Rumpi Hills

Located 80 km north of Mt Cameroon, the Rumpi Hills extend over some 1500 km2 and culminate in Mount Rata at 1768 m. Much of the area is occupied by Precambrian metamorphic and intrusive rocks that are cut by a number of Cainozoic intrusive complexes and overlain extensively by volcanics of similar age. The plutonic rocks are principally olivine gabbro, anorthositic gabbro, monzonite and syenite with equivalent dyke facies. There is a complete series from the more basic rocks to the syenites, in which alkali feldspar and sodic pyroxene are prominent. The volcanic rocks start with a basal series of alkali olivine basalt, trachyte and phonolite, which is followed by further alkali olivine basalts, hawaiite, benmoreite, trachyte, phonolite and rhyolite. Olivine phenocrysts (Fo80) and xenocrysts (Fo90) are present in the basalts while the benmoreites, phonolites, trachytes and rhyolites are characterised by the presence of anorthoclase and sanidine. Kaersutite is present in the trachytes in which arfvedsonite, aegirine and aenigmatite, and in some varieties fayalite and Al-rich ferri-annite, also occur. Jérémine (1943) noted nepheline in the phonolites, together with aenigmatite and aegirine, and Farges et al. (1994) refer to sodalite. Whole rock analyses of both the intrusive and extrusive series are given by Deruelle et al. (1991), of the volcanic rocks by Jérémine (1943) and of a phonolite by Farges et al. (1994). An XAFS study of arfvedsonite and clinopyroxene from a phonolite is reported by Farges et al. (1994).