Identification And Formation Time Of The Two Deformation Events Of The Yiwulü Metamorphic Core Complex

The YiwulüMetamorphic core complex is located in the northern margin of North China Craton,east section of Yanshan Orogenic Belt,the research about the YiwulüMetamorphic core complex and the two ductile shear zones develop on it can help to decipher the Mesozoic tectonic-thermal events in western Liaoning region and northern North China Craton,to provides a certain basis to discuss the Yanshan Movement in eastern China and the thinning history of North China Craton.Through detailed field investigation,two ductile shear deformation events can be identified on the northwestern margin of the YiwulüMetamorphic core complex,western Liaoning Province.A large number of macro and micro kinematic indicators such as augen structures,asymmetric folds,S-C fabrics and mica fishes,indicating the early ductile deformation（D₁）as the dextral shear deformation,the later extensional shear deformation（D₂）is dominated by sinistral shear,and superimposed on the early ductile deformation（D₁）.Detailed lithography,quartz EBSD analysis,zircon U-Pb dating,kinematic and rheological analysis show that the early deformation is ductile deformation with high temperature and the later deformation is ductile deformation with lower temperature.In the early high-temperature ductile deformation,the quartz EBSD fabrics take prismatic<a>slip as the dominant slip system,quartz and feldspar take GBM and BLG as the main microscopic deformation mechanism,respectively,indicating the deformation temperature is about 550～650℃,which is equivalent to the metamorphism condition of amphibolite facies.The kinematic vorticity wk value of rock samples is less than 0.71,which is formed by the general shear contributed by pure shear and simple shear.The paleo-stress value is about 18.42 MPa,and the strain rates range from 9.40×10^-10to 1.62×10^-15s^-1.In the later low-temperature ductile deformation,EBSD fabric is dominated by rhomb<a>slip system,and SGR is the main deformation mechanism of quartz grains,indicating the deformation temperature is about 400～500℃,which is equivalent to the metamorphism condition of greenschist facies.The kinematic vorticity wk value of the rock is greater than 0.71,which is formed by the general shear dominated by simple shear,the paleo-stress value is about 22.37 MPa,and the strain rates ranged from 5.00×10^-11to5.44×10^-17s^-1,superimposed on the early high-temperature ductile deformation.The K value obtained by finite strain measurement using short and long axis method and Fry method is closer to plane strain,which might represent a stress state that stretching is approximately equal to flattening;the strain strength and tensile effect of later low-temperature deformation are stronger.Zircon U-Pb ages of mylonite samples are157.4±1.6 Ma,156.3±2.3 Ma and 132.6±3.1 Ma,respectively.Combined with previous isotopic dating data,it is believed that the two stages of ductile deformation mainly occurred in the Middle to Late Jurassic and Early Cretaceous,which may be related to the subduction and roll-back of Izanagi plate to Eurasian plate under the Yanshan Movement multi-plate convergence regime.The early Cretaceous extensional deformation was accompanied by the formation of extensive metamorphic core complexes and faulted basins.