Petrology and cooling rates of the Valhalla complex, British Columbia, Canada

Abstract

Rocks from the Valhalla metamorphic core complex, British Columbia, Canada, have experienced granulite facies metamorphism at conditions of 820±30°C, 8±1 kbar. Peak metamorphism was accompanied by dehydration melting of muscovite, but not biotite, followed by minor back reaction of garnet + K-feldspar + H2O = sillimanite + biotite + plagioclose. At conditions very near those of the peak, extensive shearing produced s-c (schistosité-cisaillement) fabrics, ribbon quartz and grain size reduction of garnet at several locations. Garnet-biotite Fe-Mg exchange thermometry yields temperatures that range from 580 to 1051°C. Low temperatures are calculated from biotite modified dominantly by Fe-Mg exchange with garnet; high temperatures are calculated from Ferich biotites produced from the above retrograde reaction. Geothermometry is useless in these rocks to estimate peak temperature a priori, but is very useful to help constrain the complex reaction history of biotites. Geochronology on monazite, zircon, allanite, titanite, hornblende, muscovite, biotite and apatite has been used to constrain the timing of the metamorphic peak at 67-72 Ma and the average cooling rate to 24 ±6°C/Ma. Diffusion modeling of Fe-Mg exchange between biotite inclusions and host garnet yields cooling rates of either 3-80° C/Ma or 200-2500°C/Ma, depending on the choice of diffusion coefficients. The former value is consistent with the average cooling rate of 24°C/Ma for the complex determined 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 geochronologic vs petrologic methods may not be directly comparable because petrologic methods sample near-peak metamorphic cooling rates whereas geochronologic methods sample post-peak to ambient cooling rates.