U-Pb Zircon Dating of the Mesozoic Volcanic Strata in Luanping of North Hebei and Its Significance
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摘要: 在冀北—辽西地区, 冀北的滦平地区是中生代地层最为齐全, 也是唯一的晚中生代沉积作用基本连续的地区.许多研究者认为该地区的土城子组与张家口之间存在着J3—K1界线和中生代的构造转换界面.该地区分布的主要中生代地层自下而上依次为髫髻山组、土城子组、张家口组、大北沟组、大店子组和西瓜园组.通过LAICPMS方法的锆石UPb测年, 获得了该地区中生代火山岩地层的年代格架, 即髫髻山组的顶界年龄为(162.8±3.2)Ma, 土城子组形成的主体年代范围是(142.6±1.3)~(136.4±1.9)Ma, 张家口组形成的年代范围是(135.7±1.8)~(135.2±2.3)Ma, 大店子组顶部安山岩的年龄是(131.4±3.7)~(130.2±3.0)Ma.锆石的稀土元素分析表明: 土城子组中锆石的稀土元素特征与张家口组中锆石的特征一致, 而与髫髻山组中的锆石有较大的区别.综合以上分析数据及野外地质特征可以得出以下结论: (1)该地区的土城子组与髫髻山组之间不仅时间间隔较长, 而且二者中火山岩的源区也有较大的区别; 土城子组与张家口组之间不仅时间上基本连续, 而且二者中火山岩的源区也有明显的一致性.这就表明了滦平地区的土城子组与张家口组是同一地质背景下的产物, 即该地区二者之间应不存在J3—K1界面和构造转换界面.(2)滦平地区大店子组顶部火山岩在年代上与辽西北票—义县地区的义县组底部、辽西凌源地区的张家口组顶部基本相当.Abstract: The Mesozoic strata in Luanping, North Hebei Province are the most complete in North Hebei-West Liaoning, and the Luanping area is the only area where the deposit action in the Late Mesozoic is successive. A lot of researchers think the boundary of the J3-K1 and the tectonic framework transition interface in Mesozoic Era in North Hebei-West Liaoning are located between the Tuchengzi and Zhangiiakou formations in the Luanping area. The Mesozoic strata in Luanping from the lower to the upper are the Tiaojishan Formation, Tuehengzi Formation, Zhangjiakou Formation, Dabeigou Formation, Dadianzi Formation and the Xiguayuan Formation. U-Pb zircon dating of LA-ICP-MS obtained the age frame of the Mesozoic volcanic strata in the Luanping area. The top age of the Tiaojishan Formation is (162.8±3.2)Ma, the main forming period of the Tuehengzi Formation is (142.6±1.3)Ma-(136.4±1.9)Ma, the forming period of the Zhangjiakou Formation is (135.7±1.8)Ma--(135.2±2.3)Ma, and the age of the volcanic rocks in the top of the Dadianzi Formation is (131.4±3. 7)Ma--(130.2±3. 0)Ma. Analysis shows that the rare earth element characteristics of zircons from the Tuehengzi Formation are very similar to those from the Zhangiiakou Formation, but are very different from those from the Tiaojishan Formarion. Combined with the field characteristics of the strata, the following conclusions are drawn: (1) the time interval Detween the Tuchengzi and Tiaojishan formations is longer, and the magmatic sources of the volcanic rocks are very different; the forming period between the Tuchengzi and the Zhangjiakou formations is continuous, and the magmatic sources of the volcanic rocks are very similar. The results imply that the Tuchengzi and Zhangjiakou formations were formed in the same geological background, and so are not the boundary of the J3-K1 and the tectonic framework transition interface between the Tuchengzi and Zhangjiakou formations in the Luanping area. (2) The age of the volcanic rocks in the top of the Dadianzi Formation corresponds to that of the bottom of the Yixian Formation in the Beipiao-Yixian area in West Liaoning Province, and corresponds to that of the top of the Zhangiiakou Formation in Lingyuan, West Liaoning Province.
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表 1 采样点位置及锆石测年
Table 1. Sample locations and their LA-ICP-MS U-Pb ages of zircons
表 2 样品中错石的LA-ICP-MS U-Pb分析数据
Table 2. U-Pb isotopic data of zircons from samples in Luanping area by LA-ICP-MS
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