Cooling history for North Qaidam UHPM belt: Constraints from zircon, rutile U-Pb dating and thermometry in paragneiss

We analyzed zircon and rutile U-Pb ages and trace elements using LA-ICP-MS and SIMS of paragneiss from Shaliuhe cross-section, North Qaidam ultra-high pressure metamorphic belt. Inherited zircon cores record older protolith ages of > 700Ma. While metamorphic zircon rims give weighted average U-Pb age of 425 +/- 6Ma, and corresponding TTi-in-zircon = 689 +/- 14 degrees C. The rutile give a U-Pb age of 414.0 +/- 6.3Ma, which may represent the time of the paragneiss cooled down to the closure temperature of U-Pb in rutile (here, taken as about 570 degrees C). The rutile thermometry of Zr-in-rutile gives same temperature of 685 +/- 9 degrees C as the zircon thermometry. This indicates both rutile and zircon may preserve the peak metamorphic temperature information. We can construct a T-t path that starts at conditions and time given by the zircon temperature and U-Pb age, and ends with passage through the rutile closure temperature and the rutile age. A cooling rate of about 11 degrees C/Myr is obtained through this T-t path. Compared with the literature rutile data of eclogites (Yuka and North Dulan) from this belt, country paragneiss rocks experienced similar cooling history as the enclosed eclogites.