Zoisite Eclogites From Faro Area, Yukon: Pressure-Temperature- Fluid Composition Attending Metamorphism

English Abstract:

Eclogite occurs as metre-sized lenses within metasedimentary schists and gneisses near Faro, Yukon Territory, Canada. The 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 little retrogressioni.Estimates of peak pressure (P) and temperature (T) attending metamorphism used Fe²⁺-Mg fractionation between clinopyroxene 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 1.0, the lack of stable lawsonite suggests P<~18 kbar at T~600°C. The P-T path during unroofing and cooling is difficult to estimate but the occurrence of albite suggests that some of the P-T path was in the albite 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 peak porphyroblasts survived this deformation and retrogression. This implies lack of extensive fluids during cooling and deformation.