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Petrology of the Cenozoic volcanism in the Upper Benue valley, northern Cameroon (Central Africa)

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Abstract

Thirty-one plugs of alkaline volcanic rocks of Cenozoic age (37 Ma in mean) occur in the Upper Benue valley, northern Cameroon (Central Africa). The complete alkaline series (alkaline basalts, hawaiites, mugearites, phonolites, trachytes and rhyolites) is represented. Basalts contain phenocrysts of olivine, Al-Ti-rich diopside, and Ti-magnetite, and hawaiites-abundant microphenocrysts of plagioclase. Mugearites have a trachytic texture and contain xenocrysts of K-feldspar, apatite, quartz and unstable biotite. Phonolites are peralkaline. Trachytes (peralkaline and non-peralkaline) and rhyolites are characterised by their sodic mineralogy with aegirine-augite, richterite, and arfvedsonite phenocrysts. There is a large compositional gap between basaltic and felsic lavas, except the mugearites. Despite this gap, major- and trace-element distributions are in favour of a co-magmatic origin for the basaltic and felsic lavas. The Upper Benue valley basalts are similar in their chemical and isotopic features to other basalts from both the continental and oceanic sectors of the Cameroon Line. The Upper Benue valley basaltic magmas (87Sr/86Sr≈0.7035; ε Nd=+3.9) originate from an infra-lithospheric reservoir. The Sr–Nd isotopic composition and high Sr contents of the mugearites suggest that they are related to mantle-derived magmas and that they result from the mixing, at shallow crustal levels, of a large fraction of trachytic magma with a minor amount of basaltic magma. Major-element modelling of the basalt–trachyte evolution (through hawaiite and mugearite compositions) does not support an evolution through fractional crystallization alone. The fluids have played a significant role in the felsic lavas genesis, as attested by the occurrence of F-rich minerals, calcite and analcite. An origin of the Upper Benue valley rhyolitic magmas by fractional crystallization of mantle-derived primitive magmas of basaltic composition, promoted or accompanied by volatile, halogen-rich fluid phases, may be the best hypothesis for the genesis of these lavas. These fluids also interact with the continental crust, resulting in the high Sr-isotope initial ratios (0.710) in the rhyolites, whereas the Nd isotopic composition has been less affected (ε Nd=+0.4).

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Acknowledgements

The French Ministère de la Coopération is acknowledged for providing grants to I.N. for stays in France in the Laboratoire de Magmatologie et de Géochimie Inorganique et Expérimentale, Université Pierre et Marie Curie, Paris, and for financially supporting the research work. This work is part of a Thèse de l'Université Pierre et Marie Curie (Paris VI) by I.N. Fieldwork (B.D. and I.N.) was substantially supported by the Faculty of Sciences of the University of Yaoundé (Dean, Prof. G. Valet). The isotopic measurements in the Université Libre de Bruxelles were financially supported by the Ministère des Affaires Économiques (projet SGB/NAT 91-98). The constructive comments of J. Keller and an anonymous reviewer are greatly appreciated.

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  1. Département des Sciences de la Terre, Faculté des Sciences, Université de Ngaoundéré, B.P. 454, Ngaoundéré, Cameroun

    I. Ngounouno

  2. Laboratoire de Magmatologie et Géochimie Inorganique et Expérimentale, CNRSESA 7047, IUFM Académie de Versailles, Université Pierre et Marie Curie, 4 Place Jussieu, 75252, Paris cedex 05, France

    B. Déruelle

  3. Laboratoire de Géochimie Isotopique, Université Libre de Bruxelles, 50 Avenue F.D. Roosevelt, C.P. 160/02, 1050, Bruxelles, Belgique

    D. Demaiffe

  4. École et Observatoire de Physique du Globe, UMR CNRS 7516, Université Louis Pasteur, 5 Rue Descartes, 67100, Strasbourg, France

    R. Montigny

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Ngounouno, I., Déruelle, B., Demaiffe, D. et al. Petrology of the Cenozoic volcanism in the Upper Benue valley, northern Cameroon (Central Africa). Contrib Mineral Petrol 145, 87–106 (2003). https://doi.org/10.1007/s00410-002-0438-6

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