Crustal thickening and uplift of the northwestern Lhasa Terrane, central Tibetan Plateau: Insights from Mid-Eocene volcanic rocks in the Gerze Region

The timing of raising the northern Lhasa crust to its present-day value remains controversial. Here, we address this issue through the geochemical makeup of Eocene (ca. 37 Ma; zircon U-Pb dating) volcanic rocks erupted in the Gerze Region, northwestern Lhasa Terrane. The Gerze volcanic rocks (GVRs) possess uniform whole-rock (87Sr/86Sr)i (0.7045-0.7059) and epsilon Nd(t) (0.78-1.50), and zircon epsilon Hf(t) (5.25-9.83) and delta 18O (6.74%o-7.81%o) values. However, based on the differences in major and trace elements, the GVRs are classified into two groups. The Group I rocks show adakitic affinities, as evidenced by their high SiO2 (67.62-70.66 wt%), Sr (399-519 ppm), low Y (8.2-10.8 ppm), and Yb (0.78-1.15 ppm) concentrations, and absence of negative Eu anomalies. In conjunction with the intra-block setting of the study area during the Eocene, the lack of coeval mafic rocks, and the low Mg# (<41) and compatible element concentrations (e.g., Ni <35 ppm), we propose that the Group I rocks were formed by partially molten thickened lower continental crust with garnet and minor amphibole in the melting residue. The Group II rocks, in comparison, have higher SiO2 (70.08-77.48 wt%), lower Sr (155-244 ppm) values, and distinctly negative Eu anomalies, which, coupled with their similar Sr-Nd-Hf-O isotopic makeup with that of the Group I rocks, suggest they were most likely evolved from the pristine lower crust -derived magmas through removal of feldspar, biotite, and accessory minerals. Quantitative estimates of the paleo-crustal thickness (70.2 +/- 12.7 km) and elevation (5.7 +/- 0.6 km) through the whole-rock (La/Yb)N and Sr/ Y ratios suggest that the northern Lhasa Terrane, at least part of its western segment, had achieved its present-day thickness and elevation by the Mid-Eocene, as a result of horizontally tectonic shortening of continental crust according to a synthesis of our new and the regionally tectonmagamtic data.