New structural and petrological data unveil a very complicated ductile deformation history of the Xiongdian-Suhe HP metamorphic unit, north-western Dabie Shan, central China. The fine-grained symplectic amphibolite-fa...New structural and petrological data unveil a very complicated ductile deformation history of the Xiongdian-Suhe HP metamorphic unit, north-western Dabie Shan, central China. The fine-grained symplectic amphibolite-facies assemblage and coronal structure enveloping eclogite-facies garnet, omphacite and phengite etc., representing strain-free decompression and retrogressive metamorphism, are considered as the main criteria to distinguish between the early-stage deformation under HP metamorphic conditions related to the continental deep subduction and collision, and the late-stage deformation under amphibolite to greenschist-facies conditions occurred in the post-eclogite exhumation processes. Two late-stages of widely developed, sequential ductile deformations D 3 and D 4, are recognized on the basis of penetrative fabrics and mineral aggregates in the Xiongdian-Suhe HP metamorphic unit, which shows clear, regionally, consistent overprinting relationships. D 3 fabrics are best preserved in the Suhe tract of low post-D 3 deformation intensity and characterized by steeply dipping layered mylonitic amphibolites associated with doubly vergent folds. They are attributed to a phase of tectonism linked to the initial exhumation of the HP rocks and involved crustal shortening with the development of upright structures and the widespread emplacement of garnet-bearing granites and felsic dikes. D 4 structures are attributed to the main episode of ductile extension (D 1 4) with a gently dipping foliation to the north and common intrafolial, recumbent folds in the Xiongdian tract, followed by normal sense top-to-the north ductile shearing (D 2 4) along an important tectonic boundary, the so-called Majiawa-Hexiwan fault (MHF), the westward continuation of the Balifan-Mozitan-Xiaotian fault (BMXF) of the northern Dabie Shan. It is indicated that the two stages of ductile deformation observed in the Xiongdian-Suhe HP metamorphic unit, reflecting the post-eclogite compressional or extrusion wedge formation, the subhorizontal ductile extension and crustal thinning as well as the top-to-the north shearing along the high-angle ductile shear zones responsible for exhumation of the HP unit as a coherent slab, are consistent with those recognized in the Dabie-Sulu UHP and HP metamorphic belts, suggesting that they were closely associated in time and space. The Xiongdian-Suhe HP metamorphic unit thus forms part of the Triassic (250-230 Ma) collision orogenic belt, and can not connect with the South Altun-North Qaidam-North Qinling UHP metamorphic belt formed during the Early Paleozoic (500-400 Ma).展开更多
文摘New structural and petrological data unveil a very complicated ductile deformation history of the Xiongdian-Suhe HP metamorphic unit, north-western Dabie Shan, central China. The fine-grained symplectic amphibolite-facies assemblage and coronal structure enveloping eclogite-facies garnet, omphacite and phengite etc., representing strain-free decompression and retrogressive metamorphism, are considered as the main criteria to distinguish between the early-stage deformation under HP metamorphic conditions related to the continental deep subduction and collision, and the late-stage deformation under amphibolite to greenschist-facies conditions occurred in the post-eclogite exhumation processes. Two late-stages of widely developed, sequential ductile deformations D 3 and D 4, are recognized on the basis of penetrative fabrics and mineral aggregates in the Xiongdian-Suhe HP metamorphic unit, which shows clear, regionally, consistent overprinting relationships. D 3 fabrics are best preserved in the Suhe tract of low post-D 3 deformation intensity and characterized by steeply dipping layered mylonitic amphibolites associated with doubly vergent folds. They are attributed to a phase of tectonism linked to the initial exhumation of the HP rocks and involved crustal shortening with the development of upright structures and the widespread emplacement of garnet-bearing granites and felsic dikes. D 4 structures are attributed to the main episode of ductile extension (D 1 4) with a gently dipping foliation to the north and common intrafolial, recumbent folds in the Xiongdian tract, followed by normal sense top-to-the north ductile shearing (D 2 4) along an important tectonic boundary, the so-called Majiawa-Hexiwan fault (MHF), the westward continuation of the Balifan-Mozitan-Xiaotian fault (BMXF) of the northern Dabie Shan. It is indicated that the two stages of ductile deformation observed in the Xiongdian-Suhe HP metamorphic unit, reflecting the post-eclogite compressional or extrusion wedge formation, the subhorizontal ductile extension and crustal thinning as well as the top-to-the north shearing along the high-angle ductile shear zones responsible for exhumation of the HP unit as a coherent slab, are consistent with those recognized in the Dabie-Sulu UHP and HP metamorphic belts, suggesting that they were closely associated in time and space. The Xiongdian-Suhe HP metamorphic unit thus forms part of the Triassic (250-230 Ma) collision orogenic belt, and can not connect with the South Altun-North Qaidam-North Qinling UHP metamorphic belt formed during the Early Paleozoic (500-400 Ma).
