Zircon U-Pb age, geochemical characteristics and constraints on the Jinshajiang Paleo-Tethys collision of Early-Middle Triassic Malasongduo Formation volcanic rocks from the Gongjue area, Eastern Tibet

There is a large number of rock and structural information of the Late Paleozoic-Early Mesozoic retained in the Jinshajiang binding belt, which is the main window to study the evolution of ancient land subduction-land collision in the Jinshajiang.This paper reports the zircon U-Pb age and whole rock geochemical characteristics of the early Middle Triassic dacite-rhyolite in the Gongjue area, and discusses its petrogenesis and magma source area.Based on LA-ICP-MS zircon U-Pb dating, the weighted average U-Pb age of four felsic volcanic rocks is 246.2±1.7 Ma, 245.5±0.7 Ma, 244.7±6 Ma and 246.2±0.5 Ma, respectively, indicating the volcanic rocks were formed at 245~246 Ma.Geochemical analysis shows that felsic volcanic rocks in the study area are rich in Si (SiO₂=67.16%~74.62%), rich in K (K₂O=3.44%~5.94%), poor in Na (Na₂O=1.75%~2.53%) and poor in Mg (MgO=0.28%~1.04%).The A/CNK values range from 1.07 to 1.41, indicating peraluminous high-K calc-alkaline characteristics.The REE distribution curve shows a "V" shape.Large ion lithophile elements and high field strength elements Rb, K, and Th are relatively enriched, while Nb、Ta、Sr and Ti are relatively depleted, indicating that the magma was originated from the ancient crust.It is suggested that the felsic volcanic rocks in the Gongjue area are the products of partial melting of the lower crust caused by the underplating of mantle-derived basic magma under the background of westward subduction of the Jinshajiang ancient oceanic crust, which is in the transition stage of subduction to collision.Combined with regional geological data and previous studies, it is considered that the Gongjue area of Jinshajiang junction belt experienced Permian subduction, Early Triassic subduction-collision transition, Middle Triassic continent- continent collision and Late Triassic post-collision extension, until the end of orogeny in late Late Triassic.