島根県小馬木モリブデン鉱床の放射年代の変遷とジルコンU-Pb年代とその意義  [in Japanese] A history of the radiometric age determinations and zircon U-Pb age of the host rocks for the Komaki molybdenum deposits, Japan  [in Japanese]

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Abstract

島根県,小馬木鉱床産鉱物には様々な放射年代値が異なる鉱物について報告されていた. 今回,母岩のジルコンのU-Pb年代を測定したところ,74.8±0.4Ma が得られた. 主要な最近の既存値を適切な壊変定数で再計算し,新に測定したジルコンU-Pb年代と比較すると,白雲母のK-Ar年代は65.7±2.6∼63.3±2.5 Ma,黒雲母のK-Ar年代は60.8 ±2.4 Maを示し,雲母年代が明らかに若い. ジルコンのU-Pb閉鎖温度は900 °C,白雲母と黒雲母のK-Ar閉鎖温度はそれぞれ358 °Cと300 °Cと考えられているから,ジルコン-白雲母で47 °C/Ma, ジルコン-黒雲母で43 o/Maの冷却速度が求められる. 輝水鉛鉱も400 °C程度の熱水期に生成したであろうから,Re-Os年代にも雲母類と同様な若い年代が予想される. 今後は輝水鉛鉱の産状別のRe-Os年代測定をする必要がある.

Various radiometric ages proposed on tungsten-bearing molybdenum deposits of the Komaki mine, Southwest Japan, were reviewed, and U-Pb age was determined on zircon of the host granite as 74.8±0.4 Ma. Recalculated K-Ar muscovite ages range from 65.7±2.6 to 63.3±2.5 Ma and that of the biotite age 60.8 ±2.4 Ma, reflecting different closure temperatures in each system, such as 900 °C for U-Pb in zircon, 358 °C for K-Ar in muscovite and 300 °C for K-Ar in biotite. Cooling rates of the host magma are calculated to be 47 °C/Ma on the zircon-muscovite pair and 43 o/Ma on the zircon-biotite pair. The ore molybdenite appears to be precipitated around 400 °C, yet an older age of 74.3±3.2 Ma was reported on the Re-Os age. We need further Re-Os measurement on the molybdenites with different modes of °Ccurrence.

Journal

  • Shigen-Chishitsu

    Shigen-Chishitsu 65(1), 1-4, 2015

    The Society of Resource Geology

Codes

  • NII Article ID (NAID)
    130007434235
  • NII NACSIS-CAT ID (NCID)
    AN10379890
  • Text Lang
    JPN
  • ISSN
    0918-2454
  • NDL Article ID
    026292857
  • NDL Call No.
    Z15-123
  • Data Source
    NDL  J-STAGE 
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