저콘 포화온도로 추정한 남한 현생이언 화강암의 상대적인 마그마 생성온도

Relative Magma Formation Temperatures of the Phanerozoic Granitoids in South Korea Estimated by Zircon Saturated Temperature

  • 사공희 (연세대학교 지구시스템과학과) ;
  • 권성택 (연세대학교 지구시스템과학과) ;
  • 조등룡 (지질자원연구원 지질기반정보연구부) ;
  • 좌용주 (경상대학교 지구환경과학과)
  • Sangong Hee (Department of Earth System Sciences, Yonsei University) ;
  • Kwon Sung-Tack (Department of Earth System Sciences, Yonsei University) ;
  • Cho Deung-Ryong (Geology & Geoinformation Division, Korea Institute of Geoscience and Mineral Resources) ;
  • Jwa Yong-Joo (Department of Earth & Environmental Science, Gyeongsang National University)
  • 발행 : 2005.06.01

초록

최근 화강암의 옛저콘(inherited zircon)의 존재여부로써 마그마 생성온도를 상대적으로 저온화강암과 고온화강암으로 구분할 수 있음이 제안되었으며, 이는 저콘 포화온도와 밀접하게 관련이 있음이 밝혀졌다. $SiO_2-Zr$ 그림에서 남한의 현생이언 화강암들은 고Zr 화강암과 저Zr 화강암으로 구분된다. 이들 구분은 관입시기와 관련된 것으로 보인다. 트라이아스-쥬라기 화강암들은 거의 대부분 저Zr 화강암에 속한다. 반면 백악기-제3기초 화강암들은 대부분 고Zr 화강암에 해당하나, 마산-진해 화강암은 저Zr 화강암에 속하여 지리적 차이를 보인다. 주성분원소와 Zr 함량을 이용하여 저콘 포화온도를 계산하면 저Zr 화강암(마산진해 화강암, 평균 $759\times16^{\circ}C$)을 제외한 경상분지의 백악기-제3기초 화강암 $(608-834^{\circ}C,\;평균\;782\times31^{\circ}C$)이 트라이아스_쥬라기 화강암 ($642-824^{\circ}C$, 평균 $756\pm31^{\circ}C$)보다 높은 온도를 지시한다. 현재까지 알려진 트라이아스-쥬라기 화강암의 U-Pb 저콘 연대 자료는 모두 콘코디아 그림에서 디스코디아를 정의하는데 이는 옛저콘의 존재를 지시하며 낮은 저콘 포화온도와 조화적이다. 반면에 상대적으로 높은 저콘 포화온도를 가지는 백악기-제3기초 화강암의 경우 옛저콘의 존재에 대하여 아직 알려진 바가 없기 때문에 고온 혹은 저온 화강암에 해당하는지를 판단할 수 없다. 그럼에도 불구하고 저콘 포화온도는 마그마 생성, 온도를 어느 정도 반영하기 때문에 백악기-제3기초 화강암은 트라이아스-쥬라기 화강암보다 높은 온도에서 생성되었음을 시사한다.

It has recently been proposed that granites can be divided into hot and cold ones by absence and presence of inherited zircon, respectively, which is closely related to zircon saturation temperature. The Phanerozoic granites in South Korea are divided into high- and low-Zr groups in a $SiO_2-Zr$ diagram, which appears to be related to their intrusive age. Most Triassic-Jurassic granites belong to low-Zr group, whereas most Cretaceous-Early Tertiary granites belong to the high-Zr group with the exception of geographically distinct Masan and Jinhae granites that belong low-Zr group. Calculated zircon saturation temperatures using major elements and Zr contents indicate that the Cretaceous-Early Tertiary granites $(608-834^{\circ}C,\;average\; 782\pm31^{\circ}C)$ except for the Masan and Jinhae granites $(average\;759\pm16^{\circ}C)$ show higher temperature than the Triassic-Jurassic granites $(642-824^{\circ}C,\;average\;756\pm31^{\circ}C)$. U-Pb zircon isotope data of the Triassic-Jurassic granites reported so far define discordia in a concordia diagram, which indicates presence of inherited zircon and agrees with their low zircon saturation temperatures. So the Triassic-Jurassic granites appear to belong to cold granite. On the other hand, presence or absence of inherited zircon has not been known for the Cretaceous-Early Tertiary granites with relatively high zircon saturation temperature, so that their classification into hot or cold granite awaits further study. Nevertheless, the Creatceous-Early Tertiary granites may have formed at higher temperature than the Triassic-Jurassic granites, since zircon saturation temperature reflects formation temperature of magma to a certain degree.

키워드

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