Alkaline Rocks and Carbonatites of the World

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Yelele And Koten Hill


Occurrence number: 
Longitude: 34.55, Latitude: 3.08

Along the Kenya border an area of lavas of about 11x5 km is distributed around a plug centred on Yelele Mountain. Dykes cut both the lavas and basement granulites and gneisses west of the lavas. The lavas and dykes include trachyte, phonolite, nephelinite and olivine nephelinite, with tephrite and basanite only represented amongst the lavas (Nixon and Clark, 1967). The dykes are generally between 0.5 and 1.5 m in thickness but range up to 6 m; they can rarely be followed for more than 200 m. As well as the continuous outcrop around Yelele lavas also cap Koten Hill 13 km to the west, and there are several other outliers. The trachytes consist either of sodic plagioclase phenocrysts in a groundmass of aligned sanidine crystals, or a variety consisting entirely of spherulitic feldspar clusters in a groundmass of a little pyroxene, nepheline and possible olivine. Many of the phonolites contain phenocrysts of euhedral titanaugite with marginal aegirine-augite, up to 50% nepheline, alkali feldspar, titanite and biotite, in a groundmass of nepheline, sanidine, biotite, apatite, calcite and analcime, the last also occurring in veinlets. With decrease of feldspar these rocks grade into nephelinites. Olivine nephelinites are melanocratic rocks of titanaugite, sporadic variably altered olivine and rare barkevikite phenocrysts, in a groundmass of titanaugite, opaques, nepheline and sometimes biotite. The tephrites and basanites have up to one third phenocrysts of zoned titanaugite in a groundmass of labradorite, nepheline or analcime, opaques, apatite, a little biotite and occasional sanidine. The Yelele plug, which is just under a kilometre in diameter, is largely composed of ijolite, but nepheline syenite may be part of an incomplete outer ring. The ijolite, which includes melteigite, consists of nepheline, aegirine-augite, occasionally with titanaugite cores, abundant titanite, opaques, poikilitic reddish-brown biotite, carbonate and perovskite. The nepheline syenites comprise nepheline (30%), aegirine-augite and titanite in a matrix of sanidine, and Smith (1938) found a brown amphibole in one sample. Four rock analyses are given by Nixon and Clark (1967).

NIXON, P.H. and CLARK, L. 1967. The alkaline centre of Yelele and its bearing on the petrogenesis of other eastern Uganda volcanoes. Geological Magazine, 104: 455-72.SMITH, W.C. 1938. Petrographic description of volcanic rocks from Turkana, Kenya Colony, with notes on their field occurrence from the manuscript of Mr. A.M. Champion. Quarterly Journal of the Geological Society of London, 94: 507-53.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith