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- W1985742303 abstract "On the Adamawa plateau (Cameroon volcanic line), alkali basalt lava flows are dated at 6.5 ± 0.2 Ma by K-Ar method. Associated phonolitic and trachytic plugs were put in place 6.8 to 6.2 ± 0.2 Ma ago. The most recent volcanic episode aged 0.91 ± 0.06 Ma yielded maars and scoria cones with associated basaltic lava flows. The microlitic groundmass of miaskitic phonolites comprise of euhedral phenocrysts nepheline, anorthoclase, hastingsite, salite-ferrosalite, sphene and apatite. In the agpaïtic phonolites, interstitial anhedral crystals of aegirine-augite, aenigmatite and lavenite mould euhedral phenocrysts of anorthoclase, nepheline, sodalite and noseane. Through the felsic lava series, pyroxene defines an evolutionary trend from salite to aegirine augite with gradual acmite enrichment. Amphibole trend shows a limited range (kaersutite in trachytic lavas and hastingsite in miaskitic phonolites). Genetically, phonolites are linked to trachytes by crystal fractionation mechanism but this mechanism alone cannot account for the overall Na2O and MnO enrichments in lavenite-bearing agpaitic phonolites. At this stage, a migration of dissolved volatilemetal complexes may have accompanied crystal fractionation to produce lavenite: (Na, Ca)2(Mn, Fe2+)(Zr, Nb, Ti)Si2O7(OH, F).Keywords: agpa ïtic phonolites, migration of volatile-metal complexes, laveniteRésuméDans la rgion de Ngaoundr, (plateau de l'Adamaoua, Ligne volcanique du Cameroun), des coules de basaltes alcalins dats 6,5 ± 0,2 Ma sont accompagnes de necks et pitons trachytiques et phonolitiques, phonolites dates 6,8 et 6,2 ± 0,2 Ma par la mthode K-Ar. L'pisode ultime des cnes stromboliens, maars et coules basaltiques remonte 0,91 ± 0,06 Ma. L'ensemble des laves appartient une srie alcaline sodique. La msostase des phonolites miaskitiques englobe des phnocristaux automorphes d'anorthose, nphline, sodalite, nosane, hastingsite, salite-ferrosalite, sphne et apatite. Dans les phonolites agpaïtiques, les cristaux interstitiels xnomorphes de pyroxnes agyriniques, anigmatite et lavnite moulent les cristaux automorphes d'anorthose, sanidine, nphline, sodalite et nosane. Dans l'ensemble des laves felsiques, le pyroxne volue des salite et ferrosalite dans les trachytes et phonolites miaskitiques aux augite agyrinique et agyrine dans les phonolites agpaïtiques. La variation des compositions des amphiboles est limite (kaersutite dans les trachytes hastingsite dans les phonolites). Sur le plan ptrogntique, les phonolites drivent des trachytes par un mcanisme de cristallisation fractionne. A la fin de la diffrenciation, ce mcanisme a t coupl la migration des complexes mtaux-volatiles dissous dans les fluides magmatiques pour former des phonolites hyperalcalines agpatiques lavnite, subnsosilicate complexe de Ti et Zr de formule (Na, Ca)2(Mn, Fe2+)(Zr, Nb, Ti)Si2O7(OH, F). Mots cls: phonolites agpaïtiques, migration des complexes mtaux-volatiles, lavnite African Journal of Science and Technology Vol. 5(2) 2004: 113-123" @default.
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- W1985742303 date "2006-05-17" @default.
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- W1985742303 title "Donnees nouvelles sur les laves felsiques de ngaoundere (Adamaoua, ligne du Cameroun): chronologie K-Ar et petrologie" @default.
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