豫西洛峪口组凝灰岩锆石LA-MC-ICPMS U-Pb年龄及地层归属讨论

Zircon U-Pb dating of tuff bed from Luoyukou Formation in western Henan Province on the southern margin of the North China Craton and its stratigraphic attribution discussion

洛峪群—汝阳群主要分布在华北克拉通南缘,它们主要由石英砂岩夹少量页岩及白云岩组成。通过对豫西汝州洛峪口村、阳坡村附近的洛峪口组野外调查,发现洛峪口组含有多层凝灰岩夹层。采用LA-MC-ICPMS测年方法,对不同地点的洛峪口组凝灰岩夹层进行了锆石U-Pb年代学研究,分别获得(1638±9)Ma、(1634±10)Ma的年龄,表明洛峪口组形成于中元古代长城纪,并非原来认为的中—新元古代蓟县纪和青白口纪。由于洛峪口组位于洛峪群的顶部,很显然洛峪群及下伏汝阳群的时代均应归属长城纪。该组年龄的精确标定从根本上改变了原来豫西洛峪群—汝阳群划分方案。这对华北克拉通中—新元古代地层的划分与对比,对探讨大地构造演化与哥伦比亚超大陆的关系及早期生命演化均具有重要的意义。

Luoyu Group and Ruyang Group are cropped out extensively along the southern margin of the North China Craton(NCC),and are mostly composed of sandstone with minor shale and dolostone.Based on field investigation along Luoyukou andYangpo Village section,Ruzhou City,Henan Province,a few interbedded tuff layers were found in the upper part of the LuoyukouFormation.By means of LA-MC-ICPMS zircon U-Pb dating,the ages of the volcanic eruption,1638±9Ma and1634±10Ma from Luoyukou Village and Yangpo section,were respectively obtained.These data mean that Luoyukou Formation was formedduring Mesoproterozoic Changcheng period rather than during Meso– Neoproterozoic Jixian period and Qingbaikou period aspreviously held.According to the new dating data,the Luoyu Group and Ruyang Group which conformably underlies Luoyu Groupought to belong to the Mesoproterozoic Changcheng period,because Luoyukou Formation is at the uppermost part of Luoyu Group.This discovery has great significance for Meso-Neoproterozoic stratigraphic redivision and correlation in the NCC,and for thestudy of the relationship between the tectonic evolution history of the Late Paleoproterozoic to Early Mesoproterozoic geologicalrecords in the NCC and the breakup of the Columbia Supercontinent as well as for the evolution of the early life on the Earth.