冀北滦平—辽西凌源地区张家口组火山岩顶、底的单颗粒锆石U-Pb测年及意义

辽西凌源附近地区张家口组(J₃z)火山岩及以前的中生代地层以北东东向构造为主,而义县组(K₁y)沉积-火山岩以北北东向构造为主,张家口组与义县组之间为角度不整合接触.新的LAICPMS的单颗粒锆石UPb的测年结果表明,辽西凌源地区张家口组的底界年龄为132Ma,顶界年龄为129.5~130Ma; 冀北滦平地区张家口组的底界年龄为136Ma,顶界年龄为135Ma.经综合分析,得出以下的初步结论: (1)冀北滦平与辽西凌源地区的张家口组是可比的; 同时,说明了冀北滦平地区到辽西凌源地区的张家口组火山岩有明显的随时间迁移的特点.(2)凌源地区张家口组的顶界与义县组的底界有约5~6Ma的时间间隔,从年代学上证明了凌源附近地区张家口组与义县组之间为角度不整合接触的可靠性.暗示了该5~6Ma的间隔期是冀北—辽西附近地区中生代构造格架转换的主要时期,该时期也正是我国东北地区中生代岩浆作用的鼎盛时期; 暗示了该时期岩浆作用的动力学背景应与构造格架的转换有直接的联系; 同时在冀北—辽西地区,该间隔期之后发生了热河动物群的大爆发,说明该时期也是一个重要的生物界限.(3)从年代学方面厘定了冀北滦平的大北沟组在时代上应相当于张家口组的上部层位,而不是广义义县组的底部层位. 

Zircon U-Pb Ages and Significance of Bottom and Top Beds of Zhangjiakou Formation in Liaoning and Hebei Provinces

In Lingyuan region,Liaoning Province,the Zhangjiakou Formation and the Lower Mesozoic strata are in an ENE orientation,while the Yixian Formation is NNE orientated,the two formations are angular unconformity.According to the new U-Pb isotopic age analysis of zircon by LA-ICP-MS,the bottom bed of the Zhangjiakou Formation in Lingyuan is about 132 Ma,and the top bed is about 129.5-130 Ma.In Luanping region,Hebei Province,the bottom bed of the Zhangjiakou Formation is about 136 Ma,and the top bed is about 135 Ma.The Zhangjiakou Formation in the Lingyuan and Luanping regions is comparable,which reflects that the volcanic rock of the Zhangjiakou Formation from Luanping to Lingyuan becomes younger in time.The top bed of the Zhangjiakou Formation in Lingyuan has about a 5-6 Ma interval with the bottom bed of Yixian Formation,which proves that the angular unconformity of Zhangjiakou Formation and Yixian Formation is credible; suggesting that the 5-6 Ma interval period is the main period of the transition of tectonic framework in the Mesozoic in Hebei and Liaoning provinces.In the interval period,the magnetic action went up to high tide in the Mesozoic in the northeast of China.Moreover,after the interval period,the Rehe fauna developed into the erupted period,which also reflects that the interval period is an important biological interface in Hebei and Liaoning provinces.According to the isotopic age,the Dabeigou Formation in Luanping region corresponds to the upper bed of the Zhangjiakou Formation,not to the bottom bed of the Yixian Formation.