Sediment Evolution And Tectonic Significance Of The Late Triassic Yanchang Formation In Helan Mountain

Basin and orogenic belt are the most two basic structural units in continental geology,and they are interdependent.The orogenic process can be revealed from the basin sedimentary evolution,and the depositional courses of the basin can be reflected from the orogenic belt.The Late Triassic provenance analysis in Helan Mountain was an important part of the basin-mountain evolution and the restoration of the original basin in northwestern margin of the Ordos Basin.Previous studies of the provenance of Yanchang Formation in Helan Mountain showed that the provenances in different regions and layers were different,but there is lack of systematic analysis of chronology for the provenances.In this paper,based on the research of sedimentary features,the LA-ICP-MS U-Pb dating method was used to further explore the effects of the source direction,composition and variation in different part of Yanchang Formation in Helan Mountain,and temporal-spatial evolution of material source with other ananlysis ways.The tectonic evolution and basin-mountain relationship of Late Triassic Helan Mountain area can be revealed by the evolution of the source.According to the outcrop lithology and sedimentary characteristics in the field,Yanchang Formation in Helan Mountain was divided into 2 sequences:the lower section(T3y1?T3y3),with smainly coarse grain sandstone and the upper section(T3y4,T3y5)with fine siltstone and mudstones.The lower section formed a complete transgressive system with alluvial fan,braided river,shallow lake and deep lake sedimentary facie developing from the bottom to top.Among them,The first segment(T3y1)was a near-source alluvial fan deposition,the second segment(T3y2)developed "sand pack mud" braided river sedimentary facies sign,with alluvial fan terminal to braided river deposition;The third segment(T3y3)carried shallow lake to lacustrine sediments signs of the ripples,plant debris fossils,vertical wormhole,Ostracoda,micrite,turbidite.The upper section included the braided river,delta,inshore shallow to deep lacustrine facies.In the fourth segment(T3y4),there were the lenticular channel sandbodies with river erosion bottom surface deposition signs.The fifth segment(T3y5)developed ripples,plant debris fossils,vertical insect Porosity,turbidite,shellfish and Lamellibranchia fossils and other inshore shallow to deep lacustrine facies sedimentary signs.Yanchang Formation born two periods of lake facies in the segment of 13y3 and 13y5 in Helan Mountain.With the U-Pb dating of the detrital zircon of Yanchang Formation in Helan Mountain,compared with the zircon age distribution of the surrounding areas,the source of Yanchang Formation is mainly from the north,northwest and west(south)areas.The detrital zircon age peaks of the lower section were in Late Paleozoic(269Ma-305Ma),Paleoproterozoic(1.8Ga-2.1Ga),early Proterozoic to Neoarchaean(2.3Ga-2.6Ga),and the age peaks were similar with the age peaks in western part of the North China and Zhuozi Mountain,Kurzite belt and Alashan.The lower section age peaks had the similar characteristics with North China type.The main peaks of the upper source were more complex,and the detrital zircons age peaks are in Early Paleozoic(449Ma-563Ma),Neoproterozoic(709Ma-1020Ma),Middle Proterozoic to Neoarchaean(1.1Ga-2.9Ga),and with 1.1Ga-1.3Ga,6Ga-1.7Ga,2.0Ga,2.5Ga,2.8Ga secondary peaks,and the age peaks were similar with the age peaks in Qilian,the westen and southern Helan Mountain from Ordovician-Cambrian sedimentary strata and pre-Mesozoic detrital zircon from Hexi Corridor Belt.The upper source detrital zircon peaks closed to Qilian characteristics.Based on the analysis of gravel composition,microscopic observation,paleocurrent direction,heavy mineral,geochemistry and so on,combined with the age analysis of detrital zircon,it was concluded that the source of Yanchang Formation was mainly from the north,northwest,west(south).The northern provenance was mainly from the magmatic rocks of Kurzite belt.The northwestern provenance was mainly from magmatic rock and the Precambrian basement in the Langshan-Alashan area.The western provenance was mainly from the Mesozoic Mesozoic sedimentary strata of the Bayanhaote basin.In western of Helan Mountain,the provenance of Shuimogou and Dahalawugou was from pre-Mesozoic sedimentary recycling source.In northern,combination of heavy mineral assemblage in Jiucaigou and Guitougou were stable from bottom to top,the provenance was stable.It was mainly affected by the source of the Kurzite belt in northern,the magmatic rock and the Precambrian metamorphic base in northwest.In easthern,the provenance of upper section in Rujigou and Huangcaotan had complex provenances with mixed source characteristics,it was an intersection area with source from western,northern and northwestern.The detrital zircon age peaks were both similar to North China and Qilian peaks feature,and it was consistent with the upper section basin at eastern Helan Mountain.In late Triassic,Bayanhaot basin and the Helan Mountain area uplift-settling time and the"Source-to-Sink" had a coupling relationship.West provenance was overall cycle deposition in reverse chronological order,Qilian detrital zircon age transported to the Helan Mountain area.