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

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, southeastern 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/⁸⁶Sr_initial = 0.7041 ∼ 0.7050, ε_Nd(t) = 2.3 ∼ 4.0, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 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/⁸⁶Sr_initial = 0.7047 ∼ 0.7065, ε_Nd(t) = 3.9 ∼ 2.8, ²⁰⁶Pb/²⁰⁴Pb_feldspar = 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 southwestern 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.