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

In-Situ U-Pb Dating and Hf Isotopic Analyses of Zircons from Volcanic Rocks of the Houcheng and Zhangjiakou Formations in the Zhang-Xuan Area, Northeast China

冀北张家口-宣化地区(张-宣地区)分布着中生代后城组和张家口组火山岩．锆石的LA-ICPMS U-Pb年龄和LA- MC-ICPMS Hf同位素分析结果表明,后城组英安岩的结晶年龄为(130±1)Ma,为早白垩世,而不是前人所认为的晚侏罗世．张家口组流纹岩的结晶年龄为(126±1)Ma,也为早白垩世．后城组英安岩中锆石Hf同位素组成为εHf(t)=-18．8- -25．5,Hf平均地壳模式年龄为TDMC=2．78-2．37 Ga,平均2．54 Ga,与冀北分布的基底岩石Nd、Hf模式年龄相同,考虑到华北克拉通东部地壳生长的主要时期为晚太古代,我们初步认为后城组粗安岩可能主要来源于晚太古代地壳物质的重熔作用;而张家口组流纹岩中锆石Hf同位素组成为εHf(t)=-15．1--18．5,Hf平均地壳模式年龄为TDMC=2．34- 2．13 Ga,明显比后城组火山岩年轻．张家口火山岩来源于太古代的地壳物质和部分幔源物质混合．这些年代学数据和锆石 Hf同位素表明张-宣地区大面积的后城组和张家口组火山岩是华北克拉通东部晚中生代岩石圈强烈减薄作用的结果．

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 εН1 （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 εН1（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.