Diachroneity of continental subduction and exhumation: Constraints from the Permian-Triassic HP metamorphic terrane in the Tongbai orogen, central China

High-pressure (HP) metamorphic terrane in the Tongbai orogen comprises two HP slices (I and II) and a tectonic mélange zone in the northeast and a blueschist-greenschist zone in the southwest. HP slice I is represented by the northern and southern eclogite zones on the two sides of the Tongbaishan antiform. HP slice II is represented by retrograded eclogite-bearing metamorphic enclaves in Cretaceous gneissic granites in the Tongbai Complex. U-Pb, Lu-Hf, Rb-Sr and ⁴⁰Ar/³⁹Ar multichronometric data indicate that the peak metamorphism of HP slice I took place at ～255 Ma, whereas the metamorphic ages of HP slice II are as young as 232–220 Ma. By contrast, the tectonic mélange zone near the suture was metamorphosed at ～256 Ma. Such a diachroneity of different slices across the direction of the orogen in the Hong’an-Dabie-Sulu HP/UHP terrane is ubiquitous, and it can be interpreted by a syn-subduction detachment/exhumation model. Furthermore, the metamorphic age of HP slice I in the Tongbai orogen is older than that of the equivalent HP slice in the Hong’an orogen by ～15 Ma, suggesting that the diachroneity may have also existed along the direction of the orogen. A seesaw-type subduction/exhumation model is proposed to explain this age disparity and the subduction of the South China Block becomimg shallower towards the west.