The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 105, Issue 11
Displaying 1-7 of 7 articles from this issue
  • Joji Yoshimura, Hideo Takagi
    1999 Volume 105 Issue 11 Pages 739-756
    Published: November 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Karasaki mylonite unit crops out as a series of isolated klippes on the Sambagawa Belt, along its northern margin in western Shikoku. It mainly consists of strongly deformed amphibolites and subsidiary of pelitic and quartzo-feldspathic metamorphic rocks. This tectonic unit is distinguished on the basis of field occurrence, difference in metamorphic grade and in shear sence with respect to the underlying Sambagawa metamorphic rocks.The chemical composition of garnet and hornblende suggests that the protolith of the Karasaki mylonite is amphibolite facies metamorphic rocks, that have been devoid of Sambagawa metamorphism. Hornblendes from amphibolitic mylonites yield the K-Ar ages of 104±5 Ma, 119±6 Ma and 122±6 Ma. Accordingly, the Karasaki mylonite is possibly correlative to the Higo, Oshima, Yorii, Nishidohira and Takanuki metamorphic rocks of the latest Early Cretaceous age, which are considered to constitute the Paleo-Ryoke Belt interleaved between the Ryoke and Sambagawa Belts.
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  • Yodai Nakamura, Koji Kameo, Yoshihiro Asahara, Tomowo Ozawa
    1999 Volume 105 Issue 11 Pages 757-770
    Published: November 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Neogene Shimajiri Group is distributed sporadically in the Ryukyu Islands. This study focuses on the Shimajiri Group in Kumejima Island, central Ryukyu, and clarifies its stratigraphy and geologic age on the basis of 1) lithostratigraphy, 2) calcareous nannofossil biostratigraphy, and 3) strontium isotope stratigraphy.The Shimajiri Group in Kumejima Island unconformably overlies the middle Miocene Aradake Formation, and is overlain by the Pleistocene Ryukyu Group. The group is divided into three formations, namely the Maja, the Aka and the Uegusukudake Formations in ascending order, and the first two are redefined in this paper based on the new geologic evidence. The Maja Formation consists mainly of fine-grained sandstone, sandy siltstone and alternating beds of them. The Aka Formation is mainly composed of cross-stratified sandstone, pumiceous sandstone and tuffaceous siltstone, and unconformably overlies the Maja Formation. The Uegusukudake Formation, conformably overlying the Aka Formation, consists of basaltic lava, tuff breccia and andesite. On the basis of calcareous nannofossil biostratigraphy, the Maja and Aka Formations can be assigned to Zone CN 9 and Zone CN 12 b of Okada and Bukry (1980) respectively. Strontium isotope ages of the molluscan fossil specimens obtained from the Maja and Aka Formations revealed that the Maja Formation is assigned to the late Miocene (ca. 7.8-7.2 Ma) and the Aka Formation is assigned to the late Pliocene (ca. 3.2-3.1 Ma). These ages are concordant with the nannofossil biostratigraphy.The upper Miocene Maja Formation yields many molluscan fossils in which the characteristic species of the Kakegawa Fauna, such as Amussiopecten praesignis and Mimachlamys satoi are contained. The molluscan fauna of the Maja Formation is significant in understanding the origin of the Kakegawa Fauna, as the characteristic species of the Plio-Pleistocene Kakegawa Fauna already appeared in the Ryukyu Islands in the late Miocene.
