Ultrahigh-pressure peridotites record Neoarchean collisional tectonics

Abstract

It is debated when plate tectonics first operated on Earth. One of the arguments against the Archean (> 2.5 Ga) operation of plate tectonics is the lack of rock records that can be best explained to be formed at convergent plate boundaries, such as continental lithosphere metamorphosed at ultrahigh-pressures (> 2.7 GPa or 80–100 km). Here we report Archean ultrahigh-pressure peridotites in Eastern Hebei, the North China Craton. Bulk-rock and mineral compositions suggest that these peridotites are likely cumulates or slivers of metasomatized continental lithospheric mantle. Garnet pseudomorphs and pyroxene exsolution textures are preserved in these Archean peridotites, indicating decompression-induced breakdown of the original garnet and pyroxene from high pressures. We reintegrate the original garnet and clinopyroxene based on mass proportions and compositions of decompression-induced breakdown products. The reconstructed garnet and clinopyroxene compositions of these Archean peridotites indicate that they were brought up from mantle depths of 110–130 km. We propose that these ultrahigh-pressure peridotites are tectonic slivers of a collisional complex, possibly subducted to mantle depths and then exhumed to crustal levels during Neoarchean subduction and subsequent arc/continental collision, similar to those from Phanerozoic continental collisional zones. The Archean ultrahigh-pressure peridotites in the North China Craton provide direct evidence for operation of continental collisional plate tectonics since at least 2.5 billion years ago.