JOURNAL OF MINERALOGY, PETROLOGY AND ECONOMIC GEOLOGY
Online ISSN : 1881-3275
Print ISSN : 0914-9783
ISSN-L : 0914-9783
Volume 94, Issue 6
June
Displaying 1-3 of 3 articles from this issue
ORIGINAL ARTICLES
  • Tsukasa OHBA, Koji UMEDA
    1999 Volume 94 Issue 6 Pages 187-202
    Published: 1999
    Released on J-STAGE: August 07, 2006
    JOURNAL FREE ACCESS
    Northern part of Hachimantai volcanic field, northeastern Japan, comprises four stratovolcanoes, Hachimantai, Chausudake, Nishimoriyama, and Maemoriyama volcanoes, and many flank volcanoes, Appidake, Fukenoyu, Gentamori, Ebisu-Daikoku-mori, and an eroded volcano. K-Ar dating and geological survey revealed that Hachimantai and Chausudake volcanoes erupted about during 0.7-1 Ma, Ebisu-Daikoku-mori volcano erupted about 0.4 Ma, and ages of the products of Maemoriyama volcano are younger than 0.4 Ma. Although compositional range of the rocks from whole of the field is wide, chemical variation of rocks from individual volcano is relatively small. There is little correlation of chemical compositions with age, oppose to distinct spatial variation in the volcanic field, that basaltic lavas distribute in the eastern and northern part, and andesite lavas in the southern and western part. Spatial 87Sr/86Sr variation, that decreases from east to west, is corresponded with the trend of whole of the arc. The magmas were derived from lithosphere which heterogeneity was resulted in the spatial 87Sr/86Sr variation.
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  • Yasuko OKUYAMA-KUSUNOSE
    1999 Volume 94 Issue 6 Pages 203-221
    Published: 1999
    Released on J-STAGE: August 07, 2006
    JOURNAL FREE ACCESS
    The Tanohata plutonic complex, northern Kitakami Massif, Northeast Japan, is an early Cretaceous zoned pluton mainly composed of hornblende-biotite granodiorite. The complex is in a wall contact relation in its western margin and in a roof contact relation in the northern part of the complex, with surrounding sedimentary rocks of the Jurassic accretionary complex. The width of thermal aureole is about 5-7 km in the “western marginal area”. Sedimentary rocks in the “northern roof pendant area”, with an extension about 10×8 km, are thoroughly metamorphosed extensively. The aureole is divided into four progressive mineral zones, namely, the biotite, cordierite, cordierite-K-feldspar and garnet-cordierite zones, based on the paragenetic changes of pelitic rocks in the western marginal area. In the northern roof pendant area, only the cordierite-K-feldspar and garnet-cordierite zones are recognized, and the width of each zone is several times larger than that in the western marginal area. The distinct development of high-temperature zones in the northern roof pendant area indicates a selective upward heat transfer during cooling of a granodioritic magma chamber of the Tanohata complex.
         Conditions of metamorphism are estimated from the biotite-muscovite-cordierite-sillimanite-K-feldspar equilibrium at the sillimanite isograd, the Fe-cordierite-almandine-sillimanite equilibrium in the garnet-cordierite zone and the garnet-biotite geothermometry. The estimated temperatures are at about 560°C at the sillimanite isograd, and in a range of 650-750°C in the garnet-cordierite zone, at pressures of 2-3 kbar. The maximum temperatures of the Tanohata aureole are similar to those of the highest temperature zone of the Tono contact metamorphic aureole in the southern Kitakami Massif. The result implies that the thermal properties of magma and the surrounding wall rocks are quite similar between the Tanohata and Tono plutons. The estimated pressures of contact metamorphism are also quite similar between Tanohata and Tono aureoles, indicating that these two granodioritic bodies both emplaced and solidified in a deep part of upper crust (7-11 km) during early Cretaceous magmatism in the Kitakami Massif.
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  • WEE Soo-Meen
    1999 Volume 94 Issue 6 Pages 222-240
    Published: 1999
    Released on J-STAGE: August 07, 2006
    JOURNAL FREE ACCESS
    Major, trace element concentrations and Sr-Nd-Pb isotope systematics are reported for Jeungok basalts from the Choogaryong rift valley in mid-Korean peninsula. The lavas formed narrow and long basalt-plateau showing the layers of 10 to 20 m thick and about 95 km in length along the old river path. These lavas are olivine alkali basalts which formed in response to the subduction of the western Pacific plate beneath the northeastern part of the Eurasian plate margin. The Jeungok basalts are enriched in highly incompatible elements, and are enriched relative to bulk silicate earth (BSE) based on Sr, Nd and Pb isotope covariation diagrams. The basaltic rocks have very small variations in 87Sr/86Sr (0.7047-0.7056) and 143Nd/144Nd (0.51259-0.51265) ratios. The 207Pb/204Pb vs 206Pb/204Pb and 208Pb/204Pb vs 206Pb/204Pb plots show linear arrays parallel to the NHRL, and 208Pb/204Pb data points are displace considerably above NHRL.
         Trace elements and isotopic signatures indicate that the chemical composition of the rocks have not been modified by crustal contamination, and chemical variation of the rocks are mainly caused by fractional crystallization of olivine, plagioclase, clinopyroxene and magnetite. Trace element signatures such as Rb/Sr, Sm/Nd, Ba/Nb and La/Nb ratios are similar to OIB. These characteristics in conjunction with isotopic data suggest that these rocks were derived from enriched magma source result from mixing of the asthenospheric magma with lithospheric materials, and do not show any subduction related characteristics.
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