The Journal of the Geological Society of Japan
Online ISSN : 1349-9963
Print ISSN : 0016-7630
ISSN-L : 0016-7630
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Volume 111, Issue 8
Displaying 1-7 of 7 articles from this issue
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Review
  • Formative periods and source materials of Cretaceous-Paleogene granitoids from Honshu Arc
    Hiroo Kagami
    2005 Volume 111 Issue 8 Pages 441-457
    Published: 2005
    Released on J-STAGE: December 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.441
    JOURNAL FREE ACCESS
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    Rb-Sr whole rock isochron (RS-WI) is one of the major dating methods for granitoids. The ages defined by this isochron generally have large age-errors, and sometimes cannot be understood clearly. Analysis of the scattering of data in the isochron diagram is a useful tool to overcome these negative aspects. We found out that the Sr isotopic homogenization is not generally attained in whole-area of granitic mass but only within a narrow extent of several square kilometers and occurs in harmony with the shape of the mass. By careful sampling with full consideration of this fact, we will be able to obtain reliable ages indicating emplacement of granitic magma. It is needless to say that the samples must be co-genetic suites and fresh.
    Based on the RS-WI ages reported previously, we clarified the migration of magmatism from Cretaceous to Oligocene in the Honshu Arc (HA). Especially, the magmatism from Late Cretaceous (ca. 70 Ma) to Oligocene gradually migrated or concentrated in a narrow area along the Japan Sea coast. The analysis of such a migration of magmatism can be linked with other data to clarify the cause of the spreading of the Japan Sea basin at Middle Miocene.
    The oxygen and SrI and NdI characteristics of the Cretaceous-Paleogene granitoids of the HA can be divided into the North, South, Kitakami and Sado zones. The granitoids belonging to different zones are formed from isotopically different lower-crust and upper-mantle materials. Transitional zones sometimes exist between two zones.
    Nd model ages were calculated from the Paleozoic to Mesozoic sedimentary rocks in the western part of the HA. The main aim of this study is to ascertain whether the above differences in the O-Sr-Nd isotopic ratios from upper-mantle to lower-crust are also observed in the upper crustal materials.
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Articles
  • Anisotropy of magnetic susceptibility of tonalite in the Tanzawa plutonic complex and its implications for emplacement mechanism.
    Tatsuo Kanamaru, Masaki Takahashi
    2005 Volume 111 Issue 8 Pages 458-475
    Published: 2005
    Released on J-STAGE: December 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.458
    JOURNAL FREE ACCESS
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    The anisotropy of magnetic susceptibility and paleomagnetic direction of the Tanzawa tonalite and related rocks are studied in order to throw light on the emplacement mechanism of tonalite magma. The paleomagnetic directions of samples from syn-plutonic dikes in the Tanzawa main tonalite and dikes in the Jizodaira intrusion reveal that the mass of Tanzawa tonalite did not tilt greatly since its emplacement at 7 Ma. The Tanzawa main tonalite comprises ferromagnetic granite and its anisotropy of magnetic susceptibility is caused by the shape of magnetite clots and/or magnetite grains included in elongated minerals such as hornblende and biotite. On the basis of magnetic foliation, the four successively emplaced units are identified; they are the Kurokuragawa, Murokubogawa, Mizunokisawa, and Ishiwariyama units. Each unit is a laterally zoned pluton with mafic margin grading to felsic core. The dips of magnetic foliations are generally steep especially near the margin of each unit, but those around the roof pendant of the Kurokuragawa unit are rather gentle. The dip of magnetic lineation is steep along the margin of the units, indicating the position of the feeder of magma. On the contrary, the dip of magnetic lineation in the inner part of the unit is gentle. The magnetic foliation and lineation in the Ishiwariyama unit may have been disturbed by the local thermal convection induced by the input of high temperature magma through syn-plutonic dikes. The deformation in the last stage of solidification of magma by the left lateral strike-slip tectonic movement suggests that the tonalite magma was emplaced under transtensional tectonic regime. The space of each unit may have been created by the subsidence of the floor, and magma migrated upward through ring-shaped feeders along the margin of the subsiding floor, resulting in the formation of a lopolith-like pluton.
