Remnants of Earth's Oldest Continental Crust Formed by Subduction

Eoarchean (3.6 – 4.0 Ga) tonalite – trondhjemite – granodiorite (TTG) are the major component of Earth's oldest persevered continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths (e.g., Moyen and Martin, 2012), but whether this requires subduction remains enigmatic (e.g., Palin et al., 2016). Recent studies show that most early Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction (e.g., Johnson et al., 2017). We have identified a suite of Eoarchean tonalitic gneisses dated at ∼3.7 Ga from the Tarim Craton, northwestern China (Ge et al., 2018). These rocks are probably the oldest high‐pressure TTGs so far documented worldwide. Thermodynamic and trace element modelling demonstrates that the parent magma may have been generated by water‐fluxed partial melting of moderately enriched arc‐like basalts at 1.8–1.9 GPa and 800–830 °C, indicating an apparent geothermal gradient (400–450°C GPa ‐1 ) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high‐pressure TTGs were generated by partial melting of a subducted proto‐arc during arc accretion. Our model implies that modern‐style plate tectonics was operative, at least locally, at ∼3.7 Ga and was responsible for generating some of the oldest continental nuclei.