영남육괴 남서부 산청 동부지역에 분포하는 트라이아스기 변형 화강암의 U-Pb 연대측정과 그 함의

U-Pb Geochronology of the Triassic Foliated Granite Distributed in the Eastern Sancheong Area, SW Yeongnam Massif, Korea and its Implications

Abstract

영남육괴 남서부인 지리산 지역의 북동부에 위치한 산청지역에 분포하는 변형 화강암에 대해 SHRIMP 저어콘 U-Pb 연대측정을 수행하였다. 이 암체는 선캠브리아기 또는 시대미상의 고기 화성암류로 알려져 있었으나 2개의 시료에서 구한 U-Pb 일치연령은 각각 $237.8{\pm}4.0Ma$와 $230.2{\pm}3.4Ma$로 초기-중기 트라이아스기에 정치되었음을 보여준다. 이러한 결과는 이 변형 화강암이 약 238~230 Ma에 정치되었음을 지시한다. 연구 지역 변형 화강암은 현저한 엽리와 안구상 구조, 선구조들이 발달된 전단변형의 특성을 보인다. 변형 화강암이 섬장암에 의해 관입된 포획암체로 산출되고 있음이 관찰되며, 이는 변형시기가 섬장암의 관입시기인 약 220 Ma보다 이전임을 지시해준다. 이 지역 변형 화강암의 정치 및 변형 시기는 중부 영남육괴인 김천과 안동지역에 분포된 페름기-트라이아스기의 화강편마암류와 유사하다. 종합적으로 살펴볼 때 중부에서 남서부에 걸치는 영남육괴의 동쪽에는 화강암류를 관입시킨 화성활동이 약 260-230 Ma에 그리고 뒤 이은 변성-변형 작용이 약 230-220 Ma에 일어났다.

In this study, SHRIMP zircon U-Pb dating was performed on deformed granitic rocks in the Sancheong area in the northeastern part of the Jirisan area, southwest of Yeongnam Massif. Until now, these have been known as Precambrian or age-unknown old igneous rocks, but the U-Pb concordant ages obtained from two samples are $237.8{\pm}4.0Ma$ and $230.2{\pm}3.4Ma$, respectively, showing their emplacements in Early to Middle Triassic. These results indicate that the deformed granite was emplaced at about 238~230 Ma. The study area shows the characteristics of ductile deformation with prominent development of foliation, augen structure, and lineation. It is observed that the deformed granites occur as xenoliths within the syenite, indicating that the time of deformation is earlier than the intrusion of the syenite of about 220 Ma. The emplacement and deformation periods of the deformed granite is similar to that of Permo-Triassic granite gneisses distributed in the Gimcheon and Andong areas of the Yeongnam Massif. Taken together, the eastern part of the Yeongnam Massif, extending from the central part to the southwestern part, granite intrusions occurred at about 260-230 Ma, followed by metamorphism-deformation of about 230-220 Ma.

Fig. 1. Geological map of the Sancheong area, SW Yeongnam massif, Korea (modified after Kim et al., 1964 and Kang and Park, 1975) showing locations of the analyzed samples. The upper left inset shows the location of the study area in the southern Korean Peninsula. IB, Imjingang belt; GM, Gyeonggi massif; OB, Okcheon belt; TB, Taebaeksan basin; YM, Yeongnam massif; GB, Gyeongsang basin.

Fig. 2. Photographs of the deformed granite from the Sancheong area. (a) a slab of the sample 2010mt-19a showing augen structure, (b) an outcrop of the deformed granite intruded by a later intrusive rock, (c) a slab of the sample 2010mt-17b with mylonitic texture, and (d) a slab of the sample 2010mt-18c with deformed pegmatitic layers.

Fig. 3. Representative cathodoluminescence (CL) images for zircon grains from the deformed granite showing locations of analysed spots and their apparent ages in Ma. Spots are 25 μm in diameter. Scale bars are 100 μm in length.

Fig. 4. U-Pb concordia diagrams for the zircons separated from the deformed granite samples. a) sample 2010mt-17b, b) sample 2019mt-18a, and c) sample 2019mt-19c. Error ellipses are at 1σ level and the calculated ages are at 95% confidence.

Table 1. U-Pb zircon isotopic data for the sample 2010mt-17b from the deformed granite from the Sancheong area, the SW Yeongnam massif.

Table 2. U-Pb zircon isotopic data for the sample 2010mt-18c from the deformed granite from the Sancheong area, the SW Yeongnam massif.

Table 3. U-Pb zircon isotopic data for the sample 2010mt-19a from the deformed granite from the Sancheong area, the SW Yeongnam massif.