The petrogenesis of the granite porphyry of Yanhu composite batholith in the Lhasa terrane, Tibet and its constraints on the time limit of Bangonghu-Nujiang Ocean closure

Granitic magmatism is one of the key processes of continental crust evolution,explosive volcanic activity,and rare metal mineralization.Yanhu composite batholith is located in the western section of Bangonghu-Nujiang suture zone in the north of Lhasa terrane,and is an important part of the response of Bangonghu-Nujiang Tethys Ocean subduction.In this paper,the petrography,zircon U-Pb chronology and petrogeochemistry of the granite porphyry in the Yanhu composite batholith from Geji County were studied,and its petrogenesis was discussed.The results show that the zircon U-Pb age of the Yanhu composite batholith granitic porphyry is 103.8±0.8 Ma(MSWD=3.0,n=25),and the intrusion occurred in the late Early Cretaceous.Aluminum saturation index(A/CNK)of Yanhu composite batholith granitic porphyry ranges from 1.05 to 1.08,showing weak peraluminous characteristics with high K₂O,Na₂O and low Mg^#(12.43~22.36),indicating the high K calc-alkaline type Ⅰ-type granite.The Yanhu composite batholith granitic porphyry is enriched in K and depleted in Ba,Nb,P,Zr and Ti,and has strong negative Eu anomaly(δEu= 0.20~0.48).The REE distribution curves are characterized by right-leaning type,ΣREE=81.33×10^-6~121.61×10^-6,LREE/HREE=5.75~6.74,higher Y(20.14×10^-6~ 24.38×10^-6,> 18×10^-6)and Yb(2.11×10^-6~2.62×10^-6,> 1.9×10^-6),lower Sr/Y ratio(4.05~6.54,< 20),La/Yb=6.79~8.94.Rb/Sr = 0.50~1.12.The calculated zircon saturation temperature is 812~823℃,and the formation depth of magmatic melt is estimated to be 34~40 km,indicating that the granitic porphyry formed in the Yanhu composite batholith was partially melted products of basaltic lower crust under the high temperature background of the collision between the North Lhasa terrane and the South Qiangtang terrane.It belongs to a set of highly differentiated Ⅰ-type granites formed by remelting of the basic lower crust caused by plate fragmentation events.