Major and trace element mapping of garnet: Unravelling the conditions, timing and rates of metamorphism of the Snowcap assemblage, west‐central Yukon

Abstract

Abstract 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.