1.4–1.6 GPa, which are approximately the same as those of the garnet amphibolite. After the decompression of these rocks to upper crustal levels, both the garnet amphibolite and pelitic schist underwent low-P and high-T metamorphism probably caused by the contact metamorphism of Ordovician granitic intrusions. There is no significant evidence of omphacite in the garnet amphibolite as a constituent of eclogite facies mafic metamorphic rocks. These suggest that the garnet amphibolites found in the Upper Unit of the Makbal Complex are not exotic blocks of retrogressed eclogite in the pelitic schist matrix. The original rock of the garnet amphibolite is thought to share a common high-pressure metamorphism with the surrounding pelitic schist.

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  • Suwa Syntaxis and its formative age
    Tomoyoshi KOSAKA, Takao YANO
    Article type: Research Article
    2021 Volume 75 Issue 4 Pages 289-304
    Published: October 25, 2021
    Released on J-STAGE: December 07, 2021
    DOIhttps://doi.org/10.15080/agcjchikyukagaku.75.4_289
    JOURNAL FREE ACCESS
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    The upper Cenozoic system of North Fossa Magna covers the Suwa Syntaxis of basement rocks in central Honshu, with a stratigraphic and structural unconformity. This fact indicates that the syntaxis has been formed and regionally eroded before the initial deposition of the upper Cenozoic system at ca. 17 Ma. The Suwa Syntaxis is a large-scale ductile structure in order of 104 km2, so that the syntaxis has been formed through a long-term deformation in a condition of increased confining-pressure and temperature and of low strain-rate. The ductile deformation hence seems to have been occurred probably during a long period in the Yanshan-Hiroshima Movement (Jurassic-Paleogene), which movement is characterized by huge-scale tectono-magmatism on the East Asian continental margin. ract

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