Miocene potassic and adakitic intrusions in eastern central Lhasa terrane, Tibet: Implications for origin and tectonic of postcollisional magmatism

Miocene postcollisional potassic and adakitic rocks are widely distributed in the southern Lhasa terrane and western central Lhasa terrane. However, coeval potassic and adakitic rocks in eastern central Lhasa terrane were rarely recognized, and their origins and formation mechanism remain controversial. In this paper, we provide new geochronological and geochemical data for the Miocene postcollisional potassic and adakitic intrusions exposed in the Qingdu area, eastern central Lhasa terrane, southern Tibet. The Qingdu Miocene intrusions consist of quartz monzonite porphyry and biotite granite with coeval zircon U-Pb ages of 14.1 Ma and 14.0 Ma, respectively. They have high SiO2 (67.98-75.32 wt.%), Al2O3 (14.13-14.78 wt.%), and K2O (4.06-6.18 wt.%) and low MgO (0.25-1.46 wt.%) contents. They are both enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), with high (La/Yb)(N) (25.37-42.32) ratios. The biotite granite samples have low Y (7.10-9.96 ppm) and Yb (0.61-0.85 ppm) contents, and high Sr (229-384 ppm) contents and high Sr/Y (30-41) ratios, which show adakitic geochemical characteristics, whereas the quartz monzonite porphyry samples show potassic geochemical characteristics. They display initial (Sr-87/Sr-86)(i) ratios of 0.7083-0.7103, epsilon(Hf)(t) values of -6.7 to -0.1, epsilon(Nd)(t) values of -8.96 to -7.44, (Pb-208/Pb-204)(i) ratios of 39.028-39.110, (Pb-207/Pb-204)(i) ratios of 15.662-15.684, and (Pb-206/Pb-204)(i) ratios of 18.541-18.577. These signatures indicate that both the potassic and adakitic intrusions are more likely to originate from partial melting of a thickened lower crust, which were mainly the products of the binary mixing between the juvenile and ancient crust components. Based on the spatial distributions and isotopic features of the postcollisional potassic and adakitic rocks in the Lhasa terrane, we suggest that the differences of the lower crustal composition played a crucial role in causing the geochemical variations of the Miocene postcollisional adakitic and potassic rocks in Lhasa terrane.