Termination age of the chert deposits in the upper Permian Dalong Formation in middle and upper Yangtze area

Key words: 
• U-Pb dating /
• Arc volcanism /
• Dalong Formation /
• Late Permian /
• Sichuan Basin /
• Trough demise

Abstract: [Objective] Several intraplatform basins of deep-marine trough in the northern Yangtze platform development bedded chert of the Dalong Formation during the Late Permian. These 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 which started at 258.77 Ma were was related the Emeishan mantle plume waning. However, the termination age of this black cherts in the Dalong Formation and the causes for the demise of these intraplatform basins in the trough are still unclear. The corresponding research will help to gain a better understand of these deep-marine troughs evolution, so as to offer theoretical basis for the Permian shale gas exploration potential assessment. [Methods] This study performs zircon U-Pb LA-ICP-MS age dating and zircon trace elements measurements using bentonite interbeds in the upper part of the Dalong Formation chert beds. [Results] Zircon grains are euhedral and show clear zoning. Th/U ratios of the zircons are higher than 0.46. The zircon grains are characterized by enrichment in heavy area 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 pattern curves. Zircon U-Pb LA-ICP-MS age was 253.0 ± 1.3 Ma. Diagrams of Th/Nb vs. Hf/Th and Th/U vs. Nb/Hf of zircons show arc magma origin. [Conclusion] The chert deposition in the Dalong Formation in South China was terminated at 253.0 ± 1.3 Ma in the middle Changhsingian stage. The arc volcanism eruption was not related to the intra-plate Emeishan mantle plume activity, but was caused by the arc activity around the Yangtze block, which was probably the cause for related to the demise of the deep-marine troughs in the northern Yangtze.