Zircon U-Pb age, petrogenesis and tectonic model of Miocene quartz monzonite in Zuozuoxiang area, Western Lhasa terrane

The Miocene magmatism is of great significance to distinguish the Cenozoic tectonic evolution of the Tibetan Plateau.In this paper, a new set of Miocene quartz monzonite has been redefined in Zuozuoxiang area, western Lhasa terrane.Zircon U-Pb dating shows that the age of quartz monzonite is 22.38±0.46 Ma.Whole rock geochemical test shows that quartz monzonite has high SiO₂ (65.87%~66.90%), K₂O(6.19%~6.41%)contents, K₂O/Na₂O(1.7~1.8)ratios and low MgO(1.71%~1.76%)contents, belonging to potassic intermediate-acid rock.According to the high Mg^#(56.5~57.8), Nb/Ta(16.0~16.4), Cr(46.0×10^-6~48.9×10^-6), Ni(32.6×10^-6~34.8×10^-6)and a large number of dark inclusions in the rocks, we believe that Miocene quartz monzonite from Zuozuoxiang area was formed by partial melting of thickened lower crust with the addition of mantle material.In addition, based on the results of uplift studies, we believe that the tearing of Indian plate is more suitable for the interpretation of Miocene potassic-ultrapotassic rocks in Lhasa terrane than the convective thinning of lithospheric mantle, the delamination of thickened lithosphere and the break-off of Indian plate.