Plastic flow of ultrahigh pressure metamorphic rocks: microstructure and deformation mechanisms
超高压变质岩的塑性流变：显微构造和变形机制

Ultrahigh pressure (UHP) metamorphic rocks are products of deep continental subduction and have experienced long distance transportation and subjected to tectonic stresses during deep continental subduction and rapid exhumation.High strain and strong plastic deformation are mainly concentrated into ductile shear zones.Study of structures,microstructm'es and deformation mechanisms of the UHP rocks is important in understanding the rheological behavior of continental crust in the deep subduction processes.The eclogite and granitic mylonites in the ultrahigh pressure ductile shear zone at Yangkou,Shangdong Province,have recorded UHP deformation history of the rocks.Minerals stable at UHP conditions,omphacite,phengite,kyanite and K-feldspar have irregular undulatory extinction,subgrain boundary,core-mantle structure and dynamic recrystallization microstructures,indicating dislocation creep is a major deformation mechanism.TEM also reveals abundant dislocation structures,edge and screw dislocations,tilt and twist walls.In granitic mylonite rutile precipitates preferentially in the pressure shadows of porphyroclasts and garnet occnrs as fine grains in narrow zones parallel to foliation,which all suggest that fluid is present in deformation of the UHP rocks and fluid-enhance diffusion mass transfer is another important deformation mechanism during UHP deformation.Stress level is estimated at tens of MPa based on existing flow laws.