大陆俯冲隧道中的应变不均一分布:来自大别山超高压变质岩的记录OA北大核心CSTPCD

As the shear zone between the subducting and overlying plates,a subduction channel is the pathway for exhumation of high-and ultrahigh-pressure(UHP)metamorphic rocks and deep fluid/melt activity.Due to the wide exposure and variable deformation degree of UHP metamorphic rocks,the Dabie orogenic belt provides an ideal area to investigate metamorphism and deformation processes in a continental subduction channel.Here we summarized previous petrological and geochronological studies in the Shuanghe area of the central Dabie orogenic belt and carried out field mapping,strain analysis and 3D structural reconstruction in this region.Combining deformation with P-T-t paths of UHP rocks,three ductile deformation phases during exhumation of UHP rocks are identified.A kilometer-scale sheath fold of eclogite facies(D1)in the northern Shuanghe area is characterized by the SE-plunging fold axis with a dipping angle of～20° and a top-to-the-NW shear sense.This indicates that high fluid activity during the initial exhumation significantly reduced viscosity of eclogite,leading to the simultaneous ductile deformation of eclogites and surrounding rocks.The following deformation phase under amphibolite facies(D2)overprinted the D1 fabrics by the continuous top-to-the-NW shearing.Strain was localized in incompetent rocks such as gneisses,schists and marbles,whereas eclogites appeared as tectonic boudins.The top-to-the-SE detachment(D3)mainly occurred in schists under greenschist facies,which may be related with formation of the granitic gneiss dome in the northern Dabie orogenic belt during the Early Cretaceous.In the southern Shuanghe area,zircon U-Pb dating of weakly deformed granitic gneiss revealed the crystallization age of 757±14 Ma and the metamorphic age of 240～216 Ma,which are consistent with strongly sheared coesite-bearing granitic gneiss in the northern Shuanghe area.Hence the weakly deformed granitic gneiss also experienced the Triassic UHP metamorphism and the amphibolite facies retrograde metamorphism.Calculation of the effective viscosity of felsic gneiss indicates that dry alkali-feldspar granitic gneiss is stronger than biotite-plagioclase gneiss.The presence of water can significantly reduce the effective viscosity of UHP rocks.If localized fluid activity could not weaken the large-scale alkali-feldspar granite gneiss pluton,strain localization occurs in biotite-plagioclase gneiss.Therefore,strain distribution in a continental subduction channel is controlled by mineral assemblage,fluid activity and volume of the rock body.