The Neoproterozoic to Triassic tectonic evolution of Jangbong Island in the northwestern Gyeonggi Massif on the Korean Peninsula

Abstract

Jangbong Island is located in the coastal area of the northwestern Gyeonggi Massif, and its basement is composed of Paleoproterozoic gneiss. The basement is covered by Neoproterozoic metasedimentary rocks such as pelitic schist, calc-schist, and quartz schist. These rocks are intruded by mafic dikes metamorphosed under amphibolite facies conditions. In the study area, two different metamorphic conditions are confirmed: the first was intermediate-P/T peak metamorphism (680-630 °C/8.6-7.3 kbar), and the second was low-P/T metamorphism (600-560 °C/5.7-3.1 kbar) showing a clockwise P-T path. All rocks in the studied area are intruded by Triassic gabbro and granite. Age measurement using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields intrusion ages of 917 ± 33 Ma and 873 ± 32 Ma for metabasites. Two different metamorphic ages are also obtained from them: 255 ± 12 ~ 254.3 ± 1.3 Ma and 231.5 ± 1.9 ~ 229.8 ± 1.4 Ma for the first and second metamorphic episodes, respectively. The Triassic gabbro yields an intrusion age of 229.1 ± 0.57 Ma and formed in a post-collisional tectonic setting with Triassic granite. The geochemical analysis confirms that Jangbong Island metabasites formed in a within-plate tectonic setting and are all alkaline basalts and mostly tholeiitic. The 917-873 Ma rift-related mafic intrusion is well matched with the Neoproterozoic rift-related mafic intrusions in the northern Gyeonggi Massif and along the southeastern margin of the North China Craton (NCC). The 255-254 Ma intermediate-P/T peak metamorphism is well matched with the collisional event in the northern Gyeonggi Massif, and the 231-230 Ma low-P/T metamorphism and 229 Ma post-collisional igneous activity are well correlated with post-collisional events in the northern Gyeonggi Massif. Together with previous data, these data support the correlation between the northern Gyeonggi Massif and the NCC.