The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 112, Issue 6
Displaying 1-6 of 6 articles from this issue
Articles
  • Masakazu Niwa
    2006 Volume 112 Issue 6 Pages 371-389
    Published: 2006
    Released on J-STAGE: October 12, 2006
    JOURNAL FREE ACCESS
    The Kohachigagawa Complex is a Middle Jurassic accretionary complex in the Mino Belt in the Takayama area, Gifu Prefecture, central Japan. It is characterized by large amounts of oceanic rocks such as bedded chert, limestone and mafic volcanic rocks. The following two zones are identified in the Kohachigagawa Complex from lithology, mode of occurrence and deformation structure. Zone 1 is composed of slabs of oceanic rocks derived from Permian seamounts that are affected by intense cataclasis along a margin of each slab. Zone 2 is an imbricate stack of thrust sheets derived from component rocks of an oceanic plate stratigraphy including Permian to Early Jurassic ocean floor rocks. Toishi-type siliceous claystone and mafic volcanic rocks constitute the major or part of the decollements of the thrust sheets. Shear direction of the thrusts is top-to-the-east when a foliation trends east-northeast to northeast and dips steeply north, which is different from the shear direction of melange fabric and thrusts in the Late Jurassic to Early Cretaceous accretionary complex of the Mino Belt.
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  • Yoshiaki Sugamori
    2006 Volume 112 Issue 6 Pages 390-406
    Published: 2006
    Released on J-STAGE: October 12, 2006
    JOURNAL FREE ACCESS
    The Paleozoic and Mesozoic in the Kyoto Nishiyama area are divided into the Takatsuki Formation (redefined), the Shimamoto Formation (newly defined) and the Tamba Terrane. Late Permian radiolarians are extracted from felsic tuff, siliceous mudstone and mudstone of the Takatsuki Formation, which mainly consists of clastic rocks and tectonically overlies the Tamba Terrane. The Shimamoto Formation is composed of clastic sediments and one greenstone block. Middle Triassic radiolarians are extracted from mudstone of this formation. The Tamba Terrane is subdivided into the Honzanji Complex (redefined), the Izuriha Complex (redefined) and the Tano Complex in tectonically descending order. Early Late Triassic, late Late Triassic and Early Jurassic radiolarians are reported from mudstone of the Honzanji, Izuriha and Tano complexes, respectively. These sedimentary complexes are thought to be accretionary complex from their lithofacies and geologic structure. This indicates that the Tamba Terrane continuously was formed from early Late Triassic.
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  • Tatsuki Tsujimori, Keisaku Matsumoto
    2006 Volume 112 Issue 6 Pages 407-414
    Published: 2006
    Released on J-STAGE: October 12, 2006
    JOURNAL FREE ACCESS
    Glaucophane-epidote eclogite occurs in the eclogitic unit of the Omi serpentinite mélange, SW Japan. The eclogite has a peak mineral assemblage of garnet+omphacite+glaucophane+epidote+quartz+rutile ±phengite. A P-T pseudosection for a bulk composition appropriate to the eclogite has been calculated using the THERMOCALC 2.75 program in the model system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH) with quartz and water in excess. In the calculated phase diagram, a trivariant field of garnet+omphacite+glaucophane+zoisite occurs within the high-pressure divariant field of garnet+omphacite+glaucophane+hornblende (sensu lato)+zoisite - it is consistent with the previous P-T estimation of peak metamorphism at T=550-600ºC and P>1.8 GPa. The P-T pseudosection and average-P calculations using THERMOCALC reveal a significant pressure gap between two contrasting high-grade rocks in the eclogitic unit and the non-eclogitic unit. The glaucophane-epidote eclogite may have experienced higher pressure relative to the non-eclogitic paragonite-garnet amphibolite characterized by a trivariant field of garnet+paragonite+hornblende(sensu lato)+zoisite. Considering inferred thermal structure of present-day subduction zones, glaucophane-epidote eclogite may record subduction metamorphism of young and warm slab.
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  • Minoru Utada
    2006 Volume 112 Issue 6 Pages 415-429
    Published: 2006
    Released on J-STAGE: October 12, 2006
    JOURNAL FREE ACCESS
    Seven cauldrons of Paleogene age occur in the San'in district, western Honshu, Japan. They are mainly constituted by andesitic and felsic volcanic and volcaniclastic rocks. Alteration of these rocks was investigated by means of X-ray diffractometry using Cu-Kα radiation. They are classified into 4 types of alteration: caldera-type intermediate alteration, hornfelsic alteration, hydrothermal alteration, and weathering.
    The first type of alteration that is characterized by the presence of chlorite-smectite series is widespread in all cauldrons. It is further classified into 4 zones: weakly alteration, smectite-type, mix-type (smectite and chlorite), and chlorite-type zones. These 4 types of zones are distributed irrespective of stratigraphic order. This feature is different from diagenetic zoning. The second type of alteration (hornfelsic) is characterized by the presence of hornblende or biotite in addition to the assemblage of the chlorite-type zone. It is usually distributed surrounding to intrusive masses of rocks. Both alteration of caldera-type intermediate and hornfelsic may have been formed under heterogeneous distribution of heat, partly relating to intrusive masses. The third type of alteration (hydrothermal) is superimposed upon caldera-type intermediate and hornfelsic alteration. It is classified into 10 zones. Eight zones are characterized by the presence of hydrothermal silicate minerals and other two are characterized by the presence of carbonates and sulfates, respectively. The kind and frequency of above zones are different in each cauldron.
    The magnetic susceptibility of altered rocks in all cauldrons significantly decreases in the course of each alteration. That of hornfelsic altered rocks is the largest in averages, whereas that of the hydrothermally altered rocks is the smallest. That of the caldera-type intermediate altered rocks is intermediate, although there is no clear difference among 4 zones of this type of alteration.
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Note
  • Hiroyoshi Arai, Tohru Ohta
    2006 Volume 112 Issue 6 Pages 430-435
    Published: 2006
    Released on J-STAGE: October 12, 2006
    JOURNAL FREE ACCESS
    The Woronow-Love-Schedl method is a logratio-based statistical assessment for detecting immobile variables in compositional data. The method was designed in order to address the constant-sum constraint. This method is effective, but difficult to apply without special statistical software. A computer program to process the method for open source statistical environment ‘R’ was developed. This program enables a user to assess the immobile variables in given data set. Care must be taken, however, because the program just gives some statistical information about the candidates for immobile variables. Hence, additional geological evidence is necessary for a definite decision about immobility.
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