In situ U-Pb zircon dating of Devonian sandstones and Paleoproterozoic gneissic granites in the Imjingang Belt: Tectonic implications for the Korean Peninsula and North China

摘要

The Imjingang Belt in the Korean Peninsula (KP) attracts much attention because of its critical position in determining the tectonic affiliation of the peninsula. The key to clarifying current controversy lies in the understudied Archaean–Proterozoic basement rocks and Devonian- Carboniferous sedimentary series (i.e.,Imjin Group) in the North Korean part of the belt. In this study, we present geochronological study on magmatic zircons in three Paleoproterozoic gneissic granites and detrital zircons in four clastic rocks of the Imjin Group. SIMS U-Pb zircon analyses on the former reveal that three gneissic granites formed at 1884?±?10?Ma, 1875?±?10?Ma and 1885?±?10?Ma, respectively. They are reminiscent of similarly aged granitic magmatism from the North China Craton (NCC) and in the Nangrim, Gyeonggi and Yeongnam massifs of the KP, thus defining a consistent Paleoproterozoic granitic magmatic event across the NCC and the KP. Detrital zircons from the Puap Formation of the Imjin Group yield age populations of 2.5–2.3?Ga (~19%), 2.1–1.5?Ga (~15%), 1424–1176?Ma (~9%), 1108–676?Ma (~31%) and 591–388?Ma (~26%). The youngest group gives a weighted mean206Pb/238U age of 401?±?3?Ma (n?=?15), confirming a depositional age of younger than 400?Ma for the Puap Formation. Together with typical Middle Devonian Givetian and Upper Devonian Famennian standard fossil assemblage in the formation, we constrain the formation to be of Late Devonian. The Puap and Misan formations in the Imjingang Belt show consistent detrital zircon age spectra with those of representative Paleozoic sequences in the Pyongnam and Taebaeksan basins of the KP and the Paleozoic basins in the NCC, implying that the NCC and KP might share congruous tectonic history in the Paleozoic. This can lead to further inferences that the Nangrim and Gyeonggi massifs were connected at least before Late Devonian and that the Imjingang Belt might not be the eastward extension of the Qinling-Dabie-Sulu collisional belt.