Rapid Incorporation of Metasediments Into Lower Crust of the Gangdese Magmatic Arc, Southern Tibet

Deeply exhumed crustal terranes of continental arcs worldwide commonly contain metasedimentary components, but the nature of these lithologies and how they became incorporated into the lower crust are not fully understood. Here, we present petrological, geochemical and geochronological data from exhumed deep-crustal metapelites and orthogneisses from the eastern Gangdese magmatic arc, southern Tibet. Geochemical and geochronological affinity between metapelites and forearc sediments indicates that their protoliths were probably deposited in an Early Cretaceous (<120 Ma) forearc basin of the Gangdese arc, while the orthogneisses represent metamorphosed equivalents of Late Jurassic (157–140 Ma) arc-type magmatic rocks. Petrological modeling and zircon U-Pb dating show that metapelites and orthogneisses simultaneously experienced high-pressure granulite-facies metamorphism at peak pressure-temperature conditions of 810–840°C and 12–16 kbar at 87–85 Ma. Our findings suggest that both the Mesozoic forearc sediments and igneous rocks that were initially emplaced into the upper crust of the Gangdese arc were subsequently transported to its lower crust within c. 25 Myr by crustal shortening, folding and underthrusting during the Late Cretaceous. When combined with previous data collected from the region, we propose that tectonic burial within arc crust and slab subduction-related mechanisms most likely operate together in magmatic arcs, promoting crustal recycling. The transport of surficial silica-rich materials into the lower crust is likely a basic process that has driven the growth and differentiation of continental arcs throughout geological time.