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

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.

Latest Triassic–Early Jurassic Stikine–Yukon-Tanana terrane collision and the onset of accretion in the Canadian Cordillera: Insights from Hazelton Group detrital zircon provenance and arc–back-arc configuration

“…Early Jurassic garnet growth ages in the Yukon-Tanana terrane (Fig. 12;Clarke, 2017;Dyer, 2020;Gaidies et al, 2021) attest to Barrovian metamorphism during Early Jurassic tectonic burial. Ductile sinistral motion on the Llewellyn shear zone (Currie and Parrish, 1993) shows relative northward motion of the Stikinia block as an indentor into the western Yukon-Tanana terrane.…”

Triassic–Jurassic Accretionary History and Tectonic Origin of Stikinia From U‐Pb Geochronology and Lu‐Hf Isotope Analysis, British Columbia

“…In the northern portions of Stikinia, where collision would have been focused, deformation and crustal thickening are indicated by tectonic burial of the Yukon-Tanana terrane at ca. 200-190 Ma (Berman et al, 2007;Clark, 2017;Gaidies et al, 2020;Sack et al, 2020;Tafti & Mortensen, 2003) and a related isotopic pull-down in detrital and igneous zircons (van Drecht, 2019) followed by widespread exhumation throughout central Yukon-Tanana (mica cooling ages of ca. 190-185 Ma; Colpron et al, 2015).…”

“…200 Ma pulldown. More likely, however, the disparity likely reflects significant crustal thickening at the northern apex of Stikinia that did not persist farther south (Clark, 2017;Gaidies et al, 2020;Sack et al, 2020).…”

Garnet as a monitor for melt–rock interaction: Textural, mineralogical, and compositional evidence of partial melting and melt‐driven metasomatism

“…Trivalent cations, like rare earth elements (REEs), have lower diffusion coefficients than major divalent cations. They are therefore more resistant to diffusional modification and allow the preservation of prograde and high-T processes (Carlson, 2012;Gaidies et al, 2020;George & Gaidies, 2017;Moore et al, 2013;Rubatto et al, 2020), including those involved during partial meting (Dumond et al, 2015).…”

“…With the recent development of in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), it is now possible to rapidly and quantitatively map the 2D distribution of trace elements across major and accessory minerals of interest (George et al, 2018;Raimondo et al, 2017;Ubide et al, 2015). Yet, despite the known benefits of analysing trace elements in garnet for the purposes listed above, there remains a limited (but increasing) number of studies using this approach to understand the grain-scale controls on garnet crystallization (Gaidies et al, 2020;George et al, 2018;Rubatto et al, 2020).…”