Constraints On Structural Evolution And Chronology Of Danfeng Group In Danfeng Area Of ​​Eastern Qinling

As the demarcation line between south and north Qinling mountains, Shangdan Tectonic Belt is the main suture zone of Qinling Paleocene subduction. Ophiolitic melange, migmatite, gneiss, metasedimentary, marble and granite are outcropped here. There are both sharp differences on ophiolitic malange between the Danfeng Group and the north Qinling Group and on petrographic formation between Danfeng Group and Liuling Group which located in the south of Shangdan Tectonic Belt. Considering of these differences, the field tectonic deformation and vein materials of stages, This paper choose Danfeng Group as the ideal area to study on the deformation and geological evolution.This thesis clarified the tectonic deformation stage of Danfeng Group by analyzing the deformation trace on the basis of a comprehensive collection of the structural element, deformation character and intrusive vein; deduced the closed collision method of Qinling Paleocene by measuring strain and estimating the deformation temperature of the rocks that are mainly formed of mylonite according to their mineral association and deformation character. In addition,the thesis concluded the research findings on the subduction-colse orogeny of Qinling paleoocene and analyzed the deformation character on different stages of Danfeng Group from the perspective of its evolution. Except for the traditional trace statistics and analysis of tectonic deformation, this thesis adopted the vein material zircon method U-Pb to prescribe a limit to the activity area.After the research of Danfeng Group ophiolite on the impact of field outcropping and area geological background, the paper summarized that the closure pattern of Qinling paleoocene is a irregular pattern which subducts and closes from east to west gradually. Combined with regional geological features, Danfeng Group can be divided into four stages: The first stage is the thrust napping effect and the folding effect; on the second stage 423.2+ 1.3Ma-410.5±2.3, there was a shearing action from the left and the ophiolite fragmentation was folded; the third stage 408±2.6Ma-360.6±3.2Ma, there was a shearing action from the left; the fourth stage is the late brittle deformation superposition.