단양 지질공원 구담봉-사인암 지질명소 화강암의 SHRIMP 저어콘 U-Pb 연령과 지구화학

SHRIMP Zircon U-Pb Age and Geochemistry of Granites in the Gudambong-Sainam Geosites, Danyang Geopark

Abstract

단양 지질공원 지질명소 구담봉과 사인암을 구성하고 있는 화강암에 대한 저어콘 U-Pb 연령측정과 전암 지구화학 성분 분석을 수행하였다. 이들 화강암은 단양군의 서부와 남부지역에서 산출하며, 영월층군과 태백층군의 퇴적암을 각각 관입하고 있다. 고분해능 이차이온질량분석기를 사용하여 측정한 구담봉과 사인암화강암의 저어콘 U-Pb 동위원소 성분은 각각 $90.4{\pm}0.5Ma(t{\sigma})$와 $90.0{\pm}1.5Ma(t{\sigma})$ 백악기 관입 연령을 정의한다. 두 화강암 시료의 지구화학 성분 분석 결과는 전형적인 A-타입 화강암의 특성을 나타내며, 불국사 조산운동 후기 또는 비조산대에서 형성된 것으로 해석된다. 구담봉과 사임암 화강암 시료의 저어콘 연령과 지구화학적 특성은 기존 보고된 백악기 무암사 및 월악산 화강암체의 결과와 일치한다.

We carried out the sensitive high resolution ion microprobe zircon U-Pb age dating and whole rock geochemical analysis of granites in the Gudambong and Sainam geosites, Danyang Geopark. The granites crop out in the western and southern parts of Danyang County, and intruded sedimentary successions of the Yeongweol and Taebaek Groups, respectively. The U-Pb isotopic compositions of zircon from the Gudambong and Sainam granite samples yielded the Cretaceous intrusion ages of $90.4{\pm}0.5Ma(t{\sigma})$ and $90.0{\pm}1.5Ma(t{\sigma})$, respectively. The major and trace elements compositions of the samples showed an affinity of typical A-type granite, indicating their petrogenesis during the late stage of the Bulguksa orogeny or a tectonic dormancy. The geochronologic and geochemical results are identical to those of granites previously reported from the Cretaceous Muamsa and Wolaksan suites.

Fig. 3. Cathodoluminescence images of zircon from (a) Gudambong and (b) Sainam. Spot numbers and apparent ²⁰⁶Pb/²³⁸U ages in Table 2 are shown in each image. Ellipse denotes the analytical spot.

Fig. 4. Tera-Wasserburg plots and weighted mean ²⁰⁶Pb/²³⁸U ages of (a, b) Gudambong and (c, d) Sainam sample, respectively. Error ellipses of data points in Tera-Wasserburg plots are at 95% confidence level.

Fig. 7. Tectonic discrimination diagrams based on (a) Rb vs. Y+Nb (b) Nb vs. Y, (c) Rb vs. Yb+Ta, and (d) Ta vs. Yb diagrams (Pearce et al., 1984) together with compiled data of the Muamsa, Weolaksan and Sokrisan granites (Lee et al., 2010).

Fig. 1. (a) Tectonic map of the south of Korea showing the distribution of granites. (b) Geologic map of the Danyang area with sample locations modified from Won and Lee (1967).

Fig. 2. (a, b) Landscape photographs of Gudambong and Sainam. (c, d) Thick section and outcrop photogrphs of the Gudambong and Sainam granite samples, respectively. (e, f) Photomicrographs of the Gudambong and Sainam samples under cross-polarized light, respectively.

Fig. 5. (a) TAS diagram (Irvine and Baragar, 1971; Wilson, 1989). (b) A/NK [molar ratio Al₂O₃/(Na₂O+K₂O)] vs. A/CNK [molar ratio Al₂O₃/(CaO+Na₂O+K₂O)] diagram (Maniar and Piccoli, 1989). (c) Chondrite-normalized rare earth element (REE) variation diagram (Sun and McDonough, 1989). (d) Primitive mantle-normailized spider diagram (McDonough and Sun, 1995). Grey areas in each diagram represent compilation of geochemical data from the Muamsa, Weolaksan and Sokrisan granites (Lee et al., 2010).

Fig. 6. (a) R1-R2 diagram (dashed lines; Batchelor and Bowden, 1985) and rock classification diagram (solid lines; De la Roche et al., 1980). (b) SiO₂ vs. FeO_t/MgO diagram (after Whalen et al.,1987; Wang et al., 2004). (c) 10000xGa/Al vs. FeO_t plots of A-type granite (Whalen et al., 1987), (d) Zr+Nb+Ce+Y vs. (K₂O+Na₂O)/CaO of A-type granite (Whalen et al., 1987). Abbreviations: FG, fractionated felsic granite; and OGT, unfractionated M-, I-, S-type granites.

Table 2. Major and trace element compositions of the Gudambong and Sainam granite samples

Table 1. U–Th-Pb isotope compositions of zircons

Table 1. Continued