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

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 similar to 515 degrees C and 4 kbar to metamorphic peak conditions of similar to 600 degrees C and 6 kbar. Lu-Hf garnet-whole geochronology points to initial garnet growth atc. 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 atc. 172.9 +/- 2.4 Ma. Older garnet growth atc. 245.3 +/- 0.8 Ma during a low-P-high-Tevent 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 similar to 50 mu m thin segments of the Early Triassic garnet cores reflects a short garnet growth episode in the presence of melt at peak conditions of similar to 710 degrees 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.

Automated summary

The simulation of garnet crystallization in the MnNCKFMASHT model system revealed widespread Barrovian-type metamorphism and examined the nucleation and growth processes of garnet, combining multiple data sources and analytical techniques for in-depth investigation.