The Tongbai Hp Metamorphic Terrane: Constraints On The Architecture And Subduction/Exhumation Of The Tongbai-Dabie-Sulu Hp/Uhp Metamorphic Belt

The Tongbai orogen is situated at the junction between the Qinling and Dabie-Sulu orogenes; hence it may hold a key to understanding the geological relationships and tectonic evolution of different terranes in the Qinling-Tongbai-Dabie-Sulu orogenic belt. The Tongbai high-pressure (HP) metamorphic terrane comprises two HP slices (I and II) and a tectonic melange zone in the north and a blueschist-greenschist zone in the south. The HP slice I is represented by the northern and southern eclogite zones occurring on the two sides of the Tongbaishan antiform. The peak metamorphic conditions are estimated to be 530 similar to 610 degrees C and 1. 7 similar to 2. 0GPa for the northern zone and 460 similar to 560 degrees C and 1. 3 similar to 1. 9GPa for the southern zone. The HP slice II is represented by the metamorphic enclaves in gneissic granites in the Tongbai complex. Their peak metamorphic conditions are inferred to be within the eclogites facies field of <700 degrees C and > 1. 2GPa, and retrograde P-T conditions at 660 similar to 700 degrees C and 0.80 similar to 1. 03GPa. U-Pb, Lu-Hf, Rb-Sr and Ar-Ar multichronometric data indicate the peak metamorphism of the HP slice I taking place at 255Ma, whereas cooling to the closure temperature of muscovite at 238Ma. By contrast, the metamorphic ages of the HP slice II are as young as 232 similar to 220Ma, whereas gneissic granites dominating the Tongbai complex were emplaced at 140Ma. This suggests that, when the HP slice II was subducted to deep levels and underwent HP metamorphism, the HP slice II overlain it might have been exhumed to mid- to upper-crustal levels. This testifies a diachronous (differential) subduction/exhumation model of different HP and ultrahigh-pressure ( UHP) slices proposed for the western Dabie, eastern Dabie and Sulu regions. Furthermore, the above data demonstrate the commencement of subduction of the South China continental crust at the Late Permian, which also represents the transitional time of oceanic to continental subduction between the North and South China blocks. Based on the similarity between the Tongbai and North Dabie complexes, we consider that a lower P-T environment and the absence of UHP rocks in the Tongbai HP metamorphic terrane might be resulted from the shallower subduction of the South China block towards the west. The final west-narrowing architecture of the Tongbai-Dabie HP/UHP metamorphic belt is related to the Early Cretaceous extrusion towards the east.