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

Published online by Cambridge University Press:  05 July 2018

J. Berger ,
N. Ennih ,
J.-C. C. Mercier ,
J.-P. LiéGeois  and
D. Demaiffe
J. Berger*
Affiliation:
Section de Géologie Isotopique, Musée Royal de l’Afrique Centrale, 3080 Tervuren, Belgium Laboratoire de Géochimie Isotopique et Géodynamique Chimique, CP160/02, Université Libre de Bruxelles (U.L.B.), 1050 Brussels, Belgium UMR CNRS 6250 “LIENSs”, Université de La Rochelle, ILE, 2 rue Olympe de Gouges 17042 La Rochelle Cedex- 1, France
N. Ennih
Affiliation:
Laboratoire de Géodynamique, Université d’El Jadida, BP20, 24000 El Jadida, Morocco
J.-C. C. Mercier
Affiliation:
UMR CNRS 6250 “LIENSs”, Université de La Rochelle, ILE, 2 rue Olympe de Gouges 17042 La Rochelle Cedex- 1, France UMR CNRS 6112 “Planétologie et Géodynamique (LPGN)”, Université de Nantes, BP 92205, 2 rue de la Houssinière, 44322 Nantes, France
J.-P. LiéGeois
Affiliation:
Section de Géologie Isotopique, Musée Royal de l’Afrique Centrale, 3080 Tervuren, Belgium
D. Demaiffe
Affiliation:
Laboratoire de Géochimie Isotopique et Géodynamique Chimique, CP160/02, Université Libre de Bruxelles (U.L.B.), 1050 Brussels, Belgium
*
* E-mail: juberger@ulb.ac.be
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Abstract

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

nepheliniteperalkaline phonolitecarbonatiteDaly gaphainitefractional-crystallization modellingMio-Pliocene volcanism.
Type
Research Article
Information
Mineralogical Magazine , Volume 73 , Issue 1 , February 2009 , pp. 59 - 82
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2009

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