Mount Elgon

Mount Elgon is the largest of the group of Tertiary volcanoes of eastern Uganda, although nearly half of the volcano lies in Kenya. Its height is over 4000 m and the total area covered by volcanic rocks is in excess of 3000 m2. It has a very low profile the slope of the cone being only four degrees. According to Odman (1931) towards the summit there is a 7x8 km depressed area that he considers to represent a caldera (op. cit., Plate 2), although this is neither depicted nor referred to by Davies (1952). In most places the volcanic rocks lie directly upon the petrographically varied Precambrian basement. At a few localities, however, particularly on the western flank, occur basal sediments including coarse breccia, conglomerate, arkose, feldspathic grit, limestone, ashy sediments and tuffs. Most of these sediments are overlain by mudflows which at one locality reach nearly 200 m in thickness. By far the greater bulk of the volcano is built of tuffs and agglomerates with only about 1% composed of lava flows. Blocks in the agglomerates may reach a metre or more across but generally are less than 10 cm. The blocks are principally of nephelinite and phonolite but towards the bottom of the succession basement fragments are found and various types of pyroxenite also occur. Well bedded tuffs are present throughout the succession. More than half the lavas are nephelinite and melanephelinite with the rest phonolite and trachyte. The trachytes are concentrated in the lowest part of the succession and consist of alkali feldspar, which sometimes forms phenocrysts, and the sparse mafic minerals may include amphibole, pyroxene or biotite. The phonolites are also mainly in the lower part of the succession and, like the trachytes, may form large flows. Feldspar varies modally very widely, from 5-70%, and the colour index is similarly variable. Most are aphanitic rocks but occasional phenocrysts of feldspar, nepheline and pyroxene occur, the last varying from aegirine-augite to aegirine. Biotite may also be present as may 'barkevikite' and zircon; apatite and titanite are accessory but opaque oxides are rare. The melanephelinites and nephelinites are approximately equal in abundance and nepheline varies modally between 20 and 80%, the other dominant mineral being pyroxene; both phases may form phenocrysts. The pyroxenes vary from diopsidic varieties to aegirine-augite; barkevikitic amphibole occurs in some flows, as does biotite. Perovskite is present in almost all the lavas, magnetite and ilmenite may form as much as 15%, and pyrite, titanite and apatite are accessory. There is widespread replacement by calcite and zeolite and amygdales of these minerals are common. Melilite nephelinite formed about an eighth of the large collection studied by Davies (1952) and these grade into rocks with olivine in addition. There are also olivine-bearing melanephelinites which are free of melilitite. Plugs, dykes and sills are common over the whole of the mountain and consist most commonly of ijolite or phonolite having mineralogies similar to those of the lavas, but melilite-bearing dykes, called bergalites by Odman (1931), are particularly abundant around the 'caldera'. The general geology is fully described and illustrated by Odman (1931) who supplies considerable petrographical detail and rock analyses, while further descriptions and analyses will be found in Davies (1952). A detailed descriptive account of the opaque minerals in the lavas is given by Odman (1932). Simonetti and Bell (1995) give a full account of the Pb, Nd and Sr isotopes and include whole rock and pyroxene phenocryst analyses.