Kornerupine in Mg-Al-rich gneisses from Namaqualand,South Africa: mineralogy and evidence for late-metamorphic fluid activity |
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| Authors: | D J Waters J M Moore |
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| Institution: | (1) Geology Department, University of Cape Town, 7700 Rondebosch, South Africa |
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| Abstract: | Three kornerupine occurrences are reported in distinctive SiO2-poor, MgO- and Al2O3-rich paragneisses from the Namaqualand Metamorphic Complex in South Africa. Kornerupine coexists stably with phlogopite, cordierite, orthopyroxene, gedrite, sapphirine, sillimanite and plagioclase and, in sapphirine-free rocks, with spinel and corundum. Tourmaline of a texturally older generation than kornerupine is commonly present in the same samples.Ten analysed kornerupines show a variation in total Fe as FeO from 1.8 to 10.9 weight per cent. B2O3 contents are estimated from x-ray data and a few spectrochemical analyses to range from 0.9 to 3.5 weight per cent. There is a strong inverse correlation between B3+ and Al3+. Total iron content has a strong and systematic effect on refractive index, colour and dispersion. Fe and Mg are systematically partitioned with the other minerals, and Mg/(Mg+Fe) ratios increase as follows: spinel orthopyroxene< sapphirine  kornerupinetourmalinephlogopite
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