Alkaline Rocks and Carbonatites of the World

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Mount Kenya

stripes

Occurrence number: 
085-00-058
Country: 
Kenya
Location: 
Longitude: 37.35, Latitude: -0.25

At 5200 m (17,058 feet) the Mount Kenya volcano is the second highest mountain in Africa, although Baker (1967) estimates that originally there must have been a crater at over 6000 m and hence that it was higher than Kilimanjaro. The upper parts of the mountain have been heavily glaciated, the present small glaciers being remnants of a former ice sheet covering some 400 km2. Erosion has cut deeply into the summit area revealing a central plug of nepheline syenite and phonolite which forms spectacular, precipitous peaks (Baker, 1967, Plates 1 and 2). The volcanic rocks that erupted from a central conduit, now occupied by the central plug, and satellite volcanoes on its flanks are referred to by Baker as the Mount Kenya Suite and these occupy an approximately circular area some 100 km across. To the north the Kenya Suite overlaps the volcanic rocks of the Nyambeni Hills (No. 085-00-037) and to the west and southwest overlies volcanic rocks of the Aberdare Range and Laikipia Plateau (No. 085-00-032 ) that occupy part of the eastern shoulder of the Gregory Rift. Elsewhere the Mount Kenya Suite rests on Precambrian gneisses and schists. The only detailed work on Mount Kenya before the comprehensive account of Baker (1967) was that of Gregory (1900) who mounted an expedition in 1893; his work is summarised in his book Rift valleys and geology of East Africa (1921, pp. 144-53). Gregory proposed the name kenyte for a type of glassy phonolite. Baker (1967) divided his Mount Kenya Suite into three extrusive episodes namely an initial main eruptive event, which was followed by a period of fissure eruptions and closed by one in which activity was confined to parasitic vents. Baker summarised the succession and thicknesses of the principal units as in the following Table.The complete succession is not, of course, represented in any one area and is based mainly on observations made on the upper slopes of the mountain, where deeper erosion and better exposure afford clearer evidence. The lower basalts are generally aphyric with only small, usually altered, olivine crystals but specimens of this group described by Baker (1967) from one area contain aegirine-augite and oligoclase-andesine indicating development of an evolved alkaline series. The rhomb porphyries are characterised by the presence of partially absorbed anorthoclase phenocrysts that are usually about 1.5 cm in diameter but may reach 4 cm in some flows. The porphyritic phonolites are similar but also contain nepheline phenocrysts. Zeolite is invariably present together with pyroxene and an opaque phase. These rocks were erupted from a centre near Polish Man’s Tarn and form a 450 m diameter plug of nepheline syenite which is a strongly porphyritic rock comprising 4 cm diameter alkali feldspars in a matrix of alkali feldspar, zoned aegirine-augite, a brown amphibole with aegirine margins, and minor anhedral nepheline. Although mainly lavas there are also pyroclastic rocks in this area of the succession. The most voluminous part of the exposed section of the main volcanic episode comprises porphyritic phonolites, kenytes and agglomerates and these were erupted from the main vent. The kenytes are confined to the vicinity of the central peak and are glassy varieties of the porphyritic phonolite and agglomerates, with every gradation between. The phonolites carry 5-40% of phenocrysts of alkali feldspar (0.5 to 4 cm long) and nepheline (0.25 to 2 cm long), both of which are generally rounded and embayed, and olivine, apatite and aegirine-augite microphenocrysts occur in many rocks. The groundmass is generally very fine-grained but in coarser examples alkali feldspar laths, mossy patches of aegirine, nepheline and aenigmatite can be distinguished and fayalite, sodic amphibole and sodalite may occur (Baker, 1967; Price et al., 1985). The kenytes are confined to the higher slopes of the mountain and near the main vent form flows 3-6 m thick but generally agglomerates are more abundant than flows and there are tuffs containing kenyte bombs. Feldspar phenocrysts are up to 2 cm long but the less abundant nephelines 0.2 to 1 cm; microphenocrysts of aegirine-augite, olivine, magnetite and apatite are generally present. The matrix is usually of flow-banded glass but may be microcrystalline. The phonolitic and trachytic tuffs of the main eruptive episode include trachytes of various types that Baker (1967) describes in some detail in terms of both type and locality. They include agglomerates and tuffs as well as lavas and are interbedded with phonolites. The trachyte lavas generally contain alkali feldspar phenocrysts set in a matrix of albite-oligoclase laths, grains, and in some rocks mossy patches, of aegirine-augite and aegirine and minor sodic amphibole; aenigmatite may be present (Price et al., 1985). One group of trachytes is distinguished by the presence of fayalite and also forms a substantial plug at one locality. Nepheline occurs in some varieties, which grade into phonolites. Another group, distinguished as pantelleritic trachytes by Baker (1967), forms both small local flows and dykes high on the volcano and is characterised by the presence of clusters of riebeckite crystals and pools of quartz. There are minor occurrences of mugearite and olivine basalt, particularly on the upper slopes, and these represent the most recent activity on the mountain. Dykes and sills of all these rock types occur. Price et al. (1985) describe a basanite containing lherzolite nodules up to 5 cm in diameter. The parasitic (satellite) vents are separated from the main suite in time, mode of eruption and, to some extent, spatially and, indeed, some of them may be more closely related to the Nyambeni Hills (No. 085-00-037) volcanism. The main groups are listed in the Table.The Mount Kenya plug (Baker, 1967, Fig. 7) is approximately circular, about 2 km in diameter and exposed vertically for nearly 1000 m. The central nepheline syenite is partially enveloped by an outer phonolite cylinder which is penetrated in places by syenite veins. The nepheline syenite includes a central coarse-grained area, a porphyritic marginal facies and a chilled microsyenite which intrudes adjacent phonolites. The coarsest rock consists of tabular perthite up to 1.5 cm long, nepheline, which forms scarce phenocrysts and interstitial areas, together with poikilitic sodic amphibole enclosing aegirine, pyroxene crystals zoned to aegirine, and aenigmatite; olivine, sodalite and apatite are minor but locally may be major phases, while orange biotite and riebeckite have been found in some samples (Price et al., 1985). Rock (1976a) has described olivine-bearing nepheline syenite xenoliths from the central plug which contain large poikilitic richterite crystals; he gives whole rock and microprobe analyses of the principal mineral phases. Price et al. (1985) give analyses of pyroxene, olivine, spinel, aenigmatite and amphibole, numerous whole rock analyses, including REEs, and 87Sr/86Sr ratios for 26 rocks, and Rock (1976b) also presents a range of initial 87Sr/86Sr ratios. The glaciation of the mountain is discussed by Mahaney (1989) who differs in some of his interpretations from Baker (1967).

