Triassic geodynamics of the Korean Peninsula: Insights from petrogenesis of plutonic rocks in the Okcheon Belt and Gyeonggi Massif

Triassic magmatism on the Korean Peninsula occurred in response to pre- to post-collisional geodynamic events. The Triassic magmatism on the peninsula has been little studied in terms of its age and radiogenic isotopic compositions. We sampled Triassic igneous rocks in the Okcheon Belt and Gyeonggi Massif, and obtained new whole-rock geochemical and Sr–Nd–Pb isotopic, and zircon U-Pb age and Hf isotope data for the high-K calc-alkaline foliated Daegang and non-foliated Yeongjongdo granites. Zircon U-Pb ages reveal that plutonism occurred during 217–222 and 222–232 Ma in the Daegang and Yeongjongdo regions, respectively. The geochemical features of these granites are similar to those of the upper continental crust, but the former and latter granites are A₂- and A₁-subtype granites, respectively. The Daegang granites have ²⁰⁶Pb/²⁰⁴Pb_(t) = 17.31–17.98, ²⁰⁷Pb/²⁰⁴Pb_(t) = 15.56–15.65, and ²⁰⁸Pb/²⁰⁴Pb_(t) = 37.62–37.98, with εNd_(t) = −18.2 to −15.2. The Daegang zircons have εHf_(t) values of −10.8 to +12.1, with a mean of +1.1 ± 3.5 (1σ). The initial ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of the Yeongjongdo granites overlap those of the Daegang samples, and are 15.52–15.58 and 37.40–38.08, respectively, with ²⁰⁶Pb/²⁰⁴Pb_(t) = 16.68–17.68. The Yeongjongdo granites have negative whole-rock εNd_(t) (−17.5 to −15.7), and zircon εHf_(t) values (−20.7 to −2.8). These isotopic data and the high Nb, Rb, and Ta contents of the Yeongjongdo granites are inconsistent with a purely mantle source. We propose that reworking of OIB-like old continental crust, which had a source previously metasomatized by Nb-rich deep-seated melts, along with large degrees of crustal cannibalizing and/or flirting magma with stagnation, are the most plausible explanations for the A₁-type features of the Yeongjongdo granites, rather than extreme differentiation of mantle-derived basaltic magmas.