SHRIMP U-Pb ages of detrital zircons in metasandstones of the Taean Formation, western Gyeonggi massif, Korea: Tectonic implications

摘要

Detrital zircons in three metasandstones of the Taean Formation, western Gyeonggi massif, Korea, were analyzed using a sensitive high-resolution ion microprobe (SHRIMP), in order to provide geochronologic constraints for the depositional age as well as the protolith ages. This formation mainly comprises the sandstone–mudstone units which experienced regional metamorphism at the transitional greenschist–amphibolite facies condition. The majority of detrital zircons yielded concordant ages defining four major age components: (1) Late Archean–Early Proterozoic (2.5–2.4 Ga); (2) Middle to Earliest Late Proterozoic (1.0–0.9 Ga); (3) Middle Late Proterozoic (820–750 Ma); and (4) Silurian (~430 Ma). The youngest group gives a weighted mean ~(206)Pb/~(238)U age of 431.3 ± 4.1 Ma (n = 17), confirming the Upper Paleozoic deposition of the Taean Formation. Together with this Silurian peak, the presence of Middle to Late Proterozoic ages is characteristic for the source rock, suggesting that the western Gyeonggi massif could be correlative with the South China craton. On the other hand, the lack of Paleoproterozoic age of ~1.87 Ga is in marked contrast with its predominance in the basement rocks of central to eastern Gyeonggi massif. The overall distribution pattern of detrital zircon ages is consistent with that of the Devonian metasedimentary rock in the Imjingang belt, suggesting an extension of this collisional belt towards the south.