Geochemical Characteristics and Depositional Environment of Trace Elements and Rare Earth Elements of Sedimentary Rocks of Middle-Upper Permian Boundary in Xing-Meng Area

Abstract

The Permian Zhesi and Linxi Formation in Xing-Meng area of Inner Mongolia widely developed a set of dark mudstone, sandstone and carbonate rock assemblages, with a high content of TOC and Type II Kerogen. A systematic analysis of rare earth elements (REE) and trace elements (TE) in sedimentary rocks near the boundary between Zhesi and Linxi Formation on the section of Linxi Formation in Guandi Town, Linxi County, is carried out. The results show that the composition of TE and REE in sedimentary rocks above and under the boundary of the Zhesi and Linxi Formation are obviously different. The average value of total rare earth elements (∑REE) of carbonate rocks in the Zhesi Formation is 30.4 × 10−6, which is significantly lower than that of Linxi Formation (62.1 × 10−6). After PAAS standardization, both of them are deficient in light REE relative to medium and heavy REE, and the Zhesi Formation shows larger positive δEu and δCe anomalies relative to the carbonate rocks of Linxi Formation, which are different from normal marine and lacustrine carbonate rock in REE distribution pattern. Compared with carbonate rock, the mudstone near the boundary has relatively consistent REE composition. The ∑REE is higher (average value is 155.6 × 10−6), and showing weak negative anomaly of δEu (average value is 0.90) and weak positive or negative anomaly of δCe (average value is 1.00). The content of TE in carbonate rocks near the boundary of Zhesi and Linxi Formation show a sudden change. And, the contents of Cu, Zn, Rb and Zr, which characterize the continental contamination, increase significantly above the boundary. The characteristic parameters of Sr/Ba > 1, V/Cr > 4.25 and V/(V+Ni) > 0.54, indicate that the main depositional environment at the turn of Middle and Late Permian in the study area are marine facies and strong reduction environment. The subduction and magmatism of the Paleo-Asian Ocean plate, and the extremely strong reductive environment and hydrothermal fluid produced by them, are the main reasons for the significant positive anomaly of δEu and the general positive anomaly of δCe near the boundary of Zhesi and Linxi Formation.