Metamorphism, partial preservation, and exhumation of ultrahigh-pressure belts

The Dabie-Sulu belt of east-central China, the Kokchetav Complex of norther n Kazakhstan, the Maksyutov Complex of the South Urals, the Dora Maira Mass if of the Western Alps, and the Western Gneiss Region of southwestern Norwa y lie astride intracontinental suture zones. All represent collisional moun tain belts. Adjoining Eurasian regions exhibit little or no evidence of a c oeval calc-alkaline are. Each metamorphic complex contains mineralogic and textural relies of the presence or former existence of coesite+/-diamond. O ther ultrahigh-P moderate-T metamorphic phases, including K-rich clinopyrox ene, Mg-rich garnet, ellenbergerite, lawsonite, Al-rutile, glaucophane, hig h-Si phengite, and associations such as coesite+dolomite, magnesite+diopsid e, and talc+ kyanite, diopside, jadeite, or phengite also testify to pressu res approaching or exceeding 2.8GPa. Each of the five well-studied Eurasian ultrahigh-pressure complexes consists chiefly of old, cool continental cru st. Deep-seated recrystallization took place during the Phanerozoic. Subduc tion zones constitute the only known plate-tectonic environment where such high-P, low-T conditions exist. A model involving underflow of a salient of con tinental crust imbedded in oceanic crust-capped lithosphere explains t he ultrahigh-pressure metamorphism. Partly exhumed ultrahigh-pressure terra nes consist of relatively thin sheets 7+/-5km thick. During early stages of plate descent, hydration of relatively anhydrous units occurs, and volatil es are expelled from hydrous rocks. If present, aqueous fluids markedly cat alyze reactions. Experimental studies on MORE bulk compositions demonstrate that, for common subduction-zone P-T trajectories, amphibole (the major hy drous phase in metabasaltic rocks) dehydrates at less than similar to 2.0 G Pa; accordingly, mafic blueschists and amphibolites expel HBO at great dept h and, except for some coarse-grained, dry metagabbros, tend to recrystalli ze to eclogite. Serpentinized mantle beneath the oceanic crust devolatilize s at comparable pressures. In contrast, phengite and biotite remain stable to pressures exceeding 3.5 GPa in associated quaptzofeldspathic rocks. So, under ultrahigh-pressure conditions, the micaceous lithologies that dominat e the continental crust fail to evolve significant H2O, and may transform i ncompletely to eclogitic assemblages. Although hydrous rocks expel volatile s during compaction and shallow burial, very deep underflow of partly hydra ted oceanic crust+mantle generates most of the volatile flux along and abov e a subduction zone prior to continental collision. As large masses of sial ic crust enter the convergent plate junction, fluid evolution at deep level s severely diminishes, and both convergence and dehydration terminate. Afte r cessation of ultrahigh-pressure recrystallization, tectonic slices of sia lic massifs return to shallow depths along the subduction channel, propelle d by buoyancy; collisional sheets that retain ultrahigh-pressure effects lo se heat efficiently across both upper (extensional, normal fault) and lower (subduction, reverse fault) tectonic contacts. These sheets ascend to midc rustal levels rapidly at average exhumation rates of 2-12 mm/year. Survivin g ultrahigh-pressure relies occur as micro-inclusions encased in dense, str ong, impermeable, unreactive mineralogic hosts, and are shielded during ret urn towards conditions characteristic of midcrustal levels. Rehydration att ending decompression is incomplete; its limited extent reflects the coarse grain size and relative impermeability of the Pocks undergoing retrogressio n, as well as declining temperature and lack of aqueous fluids.