HR: 13:45h
AN: T23C-01    [Abstracts]
TI: Preservation/exhumation of ultrahigh-pressure (UHP) complexes
AU: * Ernst, W G
EM: ernst@geo.stanford.edu
AF: Dept. Geological & Environmental Sci, Bldg. 320, Room 118, Stanford University, Stanford, CA 94305-2115 United States
AB: Recognized UHP terranes around the World share several unique characteristics: (1) All reflect descent of superjacent segments of continental crust to depths of ~90 km or more in compressional orogens. (2) All are thoroughly overprinted by lower pressure mineral assemblages, and rare relict UHP phases are preserved only in kinetically inhibiting circumstances such as inclusions in refractory host minerals. (3) Preserved UHP complexes consist exclusively of thin, allochthonous sheets. (4) Dense mafic + peridotitic rock types make up less than ~10 % of each UHP terrane, the major lithologies being lower density quartzofeldspathic units ñ serpentinites. (5) Prograde + retrograde P-T paths are completed in ~5-10Myr, and rates of ascent approximate descent velocities. In addition, domical uplifts from mid-crustal levels characterize many UHP complexes. Continental crust can be carried down to great depths only in subduction zones, reflecting underflow of a chiefly oceanic plate prior to continental collision/suturing. Because exhumation involves decompression through the P-T stability fields of much lower pressure metamorphic facies, UHP relics are partially preserved mainly in tough, watertight minerals (e. g., zircon and garnet) in which their isolation prevents back-reaction. Thin-aspect-ratio, ductile-deformed nappes, generated by subduction-zone shear forces, conduct heat away from UHP complexes as they rise along the subduction channel, effectively cooling the sheets. Ascent is driven by buoyancy propulsion because the aggregate density of a dominantly quartzofeldspathic slice is considerably less dense than the mantle it had displaced. Relatively rapid ascent is required to defeat the establishment of a more normal geothermal regime in the subduction-zone environment. Tectonic aneurysms locally carry mid-crustal UHP d‚collements surfaceward. Where any one of these situations is not satisfied, UHP complexes will be totally transformed to lower pressure mineral assemblages, obliterating all evidence of past burial.
DE: 8102 Continental contractional orogenic belts
DE: 3660 Metamorphic petrology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting