Geochemical and Sr–Nd–Pb isotopic investigation of Triassic granitoids and basement rocks in the northern Gyeongsang Basin, Korea: Implications for the young basement in the East Asian continental margin

Abstract

We present chemical and Sr–Nd–Pb isotopic compositions of three Triassic (226–241 Ma) calc‐alkaline granitoids (the Yeongdeok granite, Yeonghae diorite and Cheongsong granodiorite) and basement rocks in the northern Gyeongsang basin, south‐eastern Korea. These plutons exhibit typical geochemical characteristics of I‐type granitoids generated in a continental magmatic arc. The Yeongdeok and Yeonghae plutons have similar initial Sr, Nd and Pb isotope ratios (87Sr/86Srinitial = 0.7041 ~ 0.7050, ɛNd(t) = 2.3 ~ 4.0, 206Pb/204Pbfeldspar = 18.22 ~ 18.34), but distinct rare earth element patterns, suggesting that the two plutons formed from partial melting of a similar source material at different depths. The Cheongsong pluton has slightly more enriched Sr–Nd–Pb isotopic compositions (87Sr/86Srinitial = 0.7047 ~ 0.7065, ɛNd(t) = 3.9 ~ 2.8, 206Pb/204Pbfeldspar = 18.24 ~ 18.37) than the other two plutons. The Nd model ages of the basement rocks (1.1 ~ 1.4 Ga) are slightly older than those of the plutons (0.6 ~ 1.0 Ga). The initial Sr and Nd isotopic ratios of the plutons can be modeled by the mixing between the mid‐oceanic ridge basalt‐like depleted mantle component and the crustal component represented by basement rocks, which is also supported by Pb isotope data. The Sr and Nd isotope data from granitoids and basement rocks suggest that the Gyeongsang basin, the Hida belt and the inner zone of south‐western Japan share relatively young basement histories (middle Proterozoic), compared with those (early Proterozoic to Archean) of the Gyeonggi and Yeongnam massifs and the Okcheon belt. The Nd isotope data of basement rocks suggest that the Hida belt might be better correlated with the basement of the Gyeongsang basin than the Gyeonggi massif, the Okcheon belt or the Yeongnam massif, although it may represent an older continental margin of East Asia than the Gyeongsang basin considering its slightly older Nd model ages.

Petrological Characteristics of the Satkatbong Pluton, Yeongdeok, Korea

“…It had been reported that the age of the Satkatbong pluton is 70~75 Ma through the fission track method, then reported as 118.4±4.0 Ma with K-Ar chronological analysis (Kim et al, 1999). On the other hand, the age for the Satkatbong pluton when described as the Yeonghae diorite was reported from 178.0±4.3 Ma by the K-Ar method (Kim, 1997) to 238±14 Ma by Nd-Sm isochron (Cheong et al, 2002), which is actually included in the Jurassic. Actual field geology of the Satkatbong pluton, however, shows that it resembles the Yeonghae diorite rather than the Jangsari granite, so the age representing the Jurassic is preferred.…”

“…1. Simplified geological map of the study area in Yeongdeok (modified after Hatae, 1937;Kim 1968;Chang, 1990;Cheong et al, 2002). Sampling site is presented in appendix.…”

“…Geochemical data is compared with other plutons existing in south Korea for detailed determination of geochemistry. The Yeongdeok granite (Cheong et al, 2002), near the Satkatbong pluton, Daebo and Bulguksa granitoid suits in south Korea (Kim, 1992) are used in comparison. Former data of 2 samples of the Satkatbong pluton is also plotted on following graphs (Cheong et al, 2002).…”

“…In addition, a wide range of research has been conducted on these plutons such as stratigraphy, geochronology (Hatae, 1937;Jin et al, 1984;Kim, 1971;Kim, 1988;Kim et al, 1999;Cheong et al, 2002;Yi et al, 2012), geochemistry, mineralogy including isotopic chemistry (Kim, 1992;Jwa, 1996;Cheong et al, 1998Cheong et al, , 2002Yi et al, 2012), and tectonic interpretative research (Cheong et al, 1998(Cheong et al, , 2002Kim et al, 1999). Among them, Yi et al (2012) was the first to name this pluton, the Satkatbong pluton, separating it from the Yeonghae diorite.…”

Petrological characteristics of the Yeongdeok granite

“…연구지역에서는 연대측정 (Kim et al, 1999, Yi et al, 2012a, 지화학 분석 (Cho and Kwon, 1994;Cheong et al, 2002), 화강암의 분류 (Jwa et al, 2005), 흑운모의 진화경로 (Lee and Kim, 1990) 및 심성암체내 알칼리장석의 상전이 루어져 있으며, 연구지역은 영덕 화강암이라 불리는 화강암질암류이다 (Fig. 1) (Kim, 1971;Won et al, 1978)으로 의견이 분분하 였으나 이후 절대연령 측정 연구 (Cheong et al, 2002;Sagong et al, 2005, Yi et al, 2012a (Fig.…”