Granitic rocks related to 1.18 to 1.13 Ga anorthosite-mangerite-charnockitegranite plutonism stitch three terranes in the southwestern Grenville Province (Adirondack Highlands–Morin terrane, Frontenac terrane, Elzevir terrane). Because of the refractory nature of zircon (Zrn), analysis of oxygen-isotope ratios of dated igneous zircon from these rocks allows calculation of δ 18 O values of original magmas even if the rocks were subjected to late magmatic assimilation, postmagmatic alteration, or metamorphism. Documented variability in δ 18 O(Zrn) for these granitic rocks corresponds to their geographic location. Seven plutons from the central Frontenac terrane (Ontario) have a high average δ 18 O(Zrn) = 11.8 ± 1.0‰, which corresponds to δ 18 O magma values of 12.4–14.3‰. In contrast, twenty-seven other plutons and dikes of this suite (New York, Ontario, and Québec) average δ 18 O(Zrn) = 8.2 ± 0.6‰, with a typical igneous range of 8.6 to 10.3‰ for δ 18 O magma values. High δ 18 O values in the Frontenac terrane are some of the highest magmatic oxygen-isotope ratios recognized worldwide, but these plutons are not unusual with respect to whole-rock chemistry or radiogenic isotope compositions. Such high δ 18 O values can result from mixing between paragneiss (δ 18 O ≈ 15‰) and hydrothermally altered basalts and/or oceanic sediments (δ 18 O ≈ 12‰) in the source region. We propose that high-δ 18 O, hydrothermally altered basalts and sediments were subducted or underthrust to the base of the Frontenac terrane during closure of an ocean basin between the Frontenac terrane and the Adirondack Highlands at or prior to 1.2 Ga.