Middle Jurassic tectono-magmatic evolution in the southwestern margin of the Gyeonggi Massif, South Korea

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dates and geochemical and isotopic (Sr and Nd) data were obtained for Middle Jurassic granitoids from the Buyeo area along the southwestern margin of the Gyeonggi Massif, South Korea. Foliated 179 Ma granitoids are dominated by granite and diorite, non-foliated 167 Ma granitoids include granite to alkali-feldspar granite, and the youngest, non-foliated 165 Ma granitoids consist of alkali-feldspar granite. The 179 Ma and 167 Ma rocks are geochemically typical of I-type granitoids, and their LILE enrichment and negative Nb-Ta-Ti-P anomalies are common features of subduction-related granitoids. In contrast, 165 Ma rocks are A-type granitoids that show HREE-enriched patterns with significant depictions of Sr, P, Eu, and Ti. I-type granitoids dated at 179 Ma and 167 Ma have I-Sr = 0.7092-0.7158, 0.7206-0.7263 and epsilon(Nd)(T) = -13.96 to -13.55, -21.47 to -19.51, respectively. A-type, 165 Ma granitoids have relatively lower I-Sr (0.7052) and strongly negative epsilon(Nd)(T) (-20,47), reflecting melt derivation from the lower crust. These results suggest that the formation of granitoids along the southwestern boundary between the Gyeonggi Massif and the Okcheon Belt took place at ca. 179-167 Ma by the subduction of the paleo-Pacific plate. The 165 Ma post-orogenic A-type granites intruded in an extensional setting, providing a critical time constraint on the termination of Jurassic plutonism in South Korea. The 179 Ma foliated granitoids are similar to syn-kinematic granitoids in and around the Honam shear zone, indicating that ductile shear movement may have occurred at ca. 179 Ma. Biotite K-Ar ages of deformed Precambrian rocks and Middle Jurassic granitoids from the Buyeo area generally yield ages of 144-152 Ma and 124-135 Ma. These results probably reflect pervasive reactivation during Latest Jurassic to Early Cretaceous deformation that postdated the main ductile shearing event along the southern boundary of the Gyeonggi Massif.