Chronology and geochemical characteristics of the Lapeiquan Formation rhyolite in the Altun Kaladawan area, Xinjiang, and implications for tectonic evolution of the northern margin of Altun

Due to the low level studies on the Altun Lapeiquan Formation in Xinjiang, its depositional age and tectonic genesis are still in doubt.Chronological and geochemical studies were carried out on the rhyolites of the Lapeiquan Formation.The results of zircon LA-ICP-MS U-Pb dating show that the ages of the second and third member rhyolites of the Lapaiquan Formation are 497±2.0 Ma and 483.4±1.9 Ma, respectively.The petrogeochemical studies show that the samples are Si-rich(70.07%~78.55%), Mg-poor(0.32%~0.58%), and Mg^#-low(24~30).The rare earth elements exhibit the characteristics of enrichment of light rare earth elements and relatively deficient in heavy rare earth elements((La/Yb)_N=10.23~12.73), and the negative Eu is abnormally obvious(δEu=0.10~0.19);the analysis of trace elements shows that the samples are obviously enriched in La, Nd, Zr, Ce, Sm, U, Th, Hf, etc., and relatively deficient in Sr, Nb, Ti, etc.Combined with the previous researches, the depositional age of the Lapeiquan Formation is determined to be Late Cambrian-Early Ordovician.The second member of rhyolite has the characteristics of A-type granite, and it most likely came from partial melting of crustal materials, and the tectonic environment is a post-arc extensional environment caused by the reversal of the North Altun Ocean.