| Xiong?ershan area is located in the south margin of the North China plate,and the joint area between the Qinling Dabie orogenic belt and the North China plate.In this area,the metamorphic basement of the Taihua Group and the cap rocks represented by Xiong?er group were exposed,and a lot of Mesozoic rocks were developed.Xiong?ershan area has experienced a complex structural evolution history since the Proterozoic,and rocks of different ages have recorded the structural deformation process at key structural stages.This area has become a heated research area due to gold mineralization,and the structural characteristics related to mineralization have also been studied to some extent.However,the existing investigations mainly focused on two aspects: the discussion on the Xiong?er rift system,and the extensional structural deformation in the late Mesozoic.The related investigations were lack of systematic.There has been a consensus on the occurrence of Paleoproterozoic orogeny and subsequent extension events in the region,and the Xiong?ershan area is also considered to be significantly involved in the early Mesozoic orogeny event.However there is still a lack of systematic determination of the key structural deformation sequence and its deformation characteristics in each stage since the Proterozoic,and there is a lack of due research on the deformation mode and deformation age of the key structural period.Besides,although the predecessors have carried out detailed structural analyses on the late Mesozoic extensional deformation,their deformation characteristics have not formed a unified understanding,and the age of structural deformation is still lack of effective chronological constraints.This study is based on a lot of fieldwork,combined with indoor microstructure,isotopic chronology,and other comprehensive studies.Four key structural deformations in the Xiong?ershan area since the Proterozoic are determined,including Paleoproterozoic orogenic events,Paleoproterozoic extensional events,early Mesozoic orogenic events,and late Mesozoic extensional events.In the Taihua Group metamorphic rocks of the Xiong?ershan area,the compressional deformation of the hanging wall NWW movement is generally developed.The deformation in this period is often superimposed or replaced by the later structure and has a relatively stable lineation orientation and variable foliation occurrence.The structure of this period developed in the process of collision and orogeny between the eastern and western blocks of North China in Paleoproterozoic,which represents the block collage product of early North China cratonization.Zircon U-Pb geochronology indicates that the tectonic deformation occurred between 1965 ~ 1840 Ma,which represents the time of the Paleoproterozoic syn orogenic compressional deformation and is the product of basement detachment during the process of compressional orogeny.In Taihua Group,nearly SN trending extensional deformation is also widely developed.The structure of this period is characterized by the development of gently dipping foliation and SN trending lineation.The deformation fabrics are often superimposed on the early NWW compressional fabrics,replacing or partially replacing the early fabrics.The structural deformation developed during the formation of the Xionger rift system,which was the product of intense extension during the rift period,in the same period as the large-scale volcanic activity of the Xionger group.Zircon geochronology of the deformed and undeformed dyke through this period shows that the extensional deformation developed between 1788 ~ 1724 Ma.The NNE trending and near trending ductile shear zones represent the ductile detachment deformation of the orogenic front in the Late Triassic.The deformation developed in Taihua Group in this period,with the dip direction of lineation between SN to NNE,is characterized by the movement of hanging wall to SSW and is a typical Indosinian synorogenic compressional structure.In the northern margin of Xiong?ershan,where the outcropping level is shallower,there are contemporaneous compressional folds.The axial trace of the folds is NWW,and the involved strata are Xionger group Upper Triassic.They respectively represent the compressional deformation fabric of the deep and shallow crust in the northern margin of the orogenic belt during the collision and orogeny of the South China and North China plates in the Indosinian period.Near the contact zone between the Xiong?er group and Taihua Group,there is also a NE trending ductile shear zone with an NW-SE lineation direction.In accordance with the strike and orientation of the ductile shear zone,there are a large number of normal faults in the same direction,which represents the extensional structural deformation in the early Cretaceous.Its extensional orientation is consistent with that of the contemporaneous extensional structure represented by the metamorphic core complex,which is widely developed in eastern China.This indicates that the extensional events in the late Mesozoic in eastern China can at least extend westward to the Xiong?ershan area and still have strong activity.At the same time,large-scale intermediate acid magmatism developed,and syntectonic deformation can be seen at the edge of the rock mass.Zircon geochronology of the synmorphic body indicates that the extensional structure developed at about 131MaThis study shows that in the Paleoproterozoic collisional orogeny of Xiong?ershan area,a detachment type compression structure in the direction of EW was formed;then,in the rift activity under the background of post-orogenic extension,an extension structure in the direction of SN was developed;after a long period of structural calm,in the collision orogeny of North China and South China in the Late Triassic,a basement detachment and an axial trace in the caprock in the direction of NNE were formed The NWW-SEE fold is the representative compressional structure;and in the early Cretaceous regional extension event,the NW-SE extension deformation represented by metamorphic core complex was developed. |
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