Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences



Occurrence number: 
The Wasaki Peninsula
Longitude: 34.4, Latitude: -0.48

Nyamaji is a volcanic complex of about 4x2 km situated in the central part of the Wasaki Peninsula (Le Bas, 1970 and 1977). A basal pyroclastic group is partly covered by 300 m of nephelinitic and phonolitic lavas, which were fed from a reticulated system of dykes. Some 60 phonolite dykes, up to 3 km long and generally 1 to 15 m thick, have been identified and are thought to have been the feeders for the lavas. Le Bas (1970) estimated the volumetric proportions of the volcanic rocks comprising the complex at tuff and agglomerate 17%, trachy-phonolite 23%, porphyritic phonolite 38%, xenolithic phonolite 18% and nephelinite 4%. The pyroclastic rocks comprise phonolitic and nephelinitic tuffs and agglomerates, which are at least 300 m thick near the vent but are much thinner elsewhere. The agglomerates include blocks of ijolite, phonolite, carbonatite and basement rocks. However, some of the agglomerates on the Wasaki Peninsula originated in the Kisingiri volcano, which was active contemporaneously with Nyamaji, and these can be distinguished by the presence of blocks of mugearite and other rock types (Le Bas, 1977). There was also intermingling of Kisingiri and Nyamaji lavas, particularly on the western side of the Wasaki Peninsula. Most of the Nyamaji phonolitic lavas and dykes contain varying combinations of phenocrysts of nepheline, K-feldspar, aegirine-augite zoned to aegirine, melanite rimmed by aegirine, wollastonite and nosean. The groundmass consists of alkali feldspar, nepheline, aegirine-augite, opaque phases and zeolites.

Whole rock K-Ar determinations on phonolite from a lava and from a plug gave 18.3±0.5 and 10.6±0.3 Ma (Le Bas, 1970).

LE BAS, M.J. 1970. A combined central- and fissure-type phonolitic volcano in western Kenya. Bulletin Volcanologique, 34: 518-36.LE BAS, M.J. 1977. Carbonatite-nephelinite volcanism: an African case history. John Wiley, London. 347 pp.

Fig. 3_110 General geology of the Wasaki Peninsula (after Le Bas, 1977, Fig. 10.1).
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