PETROLOGY AND COOLING RATES OF THE VALHALLA COMPLEX, BRITISH-COLUMBIA, CANADA

Rocks from the Valhalla metamorphic core complex, British Columbia, Ca nada, have experienced granulite facies metamorphism at conditions of 820 +/- 30 degrees C, 8 +/- 1 kbar. Peak metamorphism teas accompanied by dehydration melting of muscovite, but not biotite, followed by min or back reaction of garnet + K-feldspar + H2O = sillimanite + biotite + plagioclase. At conditions very near those of the peak, extensive sh earing produced s-c (schistosite-cisaillement) fabrics, ribbon quartz and grain size reduction of garnet at several locations. Garnet-biotit e Fe-Mg exchange thermometry yields temperatures that range from 580 t o 1051 degrees C. Low temperatures are calculated from biotite modifie d dominantly by Fe-Mg exchange with garnet; high temperatures are calc ulated from Fe-rich biotites produced from the above retrograde reacti on. Geothermometry is useless in these rocks to estimate peak temperat ure a priori, but is very useful to help constrain the complex reactio n history of biotites. Geochronology on monazite, zircon, allanite, ti tanite, hornblende, muscovite, biotite and apatite has been used to co nstrain the timing of the metamorphic peak at 67-72 Ma and the average cooling rate to 24 +/- 6 degrees C/Ma. Diffusion modeling of Fe-Mg ex change between biotite inclusions and host garnet yields cooling rates of either 3-80 degrees C/Ma or 200-2500 degrees C/Ma, depending on th e choice of diffusion coefficients. The former value is consistent wit h the average cooling rate of 24 degrees C/Ma for the complex determin ed from geochronology, but the faster rate cannot be ruled out and may indicate initial very rapid cooling by thrusting of the complex onto cooler basement. It is suggested that cooling rates determined from ge ochronologic vs petrologic methods may not be directly comparable beca use petrologic methods sample near-peak metamorphic cooling rates wher eas geochronologic methods sample post-peak to ambient cooling rates.