Earth's Hadean crust formed via operation of convergent tectonics.

Determining the composition, formation mechanisms and stability of the Hadean continental crust is essential for understanding the early geological history of Earth. Detrital zircons, largely from Jack Hills of Western Australia, provide the dominant direct records for the nature of continental crust during the Hadean eon and its formation processes. Although isotope and trace element compositions of these zircons are extensively determined, the major and trace element compositions of their host rocks and corresponding parental magmas remain largely debated, making the nature and evolution of the early Earth's crust ambiguous. Here, based on the comprehensive datasets for global magmatic zircons and their host rocks, we have developed machine learning models to reconstruct multiple major and trace element concentrations of the parental magmas from which Jack Hills zircons grew. The results show that the Hadean continental crust had SiO₂ contents ranging from 58 to 78 wt% with K₂O/Na₂O and Sr/Y ratios in the range of 0.1-1.2, and 1-103, respectively. It was generally not andesitic in lithochemistry, but rather felsic and dominated by low- to medium-pressure tonalite-trondhjemite-granodiorite (TTG) and potassic granites. These rocks would be derived from partial melting of low- and high-potassium mafic proto-crust, respectively, with the latter also incorporating contributions from tonalite. The lack of high-pressure TTGs does not preclude their formation at convergent plate margins, but suggests that the Hadean felsic crust would originate from the collisionally thickened rather than the deeply subducted oceanic crust. Therefore, the formation of continental crust on the Hadean Earth can be explained by the operation of convergent tectonics, outlining a petrogenetic model for Archean TTG rocks.