The most ancient rocks in the geological record provide insights into the processes that shaped the evolution and composition of the first continental masses. To better constrain these processes, we made a detailed study of a ca. 3.86 Ga felsic meta-igneous rock from the Eoarchean Saglek Block (North Atlantic Craton) that experienced high-grade metamorphism at ca. 3.5 Ga. Our robust zircon-isotope plus trace-element analyses reveal metamorphic zircon domains with δ18O values up to +9‰ at ca. 3.5 Ga, which are the highest values so far measured in any pre–3.0 Ga zircons, metamorphic or igneous, extracted from unambiguous (meta)igneous host rocks. Such elevated zircon δ18O signatures clearly document the involvement of mature supracrustal precursors (mafic volcanics ± clastic/chemical sediments) during the reworking of 3.86 Ga crust at ca. 3.5 Ga. This study provides unequivocal evidence for hydrosphere–crust interactions and reworking processes resulting in metamorphic zircon growth at ca. 3.5 Ga, namely 1 Ga before the Archean-Proterozoic transition.