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

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