How the Archean felsic crust grew from its mafic precursors between 3.8 and 2.5 billion years ago remains elusive and the subject of great debate. Here, we present silicon isotopic constraints on Tonalite-Trondhjemite-Granodiorite (TTG) and Granite-Monzonite-Syenite (GMS) plutons from the Kaapvaal craton, which range in age from 3.55 to 2.69 Ga. We identified very consistent isotopic signatures, all uniformly heavier than those ever determined for rocks which comprise most of the Bulk Silicate Earth, the Si-rich end-members of the modern continental crust and dacite-rhyolite liquids differentiated from basalts. This unusual composition is explained by the melting of a mafic source that included significant proportions (20-35wt%) of silicified metabasalts, which were common supracrustal rocks prior to 3 billion years. Thus, differentiation of the early continental crust may have been enabled by enrichment of the mafic source rocks in silica from interaction with silica-saturated early oceans prior to the melting event that formed the granitoids. Addition of silica depresses the stability of amphibole at similar water activity, allowing TTG-like melt production at lower temperatures. This may explain why early silicic continental granitoids are unique to Earth and did not emerge on other rocky planets.