P-T-t conditions of Grenvillian metamorphism were derived from mineral assemblages, growth zoning in garnet and monazite geochronology from metapelites of the Flinton Group in southeastern Ontario. Initial conditions of garnet growth determined through isopleth thermobarometry yielded 3.7 kbar and 515°C and geothermal gradients of 40°C km-1. Phase equilibria modeling revealed an increase from 615 to 715°C and 5.6 to 7.9 kbar across the study area. Forward modeling of growth zoning in a population of garnet porphyroblasts with the THERIA_G software revealed a clockwise path over the P-T interval 3.7 to 5.9 kbar and 513 to 615°C. Diffusional relaxation of growth zoning in relatively small porphyroblasts of the population provided an average heating rate of 2°C Ma-1. In situ U-Pb monazite geochronology revealed a major age population at 977 ± 4 Ma, interpreted to represent monazite growth at the expense of allanite near the peak conditions of garnet growth and metamorphism.