Discovery of the Late Triassic-Early Jurassic volcanic rocks of Xionglai Formation and constraint on the tectonic evolution of Zhikong-Sumdo Paleo-Tethyan orogen in the Riduo area of Tibet

Zircon U-Pb dating, geochemical and Hf isotope analyses were carried out on the volcanic rocks of Xionglai Formation newly discovered in Riduo area, the eastern part of the Gangdese magmatic belt.The volcanic rocks of Xionglai Formation in Riduo area in the eastern part of Lhasa massif are mainly intermediate basic lava and pyroclastic rock.Zircon U-Pb dating of andesite and basaltic andesite yields weighted mean ages of 201.3 ±6.0 Ma and 184.5 ±4.4 Ma respectively, indicating that the Xionglai Formation volcanic rocks were formed in the Late Triassic-Early Jurassic.Geochemically, the basic volcanic rocks are characterized by low SiO₂(48.53%~49.92%), high MgO(10.68%~11.43%), Mg^#(72.67), Cr(224.7×10^-6~347.8×10^-6), and Ni(90.1×10^-6~109.7×10^-6), enriched in large ion lithophile elements(Rb, Th, U, Sr) and light rare earth elements(LREEs), and depleted in high field strength elements(Nb, Ta, P, Ti).The intermediate volcanic rocks are characterized by high SiO₂(53.93%~60.54%), Al₂O₃(14.74%~16.12%), low MgO(2.45%~8.12%).They are enriched in light rare earth elements(LREEs), large ion lithophile elements(LILEs, e.g., Rb, Th, U and Ba), but depleted in high-field strength elements(e.g., Nb, Ta, P, Ti).Isotope analyses reveal that most samples have low zircon ε_Hf(t) (-12.25~-3.64), and the values of t_DM^C range from 1621 Ma to 2004 Ma.Combining with regional geology, geochronological, geochemical and zircon Hf isotopic data, it is proposed that the Late Triassic-Early Jurassic volcanic rocks of Xionglai Formation were formed in post-collision tectonic setting as a magmatic response to post-collision and extension of Zhikong-Sumdo Paleo-Tethyan orogen.The basic volcanic rocks were derived from partial melting of a depleted lithospheric mantle, while the intermediate volcanic rocks were derived from partial melting of ancient lower crustal material and contaminated by ancient lower crust during ascending.