Ultrahigh-pressure mantle metasomatism in continental collision zones recorded by post-collisional mafic rocks

Post-collisional mafic magmas are melts of mixed sources formed by the interaction between crust and mantle. We report the first identification of metasomatic sources formed at >200 km in continental collision zones that led to the Triassic post-collisional Shidao mafic magmas in the Sulu ultrahigh-pressure terrane, eastern China. The Shidao magmas are ultrapotassic with low Rb/K₂O and Rb/Sr ratios, highlighting a major contribution of K-richterite to the melts, and with high Dy/ Yb ratios indicating residual garnet in the source. Experiments have demonstrated that K-richterite coexists with garnet only at pressures higher than 6-7 GPa, which indicates that the Yangtze continental crust was subducted to >200 km, where it interacted with the overlying mantle to produce K-richter-ite + garnet-bearing assemblages. This ultradeep source of the Shidao magmas spotlights an alternative mechanism for the genesis of post-collisional magmas, which are generally of shallow origin.