Termination of the Permo-Triassic Songrim (Indosinian) orogeny in the Ogcheon belt, South Korea: Occurrence of ca. 220 Ma post-orogenic alkali granites and their tectonic implications

The 'Indosinian Orogeny' is defined in a broad sense as encompassing all the Late Paleozoic to Early Mesozoic tectonic events associated with the closure of the eastern parts of the Paleo-Tethys Ocean. It includes the Permo-Triassic continent-continent collision between the North China and South China blocks that generated the Qinling-Dabie-Sulu orogen containing the largest ultrahigh-pressure (UHP) terrane in the world. The Korean Peninsula, situated at the easternmost end of the Tethyan tectonic regime, was also tectonically influenced by the Indosinian Orogeny, often known as the 'Songrim Orogeny' in Korea. The Songrim Orogeny, characterized by intense regional medium- to high-pressure metamorphism, extends from the Imjingang belt through the Gyeonggi massif to the Ogcheon belt. It was initiated as early as the Early Permian Period (ca. 290 Ma) and peaked in the Triassic Period (ca. 255 to 225 Ma). This paper presents new geochemical and SHRIMP U-Pb zircon ages for two alkali granites that postdated the Songrim Orogeny in the Ogcheon belt. Both granites are contemporaneous (219.3 ± 3.3 Ma and 219.6 ± 1.9 Ma) and are geochemically post-orogenic A-type granites intruded in an extensional setting subsequent to a major collision. The data provide a critical time constraint on the termination of the Indosinian Orogeny in the Korean Peninsula. Despite the absence of an UHP terrane in the Korean Peninsula, similarities in the duration, metamorphic characteristics, and post-orogenic magmatism between the Qinling-Dabie-Sulu and Songrim orogens, as parts of the broad Indosinian orogenic regime, are clearly alike, suggesting a genetic linkage in their time-space characteristics.