Are low-velocity zones within the Tibetan crust the result of crustal melting from at least 28 Ma?

Geophysically identified low-velocity zones (LVZs) are widespread within the Tibetan lower–middle crust, but their nature and role in models of the development of the Tibetan Plateau remain controversial. The debate stems mainly from whether the Tibetan crust was hydrous and has therefore undergone substantial melting. Here, we identify hydrous crustal xenoliths incorporated into 28 Ma syenite porphyries from central–northern Tibet. These xenoliths indicate the former existence of a cold (T = 680–790 °C) and water-rich (1.10–1.50 wt%) crustal end-member at a depth of 14–40 km in the Tibetan lower–middle crust. Our new petrological evidence indicates the LVZs are partially molten layers that have existed within the Tibetan crust since at least 28 Ma. High- to ultrahigh-temperature metamorphism since the Miocene is inferred to have triggered widespread melting of the former hydrous crust, which would have promoted the development of LVZs and resulted in a flat Tibet.