Ngaoundere

The area to the east, and to a lesser extent the west, from the town of Ngaoundere comprises an extensive plateau of basaltic lavas. The earliest flows were erupted into two broad valleys eroded into the Precambrian basement after which continued eruption built up the lava plateau (Fitton, 1987). This activity culminated in the building of the central volcano Tchabal Nganha (No. 17). There are numerous trachyte and phonolite plugs over the whole plateau as well as small outliers of trachyte. The most recent volcanism is represented by lines of cinder cones. All the basic lavas are silica undersaturated ranging from basalts to basanites and some contain peridotite xenoliths and xenocrysts of pyroxene, anorthoclase and rare zircon (Fitton, 1987). Deruelle et al. (1987b) distinguish upper, median and lower flows and have identified brown amphibole in some basalts. The trachytes have phenocrysts of sanidine, aegirine-augite and kaersutite together with less common apatite, magnetite and apatite in a groundmass of sanidine, scarce aegirine-augite and magnetite. Phonolites contain phenocrysts of nosean, sanidine, aegirine-augite and kaersutite set in a matrix of sanidine, aegirine-augite, nepheline and magnetite. Xenoliths of nepheline syenite occur in some plugs and Lee et al. (1996) describe in detail the rock and mineral chemistry of spinel lherzolite and websterite xenoliths. An analysis of phonolite (31.6% ne) is given by Fitton (1987) and Halliday et al. (1988) give Rb-Sr, Sm-Nd, U, Pb and O isotopic data for basanite and alkali basalt. Analyses of 7 basaltic rocks, including major and trace elements and Nd and Sr isotopic data, are given by Marzoli et al. (2000).