冀北张-宣地区后城组、张家口组火山岩锆石U—Pb年龄和Hf同位素

Large-scale volcanic rocks of the Houcheng and Zhangjiakou formations are developed in the Zhangjiakou-Xuanhua (Zhang-Xuan) area. Zircon LA-ICPMS U-Pb dating gives the eruption age of dacite in the Houcheng Formation as (130±1) Ma, indicating that the Houcheng Formation formed during the Early Cretaceous, not during the Late Jurassic as previously thought. Volcanic rocks of the Zhangjiakou Formation have a zircon LA-ICPMS U-Pb age of (126±1) Ma, i.e. Early Cretaceous. In-situ zircon Hf analyses show that zircons from the Houcheng Formation dacite have ε_Hf(t) values of -18.8 to -25.5 and Hf crustal model ages of 2.78 to 2.37 Ga with an average of 2.54 Ga. Their model ages are similar to those of basement rocks in the northern North China craton, representing an important stage of crustal growth in the North China craton. Thus, we suggest that the dacite was mainly derived from the partial melting of Late Archean crustal materials. Whereas, zircons from the rhyolite of the Zhangjiakou Formation have ε_Hf(t) values of -15.1 to -18.5 and younger Hf crustal model ages of 2.34 to 2.13 Ga, indicating that the rhyolite was derived from Late Archean crustal materials, but with the involvement of mantle materials. The age and Hf isotopic data suggest that the large-scale volcanic rocks in the Zhang-Xuan area were the result of crust-mantle interaction during the Early Cretaceous lithospheric thinning in the eastern North China craton.