Osumilite and a spinel + quartz association in garnet-sillimanite gneiss from Rundvågshetta, Lützow-Holm Complex, East Antarctica

This is the first report of osumilite occurring as fine isolated inclusions within garnet porphyroblasts, as observed in garnet-sillimanite gneiss from Rundvågshetta, Lützow-Holm Complex, East Antarctica. The osumilite is characterized by high Si content (10.60 and 10.95 atoms based on 30 oxygens per formula unit), low Al content (2.99 and 3.82), a high content of M site-occupying cations (2.51 and 3.03), and high X_Mg values (about 0.81). We also report a spinel+quartz association found as inclusions within garnet porphyroblasts. Spinel grains, which are in direct contact with quartz and are spatially associated with sillimanite, show extremely high Zn contents (X_Zn^Spl=0.33-0.46) and high X_Mg values (0.45-0.54). The garnet is rimmed by sillimanite, K-feldspar, plagioclase, and quartz. Biotite and cordierite are found only as inclusions within garnet porphyroblasts, where biotite coexists with spinel-quartz or with rutile. Porphyroblastic garnet contains rutile needles and has low X_Mg values (about 0.36). The sillimanite contains a high Fe content (about 1.2wt.% Fe₂O₃).The occurrence of osumilite and spinel + quartz indicates a clockwise pressure-temperature path of ultrahigh-temperature metamorphism, involving the following events: (1) the Rundvågshetta granulites suffered prograde metamorphism within the kyanite and sapphirine. + quartz fields; (2) subsequent retrograde metamorphism, involving near-isothermal decompression, occurred in the orthopyroxene + sillimanite + quartz field; (3) the granulites passed through the garnet + cordierite + sillimanite + quartz field during decreasing temperature; (4) the granulites entered the osumilite stability field at around 8. kbar and 950 °C; and (5) the granulites retain a final record of retrograde metamorphism within the biotite + sillimanite + K-feldspar and quartz field at 6.1. kbar and about 830 °C.