华北克拉通南缘汝阳群大型具刺疑源类时代再厘定及早期真核生物群演化意义*

华北克拉通南缘汝阳群中以大型具刺疑源类为代表的真核生物群的时代归属问题一直存在争议。新近在豫西汝州阳坡村洛峪群洛峪口组中获得的凝灰岩锆石SHRIMP U-Pb年龄为1639±13Ma,结合熊耳群和汝阳群已获得的其他年龄数据,将这类形态复杂的真核生物群出现的时间限定在1.75—1.64Ga,即中元古代的早期。这说明,过去一直被认为的代表进化程度较高但仅属于新元古代的一些真核生物群,其实在中元古代早期就已经出现了;由此可以进一步推测,在更古老的地层中应有更原始的微体化石出现,从而华北克拉通南缘中元古界(>1.60Ga)将成为探寻最古老真核生物祖先的重要窗口。同时,将这些在中元古代早期就已经出现并在以后较长地史时期都存在的真核生物群用于标定地层形成时代或作为相关地层的对比标志,值得重新考虑和商榷。

Recognition of geological age for acanthomorphic acritarchs from the Ruyang Group,southern margin of North China Craton and its implication for evolution of early eukaryotes

The geological age of eukaryotes represented by large acanthomorphic acritarchs from the Ruyang Group,southern margin of North China Craton,has been controversial. New zircon SHRIMP U-Pb age of tuff(ca. 1639±13Ma)was obtained from the Luoyukou Formation of the Luoyu Group,Yangpo,Ruzhou,west Henan Province. This new age,combined with the ages from the Xiong'er and Ruyang Group,can well constraint the time of these eukaryotes with complex morphology from the Ruyang Group at 1.75-1.64Ga,which belongs to early Mesoproterozoic. Moreover,it shows that some eukaryotes with relatively higher degree of evolution,which were previously thought to be preserved in the Neoproterozoic,may have appeared in the Early Mesoproterozoic. We further suspect that there might be more primitive microfossils preserved in the older strata(>1.6Ga)in the southern margin of the North China Craton,which provides an important window to explore the ancestor of the eukaryote. Thus,caution should be taken when applying these Mesoproterozoic eukaryotes for stratigraphic correlation.