Abstract The Ti-in-zircon thermometer has been widely applied to different high-grade metamorphism rocks owing to its simplicity. Based on the calibration of Ferry & Watson (2007 ; New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers. Contributions to Mineralogy and Petrology , 154 , 429–437), we used the Ti-in-zircon thermometer to estimate the metamorphic temperatures for low-temperature eclogites of North Qilian and Western Tianshan, China, and compiled HP/UHP eclogites from the literature. The Ti-in-zircon temperatures are generally higher than the estimations by other thermometers, especially when metamorphic temperatures are lower than 650°C. Although temperature exerts the dominant control on Ti content in zircon, other factors (e.g. lattice defect, other trace element substitutions in zircon, disequilibrium zircon growth, and precipitation from metamorphic fluids) may have contributed to the overestimated Ti-in-zircon temperatures.

Abstract The Altun–Qilian–North Qaidam (AQQ) orogenic system in northern Tibet is considered to be the northernmost orogenic collage of the Prototethyan domain. It is regarded as resulting from collisions between various continental terranes derived from the northern margin of Gondwana, although the AQQ orogenic system also includes abundant ophiolites, arc magmatic rocks and subduction–accretion complexes. Some researchers regard the orogenic system of north Tibet as a typical accretionary orogen built by the development of an evolving arc–accretion complex growing southwards along the margin of the Tarim and North China cratons during the Paleozoic. We propose, based on both published data and our new data, that two distinct accretion and collision orogenies developed in the AQQ during the early Paleozoic. The diagnostic marks are HP–LT metamorphic rocks in the North Altun–North Qilian Mountains and UHP metamorphic rocks in the South Altun–North Qaidam Mountains. A review of metamorphic, geochronological, geochemical and structural data indicates that the North Altun–North Qilian HP–LT metamorphic belt is related to early Paleozoic subduction–accretion and, together with ophiolite mélanges and arc metamorphic–magmatic complexes, forms an early Paleozoic accretionary orogen. By contrast, the South Altun–North Qaidam UHP metamorphic belt is associated with continental subduction and collision, accompanied by Barrovian-type metamorphic overprinting and collision-related magmatism, reflecting an early Paleozoic collisional orogeny.