Insight into the cooling history of the Valhalla complex, British Columbia

Migmatitic paragneiss and amphibolite gneiss from the Valhalla metamorphic core complex, southeastern B.C. record a complex retrograde history that includes melt-involving net transfer reactions and partial re-equilibration at lower pressure conditions. Pelitic paragneiss from ~ 1.5 km above the Gwillim Creek shear zone contains diffusion zoned garnet porphyroblasts that indicate a slow then fast cooling history that is different from fast cooling from peak recorded in samples from within the shear zone. Fast cooling is consistent with transport of this migmatitic crustal root zone of the Canadian Cordillera up a cold footwall ramp of the thrust sense Gwillim Creek shear zone, followed by exhumation via the Valkyr Shear zone–Slocan Lake detachment after melt crystallization. Forward modeling of the interdiffusivity of Fe + Mg in garnet, governed by a retrograde net transfer reaction, indicates distinctly different cooling histories for rocks above the Gwillim Creek shear zone, suggesting these rocks may have been at similar peak temperatures at different times. Thermobarometry and thermodynamic modeling of these gneisses constrain a cooling and decompression P–T path segment consistent with retrograde amphibolite facies re-equilibration. 2-D thermal modeling suggests that slow followed by fast, then slower cooling can be accommodated by movement up a shallow thrust ramp, then rapid thrusting up a steeper ramp, and finally normal-sense shearing with erosional denudation. Ductile flow onto a cool footwall, coupled with erosion, is a viable mechanism of cooling and partial exhumation of lower crustal rocks.