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.

Metamorphic P-T Path Differences between the Two UHP Terranes of Sulu Orogen, Eastern China: Petrologic Comparison between Eclogites from Donghai and Rongcheng

The Sulu Orogen constitutes the eastern part of the Sulu-Dabie Orogen formed by Triassic collision between the Sino-Korean and Yangtze plates. An HP Slice Ⅰ and two UHP slices Ⅱ and Ⅲ with contrasting subduction and exhumation histories within the Sulu Orogen were postulated. This study presents the metamorphic P-T paths of eclogites from the two UHP belts constructed by petrog- raphy, mineral chemistry and Perple_X P-T pseudosection modeling in the MnC（K）NFMASHO system. Eclogites from Slice Ⅲ mainly consist of omphacite, garnet and quartz, with minor rutile, ilmenite, amphibole and phengite. Eclogites from Slice Ⅱ show a porphyroblastic texture with epidote porphyroblasts and garnet, omphacite, phengite, quartz and rutile in matrix. Pseudosection modeling reveals that eclogites from Slice Ⅱ witness a peak metamorphism of eclogite-facies under conditions of 3.1-3.3 GPa and 660-690 ℃, and a retrograde cooling decompression process. The eclogites from Slice Ⅲ record a heating decompressive P-T path with a peak-P stage of 3.2 GPa and 840℃ and a peak-T stage of 2.4 GPa and 950 ℃, suggesting an apparent granulite-facies metamorphism overprint during exhumation. Both eclogites recorded clockwise P-T paths with peak P-T conditions suggesting a subduction beneath the Sino-Korean Plate to -100-105 km depth. Combined with tectonic scenarios from previous studies, it is concluded that the two UHP crustal slices in the Sulu terrane have a similar geodynamic evolution, but the UHP rocks in Slice Ⅱ exhumed after the eclogitic peak-pressure conditions earlier than that of Slice Ⅲ. The existence of Slice Ⅱ diminished the buoyancy force on Slice Ⅲ, resulting in a granulite-facies overprint on Slice Ⅲ. The Sulu orogenic belt is made up of different crustal slices that underwent different subduction and exhumation histories, rather than a single unit.