The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line

Abstract

About 60 volcanic-plutonic alkaline complexes of Tertiary age are located along the continental part of the Cameroon Line. This paper reports the chemical nature and evolution of six of them (Gwenfalabo, Ntumbaw, Nkogam, Nda Ali, Bana and Koupe), all located in the southern part of this line. The geochemical data (major, trace and REE) of these six complexes reveal the following general behaviour: • -differentiation from a primary magma of intermediate composition situated in the well-known Daly gap. The absence of any discrimination between the different complexes in geochemical diagrams suggests that they have a unique source. This model is defined by three co-magmatic rock-types: cumulative basic rocks composed of anorthositic gabbros, residual acidic rocks with abundant syenites and granites and primitive intermediate rocks mainly represented by monzodiorites and monozonites of the Ntumbaw complex; • -emplacement of the acidic rocks by successive pulses, as illustrated in the Rb/Sr versus Sr diagram by three subparallel trends, corresponding (with increasing Sr contents) to fayalite syenites, other syenites, other syenites and granites respectively. From Rb/Sr isotopic data, the Nkogam granite is 67 Ma-old, while the fayalite syenite of the Mount Koupe is 46 – 50 Ma-old. Rb/Sr isotopic data of acidic formations of the other complexes and stable isotopes are needed.