The Characteristics Of Metamorphism?Deformation And Geochronology Of Rocks In Qinling Group

Orogenic belts,as one of the tectonic units with the strongest plate interaction,record abundant information of crustal and lithospheric tectonic deformation in the process of continental formation and evolution.This information is not only the key to study the tectonic evolution of orogenic belts,but also a good carrier to solve the tectonic deformation of continental lithosphere and to explore the rheological mechanism and process of lithosphere.As an ancient metamorphic basement in the Qinling orogenic belt,the Qinling Group has undergone many subduction collisions and superimposed transformation of intracontinental structures in the Greenville Period,late Early Paleozoic to early Late Paleozoic and Indosinian Periods.Its metamorphic basement records and preserves abundant geological information such as tectonic deformation,metamorphism and deep melting of the lower and middle crust.After detailed field work and microstructural analysis,this study divides the deformation stages of the Qinling Group into four stages.The first stage is a regional deformation event.The macroscopic manifestation is that the horizontal fold F₁ is formed on the moving surface of bedding schistosity S₀,and the permeable foliation S₁ and linearity L₁ are produced.The second deformation is also a regional deformation event,the macroscopical manifestation is that the first stage of regional foliation S₁ is the fold surface,forming an asymmetric-tight fold F₂,and resulting in the corresponding axial foliation S₂ and mineral stretching lineation L₂,accompanied by magmatic activity and local migmatization;the third stage of deformation is ductile shearing,which shows that marble,granite,gneiss and schist near the fault zone are subjected mylonitization and produces corresponding mylonitic foliation S₃and mylonitic lineation L₃.The fourth stage of deformation is different from the previous stages,which can be seen only in some areas.Macroscopic manifestations are non-penetrating wide and gentle folds.The fourth stage of foliation S₄ occurs as the axial plane.The temperature and pressure of peak metamorphism of basement rocks in Qinling Group were obtained by means of electron probe analysis and mineral thermobarometer.The temperature and pressure conditions of peak metamorphism of garnet-bearing biotite plagioclase gneiss are T=601-680?,P=0.39-0.55Gpa.The peak metamorphic temperature and pressure conditions of garnet amphibole are T=569-584?,P=0.40-0.42 Gpa.The metamorphic degree of both of them reaches high amphibolite facies and has the characteristics of medium-pressure clockwise P-T-t path.Zircon dating analysis of migmatite and basement metamorphic rocks in the qinling group shows that the metamorphic age of the dark body of the migmatite is427±14Ma,and the crystallization age of the light body is 442.4±8.9Ma,both of which are consistent within the error range,indicating that the qinling group experienced a tectonic thermal event at about 440-420Ma.The age of 918±56Ma given by the zircon magmatic core in garnet-bearing biotite plagioclase gneiss is consistent with the upper intercept age,indicating the formation age of the gneiss.The age of metamorphic zircon is 470±15 Ma,within the error range wiht the intercept age,representing the metamorphic age of gneiss.The age of magmatic zircon from the granitic vein cutting the foliation of gneiss is 414±8.7 Ma,which constrained the upper limit of the second deformation.