Metamorphism and tectonics of a continental arc root: Insights from garnet amphibolite in the North Qaidam tectonic belt, northern Tibet

The architecture of continental arc crust from its surface to base is a crucial aspect of understanding long-term crustal growth and reworking. However, the timing and nature of metamorphism in the middle to lower crust are some of the least-understood components of continental arc systems. In this contribution, we integrate phase equilibrium modeling and zircon geochronology to determine the metamorphic history of garnet amphibolite in the North Wulan metamorphic complex, North Qaidam tectonic belt, northern Tibet, and then provide key insights into the metamorphic nature of the middle to lower crust of an early Paleozoic continental arc, and into orogenic geodynamic processes associated with the closure of the Proto-Tethyan Ocean. Garnet amphibolite contains a peak metamorphic assemblage of garnet + orthopyroxene + hornblende + plagioclase + ilmenite + quartz ± biotite + melt, which defines a peak/near-peak pressure-temperature (P-T) range of 800−860 °C at 5−8.5 kbar, based on phase equilibrium modeling. Rounded inclusions of hornblende, plagioclase, and quartz in garnet porphyroblasts suggest prograde garnet growth at the expense of amphibole. Partial replacement of garnet and hornblende by orthopyroxene suggests high-T decompression after peak pressure. Post-peak partial replacement of garnet by plagioclase−quartz−hornblende or hornblende corona suggests decompression cooling after peak temperature. These results suggest that the garnet amphibolite records a clockwise P-T path with high-T decompression. These currently exposed granulite-facies rocks from the southern part of the North Wulan metamorphic complex represent a middle to lower crustal exposure (root) of an early Paleozoic continental arc−backarc setting (depths of >16−28 km) during the subduction of the Proto-Tethyan Ocean. Zircon petrochronology in garnet amphibolite reveals that protracted metamorphism occurred at ca. 515−449 Ma. These new insights into the early Paleozoic tectonometamorphic evolution of the North Wulan metamorphic complex constrain the northward subduction polarity of the South Qilian Ocean and the tectonic evolution of the North Qaidam tectonic belt. Clockwise P-T paths in continental arc settings are related to the thinning of the thickened arc crust triggered by the steepening of subducted oceanic slab, which is accompanied by slab retreat and rollback and could reflect the migration of continental arc magmatic axes. Our new results tie together high-resolution thermal, magmatic, and metamorphic records to explain the complex evolution of the roots of continental arc systems.