Journal of the Korean earth science society (한국지구과학회지)

The Korean Earth Science Society (한국지구과학회)
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Antirapakivi Mantled Feldspar of the Albong Trachyandesite from Ulleung Island, Korea

울릉도 알봉조면안산암에서 산출되는 안티라파키비조직 장석의 성분

  • Sung-Hyo, Yun (Department of Earth Science Education, Pusan National University)
  • 윤성효 (부산대학교 지구과학교육과)
  • Received : 2022.12.11
  • Accepted : 2022.12.20
  • Published : 2022.12.31
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Abstract

In this study, the composition of feldspar phenocrysts observed in the Ulleungdo Albong trachyandesite lava dome was identified by performing point and X-ray element mapping surface analysis (EPMA). Plagioclase, which appears as a phenocryst in the Albong trachyandesite, corresponds to bytownite and labradorite, and andesine, and lath in the microphenocrysts and the matrix corresponds to andesine to oligoclase. Alkali feldspar mantled around plagioclase phenocrysts and microphenocrysts correspond to anorthoclase and sanidine. Plagioclase phenocrysts with a distinct zonal structure represent a normal structure in which the An content of the zoning decreases from bytownite to labradorite or andesine as it moves from the center of the phenocrysts to the edge. The edge of the phenocryst is surrounded by alkali feldspar, showing an antirapakivi texture. X-ray mapping of feldspar phenocrysts showed a typical antirapakivi texture. Normal zoning with distinct zoning showing a difference in component composition was clearly shown. The edges were mantled with alkali feldspar, and antirapakivi represents the texture. The antirapakivi texture of feldspar in the Albong trachyandesite may have been formed in the mixing system when alkali feldspar crystallized and mantled around plagioclase phenocrysts and microphenocrysts. This is because plagioclase phenocrysts and microphenocrysts in magma that had already crystallized are more mafic than trachyandesite magma.

본 연구에서는 울릉도 알봉조면안산암 용암돔에서 관찰되는 장석 반정의 안티라파키비 조직에 대하여 점분석 및 X-ray element mapping 면분석(EPMA 분석)을 실시하여 장석들의 성분을 파악하였다. 알봉조면안산암에서 반정으로 산출되는 사장석은 비토우나이트에서 라브라도라이트 및 안데신에, 미반정과 석기에서 래쓰로 산출되는 것은 안데신에서 올리고클레이스에 해당한다. 사장석 반정과 미반정을 감싸 얇은 띠(누대)를 이루는 알칼리장석은 안오쏘클레이스에서 새니딘에 해당한다. 뚜렷한 누대구조를 보이는 사장석 반정은 반정 중앙부에서 가장자리 쪽으로 감에 따라 누대의 An 함량이 비토우나이트에서 라브라도라이트 또는 라브라도라이트에서 안데신으로 감소하는 정상 누대구조를 나타내며, 반정의 가장자리는 알칼리장석으로 둘러싸여 안티라파키비 조직을 나타낸다. 전형적인 안티라파키비 조직을 나타내는 장석 반정에 대하여 X-ray 면분석을 실시한 결과, 누대가 뚜렷한 성분 조성의 차이를 나타내는 정상 누대구조를 잘 보여주며, 그 가장자리는 알칼리장석으로 감싸면서 안티라파키비 조직을 나타낸다. 알봉조면안산암에서 나타나는 장석의 안티라파키비 조직은 조면안산암 마그마보다 더 고철질인 마그마 속에서 이미 결정화된 사장석 반정과 미반정이 알봉조면안산암 마그마와 혼합되면서 사장석 반정과 미반정 주위를 알칼리장석이 결정화되어 맨틀링하면서 형성되었을 것이다.

Keywords

Acknowledgement

이 과제는 부산대학교 기본연구지원사업(2년)에 의하여 연구되었다. 야외조사에서 시료 채취에 도움을 주신 황상구 교수님께 감사드리며, EPMA 분석을 담당하신 부산대학교 공동실험실습관의 진미정 박사님께도 감사드린다. 이 논문을 읽고 건설적인 비평과 조언을 주신 편집위원장과 익명의 심사자에게 감사드린다.

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