The Provenance of Sandstones from the Upper Paleozoic Shihezi Formation,Eastern Ordos Basin:Petrology,Geochemistry and Zircon Dating Constraints

摘要

Provenance studies are important to identify source rocks,basin tectonic settings,and reconstruct paleo-drainage systems,etc.,which in turn may lead to an improved understanding of potential reservoir distribution.To reconstruct the upper Paleozoic sedimentary ancient water systems of the number 8 of Shihezi Formation(He 8)in eastern Ordos basin,central China,based on the analysis of petrology characteristics of composition,cathode luminescence of quartz grains and heavy minerals,and major,trace,rare earth elements compositions of sandstones and zircon dating,this paper have investigated to determine the provenance of the study area.The detrital compositions of sandstones is lithic sandstones and lithic quartz sandstone,and quartz sandstones distribute in part of the area.Colors of illuminated quartz grains slightly changes from east to west of basin,showing that western area is main composed of high grade metamorphic rocks,and central area is of low grade metamorphic rocks.Distributions of heavy minerals are more stable from north to south,which indicate their provenances are from the north.The combinations of heavy minerals in plane showed that a few paralleled ancient flows were developed in study area,and the characteristics of heavy minerals matched well with the types of source rocks in the north.Chondrite-normalized rare earth element(REE)patterns of mudstone samples from Shihezi Formation with LREE enrichment,flat HREE,and negative Eu anomaly are attributed to felsic source-rock characteristics for Shihezi Formation sandstones.REE patterns of mudstone samples from study area are very similar to those of Archeozoic and Proterozoic metamorphotic rocks from Daqing Mountain,Wula Mountain,and Yin Mountain.All in ail,different evidences suggest that provenance of study area are from North and Northeast.The deposits in the east of Ordos Basin mainly derived from the low grade metamorphic rocks,mag-matic rocks and marbles in Daqing mount areas,as well the high grade metamorphic rocks,intrusive rocks and sedimentary rocks in the west are from Wula Mountain,and Yin Mountain.General analyses suggest that at least 3 paralleled ancient flows transported sediments to the area.In this study,the zircon U-Pb dating method was applied to the provenance analysis.Together with the mineral petrology method and the rare earth element analysis,the sedimentation of He 8 in the study area was comprehensively analyzed.All the evidences confirmed each other.