Tectono-magmatic Evolution Of The Shiwandashan Structural Belt:Implications For Understanding The Indosinian Orogeny In South China

South China straddles the boundary between two of the most dynamic domains of the earth's lithosphere: the west Pacific domain to the east,and the Tethyan domain to the west.The Indosinian tectono-magmatic evolution of South China and its relation with plate subduction/collision dynamics in the two dynamic domains have sparked the interest of geologists worldwide and stimulated considerable research.In this study,we conducted field mapping coupled with radiometric dating across the Shiwandashan structural belt in southwestern South China,to investigate how this structural belt evolved and to better understand its links to plate boundary dynamics during the Late Permian to Middle Triassic.Our results reveal an episodictectono-magmatic history of the Shiwandashan structural belt.This structural belt underwent significant NW-SE shortening during the Late Permian to Middle Triassic;the shortening was expressed by NW-verging thrusts and folds,and conjugate sets of ~N-trending sinistral and ~E-trending dextral faults.In general,the structural pattern defines a thin-skinned,northwest-propagating foreland thrust belt,with an overall northwestward decreasing trend in strain intensity.The NW-SE shortening event records an Andean-type orogeny that developed in response to northwestward subduction of the Paleo-Pacific Plate.This event was a protracted yet discontinuous process which,was punctuated by a magmatic/thermal episode featured by the Darongshan granitic magma emplacement,uplift and retrograde metamorphism.The Darongshan granitic suite occurs in the central part of the Shiwandashan structural belt.It strikes northeast parallel to regional fold trend.Our geochronological data,together with published ages,indicate that the granitoids were emplaced at ~250-240 Ma.Our geochemical data suggest that the granitoids are peraluminous,enriched in LERR and LILEs(such as Rb and Th),and depleted in HFSEs(such as Nb and Ta),consistent with geochemical affinities of S-type granite.The granitoids display negative zircon ?Hf(t)and high initial 87Sr/86 Sr values,indicating a source primarily from enriched lower crust.Another significant feature manifested by the Darongshan granitic suite is the high temperature(950-1000°C)metamorphism,as expressed by extensive presence of granulite enclaves and charnockites.We propose that,during the subduction of the Paleo-Pacific plate,the upwelling of asthenosphere through a slab window supplied heat and created a high-temperature and CO2-rich fluid environment,in which intense metamorphism and crustal melting led to generations of the granulites,charnockites and S-type granites at distinct levels.In addition,our structural observations documented a ~300-km wide ductile shear zone,termed the Qinzhou-Fangcheng shear zone.The shear zone strikes NE-SW,dips steeply(55°-85°)to SE,and displays a dextral shear sense,which was formed at ~250 Ma under greenschist facies metamorphic conditions.The generation of the shear zone was genetically related to early Triassic collision of South China with Indochina.