[Objective]Several intraplatform deep-marine trough basins in the northern Yangtze Platform developed bedded chert in the Dalong Formation during the Late Permian.The organic-rich black rocks in the trough basins are shale gas exploration targets in the Sichuan Basin.The black chert deposition in the Dalong Formation started at 258.77 Ma,related to waning of the Emeishan mantle plume.However,its termination date and the cause of the dis-appearance of the intraplatform trough basins were still unclear.The results of this study help to better understand the evolution of these deep-marine troughs,offering a theoretical basis for potential assessment and exploration of Permian shale gas.[Methods]Zircon U-Pb LA-ICP-MS age dating and trace element measurement in the bentonite interbeds were carried out in the upper part of the Dalong Formation chert beds.[Results]The zircon grains are euhe-dral and show clear zoning,with Th/U ratios higher than 0.46.The zircon grains are enriched in heavy rare earth elements(HREE)with positive Ce anomalies and negative Eu anomalies,showing a rising trend from light REE to HREE in the chondrite-normalized REE patterns.The U-Pb LA-ICP-MS age of the zircon was 253.0±1.3 Ma.Dia-grams of Th/Nb vs.Hf/Th and Th/U vs.Nb/Hf in the zircon show arc magma origin.[Conclusions]Chert deposition in the Dalong Formation in South China ceased in the mid-Changhsingian Stage of the Late Permian at 253.0±1.3 Ma.The arc volcanism eruption was not related to intraplate Emeishan mantle plume activity,but was caused by arc activity around the Yangtze Block,and was probably related to the disappearance of the deep-marine troughs in the northern Yangtze area.
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