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  • Fission track ages of welded tuffs of the Gongenzaki Formation in the Tsugaru district, Aomori Prefecture, northeast Japan
    Tohru Danhara, Hiroyuki Hoshi, Hideki Iwano, Tetsu Yoshioka, Yuji Orih ...
    2005 Volume 111 Issue 8 Pages 476-487
    Published: 2005
    Released on J-STAGE: December 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.476
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    Fission track (FT) dating has been conducted for three dacitic welded tuff samples of the Gongenzaki Formation, the lowermost volcanic formation of Tertiary green tuff sequences in the Tsugaru district. The samples were collected from the northern coast of the Kodomari Peninsula. FT ages of 19-23 Ma were determined for internal zircon surfaces, and we interpret these, based on analyses of FT length measurements, as representing emplacement ages of pyroclastic materials. FTs on external surfaces provided ages of 22-31 Ma, one of which is significantly older than that for internal surfaces. Experimental results show that the disagreement is caused by external effect. The onset of deposition of the formation is defined as~23 Ma or older, and the uppermost part can be dated as late Early Miocene (17-20 Ma). U-Pb dating by a LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) reveals that a sample contains zircons having a single age population of about 33 Ma (Early Oligocene), which is much older than the FT age (~23 Ma: early Early Miocene). It is suggested that the zircons were originally crystallized in Early Oligocene time and, nearly 10 million years later, picked up by a dacitic pyroclastic flow or within its source magma.
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  • Assessment of environmental radiation based on the geographical distributions of background γ-ray intensity and soil-type in Tokai-mura Village and Hitachinaka City, Ibaraki Prefecture, Japan
    Michio Tagiri, Masaaki Ikeda, Shin-ichi Kikuchi, Sanae Kimura, Akiko G ...
    2005 Volume 111 Issue 8 Pages 488-497
    Published: 2005
    Released on J-STAGE: December 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.488
    JOURNAL FREE ACCESS
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    The background γ-ray intensity and chemical composition of soil was determined in the Naka terrace of Tokai-mura Village and Hitachinaka City, where many nuclear facilities are located. The aim of the sampling was to assess environmental radiation within the Naka terrace and the sites of nuclear facilities. The study was initiated following the JCO Criticality Accident on September 30 th, 1999. γ-ray measurement and soil sampling were conducted at the nodes of a 500 m grid. The γ-ray intensity recorded at all sample points is less than the average radiation of Japanese granites and less than the typical radiation of humans as assessed by UNSCREA. The chemical characteristics of the soil are divided into 3 types. The distribution of both the soil types and background γ-ray intensity is controlled by geographical and geological factors. K2O and Rb concentrations within the soil show a good correlation with background γ-ray intensity. Our observations indicate that environmental radiation within the study area is derived principally from natural radioisotope radiation.
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  • The occurrence of the fossil mytilid bivalve Mytilus tichanovitchi Makiyama from the Miocene Shimonita Formation, Gunma Prefecture, central Japan: Re-examination of its biostratigraphic and marine paleoclimatic significance
    Yukito Kurihara, Takanori Nakano, Kenshiro Ogasawara
    2005 Volume 111 Issue 8 Pages 498-507
    Published: 2005
    Released on J-STAGE: December 01, 2005
    DOIhttps://doi.org/10.5575/geosoc.111.498
    JOURNAL FREE ACCESS
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    The occurrence of the fossil mytilid bivalve Mytilus tichanovitchi Makiyama is reported from the Iwayama Sandstone Member of the Shimonita Formation in the Shimonita district, Annaka-Tomioka area, Gunma Prefecture, central Japan. The mode of fossil occurrence and its associated molluscs indicate that M. tichanovitchi lived in sandy bottom of the upper sublittoral zone. Compilation of the published bio-and chrono-stratigraphic data indicates that the stratigraphic range of this species in Japan is evidently lower than that of the warm-water Kadonosawa-type Fauna and is restricted to the Lower Miocene. Strontium isotope ratios (87Sr/86Sr) of M. tichanovitchi shell from the Shimonita Formation also suggest Early Miocene age. Faunal character of M. tichanovitchi-bearing molluscan assemblages in Japan indicates the mild- to cool-temperate marine condition. The occurrence of M. tichanovitchi in central Honshu may have been related to a global cooling event at approximately 18 Ma.
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Errata
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