Removal of deep lithosphere in ancient continental collisional orogens: A case study from central Tibet, China

Abstract

Widespread but small-volume Late Cretaceous volcanic rocks in central Tibet contain important information on the Lhasa-Qiangtang collision process. In this contribution, we focus on Late Cretaceous volcanics in the southern Qiangtang subterrane and present zircon LA-ICP-MS U-Pb ages, whole-rock major and trace element compositions, and Sr-Nd isotopic data. Zircon LA-ICP-MS U-Pb dating yielded a concordant age of 80 Ma, which postdates the Early Cretaceous collision of the Qiangtang and Lhasa terranes. The volcanic rocks are potassium-rich alkaline andesites with high contents of K2O (3.45–5.11 wt %) and Th (13.39–25.02 ppm), as well as high K2O/Na2O ratios (0.6–0.9). They have higher REE and HFSE contents than coeval Mg-rich and adakite-like magmatic rocks that can be related to partial melting of a thickened lower crust. Moreover, they have higher values of Mg# and lower contents of SiO2 than lower continental crust-derived rocks in central Tibet and experimental data of mafic rocks. We argue that the andesites were generated after the removal of thickened lithospheric mantle and subsequent to the final Lhasa-Qiangtang amalgamation in a postcollisional setting. The high-K characteristics can be explained by producing the primitive andesite magmas from partial melting of the residual and shallow metasomatized lithospheric mantle (the K-rich layer) during heating by upwelling asthenosphere; subsequently, these primitive andesite magmas were subjected to fractional crystallization to generate the Amdo andesites. The way in which these andesites were formed provides evidence for the lithospheric thickening and uplifting of central Tibet during the Late Cretaceous prior to India-Asia collision.