The Hualiangting-Mituo Ductile Shear Zone In The Dabie Mountains:Implications For Oblique And Differential Exhumation Of Ultrahigh-pressure Metamorphic Slices

Exhumation processes of ultrahigh-pressure(UHP)metamorphic rocks are critical for the evolution of continental collision boundaries.The Dabie-Sulu orogenic belt is the largest and the best preserved UHP terranes in the world.The Hualiangting-Mituo shear zone is the boundary between the South Dabie low-temperature/UHP zone and the Central Dabie mid-temperature/UHP zone.However,its deformation characteristics and relationship with the exhumation history of UHP rocks is still not clear.In this study,structural analysis along three sections across the Hualiangting-Mituo shear zone was carried out.The foliation of the Hualiangting-Mituo shear zone dips south to SSW,with the dipping angle between 45° and 60°.The stretching lineation is characterized by SE plunge direction,nearly equal thrust and strike slip components.At outcrops,the asymmetrical folds and felsic veins,the S-C fabric in quartz schists and gneisses indicate a top-to-the-NW shear sense.The electron backscatter diffraction analysis shows that quartz in the Hualiangting-Mituo shear zone developed three dominant dislocation slip systems:prismatic slip system {m}<a>and rhombohedral slip system {r}<a>at moderate temperature,and basal slip system(0001)<a>at low temperature.Quartz fabric from most samples have a top-to-the-NW shear sense due to the relatively fast southward exhumation of the Central Dabie mid-temperature/UHP zone.In contrast,the top-to-the-SE shear sense of quartz may be related to the extensional deformation caused by the Dabie granitic dome in the early Cretaceous.The petrological and geochronological data in the Dabie Mountains was summarized to constrain the along-strike variations of the Dabie orogen.The results reveal the decreasing U-Pb zircon ages of both prograde metamorphism and UHP peak metamorphism from south to north.The South and Central Dabie UHP zones experienced coeval prograde metamorphism at 241?245 Ma,earlier than 238±2 Ma in the North Dabie high-temperature/UHP zone.The U-Pb zircon ages for the UHP peak metamorphism in the South,Central and North UHP zones are 228?233 Ma,225?232 Ma,and 218?229Ma,respectively.Both the South and Central Dabie UHP zones were subjected to high-pressure eclogite facies metamorphism at 215-225Ma,slightly earlier than 210?216 Ma in the North Dabie UHP zone.According to 40Ar/39Ar cooling ages of muscovite and biotite,greenschist facies metamorphism occurred at 191?195 Ma in the South and Central Dabie UHP zones,and only locally affected by the early Cretaceous magmatism.However,the Ar-Ar system of UHP zones in the North Dabie UHP zone has been totally reactivated by the early Cretaceous magmatism and migmatitism.The isobaric and isothermal contours during the peak metamorphism in the South and Central Dabie UHP zones are oblique to the trend of the Hualiangting-Mituo shear zone,with the higher pressure and temperature in the east part.This suggests oblique exhumation of the UHP rocks.According to Sm-Nd age and zircon U-Pb ages,the South Dabie low-temperature and UHP zone was first exhumed towards SE as a tectonic slice at?236 Ma,which was followed by fast exhumation of the Central and North Dabie UHP zones.The Dabie UHP terrane is composed off tectonic slices with different exhumation rates.The oblique and differential exhumation of UHP metamorphic slices was adjusted by ductile shear zones...