Lithofacies Association And Provenance Characteristics Of Xiajiang Group In The Southeastern Guizhou And Its Tectonic Implication

The South China Block was formed through the collisional orogeny between the Cathaysia Block and Yangtze Block.The collision between the latter two formed an arcshaped belt which called Jiangnan orogeny on the southeastern margin of the Yangtze Block.The formation and evolution of this orogeny has attracted geologists' attention for a long time.The Xiajiang Group distributed in western part of Jiangnan orogeny,include the border areas of Hunan,Guizhou and Guangxi.It consisted of low metamorphic normal sedimentation inter-bedded with igneous rocks,overlay the Fanjingshan Group with unconformity and underlay the Nanhua glacial deposition with parallel unconformity.There has been a lot of controversy about the age,basin type and tectonic setting of the stratum.A detailed study of it can effectively explore the evolution process of the western part of the Jiangnan orogenic belt.This paper takes Xiajiang Group which distributed in the southeastern Guizhou and its adjacent areas as the research object.Based on detailed field geological investigation,we used lots of research measures such as sedimentary petrology,geochemistry and detrital zircon geochronology analysis to conduct a comprehensive study on the sedimentary facies and provenance.On the basis of previous research results,this paper will systematically analyzes the basin properties and tectonic setting of the Xiajiang Group.Through the identification of facies marks and study on the lateral and vertical variations of lithology,it is found that the Xiajiang Group in the study area is a sedimentary product of synchrono-heteropic facies.This group in Fanjingshan and Zhijiang area is a set of shoreshallow shelf facies;in Liping-Tianzhu region formed in shelf-slope environment.On the whole,it reflects the basin paleogeographic pattern higher in the north and lower in the south.The microscopic identification results show that there are many quartz,feldspar,felsic volcanic rocks and meta-sedimentary rock fragments,and the rock types are mainly feldspathic litharenite and lithic arkose.Rare earth and trace elements in source rock discrimination diagrams show Xiajiang Group source rock type is felsic magmatic rocks.Dickinson provenance discrimination triangular diagrams and trace elements discrimination diagrams show the tectonic setting of Xiajiang Group's provenance is mainly magmatic arc and the lower Xiajiang Group's is recycled orogeny.These characteristics suggest that the provenance of the group may come from intermediate to acidic volcanic rocks in magmatic arc.The U-Pb dating of tuff zircons from the bottom Qingshuijiang formation in Fanjingshan area is 777±9Ma.The detrital zircon U-Pb age of Xiajiang Group mainly range from 810 to 760 Ma,with a primary peak(794Ma)and a low peak(2016Ma).Euhedral zircons with oscillatory zones and Th/U >0.4 characteristic of magmatic origin.Qualitative comparisons of age spectrak reveal that the provenance of Xiajiang Group is closely related to the Yangtze Block.In combination with the southward flow direction and source rock characteristics,we considered that the source area of the group should be the Fanjingshan Group and intermediate-felsic volcanic rocks on the northern margin of the Yangtze Block.The rock assemblage,sedimentary facies distribution and provenance characteristics indicate that the sedimentary basin of Xiajiang Group is wedge-top basin.The evolution of the basin had gone through three subsidence period,include slow settling period(Jialu to Wuye formation and Hengluchong to Tongtawan formation),the initial rifting stage(Fanzhao to Qingshuijiang formation and Wuqiangxi to Duoyitang formation)and strong rifting stage(Pinglue to Longli formation and Baihelong to Niuguping formation).Yangtze Block and Cathaysia Block couldn't assemblage until the end of the Early Paleozoic.