Garnet crystallization has been simulated in the MnNCKFMASHT model system using a simple nucleation and growth scenario, calibrated with three‐dimensional garnet crystal size distribution data as well as garnet compositional data obtained by electron probe micro‐analysis and laser ablation inductively coupled plasma mass spectrometry. Results indicate wide‐spread Barrovian‐type metamorphism for garnet‐zone rocks from the Snowcap assemblage along a hairpin‐shaped pressure–temperature loop with garnet growth from ~515°C and 4 kbar to metamorphic peak conditions of ~600°C and 6 kbar. Lu–Hf garnet‐whole geochronology points to initial garnet growth at c. 192.2 ± 4.7 Ma. Sm–Nd garnet–whole‐rock geochronology applied to a sample with garnet rims enriched in Sm indicates that the metamorphic peak conditions have been attained at c. 172.9 ± 2.4 Ma. Older garnet growth at c. 245.3 ± 0.8 Ma during a low‐P–high‐T event has been preserved as garnet cores separated from the Jurassic garnet rims by a sharp microstructural and compositional discontinuity. These polyphase garnets are restricted to Mn‐rich metapelitic lithologies. Trace element zoning in the outermost ~50 μm thin segments of the Early Triassic garnet cores reflects a short garnet growth episode in the presence of melt at peak conditions of ~710°C and 2.5 kbar, supported by phase equilibrium and diffusion geospeedometry calculations. Diffusion simulations across the interface between the Early Triassic garnet core and the Jurassic garnet rim indicate that the Barrovian‐type metamorphism during the Jurassic lasted for 20–25 Myr, in line with the radiometric data.

Amphibolite-facies Snowcap assemblage rocks in the Stewart River area (west-central Yukon) preserve significant information on the tectonometamorphic history of the Yukon-Tanana terrane. Garnet-bearing metapelites were analyzed to obtain detailed metamorphic pressure-temperature-time (P-T-t) information through the integration of whole-rock geochemistry, electron-microprobe analysis, X-ray micro-computed tomography, phase equilibria, and garnet-crystallization modelling. The bulk chemistry of the Snowcap assemblage rocks indicates that their protoliths are shale and wacke that were deposited in a passive-margin setting. Textural observations indicate at least three episodes of deformation, forming an early schistosity (SM-2) overprinted by a crenulation cleavage (SM-1) defined by quartz and ilmenite inclusions in garnet, and a dominant transposition foliation (SM) mainly recorded by micaceous layers. Compositional zoning of garnet with respect to its Fe, Mg, Mn, Ca, and Y contents indicates chemical disequilibrium across the garnet volume and between garnet and the rock matrix. The zoning is interpreted to result from chemical fractionation associated with garnet growth and relatively slow intracrystalline diffusion. Calculations on a key sample indicate garnet started to crystallize at approximately 510°C and 3.75 kbar and finished growing at about 670°C and 6.8 kbar. These conditions correspond to the peak P-T conditions experienced by the rock. The chemical composition of the garnet core indicates significant modification of the initial garnet chemistry by intracrystalline diffusion at temperatures of 600°C or higher for at least 20 Ma. There are no current age constraints on the samples analyzed, but previous studies indicate Permian and Jurassic diachronic and/or episodic regional metamorphism.

Quantitative and qualitative analyses of Snowcap Assemblage metapelitic rocks of the Yukon-Tanana terrane (YTT) in west-central Yukon results in the identification of three phases of metamorphism and deformation through garnet crystallization modeling coupled with garnet age dating. A first Permo-Triassic metamorphic event (ca. 245 Ma), associated with a heating event occurring during the emplacement of Permian granitoids, reached low P amphibolite facies metamorphic conditions. The second metamorphic event dated at the Jurassic (ca. 197-188 Ma) reached peak P-T conditions of 7 kbar and 650-685 °C with a heating rate of 20 °C/My and is followed by a dwell period at T > 600 °C. This event follows typical Barrovian-type metamorphism and is interpreted to be associated with the accretion of the YTT onto Laurentia. The third metamorphic event, post-Jurassic in age, reached peak P-T conditions of 6 kbar and 580 °C with a heating rate of 20 °C/My. iii

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