大别山花凉亭-弥陀剪切带与超高压变质岩片的斜向差异折返

超高压变质岩的折返过程是陆陆碰撞边界演化的关键问题。南倾的花凉亭-弥陀剪切带位于南大别低温-超高压变质 带和中大别中温-超高压变质带之间，矿物拉伸线理倾伏向为SE，逆冲和走滑分量大致相等。电子背散射衍射分析表明： 花凉亭-弥陀剪切带大多数样品的石英组构记录了上盘向NW的剪切变形，反映了中大别超高压变质岩向SE的快速折返， 而部分样品的石英具有上盘向SE的剪切指向，与早白垩世花岗岩穹隆发育导致的区域伸展有关。对前人的岩石学和年代学 成果进行总结，发现大别山进变质和超高压变质峰期/退变质的锆石U-Pb年龄从南往北逐渐变新，南大别和中大别在215~ 225 Ma同时经历了高压榴辉岩相退变质作用，在191~195 Ma经历了绿片岩相变质作用。超高压变质岩的白云母和黑云母的 40Ar/39Ar年龄靠近郯庐断裂时偏年轻，可能受到郯庐断裂活动的影响。南大别和中大别变质峰期温压的等值线与花凉亭-弥 陀剪切带的走向斜交，反映了超高压变质岩的斜向折返。因此，南大别低温-超高压变质带在~236 Ma最先开始折返，之后 中大别和北大别依次发生快速折返，具有不同折返速率和折返角度的构造岩片通过韧性剪切带调节相对运动。

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 evolution of continental collision boundaries. The south-dipping Hualiangting-Mituo shear zone is the boundary between the South Dabie low-temperature/UHP zone and the Central Dabie mid-temperature/UHP zone. It is characterized by SE-trending stretching lineation, and nearly equal thrust and strike slip components. The lattice-preferred orientation of quartz from the Hualiangting-Mituo shear zone was determined using the electronic backscatter diffraction (EBSD) technique. Most samples show the top-to-NW shear sense due to the fast SE-ward exhumation of the Central Dabie mid-temperature/UHP zone. However, some samples developed the top-to-SE shear sense because of intrusion of granitic domes and regional extension in the Early Cretaceous. A summary of petrological and geochronological data in the Dabie Mountains reveals the decreasing U-Pb zircon ages of the prograde metamorphism and UHP peak metamorphism from south to north. The South and Central Dabie UHP zones experienced coeval high-pressure eclogite facies metamorphism at 215~225 Ma and greenschist facies metamorphism at 191~195 Ma. The relatively younger 40Ar/39Ar ages of muscovite and biotite near the Tanlu fault may be affected by ductile activation of the Tanlu fault. The isobaric and isothermal contours of the peak metamorphism in the South and Central Dabie UHP zones are oblique to the trend of the Hualiangting-Mituo shear zone, suggesting oblique exhumation of UHP rocks. We proposed that the South Dabie low-temperature/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 relative motion (i.e., different exhumation rates and oblique exhumation angels) between these UHP metamorphic slices was adjusted by ductile shear zones.