The blueschist-greenschist facies transition for a model basaltic system Na2O-CaO- MgO-Al2O3-SiO2-H 2O is defined by a univariant reaction: 6 clinozoisite + 25 glaucophane + 7 quartz + 14 H2O = 6 tremolite + 9 chlorite + 50 albite; for the Fe2O3-saturated basaltic system, by a discontinuous one: 4 epidote + 5 Mg-riebeckite + chlorite + 7 quartz = 7 hematite (magnetite) + 4 tremolite + 10 albite + 7 H2O. These two reactions were experimentally investigated to determine the nature of the blueschist-greenschist transition. The results have located the first reaction at 350 ± 10°C, 7.8 ± 0.2 Kb and 450 ± 10°C, 8.2 ± 0.4 Kb. Reconnaissance experiments for the second reaction indicate that the minimum pressure for the occurrence of epidote + Mg-riebeckite + chlorite + quartz is about 4 Kb at 300°C for fO2 defined by the hematite-magnetite buffer. The presently determined P-T location for the blueschist-greenschist transition in the Fe-free basaltic system is about 3 Kb lower than the minimum pressure limit of glaucophane of Carman and Gilbert (1983), but is compatible with the revised stability field of jadeite + quartz determined by Holland (1980). Introduction of Fe3+ into the model basaltic system significantly lowers the minimum pressure limit for occurrence of the buffered assemblage sodic amphibole + epidote + actinolite + chlorite + albite + quartz, and the participating phases gradually increase their Fe 3+/Al ratio with decreasing pressure. Isopleths of sodic amphibole composition in the buffered assemblage in terms of XGi are delineated and the effect of Fe2+ and temperature on the isopleths are discussed. The AI2O3 content of sodic amphibole coexisting with epidote + actinolite + chlorite + albite + quartz decreases systematically with decreasing pressure and hence can be used as a geobarometer. Pressure estimates for metabasites at Ward Creek of the Franciscan terrane, the Mikabu greenstones of the Sanbagawa belt, the Otago schists of Lake Wakitipu, New Zealand, and the blueschists at Ouegoa, New Caledonia, based on the proposed glaucophane geobarometry, are in agreement with those derived from sodic pyroxene geobarometry.
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