Sedimentology And Provenance Analysis Of Devonian-Silurian Strata At The Northwestern Margin Of The Tarim Craton

The South Tianshan orogenic belt is located on the southwest side of the Central Asian orogenic belt.It was formed by the collision between the Central Tianshan Block(CTB)and the Tarim Craton caused by the closure of the South Tianshan Ocean.The tectonic evolution history of the South Tianshan Ocean is quite controversial.For example,there are many different views on the opening time of the South Tianshan Ocean,from Neoproterozoic to Early Devonian.Especially in recent years,studies on the provenance of detrital zircons show that the South Tianshan Ocean opened during the deposition of the Keziertage Formation in the Early Paleozoic.This thesis takes the opening time of the Southern Tianshan Ocean as the key scientific issue,and comprehensively uses methods such as sedimentology,thin section petrology,petromagnetism,and detrital zircon chronology,to study the sedimentary environment and provenance characteristics of the Early Paleozoic Keziertage Formation in the Keping-Bachu area of the northwestern margin of the Tarim Basin,and to identify the sedimentary source response of the Keziertage Formation during the opening of the South Tianshan Ocean.The analysis of lithofacies and sedimentary facies shows that the Keziertage Formation mainly develops red and reddish orange coarse-to fine-grained sandstone,siltstone and mudstone interbedded with thin pale green sandstone in the Keping area.Parallel lamina,planar and trough cross-bedding are common,interpreted as braided river delta deposits;in the Bachu area,there are mainly red medium-to finegrained sandstone,siltstone and mudstone interbedded with thin green siltstone,with small-scale ripples,planar cross-bedding and parallel laminae,interpreted as tidal flat deposits.Based on the measurement and statistics of the 120 paleocurrent data from cross-bedding,combined with the analysis of the magnetic fabric data of 495 paleomagnetic samples,the paleocurrent direction of the Keziertage Formation in the Subashi section was restored.The results show that in the middle and upper part of the Keziertage Formation,the direction of paleocurrent changed from southwest to northeast.The Dickinson triangle diagram shows that the provenance type of the Keziertage Formation has changed from the recycled orogenic belt to stable craton.Combined with previous studies,the detrital zircon chronology results show that there are five main populations in the lower part of the Keziertage Formation in the Keping area: 2.7-2.4Ga,2.2-2.0Ga,1850-1300 Ma,1250-700 Ma and 500-400 Ma,which are relatively consistent with CTB.The youngest zircon age is calculated by YC2?(3+),constrains the maximum sedimentation age of the Keziertage Formation to be 456±5 Ma;There is no significant change in the upper part.The youngest zircon age is calculated by YC2?(3+),constrains the maximum sedimentation age of the Keziertage Formation to be 421Ma±5Ma;the age spectrum peaks of the Kaziertage Formation in the Bachu area are more consistent with those in the West Kunlun Orogenic Belt and the North Tarim uplift.The potential source area changed from CTB to the West Kunlun orogenic belt and the North Tarim uplift.In summary,(1)During the deposition of the Keziertage Formation,the sedimentary environment changed from braided river delta to tidal flat;(2)The direction of paleocurrent changed from southwest to northeast,and the provenance changed from recycling orogenic belts.(3)The sedimentary environment and provenance transition time constrained the opening time of the Southern Tianshan Ocean to the middle and upper sedimentary period of the Keziertage Formation in the late Silurian,lays the foundation for the establishment and improvement of the tectonic evolution history of the Southern Tianshan Ocean.