Geological Characteristics Of Mafic Rocks From The Ediacaran System In Aksu,NW Tarim,and Their Stratigraphic Significance

Aksu region is located in the northwestern margin of Tarim basin,is a former Cambrian-one of the best areas in upper Paleozoic strata outcropped,and is regarded as one of the oldest well-substantiated Precambrian blue terranes and various types of mafic rocks,and the formation of Aksu area has long been a lack of biological formation and other sedimentary age restriction,making it difficult to through the biostratigraphy and reliable horizon control,As a result,systematic regional stratigraphic framework has not been established in this area,which makes it impossible to carry out effective regional stratigraphic correlation.In order to solve this problem,researchers often restrict the age of sedimentary strata according to the formation time of extrusive rocks developed in the strata,and establish the stratigraphic chronology framework of specific regions through regional stratigraphic correlation.Different layers of mafic rocks are widely developed in the clastic rocks of Sugetbrak Formation in the lower Ediacaran system in Aksu area,however,whether these mafic rocks are basalt or diabase exists different understanding at present,which leads to their stratigraphic significance is not clear.Field geological survey and laboratory study of the exposed mafic rocks in Dongergou section,Linkuanggou section and Yuermeinark section in Aksu area are studied in detail in this paper.Three section of rock in addition to Yuermeinark section Sugetbrak Formation at the top of the mafic rock,others can observed intrusive rocks in the surrounding rock formation.Each layer of mafic rock has the typical petrographic change characteristics,the mineral particles in the rock from the edge phase to the internal crystalline particles from fine to coarse,and there is an obvious baking zone phenomenon at the contact boundary of the upper sandstone layer.The mafic rocks in the three typical sections are mainly composed of pyroxene(45%?50%)and plagioclase phenocrysts(45%?50%),and with small amounts of Fe-Ti-oxides(5%),showing typical diabasic structures and ophitic texture,no typical flow-top,vesicular,or amygdaloidal structures were observed during field work.The geochemical composition of the mafic rocks in this region is similar,characterized by high Ti O₂ content(1.47?3.59wt.%),low Mg O content(3.52?7.88wt.%),K₂O content(0.12?1.21wt.%),and P₂O₅ content(0.32?0.67wt.%).In addition,Na₂O content is greater than K₂O content.Rb,Sr and Cs of large ionic lithophile elements(LILEs)are in deficit,among which Sr negative anomaly is the most obvious,and high field strength elements(HFSEs)are relatively enriched.The rare earth elements(REEs)were enriched by light rare earth elements(LREEs)and the heavy rare earth elements(HREEs)are loss,furthermore,the mafic rocks of Aksu area similar mineral composition,on the Harker diagram with continuous change trend,and in a spider diagram of trace elements and rare earth element distribution curve with a similar trend.Combined with their close distribution distance,similar outcropping characteristics and contact relationship with the strata,it can be inferred that these mafic rocks may originate from the same source area and are the result of a period of sub-magmatism under the same tectonic background.Through the discussion of the detailed chronological studies in recent years,it is concluded that the origin of the mafic rocks at the top of the Yuermeinark section is still controversial and needs further study.The others are all intrusive diabase,so they cannot be used as marker beds for regional stratigraphic correlation,and their diagenetic ages cannot be used to constrain the stratigraphic ages of the Ediacaran system they intruded into.