The Late Permian Sedimentary Environments of Linxi Formation in Xingmeng Area: Constraints from Carbon and Oxygen Isotopes and Trace Elements
ZHAI Da-xing;ZHANG Yong-sheng;TIAN Shu-gang;XING En-yuan;WU Fei-meng;ZHU Chang-wei;JIANG Su-yang;MLR Key Laboratory of Saline Lake Resources and Environments, Institute of Mineral Resources,Chinese Academy of Geological Sciences;School of Earth Science and Resources, China University of Geosciences(Beijing);Institute of Geology, Chinese Academy of Geological Sciences;School of Geosciences, Yangtze University;
The Late Permian sedimentary environment in Xingmeng area has long been a topic of controversy. Most researchers believe it is continental deposit, while a few people hold that it had been a closed marine basin at the early stage, and then changed into a lacustrine basin. In order to settle down this dispute, the authors measured the section of Guandi in Linxi area and discovered large quantities of limestone lenses containing calcium algae, bryozoans and some other marine fossils. Limestone lenses were collected systemically, and their elements and carbon and oxygen isotopic compositions were analyzed. The results show that the limestone lenses are characterized by depletion of LREE relative to MREE and HREE, obvious positive Eu anomalies, minor positive La, Ce anomalies and lack of Gd and Y/Ho anomalies. Si O2 values are high, and Zr and REE have obvious positive correlation, suggesting a near-coastal deposition environment. The depositional water was derived from three sources, i.e., normal seawater, river or atmospheric precipitation and hydrothermal solution. The sedimentary sequence, the assemblages of fossils in the section and the composition of carbon and oxygen isotope in limestone also provide further support for the near-shore environment. In Late Permian, there was still a narrow marine basin with hydrothermal activity in the Xingmeng-Jilin area. The sea water circulation was probably blocked in the coastal area, which caused the domination of evaporation or river discharge in a short period, thus resulting in a substantial shift of carbon isotopes. However, the oxygen isotope excursion was caused mainly by diagenesis.