冀北承德盆地侏罗系九龙山组凝灰岩LA -ICP-MS锆石U-Pb年龄与地球化学特征

对冀北承德盆地中生代侏罗纪各地层单位的时代归属一直存在争议。研究获得承德盆地九龙山组凝灰岩LA -ICP-MS锆石206Pb/238U年龄加权平均值为163.4Ma±1.1Ma（MSWD=3.3），结合研究区中生代侏罗纪其他地层测年结果，认为九龙山组的时限可能为158~164Ma，属中侏罗世晚期—晚侏罗世早期，对应于国际地层表的牛津期—卡洛维期。该组中的凝灰岩具有较高的SiO2、K2O和Al2O3含量，A/CNK=6.31~16.22，反应岩石高钾偏碱性强过铝质的特点；稀土元素总量高，REE配分曲线呈右倾型，具有明显的Eu和Sr负异常；富集大离子亲石元素（Rb、Ba、Th、U），亏损高场强元素（Nb、Ta、Zr）。凝灰岩具有与上地壳相似的稀土和微量元素特征，具典型的壳源成因类型。

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.