Late Cretaceous magmatism in the NW Lhasa Terrane, southern Tibet: Implications for crustal thickening and initial surface uplift

Crustal thickening and uplift of southern Tibet have been widely associated with India-Asia continental collision during the Cenozoic. However, recent studies indicated that the crust of the northwestern (NW) Lhasa Terrane was thickened during the late Mesozoic. Here we report geochronological and geochemical data for the Gaerqiong diorite porphyries (GPs) and Xiongma plutons (XPs) in the NW Lhasa terrane, southern Tibet. Zircon U-Pb dating suggests that these intrusive rocks were generated at ca. 85 and ca. 88 Ma, respectively. The GPs are characterized by high MgO, Cr, and Ni contents, and they have adakitic affinities. These geochemical features, combined with their depleted epsilon Nd-(t) (+1.7 to +2.0), Sr-87/Sr-86((i)) (0.705103-0.705259), and zircon epsilon Hf-(t) (+5.2 to +10.2) isotopic compositions, indicate that the GPs were produced by partial melting of the delaminated juvenile continental crust. In contrast, the XPs are composed of host granites and mafic microgranular enclaves (MMEs). The MMEs have low SiO2 and high MgO contents, and low epsilon Hf-(t) (-14.0 to -5.8) values, indicating that their parental magmas were derived from an enriched mantle. The host granites have high SiO2 and low MgO contents, and variable epsilon Nd-(t) (-7.4 to -6.3) and zircon epsilon Hf-(t) (-11 to -4.1) values. These observations, combined with the presence of MMEs in the Xiongma granites, suggest that the host granites were the result of mixing of crust- and mantle-derived magmas. Detailed study of these two plutons, combined with the previous researches, suggests that Late Cretaceous (ca. 90 Ma) magmatism in the NW Lhasa Terrane occurred in a post-collisional extensional setting related to delamination of the regionally thickened lithosphere after collision of the Lhasa-Qiangtang Terranes. We propose that the crust of the NW Lhasa Terrane reached a maximum thickness (average of >50 km) before the Late Cretaceous (ca. 90 Ma). This crustal thickening was caused by underplating of mafic magmas during slab roll-back and break-off of the southward-subducting Bangong-Nujiang oceanic lithosphere and subsequent tectonic thrusting during Qiangtang-Lhasa Terrane collision, respectively. Given that crustal thickening generally results in elevated terrain, the regional uplift (driven by isostasy due to crustal thickening) probably commenced before the Late Cretaceous (ca. 90 Ma).