Amphiboles from alkaline rocks along the Cameroon Line, Central Africa: a review

The current work presents the composition and evolutionary trends of amphiboles from alkaline volcanic and subvolcanic complexes along the Cameroon Line (CL). The chemical compositions of amphiboles from the CL cover the domains of calcic, sodi-calcic, sodic, and incidentally ferromagnesian amphiboles. Calcic amphiboles are ubiquitous and cover the full range of composition both in plutonic and volcanic rocks. Sodi-calcic (richterite—Fe-richterite—taramite—Fe-winchite) to sodic amphiboles (arfvedsonite—riebeckite—Fe-eckermannite) are found preferentially in intermediate and felsic rocks, whereas Fe–Mg amphiboles straddle the cummingtonite-grunerite series with Si ranging from 7 to 8 apfu. The crystallization process of amphiboles along the Cameroon Volcanic Line (CVL) is either magmatic or hydrothermal. Two crystallization sequences can be distinguished in the alkaline rocks from the CL: (i) an early sequence related to early magmatic stages during which calcic amphiboles crystallize directly either from magma or from pyroxenes; and (ii) a second sequence characterizes the late stages which corresponds to the crystallization of sodi-calcic amphiboles. These amphiboles are usually associated with clinopyroxene, and the anteriority of one of the above minerals relative to the other might depend on their chemical composition: calcic and sodic-calcic amphiboles crystallize later relative to calcic and sodic-calcic pyroxenes, respectively, while the crystallization of sodic amphiboles is prior to that of sodic pyroxenes. Their main transformations are pseudomorphosis into opaque minerals or pyroxene, chloritisation, and destabilization into calcite.