Geological Significance Of Flysch In Zhikong-songduoxiong Area,Tibet

Yang jingsui discovered eclogite in Songduo area,with some ophiolite units exposed in the Songduo eclogite,and Liu hongfei found blue schist near Zhikong hydropower station.Since then,more and more geologists have carried out in-depth studies on the eclogite,blue schist and green schist in this area,but the studies on the sedimentary strata of Zhikong-Songduo Ocean are relatively scarce.This work based on chronology characteristics,style and other comprehensive analysis,with reference to the theory of "Oceanic Plate Stratigraphy" method is a new division in the study area,stratigraphic structure show local orderly,overall the characteristics of the disorder,slope basin(flysch)unit(SB)is one of very important unit.Its characteristics are mainly a set of clastic rocks with the characteristics of complex rocks,the lithology is mainly divided into: medium-thick layer,siliceous rock and silt-fine sand interlayer;Interbedded mudstone and fine sandstone;The ratio of sand to mud is about 2-3:1.The U-Pb dating of detrital zircon in the continental slope basin(flysch)unit(SB)shows that the weighted average age is 248.7±4.8Ma,It shows that the sedimentary age of the complex rocks in Zhikong-Songduoxiong area is no earlier than 248.7±4.8Ma(early Triassic).Flysch deposition time during eclogite two subduction turn-back role,is in a stable ocean sedimentary environment under the background of sedimentary strata,is the straight hole-more ocean is the most direct deposit record,indicates how the Zhikong-Songduo Ocean at 248.7±4.8Ma(early Triassic)is in a stable ocean background,sedimentary environment of continuous evolution.A variety of different lithology in the study area suggest the evolution process of Zhikong-Songduo Ocean.The late carboniferous Songduo formation was deposited along with the Zhikong-Songduo Ocean extension.The basalt and andesite of Pairigang island in the early Permian proved that the Zhikong-Songduo Ocean became a mature ancient ocean.The rapid and deep subduction of the middle Permian and middle Triassic Zhikong-Songduo Ocean was accompanied by high pressure metamorphism,and the eclogites metamorphosed and retraced.In the early Triassic,the Zhikong-Songduo Ocean was relatively stable,Flysch is under the background of ocean stable sedimentary environment,the product of continuous evolution.In the late Triassic—early Jurassic,the Xionglai formation corresponded to the sedimentary events of Zhikong-Songduo Ocean subduction and closure.