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

Abstract

The Dabie‐Sulu belt of east‐central China, the Kokchetav Complex of northern Kazakhstan, the Maksyutov Complex of the South Urals, the Dora Maira Massif of the Western Alps, and the Western Gneiss Region of southwestern Norway lie astride intracontinental suture zones. All represent collisional mountain belts. Adjoining Eurasian regions exhibit little or no evidence of a coeval calc‐alkaline arc. Each metamorphic complex contains mineralogic and textural relics of the presence or former existence of coesite ± diamond. Other ultrahigh‐P, moderate‐T metamorphic phases, including K‐rich clinopyroxene, Mg‐rich garnet, ellenbergerite, lawsonite, Al‐rutile, glaucophane, high‐Si phengite, and associations such as coesite + dolomite, magnesite + diopside, and talc + kyanite, diopside, jadeite, or phengite also testify to pressures approaching or exceeding 2.8 GPa. Each of the five well‐studied Eurasian ultrahigh‐pressure complexes consists chiefly of old, cool continental crust. Deep‐seated recrystallization took place during the Phanerozoic. Subduction zones constitute the only known plate‐tectonic environment where such high‐P, low‐T conditions exist. A model involving underflow of a salient of continental crust imbedded in oceanic crust‐capped lithosphere explains the ultrahigh‐ pressure metamorphism. Partly exhumed ultrahigh‐pressure terranes consist of relatively thin sheets 7 ± 5 km thick. During early stages of plate descent, hydration of relatively anhydrous units occurs, and volatiles are expelled from hydrous rocks. If present, aqueous fluids markedly catalyze reactions. Experimental studies on MORB bulk compositions demonstrate that, for common subduction‐zone P–T trajectories, amphibole (the major hydrous phase in metabasaltic rocks) dehydrates at less than ~ 2.0 GPa; accordingly, mafic blueschists and amphibolites expel H2O at great depth and, except for some coarse‐grained, dry metagabbros, tend to recrystallize to eclogite. Serpentinized mantle beneath the oceanic crust devolatilizes at comparable pressures. In contrast, phengite and biotite remain stable to pressures exceeding 3.5 GPa in associated quartzofeldspathic rocks. So, under ultrahigh‐pressure conditions, the micaceous lithologies that dominate the continental crust fail to evolve significant H2O, and may transform incompletely to eclogitic assemblages. Although hydrous rocks expel volatiles during compaction and shallow burial, very deep underflow of partly hydrated oceanic crust + mantle generates most of the volatile flux along and above a subduction zone prior to continental collision. As large masses of sialic crust enter the convergent plate junction, fluid evolution at deep levels severely diminishes, and both convergence and dehydration terminate. After cessation of ultrahigh‐pressure recrystallization, tectonic slices of sialic massifs return to shallow depths along the subduction channel, propelled by buoyancy; collisional sheets that retain ultrahigh‐pressure effects lose heat efficiently across both upper (extensional, normal fault) and lower (subduction, reverse fault) tectonic contacts. These sheets ascend to midcrustal levels rapidly at average exhumation rates of 2–12 mm/year. Surviving ultrahigh‐pressure relics occur as micro‐inclusions encased in dense, strong, impermeable, unreactive mineralogic hosts, and are shielded during return towards conditions characteristic of midcrustal levels. Rehydration attending decompression is incomplete; its limited extent reflects the coarse grain size and relative impermeability of the rocks undergoing retrogression, as well as declining temperature and lack of aqueous fluids.