Monazite and Zircon Record Proterozoic and Paleozoic Metamorphism of Paragneiss, North Qaidam UHP Terrane, NW China

Amphibolite-facies felsic gneisses near Dulan, at the southeast end of the North Qaidam terrane, enclose minor (<10 vol%) eclogite and peridotite which record ultra-high pressure (UHP) metamorphism. Field relations, and coesite inclusions in zircons from paragneiss suggest that felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. SHRIMP-RG U-Pb geochronology and trace element analyses of four paragneiss samples constrains the ages of inheritance and metamorphism. Inherited zircon cores contain high Th/U, and REE concentrations consistent with magmatic zircon of detrital origin. Ages are variably discordant, dominantly 1.4-2.5 Ga, and suggest that the sedimentary protoliths were derived from cratonal continental crust. For three samples, zircon rims and whole grains are medium to dark in CL , with high U and low Th/U (0.04- 0.004). Negative Eu-anomalies are absent, and HREE are depleted relative to detrital cores, suggesting growth in the absence of plagioclase and in the presence of garnet in the eclogite-facies. Monazite is weakly zoned in BSE images. Weighted mean ages are: 430 ± 4 Ma (Zrn) and 438 ± 4 Ma (Mnz); 427 ± 5 Ma (Zrn) and 419 ± 3 Ma (Mnz); and 421 ± 3 Ma (Zrn; no Mnz from this sample). Zircon rims in the fourth sample contain higher Th/U (~0.1), negative Eu-anomalies, and are enriched in HREE, suggesting growth in the presence of plagioclase and in the absence of garnet. With the exception of two Paleozoic analyses, zircon rims yield a weighted mean age of 927 ± 6 Ma. Monazite from this sample yields a weighted mean age of 433 ± 3 Ma, consistent with the two Paleozoic zircon analyses. The Proterozoic zircon age is identical to ages of metagranites from this area, suggesting metamorphism of sediments accompanied granite intrusion, as has recently been reported from the Xitieshan and Lüliangshan localities 200-300 km to the northwest. The Paleozoic zircon and monazite ages overlap the 449-422 Ma range of zircon ages from eclogite enclosed in the paragneiss, consistent with in situ metamorphism of both rock types during ~25 m.y. duration of eclogite-facies conditions.