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

Abstract

A B S TR A CT 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.

The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy

“…The temperature obtained using Powell (1985) geothermometer, however, is systematically 20 40 C higher than that obtained using Krogh (1988) geothermometer for less calcic garnet (Xgrs < 0.35), but it gives a significantly higher temperature for calcic garnet (Xgrs > 0.35). As Carswell et al (1997) pointed out that temperature estimation is better using Powell (1985) geothermometer for less calcic garnet (Xgrs < 0.35), and Krogh (1988) geothermometer for calcic garnet (Xgrs > 0.35), and we concur with their suggestion. The KD Xgrs relationships used to the temperature estimation of the Sesia samples in the literature is also plotted in Figure 9 (Ungaretti et al, 1983;Oberha nsli et al, 1985;Pognante et al, 1987;Castelli, 1991).…”

Peak conditions of kyanite-bearing quartz eclogites in the Sanbagawa metamorphic belt, central Shikoku, Japan

“…This P T estimations are consistent with the occurrence of paragonite inclusion in garnet and omphacite. The combination of the reaction (1) and Grt Cpx geothermometer (GCPQ), that has been extensively employed for the P T estimation of HP rocks (e.g., Carswell et al, 1997;Cuthbert et al, 2000;Nowlan et al, 2000), gives slightly more scattered P T conditions of 2.2 2.5 GPa/665 830 °C than the GCKPQ geothermobarometer. Enami (1996) and Ota et al (2004) reported P T conditions of 1.9 2.6 GPa/700 760 °C for the kyanite quartz eclogite and 1.4 2.5 GPa/550 790 °C for eclogite assemblages from the Western and Eastern Iratsu bodies, respectively.…”

Prograde and retrograde evolution of eclogites from the Bantimala Complex in South Sulawesi, Indonesia

“…White-filled symbols refer to estimates for the barroisite-bearing eclogite, whereas small gray-filled symbols are estimates for the barroisite-free eclogite. Garnet-clinopyroxene-phengite geobarometry symbols: diamond and plus from Carswell et al (1997) and Ravna and Terry (2004), respectively. White black-filled symbols refer to minimum and maximum values of Si contents in phengite, respectively, from the barroisite-bearing eclogite.…”

“…The barometries proposed by Carswell et al (1997) and Ravna and Terry (2004) were used to calculate the P condition. The peak pressure and temperature were determined using the pyrope + 2 grossular + 3 celadonite = 6 diopside + 3 muscovite equilibrium in the coexisting garnet, omphacite, and phengite assemblages.…”