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  • Takashi Akatsuka, Masaaki Obata, Hisayoshi Yokose
    1999 Volume 105 Issue 11 Pages 771-788
    Published: November 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Murotomisaki gabbroic complex is a sill-like, sheeted layered intrusion, located in Cape Muroto, Kochi Prefecture, and is composed of gabbro, olivine gabbro and dolerite. In order to investigate the origin of the layered structure, whole rock chemical analyses and mineral chemical analyses were carried out for a series of constituent rock types. On the basis of mass balance calculations using the analytical data, a quantitative assessment was made for the crystallization and crystal settling processes. In particular, "picrite gabbros" which are situated in the lower and the upper parts of the intrusion, just above and below the chilled margins respectively, were examined in detail. The composition of the melt phase in the initial magma was estimated from the whole rock and mineral chemical analyses of the chilled margins. It was shown that olivine crystals (Fo83) accumulated (20 vol%) to form the lower picrite gabbro with an interstitial melt up to 74 vol%. Observed REE abundance data are consistent with this model. Whereas, accumulation of more Fe-rich olivine and plagioclase are necessary to account for the the upper picrite gabbro, where the volume fraction of the melt was estimated to be 68 vol%.
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  • Toshiro Takahashi, Kenji Shuto
    1999 Volume 105 Issue 11 Pages 789-809
    Published: November 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    The Early Miocene volcanic rocks (the Iwaine Formation) are widely distributed in the Hokuriku Green Tuff Province. The volcanic rocks in the Iwaine Formation in the southern part of Toyama Prefecture can be divided into two groups in the FeO*/MgO-SiO2 diagram ; andesite group I has a tholeiitic trend showing enrichment in FeO*/MgO ratio from mafic to felsic andesite, whereas andesite group II has a calc-alkaline trend on which SiO2 content increases from about 54 to 68 wt.% despite a little variation in FeO*/MgO.Petrographic and chemical data indicate that andesite group I was formed from tholeiitic basalt magma by fractional crystallization of its phenocryst minerals.Andesite group II is composed mainly of calc-alkaline andesite with subordinate amounts of high-magnesian andesite and adakitic andesite. Most phenocrystic plagioclases in calc-alkaline andesite are dusy and show reverse zoning in terms of Ca, suggesting that the andesite was of magma mixing origin. Adakitic andesite is characterized by higher Sr and lower Y contents, and higher Sr/Y ratio than common andesites in island arcs, and is also characterized by higher MgO, Ni and Cr contents compared with typical adakite. The high-magnesian andesite shows lower K2O and Rb contents, and higher Sr/Y ratio than the Setouchi high-magnesian andesite and boninite. Adakitic andesite may result from modification of felsic (adakitic) magma generated by partial melting of the subducted oceanic crust, due to interaction with the overlying mantle peridotite or mixing with mafic magma during its rise to the surface. The high-magnesian andesite seems to have a genetical close relation to adakitic andesite.K2O content of basaltic magmas in the Iwaine Formation tends to increase from the study area (eastern margin of the Hokuriku Green Tuff Province) to the western area. Thus, it is possible to consider that the study area occupied the volcanic front in the Early Miocene Hokuriku Green Tuff Province.
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  • Jun'ichi Ito, Keiichi Shiraki
    1999 Volume 105 Issue 11 Pages 810-813
    Published: November 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
    Picrite basalts containing MgO up to 16 wt.% were found in the lowest part of the Pliocene Uegusukudake Formation on the north coast of Kumejima island ; in the close proximity northwest of the island lies an active back-arc basin, Okinawa Trough. The high Fo (92.2) of olivine and Cr2O3 (56.5%) of spinel in the picrite basalts indicate the existence of a magma with >13% MgO and >0.15% Cr2O3. The Kumejima picrite basalts are tholeiitic arc picrites having relatively high SiO2 and low incompatible element abundances.They may have formed accompanying the opening of the Okinawa Trough.
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  • Masaki Enami
    1999 Volume 105 Issue 11 Pages 814-815
    Published: November 15, 1999
    Released on J-STAGE: April 11, 2008
    JOURNAL FREE ACCESS
  • [in Japanese], [in Japanese], [in Japanese], Ömer Emre, Tamer Yig ...
    1999 Volume 105 Issue 11 Pages XXI-XXII
    Published: 1999
    Released on J-STAGE: November 26, 2010
    JOURNAL FREE ACCESS
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