Large swaths of juvenile crust with tonalite-trondhjemite-granodiorite (TTG) composition were added to the continental crust from about 3.5 billion years ago. Although TTG magmatism marked a pivotal step in early crustal growth and cratonisation, the petrogenetic processes, tectonic setting and sources of TTGs are not well known. Here, we investigate the composition and petrogenesis of Archaean TTGs using high field-strength-element systematics. The Nb concentrations and Ti anomalies of TTGs show the overwhelming effects of amphibole and plagioclase fractionation and permit constraints on the composition of primary TTG melts. These melts are relatively incompatible element-poor and characterised by variably high La/Sm, Sm/Yb and Sr/Y, and positive Eu anomalies. Differences in these parameters are not indicative of melting depth, but instead track differences in the degree of melting and fractional crystallisation. Primary TTGs formed by the melting of rutile- and garnet-bearing plagioclase-cumulate rocks that resided in proto-continental roots. The partial melting of these rocks is part of a causal chain that links TTG magmatism to the formation of sanukitoids and K-rich granites. Together, these processes explain the growth and differentiation of the continental crust during the Archaean without requiring external forcing such as meteorite impact or the start of global plate tectonics.