Meeting of Volcanic Studies Group University of Leicester 15 April 1971 Chairman Professor E. A. Vincent.
1. ‘Kenya phonolites’, Kenya ‘phonolites’ and ‘Kenya’ phonolites –a classificatory and petrogenetic problem By G. R. Chapman, Bedford College, University of London
Mapping and 160 new chemical analyses of volcanic rocks from the northern Kenya Rift show that the ‘phonolites’ comprise two distinct types;
(a). Strongly undersaturated aphanitic lavas with abundant groundmass nepheline. Large phenocrysts (4 mm. +) of nepheline and alkali feldspar are usually present. Occasional phyric biotite, pyroxene and, rarely, sphene also occur. Only these lavas, in which modal nepheline exceeds 15%, should be classed as ‘phonolite’.
(b). Less undersaturated ‘trachytoid’ lavas with nepheline or other feldspathoid occuring only as microphenocrysts. The name ‘trachy-phonolite’, defined by the presence of between 10 and 15% modal feldspathoid, should be used for these rocks.
Although the phonolites are strongly alkaline they have high (19% +) alumina contents, with the result that metaluminous examples are as common as peralkaline; normative nepheline averages more than 20% and is invariably more than 14%. In contrast, both alkali and alumina content is lower in the trachyphonolites but this type is usually peralkaline; normative nepheline is less than 10%. Oxide variation diagrams clearly illustrate a chemical hiatus between the two types with respect to silica, alumina and alkalis.
The phonolites are confined to the older part of the succession; the thickest phonolite sequence (1600 m.) occurs on the Kamasia Range where the highest flows are indicated by K–Ar dates