Publication details

Corundum genesis at the Blue Jay Sapphire occurrence (British Columbia, Canada) as a record of metamorphism and partial melting in the Monashee Complex



Year of publication 2023
Type Article in Periodical
Magazine / Source Lithos
MU Faculty or unit

Faculty of Science

Keywords Corundum; Muscovite; Leucosome; Metamorphic core complex; Partial melting; Metamorphism
Description The sapphire (corundum) occurrence near Revelstoke, British Columbia, Canada, occurs in the Monashee Complex of the Omineca Belt of the Canadian Cordillera. Corundum occurs in boudins or pod-like zones of restite within a garnet-bearing leucosome. Both zones are peraluminous and contain plagioclase, K-feldspar, and biotite and muscovite as the dominant mineral phases. Relative to the leucosome, the corundum-pods are slightly depleted in SiO2 (55.28 wt% in corundum-pods vs. 62.7 wt% in leucosome) and K2O (4.74/7.05 wt%) and enriched in Al2O3 (22.57/21.97 wt%), Fe2O3 (5.96/2.11 wt%), TiO2 (0.71/0.21 wt%), CaO (0.68/0.60 wt%), and MgO (0.59/0.28 wt%). The peraluminous nature of the corundum-bearing pods is consistent with partial melting of a metapelitic protolith. Similar chemical compositions between the corundum-pods and garnet-leucosome are interpreted to infer a single, in situ partial melting event. Coinciding melt-compatible and incompatible elements suggest a closed system. Igneous textures in thin section (coarse-grained, euhedral corundum; twinning in plagioclase; small cuspate-shaped biotite, and myrmekitic intergrowths of feldspars) confirm a partial melt origin. Petrography and thermodynamic models reveal that corundum growth and partial melting was fueled by the breakdown of muscovite along the prograde P-T path. Whole-rock geochemistry of the leucosome groundmass devoid of garnet shows high Sr/Y and La/Yb ratios indicative of garnet fractionation, and garnet mineral chemistry are Ca-poor indicating garnets crystallized from the peraluminous partial melts at peak P-T conditions. 206Pb/238U ages of garnet-equilibrated zircon indicate that peak metamorphic conditions occurred at or before ~60.69 Ma. Continued zircon growth until 50.22 Ma tracks the presence of continued partial melt following peak metamorphism during nearly-isothermal decompression. A resulting P-T-t path for basement rocks form the Thor-Odin dome constrains the timing of peak metamorphism and ensuing partial melting events during Cordilleran orogenesis.

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