Geological, geochemical characteristics, U- Pb ages of the volcanic rocks from Lower Devonian Tousuquan Formation in Yaleman area, Eastern Tianshan Mountains, and their tectonic significance

A set of intermediate and acid volcanic lava, volcanic tuff and volcanic breccia are developed in Yaleman Tousuquan Formation in the Eastern Tianshan Mountains. The magma evolved from neutral to acidic from early to late, and developed explosive facies, effusive facies and subvolcanic facies, characterized seven eruption rhythmic cycles. According to LA- ICP- MS zircon U- Pb isotopic dating, the rhyolite age of the Tousuquan Formation is 406. 0±3.0 Ma (MSWD=0. 35), and the age is early Devonian. Rock geochemistry shows that the volcanic rocks of Tousuquan formation have the characteristics of medium aluminum (13. 2%~18. 10%), rich in sodium and poor in potassium (K2O/Na2O=0. 10~0. 67), belonging to the high differentiation, low- to medium- K calc- alkaline series; The total amount of rare earth elements is relatively high (∑ REE=98. 7×10-6~214×10-6), LREE is enriched, HREE is depleted (LREE/HREE=3. 75~4. 58), Eu shows weak negative anomalies (δEu=0. 62～0. 92), large ion lithophile elements Rb, K, Ba are significantly enriched, and high field strength elements Nb, Ta and Ti are depleted. These geochemical characteristics exhibit signatures of mantle- derived remelted volcanic rocks that assimilated crustal materials during magma ascent, and indicate a continental margin arc tectonic setting related to subduction.