紀伊半島西部の御荷鉾帯および三波川帯泥質片岩の<SUP>40</SUP>Ar/<SUP>39</SUP>Ar全岩年代

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  • 40Ar/39Ar whole-rock dating of metapelites from the Mikabu and Sambagawa Belts, western Kii Peninsula, Southwest Japan.

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^<40>Ar/39Ar whole-rock age spectra have been obtained from three metapelite samples from the Mikabu and Sambagawa Belts in the western Kii Peninsula (Wakayama Prefecture) as a feasibility study. All samples yielded 2σ plateau ages and the integrated 40Ar/39Ar ages are concordant with the K-Ar ages reported by Kurimoto (1993).A phyllite with a shear band cleavage from the spotted zone of the Sambagawa Belt (Iimori Unit) yielded a plateau age of 73.0±0.8 Ma (82% 39Ar release). This result is interpreted as the age of the isotopic closure of white mica during retrograde recrystallization in this greenschist facies metamorphic rock.Two slate samples from the Mikabu Belt (Kebara Unit) yielded 96.4±1.0 Ma (54% 39Ar release) and 102.9±1.0 Ma (49% 39Ar release) plateau ages. The 7 Ma age difference between the Kebara samples may be interpreted by the smaller grain size of the white mica in the youngest sample. Alternatively, the age difference indicates that the main tectono-metamorphic recrystallization in both samples did not occur at the same time. Both interpretations imply that the main tectono-metamorphic phase in the Kebara Unit occurred significantly earlier than in the Iimori Unit.The age spectra of the Kebara samples show progressively increasing apparent ages during early incremental degassing climbing to age plateaux at intermediate- and high-temperature 39Ar release. Such staircase age spectra may be interpreted by partial loss of radiogenic 40Ar due to thermo-tectonic resetting related to movement along the Aridagawa Tectonic Line, that runs parallel to the Kebara Unit.

収録刊行物

  • 地質学雑誌

    地質学雑誌 106 (10), 703-712, 2000

    一般社団法人 日本地質学会

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