Abstract New mineral data for kyanite-bearing eclogites supplement existing data for eclogite nodule suites from the Roberts Victor and Bellsbank kimberlite intrusions, South Africa. Calculation of equilibration temperatures using the refined K D calibration of Ellis and Green (1979) indicates that the majority of these eclogites equilibrated over a considerably narrower range (940–1185 °C) than implied by earlier estimates which did not take account of the influence of the Ca-component of garnet on K D . Most of the Roberts Victor diamondiferous eclogites appear to have equilibrated in the lower part of this range, whilst the equilibration temperatures for the Roberts Victor diamond-free eclogites extend to higher values than those from Bellsbank. The petrogenetic implications of the (calculated) temperature and pressure equilibration conditions for these eclogite suites are discussed with particular reference to possible uppermantle geotherms and available data on the graphitediamond transition boundary.
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Synopsis This paper documents the first identified occurrence of a garnet-bearing olivine-web-sterite rock within the Eastern Glenelg Lewisian of the Glenelg inlier. Bulk chemistries of the rock and associated websterites are comparable with komatiite or ultrabasic intrusive rocks. However, comparison with zoned ultrabasic–basic gneisses from the Scourie area suggests that the rock may be genetically linked to associated eclogites and represents a cumulate from a magma of basaltic composition. Geothermobarometry indicates that the rock equilibrated at eclogite facies P–T conditions: 20 ± 3 kbar, 730 ± 50°C. It is suggested that the magmatic protolith for this rock was emplaced during the Scourian, and from the textural evidence of the pyroxene grains, experienced granulite facies conditions followed by cooling prior to being subject to eclogite facies metamorphism during Grenville events.
Pressure–temperature conditions for formation of the peak metamorphic mineral assemblages in phengite‐bearing eclogites from Dabieshan have been assessed through a consideration of Fe 2+ –Mg 2+ 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 Fe 2+ –Mg 2+ partitioning between mineral phases, due to uncertainties over Fe 3+ /Fe 2+ 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.
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