The western Korean Peninsula preserves evidence for multiple Proterozoic to Phanerozoic orogenic processes along the convergent continental margins. After the Late Palaeozoic to Early Mesozoic collisional stage within the long-lived tectonic Wilson cycles, the area underwent post-collisional extensional tectonics, creating the Late Triassic to Early Jurassic basins on top of the collisional orogenic belt. Here, we present the LA-MC-ICP-MS U-Pb ages of detrital zircons and bulk-rock geochemical data from post-collisional basin fills (viz., the Nampo and Gimpo groups and the Seokmun Formation) and the overlying volcano-sedimentary sequence (Oseosan Volcanic Complex). Our results suggest that (1) the Nampo Group was supplied with detritus mainly from nearby felsic igneous rocks or geochemically equivalent migmatitic basement gneisses, while the Gimpo Group and Seokmun Formation were predominantly sourced from (meta-)sedimentary rocks, likely reflecting their structural positions in the orogenic belt in terms of the exhumed basement-dominant hinterland side vs. the cover rock-dominant foreland side of the mountain belt; (2) the ages of the youngest zircon group in the Nampo Group and the overlying Oseosan Volcanic Complex (ca. 1854 age peak in the lowermost unit and ca. 164 Ma age peak in the uppermost unit) tend to approach the sedimentation ages with ascending stratigraphic order, indicating a transition from a collisional to a convergent tectonic setting during sedimentation. The results from this study provides important insights into the sedimentary response to the evolution of the collisional orogenic belt along the western Korean Peninsula and elucidates the Mesozoic tectonic transition from collisional (closure of the Paleo-Tethys Ocean) to convergent (subduction of the Paleo-Pacific Ocean) processes in the Far East Asian continental margin.

In response to orogenic cycles, the ductile shear zone records a complex crustal deformation history. In this study, we conducted a microstructural analysis of two NW–SE trending ductile shear zones (Deokjeok Shear Zone (DSZ) and Soya Shear Zone (SSZ)) in the Late Triassic post-collisional granites along the western Gyeonggi Massif in the Korean Peninsula. The DSZ, overlain by the Late Triassic to the Early Jurassic post-collisional basin fill (Deokjeok Formation), has asymmetric microstructures indicative of a top-down-to-the-northeast shear. Depending on the structural position, the SSZ, which structurally overlies the Deokjeok Formation, exhibits two contrasting styles of deformation. The lower portion of the SSZ preserves evidence of top-up-to-the-southwest shearing after top-down-to-the-northeast shearing; on the other hand, the upper portion only indicates a top-up movement. Given the primary deformation mechanisms of both quartz and feldspar, the deformation temperatures of DSZ and SSZ were estimated at ~300–350 °C and ~350–400 °C, respectively, indicative of the mid-crustal condition. New zircon U-Pb isotopic ages from mylonitic granite in the SSZ and volcanic rocks in the Deokjeok Formation, combined with previously published geochronological data, indicate that the post-collisional granites and volcano-sedimentary sequence were nearly contemporaneous (ca. 223–217 Ma) and juxtaposed because of the Late Triassic orogenic collapse and subsequent new orogenic event. In this study, we highlight the role of the extensional DSZ as a detachment propagated into the middle crust during the Late Triassic orogenic collapse. Our results report a deformational response to a transition from the collisional Songrim Orogeny to the subduction-related Daebo Orogeny in the western Gyeonggi Massif. This, in turn, provides essential insight into cyclic mountain building/collapse in the East Asian continental margin during the Mesozoic time.

In this contribution, we report the metasomatic characteristics of a lamprophyre dyke–marble contact zone from the Hongseong–Imjingang belt along the western Gyeonggi Massif, South Korea. The lamprophyre dyke intruded into the dolomitic marble, forming a serpentinized contact zone. The zone consists of olivine, serpentine, calcite, dolomite, biotite, spinel, and hematite. Minor F and Cl contents in the serpentine and biotite indicate the composition of the infiltrating H₂O-CO₂ fluid. SiO₂ (12.42 wt %), FeO (1.83 wt %), K₂O (0.03 wt %), Sr (89 ppm), U (0.7 ppm), Th (1.44 ppm), and rare earth elements (REEs) are highly mobile, while Zr, Cr, and Ba are moderately mobile in the fluid. Phase equilibria modelling suggests that the olivine, spinel, biotite, and calcite assemblage might be formed by the dissolution of dolomite at ~700 °C, 130 MPa. Such modelling requires stable diopside in the observed conditions in the presence of silica-saturated fluid. The lack of diopside in the metasomatized region is due to the high K activity of the fluid. Our log activity K₂O (a_K2O)–temperature pseudosection shows that at a_K2O~−40, the olivine, spinel, biotite, and calcite assemblage is stable without diopside. Subsequently, at ~450 °C, 130 MPa, serpentine is formed due to the infiltration of H₂O during the cooling of the lamprophyre dyke. This suggests that hot H₂O-CO₂ fluids with dissolved major and trace elements infiltrated through fractures, grain boundaries, and micron-scale porosity, which dissolved dolomite in the marble and precipitated the observed olivine-bearing peak metasomatic assemblage. During cooling, exsolved CO₂ could increase the water activity to stabilize the serpentine. Our example implies that dissolution-reprecipitation is an important process, locally and regionally, that could impart important textural and geochemical variations in metasomatized rocks.