Nda Ali

The Nda Ali complex intrudes the Okoroba granite and Cretaceous sedimentary rocks, which are overlain by trachytic lavas (Njonfang and Moreau, 1996). It comprises a first stage of inward-dipping cumulates of gabbro, diorite and monzonite that are cut by an intrusion of syenite which was followed by emplacement of trachyte dykes, the formation of trachyte domes and tephriphonolite plugs, with dykes and flows of phonolite, benmoreite and trachyte emplaced last (Deruelle et al., 1991). The gabbros are layered with mafic layers comprising either pyroxene or amphibole. The syenites extend over some two thirds of the complex and comprise alkali feldspar, up to 7% quartz, katophorite to eckermannite, minor biotite and, in most rocks, pyroxene that varies from salite through aegirine-augite to aegirine (Njonfang and Moreau, 1996). The trachyte and domes, the latter surrounding the syenites, contain sparse alkali feldspar phenocrysts set in a matrix of alkali feldspar, 14-21% of sodic pyroxene, and a little biotite, quartz and apatite and rare titanite. The tephriphonolite, named by Njonfang and Moreau (1996) from the rock chemistry (10.7% ne), contains phenocrysts of alkali feldspar, kaersutite and aegirine-augite in a matrix of the same minerals plus biotite. The phonolite of the dykes is characterised by the presence of euhedral to subhedral sodalite. The phonolite forming flows has phenocrysts of alkali feldspar, nepheline and less abundant pyroxene, amphibole and titanite in a matrix of the same minerals together with a little biotite. Rock analyses are available in Derruelle et al. (1991) and Njonfang and Moreau (1996), the latter also including analyses of feldspar, pyroxene and amphibole.