Sedimentary Evolution,Tectonic Setting And Provenance Analysis Of The Devonian Liuling Group In The Northern Belt Of The South Qinling Mountains

The South Qinling North Belt is adjacent to the Shangdan suture between the North China Plate and the Yangtze Plate,which main exposed stratum is the Devonian Liuling Group.Predecessors have done a lot of research on Liuling Group from different angles,whereas there are still divergences on the tectonic background and tectonic properties of the Liuling Group and the time limit of the collision between the two plates.This paper selects the Devonian Liuling Group in the Zhashui-Shanyang-Danfeng area in the northern belt of the South Qinling as the research object.Using the methods of sedimentology,petrology,geochemistry and detrital zircon chronology,the sedimentary evolution characteristics,tectonic background and provenance of the Devonian Liuling Group in the northern belt of the South Qinling Mountains are studied,in order to provide basises for solving the aforementioned problems.(1)By analyzing the sedimentary characteristics of the Devonian Liuling Group in the northern belt of the South Qinling Mountains,four types of sedimentary systems are identified: wavecontrolled coastal system,tide-controlled coastal system,storm-controlled shallow sea shelf system and semi-deep sea-deep sea system which mainly developed the turbidite system.The sedimentary sequence shows that in the early-middle Middle Devonian,the Liuling Group was a shallow sea shelf-shore sedimentary system,which sedimentary water body was shallow;in the late Middle Devonian to early-middle Late Devonian,the sedimentary environmental water body continued to deepen,which evolved into a semi-deep sea-deep sea system which mainly developed the turbidite system;in the late Late Devonian,the sedimentary environment evolved to a coastal environment again,and the water body changed to a shallow water environment;The sedimentary water environment of the Devonian Liuling Group in the northern belt of the South Qinling Mountains has generally experienced a process of deepening at first and then becoming shallow.(2)Through Dickinson diagram analysis of the sandstone clastic components of the Devonian Liuling Group in the northern belt of the South Qinling Mountains,it is determined that the provenance area is a recycle orogenic belt;The analysis results of the characteristics of major and trace geochemical elements show that the provenance region of the Devonian Liuling Group in the northern belt of the South Qinling Mountains is continental island arc and active continental margin.(3)The analysis of detrital zircon U-Pb dating method shows that the age of the main peak of detrital zircon is Early Paleozoic(445±10Ma),and the age of the secondary peak are Early Neoproterozoic(930±10Ma),Middle Neoproterozoic(800±20Ma)and Meso-Paleoproterozoic(1800±50Ma,2500±50Ma)in the Devonian Liuling Group in the northern belt of the South Qinling.All of the peak age of detrital zircon in the Liuling Group is found in the North Qinling Mountains.Although there are some peak ages of the Liuling Group(800±20Ma,2500±50Ma)in the South Qinling,the main peak age source(445±10Ma)is lacking.The zircon age characteristics of the samples are all the same,indicating that the provenance of the Devonian Liuling Group in the northern belt of the South Qinling is a unilateral source of the North Qinling.(4)Andesitic lithic tuff was found in the sedimentary wedge,which zircon dating results showed a single peak shape with a peak age of 420 Ma,indicating that the island arc still existed and the Shangdan Ocean was still subducting in the Late Silurian;The study on the provenance of the Liuling Group shows that the Liuling Group has begun to accept the provenance of the North Qinling in the recycle orogenic belt in the Middle Devonian,indicating that the northern and southern Qinlings had been collided and spliced before the Middle Devonian.Therefore,the analysis suggests that the time limit of the collision and splicing of the northern and southern Qinlings was Early Devonian.(5)Although the source of sandstone debris in the Liuling Group comes from the recycle orogenic belt,the debris is mainly metamorphic rock debris,and there are few sedimentary debris and magmatic debris,reflecting that the provenance is mainly from basement metamorphic rock of North Qinling,and there no significant uplift thrusting was formed in this collision.Combined with the absence of molasses closely related to orogeny in the Devonian in the region,it indicates that the Qinling orogenic belt only spliced and collided but did not make orogens in the Early Devonian.(6)The sedimentary sequence of the Liuling Group is coastal neritic-deep sea-shore,changing from shallow to deep and then shallow,meanwhile the provenance is unilaterally from the North Qinling,combined with the overall Devonian system in the South Qinling as the extensional background,it is inferred that the tectonic setting of the Liuling Group is a post-collision extensional rift basin.