| Summary: | International audience Eclogites from the Faro area (Yukon-Tanana terrain, Canada) occur within glaucophane schists, mica gneisses, and quartzites. The rocks provide evidence of metamorphic evolution during the onset of subduction. Geothermobarometry (by the Grt-Cpx thermometer and the Cpx-Qtz-Pl barometer) points to an increase in the temperature and minimum pressure (from ∼520°C, 11 kbar to ∼690°C, 15 kbar) during the prograde stage. The retrograde stage occurred at an insignificant decompression and a temperature decrease to ∼540°C. The overall P-T metamorphic trajectory of the eclogites is counterclockwise. The Fe-Mn cores of garnet idioblasts bear small (no more than 50 μm in diameter) relics of Ca-rich garnet, which was contained in the rock prior to the prograde eclogite-facies metamorphism. The finding of the garnet relics made it possible to utilize geospeedometry to constrain the timing not only of the retrograde but also of the prograde stages. A simulation (based on the model for diffusion in a sphere) of the conditions under which the garnet relics could be preserved indicates that the total duration of the P-T metamorphic evolution of the eclogite did not exceed 0.2 Ma. The corresponding estimates for the minimum burial and heating rates are 7 cm/year and 850°C/Ma, respectively. The subsequent retrograde evolution of the eclogite proceeded with a minimum cooling rate as high as 750°C/Ma. According to the proposed thermal-tectonic model, the very rapid temperature and pressure changes in the eclogite were caused not only by the displacement of the rock but also by rapid transformations in the thermal structure of the subduction zone.
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