Neoarchean Peridotites in the North China Craton and Implications for the oneset of Plate Tectonics

The inner and outer layers of the Earth can be connected by plate tectonics with exchange of material and energy, thus shaping the habitable Earth today. However, the existence of Archean plate tectonics has been controversial. One of the reasons is the lack of rock records that can best represent the presence of the convergent plate boundaries during that time, such as continental lithosphere with ultrahigh-pressure metamorphism (> 2.7 GPa or 80–100 km). Here we investigated the peridotites from the North China Craton, and conducted a systematic investigation involving field survey, mineralogy, petrology, geochronology and geochemistry on these peridotites. Temperature and pressure conditions for protoliths of these peridotites, as well as oxygen fugacity (fO₂), were also calculated, to constrain petrogenesis, tectonic setting, and characteristics of mantle fO₂.

In situ U-Pb dating on zircons from the peridotites yields metamorphic/altered age of 2535–2517 Ma and were intruded by the unmetamorphosed granite dykes at ~2500 Ma. Garnet pseudomorphs and pyroxene with exsolved textures were identified in these peridotites, suggesting that the original garnet and pyroxene were brought from high pressures and the breakdown was induced by decompression. Reintegrating the compositions of the original garnet and pyroxene and compositions of the original garnet and pyroxene indicate that these peridotites were brought up or once seated at mantle depths of 110–130 km. The calculated dT/dP thermal gradients is around 375 ^oC/GPa, close to those of modern collisional orogens.   The occurrence of phlogopite and amphibole in the studied peridotites and the enrichment of light rare earth elements in their bulk-rock and mineral trace elements, as well as the higher contents of magnesium and aluminum in the rim, and chromium and iron in the core of spinels in some samples, which further demonstrates that the studied peridotites experienced mantle metasomatism during plate subduction. Using Olivine-orthopyroxene-spinel oxybarometry, this dissertation obtained the fO₂ of these Archean metasomatized peridotites to range from ΔFMQ +1.0 to ΔFMQ +1.7, which are more oxidized than the Archean ambient mantle, but are similar to the modern sub-arc mantle.

The ultrahigh-pressure peridotites prove that some forms of plate tectonics have been operating at least since the Neoarchean, and also indicate that the continental deep subduction could have existed at least prior to 2.5 billion years ago. During this process, the Neoarchean mantle oxidation was increased, in which subducted crustal materials would have significantly metasomatized the mantle and increased its oxygen fugacity. This process may have contributed to the Archean atmospheric redox evolution and triggered the GOE in the early Proterozoic.