The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 106, Issue 4
Displaying 1-6 of 6 articles from this issue
  • Yukiyasu Fujii
    2000 Volume 106 Issue 4 Pages 249-263
    Published: April 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Macroscopic fractures developed in the Late Cretaceous Toki Granite in the Tono district, central Japan, were analyzed in the field and under the microscope. The fractures are characterized by their morphology (open or closed, brittle and/or plastic deformation) and filled with several kinds of materials. The fracture-filling materials are authigenic minerals (iron-oxide, sericite, quartz, chlorite and epidote) and/or pulverized grains derived from the host granite (quartz, feldspar, biotite, etc.). Sericite, quartz, chlorite and epidote are hydrothermal minerals related to the activity of granitic magma. Iron-oxide, however, is considered to have been formed in a later stage of fracture filling on the basis of the occurrence.All the fractures are primarily formed as open and/or shear fractures by brittle failure, although some of them also show microstructures caused by plastic deformation, such as dynamically recrystallized quartz. This type of fractures are interpreted to have originated from brittle fractures which were subsequently hydrolytically weakened in the presence of hydrothermal solution and deformed plastically.
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  • Hidetoshi Hara, Katsumi Kimura
    2000 Volume 106 Issue 4 Pages 264-279
    Published: April 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The errors of the illite crystallinity (IC) and its factors were examined on samples from argillaceous rocks of the Urayama Unit of the Jurassic Chichibu accretionary complex in the Kanto Mountains, central Japan.All sedimented slides for X-ray diffractometer (XRD) were controlled in the thickness of clay film on slides and treated by the ethylene glycol solvation. Combined errors of XRD machine and intra-sample were estimated to be about 1.5% (coefficient of variation) together. The IC values of black shales have the intra-outcrop errors of 4.6%, and also the intra-area (2×2.5 km2) variations within 9%. The intra-area variations decrease up to approximately 6% except for samples near major faults bounding tectonostratigraphic units. The IC values of melange-type rock show the intra-outcrop error of 11%, which are much larger than those of black shale. Quantitative analyses of lithological effects for the IC values indicate that the IC values tend to decrease by increase of detrital muscovite and sandstone fragments, and vary with distribution of grain size (<2μm). These data show that both effects of faults and lithological characters of melange-type rock cause to increase the IC values errors.The above mentioned results suggest that in case of the IC analysis for regional thermal maturity of the accretionary complexes, we should avoid sampling rocks near faults and we need to estimate the intra-outcrop errors of the IC values for melange-type rock and black shale.
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  • Masayuki Fukase, Kenji Shuto
    2000 Volume 106 Issue 4 Pages 280-298
    Published: April 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Oligocene volcanic rocks from the middle part of the Monzen Formation in the Oga Peninsula, northeastern Japan consist of lava flows and pyroclastic rocks. These volcanics are composed of basalts and andesites which can be divided into four groups based on differences in their TiO2 contents and petrographic characteristics ; high TiO2 basalt group (HTG), low TiO2 basalt and andesite group (LTG), quartz bearing medium TiO2 andesite group (MTG) and porphyritic andesite group (PTG).Olivine basalts of the HTG have geochemical characteristics similar to those of basaltic rocks from continental rift zone, whereas other basalts of the HTG and LTG have characteristics of both continental rift zone basalts and of island arc basalts. These compositional differences of basaltic rocks can be attributed mainly to differences in the geochemical characteristics of their source mantle. The variations in HFS (high field strength) element contents among olivine basalts in the HTG may have resulted from either differences in degree of partial melting of source mantle or differences in degree of fractional crystallization mainly of clinopyroxene from the primary basaltic magma.Andesitic rocks from the study area are characterized by the wide variations in Zr/Nb ratio and initial 143Nd/144Nd ratio (Nd I value), suggesting that the production of these andesitic rocks were affected by assimilation of crustal materials with low Zr/Nb ratio and low 143Nd/144Nd ratio. The accidental materials such as granitic fragments, quartz crystals having corroded form and phenocrystic plagioclases having core part with very low An% are contained in the MTG and LTG andesites. These features indicate that granitic rocks probably constituting wall rocks around the magma chamber are the candidate for the assimilants.
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  • Kazutaka Amano, Masanori Suzuki, Tokiyuki Sato
    2000 Volume 106 Issue 4 Pages 299-306_1
    Published: April 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Molluscan fauna of the Tentokuji Formation in Akita Prefecture was reexamined for elucidating the horizons of the Pliocene warm-water influx into Japan Sea. The Tentokuji Formation is lithologically subdivided into the lower, middle and upper parts. This formation can be assigned to the lower to middle part of NN 16 nannofossil zone (middle Pliocene). The fauna includes many characteristic species of the Plio-Pleistocene Omma-Manganji fauna and two relict species of the Miocene Shiobara-type fauna. Besides many cold-water species, 15 warm-water ones were newly found from the basal and middle parts of the formation. Shallow warm-water current over the cold-water mass is the most plausible factor explaining the characteristic occurrences of the warm-water species from the basal and middle parts of the Tentokuji Formation.
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  • Toru Takeshita, Hidemi Tanaka, Tetsumaru Itaya
    2000 Volume 106 Issue 4 Pages 308-311
    Published: April 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The 'Ishizuchiyama Tertiary System' consists of the Ishizuchi and Kuma Groups. The former Group lies on the latter one with clino-unconformity. We determined K-Ar biotite ages for two acidic tuff beds from the Kuma Group and the lower formation of the Ishizuchi Group, respectively. The tuffs from the Kuma Group give 16.9 and 15.5 Ma in age, and those from the Ishizuchi Group, 15.1 and 14.9 Ma. This small range of ages limits the timing of deformation associated with the formation of the E-W-trending Tobe thrust in the Kuma Group. The N-S-trending compression tectonics in western Shikoku at ca 15 Ma can be ascribed to an inversion tectonics from the N-S-trending extension tectonics associated with the opening of the Japan Sea, and prevailed in the whole southwest Japan.
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  • Yoshihisa Kawanabe
    2000 Volume 106 Issue 4 Pages VII-VIII
    Published: 2000
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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