Mabounie

Located 40 km east-southeast of Lambarene, the Mabounie complex is emplaced in Precambrian gneisses, migmatites and mica schists, but has a faulted western margin where it is in contact with Cretaceous sediments. A central carbonatite is surrounded to the east by a semi-circular intrusion of syenite around which is a broad arc of fenites. The 1.2 km diameter carbonatite comprises sovite and beforsite which are interleaved with rocks rich in mica and/or magnetite. Xenoliths of phlogopitised basement rocks are common. Apart from major calcite, dolomite, apatite and tetraferriphlogopite, accessory minerals include pyrochlore, ilmenite, olivine, baddeleyite, pyrite and pyrrhotite. The syenites consist of a leucocratic facies of albite, hornblende, which is partly or wholly replaced by magnesio-riebeckite, aegirine and accessory pyrochlore and opaque phases. A melanocratic facies consists of aegirine-augite, microcline, partly replaced by albite, some magnesio-riebeckite and pyrochlore. Both syenite facies contain <0.3% K2O but >10% Na2O. The fenites are characterised by the development of riebeckite and/or arfvedsonite and aegirine. Dykes cutting the syenites and fenites and extending into the country rocks include trachytes, lamprophyres and quartz-baryte rocks. The trachytes contain biotite and an analysis (Laval et al., 1988) includes 13.9% K2O. The lamprophyres have carbonated phenocrysts of phlogopite, olivine and possible clinopyroxene in a matrix of carbonate, aegirine-augite and accessories. Analyses of all major rock types are given by Laval et al. (1988).