Petrotectonics of ultrahigh-pressure crustal and upper-mantle rocks—Implications for Phanerozoic collisional orogens

Abstract

Ultrahigh-pressure (UHP) metamorphic terranes in contractional orogens refl ect descent of continental crust bonded to a dense, dominantly oceanic plate to depths of 90‐140 km. All recognized well-documented UHP complexes formed during Phanerozoic time. Rocks are intensely retrogressed to low-pressure assemblages, with rare relict UHP phases retained in tough, refractory host minerals. Resurrected UHP slabs consist chiefl y of quartzofeldspathic rocks and serpentinites; dense mafi c + ultramafi c lithologies comprise <10% of exhumed masses. Associated garnet-bearing ultramafi c lenses are of four general origins: type A peridotite + eclogite pods refl ect premetamorphic residence in the mantle wedge; type B masses were mantle-derived ultramafi c-mafi c magmas that rose into the crust prior to subduction; type C tectonic lenses were present in the oceanic lithosphere prior to underfl ow; and type D garnet peridotites achieved their deep-seated mantle mineralogy long before—and independent of—the subduction event that produced the UHP-phase assemblages in garnet peridotite types A, B, and C. Geochronology constrains the timing of protolith, peak, and retrograde recrystallization of gneissic, ultramafi c, and eclogitic rocks. Roundtrip pressure-temperature (P-T ) paths were completed in <5‐10 m.y., where ascent rates approximated subduction velocities. Exhumation from profound depth involves near-adiabatic decompression through P-T fi elds of much lower-pressure metamorphic facies. Many complexes consist of thin, allochthonous sheets, but those in eastern China and western Norway are about 10 km thick. Ductilely deformed nappes generated in subduction zones allow heat to be conducted away as sheet-like UHP complexes rise, cooling across both upper and lower surfaces. Thicker UHP massifs also must be quenched. Ascent along the subduction channel is driven mainly by buoyancy of low-density crustal material relative to the surrounding mantle. Rapid exhumation prevents establishment of a more normal geothermal regime in the subduction zone. Lack of H 2 O impedes back reaction, whereas its presence accelerates transformation