Ductile Shearing Characteristics,Geochronological Restriction And Regional Tectonic Implication In The Northwestern Of Sulu Orogen

The Sulu orogenic belt,as a product of the Triassic continental collision between the South China Block and the North China Block,has experienced multi-stage structural events,and its evolution has always been the focus of research.Intense structural deformation existed in Huanghua-Tiejueshan areas which are located in the northwestern margin of the Sulu orogen,and therefore formed ductile shear zone.Because the ductile shear zone is closely related to the orogenic process,the detailed study of the deformation characteristics and geochronology of the ductile shear zone is beneficial to deepen the understanding of the tectonic evolution of the Sulu orogen.In this paper,we analyze the geometry and kinematic of shear zone based on detailed field investigation and microstructure analysis.The results suggest that the ductile shear zone in ENE direction was geometrically shaped as an antiform and experienced two stages of ductile shear deformation.(1)The early ductile deformation is produced by subhorizontal top-to-NWW trending shear tectonism under the regional compression stress.(2)The late ductile deformation is formed due to NNW-SSE trending extensional collapse under the regional extensional stress.In combination with the mineral deformation characteristics under microscope and the EBSD image of quartz C-axis fabric in directional sample,the two stages of ductile deformation temperature are estimated.(1)The early ductile deformation was developed under a condition of low amphibolite facies to high greenschist facies,and the temperature was between 500? to 600?.The local positon was under a condition of high amphibolite faces,and the temperature was more than 600?.(2)The late ductile deformation was developed under a condition of low greenschist faces,and the temperature was between 350? to 450?.According to the ages of magmatism and deformed minerals in the study area,the age of the two periods of ductile deformation is limited.(1)A large number of pegmatite veins appear in the study area,and they are related to the early ductile deformation.A pegmatite vein is dated at 150.1±1.1 Ma by LA-ICP-MS U-Pb analysis.Synthesized previous research,the early ductile deformation was initiated no later than 150 Ma.(2)The muscovite which is related to the late ductile deformation gives plateau age of 137.09±0.32 Ma that consistent with inverse isochron age.Moreover,dioritic porphyrite which intruded into the ductile shear zone without deformation yields LA-ICP-MS U-Pb ages of 123.5±0.8 Ma and 127.1±1.6 Ma.The late ductile deformation started no later than 137 Ma,and terminated prior to 127 Ma.Taking regional geological data into consideration,this paper suggest that the early and late ductile deformation corresponds to the late stage of intracontinental orogeny and post-orogenic extension,respectively.Two stages of tectonic activity reflect the tectonic regime transition in the Sulu orogeny.The tectonic regime transition possibly is closely related to subduction of Paleo-pacific plate and crustal extension in eastern China during the late Mesozoic.