Zircon U-Pb age, geochemical characteristics and tectonic implications of detrital zircon from Linxi Formation in the Wulanhaote area, central Da Hinggan Mountains

There are a set of thick layers of rocks in Wulanhaote of Inner Mongolia which are lack of fossil and suffered from contact metamorphism. LA-ICP-MS U-Pb isotope test was performed on the zircons from the clastic rock samples. The concordant-near concordant ages of 80 zircons were obtained. The 80 detrital zircons can be divided into three groups according to age and frequency distribution characteristics:242~294Ma with peak age at 261 Ma; 301~381Ma with peak age at 348, and 454~530Ma with peak age at 487Ma. In addition, other 7 zircons have ages of 824Ma, 836Ma, 859Ma, 867Ma, 1279Ma, 1556Ma and 2447Ma. The weighted average age for the youngest age group is 263.6±3.3Ma (MSWD=4.9, n=50), consistent with the minimum peak age of 261 Ma. It is inferred that the maximum deposition age of the Linxi Formation is 261 Ma. The major chemical composition of these rocks is SiO₂ 67.81%, Al₂O₃ 18.22%, MgO 1.44%, CaO 0.41%, Na₂O 1.54% and K₂O 3.90%, with K₂O/Na₂O between 1.15 and 10.21, A1₂O₃/(CaO + Na₂O) between 4.87 and 26.38. The value of rare earth element ΣREE is in the range of 162.39×10^-6~223.46×10^-6, averagely 200.88×10^-6. δEu is in the range of 0.58~0.71, averagely 0.64. δCe is between 1.00 and 1.05. The clastic rocks are characterized by LREE enrichment and HREE depletion. Trace elements are characterized by depletion of Nb, Ta, Sr and enrichment of Rb, Ba, La, Ce, Pb, Nd, Sm.These data show that the Linxi Formation was formed in an active continental marginal structural background. Based on detrital zircon age profile information, the authors hold that sediment sources in Linxi Formation were diverse and complex, in addition to blocks in the northeast, there existed provenance information for the North China plate and the Siberia plate at the same time, suggesting that the North China plate and the Siberia plate probably began the subduction collision process during the deposition period of the Linxi Formation.