The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
Volume 107 , Issue 5
Showing 1-6 articles out of 6 articles from the selected issue
  • Masamichi Takebe
    2001 Volume 107 Issue 5 Pages 301-315
    Published: May 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    In order to elucidate factors controlling REE concentrations in deep-sea sediments from central to western Pacific, 64 samples were analyzed for major elements by XRF and rare earth elements (REEs) by ICP-MS. Statistical factor analyses were applied for the estimation of factors. The main conclusions are as follows;1) REEs, except for Ce, show remarkably good positive correlation with P2O5, which also correlates with CaO. Further in P2O5-CaO diagram, many samples are plotted near a line with a slope similar to CaO/P2O5 ratio of apatite. These facts indicate that REEs are concentrated in apatite of Pacific sediments. Statistical factor analysis suggests that apatite is the principal factor controlling REE concentrations in the Pacific sediments.2) Many researchers pointed out that airborne transport of continental materials such as loess is also important in controlling the chemistry of marine sediments in addition to the apatite. Based on REE compositions of "loess" and "calculated apatite", other factors controlling REE composition of Pacific sediments were estimated by the factor analysis. Newly identified REE pattern normalized to NASC exhibits a convexity with a peak at Eu, resembling remarkably that of diagenetic Mn nodule. No clear relationship, however, could be established between the third factor and Mn contents of samples.3) The distribution of factor scores revealed that material of loess origin contribute to the sediments mainly of northern Philippine Sea and central Pacific, apatite to those of equatorial Pacific, and the third factor to those in the vicinity of Hawaii islands and northern of Pacific.The result obtained from this study may give us a clue in clarifying the depositional environment of subduction complex on land.
    Download PDF (1766K)
  • Lawrence R. Zamoras, Atsushi Matsuoka
    2001 Volume 107 Issue 5 Pages 316-336
    Published: May 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Busuanga Island is considered an accretionary complex primarily composed of Middle Permian to Jurassic chert (Liminangcong Formation), Middle Jurassic to Early Cretaceous clastics (Guinlo Formation) and limestone units of various ages, with minor melange bodies (Bicatan Melange), collectively called as the Malampaya Sound Group. These rocks are regarded as offscraped sedimentary deposits from a subducted oceanic plate that developed imbricate structure during the Jurassic-Early Cretaceous accretion. The lithologic transition from chert to siliceous mudstone to terrigenous clastics indicates facies change from pelagic to terrigenous condition brought by a plate movement from an open ocean to the subduction zone. The siliceous mudstone units, which mark the end of chert deposition and the beginning of clastic deposition, are classified into three different ages in Busuanga Island, and found to be younging from north to south. This provides a major basis for defining three belts composing Busuanga Island : the Northern, Middle and Southern Busuanga belts. The Northern Busuanga Belt is composed of Middle Permian to Middle Jurassic (probably Aalenian) chert followed by Bathonian-Callovian (JR 4-JR 5) siliceous mudstone and Callovian (JR 5) terrigenous clastics. The Middle Busuanga Belt has its topmost chert deposit at Bajocian-lower Bathonian (JR 4), siliceous mudstone interval from upper Bathonian to lower Oxfordian (JR 5-JR 6), and terrigenous clastics at Oxfordian (JR 6). The Southern Busuanga Belt shows the transition from lower-middle Tithonian (JR 8) chert to upper Tithonian-Berriasian (KR 1) siliceous mudstone. The subsequent deposition of terrigenous clastics, supposedly overlying the siliceous mudstone, is considered occurring within the Lower Cretaceous.The offscrape accretion period of these three belts probably occurred during the Middle Jurassic for the Northern Busuanga Belt, Late Jurassic for the Middle Busuanga Belt and Early Cretaceous for the Southern Busuanga Belt. These chert-clastic sequences of the Malampaya Sound Group are also found correlative with the Togano Group of the Southern Chichibu Terrane of SW Japan.
    Download PDF (3752K)
  • Tatsumi Abe, Hideo Takagi, Koji Shimada, Shinji Kimura, Keisuke Ikeyam ...
