Dar el Beïda, Algeria
Dar el Beïda, Algeria

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Adjerid Z.,B.P. 92 | Adjerid Z.,Laboratory Of Geodynamique | Godard G.,CNRS Paris Institute of Global Physics | Ouzegane K.,Laboratory Of Geodynamique | Kienast J.-R.,University Paris Diderot
Journal of Metamorphic Geology | Year: 2013

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 Mohorovičić discontinuity. © 2012 John Wiley & Sons Ltd.

Adjerid Z.,Mohammed V University | Adjerid Z.,Laboratory Of Geodynamique | Godard G.,University Paris Diderot | Ouzegane K.,Laboratory Of Geodynamique
Lithos | Year: 2015

The Ti-N-Eggoleh area (Sérouènout Terrane, Central Hoggar, Algeria) comprises mainly a high-pressure Neoproterozoic metamorphic formation consisting of talc-kyanite-quartz whiteschists, chlorite schists, marbles, sulphide ores, partially serpentinized peridotites and partially amphibolitized eclogites, and reminiscent of an ophiolitic mélange that was metamorphosed and strongly deformed under eclogite-facies conditions. Major and trace elements indicate that the whiteschists underwent intense hydrothermal alteration, with Mg enrichment and leaching of alkalis and Ca, prior to high-pressure metamorphism. The main talc-kyanite-quartz paragenesis is stable within a large multivariant P-T field extending from high-. P amphibolite to eclogite facies; the Tschermak substitution in talc marginally constrains peak pressure conditions to P> 11. kbar and 600. < T< ~ 800. °C. The subsequent development of cordierite. ± sapphirine ± corundum symplectites and coronae at the contact between talc and kyanite was due to isochemical and almost univariant reactions (Tlc + Ky + Qtz → Crd; Tlc + Ky. → Crd + Crn; Tlc + Ky → Crd + Spr) which combined together to produce hybrid microstructures. These metamorphic reactions indicate a thermal overprint under granulite-facies conditions ( T= 650-860. °C; P< 12 kbar). The peraluminous Mg-rich sapphirine observed in the symplectites reaches one of the highest degrees of Tschermak substitution ever reported for this mineral (with n= 3.6), intermediate between the 13:19:5 ( n= 3) and 3:5:1 ( n= 4) theoretical compositions. The neighbouring eclogites record a similar P-T evolution: after the eclogite-facies metamorphic peak, they underwent partial amphibolitization and a subsequent high-temperature overprint under granulite-facies conditions that led to partial dehydration. The Ti-N-Eggoleh series is interpreted as the product of the thermal alteration of oceanic rocks that were subducted prior to the continental collision that formed the West Gondwana orogenic belt during the Neoproterozoic Pan-African orogeny. The decompression associated with the early phase of exhumation was followed by an important increase in temperature towards granulite-facies conditions, possibly determined by the intrusion of abundant mafic rocks in this region due to delamination of the lithospheric mantle. The Ti-N-Eggoleh area and its high-pressure meta-ophiolitic series apparently belong to the Sérouènout Terrane, which stretches along the eastern margin of the Western Gondwana orogenic belt and consists mainly of oceanic metasediments; they are possibly markers of an ancient, yet unidentified, subduction and suture zone. © 2015 Elsevier B.V..

Doukkari S.A.,Laboratory Of Geodynamique | Ouzegane K.,Laboratory Of Geodynamique | Arab A.,Laboratory Of Geodynamique | Kienast J.-R.,IPGP | And 4 more authors.
Journal of African Earth Sciences | Year: 2014

The Tighsi area is situated in the northern part of the Egere-Aleksod terrane (Central Hoggar, South of Algeria). The eclogites and the garnet-amphibolites occur as lenses in marble, metapelite and quartzite. The petrological study shows that the high pressure paragenesis is characterized by the assemblage omphacite-garnet-rutile-quartz and epidote. Omphacite contains epidote, rutile and quartz inclusions. Due to the decompression, omphacite is the first mineral to destabilize into very thin symplectites of albite and clinopyroxene poorer of the jadeitic component. The paragenese of the lower pressure is represented by kelyphites and symplectites of amphibole-plagioclase separating garnet from omphacite and quartz. During this evolution, rutile transforms into ilmenite then, in the latest stage sphene replaces ilmenite. The thermodynamic modeling using THERMOCALC has confirmed these textural observations and suggests that the eclogites have experienced three stages of metamorphism. Stage I is the high pressure association, an H2O under-saturated pseudosection adapted to this stage has been constructed in NCFMASHTO system. The association of high pressure corresponds to the field where garnet is in equilibrium with omphacite including epidote-rutile and quartz; The P-T conditions of 19.6kbar and 694°C have been estimated using the garnet core compositions. Stage II is the isothermal decompression where the amphibole-plagioclase kelyphites occur at the expense of garnet, omphacite and quartz at 15.5kbar and 774°C (conditions estimated using the garnet rim and the plagioclase-amphibole kelyphites compositions). Stage III is the late retrograde evolution; a second pseudosection with H2O in excess has been constructed to model this stage and show the transformation of eclogites into garnet-bearing amphibolites and explains the occurrence of sphene. The P-T conditions of 9.3kbar and 695°C have been estimated using the compositions of the plagioclase with the maximum Xan and the amphibole with the minimum concentration of AlVI. Both pseudosections show a clockwise path for eclogite evolution to garnet-amphibolites. This path characterizes the exhumation that succeeded a subduction event; therefore, the P-T path shows an isothermal decompression. Then, the geothermic gradients became normal so the decrease of pressure is accompanied with a decrease in temperature. © 2014 Elsevier Ltd.

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