Age: 
Baker et al. (1971) obtained K-Ar dates on phonolite and nepheline syenite from the central plug of 3.1 and 2.64 Ma. Rock (1976b) obtained a Rb-Sr whole rock isochron age of 4.5 Ma, based on a range of extrusive and intrusive rocks.
References: 
BAKER, B.H. 1987. Outline of the petrology of the Kenya rift alkaline province. In J.G. Fitton and B.G.J. Upton (eds) Alkaline igneous rocks. 293-311. Geological Society of London, Special Publication 30 and Blackwell Scientific Publications, Oxford. 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.GREGORY, J.W. 1900. The geology of Mount Kenya. Quarterly Journal of the Geological Society of London, 56: 205-22.MAHANEY, W.C. 1989. Glacial advances on Mount Kenya in the Middle Holocene: fact or fiction. In W.C. Mahaney (ed) Quaternary and environmental research on East African mountains. 141-53. Balkema, Rotterdam. PRICE, R.C., JOHNSON, R.W., GRAY, C.M. and FREY, F.A. 1985. Geochemistry of phonolites and trachytes from the summit region of Mt. Kenya. Contributions to Mineralogy and Petrology, 89: 394-409.ROCK, N.M.S. 1976a. Petrogenetic significance of some new xenolithic alkaline rocks from East Africa. Mineralogical Magazine, 40: 611-25.ROCK, N.M.S. 1976b. A comparative strontium isotopic composition of alkaline rocks: new data from southern Portugal and East Africa. Contributions to Mineralogy and Petrology, 56: 205-28.
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith