Thickened juvenile lower crust-derived ~ 90 Ma adakitic rocks in the central Lhasa terrane, Tibet

Abstract

The questions of why the Late Cretaceous magmatism generated and how the nature of the lower crust evolves in central Tibet remain poorly constrained. In this paper, we report the presence of early Late Cretaceous adakitic rocks from the Azhang area, northern edge of the central Lhasa subterrane, central Tibet. These rocks are rhyodacites/dacites in composition and have geochemical characteristics of adakitic rocks, e.g., high Sr (554–836ppm), Sr/Y (66–100), and (La/Yb)N (20–21), low Y (7.96–8.96ppm) and heavy rare earth elements (HREE). In situ zircon U–Pb dating for two samples yields an early Late Cretaceous age (90±1Ma and 87±1Ma). The low MgO (1.4–1.9wt.%) contents and compatible element abundances (e.g., Cr=22–30ppm; Ni=19–25ppm) indicate that these rocks were most likely derived from the partial melting of a garnet-bearing amphibolite under a thickened lower crust condition. The positive whole-rock εNd(t) (+2.5 to +5.6) and zircon εHf(t) (+8.9 to +16.0) values suggest that this thickened lower crust was juvenile. The crust beneath the central Lhasa subterrane may have been significantly thickened due to tectonic shortening in response to the Lhasa–Qiangtang collision and magma underplating before the emplacement of Azhang adakitic rocks (~90Ma). We argue that regional lithospheric delamination at ~90Ma triggered the partial melting of the lowermost garnet-bearing crust that is still attached to the middle crust to generate the Azhang adakitic rocks. The presence of the ~90Ma Azhang adakitic rocks provides valuable constraints on the origin of the early Late Cretaceous magmatism in the central Lhasa subterrane and on the crustal evolution beneath the Lhasa–Qiangtang collisional zone prior to the Cenozoic India–Asia collision.