Structural Characteristics And Evolution Of Shirenshan Block In North Qinling

The Qinling orogenic belt is a subduction collisional orogenic belt across the east to west of China.It is the result of the subduction,collision and in-land orogening of two major plates of Yangtze and North China block.Plate convergence caused strong deformation of the southern margin of the North China plate and was eventually involved in the Qinling orogenic belt as an important part of the orogenic belt.Shirenshan block is located at the junction of North Qinling and North China plate,thus recording the whole process of formation and evolution of the orogenic belt.The detailed study on the metamorphism and deformation of the plate can restore the subduction and collision of the plate,retrieve the convergence mode and kinematics of the plate,and provide important implications.Based on tectonic geology and tectonic mineralogy,the Shirenshan block is affected by the Luoluan fault and the Lushan fault.Through the field observation of geological profile,microstructural analysis,geochemical analysis,isotope chronology and other theories and methods,the metamorphic deformation and tectonic evolution of Shirenshan rock block are studied comprehensively.The macroscopic deformation characteristics in the field show that the Shirenshan block is a complex geological body with multi-stage tectonic movement.The distance from east to west is about 40 km,and the distance from north to south is about 20 km.Its composition is composed of the southern Taihua group migmatite,the central gneiss and the northern undeformed granite.The south side of the block is the Luoluan fault,the northern east side is the Lushan fault,and the north side is the Checun fault.In general,the structural deformation of the block is affected by the structural activities of the Luoluan fault,and the deformation strength of the rock gradually weakens from south to north.And the Luoluan fault is characterized by the subduction of the southern plate and left strike slip tectonic movement.The Lushan fault on the northern east side was characterized by the right strike slip and the kinematics characteristic of the south plate upward thrust.The tectonic action of the two faults restricts each other,which makes the block sandwiched in them,so that the block shows the whole westward and upward kinematics characteristics.The Shirenshan block is mainly divided into three categories: migmatite,gneiss and mylonite.The analysis of mineral deformation characteristics and metamorphicfacies of the block shows that the mineral symbiotic assemblage of the near-Luoluan fault zone is green schist facies,while it translated to the amphibolite facies in the north.The degree of metamorphism from south to north is from low to high.The study of dynamic recrystallization type found that near the Luoluan fault,the quartz mainly manifested as bulging dynamic recrystallization,from the fault zone to the north,and the quartz dynamic recrystallization type gradually translated to subparticle dynamic recrystallization until the final high temperature boundary migration dynamic recrystallization.It can be seen that near fault zone,the quartz dynamic recrystallization temperature is low,away from the fault,the dynamic recrystallization temperature gradually increased.It shows that the dynamic recrystallization of quartz is affected by the ductile shear metamorphism of the Luoluan fault.Feldspar mostly exists in the form of residual spots,near the Luoluan fault zone,and the feldspar is mainly brittle fracture and fragmentation flow from south to south.As far away from the fault zone,the feldspar gradually developed puffy recrystallization,which is also affected by the metamorphism of the Luoluan fault.according to the analysis of electron probe data,the metamorphic temperature of middle gneiss is 405?558?,and the pressure is 0.83?1.05 GPa,which indicates the metamorphic phase environment of the green gneiss-angle diorite phase and shows its large burial depth.The composition change of plagioclase from the core to the mantle to the edge also changes regularly with its corresponding grade,which reflects that the growth environment of plagioclase experienced from medium-high temperature and medium-high pressure to medium-low temperature and medium-low pressure,indicating that the growth environment of plagioclase is affected by the retrograde metamorphism.According to the analysis of strain strength and deformation mechanism,the Shirenshan block mainly experiences the structural deformation of shear tensile type,which is manifested as uniaxial cigar deformation mechanism and affected by the Luoluan fault.The finite strain strength of feldspar is strong to weak from south to north.At the same time,as far away from the Luoluan fault,the fractal dimension and strain rate of quartz gradually decrease,indicating that the degree of deformation gradually decreases and the temperature gradually increases.It can also be seen from the fractal dimension of quartz that the dynamic recrystallization of quartz is closely related to the Luoluan fault,which indicates that the formation of the Shirenshan isaffected by the retrograde metamorphism of the Luoluan fault.Vorticity analysis shows that the vorticity value of the Luoluan fault on the south side of the Shirenshan block is > 0.75,showing the structural characteristics dominated by simple shear.The vorticity value in the north-east Lushan fault zone