stripes
Lying immediately south of Oldoinyo Lengai, the composite volcano of Kerimasi forms a symmetrical cone straddling the fault scarp of the Gregory Rift valley (Guest et al., 1961) and rising approximately 1000 m above the Serengeti Plain. The cone is built principally of nephelinitic agglomerates, tuffs and flows which are surmounted by a limestone layer which thickens towards the summit. Slumping produced a large amphitheatre opening to the southwest which has exposed the earlier nephelinites. These rocks are also exposed in radial gullies cutting through the limestone carapace. The limestones consist mainly of fragments of magnetite sovite, foliated sovite and forsterite sovite in a calcite cement (Dawson, 1964b), but Hay (1983) considered that in areas studied by him 70-80% of the blocks were originally natrocarbonatite, the original nyerereite having been replaced by calcite. Photomicrographs illustrating a range of textural features will be found in Hay (1983). Carbonate lapilli tuffs containing aegirine-augite, magnetite and melilite are found on the lower western slopes (Mariano and Roeder, 1983). Hay (1983) describes a carbonatite-bearing avalanche deposit of 9x5 km on the eastern side of the volcano which is up to 8 m thick and consists of breccias with blocks up to 5 m across of what is considered to have been natrocarbonatite from a lava flow. Dawson (1964b) suggested that carbonate-rich tuffs within the Olduvai Beds and across the Serengeti Plains west of Kerimasi were probably derived from this volcano, as well as from Oldoinyo Lengai. There are analyses of carbonatite, including trace element and Sr, Nd and Pb isotope data, in Paslick et al. (1995) and Kalt et al. (1997), and analyses of carbonatite phases in Dawson et al. (1996b). The craters of Loolmurwak and Kisete to the north and northeast of Kerimasi expose deposits consisting of agglomerates with carbonatite clasts in a matrix of carbonatite ash and lapilli, and air fall tuffs. One tuff in the Kisete crater contains lenticles of natrocarbonatite which, according to Hay (1983), resemble fiamme.
DAWSON, J.B. 1964b. Carbonatitic volcanic ashes in northern Tanganyika. Bulletin Volcanologique, 27: 81-91.DAWSON, J.B., STEELE, I.M., SMITH, J.V. and RIVERS, M.L. 1996b. Minor and trace element chemistry of carbonates, apatites and magnetites in some African carbonatites. Mineralogical Magazine, 60: 415-25.GUEST, N.J., JAMES, T.C., PICKERING, R. and DAWSON, J.B. 1961. Angata Salei. Geological Survey of Tanganyika, Quarter Degree Sheet, 39.HAY, R.L. 1976. Geology of the Olduvai Gorge. University of California Press, Berkeley. 203 pp.HAY, R.L. 1983. Natrocarbonatite tephra of Kerimasi volcano. Tanzania. Geology, 11: 599-602.KALT, A., HEGNER, E. and SATIR, M. 1997. Nd, Sr and Pb isotopic evidence for diverse lithospheric mantle sources of East African Rift carbonatites. Tectonophysics, 278: 31-45.MARIANO, A.N. and ROEDER, P.L. 1983. Kerimasi: a neglected carbonatite volcano. Journal of Geology, 91: 449-55.PASLICK, C., HALLIDAY, A.N., JAMES, D. and DAWSON, J.B. 1995. Enrichment of the continental lithosphere by OIB melts: isotopic evidence from the volcanic province of northern Tanzania. Earth and Planetary Science Letters, 130: 109-26.
