Thermobarometry of phengite‐bearing eclogites in the Dabie Mountains of central China

摘要: Pressure–temperature conditions for formation of the peak metamorphic mineral assemblages in phengite-bearing eclogites from Dabieshan have been assessed through a consideration Fe2+–Mg2+ partitioning between garnet–omphacite and garnet–phengite pairs reaction equilibrium celadonite+pyrope+grossular=muscovite+diopside, which incorporates an evaluation extent strongly pressure-dependent inverse Tschermak's molecule substitution phengites. For latter equilibrium, calibration recommended activity–composition models indicated by Waters & Martin (1993) employed importantly yield results consistent with petrographic evidence stability at coesite certain samples quartz others. Confirmation that some phengite-eclogite silicate equilibrated confining pressures sufficient (and cases even diamond) rather negates previous suggestions may stabilized only very localized, possibly just intracrystalline, domains. Inherent difficulties temperatures phases, due to uncertainties over Fe3+/Fe2+ ratios minerals (especially omphacites), re-equilibration during extensive retrograde overprinting samples, are also discussed. Our indicate existence south-central phengite markedly different equilibration within two structurally distinct tectonometamorphic terranes. Thus our data do not support earlier contentions comprises coherent continental-crust terrane regional P–T  gradient signalling deepest-level subduction north. Instead, we recognize Central Dabie ultra-high-pressure (coesite eclogite-bearing) be overlain Southern high-pressure (quartz major southerly dipping shear zone along late orogenic extensional collapse appears eliminated least 20 km crustal section.