Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 36.43, Latitude: -1.67

Olorgesailie is the highest mountain in the Magadi section of the East African rift (No. 085-00-067) the whole volcano covering about 15x12 km. Inliers of lava similar to those of Olorgesailie occur on Shanamu to the west (Baker, 1958). The volcano is predominantly formed of lavas, with some agglomerates, that are mainly feldspar- and pyroxene-phyric trachytes with olivine basalt at the foot of the mountain on its eastern side. The summit area consists of ‘nephelinites’ which include abundant nepheline and fewer aegirine-augite phenocrysts in a trachytic groundmass of alkali feldspar, aegirine-augite, aegirine and titanite. Shanamu is built of trachytes, which differ from those of Olorgesailie in being more peralkaline, with aegirine-augite and aenigmatite present in some samples, while basalts are more abundant.

K-Ar on trachybasalt and nephelinite from Olorgesailie gave 2.62 and 2.21 Ma and on trachybasalt and olivine basalt from Shanamu 2.6 and 2.4 Ma (Fairhead et al., 1972). Baker et al. (1971) give a date of 2.7 Ma for a trachyte from Shanamu.
BAKER, B.H. 1958. Geology of the Magadi area. Report, Geological Survey of Kenya, 42: 1-81.BAKER, B.H., WILLIAMS, L.A.J., MILLER, J.A. and FITCH, F.J. 1971. Sequence and geochronology of the Kenya Rift volcanics. Tectonophysics, 11: 191-215.FAIRHEAD, J.D., MITCHELL, J.G. and WILLIAMS, L.A.J. 1972. New K/Ar determinations on rift volcanics of S. Kenya and their bearing on age of rift faulting. Nature Physical Science, 238: 66-9.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith