Economic and Environmental Geology (자원환경지질)

The Korean Society of Economic and Environmental Geology (대한자원환경지질학회)
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Reversely Zoned Compositional Variations and their Origins of the Andong Pluton, Andong Batholith, Korea

안동심성암체의 역누대 초성변화와 그 성인

  • 황상구 (안동대학교 지구환경과학과) ;
  • 이보현 (안동대학교 지구환경과학과)
  • Published : 2002.02.01
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Abstract

The Andong pluton in the Andong Batholith is composed of comagmatic plutonic rocks, in which the lithofacies comprise hornblende biotite tonalite in the central paft biotite granodiorite in the marginal paft and porphyritic biotite granite at the topside (noJthea~tern paft) of the pluton. The pluton is petrographically and petrochemically zoned, having more mafic center than margin and topside. Distribution pallern of the lithofacies represents a reverse zoning in the pluton. Modal and chemical data in the pluton show progressive and gradual compositional variations from the centrer via the margin to the topside. Quartz and K-teldspar increase toward the topside of the pluton, whereas hornblende, biotite and color index increase toward the center. The bulk composition in the pluton is also reversely zoned, with high $Si0_2$ and $K_{2}O$ in the topside facies, and high MnO, CaO, $Ti0_2$, $Fe_{2}O_{3}$t, MgO and $P_{2}O_{5}$ in the central facies. The reverse zoning is also evident in higher Cr. V, Ni, Sc and Sr of the more mafic tonalite in the interior. The reversely zoned pluton results from remobilization (resurgence) of the lower more mafic compositional zone into the upper more felsic zones of the pluton modified by thennogravitational diffusion and fractional crystallization. In the initial stages of evolution, the pluton was a petrochemical system that fonned chemical compositional zonation with mafic tonalitic magma in the lower. granodioritic one in the middle and granitic one in the upper paft of the magma chamber. Periodic influxes of more mafic magma from the ba~e resulted in mingling of liquids and redistribution of minerals, and may have triggered the remobilil.ation of the lower compositional zone into the upper more felsic zones.

안동심성암체는 3개 암상으로 구분되는 동원성 마그마로부터 유래되었다. 이 암체는 전체적으로 중앙부에 각섬석 흑운모 토날라이트가 노출되고 이의 연변부에 흑운모 화강섬록암이 놓이며, 상단부(복동부)에 반상 흑운모 화강암이 놓인다. 결과적으로 이들은 토날라이트를 중심으로 하는 동심원상 분포를 하며, 중앙부에 고온 광물군의 고철질 암석이 우세하고 연변부와 상단부로 갓수록 보다 저온 광물군의 규장질 암석이 우세한 역누대를 나타낸다. 이 암체의 모드 및 화학 자료는 중앙부에서 연변부, 상단부 순으로 역누대 조성변화를 보여준다. 이 조성변화는 인접하는 암상간에서 매우 점이적으로 변화한다. 각섬석과 흑운모 등의 고철질 광물은 중앙부의 토날라이트에서 가장 풍부하고 연변부로 갈면서 감소하고 상단부로 갈수록 더욱 감소한다. 석영과 K-장석 등의 규장질 광물은 이와 반대로 변화한다. 화학적으로도 고철질 광물을 지배하는 원소들은 중앙부에서 최고치이고 이로부터 연변부와 상단부로 갈수록 점차 감소되는 양상을 보이며, 반면에 규장질 광물을 주로 지배하는 원소들은 이와 반대 양상을 나타낸다. 이 역누대 심성암체는 열중력확산작용과 분별결정작용에 의해 수정된 마그마챔버의 상부 규장질 부분 속으로 하부의 더 고철질 부분이 재이동(소생)된 결과이다. 진화의 초기 단계에 , 이 마그마챔비는 하부에 토날라이트질, 중부에 화강섬록암질, 상부에 화강암질 마그마를 갖는 화학적 조성누대를 이룬 암석화학계이었다. 후기 단계에 챔버 기저에서 더 고철질 마그마의 유입에 의해 하부의 토날라이트질 조성대가 상부의 규장질 조성대 속으로 재이동을 야기시켰다.

Keywords

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