Early Palaeozoic North Qaidam UHP metamorphic belt on the north‐eastern Tibetan plateau and a paired subduction model

Abstract

ABSTRACT An Early Palaeozoic UHP metamorphic belt was recently discovered in the North Qaidam Mountains in the north‐eastern part of the Tibet Plateau. The belt contains abundant eclogite with lesser amounts of garnet peridotite and gneiss. The paragneiss contains abundant zircon with coesite inclusions whereas the eclogite has garnet with coesite pseudomorphs and omphacite with exsolved quartz. The calculated peak metamorphic conditions for the eclogite are T = 730 °C and P = 2.8 GPa. The garnet peridotites are magnesium‐rich with Mg#s [100 × Mg/(Mg + Fe)] up to 92. Olivine in the peridotites is highly magnesian (Fo > 91.5) and TiO 2 ‐poor and is typical of low‐ T garnet peridotites. Calculated peak metamorphic conditions for the peridotites are T = 837 °C and P = 2.5 GPa. U–Pb and Sm–Nd isotope dating indicate that continental subduction started during the early Ordovician (∼ 495 Ma). The North Qaidam UHP, together with the North Qilian HP belt about 300 km to the north, constitute a pair of genetically related early Palaeozoic subduction zones along the northern margin of the Tibet Plateau.