Geochemistry of Basalt and Andesitic Porphyrite in Longbaesang Area, Tibet: Implications for the Tectonic Evolution of the Bangonghu-Nujiang Ocean

The tectonic nature of Shiquanhe-Yongzhu-Jiali ophiolitic mélange zone andits relationship to the Bangonghu-Nujiang suture zone are key to understanding the Mesozoic tectonic evolution of the Bangonghu-Nujiang Ocean. Geochemistry of basalt and andesitic porphyrite in Longbaesang area are reported in this paper. The results show that the basalt are tholeiitic rocks,and enriched in light rare earth elements and large ion lithophile elements (Rb、Ba、K、Sr、Pb),and depleted Nb,Ta,Ti. All of the geochemistry characteristics of the basalt rocks are the same as typical of island arc tholeiite.The andesitic porphyrite are tholeiitic rocks,which has the trend of evolution to calc alkaline rocks,enriched large ion lithophile elements (Rb、Ba、K、Sr、Pb、U),depleted Nb,Ta,clearly displaying compositional characteristics of island-arc volcanic rocks. Additionally,the compositional characteristics of lower ΣREE (11.8×10^-6-13.8×10^-6)、(La/Yb)_N(0.37-0.43) and depleted in light rare earth elements,which are also Similar to normal mid-ocean ridge basalt,Therefore,the andesitic porphyrite may be formed in a immature back-arc basin. Combined with the regional geological data,the tectonic environment of the basalts and andesitic porphyrites both are related to subduction,which respectively,may be formed in the oceanic or the active continental margin of island arc environment and the immature back-arc basin environment,related to the South subduction of Bangonghu-Nujiang ocean crust,which are record the recycling of the Southern subducted Bangonghu-Nujiang oceanic crust during the late Jurassic to the early Cretaceous.