Alkaline Rocks and Carbonatites of the World

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Ngong Hills


Occurrence number: 
Longitude: 36.68, Latitude: -1.4

The Ngong Hills, 24 km southwest of Nairobi, are the remnants of a volcano that probably had a diameter of about 11 km (Saggerson, 1991). It consists principally of lavas of basanite and tephrite but the small Ngoroi Hill on the eastern side of the complex consists of biotite-olivine melanephelinite (ankaratrite) and feldspar-bearing melanephelinite (tannbuschite). The ankaratrite contains abundant phenocrysts of olivine up to 1 cm across and some of biotite set in a matrix of pyroxene, generally zoned with green terminations of aegirine-augite, magnetite and nepheline. The tannbuschite is similar but for the presence of a little feldspar. The most widespread lavas are basanites, with some nephelinites, which contain large phenocrysts of zoned titanaugite and olivine pseudomorphs in a groundmass of augite, labradorite, magnetite and nepheline with, in some rocks, biotite. The tephrites are similar but lack olivine and contain phenocrysts of barkevikite.

A sample of basanite gave a K-Ar date of 2.58±0.17 Ma (Saggerson, 1991).
SAGGERSON, E.P. 1991. Geology of the Nairobi area. Report, Geological Survey of Kenya, 98: 1-91.
Fig. 3_122 General map showing the distribution of volcanic rocks in south-central Kenya. The areas covered include the Narok and Mara River areas (No. 59), the Suswa area (No. 60), the Nairobi area (No. 62), and the Magadi area (No. 67). The volcanoes of Suswa (No. 61), Ngong (No. 63), Ol Esayeiti (No. 64), Olorgesailie (No. 68), Lenderut (No. 69) and Shombole (No. 70) are also indicated. (based on Geological Map of Kenya, 1:1,000,000, 1987).
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