Differential exhumation and cooling history of North Qaidam UHP metamorphic rocks, NW China: Constraints from zircon and rutile thermometry and U-Pb geochronology

We analyze zircon and rutile U-Pb ages and trace elements using LA-ICP-MS and SIMS of eclogites from the North Qaidam ultra-high-pressure metamorphic belt in NW China. Zr-in-rutile and Ti-in-zircon thermometries give consistent peak metamorphic temperatures for two fresh eclogite samples. Zircon and rutile U-Pb dating from the Yuka eclogite yielded ages of 430.2±3.7Ma and 407±16Ma respectively. Zircons from North Dulan eclogite record two distinct eclogite-facies metamorphic events at 446.6±4.1 and 430.5±4.7Ma, with T_Ti-in-zircon=684±16°C and 649±26°C respectively, while rutile U-Pb dating for the same North Dulan eclogite sample yielded an age of 416±10Ma. These rutile U-Pb ages are interpreted to record the time when these rocks cooled down to the closure temperature of Pb in rutile (here, taken to be ca. 570°C). Thus, we obtain a slow cooling rate of ~3-4°C/Ma for Yuka and North Dulan terranes. For one granulite-facies overprinted South Dulan eclogite sample, rutile U-Pb dating gave a younger age of 383±10Ma. Combined with literature chronological and temperature data for South Dulan, the data imply later exhumation than the other samples with a faster cooling rate of ~6.6°C/Ma, after the granulite-facies event. The T-t paths obtained show that the North Qaidam eclogites experienced considerably slower cooling history than previously assumed, and also that while the thermal histories of the Yuka and North Dulan eclogites resemble one another, the South Dulan eclogite is quite distinct. So, a differential exhumation and emplacement of UHP metamorphic rocks occurred in the North Qaidam.