华北古陆南缘豫西新元古代大型疑源类及古地理环境分析

The discovery of large-scale acanthomorphic acritarch assemblage on the southern margin of North China old land and an analysis of its paleogeographic environment

华北古陆南缘豫西元古宙地层中白草坪组出现大量大型具刺疑源类,通常大型疑源类首现于新元古代地层,而华北古陆这类的疑源类却首现于中元古地层,因此,其起源和演化谱系就成为许多学者仍需深入探讨的未解之谜。近年来,蓟县中、新元古代地层一直是视为标准剖面,但是,下马岭组新的锆石SHRIMP U-Pb年龄打破了该组原有的中、新元古代地层的位置,同时也打破了生物演化序列的完整性。新的年龄意味着蓟县剖面中、新元古代地层有可能缺失1200～900 Ma地质记录。从生物地层上分析,豫西的汝阳群和洛峪群中出现大量大型疑源类有可能填补蓟县剖面中生物的空缺。本文通过生物地层学、化学地层学研究和岩相分析,探讨其发生可能和地层及构造意义。

Numerous large-scale acanthomorpic acritarchs have been found in Neoproterozoic Ruyang Group on the southern margin of North China old land.They are acritarchs with double layers,complex ornaments on the outer layer,dense hollow or nets on the inner layer and nucleus in the cell.Some geologists and palaeogeologists are doubtful why these acritarchs occur in Mesoproterozoic strata in that they usually occur in Neoproterozoic strata.However,scientists fail to know their origin and evolutional pedigree.In recent years,the Meso-and Neoproterozoic section in Jixian has been regarded as a stratotype section in China;nevertheless,new SHRIMP zircon U-Pb dating（1368±12 Ma） was performed for Xiamaling Formation,which gives the formation a new location in the Meso-and Neoproterozoic column and breaks the old evolutional sequence.The new dating suggests that there is a gap in the geological record in 1200～900 Ma.According to a biostratigraphic analysis,the Neoproterozoic strata with large-scale acanthomorpic acritarchs will fill up the gap of the Jixian section.This paper deals with the appearance of the acritarchs,stratigraphic correlation,and the possible tectonic implication of the acritarchs in the light of biostratigraphy,geochemistry and lithology.