The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 115, Issue 3
Displaying 1-6 of 6 articles from this issue
Articles
  • Tsuyoshi Sakurai, Akira Takasu
    2009 Volume 115 Issue 3 Pages 101-121
    Published: March 15, 2009
    Released on J-STAGE: August 07, 2009
    JOURNAL FREE ACCESS
    The Sambagawa belt is a typical high-P/T metamorphic belt. Several tectonic blocks of various size, protoliths, and metamorphic histories are scattered in the high-grade portions of the belt in the Besshi district of central Shikoku, Japan. The Gazo mass is a tectonic block located at the northeastern margin of the largest tectonic block, the Iratsu metagabbro mass. The Gazo mass consists mainly of pelitic and basic schists with intercalated layers and lenses of eclogite within layers of basic schist. Typical eclogite in the Gazo mass consists of garnet, clinopyroxene, epidote, phengite, paragonite, and quartz. Four stages of metamorphism are distinguished based on eclogite texture: (1) Pre-peak metamorphism at conditions of the eclogite facies, as indicated by inclusions in garnet and clinopyroxene. The inclusions include clinopyroxene (XJd=0.07-0.35), epidote, phengite, Na-Ca amphibole, quartz, titanite, and rutile. Garnet and omphacite crystals contain resorbed cores and overgrown rims, suggesting the occurrence of a substage of decreasing pressure and temperature or H2O infiltration during prograde metamorphism. (2) Peak metamorphism at conditions of the eclogite facies (12-25 kbar, 510-580°C), with the coexistence of garnet and omphacite (XJd=0.20-0.51). (3) Retrograde metamorphism at conditions of the epidote-amphibolite facies, producing symplectitic aggregates of clinopyroxene (XJd=0.01-0.11) + albite ± Na-Ca and Ca amphibole after omphacite. (4) Sambagawa prograde metamorphism at conditions of the epidote-amphibolite facies, producing porphyroblastic amphibole after omphacite of the eclogitic stage, with similar chemical compositions and zoning patterns to those of amphibole in the Sambagawa basic schists.
    We propose that the Gazo mass became united with the Iratsu mass during exhumation of the latter, which had undergone high-P/T metamorphism at much deeper levels in the subduction zone. The sutured Gazo-Iratsu mass was then emplaced into the Sambagawa schists, followed by prograde metamorphism of the entire rock mass.
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  • Yasuyuki Tsujino
    2009 Volume 115 Issue 3 Pages 122-129
    Published: March 15, 2009
    Released on J-STAGE: August 07, 2009
    JOURNAL FREE ACCESS
    The uppermost part of the Cretaceous Yezo Group exposed in the Kotanbetsu area of Hokkaido is defined as the Hakobuchi Formation. The formation consists of alternating beds of fine- to medium-grained sandstone with hummocky cross-stratification and thin mudstone, and conformably overlies muddy deposits of the Haborogawa Formation of the Yezo Group. The present investigation found several ammonoids and inoceramids within the formation, but no reliable age-diagnostic mega-fossils. In the Kotanbetsu and Haboro areas, however, Sphenoceramus orientalis and S. schmidti, which are indicative of the middle-upper part of the lower Campanian, have been reported from the uppermost Haborogawa Formation. Therefore, the biostratigraphic position of the Hakobuchi Formation in the Kotanbetsu area is thought to correspond to the lowest part of the S. orientalis-S. schmidti Zone. Based on the distribution of the Hakobuchi Formation in the Kotanbetsu area and the coarsening-upward sequence in the uppermost part of the Haborogawa Formation of the Haboro and Tappu areas, it is possible that the Hakobuchi Formation was deposited over a wide part of the Haboro-Kotanbetsu-Tappu area.
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  • Kazunobu Terabe, Atsushi Matsuoka
    2009 Volume 115 Issue 3 Pages 130-140
    Published: March 15, 2009
    Released on J-STAGE: August 07, 2009
    JOURNAL FREE ACCESS
    A total of 25 species belonging to 21 genera of marine bivalve fossils were obtained from the Sebayashi Formation of the Sanchu Cretaceous System in the Chichibu Composite Belt, Sebayashi area, Gunma Prefecture, Kanto Mountains. The bivalve fauna includes Neithea matsumotoi and Nemocardium yatsushiroense indicative of the Tethyan fauna typically found in the Albian Yatsushiro Formation of the Pre-Sotoizumi Group in west Kyushu. The same locality yielded an ammonoid fauna of Barremites otsukai, Pseudohaploceras japonicum and Crioceratites (Paracrioceras) asiaticum. These co-occuring ammonoids suggest that Neithea matsumotoi, Nemocardium yatsushiroense and Yabea akatsui range down into the Barremian. The Tethyan bivalve fauna in the Sebayashi Formation cannot be explained soley by sinistral translation along the Kurosegawa Tectonic Zone. The pattem of bivalve occurrence suggests that the Sebayashi Formation was deposited in an environment storongly affected by increasing seawater temperature.
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Note
  • Tohru Danhara, Hideki Iwano
    2009 Volume 115 Issue 3 Pages 141-145
    Published: March 15, 2009
    Released on J-STAGE: August 07, 2009
    JOURNAL FREE ACCESS
    We recently determined new zeta values of zircon and apatite for fission-track dating using diallyl phthalate (DAP) detectors. Thermal neutron irradiation was performed using the pneumatic tube No.2 at the JRR-3 reactor of Japan Atomic Energy Agency (JAEA), for which the cadmium ratio for Au is 24. The resulting zeta values are 416 ± 3 (ED1) and 371 ± 3 (ED2) for zircon with Q-DAP detectors by an observer (TD), 414 ± 3 (ED1) and 391 ± 4 (ED2) for zircon with Q-DAP detectors, and 337 ± 4 (ED1) for apatite with S-DAP detectors by a second observer (HI). These values are 6-11 % higher than those obtained for the pneumatic tube at the JRR-4 reactor of JAEA, with a cadmium ratio of 3.6. The significant difference in zeta values between the two irradiation sites can be partly attributed to the effect of the fast neutron-induced fissions of 232Th and 238U in SRM612 glass at JRR-4, with its relatively poor thermalisation.
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