The Deformational Features And Tectonic Implications Of The Chongshan-Bluoxueshan Deformation Zone In Wayao Area, Western Yunnan, SW China

Southwest Sanjiang area is located in the southeast of the Qinghai-Tibet Plateau,east part of the eastern Himalayan Syntaxis,which has experienced strong tectonic deformation and formed such complex tectonic characteristic in Cenozoic.Based on the study of granite mylonite,we develop a collision extrusion model that the deformation is formed in the boundary of the block while the filler in Mesozoic-Cenozoic basin indicate a dispersively gradual decrease strain style.In this way,how to combine this two tectonic models and explain the deformation effect of the collision between India and Eurasia is a significant problem.This article is focused on the Chongshan-Biluoxueshan deformation zone,taking the fillings in the Cretaceous Jingxing Formation and the granitic rocks intruded into the formation as the object of study,,using zircon U/Pb dating to limite the granite emplacement age,and discussing the deformation history by mica Ar/Ar dating.New understanding are listed as follows:(1)The Wayao Area of the Chongshan-Biluoxueshan deformation zone expose early Cretaceous Jingxing Formation,low to medium-grade metamorphic rocks(mainly mica schist)of which the formation age is unknown are leucogranite from the west to the east.Leucogranite has intruded into the metamorphic rocks.The three kinds of rocks have experienced strong deformation while the deformation history is the most complicated.(2)Leucogranite has deformed into mylonite leading to strong flattening and stretched quartz and fragmentated feldspar and formed core-mantle structure and deformation bands gradually.Observation of Z-X and Y-Z plane sections showed that the elongation ratios of minerals of these two directions are similar showing S mylonite type characteristics.Feldspar residual plaque morphology results revealed that K values of the mylonite are far less than 1,between 0.02 and 0.22,indicating pure shear deformation.(3)Early Cretaceous deformation Jingxing Formation suggest a result of fold structure.Section analysis and structure plane occurrence projection show that it is B-type fold,a product of the buckling effect.Hub and axial plane occurrence indicates nearly east-west extrusion.(4)Cenozoic sedimentary rocks which have been intruded by the leucogranite exhibit no evidently deformation phenomenon indicating that the emplacement depth of leucogranite is not deep,and the mylonitization should occur in the same time.Granite zircon U-Pb dating results was 22.9 ±0.82 Ma,while muscovite 40Ar/39 Ar age is 16.75±0.22 Ma.(5)Combined the regional geology characteristics,we infer that the ChongshanBiluoxueshan deformation zone is the result of collision between India and Eurasia Block.Deformation begins in 55 Ma.In 17 Ma,deformation reaches to the peak This tectonic event resulted in uneven uplift of the rocks.The Wayao Section of this belt mainly contains pure shear strain,which is not consistent with plot escape model.Moreover,the overall deformation effect of the India-Eurasia collision still remains to be further detailed study.