Geochemistry and Geochronology of the Nlonako Alkaline Complex, Central Africa

Abstract

The Nlonako Anorogenic Complex (NAC) is located in the southern continental sector of the Cameroon Line (CL). It intrudes the Cretaceous sediments and the Pan-african granitogneissic basement and is characterized by an association of diversified plutonic and volcanic rocks. Geochemical studies of major, trace and rare earth elements reveal an alkaline series which corresponds to a gabbro-diorite-monzonite-syenite-granite type. This series comprises two suites: a metaluminous suite made of basic and intermediate rocks, fayalite alkali granite and syenites; and a peralkaline suite including rhyolite and other granites. Both suites are derived from a single, hot and dry magma of basaltic composition. The evolution of the magma is dominated by fractional crystallization of mantle material and was progressively affected by crustal contamination at the end of the differentiation process. The conventional whole-rock Rb/Sr method was performed on three syenites and five granites. The ages obtained range between 44.6 and 49.7 Ma, confirming the Tertiary time of emplacement of the NAC.

Keywords: Nlonako Anorogenic Complex (NAC), alkaline series, magma, fractional crystallization, crustal contamination, tertiary age.