Relict clinopyroxenes from metadolerites of the Early Permian Petroi Metabasalt formation, studied by electron microprobe, show a limited compositional range near the diopside-augite boundary in the pyroxene quadrilateral. Clinopyroxene analyses from three metadolerites, grouped in approximately equal Fs contents, define an overall smooth trend between Fs10 and Fs16. This is typical of clinopyroxenes from mildly alkaline basic magmas. Pyroxene stoichiometry suggests high Fe3+ contents (0.04 to 0.20 a.f.u.), which with high Al (0.10 to 0.31 a.f.u.) and Ti (0.03 to 0.08 a.f.u.) implies that CaTiAl2O6 and CaFe3+AlSiO6 are important ‘other components’. Relative Alz in CaFe3+AlSiO6 decreases and consequently CaTiAl2O6 increases with progressive fractionation. This, with the Fe2+:Fe3+ ratios in the Petroi clinopyroxenes, suggests falling ƒO2 in the magma with fractionation. The ƒO2 controlled entry of Alz, Tiy and Nax into the clinopyroxenes and hence the Petroi clinopyroxene trend.

The Pandé massif is a small (4.9×63.4 km) subvolcanic complex of the Cameroon Line striking W – E and intrudes a Panafrican granite basement. It comprises a syenite-granite suite, where coarse- to finegrained syenites are predominant and the granites are the product of residual melt after syenite crystallization, and two volcanic (trachyte-rhyolite and trachyte) sequences. Amphibole and pyroxene are the dominant mafic silicates, the first occurring mainly in rhyolites and coarse- to medium-grained syenites, and the second, principally in all syenites, trachytes and granites. Rare biotite flakes are encountered in the coarse-grained or alkaline syenites and fayalite rimmed with oxides occurs in trachyte from the first volcanic sequence (T1). Apatite and zircon are common accessories, whereas some titanite occurs in the medium-grained syenites. The plutonic rocks are drusy, intrude the first volcanic sequence but pre-date the second (T2).

All the mafic minerals are Fe-rich. Detailed studies of amphibole and pyroxene show that their compositions define relatively limited trends, amphibole varying from ferro-richterite to arfvedsonite and pyroxenes along the acmite-hedenbergite join of the Ac-Hd-Di diagram, in both the intrusive suite and volcanic rocks. Where the two minerals coexist, pyroxene crystallized subsequent to amphibole, a situation generally found in late-stage or subsolidus aegirines. The overlap in plutonic and volcanic pyroxene trends suggests their crystallization from magmas of the same composition. However, the presence of quartz and fayalite in T1 and of pure aegirine in T2 and the occurrence of Zr-bearing aegirine (NaZr0.5Fe0.52+Si2O6) in the early crystallizing alkaline syenites evolving towards pure aegirine from medium- to fine-grained quartz syenites and granites, are consistent with changes in oxygen fugacities during magmatic differentiation. Two stages are distinguished: fO2 increasingly decreased from T1 to alkaline syenite emplacement (from 10−16 to 10−24 bracketed by WM and QFM buffers) where a disequilibrium, probably caused by water dissociation with volatile loss (H2) during magma degassing, favoured crystallization of Zr-bearing aegirine; a decrease in amphibole proportions towards medium-grained quartz syenites and an increase in fO2 from the medium-grained quartz syenites to granites and T2 sequence.

The Mg-poor nature of all the mafic silicates, subsolidus origin of amphiboles, crystallization of pyroxene subsequent to amphibole and subsolidus trends defined by pyroxenes are compatible with the parental magma having itself been a late-stage derivative magma, e.g. the last product of an alkaline melt from which the voluminous Mayo Darlé granite bodies crystallized.