Deformation Evolution Of Nanduan Formation And Lancang Group In Xiaoheijiang Area

The Changning-Menglian suture zone is located in the southwest of the“Sanjiang” region of China and is an important polymetallic metallogenic belt in the“Sanjiang” region.The deformation in the suture zone is very complicated and has always been a hot spot in disputes in the “Sanjiang” region.There are still many debates about the developmental time and structural properties of the Changning-Menglian suture zone.Regarding the timing of Changning-Menglianyang's initial dive,the basic consensus was in the Early Permian or Late Carboniferous.There has also been controversy over the sources of provenance in the southern section.There have also been different opinions on the age of the Tatars.In addition,the work of the predecessors was not exhaustive in the evolution of the deformation of the Xiaohei River region.This paper takes the Lancang group and Nanduan formation in the Xiaohejiang area of the Changning-Menglian suture zone as the research object,through field observation records,measurement statistics,indoor observation and analysis and dating experiments,and obtains some new insights.Comparison of zircon U-Pb geochronological experiments with regional geology shows that the Lancang group and Nanduan formation have concentrated peaks of about 500-600 Ma and about 1000-1200 Ma,indicating that they have received a large amount of detrital material from the Neoproterozoic and Cambrian.The minimum age of the Lancang group is about 460 Ma,indicating that its formation was later than the Ordovician,and was not the Neoproterozoic strata;the minimum age of the Nanduan formation was 346 Ma,which was later than the Early Carboniferous.The age spectrum diagrams of two are in good agreement with the Yangtze plate,indicating that most of their original rock source originated from the appearance of the block.The macrostructure analysis shows that the Lancang group is characterized by tightly developed folds and co-axial superimposed folds,while the Nanduan formation is characterized by lensization,turbulence,and asymmetrical folds..Based on the occurrence statistics of the fold center and wrinkle line,the Lancang group's closed folds and the co-axial superimposed wrinkling hinges are oriented approximately at 330°,and the part of the Nanduan formation with asymmetric folds is the same..It shows that the two are deformed under the same tectonic stress field.This deformation developed in the early Triassic oceanic subduction,and different deformation environments were formed due to the different depth environments.The Lancang group and the Nanduan formation mainly experienced three stages of deformation,which were related to the subduction of the paleo-Tethys ocean.The main deformation in D1 phase occurred in the Early Triassic.The subduction effect brought the material of the lower plate and the material at the bottom of the upper plate into the deeper crust.The metamorphism and deformation of the blue schist facies occurred,and the blue schist facies of the Lancang group was metamorphic and tight.The development of closed folds formed at this stage;D2 phase deformation occurred in the middle and late triads,characterized by penetrative infiltration developed in the Nanduan formation metamorphic quartz sandstone;D3 phase deformation occurred after the Late Triassic.The Nanduan formation and the Lancang group are characterized by widespread development of folds.The microstructural and quartz c-axis texture analysis shows that the Lancang group's muscovite quartz schist undergoes grain boundary migration recrystallization of the quartz grains with a deformation temperature of about 550°C;quartz c-axis fabric results show that it has occurred Cylindrical <a>Slip and bottom <a>slip,that is,it has undergone one stage of intermediate temperature deformation(about 550-650°C)and superimposed one stage of low temperature deformation(about 400°C).The quartz particles of Nanduan formation have undergone the recrystallization of subgrains,and the deformation temperature is about 400°C.The quartz c-axis fabric results show that a period of bottom <a> slip occurred and the temperature was about400°C.The dating test of the Lancang group determined that the formation age of the Lancang group was not Neo-Ancient but the Ordovician;the experimental analysis of the quartz C-axis fabric yields that the Lancang group and Nanduan formation have experienced about 550°C and about 400 °C metamorphism respectively.These preliminary understandings are expected to provide some support for the study of the Changning-Menglian suture zone and Paleo-Tethys.