Petrogenesis And Geological Significance Of Eocene Rhyolites In The Wuma Area,Tibet

The uplift of the Tibetan Plateau is closely related to the global climate and environmental changes since the Cenozoic,and has always been the focus of geologists at home and abroad.The uplift of the Tibetan Plateau has been studied for nearly 100 years,but the time,process and mechanism of the uplift are still controversial.In this paper,the Eocene rhyolites in Wuma area of Northern Lhasa terrane in the Central Tibetan Plateau were studied by zircon U-Pb dating,zircon Lu-Hf isotope,whole rock geochemistry and Sr-Nd isotope,and their petrogenesis was discussed.Combined with regional geological data,the uplift mechanism of Northern Lhasa terrane in the Central Tibetan Plateau during Eocene was constrained.The results show that Wuma rhyolite was formed at 42-38Ma(Eocene),with high Si O₂(68.5-71.5 wt.%),Al₂O₃(15.1-16.2 wt.%),Na₂O(1.87-4.01 wt.%)and K₂O(5.46-6.81 wt.%),low Mg O(0.20-0.48 wt.%),Ti O₂(0.36-0.42 wt.%)and P₂O₅(0.09-0.13 wt.%),enrichment of LILE and LREE,depletion of HFSE and HREE,positive zircon?Hf(t)(+5.3-+8.5)and whole rock?Nd(t)(+0.8-+1.5).Combined with the Eocene magmatic rocks of the Northern Lhasa terrane reported by predecessors,we speculate that Wuma rhyolites were formed by partial melting of the new mafic lower crust,with garnet and amphibole remaining in the magma source area.The Eocene rhyolites in the Northern Lhasa terrane have lower(La/Yb)_N,Sr/Y and zircon Eu/Eu*than those in the Southern Central Lhasa terrane and the Qiangtang terrane in the central Tibetan Plateau,indicating that the lower crust depth of the Eocene rhyolites in the Northern Lhasa terrane is shallower than that in the Southern Central Lhasa terrane and the Qiangtang terrane.According to the gravity equilibrium hypothesis and previous paleogeographic elevation data,we conclude that the paleoelevation of the Northern Lhasa terrane in Eocene is significantly lower than that of the Southern Central Lhasa terrane and Qiangtang terrane on both sides.Based on the significant topographic relief,the similar geochemical and Sr-Nd isotopic characteristics of the Eocene magmatic rocks in the Central Tibetan Plateau,and the weak Eocene shortening of the middle-upper crust in the Northern Lhasa terrane,we propose lower crust channel flow to explain uplift of Northern Lhasa terrane during Eocene.