刚果盆地西南缘Inkisi组碎屑锆石U-Pb年代学:对地层时代及物源的约束

Detrital Zircon U-Pb Chronology of the Inkisi Formation in the Southwestern Margin of the Congo Basin:Constraints on Stratigraphic Age and Provenance

Inkisi组是泛非运动之后刚果盆地最早的沉积地层之一。确定Inkisi组地层时代、物源及恢复构造演化过程,对研究刚果盆地的演化具有重要地质意义。本文通过对Inkisi组碎屑锆石进行U-Pb年代学研究,深入探讨了其形成时代,对泛非运动进行了约束,并进一步探讨了其物源。研究发现,Inkisi组下段与上段碎屑锆石最小年龄分别为526±6 Ma和534±14 Ma,最年轻的4颗锆石的加权平均年龄为532±9.4 Ma,由此限定了Inkisi组的最大沉积时代。因此将其时代划归早寒武世,并将其从新元古界Xisto-Gresoso群解体。另外,Inkisi组岩石并未发生变质变形,也说明其形成于泛非运动之后。碎屑锆石年龄谱结果显示,Inkisi组碎屑锆石的年龄变化范围较广,整体呈现五个年龄峰值:>2800 Ma、2800~2400 Ma、2100~1750 Ma、1500~900 Ma、850~500 Ma。通过将年龄峰值及周围地质体进行对比,认为西刚果造山带、卡赛克拉通、安哥拉克拉通是其主要的物源,卢弗里安构造带等是其次要物源。

The Inkisi Formation is one of the earliest sedimentary strata in the Congo Basin after the Pan-African movement.The determination of stratigraphic age,provenance and tectonic evolution of Inkisi Formation is of great geological significance to the study of the evolution of Congo Basin.According to the U-Pb geochronology of detrital zircons from the Inkisi Formation,the formation age and Pan-African movement of the Inkisi Formation were constrained,and the provenance was discussed.Results show that the minimum ages of detrital zircons in the lower member and upper member of the Inkisi Formation are 526±6 Ma and 534±14 Ma,respectively.Furthermore,the weighted average age of the four youngest zircons is 532±9.4 Ma,which restricts the earliest deposition age of the Inkisi Formation.The Inkisi Formation is therefore assigned to the Early Cambrian and disassembled from the Neoproterozoic Xisto-Gresoso Group.In addition,the rocks of Inkisi Formation did not undergo metamorphism and deformation,indicating that it was formed after the Pan-African movement.The results of detrital zircon age spectrum show that the detrital zircons of Inkisi Formation have a wide age range,showing five age peaks:>2800 Ma,2800~2400 Ma,2100~1750 Ma,1500~900 Ma,and 850~500 Ma.By comparing the age peaks with the surrounding geologic bodies,it is concluded that the Western Congo Orogenic belt,Kasai Craton and Angola Craton are the main provenance,and the Lufilian tectonic belt is the secondary provenance.