Two stages of granulite facies metamorphism in the eastern Himalayan syntaxis, south Tibet: petrology, zircon geochronology and implications for the subduction of Neo-Tethys and the Indian continent beneath Asia

The eastern Himalayan syntaxis in southeastern Tibet consists of the Lhasa terrane, High Himalayan rocks and Indus-Tsangpo suture zone. The Lhasa terrane constitutes the hangingwall of a subduction zone, whereas the High Himalayan rocks represent the subducted Indian continent. Our petrological and geochronological data reveal that the Lhasa terrane has undergone two stages of medium-P metamorphism: an early granulite facies event at c. 90 Ma and a late amphibolite facies event at 36-33 Ma. However, the High Himalayan rocks experienced only a single high-P granulite facies metamorphic event at 37-32 Ma. It is inferred that the Late Cretaceous (c. 90 Ma) medium-P metamorphism of the southern Lhasa terrane resulted from a northward subduction of the Neo-Tethyan ocean, and that the Oligocene (37-32 Ma) high-P (1.8-1.4 GPa) rocks of the High Himalayan and coeval medium-P (0.8-1.1 GPa) rocks of the Lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the Indian continent beneath Asia. Our results provide robust constraints on the Mesozoic and Cenozoic tectonic evolution of south Tibet.