F-rich phlogopite stability in ultra-high-temperature metapelites from theNapier Complex, East Antarctica

Fluorine-rich phlogopite [F content up to similar to8 wt%; F/(F + OH) simil ar to0.9] in ultra-high-temperature metapelitic granulites from the Napier Complex, East Antarctica is associated with aluminous orthopyroxene, osumil ite, sapphirine, garnet, and quartz. Textural relationships imply that some or the phlogopite is of primary origin and stable under ultra-high-tempera ture conditions. This is in accord with recent experimental evidence on the stability of F-rich phlogopite. Because the F-rich phlogopite also occurs as rounded inclusions in aluminous orthopyroxene (Al2O3 up to 12.8 wt%), sa pphirine, osumilite, and garnet, it is inferred that the ultra-high-tempera ture mineral assemblages, which includes these minerals formed during progr ade partial melting reactions at the expense of phlogopite, at a depth of l ess than 30 km. Thus the coarse-grained peak metamorphic assemblages formed below 9 kbar, and there is no evidence the rocks underwent any significant degree of decompression during or soon after peak metamorphic conditions. The phlogopite breakdown reactions we suggest on the basis of textural argu ments differ from those postulated from experiments on F-free systems.