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.iii

Pressure–temperature–time (P–T–t) estimates for the Montresor Belt, obtained using phase equilibria and geospeedometry modelling integrated with in situ U–Th–Pb monazite geochronology, shed new light on the tectonometamorphic effects of the Snowbird phase of the Trans-Hudson orogeny. Typical metapelitic assemblages of the lower Montresor group consist of white mica, biotite, plagioclase, quartz, and andalusite, which in some rocks is partly or completely pseudomorphed by white mica. The observed assemblages reflect peak P–T conditions centring at approximately 575 °C and 3 kbar. Rocks with high bulk Fe/Mg contents contain compositionally zoned garnet, permitting the addition of further constraints on the conditions of metamorphism in the Montresor Belt: Core compositions of earliest-grown garnets indicate initial garnet crystallization at approximately 535 °C and 2.3 kbar, suggesting a nearly isobaric P–T path of prograde metamorphism with a gradient of approximately 50 °C·kbar–1. Chemical age-dating of monazite inclusions in garnet yields ages of ca. 1870 ± 9 to 1837 ± 9 Ma. Retrograde, pseudomorphic andalusite replacement by white mica at approximately 540 °C is inferred to have been controlled by variations in bulk rock chemistry. Morphologically corroded and chemically heterogeneous monazite adjacent to white mica pseudomorphs suggests that andalusite replacement took place at ca. 1792 ± 10 Ma, possibly associated with extension and movement along the detachment fault separating the upper and lower Montresor groups. Simulations of diffusion across chlorite- and biotite-filled cracks in garnet assumed to be coeval with andalusite replacement suggest that the rocks have experienced the retrograde event for at least 20 My.