The occurrence of osumilite in pelitic granulites of the Namaqualand Metamorphic Complex, South Africa

摘要

The rare mineral osumilite occurs in Mg-rich metapelitic granulites near Bitterfontein in Namaqualand, South Africa. Associated metapelitic rocks contain the assemblage spinel + quartz which has been used to delineate the upper granulite facies zone of the Namaqualand Metamorphic Complex. The osumilite can be described as very Mg-rich, with most of the iron in the state of Fe~(3+). The presence of osumilite is largely due to a high oxidation state, high Mg/(Mg+Fe~(2+)) ratio, and low H_2O activity. Textural evidence exists for the following three osumilite-forming reactions: biotite + cordierite + K-feldspar = osumilite + me cordierite + orthopyroxene + K-feldspar + quartz = osumilite; biotite + cordierite + quartz = osumilite + orthopyroxene + K-feldspar + melt. Osumilite formation took place at peak metamorphic conditions around 870℃ and 5 kbar, and was followed by approximately isoboric cooling resulting in the freezing-in of mineral equilibria at around 760℃.