P-T-t conditions are determined for the Montresor belt (Rae Province, Nunavut) using modern thermodynamic modeling and in-situ U-Th-Pb monazite geochronology. Phase equilibrium modeling yields peak P-T estimates of 355 - 455°C and <4200 bar for the upper Montresor belt and 500 - 580°C and <4000 bar for the lower Montresor belt, with a high geothermal gradient of ca. 57°C/km during metamorphism. Forward modeling of garnet crystallization suggests growth along a clockwise, nearly isobaric P-T path from ca. 539 – 544°C and 3090 – 3140 bar. Chemical age dating of metamorphic monazite indicates peak P-T conditions occurred at ca. 1837 ± 8.6 Ma. Altered monazite domains suggest a fluid-mediated alteration event during cooling at ca. 1792 ± 10.1 Ma, associated with the emplacement of dykes into the lower Montresor group. These fluids increased the bulk rock K and caused modification to garnet growth zoning and mechanical fracturing of garnet.