Geological Characteristics And Formation Age Of Metamorphic Strata In The North-side Of Mianlue Tectonic Zone,South Qinling

The South Qinling Mianlue tectonic belt is the main joint belt of the North and South continental blocks in China.Its geological characteristics,structural properties and tectonic evolution are of great significance to the formation and evolution of the Qinling orogenic belt and the collisional orogenic process of the Yangtze plate and the North China plate.In this paper,the metamorphic strata exposed in the Xiaobianhe-Changba area,Mianxian County,north of the Mianlue tectonic belt were selected as the research object.A systematic research of field geological survey and indoor petrology,rock geochemistry and zircon U-Pb isotope chronology were carried out.The main achievements and understanding are:1.The meta-sedimentary rocks in the Xiaobianhe-Changba area are mainly a combination of mica quartz schist,biotite gneiss and light gray thick layered marble rock.The main characteristics of metamorphic minerals are garnet,cross stone,kyanite,biotite and muscovite.Combined with the inclusion relationship between metamorphic minerals through microscopic petrographic observation,the distribution of metamorphic phases is determined from greenschist to low Hornblende facies:the temperature and pressure conditions of the greenschist facies metamorphism constraining the metamorphic sedimentary rock are T=460?560?,P=0.39?0.6 GPa;the temperature and pressure conditions of the hornblende facies metamorphism are T=580?663?,P=0.65?0.73 GPa;the temperature and pressure conditions of the late degeneration process are T=470?570?,P=0.35?0.5 GPa.2.The minimum age-weighted average value of detrital zircon in the metamorphic sedimentary strata in the Xiaobianhe-Changba area is 576.5±12.4 Ma,which represents the lower limit of the sedimentary age of the set of metamorphic sedimentary strata.Its formation time should not be earlier than the end of Neoproterozoic.Previous studies have shown that its sedimentary age should be no later than the early Devonian,representing the upper limit of the sedimentary age.It is concluded that the main body of the formation age of the metamorphic sedimentary strata should be the Early Paleozoic.The age spectrum of detrital zircon mainly includes 76 Neoproterozoic ages,90?595 Ma;1 Mesoproterozoic age,1018Ma;and 2 Paleoproterozoic ages,2026 Ma and 2435 Ma.Comprehensive research suggests that the source of the metamorphic sedimentary strata in the Xiaoyuehe-Changba area of the study area is mainly from the volcanic rocks in the Mianlue tectonic belt and the Neoproterozoic magmatic rocks in the northwestern margin of the Yangtze block.3.The meta-sedimentary rocks in the Xiaobianhe-Changba area have undergone redeposition,and the source rock types are relatively complex.They can be obtained by combining the field macro-features,rock assemblage characteristics,geochemical discrimination parameters and discrimination diagrams.The main body of metamorphic rocks is a set of low-intermediate metamorphic parametamorphic rocks.The primary rock components of parametamorphic rocks are mainly mixed sandstone and argillaceous siltstone.The material composition mainly comes from felsic rock and mixed sandstone,and the medium-low weathering degree reflects the low maturity and structural maturity of the material composition.The climate and tectonic environment are in an unstable state,and have experienced different tectonic environment depositions.The main tectonic environment during the deposition was rifting troughs or deep-water rifting deposits on the continental margin.4.The macro-structural deformation characteristics of the metamorphic sedimentary strata in the Xiaobianhe-Changba area are analyzed,divided into four structural deformation stages:the first structural deformation(D₁)is the plastic solid rheological deformation in the deep structural layer,manifested in the early face and the formation of the same structure deep melt veins(S₁);the second structural deformation(D₂)is at the mid-deep tectonic level and is also the main fold formation period,manifested as a medium-scale back shape formed by the early permeability surface theory(S₁)controlled by the thrust shear,directional fold structure and outcoherent scale inconsistent folds,tectonic lens,B-type wrinkle line and new surface texture(different types of fold cleavage S₂),etc;the third-stage tectonic deformation(D₃)is a ductile strike-slip shear deformation in the middle tectonic level,formed during the intracontinental strike-slip adjustment stage,and manifested as an early surface texture(S₁?S₂)forming outcrop under the action of late left strike slip asymmetrical structure of the scale lens body,rootless folds,tensile lines,inconsistent folds and the accompanying steep tilt lines;the fourth-stage tectonic deformation(D₄)is a brittle deformation at the superficial tectonic level,formed during the intracontinental deformation stage of the Qinling orogenic belt,manifested as a series of thrust faults from north to south.