HIGH-PRESSURE AND ULTRAHIGH-PRESSURE METAMORPHIC BELTS - SUBDUCTION, RECRYSTALLIZATION, EXHUMATION, AND SIGNIFICANCE FOR OPHIOLITE STUDY

Abstract

Alpine-type orogenic belts are produced by short-lived subduction of an ocean basin and the underflow of continental crust, resulting in suturing. Old, relatively competent sialic basement and superjacent units characterize these complexes; ophiolites, chiefly enriched mid-ocean ridge basalt and suprasubduction-zone complexes are common in some belts, but are rare in others. Metamorphism of deeply subducted (60-140 km) parts of the orogen ranges from high pressure to ultrahigh pressure , and is not paired. Coeval calcalkaline arc rocks are uncommon. In contrast, paired Pacific-type orogenic belts develop within and landward from long-lived subduction zones. They consist of an outboard trench + accretionary prism deposited on oceanic crust, and an inboard volcanic-plutonic continental margin or island arc. The trench assemblage consists of graywacke + shaley mélanges, minor but widespread deepwater cherts and/or carbonates, and ubiquitous disaggregated ophiolites, especially normal mid-ocean basalt and ocean island basalt varieties, all recrystallized under high-pressure conditions at depths of 15-70 km. A massive, coeval calcalkaline arc dominates the subparallel terrane landward from the trench, and high temperatures characterize the associated metamorphism. In both high-pressure-ultrahigh-pressure Alpine and high-pressure Pacific-type metamorphic terranes, outboard thrust faults chiefly dip landward beneath the stable, nonsubducted plate, and fold vergence is seaward, reflecting similar convergent plate-tectonic settings; inboard, antithetic thrusting characterizes some contractional realms. High pressures and low temperatures in these belts attest to dynamic subduction-zone conditions. Because of their structural integrity, some Alpine-type microcontinents, island arcs, or continental promontories are carried down as much as 140 km, well into the brittle-ductile transition region, before decoupling from the sinking mantle lithosphere. In sharp contrast, incompetent Pacific-type graywacke + shale terranes generally separate from descending oceanic lithosphere at much shallower depths. The ascent of packets of subducted material as thin aspect-ratio sheets 1-5 km thick, combined with normal faulting above and subduction-zone thrusting below, promotes conduction cooling and partial preservation of decompressing high-pressure and ultrahigh-pressure metamorphic complexes. Worldwide, subduction-zone terranes consist of dominantly of small masses of high-density mafic blueschist-eclogite and/or anhydrous peridotitic lenses surrounded by voluminous, low-density quartzofeldspathic ± serpentinitic material, hence the bulk terrane density is less than that of unaltered, dynamically displaced mantle; exhumation to mid-crustal levels is largely buoyancy driven. Partial preservation of high-pressure-ultrahigh-pressure relict phases reflects rapid ascent combined with mineralogic armoring and a general lack of catalytic aqueous fluid. Exposure of rising high-pressure and ultrahigh pressure metamorphic belts is a consequence of erosional decapitation and gravitational collapse of the subduction complex. Many exhumed high-pressure-ultrahigh- pressure sheets are of relatively small volume, and for these, only modest amounts of sedimentary debris are provided to successor basins.