Metamorphic records reveal lithospheric foundering as a geodynamic mechanism for breakup of supercontinent Rodinia

Assembly and breakup of supercontinents play an important role in the evolution of the geosphere, hydrosphere, atmosphere and biosphere. Yet the geodynamic mechanism of supercontinent breakup remains highly controversial between two modes of mantle convection (mantle plume vs. asthenospheric upwelling). Because supercontinent breakup is associated with intensive disturbance of the asthenosphere-lithosphere system along fossil suture zones, an increase of metamorphic thermal gradients is expected to occur in crustal rocks at rifted continental margins, leading to a series of differences in metamorphic products between the short-term and long-term processes. Thus, the long-term tectonothermal evolution from supercontinent assembly to breakup can be revealed by specific metamorphic records. This is illustrated by the present study of petrology and geochronology for poly-metamorphic records in Neoproterozoic metapelites from the northern margin of the South China Block, which was successfully rifted from supercontinent Rodinia in the middle Neoproterozoic. The metapelites were originally produced by Barrovian type metamorphism at ca. 870 Ma through deep burial of sedimentary rocks during the Rodinia assembly, and then overprinted under upper amphibolite-facies conditions for Buchan type metamorphism at about 830 Ma when the South China Block would attempt but fail to break up from Rodinia due to lithospheric thinning. The high temperature was maintained in the continental crust for Buchan type metamorphism till ca. 760 Ma, eventually resulting in breakup of the South China Block from Rodinia. Thinning of the lithospheric mantle is spatiotemporally associated with heating of the overlying continental crust during the ca. 70 Myr development of continental rifting from failure to success. Such long-term tectonic processes are prominent during the Rodinia breakup, and their validity is tested by cation diffusion modelling from the poly-metamorphic records. The results indicate that lithospheric foundering is the feasible geodynamic mechanism of Rodinia breakup, which can be constrained by the petrochronological study. This provides insights into the temporal effect of mantle poloidal convection on continental rifting along the fossil suture zone.