Post-collisional lithosphere delamination of the Dabie-Sulu orogen

Abstract

The consistence between the first rapid cooling time (226–219 Ma) of the untrahigh pressure metamorphic (UHPM) rocks in the Dabie Mountains and the formation time (205–220 Ma) of the syncollisional granites in the Qinling and Sulu areas suggests that the first rapid cooling and uplift of the UHPM rocks may be related to breakoff of subducted plate. Therefore the second rapid cooling and uplift (180–170 Ma) of the UHPM racks needs a post-collisional lithosphere delamination which resulted in the granitic magmatism with an age of about 170 Ma. In addition, the rapid rising of the Dabie dome in the early Cretaceous (130–110 Ma) and the corresponding large-scale magmatism in the Dabie Mountains need another lithosphere delamination. The geochronology of the post-collisional mafic-ultramafic intrusions and geological relationship between the mafic-ultramafic intrusions and granites suggest that partial melting was initiated in the mantle, and then progressively developed in the crust, suggesting a mantle upwelling underneath the Dabie Mountains. The unusual fractional trend of the gabbros characterized by lower SiO2 content (46.24%) corresponding to lower MgO content (4.53%) and their typical geochemistry features of the lower crust suggest underplating of the mantle derived magma and interaction between the magma and lower crust before their intrusion. Lithosphere delamination could be the dynamic cause of the mantle upwelling and underplating. The seismic tomography results of the Dabie Mountains and adjacent areas clearly show lithosphere thinning below the north and south sides of the Dabie Mountains. Because there is no Cenozoic magma event in the Dabie Mountains, the lithosphere thinning may result from delamination of thickened lithosphere mantle after collision. In addition, both the lower velocity zone in the 40 km depth and the basin + dome + basin coupling relationship in the Dabie Mountains also suggest the lithosphere delamination and underplating on the two sides of the orogen.