Deformation behavior and fluid action of quartz veins in the Xuelongshan metamorphic complex,Western Yunnan

The Xuelongshan metamorphic complex underwent Cenozoic sinistral strike-slip shearing and exhumation from deep to shallow in the southeast margin of the Tibet Plateau.We studied the deformation behavior and fluid action of quartz veins in the metamorphic complex in depth based on macroscopic observation,electron backscatter diffraction fabric,and fluid inclusion analysis.The mylonitic samples are characterized by strongly elongated and orientated porphyroclasts and a finegrained matrix with a shear strain of 2.4.The ultramylonitic samples are characterized by a strongly fine-grained matrix with a shear strain of 5.8.The{m}glide controls the quartz deformation in the mylonites and ultramylonites.As the dominant recovery mechanism,subgrain rotation dynamic recrystallization reduces the grain size and promotes rheological weakening.The porphyroclasts restrict the crystal orientation of the recrystallized quartz matrix.The mylonitic quartz veins have abundant single-phase and two-phase fluid inclusions.However,the ultramylonitic quartz veins have a few fluid inclusions,most of which are small and irregular toward the grain boundaries.The temperature and pressure of fluid activity in quartz veins are 550–500℃and 391–218 MPa for ultramylonitic quartz veins,with a depth of 14.2–7.9 km and 450–400℃and 236–91 MPa for mylonitic quartz veins,with a depth of 8.6–3.3 km.The deformation process changes the shape of fluid inclusions from mylonitization to ultramylonitization and causes fluid leakage,an increase in local fluid pressure at the boundary,and hydrolysis weakening,promoting lattice dislocation glide and recrystallization,which is directly related to regional continuous shear deformation and exhumation.