Metamorphic History of Sapphirine-bearing and Related Magnesian Gneisses from Namaqualand, South Africa |
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| Authors: | WATERS D J |
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| Institution: | Department of Geology, University of Cape Town Rondebosch, 7700 South Africa |
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| Abstract: | Sapphirine occurs with cordierite, phlogopite, spinel, sillimanite,corundum, orthopyroxene, and gedrite in granulite facies Mg-and Al-rich paragneisses within the low P, high T NamaqualandMetamorphic Complex. The gneisses reveal a three-stage texturalhistory. Sapphirine appeared during a second stage of progrademineral growth which produced nodular structures and intergrowthsinvolving spinel, corundum, and sillimanite, pseudomorphingan earlier generation of coarse, amphibolite facies minerals.A third generation of coarse, cross-cutting, mainly hydrousminerals (gedrite, kornerupine, phlogopite) is sporadicallydeveloped. The wide variety of cofacial mineral assemblages allows thedelineation of the stable mineral associations of sapphirinein the system K2O-MgO-FeO-Al2O3-SiO2-H2O under P-T conditionsindependently estimated at about 5 kb, 750–800 °C.The natural assemblages provide constraints which, taken togetherwith existing thermodynamic and experimental data, allow theestimation of P-T slopes of sapphirine equilibria. The mineraltextures thus indicate sapphirine growth under increasing T,decreasing a(H2O), and constant or slightly increasing P. The preservation of prograde reaction textures during fine-grainedmineral growth probably results from the reduced importanceand/or more CO2-rich composition of the metamorphic fluid undergranulite facies conditions in these refractory rocks. Aqueousfluids were locally reintroduced after the metamorphic peak. |
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