Subduction, ultrahigh-pressure metamorphism, and regurgitation of buoyant crustal slices — implications for arcs and continental growth
Intracontinental collisional belts that retain mineralogic relics of Phanerozoic ultrahigh-pressure (UHP) metamorphism are increasingly being recognized. Adjacent regions generally lack evidence of coeval calcalkaline volcanism/plutonism. Following consumption of intervening oceanic lithosphere, each orogen marks the site of a deeply subducted microcontinental promontory, fragments of which later returned to midcrustal levels; mafic and ultramafic rocks are volumetrically minor. Pressures in UHP complexes approached or exceeded 2.8 GPa at 600–900°C. Subduction zones involve low-T prograde gradients, and constitute the only plate-tectonic realm where such conditions exist. Although internal portions of descending plates may have even lower geothermal gradients, crustal upper margins are typified by P–T trajectories of 5–10°C/km. Attending underflow, clinoamphibole-rich metabasaltic rocks dehydrate at pressures >2.2–2.5 GPa. This accounts for the kinetically favorable solid state production of mafic eclogites and partial fusion of MORBs to form hydrous andesitic melts. Devolatilization of partly serpentinized mantle lithosphere also introduces H2O to the magmagenic zone. As large masses of continental crust enter deeper portions of subduction zones, mineralogic transformations slow or cease because micas, the major OH-bearing phases in such rocks, retain volatile contents to pressures >4 GPa. Persistence of hydroxyl-bearing layer silicates may explain the metastable preservation of low-density assemblages in continental crust subjected to UHP conditions, and the dearth of calcalkaline magmatism in most collision belts. Exhumed UHP terranes are intensely deformed and disaggregated. All consist dominantly of quartzofeldspathic compositions; mafic eclogites±garnet peridotites constitute only a few volume percent. Eventual decoupling of the relatively low-density subducted crust promotes buoyancy-propelled transport to shallow depths along the subduction channel, with the hanging-wall lithosphere acting as a stress guide. Extensional collapse and erosional unroofing of rising masses result in the continued ascent of deeply subducted but buoyant crustal material. Although equidimensional massifs must return surfaceward, large volume-to-surface areas render the contained UHP phase assemblages virtually unquenchable. Surviving UHP terranes, thus, consist of relatively thin slabs of continental crust. Allochthons that retain UHP relics apparently lost heat by conduction across both upper, normal fault and lower, reverse fault contacts. Based on present exposures, estimated thicknesses of UHP sheets are: Qinling–Dabie–Sulu belt=5 km; Kokchetav complex=1–3 km; Dora Maira Massif=1–2 km; Western Gneiss Region=>1 km. Slices evidently rose to midcrustal levels at exhumation rates approaching 10 mm per year. Back reaction attending decompression in all cases was nearly complete. Where UHP mineral inclusions persisted, retrogression apparently was limited by the relative impermeability of the rocks to catalytic aqueous fluids and by declining temperatures; the coarse grain size of host phases also disfavored complete retrogression.