Lithospheric structure under the Dabie orogen and another model of exhumation of UHP rocks

Abstract：Study of helium isotrope of UHP rocks in the Dabie area shows that minerals of UHP rocks such as eclogite in the Dabie area were not derived from the mantle but formed at top of the lithospheric mantle (LID). According to the deep geophysical data, the authors propose a new model for the formation of UHP minerals at the lithospheric mantle lid, i.e. when supracrustal rocks were subducted to the LID, UHP metamorphic rocks were formed at strong pressures caused by convergence of plates and high temperatures produced by the mushroom cloud mantle, and then were exposed at the surface due to doming and erosion of the crust. The available literature indicates that when visco-plastic continental plates collided and were subducted the deformation of the LID was much more complex than the deformation due to subduction of a rigid plate. The direction of subduction was mostly vertically downward, and the subducted plate and retro-side wedged each other like fried dough twist and the subducting direction might change at depth. When the LID partially melted, subducted materials would spread in the locally molten layer and formed UHP metamorphic rocks at high pressures and temperatures. The global distribution of main UHP metamorphic rocks is associated with the distribution of the Paleo-Tethys Ocean. However, it is not certain whether both of them were soft collision between visco-plastic continental plates and whether there were high-temperature regions at the LID near the collision zone.