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Published online 15 June 2009
Mineralogical Magazine; February 2009; v. 73; no. 1; p. 59-82; DOI: 10.1180/minmag.2009.073.1.59
© 2009 Mineralogical Society of Great Britain and Ireland
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Right arrow Articles by Berger, J.
Right arrow Articles by Demaiffe, D.

The role of fractional crystallization and late-stage peralkaline melt segregation in the mineralogical evolution of Cenozoic nephelinites/phonolites from Saghro (SE Morocco)

J. Berger1,2,3,*, N. Ennih4, J.-C. C. Mercier3,5, J.-P. Liégeois1 and D. Demaiffe2

1 Section de Géologie Isotopique, Musée Royal de l'Afrique Centrale, 3080 Tervuren, Belgium
2 Laboratoire de Géochimie Isotopique et Géodynamique Chimique, CP160/02, Université Libre de Bruxelles (U.L.B.), 1050 Brussels, Belgium
3 UMR CNRS 6250 "LIENSs", Université de La Rochelle, ILE, 2 rue Olympe de Gouges 17042 La Rochelle Cedex-1, France
4 Laboratoire de Géodynamique, Université d'El Jadida, BP20, 24000 El Jadida, Morocco
5 UMR CNRS 6112 "Planétologie et Géodynamique (LPGN)", Université de Nantes, BP 92205, 2 rue de la Houssinière, 44322 Nantes, France

* E-mail: juberger{at}ulb.ac.be

The Saghro Cenozoic lavas form a bimodal suite of nephelinites (with carbonatite xenoliths) and phonolites emplaced in the Anti-Atlas belt of Morocco. Despite the paucity of samples with intermediate composition between the two main types of lava (only one phonotephrite flow is reported in this area), whole-rock major element modelling shows that the two main lithologies can be linked by fractional crystallization. The most primitive modelled cumulates are calcite-bearing olivine clinopyroxenites, whereas the final stages of differentiation are characterized by the formation of nepheline-syenite cumulates. This evolution trend is classically observed in plutonic alkaline massifs associated with carbonatites. Late-stage evolution is responsible for the crystallization of hainite- and delhayelite-bearing microdomains, for the transformation of aegirine-augite into aegirine (or augite into aegirine-augite), and for the crystallization of lorenzenite and a eudialyte-group mineral as replacement products of titanite. These phases were probably formed, either by crystallization from late residual peralkaline melts, or by reaction of pre-existing minerals with such melt, or hydrothermal peralkaline fluid.

KEYWORDS: nephelinite, peralkaline phonolite, carbonatite, Daly gap, hainite, fractional-crystallization modelling, Mio-Pliocene volcanism






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