Pressure-temperature-time constraints on metamorphism in the southeastern Taltson Domain, Saskatchewan, Canada

Abstract

The southeastern Taltson Domain is located on the southern margin of the Rae Craton in northern Saskatchewan, Canada, and encompasses the region south of the Athabasca Basin and west of the Snowbird Tectonic Zone. There has been relatively little metamorphic geochronology and no quantitative pressure–temperature (P–T) constraints from the southeastern Taltson Domain, which hinders attempts to link its metamorphic history to surrounding regions. New in situ LA-ICP-MS U–Pb monazite and zircon ages from rare metasedimentary rocks from the southeastern Taltson Domain reveal a polymetamorphic history. A cordierite–sillimanite–spinel-bearing migmatite from Fournier Lake records high thermal gradient metamorphism at 1988 ± 21 Ma, reaching P–T conditions of 4–6.4 kbar and > 900 ◦C. The cordierite–sillimanite–spinel assemblage overprints an earlier bio- tite–sillimanite-bearing assemblage that formed at 2075 ± 31 Ma. An orthopyroxene–garnet gneiss from within the Snowbird Tectonic Zone has a dominant monazite population at c. 1.90 Ga, but contains rare monazite ages between c. 2.19–2.01 Ga that support a phase of metamorphism prior to 2.0 Ga. The main metamorphic event to have affected the southeastern Taltson Doman occurred at c. 1.94–1.90 Ga. Phase equilibria modelling suggests peak P–T conditions were 8.3–11.2 kbar and 800–930 ◦C. Monazite with high HREE contents yields weighted mean ages of 1926–1918 Ma, interpreted to reflect prograde metamorphism prior to significant garnet growth. Zircon from an extremely garnet and zircon rich sample (now silicified and chloritized as a result of younger alteration) formed at c. 1918 Ma. REE compositions of the zircon indicate that it formed in equilibrium with garnet at temperatures of ~ 830 ◦C, suggesting peak metamorphism occurred around c. 1.92 Ga. HREE-poor monazite with weighted mean ages of c. 1910–1895 Ma formed after the majority of garnet growth and either reflect the timing of melt crystallisation or recrystallisation during retrogression and fluid flow along major shear zones in the region. The P–T conditions and c. 1.93–1.90 Ga ages from the southeastern Taltson Domain are very similar to other rocks in the Dodge–Snowbird Domain of the Rae Craton, and demonstrate that the footprint of mid-P granulite facies metamorphism extends south and west into the southeastern Taltson Domain.