Middle Triassic Post-collisional Granitoid in the Western Gyeonggi Massif, Korea: Crustal Melting during the Permo-Triassic Orogenic Collapsing

This study presents field, petrology, geochemistry, and geochronology data of granitoid intrusions in the Gukhwado area, a coastal island in the western Korean Peninsula, to better understand their geologic and tectonic significance. These granitoids intruded into the complexly folded psammitic and calcareous metasedimentary rocks, which were deposited during the Middle Paleozoic and metamorphosed during the Permo-Triassic crustal thickening of the Western Gyeonggi Massif. The granitoids, rich in quartz, K-feldspar, muscovite, and biotite, show variation in clinopyroxene and amphibole content, categorizing them as intermediate to felsic in composition. Geochemically, they exhibit a strongly peraluminous composition, with Al2O3, (Na2O+K2O), and CaO ranging from 9-20 wt.%, 3-6 wt.%, and 0.02-0.71 wt.%, respectively. Their trace and rare earth element patterns indicate characteristics of postcollisional granitoids, likely derived from the anatexis of crustal rocks. U-Pb zircon age dating suggests that these intrusions occurred during the Middle Triassic (ca. 233 Ma). Our findings imply that these granitoids may be important indicators of Middle Triassic crustal melting during orogenic collapse in the Western Gyeonggi Massif, following the peak metamorphic and crustal thickening events of the Permo-Triassic collision.