The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 116, Issue 6
Displaying 1-5 of 5 articles from this issue
Articles
  • Sumiaki Machi, Akira Ishiwatari
    2010 Volume 116 Issue 6 Pages 293-308
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    Ultramafic bodies in the Kotaki area of the Hida Marginal Belt are composed of Paleozoic ophiolites and Paleozoic-Mesozoic sediments. The ultramafic rocks can be classified into two major types. Type 1 consists of primary ultramafic rocks that preserve primary textures and primary minerals of mantle peridotite. Type 2 consists of regionally metamorphosed peridotite that has a schistose or mylonitic texture. Both types were locally affected by later contact metamorphism.
    The primary mantle peridotite (Type 1) is subdivided into a high-Al group [spinel Cr#, Cr/ (Cr+Al), of 0.33-0.38] and a high-Cr group [Cr# of 0.48-0.55]. The high-Cr group is pervasive among ultramafic bodies of the Oeyama ophiolite. The high-Al group is identical to lherzolitic peridotite that has been found only in the Oeyama ultramafic body.
    Regionally metamorphosed peridotite (Type 2) can be subdivided into two subtypes based on mineral assemblage and texture. Type 2A contains olivine (ol) +tremolite (tr) ±antigorite (atg) ±orthopyroxene (opx). Type 2B contains ol+atg+clinopyroxene (cpx). Types 2A and 2B are similar to the peridotite mylonite and serpentinite mylonite, respectively, reported from the Happo ultramafic body to the south. Type 2A metaperidotite contains Na-rich tremolite (up to 2.53 wt.% Na2O) produced by metasomatism, possibly related to the slab-derived fluid that penetrated through the wedge mantle above an early Paleozoic subduction zone. Type 2B metaperidotite may represent the wedge mantle metamorphosed at a lower temperature, and Type 1 peridotite may represent the mantle portion that was unaffected by metamorphism and metasomatism.
    Serpentinite hornfelses that formed by later contact metamorphism vary in their mineral assemblages, defined by atg, ol+cpx, ol+tr, ol+tr+talc, and ol+tr+opx zones that are arranged from west to east. The zones indicate that metamorphic temperatures generally increased toward the east. This finding suggests that the hornfelsic metamorphism of previously serpentinized peridotites of Type 1 and Type 2 was caused by a concealed plutonic intrusion beneath the Ishizaka rhyolite, which is exposed to the east.
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  • Takatsune Sato, Hideo Takagi
    2010 Volume 116 Issue 6 Pages 309-320
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    This paper discusses the paleostress field and its history in the Tanzawa tonalite body in the Izu arc-Honshu arc collision zone, as determined from the 3-D orientation of healed and sealed intracrystalline microcracks in quartz grains, in conjunction with microthermometry of fluid inclusions in healed microcracks. Crosscutting relationships indicate that the healed microcracks formed prior to the formation of sealed (and possibly open) microcracks. Both healed and sealed microcracks strike NNE-SSW and dip vertically. It is estimated that healed microcracks were formed at a pressure range of 0.20-0.29 GPa and a temperature range of 275-410°C, based on isochores inferred from microthermometry of the fluid inclusions and with reference to the geothermal gradient (30-50°C/km). The formation temperature of the healed microcracks can be correlated with the closure temperature of the biotite K-Ar system, considering that the Azegamaru and Yushin plutons of the Tanzawa tonalite body yield K-Ar biotite ages of 5-4 Ma. The healed microcracks probably formed in the early Pliocene during collision of the Tanzawa block against the Honshu arc. Because the dominantly NNE-SSW trending microcracks were rotated clockwise by 10° during the collision of the Izu block against the Honshu arc at about 1 Ma, as deduced from reported paleomagnetic data, the orientation of maximum horizontal stress (σHmax) in the Tanzawa tonalite body during and after its collision is inferred to have trended nearly N-S. Recently reported in situ stress measurements in the Tanzawa tonalite body indicate that σHmax trends NNE-SSW. Accordingly, the N-S to NNE-SSW trends in σHmax have not changed greatly during or after the formation of healed microcracks in the Tanzawa tonalite body, which occurred at about 5-4 Ma.
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  • Hiroyoshi Sano, Kiyoko Kuwahara, Akira Yao, Tetsuji Onoue
    2010 Volume 116 Issue 6 Pages 321-340
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    Siliceous micrite crops out in close association with bedded chert and nodular chert in the Middle Jurassic melange of the Mino terrane in the Mt. Funabuseyama area, central Japan. The siliceous micrite and chert form a characteristic lithologic association of a siliceous micrite-chert facies (ca. 60 m thick) in a bedded chert succession. The siliceous micrite is light gray and occurs as thin, lenticular beds in thin-bedded chert and as irregular-shaped blocks in thick-bedded to massive chert. The siliceous micrite is described as radiolarian lime-mudstone that includes subordinate thin-shelled bivalves within a lime-mud matrix. Conodont and radiolarian fossils indicate a Carnian to lower Norian age for the siliceous micrite and associated bedded chert.
    We interpret the Upper Triassic siliceous micrite as deep-water sediment deposited in a pelagic setting within a mid-oceanic realm. This interpretation implies that short-lived fluctuations of the carbonate compensation depth occurred in Carnian to early Norian time, probably due to the intermittent accumulation of lime mud derived from calcareous nannoplankton.
    The Upper Triassic siliceous micrite-chert facies differs in containing siliceous micrite from the coeval bedded chert of the oceanic plate stratigraphy of the Mino terrane. We postulate that compared with the Upper Triassic chert, the siliceous micrite-chert facies accumulated on a topographic high at relatively shallow water-depths, probably near the carbonate compensation depth. The Upper Triassic siliceous micrite-chert facies is inferred to have accumulated on the lower slope of a mid-oceanic seamount. We suggest an alternative age of Early Permian or Middle-Late Triassic for the formation of the mid-oceanic seamount.
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Note
  • Shunichi Suzuki
    2010 Volume 116 Issue 6 Pages 341-346
    Published: 2010
    Released on J-STAGE: October 13, 2010
    JOURNAL FREE ACCESS
    Gold placer was discovered at Nonodake Hill in the mid-eighth century, making it the site of the earliest gold diggings in Japan. Watanabe (1935) was the first geologist to discuss the source of the placer gold, suggesting that it was derived from a Tertiary conglomerate containing rounded fragments of vein quartz, and that the primary source was pre-Tertiary gold veins in the Kitakami Mountains. Onoda (1942) agreed with Watanabe’s view. However, Yagyu (1953) suggested that the placer gold was derived from Tertiary gold veins on the northern side of Nonodake Hill. Taguchi and Ozaki (1994) undertook chemical analyses of particles of placer gold using a scanning electron microscope with an X-ray microanalyzer. Their results support Watanabe’s view on the primary source of the placer gold.
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