광물과 암석 (Korean Journal of Mineralogy and Petrology)

  • 제33권2호
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  • Pages.87-97
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  • 2020
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  • 2734-0333(pISSN)
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  • 2734-0538(eISSN)
한국암석학회 & 한국광물학회 (The Petrological Society of Korea & The Mineralogical Society of Korea)
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산방산용암돔 조면암에서 산출되는 장석의 안티라파키비 조직

Antirapakivi Mantled Feldspars from Sanbangsan Trachyte Lava Dome, Jeju Volcanic Field, Korea

  • 윤성효 (부산대학교 지구과학교육과)
  • Yun, Sung-Hyo (Department of Earth Science Education, Pusan National University)
  • 투고 : 2020.06.01
  • 심사 : 2020.06.29
  • 발행 : 2020.06.30
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초록

산방산조면암에서 반정으로 산출되는 장석은 라브라도라이트(An53.6)에서 안데신(An35.4), 미반정과 기질부에서 래쓰로 산출되는 것은 안데신(An31.2)에서 올리고클레이스(An18.7)에 해당한다. 장석 반정과 미반정을 감싸 얇은 띠(누대)를 이루는(mantled) 장석은 안오소클레이스(Or20.5An9.4)에서 새니딘(Or49.2An1.4)이다. 뚜렷한 누대구조를 나타내는 사장석 반정은 반정 중앙부에서 가장자리 쪽으로 감에 따라 누대의 An함량이 안데신(An39.3)에서 라브라도라이트(An51.3) 사이에서 감소 증가를 반복하는 진동누대구조를 나타내며, 반정의 가장자리는 알칼리장석(Or31.9-39.4Ab63.2-57.0An4.9-3.7)으로 둘러싸여 안티라파키비 조직을 보여준다. 안티라파키비 조직을 나타내지 않고 미반정으로 산출되는 사장석의 누대구조는 중심부에서 가장자리로 감에 따라 An함량이 감소하는(An36.4→An25.6) 정상누대구조를 나타낸다. 전형적인 누대구조를 나타내는 장석 반정에 대하여 K, Ca, Na 원소에 대하여 면분석(X-ray mapping) 결과, 6개의 누대가 뚜렷한 성분 조성의 차이를 나타내는 진동누대구조를 잘 보여주며, 그 가장자리는 알칼리장석으로 감싸면서 안티라파키비 조직을 나타낸다. 기질부는 K-부화, Ca-결핍된 알칼리장석으로 구성된다. 산방산조면암에서 나타나는 장석의 안티라파키비 조직은, 조면암질 마그마보다 더 고철질의 마그마 속에서 이미 결정화된 사장석 반정과 미반정이 산방산 조면암질 마그마와 혼합되면서 사장석 반정과 미반정 주위를 알칼리장석이 결정화되어 맨틀링하면서 형성되었을 것이다.

The compositions of the phenocrystic feldspars of the Sanbangsan trachyte range from labradorite(An53.6) to andesine(An35.4), and of the microphenocrysts and laths range from andesine(An31.2) to oligoclase(An18.7). Mantled feldspar which forms a thin rim around the phenocrysts and microphenocrysts, is anorthoclase(Or20.5An9.4) to sanidine(Or49.2An1.4). Phenocrystic plagioclase, which shows a distinct zonal structure, represents an oscillatory zoning in which the An content of the zone repeatedly increases or decreases between andesine (An39.3) and labradorite (An51.3) from the core toward the rim, and the rim of the phenocrysts is surrounded by alkali feldspar(Or31.9-39.4Ab63.2-57.0An4.9-3.7), showing the antirapakivi texture. Microphenocryst which does not represent the antirapakivi texture, shows the normal zoning with a decreasing An content (An36.4→An25.6) as it moves outward from the center of a crystal. As a result of X-ray mapping of K, Ca, and Na elements for the feldspar phenocrysts representing the typical zonal structure, shows the oscillatory zoning that six zones show the distinctive compositional differences, and the rims are mantled by alkali feldspar to indicate the antirapakivi texture. The groundmass is composed of K-enriched, Ca-poor alkali feldspar. The antirapakivi texture of feldspar which appears in Sanbangsan trachyte, may have been formed in mixing systems as a result of the juxtaposition of near liquidus melt, rich in alkali feldspar components(trachytic magma), with plagioclase phenocrysts and microphenocrysts already crystallized in a more mafic system.

키워드

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