The Journal of the Petrological Society of Korea (암석학회지)

The Petrological Society of Korea (한국암석학회)
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Jurassic (~170Ma) Zircon U-Pb Age of a "Granite Boulder" in the Geumgang Limestone, Ogcheon Metamorphic Belt, Korea: Reinterpretation of its Origin

옥천변성대 금강석회암 내 "화강암 거력"의 쥬라기(~170 Ma) 저어콘 연대: 성인에 대한 재해석

  • Cheong, Wonseok (Department of Earth and Environmental Sciences, Chungbuk National University) ;
  • Cho, Moonsup (Department of Earth Science Education, Seoul National University) ;
  • Yi, Keewook (Division of Environmental and Material Sciences, Korea Basic Science Institute) ;
  • Lee, Min Sung (Department of Earth Science Education, Seoul National University) ;
  • Kim, Yoonsup (Department of Earth and Environmental Sciences, Chungbuk National University)
  • 정원석 (충북대학교 지구환경과학과) ;
  • 조문섭 (서울대학교 지구과학교육과) ;
  • 이기욱 (한국기초과학지원연구원 환경.소재분석본부) ;
  • 이민성 (서울대학교 지구과학교육과) ;
  • 김윤섭 (충북대학교 지구환경과학과)
  • Received : 2016.01.25
  • Accepted : 2016.03.03
  • Published : 2016.03.31
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Abstract

A "granite boulder", ~70 cm in size, was reported from the Geumgang Limestone, and has been considered as a glaciogenic dropstone. Since this interpretation has enormous implications for unraveling the evolution history of the Ogcheon Metamorphic Belt, we re-examined the contact relationship and structure of the "granite boulder", and estimated its emplacement age based upon SHRIMP U-Pb zircon dating. The weighted mean $^{206}Pb/^{238}U$ age pooled from 6 spot analyses of two specimens is $170{\pm}2Ma$ ($2{\sigma}$, MSWD=2.2). This zircon age suggests that the "granite boulder" in the Geumgang Limestone is a part of Jurassic granite, rather than a glaciogenic dropstone.

옥천변성대의 금강석회암 내에서 보고된 소위 "화강암 거력"은 장축 길이가 약 70 cm에 달하며 빙하기원의 낙하석으로 제안된 바 있다. 이러한 해석은 옥천대의 진화사를 밝히는데 매우 중요하므로, 우리는 이 "화강암 거력"의 산상을 재검토하고 저어콘을 분리해 SHRIMP U-Pb 연대를 구하였다. 두 개 시료의 6개점 분석 자료로부터 얻은 저어콘의 $^{206}Pb/^{238}U$ 가중평균 연령은 $170{\pm}2Ma$ ($2{\sigma}$, MSWD=2.2)이다. 이 연령은 "화강암 거력"이 쥬라기 화강암의 일부이며, 빙하기원이 아님을 말해준다.

Keywords

References

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