LA-ICP-MS zircon U-Pb geochronology, geochemical characteristics and genetic study of Yeba Formation lavas in Demingding area, southern Tibet

The Yeba Formation lava, generally considered to be the earliest volcanic rock in southern Tibet, is important for understanding the subduction of the Neo-Tethys Ocean and the evolution of the Tibet before India-Asia collision. Previously published zircon U-Pb dating results for the Yeba Formation felsic rocks indicated that they were developed during Early-Middle Jurassic. However, so far, the precise ages of intermediate-basic rocks are absent. Meanwhile, the geodynamic setting for the generation of the Yeba Formation lavas remains controversial. The result of LA-ICP-MS zircon U-Pb dating of andesite of the Yeba Formation from Demingding area is 188.8 ± 1.8 Ma, which is consistent with the formation age of acidic rocks. This suggests that the Yeba Formation lavas was mainly developed during Early-Middle Jurassic. The geochemical data of Yeba Formation lavas in Demingding area show an affinity to arc magma, such as enrichment of large ionic lithosphere elements (e.g., Rb, U) and light rare earth elements (e.g., 3.3<(La/Yb)N<9.9), and depletion of high field strength elements (e.g., Nb, Ta, and Ti), Sr and Eu (δEu= 0.75~0.83). The new data provided by this study, combined with previous results, indicate that the generation of the Yeba Formation lavas was probably in a continental arc setting that was related to the northward subduction of the Neo-Tethys Ocean during Early-Middle Jurassic.