Journal of the Geological Society of Korea (지질학회지)

The Geological Society of Korea (대한지질학회)
권호 표지

Metamorphism of the Gonamsan area in the western Imjingang belt: Anti-clockwise P-T-t path

서부 임진강대 고남산 지역의 고압 변성작용: 반시계 방향의 P-T-t 경로

Jeon, Eun-Yeong;Gwon, Seong-Taek
전은영;권성택

  • Published : 19990300

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Abstract

In order to better understand the tectonic evolution of the Imjingang belt, a possible extension of the Chinese collision belt, we studied geologic relations, petrography and mineral chemistry of the Gonamsan metamorphic rocks near southeastern end of the western Imjingang Belt. The N-S trending Gonamsan metamorphic rocks, considered to be an extension of the Samgot Formation of the Yoncheon Group, consist of amphibolite, calc-silicate and metapelite to metapsammite. Mylonites with a sinistral shear sense occur in the amphibolite and have foliation subparallel to the regional structural trend. Major mineral assemblages of the amphibolite are amphibole + plagioclase + quartz + magnetite $\pm$ clinopyroxene $\pm$ garnet $\pm$ biotite $\pm$ K-feldspar. Reaction textures in the amphibolite suggest that amphibole coronas were formed by the hydration reaction (clinopyroxene + plagioclase + $H_2O$ = amphibole + quartz), while garnet coronas were formed either by the dehydration reaction (amphibole + plagioclase = garnet + quartz + $H_2O$) or by the anhydrous reaction (ilmenite + plagioclase + quartz = garnet + rutile). We suggest the following sequence of an anti-clockwise P-T-t path for the metamorphic evolution of the study area from structural relationship, rock texture, and estimated metamorphic temperature and pressure: (1) peak metamorphism at about $750-800^{\circ}C$ and 10 kbar, and subsequent isobaric cooling to about $700^{\circ}C$, (2) isothermal burial to about 13-15 kbar, and (3) uplift with cooling to about $650^{\circ}C$ and 10-12 kbar. This path and our preliminary Sm-Nd garnet age of $263.2\;{\pm}\;5.8\;Ma$, indicate a crustal thickening event in late Permian. Therefore, this study, along with the previous one for the Yoncheon area, further supports the possibility of the Imjingang belt to be the extension of the Chinese collision belt.

우리는 중국 대륙 충돌대의 동쪽 연장으로 생각되는 임진강대의 지구조적 진화를 보다 잘 이해하기 위해 서부 임진강대 남동단의 고남산 지역 변성암류에 대한 지질 관계, 암석기재 및 광물 화학을 연구하였다. 남-북 주향의 고남산 지역 변성암들은 연천층군 삼곶층의 연장부로 생각되어지며, 각섬암, 석회질 규산염암, 변성 이질 내지 사질암 등으로 구성된다. 각섬암체 내에는 좌수향 연성 전단대가 발달하고 주변암의 엽리 구조 방향과 조화적이다. 각섬암은 주로 각섬석 + 사장석 + 석영 + 자철석$\pm$단사휘석$\pm$석류석$\pm$혹운모$\pm$알칼리 장석의 광물 조합을 보인다. 각섬암내에는 특징적으로 다양한 코로나 조직이 관찰되는데, 각섬석 코로나는 단사휘석$\pm$사장석 + $H_2O$= 각섬석 + 석영의 수화 반응에 의해, 석류석 코로나는 각섬석 + 사장석 = 석류석 + 석영 + $H_2O$의 탈수 반응 그리고/혹은 일메나이트 + 사장석 + 석영 = 석류석 + 금홍석의 무수 반응에 의해 형성되었음을 시사한다. 고남산 지역 변성암의 구조적인 관계, 암석의 조직, 변성 온도와 압력 등으로부터 약 $750-800^{\circ}C$ 온도, 10 kbar 압력에서의 최고 변성작용 이후, 약 $700^{\circ}C$로의 등압감온, 약 13-15 kbar로의 등온증압을 거쳐, 약 $650^{\circ}C$, 10-12 kbar로의 감온감압되는 반시계 방향의 경로를 추정할 수 있다. 이러한 변성 진화 경로는 석류석 Sm-Nd 연대 $263.2\;{\pm}\;5.8\;Ma$와 더불어 페름기 말에 지각이 두꺼워진 사건을 지시한다. 따라서, 이 연구는 기존 연천 지역의 연구 결과와 더불어 임진강대가 중국 대륙 충돌대의 연장일 가능성을 더욱 뒷받침한다

Keywords

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