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The mineralogy and geochemistry of a subvolcanic alkaline complex from the Cameroon Line: The Nda Ali massif, southwest Cameroon

The Nda Ali massif is one of the 60 Tertiary pluntonic anorogenic complexes known as ‘granites ultimes’ of the Cameroon Line. It forms a plutono-volcanic complex comprising a gabbro-diorite-monzonite-syenite suite and associated basanite-trachyte-phonolite volcanic dykes and lava flows. The mineralo...

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Bibliographic Details
Published in:Journal of African earth sciences (1994) 1996-02, Vol.22 (2), p.113-132
Main Authors: Njonfang, Emmanuel, Moreau, Christian
Format: Article
Language:English
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Summary:The Nda Ali massif is one of the 60 Tertiary pluntonic anorogenic complexes known as ‘granites ultimes’ of the Cameroon Line. It forms a plutono-volcanic complex comprising a gabbro-diorite-monzonite-syenite suite and associated basanite-trachyte-phonolite volcanic dykes and lava flows. The mineralogical and geochemical data indicate: 1. i) Alkaline affinity of both plutonic and volcanic rocks. 2. ii) Early development of peralkalinity in syenites marked by the presence of aegirine (only one sample contains normative acmite); its crystallization after alkali amphibole indicates that the high activity of sodium may be the controlling factor. 3. iii) Penecontemporaneity of trachytes and basanite dykes with plutonic rocks; both are chemically similar. The other lavas form a later volcanic sequence with both undersaturated (tephri-phonolite to phonolites) and saturated (only represented by trachyte domes) trends. 4. iv) Evolution from an undersaturated alkaline magma near syeno-monzonitic or basanitic composition; both rocks have 100× Rb Ba ratios (6.7 and 6.5) in the range of those obtained for mantle liquids. This evolution is dominated by fractional crystallization, the process involving feldspars, pyroxenes, amphiboles and oxides together with apatite for plutonic rocks and sphene for the volcanic sequence. 5. v) Moderate and negligible crustal contamination, respectively, for plutonic rocks and associated dykes and for the undersaturated volcanic trend. Isotopic data, not yet available, are needed here to quantify the effect of crust involvement. Le massif de Nda Ali est l'un des soixantes complexes anorogéniques d'âge Tertiare dénommés ‘granites ultimes’ de la Ligne du Cameroun. C'est un complexe pluto-volcanique comprenant une suite de type gabbrodiorite-monzonite-syénite à laquelle sont associés les dykes volcaniques (basanite-trachyte-phonolite) et les laves. Les données minéralogiques et géochimiques indiquent: 1. i) Une affinité alcaline des roches plutoniques et volcaniques. 2. ii) Un développement précoce de l'hyperalcalinité dans les syénites marqué par la présence de l'aegyrine (un seul échantillon contient de l'acmite normative); sa cristallisation après l'amphibole alcaline indique une grande activité du sodium. 3. iii) Que les dykes (basanite et trachyte) sont pénécontemporains des plutonites, les autres laves formant une séquence tardive avec deux lignées sous-saturée (téphri-phonolite à phonolites) et saturée (représentée seulement par les
ISSN:1464-343X
1879-1956
DOI:10.1016/0899-5362(96)00126-1