Relationships at Oldoinyo Lengai, Tanzania

The younger (<2000 a) nephelinites at Oldoinyo Lengai are cano, and correlated with the Naisiusiu Beds of the Olduvai Gorge succession dated at 22–15 ka (Hay,peralkaline, with alkalinity indices ([Na + K]/Al) ranging from 1·43 in wollastonite nephelinite to 2·15 in combeite nephelinite. 1989), consist of nepheline, clinopyroxene, garnet, ap-atite, magnetite and glass (Donaldson et al., 1987). InThey have a common set of phenocrysts (nepheline, clinopyroxene, Ti-andradite, combeite and apatite) but some phenocrysts and contrast, the nephelinites forming the thin veneer of later black pyroclastics, correlated with the Namorod Ash ofgroundmass phases are different (e.g. wollastonite phenocrysts and groundmass melilite in wollastonite nephelinite, and phenocrystal the Olduvai succession dated at 2000–1250 a (Hay, 1989), and later small flows and scoria cones [the me-sodalite and groundmass lamprophyllite, ‘delhayelite ’ and aegirine-rich pyroxene in combeite nephelinite). Examination of silicate – lanephelinite unit of Dawson (1962a)], are more highly evolved, with higher (Na+K)/Al values; additionally,carbonate spherules from the 1993 eruption showed that na-trocarbonatite unmixed from (formerly) carbonate-saturated they contain wollastonite and/or combeite (Na2Ca2-Si3O9), and the most peralkaline contain even morewollastonite nephelinite, with strong partitioning of alkalis, Ba, Sr and halogens into the carbonate melt. Compared with wollastonite unusual phases such as baryto-lamprophyllite, Na–Sr-rich phosphate and a phase akin to delhayelite (K4Na-nephelinite, combeite nephelinite is enriched in these elements, and the model proposed here is that their enhancement in combeite Ca2[Al2Si6O19]F,Cl,OH) (Dawson & Hill, 1998). Don