stripes
In the area between the southern end of Lake Natron and the village of Engaruka, but concentrated in the area between the volcanoes of Gelai, Oldoinyo Lengai and Kerimasi, is a concentration of tuff rings, tuff cones and maar type explosion craters (Dawson and Powell, 1969). These authors describe and illustrate the forms of the Oldoinyo Loolmurwak, Kisete and Eledoi craters. Most of these features comprise tuffs, often cemented with carbonate, containing numerous blocks, while large crystals of kaersutite, augite and phlogopite occur in the tuffs surrounding the explosion craters of Kisete, Loluni and Loolmurwak. Small lava flows are found at Oldoinyo Loolmurwak, Armykon Hill and form a small flow north of Lalarasi Hill. The lava flow at Loolmurwak is an olivine-melilite nephelinite with phenocrysts of olivine, partly altered melilite and aegirine-augite, in a groundmass of clinopyroxene, nepheline, magnetite, rare perovskite and glass. The Lalarasi flow is an olivine melilitite with phenocrysts of olivine and aligned melilite in a matrix of glass with some magnetite and perovskite. The flow at Armykon Hill is also an olivine melilitite. Analyses of these three lavas are given by Dawson and Powell (1970) and Dawson et al. (1985) who also give analyses of the mineral phases and further data, including REE. The blocks in the tuff cones comprise basalts, members of the urtite-melteigite series, those occurring on the lower slopes of Kerimasi being considered by Dawson and Powell (1970) to derive from Kerimasi pyroclastic rocks, which the cones cut. A suite of ultramafic mantle xenoliths occurs at Pello Hill and Eledoi Crater which have been the subject of intense study. The xenoliths are up to 30 cm across and include very fresh lherzolite, harzburgite and dunite. Most contain diopside-spinel aggregates and crystals of phlogopite and amphibole. Amphibole, phlogopite, diopside and ilmenite often form vein networks which are described and illustrated in some detail by Dawson and Smith (1988), who give analyses of host rock (katungite), xenoliths and the constituent mineral phases. Johnson et al. (1997) have described in detail ultramafic xenoliths and megacrysts from the Deeti melilitite tuff cone. The upper part of the cone is cemented by carbonate and includes golf ball-sized lapilli which have cores that generally consist of debris from disrupted mantle xenoliths surrounded by concentric layers of carbonate-bearing melilitite then carbonate-rich material. Xenoliths include ijolite and mantle types amongst which some olivine-pyroxene rocks are cut by networks of amphibole and mica; much mineralogical data are given. The whole rock and phase chemistry of olivine-biotite pyroxenite xenoliths (metasomatised upper-mantle peridotite) from the Loluni tuff ring are described by Dawson and Smith (1992). Pb, Nd and Sr isotopes and U, Pb, Nd, Rb and Sr data for spinel lherzolites from Pello Hill and Eledoi Crater are given by Cohen et al. (1984).
COHEN, R.S., O'NIONS, R.K. and DAWSON, J.B. 1984. Isotope geochemistry of xenoliths from East Africa: implications for development of mantle reservoirs and their interaction. Earth and Planetary Science Letters, 68: 209-20.DAWSON, J.B. and POWELL, D.G. 1970. The Natron-Engaruka explosion crater area, northern Tanzania. Bulletin Volcanologique, 33: 791-817. DAWSON. J.B. and SMITH, J.V. 1988. Metasomatised and veined upper-mantle xenoliths from Pelo Hill, Tanzania: evidence for anomalously-light mantle beneath the Tanzanian sector of the East African Rift Valley. Contributions to Mineralogy and Petrology, 100: 510-27.DAWSON. J.B. and SMITH, J.V. 1992. Olivine-mica pyroxenite xenoliths from northern Tanzania: metasomatic products of upper-mantle peridotite. Journal of Volcanology and Geothermal Research, 50: 131-42.DAWSON, J.B., SMITH, J.V. and JONES, A.P. 1985. A comparative study of bulk rock and mineral chemistry of olivine melilitites and associated rocks from east and South Africa. Neues Jahrbuch fur Mineralogie Abhandlungen, 152: 143-75.JOHNSON, L.H., JONES, A.P., CHURCH, A.A. and TAYLOR, W.R. 1997. Ultramafic xenoliths and megacrysts from a melilitite tuff cone, Deeti, northern Tanzania. Journal of African Earth Sciences, 25: 29-42.MACINTYRE, R.M., MITCHELL, J.G. and DAWSON, J.B. 1974. Age of fault movements in Tanzanian sector of East African rift system. Nature, London, 247: 354-6.
