Liou, JG; Zhang, RY; Ernst, WG; Liu, J; Mclimans, R
Long-duration piston-cylinder phase-equilibrium experiments in the basalt-H2O system at 600-950 °C, 0.8-3.0 GPa demonstrate that amphibolite transforms to eclogite with increasing pressure by three reversed reactions: appearance of garnet, breakdown of plagioclase, and decomposition of amphibole. The garnet-in, plagioclase-out, and amphibole-out phase boundaries, coupled with the presence of titanium-rich phases, divide P-T space into four regions corresponding to the amphibolite, garnet amphibolite, hornblende-eclogite, and eclogite facies. For MORB bulk-rock compositions, rutile occupies a broad P-T synthesis field at high pressures, titanite occurs at low P and T, and ilmenite is confined to low P and high T. At Paimpaludo in the Western Ligurian Alps, an eclogite ferrogabbro layer at least 120 m thick is encased in the Beigua antigoritic serpentinite. The eclogite core is massive, relatively unaltered, and homogeneous, whereas the margin of the body is strongly foliated and retrograded. The TiO2 content of the eclogite averages 6%, dominantly as rutile, but massive eclogite seems to be richer in rutile than the foliated margins. Omphacite + garnet + rutile rocks are differentiated Fe-Ti rich gabbros metamorphosed to eclogitic assemblages, followed by amphibolite/blueschist- and later greenschist-facies overprinting. Titanium resides principally in rutile, ilmenite is common, and titanite is locally significant. Rutile is rimmed by ilmenite; cracks within garnet contain ilmenite, albite and chlorite. Titanite is the latest Ti phase, forming after ilmenite and rutile. Rutile exhibits different colors and modes of occurrence, but it is remarkably homogeneous, containing minor FeO (0.23-0.61 wt%), MnO (0.0-0.49 wt%) and CaO (0.0-0.46). In contrast, ilmenite shows marked compositional variations, with MnO (0.31-5.73 wt%), MgO (0.17-0.69 wt%), CaO (0.0-0.48 wt%) and detectable ZnO. Titanites contain 0.78-1.32 wt% Al2O3 and 0.22-0.57 wt% FeO. The occurrence of garnet + omphacite ± talc + rutile as the main eclogite assemblage suggests high-pressure (>1.5 GPa) metamorphism, but garnet-clinopyroxene geothermometers indicate recrystallization at only 425 ± 75 °C. Because the eclogitic rocks contain abundant zoned amphiboles and widespread retrograde assemblages, garnet and clinopyroxene probably partly re-equilibrated during the amphibolite/blueschist-facies overprint, which might explain the low-temperature estimates.