LA-ICP-MS zircon U-Pb age and geochemical characteristics of tuff of Jiulongshan Formation from Chengde basin, northern Hebei

Controversy has long existed over the ages of Jurassic strata in Chengde basin，northern Hebei. The authors obtained for the first time the U-Pb age (163.4Ma±1.1Ma) of zircon from tuff of the Jiulongshan Formation. In combination with ages of other Jurassic strata, the authors hold that the age of the Jiulongshan Formation is most likely in the range of 159~165Ma, suggesting an epoch between late Middle Jurassic and early Late Jurassic. This result indicates that the Jiulongshan Formation should belong to Callovian-Oxfordian period. Petrochemical and geochemical analyses show that the tuff is of peraluminous, K-high, and calc-alkaline nature, with A/CNK being 6.31~16.22. The ∑REE values range from 190.96×10-6 to 340.68×10-6, and the REE patterns show right-oblique feature and obvious negative Eu, Sr anomalies. Trace elements show enrichment of LILE (large ion lithophile elements) such as Rb, Ba, Th and U and depletion of HFSE (high field strength elements) such as Nb, Ta and Zr. The REE distribution patterns and trace elements characteristics of tuff in Jiulongshan is similar to those of the upper crust which had been partially melted by greywacke.