Geochemistry, zircon U[sbnd]Pb ages and Lu[sbnd]Hf isotopes of Triassic plutons in the eastern Gyeonggi Massif, Korean Peninsula: Magma genesis and geodynamic implications for East Asia

Abstract

The spatial-temporal distribution of the Triassic post-collision-related and arc-related plutons in the Korean Peninsula is key to unraveling the positions of the Triassic collisional boundary and subduction zones in East Asia. Zircon U[sbnd]Pb dating of the Hajodae Granite, part of the granite-syenite-gabbro complex in the Yangyang area of the northeastern Gyeonggi Massif, gives a weighted mean 206Pb/238U age of 228.0 ± 2.3 Ma as the time of magmatism with a low zircon crystallization temperature of ∼770 °C. The Hajodae Granite and two-mica granites from the same complex belong to the shoshonitic to high-K and ferroan series and show high Rb/Sr and Rb/Ba ratios, suggesting melting of upper crustal materials and subsequent fractional crystallization. They plot in the post-collisional tectonic setting fields in trace element discrimination diagrams. Monzogabbros from the same complex are characterized by enrichment in LREEs and depletion in HREEs similar to those of OIB, with inherited subduction enrichments, suggesting their post-collisional tectonic setting. The southern limit of the post-collision-related Triassic plutons (∼235–225 Ma) in the Korean Peninsula marks a clear line defined as the Hongseong-Yangpyeong-Odesan (HYO) Belt within the Gyeonggi Massif, implying that the belt is the collisional boundary between the North China and South China Cratons. The Namhangjin Diorite in the southeastern Gyeonggi Massif yields weighted average zircon 206Pb/238U ages of 224.9 ± 2.0 Ma and 220.2 ± 2.1 Ma as the time of magmatism. They are characterized by low Y contents (9–12 ppm) and high Sr/Y ratios (91–135), indicating their adakitic nature. They are highly depleted in HREEs with convex-downward patterns between MREEs and HREEs in the C1 chondrite-normalized diagram, suggesting the presence of residual garnet and hornblende. They are characterized by high contents of compatible elements, including Cr (150–290 ppm), Ni (50–70 ppm), and MgO (3.47–5.24 wt%), have high Mg# of 60.4–63.4, and show higher zircon crystallization temperatures of ∼820–850 °C, suggesting that the Namhangjin Diorite was generated by the partial melting of delaminated lower crust due to extension related to slab roll-back of the subduction zone along the margin of northeastern Asia. Together with previous data, this study indicates that the Triassic igneous activity in the northern Gyeonggi Massif occurred in a post-collisional tectonic setting, whereas that in the southern Gyeonggi Massif, Ogcheon Belt, and Yeongnam Massif occurred in a subduction- or slab roll-back-related tectonic setting.