Abstract:
Two apparently distinct, sub-parallel, paleo-subduction zones can be recognized along the northern margin of the Tibetan Plateau: the North Qilian Suture Zone (oceanic-type) with ophiolitic mélanges and high-pressure eclogites and blueschists in the north, and the North Qaidam Belt (continental-type) in the south, an ultrahigh-pressure (UHP) metamorphic terrane comprising pelitic and granitic gneisses, eclogites and garnet peridotites. Eclogites from both belts have protoliths broadly similar to mid-ocean ridge basalts (MORB) or oceanic island basalts (OIB) in composition with overlapping metamorphic ages (480-440Ma, with weighted mean ages of 464 ± 6Ma for North Qilian and 457 ± 7Ma for North Qaidam), determined by zircon U-Pb sensitive high-resolution ion microprobe dating. Coesite-bearing zircon grains in pelitic gneisses from the North Qaidam UHP Belt yield a peak metamorphic age of 423 ± 6Ma, 40Myr younger than the age of eclogite formation, and a retrograde age of 403 ± 9Ma. These data, combined with regional relationships, allow us to infer that these two parallel belts may represent an evolutionary sequence from oceanic subduction to continental collision, and continental underthrusting, to final exhumation. The Qilian-Qaidam Craton was probably a fragment of the Rodinia supercontinent with a passive margin and extended oceanic lithosphere in the north, which was subducted beneath the North China Craton to depths >100km at 423Ma and exhumed at 403Ma (zircon rim ages in pelitic gneiss). © The Author 2005. Pubished by Oxford University Press. All rights reserved.
