Growth of early continental crust by partial melting of eclogite

The tectonic setting in which the first continental crust formed, and the extent to which modern processes of arc magmatism at convergent plate margins were operative on the early Earth, are matters of debate1,2. Geochemical studies have shown that felsic rocks in both Archaean high-grade metamorphic (‘grey gneiss’) and low-grade granite-greenstone terranes are comprised dominantly of sodium-rich granitoids of the tonalite-trondhjemite-granodiorite (TTG) suite of rocks3,4,5,6,7. Here we present direct experimental evidence showing that partial melting of hydrous basalt in the eclogite facies produces granitoid liquids with major- and trace-element compositions equivalent to Archaean TTG, including the low Nb/Ta and high Zr/Sm ratios of ‘average’ Archaean TTG8, but from a source with initially subchondritic Nb/Ta. In modern environments, basalts with low Nb/Ta form by partial melting of subduction-modified depleted mantle9,10, notably in intraoceanic arc settings in the forearc11,12 and back-arc13,14 regimes. These observations suggest that TTG magmatism may have taken place beneath granite-greenstone complexes developing along Archaean intraoceanic island arcs by imbricate thrust-stacking15 and tectonic accretion16 of a diversity of subduction-related terranes. Partial melting accompanying dehydration of these generally basaltic source materials at the base of thickened, ‘arc-like’ crust would produce compositionally appropriate TTG granitoids in equilibrium with eclogite residues.