Jangbong Island is located in the coastal area of the northwestern Gyeonggi Massif. The basement of Jangbong Island is covered by Neoproterozoic metasedimentary rocks such as pelitic schist, quartz schist, and calc schist. These rocks were intruded by mafic dikes that were later metamorphosed into amphibolites. From pelitic schist and amphibolite, two different metamorphisms and a clockwise P-T path are confirmed: the first metamorphism was an intermediate-P/T type peak metamorphism (680-630 ℃/8.6-7.3 kbar), and the second was a low-P/T type metamorphism (600-560 ℃/5.7-3.1 kbar). All rocks on Jangbong Island and in the northern Gyeonggi Massif were intruded by Triassic igneous rocks including gabbro. Age dating using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields intrusion ages of 917-873 Ma for amphibolites. Age of 255-254 Ma and 231-230 Ma are also obtained for the first and second metamorphic episodes, respectively. The Triassic gabbro yields an intrusion age of 229 Ma. The geochemical analysis confirms that Neoproterozoic amphibolites formed in a within-plate tectonic setting and that the Triassic igneous rocks formed in a post-collisional tectonic setting. The 917-873 Ma rift-related mafic dikes are well correlated with the Neoproterozoic rift-related mafic dikes in the northern Gyeonggi Massif and along the southeastern margin of the North China Craton (NCC). The 255-254 Ma intermediate-P/T type peak metamorphism is well matched with the continental collision event within the Gyeonggi Massif, and the 231-230 Ma low-P/T type metamorphism and 229 Ma post-collisional igneous activity are well related with post-collisional events in the northern Gyeonggi Massif. Together with previous data, this study supports the correlation between the northern Gyeonggi Massif, NCC and Permo-Triassic collision within the Gyeonggi Massif. During the Permo-Triassic, post-collision igneous activities occurred in the northern Gyeonggi Massif, whereas on the southern margin of the Korean Peninsula, including the Yeongnam Massif, subduction-related granitoids intruded at 283-222 Ma and were well correlated with subduction-related igneous rocks (265-207 Ma) on the southern margin of the South China Craton (SCC). This information suggests that subduction along the southern margin of the Korean Peninsula started due to continental collision between the NCC and SCC in the Gyeonggi Massif.