Journal of the Mineralogical Society of Korea (한국광물학회지)

The Mineralogical Society of Korea (한국광물학회)
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Mineral Chemistry of Magnetite from the Hongcheon Carbonatite-Phoscorite Complex, Korea

홍천 카보나타이트-포스코라이트 복합체에서 산출되는 자철석의 광물화학

  • Received : 2013.12.03
  • Accepted : 2013.12.25
  • Published : 2013.12.31
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Abstract

Magnetite, a major constituent mineral of the Hongcheon carbonatite-phoscorite complex, was produced over three stages in each rock type and decreased in quantity toward the late stage. Electron microprobe analyses for magnetite revealed that Ti and V were detected in traces, but showed increasing tendency from early to late stage. On the contrary, Mg and Mn decreased distinctly, and it is the general differentiation trend of carbonatitic magma. Al also showed decreasing tendency in carbonatite and phoscorite, and Cr was mostly below detection limit except late phoscorite. In early stage, $Fe^{2+}$ was largely replaced by $Mg{2+}$ and $Mn^{2+}$, and $Fe^{3+}$ by $Al^{3+}$ in magnetite, but it has nearly pure composition in late stage. Tendency of increase in V and decrease in Mn toward late stage represents that magma differentiation progressed under the condition of decreasing oxygen fugacity. Low concentrations of Mg, Al, Cr and Ti, as well as the absence of olivine and phlogopite, suggest that the Hongcheon carbonatite-phoscorite complex was generated from depleted magma. Especially, lower concentrations of Mg in magnetite compared to other typical carbonatite-phoscorite complex, and abundant occurrence of Fe-carbonate minerals and quartz in late stage, suggest that magma differentiation of the Hongcheon carbonatite-phoscorite proceeded to the latest stage.

홍천 카보나타이트-포스코라이트 복합체를 형성하는 주 구성광물인 자철석은 각각 세 시기에 걸쳐 정출되었으며 후기로 가면서 함량이 점차 감소한다. 자철석에 대한 전자현미분석결과 Ti, V은 미량 검출되지만 초기에서 후기로 가면서 증가하는 경향을 보여준다. 반면, Mg, Mn은 뚜렷이 감소하는데 이는 일반적인 카보나타이트질 마그마 분화특성을 잘 나타낸다. Al 또한 카보나타이트와 포스코라이트에서 감소하는 경향을 보여주며, Cr은 대부분 검출한계 미만을 나타내나 후기 포스코라이트에 와서는 미량 정출된다. 자철석은 초기에는 $Fe^{2+}$가 주로 $Mg{2+}$와 $Mn^{2+}$에 의해 치환되고, $Fe^{3+}$는 $Al^{3+}$에 의한 치환되는 양상이 주를 이루었으나 후기에 와서는 감소하면서 거의 순수한 자철석 조성을 갖게 된다. V의 증가와 Mn의 감소는 마그마 분화가 산소분압이 점차 감소하는 환경에서 진행되었음 나타내고, 감람석, 금운모의 부재와 더불어 자철석의 Mg, Al, Cr 및 Ti 원소들의 함량이 낮은 것은 홍천 카보나타이트-포스코라이트 복합체가 결핍된 모마그마로부터 생성되었음을 지시한다. 특히, 후기로 가면서 철질 탄산염광물과 석영의 산출이 두드러지면서 전형적인 카보나타이트-포스코라이트 복합체에 비해 연구지역 자철석의 Mg 함량이 적게 산출되는 것은 마그마 분화가 최후기까지 진행되었음을 시사한다.

Keywords

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