Metamorphism of the Amdo metamorphic complex, Tibet: Implications for the Jurassic tectonic evolution of the Bangong suture zone

Tibet consists of several terranes that progressively collided with the southern margin of Asia during the Mesozoic following the closure of intervening ocean basins. This Mesozoic amalgamation history, as well as the extent to which it may have contributed to crustal thickening prior to the Cenozoic Indo-Asia collision, remains poorly constrained and strongly debated. Here, we present a metamorphic petrological and U-Pb zircon geochronological study of the Amdo metamorphic complex, one of the few exposures of high-grade metamorphic rocks in central Tibet, located along the Bangong suture between the Qiangtang terrane to the north and the Lhasa terrane to the south. U-Pb ages of metamorphic zircon in gneiss constrain the timing of peak metamorphism at c. 178 Ma, prior to the Early Cretaceous collision between the two terranes. Peak P-T conditions of gneiss within the metamorphic complex are constrained by conventional as well as multi-equilibrium (THERMOCALC v.3.21 and v.3.33) geothermobarometry of two samples of garnet-amphibolite. Whereas THERMOCALC v.3.21 yields similar results as conventional geothermobarometry, THERMOCALC v. 3.33 yields dramatically lower pressures, mostly due to the change in the amphibole activity model used. Using THERMOCALC v.3.21, the two garnet-amphibolite samples yield similar P-T conditions of 0.83 ± 0.06 GPa at 646 ± 33 °C and 0.97 ± 0.06 GPa at 704 ± 35 °C. Plagioclase coronas on the garnet-amphibolite sample with lower peak P-T conditions indicate a period of isothermal decompression. Additional geothermometry on two garnet-free amphibolites yielded similar temperatures of 700-750 °C and suggests similar P-T conditions across most of the complex. However, two exposures of garnet-kyanite schist located along the southern edge of the metamorphic complex yielded slightly lower peak conditions of 0.75-0.85 GPa and 550-610 °C. Petrographic and field relations suggest the difference in metamorphic grade between the schist and gneiss is due to an intervening thrust fault. The existence of the thrust fault indicates that at least part of the exhumation of the complex was due to contractional deformation, possibly during the Lhasa-Qiangtang collision. Our P-T-t results indicate the occurrence of a significant Early Jurassic tectonothermal event along the southern, active margin of the Qiangtang terrane that deeply buried the Amdo rocks. We suggest that the metamorphism is a result of arc-related tectonism that may have been regionally extensive along the southern Qiangtang terrane; geological records of this tectonism may be rarely exposed along strike because of a lack of exhumation or subsequent depositional and structural burial.