Giant continental ultra-high pressure terranes: continental remodeling and recycling by subduction of continental margins

Giant ultra-high pressure (UHP) metamorphic terranes such as the Western Gneiss Complex in the Scandinavian Caledonides and Dabieshan in central China probably resulted from wholesale subduction of passive continental margins (PCM) during continental collision. PCM sediments are large repositories of volatiles including hydrates, nitrogen species, carbonates and hydrocarbons. Sediments and upper/mid-crustal basement are rich in incompatible elements and are fertile for melting. Lower crust may be more mafic and refractory. Juvenile rift-related mafic rocks also have the potential to generate substantial volumes of granitoid melts, especially if they have been hydrated. Exposed giant UHP terrains demonstrate the largely coherent return (eduction) of continental crust from mantle depths. They show evidence for substantial fluxes of aqueous fluid, anatexis and, in entrained orogenic peridotites, metasomatism of mantle rocks by crust- derived C-O-H fluids. However, substantial bodies of continental material may never return to the surface as coherent masses of rock, but remain sequestered in the mantle where they melt or become entrained in the deeper mantle circulation.

During subduction, PCM’s become partitioned by a range of mechanisms. Mechanical partitioning strips away weaker sediment and middle/upper crust, which circulate back up the subduction channel and never reach depths at which high pressure metamorphism and anatexis occur. However, denser, stronger transitional pro-crust and lower crust may “stall” near the base of the lithosphere or be irreversibly subducted to join the global mantle circulation. Under certain conditions some sediment and upper crustal basement may reach depths for UHP metamorphism. Further partitioning then takes place by anatexis, which either aids stripping and exhumation of the more melt-prone rock-masses through mechanical softening, or separates melt from residuum so that melt escapes and is accreted to the upper plate leading to “undercrusting”, late-orogenic magmatism and further refining of the crust. Melt that traverses sections of the mantle wedge will interact with it causing metasomatism and refertilisation. Partitioning also takes place by solid-fluid and melt-fluid partitioning; dehydration may take place both during subduction and exhumation, and fluxes between dehydrating and hydrating rock masses influence the internal fluid budget of the orogen
(essential for eclogitisation and densification of mafic lithologies). Ascending granitic melts advect dissolved water to shallow levels, or even the atmosphere. Irreversible subduction of PCM sediment carries water plus nitrogen and carbon species to the deeper mantle. Decarbonation of voluminous PCM carbonates depends on the thermal regime and in warm regimes may release a pulse of CO2 to the atmosphere, but this is limited in colder subduction
zones, which therefore have the potential to transfer significant volumes of carbon to the deep mantle.

Exhumation of giant UHP terranes appears to be largely dominated by tectonic processes rather than erosion. Evidence from sedimentary provenance suggests that their sediment yield
is limited and hence erosion of giant UHP terranes is not likely to be a major contributor to continental recycling.