BULLETIN OF THE GEOLOGICAL SURVEY OF JAPAN
Online ISSN : 2186-490X
Print ISSN : 1346-4272
ISSN-L : 1346-4272
Volume 64 , Issue 3-4
Showing 1-3 articles out of 3 articles from the selected issue
Article
  • Shunsuke Endo, Toru Yamasaki
    2013 Volume 64 Issue 3-4 Pages 59-84
    Published: May 14, 2013
    Released: June 26, 2013
    JOURNALS FREE ACCESS
        Geological mapping (7×10 km2) of the Ryoke Plutono−Metamorphic Complex exposed in the Tsukude area, central Japan was carried out to provide information about geological structure, the nature of plutonism and metamorphism, and their relationships with the neighboring districts. The Ryoke Metamorphic Complex (RMC) of this area is a 4,000 m−thick sequence of north−dipping strata composed mainly of metapelite, metapsammite and metachert, which corresponds to the upper unit of RMC in the Mikawa Plateau. RMC of the Tsukude area was intruded by the post−metamorphic Ryoke granitic rocks, including the Shinshiro Tonalite, Mitsuhashi Granodiorite and Busetsu Granite. In addition, the distribution of pyroxene− and calcic plagioclase−bearing plutonic rocks was newly identified as the Tsukude mafic rocks. Metamorphism in this area is divided into biotite zone and K−feldspar−cordierite zone. The K−feldspar−cordierite zone represents contact aureoles of the Ryoke plutonic rocks. The Shinshiro Tonalite is surrounded by an unusually wide contact aureole and recrystallization conditions of rocks lying above the K−feldspar−cordierite isograd are estimated to be 230−240 MPa and >600 ºC, implying emplacement of the Shinshiro Tonalite at 8.5−9.0 km depth.
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Report
  • Satoshi Nakae
    2013 Volume 64 Issue 3-4 Pages 85-112
    Published: May 14, 2013
    Released: June 26, 2013
    JOURNALS FREE ACCESS
        The Nanjō Mountains located in the central region of Fukui Prefecture, Southwest Japan, are chiefly underlain by a sedimentary complex consisting of various rock-types such as basalt, limestone, chert, mudstone and sandstone. Among these rocks within the mountains, 27 chert outcrops were explored for their radiolarian content. In the studied Imajō district, the 14 rock samples from 10 localities yielded moderately- to poorly-preserved radiolarian remains as a result. Most of the samples contained Triassic to Middle Jurassic species, with Spumellaria and Entactinaria dominant among the Triassic faunas and Nassellaria dominant among the Jurassic faunas. The description and faunal analysis of these radiolarians revealed that the cherts in the Imajō district indicate a long range in age from the Olenekian? stage to the Bajocian stage.
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Note and Comment
  • Chikao Kurimoto
    2013 Volume 64 Issue 3-4 Pages 113-119
    Published: May 14, 2013
    Released: June 26, 2013
    JOURNALS FREE ACCESS
        In the northwestern part of the Kii Peninsula, strata of the Sambagawa (Sambagawa Belt in strict sense and Mikabu Belt), Chichibu and Shimanto Belts are distributed from north to south based on metamorphic grade, geologic age and lithologic characters. The Mikabu Belt is occupied by the Oishi, Numata and Saga Formations from north to south.
        K‒Ar ages were determined on white mica grains for 23 samples from the Mikabu Belt, 22 samples of which are pelitic schists and one sample is a greenschist.
        K‒Ar ages of the Oishi, Numata and Saga Formations range over 81.7‒98.2 Ma, 96.7‒117.0 Ma and 113‒118 Ma, respectively. The Numata Formation ranges over 96.7‒125.1 Ma, including the data of Kurimoto (1995). Judging from metamorphic grade and closure temperature of the K‒Ar system in muscovite, it is believed that K‒Ar ages from these Mikabu samples show the time of metamorphism.
        The K‒Ar age of the Oishi Formation is roughly concordant with that of the Kebara Formation (Kurimoto, 1993) which is considered to belong to the Mikabu Belt. On the other hand, the K‒Ar ages of the Saga and the Numata Formations are older than those of the Oishi and the Kebara Formations.
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