Fast Cooling and Exhumation of the Valhalla Metamorphic Core Complex, Southeastern British Columbia

High-grade, migmatitic paragneisses (820°C, 8 kbar peak conditions) of the Valhalla metamorphic core complex experienced the retrograde net transfer reaction (ReNTR): garnet + melt + K-feldspar = sillimanite + biotite + plagioclase during cooling and crystallization of the melt, resulting in an increase in the Fe/(Fe + Mg) of both garnet and biotite at their margins. Diffusion profiles generated by this increase in Fe/(Fe + Mg) have been modeled numerically. Optimal fits were obtained with an initial, short period of rapid cooling (> 200°C/m.y. for < 0.5 m.y.) followed by slower (but still rapid) cooling at 20–30°C/m.y. for 10–20 m.y. Four generations of monazite have been observed. Monazite inclusions within garnet contain only generations 1 and 2, and generations 3–4 are found only in matrix monazites. Generations 1–2 record ages of 75–85 Ma, which is believed to be represent the prograde metamorphism of the complex. A single age of ~105 Ma may represent an early contact metamorphic episode. Third-generation monazite is 60 ±2 Ma, and reflects near-peak conditions. Generation 4 monazite, which occurs only on the outer rims of monazite, has not been dated, but is believed to represent monazite produced during melt crystallization. A hornblende K-Ar age of 58±2 Ma suggests a cooling rate of several tens to several hundreds of degrees/Ma immediately following the metamorphic peak. The extremely rapid cooling (hundreds of degrees/m.y.) at ~60 Ma must have been tectonically induced and it is suggested that transport of the hot Valhalla complex onto cooler basement up a thrust ramp along the underlying Gwillim Creek shear zone was the cause. Two-dimensional thermal modeling indicates that thrust transport of several centimeters/year up a 20° ramp will produce the observed cooling rates. The subsequent slower cooling rate (tens of degrees/m.y.) is believed to have been the result of extensional unroofing by low-angle normal faulting along the Slocan Lake fault.