The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 106, Issue 5
Displaying 1-7 of 7 articles from this issue
  • Nobuyoshi Tomioka, Akira Ishiwatari, Atsushi Tanase, Satoshi Shimizu, ...
    2000 Volume 106 Issue 5 Pages 313-329
    Published: May 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Arashimadake cauldron is a concentrically zoned volcano-plutonic complex (7.5×5 km in size) located in the southeastern margin of the Tertiary Green Tuff region along the Japan Sea coast. The cauldron is composed of a central pluton (quartz monzodiorite), surrounding andesitic and basaltic volcanic rocks, and outer ring dikes (two-pyroxene porphyrite), and is inset in the basement rocks such as Mesozoic sedimentary rocks, Mesozoic granite and Paleozoic gneiss. The volcanic rocks are composed of alternating pyroclastic rocks and lavas intercalated with minor clastic sedimentary rocks, and dip toward the rim of the cauldron. Dikes of plagioclase porphyrite and dolerite are abundant in the basement rocks surrounding the cauldron, in which those dikes are rare. The dike swarm may be feeders of the stratovolcano, which existed prior to the cauldron subsidence.K-Ar whole rock ages and geologic relationship indicate following history. The feeder dikes and stratovolcano formed around 20 Ma. Its central part subsided along a circular fracture, along which the ring dikes intruded. Resurgent dome uplift may have taken place in the subsided volcanic block in relation to the subsequent intrusion of the central pluton at about 18 Ma. The resurgent doming is characteristic of the Valles-type cauldrons, although this cauldron is smaller and lacks dacitic or rhyolitic welded tuff, which marks the collapse.The volcano-plutonic rocks mostly belong to the medium- and high-K calc-alkaline series. Their initial 87Sr/86Sr ratios (SrI) range from 0.705 to 0.707, which are higher and more variable than that of the Tertiary tholeiitic basalt in the Japan Sea and Noto Peninsula (<=0.704). The correlation between SrI and Sr content suggests mixing of the alkali basaltic and tholeiitic magmas of mantle origin as well as assimilation of high 87Sr/86Sr, low-SiO2 crustal material such as pelitic gneiss.
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  • Satoshi Okamura, Haruhiko Sekine, Kazuo Arai, Masatsugu Yamamoto, Hiro ...
    2000 Volume 106 Issue 5 Pages 330-346
    Published: May 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    K-Ar ages and chemical compositions of the Cenozoic volcanic rocks from Hamamasu area at western margin of central Hokkaido have been determined with the purpose of obtaining constraints on the tectono-magmatic evolution at a junction between the Kuril and Northeast Honshu arcs. The periods of eruption from this area is distinguished into the following four stages of island arc volcanism : (1) the early Miocene (19 Ma) ; (2) middle to late Miocene (14∼7 Ma) ; (3) late Miocene (7∼6 Ma) ; and (4) Pliocene (4∼2 Ma). The early to late Miocene (19-7 Ma) volcanic rocks have more enriched isotopic ratios (higher 87Sr/86Sr and lower 143Nd/144Nd) and lower Zr/Nb ratio than those of the late Miocene to Pliocene (7-2 Ma) volcanics. The geochemical trends from 19 Ma to 2 Ma in volcanic rocks suggest that the sites of magma generation beneath Hamamasu moved from the lithosphere to the asthenosphere. The early Miocene volcanism along the western margin of central Hokkaido including Hamamasu area is characterized by the bimodal volcanic rocks. We propose that the volcanism may have been resulted from the collision between the Eurasian Plate and the Okhotsk Plate, followed by the dextral strike-slip faulting associated with the formation of the Kuril Basin. The change from an enriched to a depleted source region during late Miocene time is correlated with an inactive interval of the volcanism during 6-4 Ma. The Pliocene volcanism is attributed to the upwelling of asthenosphere in the arc-arc junction.
