The Loon Lake pluton is a small, reversely zoned pluton with subordinate isolated bodies of diorite and granodiorite gneiss. This pluton is located within the Hastings Basin of the Grenville Province and intrudes the structurally complex Grenville Supergroup.A synthesis of the oxygen and strontium isotopic and trace element data for the monzonite core and quartz monzonite rim indicates that these units are cogenetic but the margin of the pluton has interacted with a CO2–H2O fluid derived from the country rock. This interaction influenced the REE pattern and produced a noticeable enrichment of Rb, 18O, and possibly SiO2 in the rim but did not significantly influence the strontium isotopic systematics. A composite isochron including samples from both units indicates an emplacement age of 1065 ± 13 Ma with an initial strontium ratio of 0.7034 ± 0.0004. Samples from the isolated mafic units indicate an older age and therefore are not interpreted as cogenetic endmembers.The uniform but abnormally high δ18O in the monzonite core and low initial strontium ratio are interpreted to reflect a source composition in the lower crust.