Petrotectonic Significance of High and Ultrahigh-Pressure Metamorphic Belts: Inferences for Subduction-Zone Histories

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Abstract
The 900-km-long Franciscan accretionary prism of the California Coast Ranges consists chiefly of greywackes and dark shales of late Mesozoic age. The complex is characterized by monotonic growth toward the edge of the continent and by the systematic, ocean ward decrease in metamorphic intensity. High-pressure (HP) mineral assemblages include lawsonite, aragonite, glaucophane, and/or jadeitic pyroxene. Systematic seaward vergence and younging of the Franciscan, as well as progressively lower metamorphic grade, indicate that landward parts of the belt have been exhumed more profoundly than sections farther outboard. Mapped metamorphic/structural discontinuities reflect juxtaposition of spatially associated, co-evolving continental margin lithotectonic units, and, except for far-travelled oceanic entities, do not imply amalgamation of exotic, unrelated terranes. Pacific-type underflow, metamorphism, and tectonic imbrication characterize belts in which oceanic lithosphere has returned to the mantle. Resurrected, largely metasedimentary, trench complexes display mineral parageneses suggesting maximum retrieved burial depths of ∼ 25-30 km. Analogous HP metamorphic belts fringe the Pacific Basin (e.g., Indonesian Archipelago, Sanbagawa belt of SW Japan, Kamuikotan terrane of Hokkaido and Sakhalin Island, Chugach-Kenai-Kodiak belt of southern Alaska, and Western Series of the Chilean Coastal Range). The Dabie Mountains of east-central China represent a segment of the more than 2000-kmlong Qinling-Dabie-Sulu suture zone between the Sino-Korean and Yangtze cratons. Eastern extensions of the belt apparently cross the Korean Peninsula, and crop out along the Japan Sea margin of Kyushu and western Honshu; a NE tract may be present in the Sikhote-Alin of the Russian Far East. This orogenic belt reflects the latest Paleozoic/early Mesozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs. The preservation of coesite and diamond as inclusions in strong, refractory mineral constituents in eclogite facies ultrahigh-pressure (UHP) metamorphic crustal rocks (mafic eclogites, quartzofeldspathic gneisses, and marbles) attests to profound subduction of the Dabie-Sulu area, a leading salient of the old, cold Yangtze craton. The northward increase in intensity of subsolidus recrystallization of the suture complex is comparable to the internal progression in grade of HP and UHP metamorphism in the Western Alps. Plate-tectonic histories of both regions evidently included continental collision and underflow, ultrahigh-pressure metamorphism, and then return toward midcrustal levels of low-density (but coherent) microcontinental blocks as a result of decoupling and continued descent of the higher-density oceanic plate. Other UHP metamorphic complexes include the Kokchetav complex of Kazakhstan, the Maksyutov massif of southern Russia, and the Western Gneiss Region of Norway. In all cases, fragments of ancient sialic-crustcapped lithosphere subducted to depths in excess of 80-100 km have been recovered. Pacific-style HP metamorphic belts now exposed at the surface recrystallized under physical conditions up to about 400-500 °C and 12-14 kbars; Alpine-type UHP parageneses include similar phases and assemblages but, in addition, contain traces of others requiring conditions of 600-800 °C and 28-35 kbars or more. Broad tracts of buoyant, quartzofeldspathic debris carried downward on oceanic lithosphere (Pacific-type subduction) tend to choke trenches with voluminous, weak materials that decouple at 25-30 km depth. In contrast, as they enter the suture zone, narrow salients of subducted continental crust (Alpine-type subduction) appear to be negatively buoyant in regional terms, are therefore carried to much greater depths, remain intact, and may rise differentially as coherent blocks. In both cases, low-density quartzofeldspathic material displaces denser mantle, hence the return to mid and upper crustal levels is propelled by body forces.