SHRIMP U-PB AGES OF DETRITAL ZIRCONS IN METASEDIMENTARY ROCKS OF THE CENTRAL OGCHEON FOLD-THRUST BELT, KOREA: EVIDENCE FOR TECTONIC ASSEMBLY OF PALEOZOIC SEDIMENTARY PROTOLITHS

We analyzed detrital zircons in four metasandstones and two metadiamictites of the central Ogcheon Metamorphic Belt (OMB) in order to provide geochronologic constraints on the time of deposition and protolith age distributions. SHRIMP U-Pb ages of the newly analyzed zircons range from Archean to Early Paleozoic, and their age patterns permit the definition of four lithotectonic slices: upper and lower Pibanryeong units consisting primarily of quartzite and metasandstone; an upper Poeun unit of black slate containing Early Permian plant fossils; and a lower Poeun unit made up of metasandstone and metadiamictite. A quartzite of the upper Pibanryeong unit yielded two major zircon age populations at 965 ± 11 Ma and 447 ± 5 Ma, whereas two lower Pibanryeong metasandstones show major age concentrations at ~2.5 Ga, 1.0–0.9 Ga, and 545–535 Ma. The youngest components of these units suggest maximum ages of sedimentation as Late Ordovician and Early Cambrian, respectively. These upper and lower Pibanryeong zircon age distribution patterns are consistent with those of Ordovician and Devonian sedimentary strata, respectively from the South China Craton (SCC). On the other hand, three samples of the lower Poeun unit yielded two distinct age populations at ~1.87 Ga and 755 ± 7 Ma. In addition, a metadiamictite contains abundant Mesoproterozoic zircons with two broad peaks centered at ~1.3 and 1.6 Ga. Protolith ages and distribution patterns are profoundly different between the Pibanryeong and Poeun lithotectonic packages, suggesting the presence of a major thrust fault juxtaposing allochthons that are also characterized by large P-T contrasts. Our detrital zircon results for the Pibanryeong unit are compatible with derivation from the SCC, in apparent contrast with the Poeun occurrence of Early Permian plant fossils most likely correlative with the North China Craton (NCC). Taken together, the suturing of Paleozoic metasedimentary terranes in the OMB is a product of Permo-Triassic orogeny reflecting collision of the North and South China cratons.