The mafic mineralogy of the Pande massif, Tikar plain, Cameroon: implications for a peralkaline affinity and emplacement from highly evolved alkaline magma

摘要

The Pande massif is a small (4.9 x 3.4 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 fine-grained 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 (NaZr_0.5Fe_0.5~(2+)Si_2O_6) 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: f_(O_2), increasingly decreased from T1 to alkaline syenite emplacement (from l0~(-16) to l0~(-24) bracketed by WM and QFM buffers) where a disequilibrium, probably caused by water dissociation with volatile loss (H_2) during magma degassing, favoured crystallization of Zr-bearing aegirine; a decrease in amphibole proportions towards medium-grained quartz syenites and an increase in f_(O_2) 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 Darle granite bodies crystallized.