Maximum Sedimentary Age And Provenance Of Pingdingshan Sandstone In Yiyang Area,Southern Part Of North China Block

Pingdingshan sandstone is a prominent landmark in the upper part of Late Paleozoic strata of North China Block,it is well known for its extensive distribution,special lithology and significant division of sedimentary lithology for the late Paleozoic strata,suggesting that Pingdingshan sandstone may record a geological event,but its sedimentary age and sedimentary environment has always been the focus of controversy.In this study,Pingdingshan sandstone and its underlying Shangshihezi Formation samples in Yiyang area,southern North China Block are selected,based on LA-ICP-MS U-Pb dating of detrital zircon,the maximum sedimentary age and source area of Pingdingshan sandstone are analyzed,on this basis,the sedimentary environment of Pingdingshan sandstone is further discussed by means of rock slices,electron microscopy and cathodoluminescence.The results show that the detrital zircon of fine feldspar quartz sandstone sample from the upper of Shangshihezi Formation,Yiyang area in the southern part of North China Block,is mainly from the Palaeozoic magmatic zircons and the Precambrian metamorphic zircons,the Paleozoic zircons account for 29% with the peak age of ~278Ma and ~304Ma,the Precambrian zircons account for 71% with distinct peak ages of ~1894Ma and ~2520Ma;the detrital zircons from three consecutive samples(medium-grained feldspar quartz sandstone,fine-grained feldspar quartz sandstone and silty mudstone)from the lower part of Pingdingshan sandstone,Yiyang area in the southern part of North China Block,are mainly from the Palaeozoic magmatic zircons and the Precambrian metamorphic zircons,the Paleozoic zircons account for 14% with the peak age of ~262Ma,~326Ma and ~458Ma,the Precambrian zircons account for 86% with distinct peak ages of 1866 Ma and ~2542Ma,in addition,there are three zircons with ages ranging from ~778 Ma,~874 Ma and ~1088 Ma,respectively.using the youngest single detrital zircons age(YSG)limits the maximum sedimentary age of the stratum,the YSG age of the sandstones at the upper of the Shangshihezi Formation underlying Pingdingshan sandstone are determined to be 266 +8 Ma,which are consistent with the Changxing-Wujiaping Period of the Late Permian determined by paleontological data,the YSG age of three consecutive samples is 259±8Ma,248±7Ma and 252±7Ma respectively,which is close to the Permian/Triassic boundary age 251±0.4Ma;therefore the bottom of Pingdingshan sandstone may be a contact boundary between Permian and Triassic,and the geological age of the Pingdingshan sandstone in the southern part of North China Block should belong to early Triassic.Based on the comparison between the age spectrum of detrital zircon and the age map of potential provenance area,the sedimentary and tectonic background of Pingdingshan sandstone are analyzed,the main material source area of Pingdingshan sandstone and its underlying upper sandstone of Shangshihezi Formation is from the late Paleozoic intrusive rocks and the metamorphic basement of North China Block in the Inner Mongolia Uplift at the northernpart of North China Block,however,Pingdingshan sandstone is mixed with a large number of the Caledonian middle-acid granitic intrusive rocks and and ~1000Ma granite in the North Qinling orogenic belt or the main metamorphic strata(Qinling Group,Erlangping Group and Kuanping Group)in the intruded area.Based on the analysis of sandstone sediment provenance of Pingdingshan sandstone and its underlying Shangshihezi Formation,with the facts that the provenance of the lower strata(Shanxi Formation,Xiashihezi Formation and Shangshihezi Formation)almost all from the Inner Mongolia uplift in the northern part of North China Block,and the provenance of the upper strata of Shiqianfeng Formation(except Pingdingshan sandstone)mainly from the North Qinling orogenic belt,it is inferred that the Pingdingshan sandstone may be the material record of the sedimentary and tectonic environment change between Permian and Triassic in the southern part of the North China Block;based on the above analysis,the sedimentary environment of Pingdingshan sandstone is discussed in combination with the sedimentary structure and petrological analysis of Pingdingshan sandstone,it is considered that Pingdingshan sandstone may be a littoral facies deposit formed by re-deposition of fluvial or delta facies sandstones transformed by strong hydrodynamics.