ZIRCON GEOCHRONOLOGY OF ECLOGITE, ORTHO-, AND PARAGNEISS, NORTH QAIDAM UHP TERRANE, NORTHWEST CHINA

Amphibolite-facies felsic gneisses of the North Qaidam Mountains enclose minor (<10 vol.%) eclogite, peridotite and pyroxenite (±garnet) which record ultra-high pressure (UHP) metamorphism. Field relations, and the presence of coesite inclusions in zircons from paragneiss suggest felsic, mafic, and ultramafic rocks all experienced UHP metamorphism and a common amphibolite-facies retrogression. Cathodoluminescence (CL) and SHRIMP-RG U/Pb and REE analyses of zircons from two granitic orthogneisses (collected near Dulan, at the southeast end of the North Qaidam UHP terrane) reveal oscillatory zoning, moderate to high Th/U, U, and REE concentrations, and large Eu anomalies, consistent with magmatic crystallization at ca. 936 Ma (all ages are ²³⁸U/²⁰⁶Pb, ²⁰⁷Pb corrected for common Pb, except as noted). Zircon cores from two paragneisses contain high Th/U, moderate to high U and REE concentrations, prominent Eu anomalies, and yield discordant Early Proterozoic ²⁰⁷Pb/²⁰⁶Pb ages, and are probably of detrital origin, but zircon rims contain very low Th/U, high U, low REE abundances, and Eu anomalies are small or absent, suggesting eclogite-facies growth. Weighted mean ages of the rims are 418±3 Ma and 411±3 Ma. Two fresh eclogites contain zircon with CL dark cores and rounded grey overgrowths. Analyses of cores (partial overlap on rims) yield moderate to high Th/U and U, and ages up to 475 Ma, which places a minimum age on eclogite protolith crystallization; the rims yield low Th/U, moderate to high U, and weighted mean ages of 449±3 Ma and 440±4 Ma, reflecting metamorphic growth. Analyses of zircon from two retrogressed eclogites reveal CL bright, mottled cores (± faint oscillatory zoning) surrounded by irregular, dark rims characterized by low Th/U, and low to very low U, which yield weighted mean ages of 421±5 Ma and 415±12 Ma, reflecting metamorphic growth. The spread in metamorphic ages (449-411 Ma) is probably too large to be explained by a single metamorphic event, and suggests that some of the mafic enclaves record polymetamorphic/tectonic histories prior to their incorporation in the surrounding gneisses. Juxtaposition of the different rock types probably occurred during exhumation from UHP conditions.