Early Paleozoic polymetamorphism and anatexis in the North Qinling orogen: Evidence from U-Pb zircon geochronology

High-grade metamorphic rocks in North Qinling orogen, traditionally regarded as the Qinling Group, consist of eclogites, high pressure granulites, medium-low P/T. granulite-amphibolite facies metamorphic rocks and experienced intensive migmatization. This paper presents a combined study of CL image, LA-ICP-MS U-Pb age, and trace element of zircon in migmatites from Shuanglong area in the Qinling Group. In a migmatitic gneiss, the inherited magmatic cores of zircon have oscillatory zoning and yield U-Pb ages of 941 +/- 11Ma (2 sigma, MSWD = 0. 59), representing the protolith age. The igneous zircon cores from a garnet biotite schist yield U-Pb age of 756 +/- 9. 9Ma ( 2 sigma, MSWD = 1. 07), indicating that the protolith of the metabase formed in the Neoproterozoic. Analyses of metamorphic zircon domains from the metabase yield a weighted mean Pb-206/U-238 ages of 484 +/- 9. 6Ma (2 sigma, MSWD = 0. 88); the metamprohic zircon rims from the gneiss also yield an age of 498 +/- 11 Ma. Most of these metamorphic zircons show typical metamorphic growth zoning, low Th/U ratios, flat HREE patterns, and weak Eu anomalies. All these characteristics and the ages are comparable with those of the metamorphic zircons in Qinling eclogites and high pressure granulites, indicating that the zircon crystals grew in the presence of garnet and absence of feldspar and thus under UHP-HP conditions. The zircon grains from a leucosome vein show weak cathodoluminescence, low Th/U ratios, flat HREE patterns, and Eu anomalies. They give a weighted mean U-Pb age of 422 +/- 4. 0Ma (2 sigma, MSWD = 0. 61), which was taken as registering the crystallization of the melt of partial melting. A single spot on the outer rim of zircon in metabase also yields a Pb-206/U-238 age of 424 +/- 9. 1Ma. These ages are consisting with the retrograde ages of the medium-low P/T granulite-facies metamorphism of Qinling Group. Combined with previous results, we suggest that Qinling Group widely suffered 500 similar to 480Ma HP-UHP metamorphism, and subsequently overprinted by the 440 similar to 400Ma medium-low P/T metamorphism, anatexis and magmatism. The Qinling Group may be a microcontinent, which was splitted from the South China Block or similar block in response to the global-scale breakup of the Rodinia supercontinent, and drafted to the south margin of the North China Carton. The North Qipling orogen experienced a Late Cambrian-Early Ordovician subduction and collision, resulting in HP-UHP metamorphism; followed by Silurian HT metamorphism, anatexis and magmatism during the northward subduction of Shangdan ocean lithosphere beneath the north Qinling terrane.