The Journal of the Petrological Society of Korea (암석학회지)

The Petrological Society of Korea (한국암석학회)
권호 표지

Origin and Evolution of Leucogranite of NE Yeongnam Massif from Samcheok Area, Korea

삼척지역 북동 영남 육괴에 분포하는 우백질 화강암의 기원 및 진화

  • Cheong, Won-Seok (Department of Earth & Environmental Sciences, College of natural Science, Chungbuk National University) ;
  • Na, Ki-Chang (Department of Earth & Environmental Sciences, College of natural Science, Chungbuk National University)
  • 정원석 (충북대학교 자연과학대학 지구환경과학과) ;
  • 나기창 (충북대학교 자연과학대학 지구환경과학과)
  • Published : 2008.03.30
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Abstract

We study metamorphism of metasedimetary rocks and origin and evolution of leucogranite form Samcheok area, northeastern Yeongnam massif, South Korea. Metamorphic rocks in this area are composed of metasedimentary migmatite, biotite granitic gneiss and leucogranite. Metasedimentary rocks, which refer to major element feature of siliclastic sediment, are divided into two metamorphic zones based on mineral assemblages, garnet and sillimanite zones. According to petrogenetic grid of mineral assemblages, metamorhpic P-T conditions are $740{\sim}800^{\circ}C$ at $4.8{\sim}5.8\;kbar$ in the garnet zone and $640-760^{\circ}C$ at 2.5-4.5kbar in sillimanite zone. The leucogranite (Imwon leucogranite) is peraluminous granite which has high alumina index (A/CNK=1.31-1.93) and positive discriminant factor value (DF > 0). Thus, leucogranite is S-type granite generated from metasedimentary rocks. Major and trace element diagram ($R_1-R_2$ diagram and Rb vs. Y+Nb etc.) show collisional environment such as syn-collisional or volcanic arc granite. Because Rb/sr ratio (1.8-22.9) of leucogranites is higher than Sr/Ba ratio (0.21-0.79), leucogranite would be derived from muscovite dehydrate melting in metasedimentary rocks. Leucogranites have lower concentration of LREE and Eu and similar that of HREE relative to metasedimentary rocks. To examine difference of REEs between leucogranites and metasedimentary rocks, we perform modeling using volume percentage of a leucogranite and a metasedimenatry rock from study area and REE data of minerals from rhyolite (Nash and Crecraft, 1985) and melanosome of migmatite (Bea et al., 1994). Resultants of modeling indicate that LREE and HREE are controlled by monazites and garnet, respectively, although zircon is estimated HREE dominant in some leucogranite without garnet. Because there are many inclusions of accessary phases such as monazite and zircon in biotites from metasedimentary rocks. leucogranitic magma was mainly derived from muscovite-breakdown in metasedimenary rocks. Leucogranites can be subdivided into two types in compliance with Eu anomaly of chondrite nomalized REE pattern; the one of negative Eu anomaly is type I and the other is type II. Leucogranites have lower Eu concetnrations than that of metasedimenary rocks and similar that of both type. REE modeling suggest that this difference of Eu value is due to that of components of feldspars in both leucogranite and metasedimentary rock. The tendency of major ($K_2O$ and $Na_2O$) and face elements (Eu, Rb, Sr and Ba) of leucogranites also indicate that source magma of these two types was developed by anatexis experienced strong fractionation of alkali-feldspar. Conclusionally, leucogranites in this area are products of melts which was generated by muscovite-breakdown of metasedimenary rock in environment of continetal collision during high temperature/pressure metamorphism and then was fractionated and crystallized after extraction from source rock.

삼척 원덕읍에 분포하는 영남육괴 변성퇴적암류에 대한 변성작용을 판단하고 이에 따른 우백질 화강암의 기원과 진화과정을 규명하였다. 변성퇴적암류는 광물 조합에 따라 크게 석류석대와 규선석대로 나눌 수 있다. 규산질 퇴적암의 특징을 나타내는 변성퇴적암류는 암석성인격자를 바탕으로 석류석대는 $4.8{\sim}5.8\;kbar$, $740{\sim}800^{\circ}C$, 규선석대는 2.5-4.5 kbar, $640-760^{\circ}C$의 변성작용을 받았다. 이 지역에 분포하는 우백질 화강편마암류(임원 우백질화강암)는 A/CNK=1.31-1.93이고 DF(discriminant factor)>0인 과알루미늄질 화강암이다. 따라서 이는 S-type의 화강암류에 속하며 이의 기원은 주변의 변성퇴적암류이다. 주원소 및 미량원소 성분들은 우백질 화강암이 충돌대 또는 화산호 화강암 같은 대륙의 충돌 환경과 관련성을 나타낸다 우백질 화강암의 Rb/Sr의 비율(1.8-22.9)은 Sr/Ba 비율(0.21-0.79)에 비해 크기 때문에 백운모의 탈수 용융작용으로 우백질 마그마가 형성되었다. 우백질 화강암의 REE 함량은 전반적으로 변성퇴적암류보다 낮은 LREE 함량과 비슷한 HREE 함량을 갖는다. 이러한 형성 과정을 확인하기 위해 일부 변성퇴적암 및 우백질화강암 시료의 광물 함량비율과 기존 연구의 유문암 및 미그마타이트에 들어 있는 광물의 REE 함량을 이용하여 모델링을 수행했다. 이에 따르면 일부 우백질 화강암의 HREE를 저어콘이 조절했을 가능성도 보여주나, 대부분의 우백질 화강암의 LREE 조절자는 모나자이트이고 HREE 조절자는 석류석으로 판단된다 변성퇴적암에서 부수광물들 모나자이트 및 저어콘 같은 부수광물들은 주로 흑운모의 포유물로 확인되기 때문에 변성퇴적암으로부터 형성된 우백질 마그마는 주로 백운모의 붕괴 작용으로 형성된 것이다. 콘드라이트로 표준화한 REE 패턴에서 우백질 화강암은 음의 Eu 이상치를 갖는 것(Type I)과 양의 이상치를 갖는 것(Type II)로 구분할 수 있다. 우백질 화강암은 변성퇴적암류에 비해 낮은 Eu 함량을 갖으며 REE 형태와 관계없이 비슷한 Eu 함량을 갖는다. 이는 REE 모델링에서 변성퇴적암과 우백질 화강암의 장석 성분과 관련이 깊은 것으로 나타난다. 또한 주원소 ($K_2O$ and $Na_2O$) 및 미량원소(Eu, Rb, Sr, Ba) 역시 강한 알칼리 장석의 분화작용을 지시한다. 결론적으로 본 연구지역에 분포하는 우백질 화강암은 대륙충돌 환경에서 변성퇴적암류가 고온변성작용 중에 발생한 백운모 탈수 용융작용으로 발생된 용융체가 이후 분화과정을 겪어 산출된 것으로 판단된다.

Keywords

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