Varitation Of Sediment Provenance And Paleoenvironment In The Weihe Basin Since Late Miocene Revealed By Detrital Zircon Dating

The evolution of the East Asian monsoon and arid climate in late Cenozoic and the driving force behind has been a subject of considerable debate.The Weihe Basin,located to the north of Qinling Mountain in Central China,preserved Cenozoic deposition since late Eocene.This Cenozoic depositon provides an excellent archive of climate and geomorphology evolution in the late Cenozoic in East Asia.The age framework of sediments at last 11 Ma is established by fossil mammals and magnetostratigraphy.However,the provenance of the sediment in since late Miocene is poorly studied.Detrital zircon U-Pb age is one of the most effective methods to trace the provenance of sediment in basin.The peak position and relative peak intensity of the detrital zircon age distribution of a certain block contains information about the time and relative intensity of the tectonic movement this block has been experience.Therefore,detrital zircon age distribution can be used to estimate relative contribution of different provenance.The potential source areas of sediments in the Weihe basin include the blocks/belts around the basin,such as North Qinling,West Qinling,Qinlian and North China Craton.On the other hand,the arid areas surrounding the basin,such as piedmont of north Tibet Plateau,the piedmont of Gobi Altay Mountains,Tengger Desert and Mu Us sand fields,and the Chinese Loess Plateau can also contribute aeolian sediments to Weihe basin.However,compared to the belts/blocks around the basin,there is only few detrital zircon age data on arid area surrounding the Weihe basin.Therefore,this study will firstly survey sediments on the Northeastern Tibet Plateau piedmont,the Gobi Altay Mountains piedmont,Tengger Desert and Mu Us sand fields.29 surface samples are collected in these areas and detrital zircon dating is carried on them.Sediments in piedmont of Gobi Altai Mountains,and north part of Tengger Desert and Mu Us sand fields are characterized by only one dominant peak at around 405-252 Ma.Sediments in Northeastern Tibetan Plateau,south part of Tengger Desert and Mu Us sand fields exhibit a different pattern,with four main sets of peaks:350-230 Ma,550-350 Ma,1300-550 Ma,and 2.0-1.3 Ga.The 550-350 Ma and 350-230 Ma peaks are dominant in most of the samples.Detrital zircon age of sediments in east part of Mu Us sand fields exhibit dominant peaks at 2.0-1.3 Ga and2.8-2.1Ga.Based on the detrital zircon age characteristics of the arid area surrounding the Weihe basin,we propose a new approach to quantitatively estimation the contributions of different source areas.The calculation reveals that the sediments of the Tengger Desert and Mu Us sand field are spatially heterogeneous in terms of their source.In the northern part of the two deserts,43-83%of the sediments are derived from the Gobi Altai Mountains,while in the south,material from the Northeastern Tibetan Plateau comprises 51-98%of the sediments.Loess deposits from the Chinese Loess Plateau also comprise a mixture of the three different sources,with material from the Northeastern Tibetan Plateau making the dominant contribution(65-100%),with material from the North China Craton and the Gobi Altai Mountains comprising 0-35%and 0-40%of the loess deposits,respectively.Subsequently,based on detrital zircon age feature of the potential source areas obtained above,we studied the the provenance of eolian sediments in the Weihe basin since 7 Ma.As the zircon grains dated in this study are coarser than 20?m,the detrital zircon age tracing reveals the provenance of bedload and coarse part of suspendedload in the eolian sediments.The results show that the bedload and coarse part of suspendedload of Red Clay in the Weihe Basin are mainly derived from sediments around the Weihe River Basin,and there is no significant contribution from further north.This shows that the winter monsoon at that time is relatively weak and is not strong eough to transport bedload and coarse part of suspendedload from arid areas surrounding the Weihe Basin,such as Gobi Altay Mountains.The shift of eolian sediment provenance took place at 2.6 Ma,?0.9 Ma and the last glacial period.Detrital zircon from Qilian,north of Weihe Basin,made first appearance at?2.6Ma,making contribution of 21%.Detrital zircon from the Gobi Altai Mountains,which is to the north of Weihe Basin,shows up at?0.9Ma.The Gobi Altay Mountains detrital zircon contribution increased significantly at last glacial period to 23%.This is in accordance with the stepwise aridification of Central Asia and global cooling,which indicates the intensity of atmosphere circulation in late Cenozoic is influenced by the global cooling.Lastly,we studied the provenance of fluvial and lacustrine sediments at 11-7 Ma in Weihe Basin,which is also known as the Bahe Formation.The sediments of the Bahe Formation are mainly derived from the Lantian and Muhuguan granitoid plutons,the Qinling complex and the Erlangping group in the North Qinling Mountains.The contribution of the Lantian and Muhuguan granitoid plutons varies in accordance with the sedimentary facies.The contents of sediments from Lantian and Muhuguan granitoid plutons reach the maximum(29%)at thick gravel deposits in the middle of the Bahe Formation and the gravel mudstone at the top.Combined with the previous study of apatite fission track in the North Qinling area,this transformation of sedimentary facies and material composition may be related to the rapid uplift of North Qinling in late MioceneThe deposition in the Weihe basin since late Miocene reveals the stepwise aridification in East Asian since late Cenozoic.The grain size of the sediments of the Bahe Formation in the 11-7 Ma area becomes finer from bottom to top,indicating a weakened hydrodynamic environment during 11-7 Ma in the Weihe basin.This may reflect the gradual decrease of the tectonic activity in the area.Besides,this may also reflects the gradual aridifacation of the Weihe basin.Aeolian sediments began to accumulate in the Weihe Basin since?7 Ma,which also reflects the gradual aridifacation of North China.