옥천변성대 충주지역 계명산층의 SHRIMP U-Pb 저어콘 연대측정

Abstract

충주 계명산층은 문주리층과 함께 옥천 변성대의 진화과정을 이해하는데 중요한 암체임에도 불구하고 정확한 생성시기를 규명하지 못하고 있다. 계명산층은 인접한 문주리층과 함께 A-1 형의 마그마에서 진화되었음이 보고된 바 있으며 (고상모 외, 2005), 문주리층의 변성화산암은 많은 학자들에 의해 U-Pb 연대가 약 750 Ma 로 보고된 바 있으나 (Lee et al., 1998, 조문섭 외, 2004) 계명산층은 Kim et al. (2006)이 저어콘 2개 입자의 분석을 통해 얻은 약 870 Ma의 U-Pb 저어콘 연령을 보고한 것 외에는 정확한 연대 측정이 이루어지지 못하였다. 이번 연구에서는 계명산층의 생성시기를 밝히기 위해 계명산층에서 암석을 채취하여 저어콘을 분리하였고 SHRIMP를 이용하여 총 297개 분석점에 대한 U-Pb 연대를 측정하였다. 그 결과 9개의 표품에서 각각 861.2 ± 3.0 Ma (n=10), 875 ± 2.2 Ma (n=19), 873.9 ± 2.8 Ma (n=10), 863.6 ± 2.6 Ma (n=24), 857.9 ± 2.5 Ma (n=13),이 865.8 ± 4.3 Ma (n=13), 857.0 ± 5.6 Ma (n=9), 863.0 ± 5.4 Ma (n=16)로 나타났다. 알칼리 화강암의 생성 시기는 약 855.7 ± 4.6 Ma (n= 15) 로 구할 수 있었다. 계명산층에서 분리한 저어콘들이 약 850-880 Ma의 범위에 집중적인 연대 분포를 보이는 것은 이 시기의 단속적인 화산활동으로 계명산층이 형성되었음을 나타낸다. 문주리층과 계명산층의 변성화산암들은 공통적으로 대륙내 열곡대 환경을 지시하는 A-1 형 마그마의 특성을 나타내지만 약 100 Ma 이상의 생성연대 차이를 보이기 때문에 두 지역은 별개의 화성활동에 의해 생성된 것임을 알 수 있다. 또한 overgrowth zone에서 Th/U 비가 0.1 이하인 것만을 선택적으로 SHRIMP U-Pb 연대측정 한 결과 265.5 ± 4.3 Ma (n=6) 으로 옥천변성대의 변성시기를 구할 수 있었으며 이는 이전 Cheong et al. (2003) 과 Kim et al. (2007) 이 구한 결과 보다 오차를 훨씬 줄인 결과이다. 9 위에서 구한 문주리층과 계명산층의 생성 시기는 전 세계적으로 초대륙 로디니아의 분열시기에 속하며 이웃 나라인 중국에서 로디니아 분열과 관련된 연구를 통해 남중국에서 약 830 Ma 가 우세하게 나타나는 것을 알 수 있다. 하지만 옥천변성대의 특징적인 A-1형이 나타나는 문주리층과 계명산층에서는 상당한 범위를 보이는 것으로 보아 로디니아 분열은 830 Ma 뿐만 아니라 빠르게는 880 Ma 부터 상당기간 지속되었을 것으로 판단된다.

The Okcheon Metamorphic Belt is important to understand the crustal evolution of the Korean peninsula. Notwithstanding decades of study, however, the age of Okcheon Metamorphic Belt has been the subject of much controversy for a long time but has not yet been clearly resolved, which has been an obstacle to further research. The Gyemyeongsan Formation shows very peculiar geochemical characteristics of A-1 type magmatism together with nearby Munjuri Formation that is also mainly composed of metavolcanic rocks (Koh et al., 2005; Kim et al., 2006). Considering the fact that the Munjuri Formation has been dated as approximately 760 Ma (Lee et al., 1998; Cho et al., 2004; Kim et al., 2006). On the contrary, there are some opposing evidences. Kim et al. (2006) was able to obtain SHRIMP U-Pb age of approximately 870 Ma from the Gyemyeongsan Formation, but the number of analyzed zircon grains is only two. In this study, I conducted geochronological studies to determine not only the precise eruption ages for the Gyemyeongsan metavolcanics but also their timing of metamorphism utilizing SHRIMP U-Pb zircon age determinations and discussed the implications of the results for the evolution of the Okcheon Metamorphic Belt. SHRIMP U-Pb isotope analysis was carried out for zircons separated from 9 rocks including a sample of alkali granite (GMS 3) and number of analyzed points are 297 in total. The Concordia ages determined from the metavolcanics are 861.2 ± 3.0 Ma (n=10), 875 ± 2.2 Ma (n=19), 873.9 ± 2.8 Ma (n=10), 863.6 ± 2.6 Ma (n=24), 857.9 ± 2.5 Ma (n=13), 865.8 ± 4.3 Ma (n=13), 857.0 ± 5.6 Ma (n=9), 863.0 ± 5.4 Ma (n=16). And one alkali granite sample yield a concordia age of 855.7 ± 4.6 Ma (n= 15). The magma from which the Gyemyeongsan Formation was formed at approximately 875 Ma to 855 Ma ago seems to have been sustained up to 20 million years. The ages of the metavolcanic rocks of Gyemyeongsan Formation are contrast to the metavolcanic rocks of Munjuri Formation having ca. 760 Ma (Lee et al., 1998; Cho et al., 2004; Kim et al., 2006). Although these two metavolcanic formations share very peculiar characteristics of type A-1 only found from the continental-scale rift environments (Koh et al., 2005; Kim et al., 2006), their igneous activities seem to have occurred at different times. Also, I could successfully obtain the age of metamorphism approximately 265 Ma from several zircon grains despite the fact that the widths of the metamorphic overgrowth rims are generally very narrow for the SHRIMP U-Pb analysis. The timing of metamorphism obtained in this study is in accord with the previously reported metamorphic ages by Cheong et al. (2003) and Kim et al. (2007) suggesting that the regional metamorphism of the Okcheon Metamorphic Belt occurred at contemporaneously. The Gyemyeongsan Formation and the Munjuri Formation were generated during the period of supercontinent Rodinia breakup into several pieces. In case of neighboring China, igneous activities of similar periods have been reported to be associated with the break-up of Rodinia. Even though ages of igneous activities from approximately 850 Ma to 750 Ma have been reported, it is common to consider that the break-up of Rodinia started at approximately 820-830 Ma (Li et al., 2010; Wang et al., 2010; Shu et al., 2011). This study reveals that the Gyemyeongsan Formation was formed from the volcanic rocks generated during the period of 875-855 Ma and also that the Gyemyeongsan Formation has typical A-1 type magma characteristics indicating its origin related with very deep continental rift formation. Such findings naturally support an idea that the volcanic protoliths of the Gyemyeongsan Formation were produced during the break-up of Rodinia supercontinent.