In this study we contrast insights on Precambrian crustal growth and maturation from radiogenic and oxygen isotope systematics in the Superior (3.0–2.7 Ga) and Grenville (1.3–1.0 Ga) Provinces of the Canadian shield. Oxygen isotope ratios in zircon provide the best evidence of supracrustal input into ancient orogens. Archean Superior Province zircons have relatively low δ18O values and a limited range (5.7‰ ± 0.6‰), while Proterozoic Grenville Province zircons have elevated δ18O values and a wider range (8.2‰ ± 1.7‰). These data reflect fundamental differences in crustal evolution and maturation between the Superior and the Grenville Provinces. In the Grenville Province, radiogenically juvenile supracrustal material with high δ18O values was buried (or subducted) to the base of the crust within 150 m.y. of initial crust production, causing high magmatic δ18O values (δ18O [zircon] ≥ 8‰) in anorthosite suite and subsequent plutons. Information about large volumes and rapid recycling of Grenville crust is not accessible from radiogenic isotope data alone. The Grenville data contrast with the restricted δ18O values of Superior Province magmatism, where subtle (∼1‰) elevation in δ18O occurs only in volumetrically minor, late to postorogenic (sanukitoid) plutons. Differences in sediment δ18O values between the Superior and Grenville Provinces are predominantly a function of the δ18O of source materials, rather than differences in chemical maturity or erosion styles. This study shows that zircon is a robust reference mineral to compare igneous processes in rocks that have undergone radically different histories.