Zircon LA-ICP-MS U-Pb dating of and tectonic setting of rhyolites from Baishan Formation in Fengleishan area of the Beishan orogenic belt

Located at the junction of North China Plate intersection, Tarim plate and Kazakhstan plate, Beishan area is characterized by nearly EW-trending tectonic-magmatic activity. The rocks mainly include rhyolite, rhyolitic breccias-containing debris crystal tuff, dacitic crystal tuff, andesitic crystal tuff and a small amount of dacite. They can be divided into two sections based on features and rock eruption spatial superposition relationship. Geochemistry shows that rhyolites have high SiO₂ content (77.05%~77.52%) and low TiO₂(0.06%-0.19%), CaO (0.38%-1.04%), MgO (0.09%-0.17%) content with K₂O/Na₂O higher than 1. They are relatively enriched in large ion lithophile elements (LILE) Rb and K, while depleted in high field strength elements (HFSE) Nb, Ta, P and Ti. The total amount of light rare earth elements (ΣLREE average value being 127.55×10^-6) is significantly higher than that of the heavy rare earth elements (ΣHREE average value being 10.92×10^-6), and LREE fractionation obviously ((La/Yb)_N varies from 6.37 to 7.88, with an average of 7.05, whereas (La/Sm)_N varies from 4.51 to 4.61, with an average of 4.55. Eu has obvious negative anomaly (δ Eu average value being only 0.62). Petrological and geochemical characteristics indicate that the tectonic setting was a continental-margin-arc. Zircon U-Pb age is 318.5±1.2 Ma for rhyolite by LA-IC-MS method. Combined with regional geological features, the authors believe that Baishan Formation in Fengleishan formed in the Late Carboniferous.