EARLY PALEOZOIC TECTONIC AND THERMOMECHANICAL EVOLUTION OF ULTRAHIGH-PRESSURE (UHP) METAMORPHIC ROCKS IN THE NORTHERN TIBETAN PLATEAU, NORTHWEST CHINA

Abstract

Coesite-and diamond-bearing ultrahigh-pressure (UHP) metamorphic rocks represent continen-tal materials that were once subducted to depths of >90 km. Identifying how these rocks were sub-sequently returned to Earth's surface has been a major challenge. Opinions on this matter vary widely, ranging from vertical extrusion of a coherent continental slab to channel flow of tectonically mixed mélange. To address this problem, we conducted integrated research across the North Qaidam UHP metamorphic belt using structural mapping, petrologic studies, and geochronologic and thermochronologic analyses. Our regional synthesis indicates that the early Paleozoic Qilian orogen, within which the North Qaidam UHP metamorphic belt was developed, was created by protracted southward oceanic subduction. The process produced a wide mélange belt and the Qilian magmatic arc. Arc magmatism was active between 520 and 400 Ma, coeval with North Qaidam UHP metamor-phism. The North Qaidam UHP metamorphic belt also spatially overlaps the early Paleozoic Qilian magmatic arc. Petrologic, geochronologic, and geochemical studies indicate that the protolith of the UHP metamorphic rocks was a mixture of continental and mafic/ultramafic materials, derived either from oceanic mélanges or pieces of a rifted continental margin tectonically incorporated into an oceanic subduction channel. These observations require that the North Qaidam UHP metamorphic rocks originated at least in part from continental crust that was subducted to mantle depths and then transported across a mantle wedge into a coeval arc during oceanic subduction. Upward transport of the UHP rocks may have been accommodated by rising diapirs launched from a mélange channel on top of an oceanic subducting slab.