The Provenance Analysis Of Jurassic And Its Relation To Basin And Mountain In The Southern Margin Of Yili Basin,Xinjiang

The Yili basin is located in the southwestern margin of the Western Tianshan orogenic belt,and the Jurassic is a thick terrigenous clastic sedimentary rock,which records the orogenic process of the Western Tianshan Mountains and the evolution of the Yili Basin.The systematic study of the provenance of the clastic rocks can clearly reveal the relationship between the Western Tianshan Mountains and Yili basin,and it provide a reference for the further study of the western part of Mesozoic orogenic belt.At the same time,Yili basin is one of the most important sandstone type uranium deposits in China.In view of the exploration of sandstone type uranium deposit,it is very important to make clear the origin of the uranium bearing sand body,to find out the source of detrital matter,and to evaluate the uranium supplying ability of the source rock.Based on the precise measurement of the typical section of the target layer in the study area,combined with drilling data collected in the basin and on the basis of previous studies,this paper uses the principles and methods of sedimentology,high resolution sequence stratigraphy,structural geology,sedimentary geochemistry,etc.The methods of whole rock geochemistry,detrital zircon geochronology and trace element geochemistry,cathodoluminescence are using.The characteristics of clastic rock,sedimentary environment,high resolution sequence stratigraphy,the source rock composition and tectonic setting of clastic rock source area,and the coupling relationship between basin filling sedimentary characteristics and peripheral orogenic belt are studied systematically and deeply.The Jurassic in the southern margin of the Yili basin is mainly composed of medium coarse grained lithic sandstone,medium coarse grained lithic quartz sandstone and mixed sandstone.Particle sorting medium is poor.Particle roundness is sub rounded-chestnut shape.The particles are filled by the base.Part of the sandstone is calcareous cementation,particle support,pore cementation.The debris content is about 23-65%,the quartz is about 32-74%,and the feldspar content is about 2-10%.The composition and structure maturity of sandstones are low,which shows the characteristics of near source accumulation.The lithic sandstone in the southern margin of the Yili basin is mainly composed of siliceous rock debris,siltstone debris,mudstone debris,and a small number of rhyolite,granite and other intermediate acid igneous rock debris.The detrital quartz grains are mainly single crystal quartz,and most of them are non wavy extinction characteristics,and the boundaries of the polycrystalline quartz grains are in a straight line contact.The feldspar content is low,and it has been mainly derived from the K-feldspar of the intermediate acid volcanic rocks,and is rarely in the basic plagioclase.The characteristics of clastic grains indicate that the source rocks of source rocks are derived from volcanic source rocks.The Dickinson mapping points of sandstone clastic components show that the clastic rocks are formed in the environment of the recycling orogenic belt.The geochemical characteristics show that the parent rocks are felsic rocks under the background of continental island arc.The chronological characteristics of detrital zircons indicate that the clastic rocks of the Baowan formation originated from the early Middle Permian and Carboniferous rocks,the Sangonghe formation and the Xisanyao formation originated from Carboniferous rocks.In the Jurassic period,the denudation of the source area has the characteristics of the roof uncovering.The Jurassic clastic rocks in the Yili basin are derived from the environment of the recycling orogenic belt,and the parent rocks in the source rocks are mainly formed in Carboniferous continental arc environment.The source rocks of the Jurassic clastic rocks in the southern margin of the Yili basin are mainly from the southern orogenic belt of Qapqal Mountain.Fan delta,lake,braided river delta,fluvial facies are mainly developed.The Badaowan formation is mainly composed of coarse clastic fan delta deposits,and the Sangonghe formation is mainly composed of fine grained shallow lake deposits.The Xisanyao formation is mainly composed of coarse braided river delta and the Toutunhe formation is mainly composed of coarse grained fluvial deposits.The fossils of the ancient plants are mainly large and flat,mudstone shows strong Na,Ca and Sr losses,the index number of chemical weathering of mudstone(CIA)is as high as 85~91,all these characteristics indicate that the Jurassic clastic sedimentary period is warm and humid.From the late Devonian to the Late Carboniferous,affected by the northward subduction of the southern Tianshan ocean,a large number of continental island arc volcanic rocks developed in the southern margin of the Yili-Central Tianshan block.At the end of the Late Carboniferous,the Yili-Tianshan block collided with the Tarim block,and the South Tianshan orogenic belt entered the post collisional evolution stage.A large number of volcanic and clastic rocks formed.In the early Jurassic,affected by the rapid uplift of the southern margin of the basin,the Carboniferous and Permian volcanic /sedimentary rocks are uplifted and eroded.At this time,there is a large gap between the mountain and basin,the slope is steep,and the coarse clastic material fills the basin rapidly forming the fan delta deposits in the Badaowan formation;At the end of the early Jurassic,the lake rapidly expanded,the lake level rose,and the sediment supply was insufficient,which formed the Sangonghe formation of lacustrine sediments;After the Middle Jurassic,affected by the continuous denudation of the mountain and the continuous filling of the basin,the elevation difference between mountain and basin is decreasing,the lake is shrinking until it disappears,it formed the braided river delta deposit in Xisanyao formation and fluvial deposits in Toutunhe formation.The southern margin of the Yili basin in the Jurassic sedimentary period shows obvious characteristics of peneplanation.