LAUSR

Abstract In the Yanbian area, northeastern China, Late Paleozoic sedimentary successions are well developed and contain both Angaran and Cathaysian-derived floras. The flora-bearing successions were originally thought to be Permian in age and these flora fossils disappeared after the Permo-Triassic mass extinction (PTME). The Jiefangcun Formation was thus assigned a Permian age. In this study, the weighted mean 206Pb/238U ages of the youngest zircons from two samples from the Jiefangcun Formation are 246.8 ± 2 Ma (MSWD = 0.65, n = 11) and 247.2 ± 3 Ma (MSWD = 0.15, n = 7). One dacite above this formation is dated to 241 ± 16 Ma (MSWD = 2.6, n = 3). Therefore, the Jiefangcun Formation was deposited during the Middle Triassic (247–241 Ma). This result further suggested that the Angaran and Cathaysian floras did not disappear after the PTME and recovered in the Middle Triassic. Moreover, the chemical index of alteration, index of compositional variability, and plagioclase index of alteration values suggest that the sediments of the Jiefangcun Formation were derived from an intermediate-felsic igneous source with a weak degree of weathering and simple sedimentary recycling history. The Rb/K, Sr/Ba, V/Ni and Sr/Cu values indicate that this formation was deposited in micro-saline water in an arid climate. Detrital zircon ages from the North China Craton and Jiamusi-Mongolia block in this formation suggest that the proximal provenance was bidirectional. The Paleo-Asian Ocean was therefore finally closed during the Middle Triassic with the witness of floral recovery in the Yanbian area.