Perovskite U-Pb and Sr-Nd isotopic perspectives on melilitite magmatism and outward growth of the Tibetan Plateau

摘要

Mantle-derived alkaline magmatism along major strike-slip faults provides unique insights into the lateral growth of the Tibetan Plateau. Here we use the geochemistry of perovskites from the West Qinling melilitite to probe into the nature and dynamics of sub-lithospheric mantle beneath the northeastern Tibetan Plateau. The texture and chemical composition of perovskites indicate their early crystallization from a CO2-rich melilitite magma. Most perovskite crystals have moderately depleted Sr-Nd isotopic compositions, whereas a few grains exhibit high Sr-87/Sr-86(i) and low epsilon(Nd)(t). Together with the bulk-rock geochemistry of the melilitite, the perovskite Sr-Nd isotope data imply that the primary magma parental to the melilitite was most likely derived from seafloor subduction-modified asthenosphere and underwent interaction with lithospheric mantle during ascent. In situ U-Pb dating of the perovskites demonstrates the temporal correlation between the melilitite magmatism and Kunlun strike-slip faulting in the early Miocene. These findings indicate the fundamental role of India-Asia convergence in driving outward plateau growth through strike-slip extrusion and in reactivating long-lived lithospheric zones of weakness for evacuating low-volume asthenospheric melts.