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

摘要

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 °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 (ε(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 ~ 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.