Formation Age, Tectonic Setting and Geological Significance of The Jurassic Jiulongshan Formation Tuff in Xuanhua Basin, Northwest Hebei

The continental sedimentary strata of the Jiulongshan Formation in the Xuanhua basin in Northwest Hebei are well developed, but the age of the strata is lacking in evidence from fossils and isotope dating data. In the Yangjiaying−Xiangshuipu area of the study area, rhyolitic tuff interlayers develop in the middle and lower parts of the Jiulongshan Formation. In order to accurately determine the age and formation environment of the Jiulongshan Formation in the Xuanhua Basin, the author collected zircon U−Pb isotope dating samples from the rhyolitic tuff and rock geochemical analysis samples. The ²⁰⁶Pb/²³⁸U weighted average age (LA−ICP−MS method) of (161.9±0.8) Ma was obtained, which is the early Late Jurassic. The petrochemical characteristics show that the rhyolitic tuff is a high−silicon, high−potassium calc−alkaline peraluminous rock; the distribution pattern curve of rare earth elements is right−dipping, with strong enrichment of light rare earths and no obvious negative Eu anomaly; the enrichment of large ions is lithophilic elements (Rb, K, Th, U), depleted Sr and high field strength elements (Nb, Ti). The geochemical characteristics of the rocks show that it was formed in a compressional tectonic setting. The zircon Hf isotope εHf(t) value is between −33.79～−2.71, and the second−stage depletion model age (t_DM2) is between 1179～3323 Ma, indicating that the rhyolitic tuff magma is mostly Paleoproterozoic−Mesoproterozoic thickened lower crust Melted to form. According to comprehensive analysis, the Jiulongshan Formation in Xuanhua basin was formed in the late Middle Jurassic−early Late Jurassic compressional structural setting. The research results provide new data for the study of the age attribution and formation environment of the Jiulongshan Formation.