Late Cretaceous (∼81Ma) high-temperature metamorphism in the southeastern Lhasa terrane: Implication for the Neo-Tethys ocean ridge subduction

An integrated study of U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions of garnet-bearing granulite and marble from the granulite-facies domain of the Nyingchi Complex (eastern Himalayan syntaxis) has provided insights into the tectonic evolution of the southern Lhasa terrane. The peak metamorphism of the garnet-bearing granulite is marked by a mineral assemblage of garnet + orthopyroxene + high-Ti amphibole + plagioclase + quartz + rutile. Abundant exsolved rutile needles are observed within amphibole, garnet and quartz. The peak metamorphic temperatures are estimated at 803-924 degrees C. Geochemical data from the garnet-bearing granulites provide evidence for a basaltic protolith that formed in a continental-margin arc setting. Sr-Nd-Hf isotopic compositions indicate that this protolith was sourced from partial melting of a depleted mantle. LA-ICP MS U-Pb zircon dating shows that the protolith age and metamorphic age of the garnet-bearing granulites are 89.3 +/- 0.6 Ma and 81.1 +/- 0.8 Ma, respectively. The detrital magmatic zircons from the marble yield ages from 86.3 to 167 Ma. The age distribution and Hf isotopic composition (epsilon(Hf)(t) = + 5.9 to + 17.5) of the detrital magmatic zircon in the marble are consistent with the isotopic data of zircons from the Jurassic-Cretaceous Gangdese batholiths, suggesting that the clastic sediments were partially derived from these intrusives or associated volcanic rocks, and deposited in the fore-arc basin of the Gangdese arc. The metamorphic zircons in the marble yield a metamorphic age of 81.4 +/- 0.5 Ma. These results show that both the arc magmatic rocks and forearc sedimentary rocks underwent high-temperature (HT) granulite-facies metamorphism at similar to 81 Ma, indicating anomalously high heat input in the forearc region. A range of tectonic observations, including a coeval hiatus in arc magmatism and a period of regional uplift, indicate that HT metamorphism resulted from the subduction of the Neo-Tethys ocean ridge beneath the southern Lhasa terrane during the Late Cretaceous. (C) 2013 Elsevier B.V. All rights reserved.