渭河地区及周缘晚古生代-中生代碎屑锆石年代学、地球化学及构造-沉积意义

Characteristics of geochronology,geochemistry of Late Paleozoic and Mesozoic in Weihe region and its tectonic-sedimentary significance

本文在对渭河地区及周缘晚古生代-中生代残存地层分布研究的基础上,采用岩石学、锆石同位素年代学、主微量元素地球化学分析方法,对渭河地区南北两侧上古生界二叠系及中生界三叠系进行对比,进而恢复了研究区晚古生代晚期、中生代早期沉积面貌,并结合裂变径迹构造抬升的研究结果,探讨了渭河地区中生代后期改造过程及演化阶段。结果显示:渭河盆地内部主要凹陷可能仅残留小范围的、不连续的C-P地层,未发现中生代地层。岩石学、锆石U-Pb年龄、主微量元素表明鄂尔多斯南部和北秦岭地区二叠系、三叠系具有很好的对比性,两者在相同时期为同一盆地。二叠系碎屑岩源区可能为再旋回造山带及陆块源区,主要来自北秦岭中-新元古界宽坪群变质碎屑岩及南部二郎坪群火山-沉积岩;三叠系沉积岩物源主要来自北秦岭地区的宽坪群、秦岭群或同期发育的火山岩。裂变径迹资料暗示渭河地区与渭北隆起及秦岭造山带中生代抬升期次具有一致性:晚侏罗世-早白垩世末,地层以强烈的构造变形、弱抬升为主;早白垩世末以来,地层发生大规模抬升、剥蚀,致使上古生界-中生界在渭河地区残留较少。在以上研究的基础上,将渭河地区晚古生代-中生代演化过程分为晚古生代二叠纪、中生代三叠纪-早中侏罗世、晚侏罗世-早白垩世末、早白垩世末-白垩纪末几个演化阶段。

Based on the distribution of residual strata in the Weihe and its adjacent region,the Permian and Triassic to the north and south Weihe region were compared by the methods of petrology,U-Pb isotopic chronology and major-trace elements,and we reconstructed original sedimentary facies from the Late Paleozoic-Early Mesozoic in study area.Furthermore,the Late Mesozoic reformation process was discussed according to simulation of fission track(FT).The study showed that the main sedimentary centers in the Weihe basin might remain small-scale and discontinuous Cretaceous-Permian(C-P)strata without Mesozoic strata.The results of petrology,U-Pb ages of zircons and major-trace elements suggested that the Permian and Triassic in the southern Ordos and northern Qinling regions were well contrasted,both of which are the same basin.The provenance of Permian was characterized with the recycled orogenic zone and continental sources,which are mainly derived from metamorphic clastic rocks in the Kuanping Group and volcanic-sedimentary rocks in the Erlangping Group.The provenance of Triassic might derive from the Kuanping Group,Qinling Group or contemporaneous volcanic rocks.The results of fission track dating revealed that the Mesozoic uplift of Weihe region,Weibei uplift as well as northern Qinling area had consistent evolution stages.Late Jurassic-Early Cretaceous,the geological evolution was dominated by tectonic deformation and weak uplift.Since the Early Cretaceous,the strata had been extensively uplifted and eroded,which led to less residual strata of the Upper Paleozoic-Mesozoic in the Weihe region.This research suggested that the evolution phases from Late Paleozoic-Mesozoic can be divided into Permian,Triassic-Early Jurassic,Late Jurassic-Early Cretaceous,and Late Cretaceous.