    2001 Volume 107 Issue 5 Pages 337-353
    Published: May 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    This paper aims to document the principal features of the ductile shear deformation of the Sambagawa metamorphic rocks in the Ogawa-Ayukawa area, Kanto Mountains by integrating geological, petrological, microstructural and kinematic data. Metamorphic zones change northward from chlorite in the south to garnet, biotite and again garnet. Lithologic layering and metamorphic zone boundaries are generally parallel.A penetrative horizontal stretching lineation trending WNW-ESE is well developed, and is almost parallel to the trend of the Sambagawa Belt. Convincing shear sense determination using shear bands, oblique shape and c-axis fabrics of quartz grains in XZ thin sections was possible for about 14% of all samples collected. Top-to-the-west sense of shear is dominant in the study area, but minor domains with opposite sense of but shear (top-to-the-east) also occur particularly in the northern and southern areas. Quartz shape fabric and shear bands in opposite limbs of small-scale recumbent fold (F1) indicate the same sense of shear. This suggests that the shearing postdates or was synchronous with the recumbent folding. The distribution of metamorphic zoning has been interpreted as the result of a macroscopic recumbent fold with northward vergence, however, studies of the distribution of lithology, mineral assemblages and shear-sense indicators gave no evidence in support of a large-scale recumbent fold in the area. Domains showing opposite oblique quartz shape fabrics and related interference colors with the gypsum plate inserted are observed in some quartz schists. The boundaries of the domains showing inverted sense of shear indicator are subparallel to the schistosity. Quartz c-axis fabrics have cleft girdle patterns suggesting uniaxial stretching deformation. The shortest axis of each quartz grain is parallel to Y or subsidiary to Z, suggesting that not only Z-parallel shortening but also Y-parallel shortening is dominant. Minor folds (crenulation) are commonly observed in YZ sections. These lines of evidence suggest that the bulk shear flow (fundamentally westward flow) was partitioned into the domains with opposite sense of shear and that X-parallel stretching is dominant.These features suggest the dominant deformation in the area is a ductile flow with partitioning into kinematically distinct domains and associated with overall uniaxial extension. This ductile flow is probably related to a tectonic framework such as oblique subduction of oceanic plate relative to continental plate during and after the peak metamorphism of the Sambagawa Belt. No evidence of dip-slip ductile to brittle shear which support the extrusion model of the high-P metamorphic rocks has been found in the study area.
    Download PDF (5437K)
  • Yasutaka Terakado
    2001 Volume 107 Issue 5 Pages 354-357
    Published: May 15, 2001
    Released: April 11, 2008
    JOURNALS FREE ACCESS
    Re-Os dating method was applied to the Kuroko ore deposits of the Hokuroku district, Japan. The Re and Os concentrations of four Kuroko, three Oko and one Keiko ores vary from 4.1 to 233 ppb and from 9.1 to 647 ppt, respectively, and their 187Os/188Os ratios range from 0.84 to 1.43. Most reliable isochron age is 14.32±0.51 Ma, which is consistent with previously reported ages based on stratigraphical, paleontological and K-Ar studies. The initial 187Os/188Os ratio of 0.7563±0.0047 falls within the range of the middle Miocene seawater values, suggesting seawater origin of the Kuroko Os.
    Download PDF (638K)
  • Masayuki Sakakibara, Ralph Kretz, Toshisuke Kawasaki
    2001 Volume 107 Issue 5 Pages IX-X
    Published: 2001
    Released: November 26, 2010
    JOURNALS FREE ACCESS
    A corona is a small-scale structure developed around a mineral grain by reaction with its surroundings. It could indicate a change in chemical and/or physical factors during a prograde or retrograde metamorphic event, or associated with cooling from igneous temperatures (Kretz, 1994). The olivine-plagiocalse corona is one of distinct and most common examples of them in metamorphosed rocks.
    In Ontario, the Grenville Orogen comprises the Central Gneiss Belt and the structurally overlying Central Metasedimentary Belt, divided into a northeastern Elzevir terrane and a southeastern Frontenac terrane. The Chenaux gabbro intrusion is one of plutons in the Raglan gabbro belt at the base of the Elzevir terrane hanging wall (Pehrsson et al., 1996) (Fig.1). It was emplaced during the late stage of the regional metamorphism and deformation, culminated at ca.1.1 Ga.
    The Chenaux Gabbro is partly deformed and recrystallized on the amphibolite facies condition. Intrusive contacts between the gabbro and surrounding carbonate rocks are observed at various places (Fig.2 (1)). It has occasionally igneous layering (Fig.2 (2)). The principal constituents of the gabbro are plagiolclase, olivine, clinopyroxene, orthopyroxene and calcic amphiboles, with small amounts of biotite, spinel and Fe-Ti oxides.
    Download PDF (11613K)
  • Lawrence R. Zamoras, Atsushi Matsuoka
    2001 Volume 107 Issue 5 Pages XI-XII
    Published: 2001
    Released: November 26, 2010
    JOURNALS FREE ACCESS
    The Malampaya Sound Group is predominantly composed of chert (Liminangcong Formation), clastics (Guinlo Formation) and scattered limestone units that characterize the North Palawan Block (Zamoras and Matsuoka, 2001). In the Calamian Islands (grouping Busuanga, Coron, Culion and other smaller islands) the Liminangcong Formation is represented by Permian to Late Jurassic chert; the Guinlo Formation is represented by Middle-Late Jurassic to Early Cretaceous clastics.
    The common succession from chert to clastics with generally conformable contact is thus referred to as a chertclastic sequence. In Busuanga Island, linear topographic patterns reveal the parallel distribution of chert-clastic sequences dipping generally NW in the west, and NE in the central and eastern portions. This chert-clastic repetition indicates an imbricate structure typical to offscrape accretion complexes.
    Download PDF (11637K)
feedback
Top