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  • Akira Ono
    2000 Volume 106 Issue 5 Pages 347-352
    Published: May 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Oharai Formation is studied to elucidate the provenance of clastic materials. Metamorphic rocks and sedimentary rocks are commonly found as pebbles in the Oharai Formation. The lithofacies of these rocks are closely akin to those in the Tsukuba-Yamizo belts. Metamorphic grades of the pebbles range from low to middle parts of the biotite zone of the Ryoke metamorphic belt. The K-Ar age of biotite from a coarsegrained biotite schist is determined to be 99.6 Ma. The K-Ar ages for two granitic pebbles are 64 Ma and 73 Ma. The provenance of the pebbles is considered to be a terrain located at the east or south of the Tsukuba-Yamizo mountains.
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  • [in Japanese], [in Japanese], [in Japanese]
    2000 Volume 106 Issue 5 Pages 353-362
    Published: May 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Eclogitic glaucophane schist occurs as a boulder (about 4 m diameter) among many other garnet glaucophane schist and epidote glaucophane schist of the late Paleozoic Renge metamorphic belt in the Yunotani valley, western Omi area. The eclogitic glaucophane schist forms a mafic layer (1.2 m wide) intercalated within pelitic schist (garnet-paragonite-phengite schist). It consists mainly of glaucophane (modal volume : 37%), garnet (21%), omphacite (19%), epidote (19%), and small amount of quartz, albite, phengite, chlorite, rutile and titanite. The eclogite facies mineral assemblage 'garnet+omphacite+glaucophane+epidote+rutile+quartz' is slightly replaced by secondary chlorite, albite, calcite and rarely actinolite, and form penetrative schistosity (S1). The preceding epidote blueschist facies mineral inclusions 'glaucophane+epidote+titanite+quartz+albite' forming earlier schistosity (S0) are preserved in the rotated garnet core. These petrographic features show progressive transition from the epidote blueschist facies to the eclogite facies with little retrogression. Although the relict eclogite facies minerals have been previously found in some retrogressive Renge metamorphic rocks, the Yunotani eclogitic glaucophane schist first provides evidences for the progressive P-T path leading to the eclogite facies.
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  • Tsukasa Nakano, Yoshito Nakashima, Ko-ichi Nakamura, Susumu Ikeda
    2000 Volume 106 Issue 5 Pages 363-378
    Published: May 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    X-ray computerized tomography (CT) is a technique to reconstruct a CT image showing the spatial distribution of X-ray linear absorption coefficients (LAC's) of the materials in a sample. This technique allows us to observe and to analyze non-destructively, the three-dimensional precise internal structure of rocks. The LAC is a physical property which depends on density, state and chemical composition of the material and X-ray energy used in the measurement. Chemical composition of a material greatly affects the LAC for the photon energy level of the medical X-ray CT scanners. Filtered back-projection (FBP) method and convolution back-projection (CBP) method are applied to the reconstruction of a CT image from the obtained X-ray projection data. A suitable choice of a reconstruction filter in the methods enables us to enhance the target texture in the image. An artifact called beam hardening occurs in the CT image obtained by the CT scanner with polychromatic X-ray. This artifact can be reduced by the use of a suitable pad for the sample and/or by the correction of the image using spectrum data of the X-ray source used in the measurement and bulk density and chemical composition of the sample material. To apply the X-ray CT to advanced observation and quantitative analysis of the internal structure of rock, it is necessary to understand the basic theory and the detailed techniques used in the X-ray CT.
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  • Yasuo Yamamoto, Hiroshi Kurita, Takashi Matsubara
    2000 Volume 106 Issue 5 Pages 379-382
    Published: May 15, 2000
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Geologic age of the "Miocene" Iwaya Formation in Awajishima Island is examined by means of calcareous nannofossils and dinoflagellate cysts. Calcareous nannofossil flora indicates biozone NP 17 to NP 20 or CP 14 b to CP 15 b and a late Middle Eocene to Late Eocene age. The dinoflagellate cyst assemblages also show approximately the same age. These results reveal that the Iwaya Formation is the constituent of the Eocene-Oligocene Kobe Group.Molluscan assemblages resembling those from the Iwaya Formation have been known from the "Setouchi Miocene Series" in the coastal area of the eastern Seto Inland Sea. Molluscan fauna and stratigraphic correlation of the "Miocene" in this area should also be re-examined on the basis of precise geochronologic data.
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  • Aiming Lin, Zhenyu Yang, Zhiming Sun, Tianshui Yang
    2000 Volume 106 Issue 5 Pages IXI-X
    Published: 2000
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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