PRESSURE-TEMPERATURE EVOLUTION OF ZOISITE-HORNBLENDE ECLOGITES FROM FARO AREA, YUKON

Eclogite occurs as metre-sized lenses within metasedimentary schists and gneisses near Faro, Yukon Territory, Canada (Lat. 62.2°N, 133.3°W). The common mineral assemblage in eclogite is quartz-garnet-zoisite-Ca-amphibole-clinopyroxene-rutile-phengite. Secondary minerals include epidote-titanite-chlorite-glaucophane-calcite-K-feldspar and albite. Eclogites have undergone strong deformation, locally producing a mylonitic matrix. Garnet is relatively fresh but contains numerous fractures and veins of chlorite, calcite, and quartz. Zoisite is ferrian zoisite. Ca-amphiboles range from barroisite to wichite. Clinopyroxene has a maximum jadeite content near 0.45 mol fraction. Estimates of pressure (P) and temperature (T) attending metamorphism have used Fe²⁺-Mg fractionation between clinopyroxene and garnet and Ca-amphibole and garnet, Zr in rutile, clinopyroxene-garnet-phengite equilibria and isochemical phase diagram sections (also known as pseudosections). Peak P-T conditions are difficult to estimate. Using Zr in rutile, the temperature ranges from 580 to 595°C at 20 kbar, with no difference between rutile included in garnet and rutile in the matrix. For a(H₂O)= 1.0, the lack of stable lawsonite suggests P < 20.5 kbar at T ~ 580 °C using a new Zo-solid solution model that more accurately reflects the observed Zo compositions during peak metamorphism.The P-T path during unroofing and cooling is difficult to estimate. Glaucophane + epidote suggests some of the unroofing was in the blueschist facies field. The occurrence of albite suggests that some of the P-T path was in the albite P-T stability field. The host metasedimenary rocks contain zircon-quartz-rutile but suggest equilibration temperatures of about 545°C. Post-peak penetrative deformation promoted some retrogressive reactions but the coarser porphyroblasts survived this deformation and retrogression.