The Journal of the Petrological Society of Korea (암석학회지)

The Petrological Society of Korea (한국암석학회)
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Microstructure Related to the Growth of Rare-earth Mineral in the Eoraesan Area, Chungju, Korea

충주 어래산 지역에서 희토류 광물의 성장과 관련된 미구조

  • Kang, Ji-Hoon (Department of Earth and Environmental Sciences, Andong National University)
  • 강지훈 (안동대학교 지구환경과학과)
  • Received : 2019.06.07
  • Accepted : 2019.06.23
  • Published : 2019.06.30
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Abstract

The Eoraesan area, Chungju, which is located in the northwestern part of Ogcheon Metamorphic Zone, Korea, mainly consists of the Neoproterozoic Gyemyeongsan Formation and the Mesozoic igneous rocks which intruded it. The metaacidic rocks (MAR) of the Gyemyeongsan Formation show a maximum radioactive value, and the Early Jurassic biotite granite is regionally distributed in this area. In this paper is researched the microstructure related to the growth of rare-earth mineral of allanite in the MAR, and is considered the source and occurrence time of rare-earth element (REE) mineralization. The MAR is mainly composed of alkalic feldspar (mainly microcline), quartz, iron-oxidizing mineral, biotite, muscovite, plagioclase, hornblende, allanite, zircon, epidote, fluorite, apatite, garnet, (clino)zoisite etc. The radioactive elements contained in the allanite cause a dark brown hale in the surrounding biotite, and the allinte also occurs as aggregate along the regional foliation. The deflection of regional foliation and the strain shadows, which are common to the pre-tectonic porphyroblast grown before the formation of regional foliation, can't be observed around most allanites (aggregates). The grain size and orientation of ironoxidizing mineral included in the allanite aggregate are the same as those in the matrix. It is recognized the hydrothermal conversion of hornblende to biotite due to the intrusion of igneous rock, and the secondary biotite occurs and contacts with allanite, zircon, epidote etc. These microstructures indicate that the rare-earth mineral of allanite (aggregate) grew by the hydrothermal alteration due to the intrusion of igneous rock after the formation of regional foliation. It is considered that the REE mineralization is closely related to the intrusion of Early Jurassic biotite granite which is regionally distributed in this area.

옥천변성대의 북서부에 위치하는 충주 어래산 지역은 주로 신원생대 계명산층과 이를 관입하는 중생대 화성암류로 구성되어 있다. 계명산층의 변성산성암의 방사능 값은 매우 높게 나타나고, 전기 쥬라기 흑운모 화강암은 이 지역에 광범위하게 분포한다. 이 논문에서는 변성산성암을 중심으로 희토류 광물인 갈렴석의 성장과 관련된 미구조 연구를 수행하여 희토류 광화작용의 기원과 발생 시기를 고찰하였다. 변성산성암은 주로 알칼리장석(주로 미사장석), 석영, 철산화광물, 흑운모, 사장석, 각섬석, 갈렴석, 저어콘, 녹렴석, 형석, 인회석, 석류석, (사)유렴석 등으로 구성되어 있다. 갈렴석은 흑운모와의 접촉부에 방사선 손상에 의한 짙은 갈색 광륜을 형성시키기도 하고, 광역엽리를 따라 집합체로 산출되기도 한다. 대부분의 갈렴석(집합체)의 주변부에서는 광역엽리가 형성되기 이전에 이들이 성장하였음을 지시하는 변형그늘이나 광역엽리의 굴곡면은 관찰되지 않는다. 갈렴석 집합체에 포획된 철산화광물의 입자 크기와 방향성은 광역엽리를 따라 산출하는 기질부의 철산화광물과 동일하다. 후기 화성암의 관입에 의한 열수변질작용으로 각섬석의 흑운모화가 인지되고, 이차적으로 성장한 흑운모는 갈렴석, 저어콘, 녹렴석 등과 접촉 공생한다. 이러한 미구조로부터 희토류 광물인 갈렴석(집합체)은 주로 광역엽리가 형성된 이후에 화성암의 관입과 관련된 열수변질작용으로 성장하였으며, 충주 어래산 지역의 희토류 광화작용은 이 지역에 광범위하게 산출되는 전기 쥬라기 흑운모화강암의 관입과 밀접한 관련성이 있는 것으로 고찰된다.

Keywords

HGOSBQ_2019_v28n2_129_f0001.png 이미지

Fig. 2. The locality of study area in the Chungju (Kim and Lee, 1965) (upper C) and Goesan (Lee and Kim, 1972) (lower G) sheets (1: 50,000). For detailed explanation, see the 1: 50,000 Chungju and Goesan sheets.

HGOSBQ_2019_v28n2_129_f0002.png 이미지

Fig. 3. Geological map of the Eoraesan area, Chungju (Kang et al., 2017, 2018a). Numbers (167, 179, 187, 342, 431): sample sites of Figs. 4, 6~8.

HGOSBQ_2019_v28n2_129_f0003.png 이미지

Fig. 4. Microphotographs of microstructures (a, b: No. 342-1, coarse-grained type) before and (c, d: No. 167, coarse-grained type) after the formation of regional foliation (Sn) in the metaplutonic acidic rocks.

HGOSBQ_2019_v28n2_129_f0004.png 이미지

Fig. 5. Microphotographs of pre-tectonic allanite (Aln) with respect to the formation of regional foliation (Sn) in the metahypabyssal acidic rock of Namsan area, Chungju. Note the well-developed strain caps and strain shadows around the allanite porphyroblast. It indicates that later deformation formed the regional foliation which is deflected around the allanite crystal. Scale bar: 0.3 mm.

HGOSBQ_2019_v28n2_129_f0005.png 이미지

Fig. 6. Microphotographs of allanite occurring as a single crystal in the metaplutonic acidic rocks.

HGOSBQ_2019_v28n2_129_f0006.png 이미지

Fig. 7. Microphotographs of allanite occurring as aggregate crystals in the metaplutonic acidic rocks (No. 187-2, basic-bearing type).

HGOSBQ_2019_v28n2_129_f0007.png 이미지

Fig. 8. Microphotographs of microstructures showing the hydrothermal conversion of hornblende to biotite in the metaplutonic acidic rocks.

HGOSBQ_2019_v28n2_129_f0008.png 이미지

Fig. 1. (a) Tectonic province maps of the Korean Peninsula, and (b) geological map showing the division of stratigraphic sequences and structural units in the Ogcheon Belt with the locality of study area (Kang et al., 2012b). 1: Thrust, 2: Fault, B: Bonghwa, Bo: Boeun, C: Chungju, G: Gwangju, Ga: Gangneung, Je: Jeonju, M: Mungyeong, Mo: Mokpo, O: Ogcheon, P: Pyeongchang, S: Sunchang, W: Wando, Y: Yecheon, Ye: Yeongkwang.

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