Multistage progressive evolution of rare osumilite‐bearing assemblages preserved in ultrahigh‐temperature granulites from In Ouzzal (Hoggar, Algeria)

Osumilite is reported in Palaeoproterozoic Al–Mg-rich granulites from the Khanfous area (Tekhamalt, In Ouzzal, Hoggar, Algeria). The main peak assemblages are osumilite + sapphirine + biotite + orthopyroxene + sillimanite and osumilite + orthopyroxene + sillimanite + quartz ± biotite (±K-feldspar) in silica-deficient and silica-saturated granulites respectively. Osumilite coexists with F-rich biotite (XF ≈ 0.6). The observed microstructures, the mass balance of metamorphic reactions and P–T pseudosections modelled for bulk-rock and reaction-microdomain compositions indicate a clockwise P–T metamorphic evolution at ultrahigh temperatures, without substantial post-peak deformation. The peak P–T conditions recorded by the osumilite-bearing assemblages are 8.5–9.0 kbar and 930–980 °C. During retrogression, osumilite was partially or totally replaced by fine-grained pseudomorphs of cordierite + orthopyroxene + K-feldspar + quartz at ~7 kbar and ~850 °C. This study confirms that osumilite can occur only in Mg-rich metamorphic rocks that experienced ultrahigh-temperature metamorphism under anhydrous conditions. In the presence of a hydrous fluid, it is replaced, even at high temperatures, by cordierite-bearing assemblages. This important feature explains the rarity of osumilite in granulite facies rocks and its common replacement by cordierite + orthopyroxene + K-feldspar + quartz pseudomorphs. The peak conditions suggest that a delamination of the lithospheric mantle underneath the In Ouzzal crust brought the asthenosphere close to the Mohorovicic discontinuity.