METAMORPHISM AND TECTONICS OF PHANEROZOIC ULTRAHIGH-PRESSURE (UHP) COLLISIONAL COMPLEXES

UHP metamorphic terranes reflect descent of continental crust to depths of 90-140 km in now-imbricated contractional orogens. For well-documented UHP belts, recrystallization took place in Phanerozoic time. Rocks are thoroughly retrogressed to lower pressure mineral assemblages; relict UHP phases are retained only under kinetically inhibiting circumstances, mainly in tough, refractory host minerals. UHP complexes consist chiefly of quartzofeldspathic rocks ± serpentinites; mafic-ultramafic lithologies comprise less than 10 % of each exhumed subduction complex. Many are thin, allochthonous sheets, but the two largest, in China and Norway, are ~10 km thick. SHRIMP dating of zoned zircons from gneissic and eclogitic rocks constrains the timing of protolith, peak, and retrograde recrystallization. Roundtrip P-T paths are completed in ~10-20 Myr; rates of ascent to mid-crustal levels approximate descent velocities. Sialic crust may be carried to great depth as an integral part of a largely oceanic subducting plate. Exhumation typically involves adiabatic decompression and back reaction through the P-T fields of much lower pressure metamorphic facies. Thin-aspect-ratio, ductiley deformed nappes, generated by subduction-zone shear forces conduct heat away as UHP complexes rise, effectively cooling the sheets. Ascent along the subduction channel is driven by the buoyancy of a mainly quartzofeldspathic slab relative to the surrounding mantle. Rapid exhumation prevents establishment of a more normal geothermal regime in the subduction-zone environment. Combined with vigorous erosion, late-stage underplating, contraction, tectonic aneurysms and/or lithospheric plate shallowing may elevate mid-crustal UHP décollements in domical uplifts. Unless quenched, UHP complexes are totally transformed to lower pressure mineral assemblages, obliterating evidence of past profound burial. Lack of Archean and all but the youngest Proterozoic UHP complexes probably reflects planetary thermal relaxation and progressive thickening of cooler lithospheric plates.