Comparative Petrotectonic Study of Five Eurasian Ultrahigh-Pressure Metamorphic Complexes

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Abstract
The Dabie-Sulu belt of east-central China, the Kokchetav Complex of northern Kazakhstan, the Maksyutov Complex of the southern Urals, the Dora Maira Massif of the Western Alps, and the Western Gneiss Region of southwestern Norway all contain mineralogic/textural relics indicating the presence or earlier occurrence of coesite and/or diamond. Other ultrahighpressure phases, including K-rich clinopyroxene, Mgrich garnet, ellenbergerite, lawsonite, zoisite, glaucophane, and associations such as magnesite + diopside, talc + kyanite, diopside, high-silica phengite, and/or jadeite, also attest to metamorphic pressures approaching or even exceeding 25 to 30 kbar. Subduction zones constitute the only known plate-tectonic environment where such P-T conditions exist. Many of these UHP relics occur as tiny inclusions encased in dense, refractory, unreactive mineralogic hosts, and hence were preserved during return transit toward normal P-T conditions characteristic of midcrustal levels. Each of the exhumed ultrahigh-pressure complexes studied consists dominantly of old continental crust, but in all cases ultrahigh-pressure metamorphism took place during Phanerozoic time. These profoundly buried, then resurrected, terranes appear to consist of relatively thin sheets (7 ± 5 km thick). As a group, they appear to have returned to midcrustal levels rapidly (1 to 6 mm/yr) after cessation of the ultrahigh-pressure subduction-zone recrystallization. In all five intracontinental suture belts reviewed, adjoining terranes exhibit little or no evidence attesting to the former presence of a contemporaneous calc-alkaline arc.