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 plutonic 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: i) Alkaline affinity of both plutonic and volcanic rocks. 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. 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. 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. 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.