Delamination featured by partial removal of lithospheric mantle: constraints from reconstructed crustal thermal structure of the Tongbai orogen, central China

Lithospheric thickening and thinning are common processes during evolution of collisional orogens. Numerical modelling suggests that partial and wholescale removal of thickened lithospheric mantles could both trigger asthenospheric upwelling, crustal melting, and orogenic collapse. However, due to modification of original geophysical and geological records by later tectonothermal events, distinguishing partial from wholescale removal of lithospheric mantle proves challenging for an ancient, collapsed orogenic belt like the Tongbai orogen, situated in the middle segment of the Qinling-Tongbai-Hong’an-Dabie-Sulu orogen in central China. In this study, a systematic petrological study of four amphibolites in the southern Tongbai orogen and a 1–D thermal modelling were combined to ascertain if lithospheric mantle experienced partial or wholescale removal during collapse of the Tongbai orogen. P–T reconstruction based on multiple thermobarometries combined with zircon dating indicates that the amphibolites experienced ca. 240 Ma high pressure eclogite-facies metamorphism under about 500–530oC and 1.9–2.2 GPa, and ca. 137–123 Ma high temperature amphibolite- to granulite-facies metamorphism under 500–760oC and 0.7–0.9 GPa, which were associated with Triassic continental subduction and Cretaceous extensional collapse, respectively. Thermal modelling results show that the crustal thermal structure in the early Cretaceous reflected by metamorphic P–T conditions of amphibolites could be produced by thinning of lithospheric thickness to about 70–120 km. This suggests that lithospheric thinning was achieved by removing the lower part of the lithospheric mantle, which is also supported by evidence from geophysical investigations including electrical resistivity and seismic velocity models and mantle xenolith studies that indicate the presence of residual depleted lithospheric mantle beneath the Tongbai orogen. As previous studies have suggested that the lithospheric mantle beneath the neighboring Dabie-Sulu orogen has been entirely removed, it can be inferred that the thickened lithospheric mantle beneath different segments of an orogen can be removed in different scales (partial or wholescale).