Multistage progressive evolution of rare osumilite‐bearing assemblages preserved in ultrahigh‐temperature granulites from In Ouzzal (Hoggar,Algeria) |
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Authors: | Z. Adjerid G. Godard KH. Ouzegane J.‐R. Kienast |
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Affiliation: | 1. Ecole Normale Supérieure Département des Sciences Naturelles, , Alger, 16050 Algeria;2. Lab. de Géodynamique, Géologie de l'Ingénieur et de Planétologie, FSTGAT‐U.S.T.H.B., B.P. 32 El Alia, , Alger, 16111 Algeria;3. Institut de Physique du Globe, Sorbonne Paris Cité, Univ, Paris‐Diderot, UMR 7154 CNRS, , Paris Cedex 05, 75238 France;4. Univ. Paris‐Diderot, , Paris 7, France |
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Abstract: | 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 Mohorovi?i? discontinuity. |
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Keywords: | Al– Mg granulite F‐rich biotite In Ouzzal osumilite ultrahigh‐temperature metamorphism |
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