High-18O granitic plutons from the Frontenac Axis, Grenville Province of Ontario, Canada

18O/16O ratios have been determined for whole rock as well as coexisting minerals from nine syn- to post-tectonic monzonite and quartz monzonite plutons in the Westport-Gananoque area covering approximately 2500 km2. The plutons fall into two distinct isotopic groups: (I) Low-18O group: Rideau Lake, Westport, and Wolfe Lake plutons and (II) High-18O group: Lyndhurst, Perth Road, Battersea, Gananoque, South Lake and Crow Lake plutons. The 18O/16O ratios of granitic rocks from the latter group are among the highest ever reported in the literature. The isotopic compositions of samples within each pluton bear no relationship to the sample distance from the intrusive contacts nor are they affected by the types of country rocks into which they are emplaced. Oxygen isotope fractionations among coexisting quartz, feldspar, biotite and magnetite all show normal plutonic values. Thus, all lines of evidence suggest that the monzonite and quartz monzonite plutons did not acquire their characteristic 18O/16O ratios at the present level of exposure, either by isotopic exchange with the surrounding metasedimentary country rocks or by any post-crystallization low-temperature alteration processes. The results are discussed. (author)