Prograde and Retrograde Zircon Growth in HP Migmatitic Schist of the Monashee Complex (Canadian Cordillera): Assesment of the Ti-In Zircon Thermometer

Félix Gervais, Polytechnique Montréal
Jim Crowley, Boise State University

U-Pb ages and trace elements acquired simultaneously on zircon grains from a migmatitic pelitic-schist, collected in the northern part of the Frenchman Cap dome in the Monashee Complex (SE Canadian Cordillera), were linked with results of phase equilibria modeling. Three phases of zircon growth were documented. The first occurred at c. 72 Ma as the rock crossed the muscovite dehydration-melting in the kyanite field at temperatures > 725°C on its prograde path. It produced zircon with a shallow positive slope in high rare-earth elements (HREE) and a moderate Eu anomaly. The second occurred at c. 67 Ma as the rock crossed the biotite dehydration-melting that formed garnet and rutile at temperatures of 800-875°C. It produced zircon depleted in HREE and without Eu anomaly. The third took place at c. 62 Ma as the rock crossed back the latter reaction on the retrograde path and thus consumed garnet and rutile at temperatures > 800°C. It produced zircon similar to the first generation, but also enriched in Nb and Ta released during rutile breakdown. In contrast with the high temperature of zircon growth documented above, Ti-in zircon thermometry yields unrealistically low temperatures between 546 and 612°C, and this despite excellent analytical conditions and without uncertainties in buffering assemblage (rutile and quartz were present throughout). This study, therefore, demonstrate that zircon growth along a prograde path is possible and, moreover, highlights the needs to use extreme caution when interpreting results of zircon thermometry in metamorphic rocks.