Origin of Early Eocene diabase dikes in the southern Lhasa terrane: Implications for the Neo-Tethyan slab breakoff

The geodynamic evolution of the Himalayan-Tibetan orogen during the transition from Neo-Tethyan oceanic to Indian continental subduction remains unclear. Therefore, we conducted a new geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic study of post-collisional diabase dikes in the central segment of the southern Lhasa subterrane. The diabase dike samples yield zircon U-Pb ages of 51-49 Myr, consistent with the timing of the Gangdese magmatic flare-up (ca. 53-50 Myr). In addition, the diabase dikes exhibit a geochemical intraplate affinity and depleted Sr-Nd-Pb isotopic compositions. Their variable Nb contents allow, the dikes to be divided into low-Nb (<8 ppm) and high-Nb (>9 ppm) types, which represent the melting products of slab fluid-modified Gangdese lithospheric mantle and a mixed mantle source comprising Gangdese lithospheric and deep asthenospheric mantle, respectively. In addition, the wide variation in epsilon Hf(t) values (-10.0 to +12.3) of zircons from these diabase dikes is consistent with the Gangdese zircon Hf isotopic 'pull-down' at ca. 50 Ma. Integrating our new data with published results suggests that the zircon Hf isotopic 'pull-down' was a direct consequence of the involvement of subducted Indian continental components in the formation of the Gangdese continental lithosphere. The combined data further suggest that upwelling of the asthenosphere, triggered by breakoff of the Neo-Tethyan oceanic slab, resulted in extensive mantle and crustal melting in the Lhasa terrane and the Tethyan Himalaya in southern Tibet.