New evidence for prograde metamorphism and partial melting of Mg-Al-rich granulites from western Namaqualand, South Africa

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Cordierite-phlogopite gneiss with spectacular coarse pseudomorphs of sillimanite after andalusite occurs as a kind of Mg-Al-rich rock in the Dabeeb-Hytkoras area of the granulite-faces western Namaqualand Metamorphic Complex. The sillimanite pseudomorphs are always fringed by spinel-cordierite coronas, which, in turn, are surrounded by cordierite mono-mineral zones. The spinel-cordierite coronas include minor amounts of sapphirine, corundum and rutile. The matrix comprises mainly cordierite and phlogopite with minor plagioclase and orthopyroxene plus a trace of quartz. Orthopyroxene is often replaced partially by symplectitic intergrowths of phlogopite and quartz. The modes of occurrence and chemical compositions of minerals indicate that the rock followed a prograde metamorphic P-T path from the andalusite to the sillimanite stability fields and a subsequent isobaric cooling path and that partial melting took place during granulite-facies metamorphism resulting in its present geochemical character.
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