FAST COOLING OF THE VALHALLA COMPLEX

New monazite geochronology and diffusion modeling of zoning in high grade garnet constrain the cooling rate of the Valhalla metamorphic core complex, southeastern B.C. Five generations of monazite have been observed from migmatitic paragneisses (820 °C, 8 kbar peak conditions). Monazite inclusions within garnet contain only generations 1 and 2 and generations 3-5 are found only in matrix monazites. Generations 1-3 record ages of 75-85 Ma, which is believed to represent the prograde metamorphism of the complex. A single age of ca 105 Ma may represent an early contact metamorphic episode. Fourth generation monazite is 60 ±2 Ma, and reflects near-peak conditions. Generation 5 monazite, which occurs only on the outer rims of monazite has not been dated, but is believed to represent monazite produced during melt crystallization. Hornblende K-Ar age of 58±2 Ma suggest a cooling rate of several tens to several hundreds of degrees/Ma immediately following the metamorphic peak. Diffusion modeling based on zoning driven by Fe-Mg exchange reactions between garnet and biotite result in calculated cooling rates of ca 20 °C/Ma when the rocks were 500-600 °C. Zoning on the rims of garnet is, in most cases, controlled by the net transfer reaction Grt + Kfs + Melt=Bt + Sil + Plg. Modeling the diffusive zoning using this reaction for the T-X boundary condition results in calculated cooling rates of several hundred ° C/Ma over the temperature interval 820-700 °C. The composite temperature-time path from both the geochronology and the diffusion modeling therefore shows an initial period of very rapid cooling (hundreds of degrees/Ma) for a short period of time (< 1 Ma) followed by slower cooling (tens of degrees/Ma) for several Ma. The slower cooling is believed to have been the result of extensional unroofing by low angle normal faulting. The faster cooling rate must have been tectonically controlled 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 of this rapid cooling. Thermal modeling indicates that thrust transport of several centimeters/year up a 20 ° ramp will produce the observed cooling rates.