is<0.75,which shows the dominated by pure shear and a small number of characteristic components of simple shear deformation.Stress field analysis shows that the main compressive stress direction of Luoluan fault in the south side of Shirenshan block is mainly 45°?60° in the NE-SW direction,170°?180° in the SN direction,80°?85° in the EW direction.Based on the analysis of macroscopic deformation and kinematic characteristics in the field,it is concluded that the block is subjected to three stages of tectonic activity: the first stage of tectonic activity is dominated by subduction from NE to SW direction,the second stage occurs near EW horizontal sliding movement at fault zone and rock boundary,and the third stage of near-north tectonic action is more intense in the southeast of rock mass,which is characterized by thrusting from south to north.The main compressive stress direction of the Lushan fault zone on the north-east side of the block is mainly45°?50° in the NE-SW direction and 135°?145° in the NWW-SEE direction.The two groups of stresses are distributed in the north of the Shirenshan block.It is inferred that the principal stress in the NE-SW direction corresponds to the tectonic activity of the north thrust of the Shirenshan block,while the NWW-SEE stress corresponds to the right strike slip movement in the NW-SE direction of the Lushan fault and the Shirenshan block boundary.Geochemical characteristics of the Shirenshan rock block show that the Taihua Group migmatite and the central gneiss are all rocks of high potassium calc-alkaline series A type granitic composition.Not only the content of the main elements is close,but the rare earth element distribution map and trace element cobweb map all show similar curve characteristics,indicating that they both have similar geochemical behavior and common source region properties.The middle gneiss has the characteristics of rare earth elements of crustal remelting granite,which indicates that the magma forming the gneiss of Shirenshan contains partially melted crustal material,and its source rock material is closely related to the land crust.Sr-Nd and Lu-Hf of rock in Isotopic studies show that the parent magma forming Shirenshan gneiss is related to the southern Taihua Group migmatite,which is mainly part of the molten material in the crust of the Middle Proterozoic,and it is assumed that the source rockis the southern Taihua Group material.And the ratio of Lu-Hf isotopes in the middle gneiss samples of Shirenshan varies greatly,indicating that the molten material may contain some mantle-derived magma mixed,indicating that the magma forming the gneiss of Shirenshan has several non-uniform source regions.The chronology shows that the zircon of Shirenshan gneiss has typical ring zone characteristics.The nuclear age of Shirenshan gneiss is ?1500 Ma,which shows the residual zircon of the early Proterozoic rocks in the southern margin of the North China plate.According to the previous geochemistry study,the age of ?430 Ma of mantle is magmatic crystallization age,which reflects the subduction age of Luoluan fault.The Luoluan fault has the characteristics of left-line shear deformation,and the zircon age of the granite vein affected by the left-line shear tectonic activity is 406Ma?413 Ma,indicating that the left-line slip movement of the fault has been active at least since the late Paleozoic.Due to the right-line shear deformation characteristics of the Lushan fault zone,the zircon age of the deformed minerals in the sliding shear deformation lens is about 419 Ma,which indicates that the fracture experienced the tectonic deformation activity of the early Devonian-late Silurian period.The continental deformation in Qinling orogenic belt continued from the beginning of the late Silurian period until the late Paleozoic.During this period,the main structural deformation modes of the Shirenshan block in the southern margin of the North China plate are the continental slip and lateral extrusion structures.Through the detailed field survey and the analysis and research of various indoor means,it is considered that the Shirenshan gneiss formed in the tectonic environment of the north Qinling subduction to the southern margin of the North China plate.In the late Calidon period,due to the subduction of the Luoluan fault,the rocks in the middle and lower crust of the southern margin of the North China plate were partially melted,and the mantle source material was mixed to form the same tectonic magma.The magma intrudes into the north side of the Luoluan fault zone along the structural weak zone and is oriented by the dark minerals that crystallize first due to the tectonic activity of the fault,forming the primary gneiss of Shirenshan.At the later stage of subduction,left-walking slip motion occurred and the tectonic activity of thrusting from south to north was superimposed.Combined with the characteristics of right-walking slip movement in the northern and eastern Lushan fault zone,the whole block moves westward and upward.In the early Cretaceous period,a large amount of granitic magma intruded in the northern part of Shirenshan,and the heat flow causedthe replacement of the rocks and formed the Yanshanian age.Therefore,the metamorphism and deformation of Shirenshan block is a reflection of tectonic-magmatic activity in the southern margin of North Qinling and North China plate,and it is of great significance to study the edge deformation during plate subduction.