Multistage progressive evolution of rare osumilite-bearing assemblages preserved in ultrahigh-temperature granulites from In Ouzzal (Hoggar, Algeria) Z. ADJERID, 1,3 G. GODARD, 2 KH. OUZEGANE 3 AND J.-R. KIENAST 4 1 Ecole Normale Sup erieure D epartement des Sciences Naturelles, B.P. 92, Vieux Kouba, Alger, 16050, Algeria 2 Institut de Physique du Globe, Sorbonne Paris Cit e, Univ. Paris-Diderot, UMR 7154 CNRS, 1 rue Jussieu, Paris Cedex 05, 75238, France (godard@ipgp.fr) 3 Lab. de G eodynamique, G eologie de l’Ing enieur et de Plan etologie, FSTGAT-U.S.T.H.B., B.P. 32 El Alia, Dar el Beida, Alger, 16111, Algeria 4 Univ. Paris-Diderot, Paris 7, France ABSTRACT Osumilite is reported in Palaeoproterozoic Al–Mg-rich granulites from the Khanfous area (Tekha- malt, In Ouzzal, Hoggar, Algeria). The main peak assemblages are osumilite + sapphirine + bio- tite + orthopyroxene + sillimanite and osumilite + orthopyroxene + sillimanite + quartz  biotite (K-feldspar) in silica-deficient and silica-saturated granulites respectively. Osumilite coexists with F-rich biotite (X F  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 defor- mation. 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 ultra- high-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 cordie- rite + 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 Mohorovi ci  c discontinuity. Key words: Al–Mg granulite; F-rich biotite; In Ouzzal; osumilite; ultrahigh-temperature metamorphism. INTRODUCTION Osumilite is an unusual mineral occurring in granu- lite facies rocks formed under medium- to low-pres- sure and high-temperature conditions. The stability of this tectosilicate ((K, Na) (Mg, Fe) 2 (Mg, Fe, Al) 3 (Si, Al) 12 O 30 ; Miyashiro, 1956) requires a high-X Mg chemical environment and, most importantly, extre- mely dry conditions (e.g. Olesch & Seifert, 1981; Motoyoshi et al., 1993; Audibert et al., 1995; Car- rington & Harley, 1995a,b; Holland et al., 1996; Kel- sey et al., 2004; Kelsey, 2008). These are the main reasons why the occurrence of osumilite is so limited in crustal environments. Despite this uniqueness, os- umilite is one of the diagnostic minerals for ultra- high-temperature (UHT) metamorphism, and its mode of occurrence and stability provide constraints on the origin and evolution of UHT metamorphic terranes (e.g. Harley, 2008; Kelsey, 2008). Metamor- phic osumilite was first reported in contact aureoles (Chinner & Dixon, 1973; Berg & Wheeler, 1976; Berg, 1977; Maijer et al., 1977, 1981; Bogdanova et al., 1980; Kars et al., 1980; Jansen et al., 1985; Tobi et al., 1985; Arima & Gower, 1991; Dasgupta et al., 1997; Westphal et al., 2003; Hokada et al., 2008) and in xenoliths (Aramaki, 1961; Schreyer et al., 1983; Komatsu & Fukumoto, 1993; Ettinger et al., 1996; Hacker et al., 2000; Torok, 2007). As for regional-scale metamorphism, osumilite has only been observed in Precambrian granulites from Uganda (Sandiford et al., 1987), the Namaqualand Metamor- phic Complex, South Africa (Nowicki et al., 1995; All mineral abbreviations are after Kretz (1983) with the addition of Osm for osumilite. X Mg , X F and X K are Mg/(Mg + Mn + Fe 2+ ), F/(F + Cl + OH) and K/(K + Na) respectively. X Tsch is the molar proportion of the Tschermak end-members (Mg, Fe, Ca) (Al, Fe 3+ ) [AlSiO 6 ] in orthopyroxene. © 2012 Blackwell Publishing Ltd 1 J. metamorphic Geol., 2013 doi:10.1111/jmg.12031