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.

The Gyeonggi Massif is one of the major Precambrian basements in the Korean Peninsula where Paleoproterozoic orthogneiss and paragneiss were widely exposed. In this study, newly identified Rhyacian to Orosirian meta‐igneous rocks from the northwestern Gyeonggi Massif (Soyeonpyeongdo and Ganghwado areas) were investigated to elucidate their petrogenesis and tectonic implications based on zircon U–Pb–Hf isotopic data and whole‐rock geochemistry. The garnet‐bearing amphibole granite gneiss and biotite granite gneiss in the Soyeonpyeongdo area emplaced at ca. 2.06 Ga and 1.88 Ga, respectively, whereas the tonalite gneiss in the Ganghwado area formed at ca. 1.93 Ga. The 2.06 Ga garnet‐amphibole granite gneiss shows both I‐type and A ₂ ‐type features enriched in LILE and depleted in Nb–Ta, indicating generation in an arc‐related extensional regime. The 1.93 Ga tonalitic gneiss exhibits adakitic characteristics, negative εHf(t) values (−8.2 to −4.9), and thickened crustal signatures (42–74 km), which are consistent with derivation from Archean mafic lower crust thickened during a regional syn‐collisional stage. The 1.88 Ga biotite granite gneiss displays metaluminous to weakly peraluminous I‐ and A2‐type features with slightly negative εHf(t) values (−1.7 to +0.6), implying formation in a post‐collisional extensional setting. These successive magmatic events (2.06–1.85 Ga) in the Gyeonggi Massif are temporally and geochemically comparable to the Rhyacian–Orosirian magmatism in the Liao–Ji Belt of the eastern North China Craton. Therefore, the northwestern Gyeonggi Massif recorded a Paleoproterozoic tectono‐magmatic evolution broadly comparable to that of the Liao–Ji Belt.