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

Journal of Metamorphic Geology - Tập 31 Số 5 - Trang 505-524 - 2013
Zouhir Adjerid1,2, Gaston Godard3, Khadidja Ouzegane2, Jean-Robert Kiénast4
1Ecole Normale Supérieure Département des Sciences Naturelles B.P. 92, Vieux Kouba Alger 16050 Algeria
2Lab. de Géodynamique, Géologie de l'Ingénieur et de Planétologie FSTGAT‐U.S.T.H.B., B.P. 32 El Alia Dar el Beida Alger 16111 Algeria
3Institut de Physique du Globe Sorbonne Paris Cité, Univ Paris‐Diderot, UMR 7154 CNRS 1 rue Jussieu Paris Cedex 05 75238 France
4Univ. Paris-Diderot, Paris 7, France

Tóm tắt

AbstractOsumilite 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 Mohorovičić discontinuity.

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Journal of Metamorphic Geology

Tập 31 Số 5

505-524

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