Post-collisional mafic magmatism: Insights into orogenic collapse and mantle modification from North Qaidam collisional belt, NW China

Post-collisional magmatism contains important clues for understanding processes of orogenic belts, potentially including unrooting and collapse. Here we report new geochronological and geochemical data for a suite of post-collisional mafic dykes in the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt. Two groups of magmatic rocks can be distinguished: (1) Middle Devonian (392–375 Ma) basic-intermediate dykes; (2) Upper Devonian (~360 Ma) ultrabasic dykes. Whole-rock geochemistry and zircon Hf isotopes reveal that the intermediate-basic dykes are derived from a lithospheric mantle source, whereas the ultrabasic dykes are melting of asthenosphere mantle. We suggest that such mantle-derived mafic magmatism is the critical indicator for the start of post-collisional magmatism and orogen unrooting and collapse. We propose a geodynamic model explaining the activities of the lithospheric and asthenospheric mantle during the post-collisional stage, which reveals a ~ 35 million year unrooting process, from slow lithospheric mantle erosion between ~395–375 Ma, to final collapse by lithosphere delamination and asthenosphere upwelling at ~360 Ma. Addition of juvenile mantle materials to the crust by the post-collisional mafic magmatism suggests that the post-collisional stage is an important period for continental growth in Earth's history.