Nascent subduction: A record in the Yukon eclogites

Eclogites from the Faro area (Yukon-Tanana terrain, Canada) occur within gl aucophane schists, mica gneisses, and quartzites. The rocks provide evidenc e of metamorphic evolution during the onset of subduction. Geothermobaromet ry (by the Grt-Cpx thermometer and the Cpx-Qtz-Pl barometer) points to an i ncrease in the temperature and minimum pressure (from similar to 520 degree s C, 11 kbar to similar to 690 degrees C, 15 kbar) during the prograde stag e. The retrograde stage occurred at an insignificant decompression and a te mperature decrease to similar to 540 degrees 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 mu m in diameter) relies of Ca-rich garnet, which was contained in the rock prior to the prograde eclogite-fac ies metamorphism. The finding of the garnet relics made it possible to util ize geospeedometry to constrain the timing not only of the retrograde but a lso of the prograde stages. A simulation (based on the model for diffusion in a sphere) of the conditions under which the garnet relies could be prese rved 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 min imum burial and heating rates are 7 cm/year and 850 degrees C/Ma, respectiv ely. The subsequent retrograde evolution of the eclogite proceeded with a m inimum cooling rate as high as 750 degrees 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.