Alkaline Rocks and Carbonatites of the World

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Monduli-Arusha Tuff Cones


Occurrence number: 
Longitude: 36.6, Latitude: -3.37

In the vicinity of Arusha and Monduli nearly 100 small craters, tuff cones and tuff rings have been mapped (Dawson, 1964a and b; Dawson and Pickering, 1964; Dawson et al., 1970; Wilkinson et al., 1983). They are rarely more than 200 m in height and up to 3 km in diameter, though many are a kilometre or less across. They are built principally of welded scoria and lapilli but carbonate-rich pyroclastic rocks are a major component of some. The scoria is generally basaltic in composition but may contain a little nepheline. A number of cones are very rich in a variety of xenoliths including many very fresh ones of mantle origin. Lashaine is a small volcano a little over 200 m high south of Monduli (Dawson, 1964a). A crater is surrounded by a ring of red, welded, olivine-phyric ankaramite scoria which is overlain by calcite-rich tuffs and agglomerates. The latter rocks comprise up to 70% calcite rich in Sr, Ba and La, and include blocks of olivine basalt, red scoria, gneiss, granulite and ultrabasic rocks, the last of which are rarely greater than 30 cm in diameter, most being much smaller. The ultrabasic xenoliths include garnet and spinel peridotites, harzburgite, websterite and dunite and numerous papers have been published describing them including Dawson et al. (1970), Dawson and Smith (1973), Reid et al (1975b), Rhodes and Dawson (1975), Ridley and Dawson (1975), Pike et al. (1980) and Henjes-Kunst and Altherr (1992); granulite xenoliths are described by Jones et al. (1982a). Data on He and Sr isotopes in xenoliths will be found in Porcelli et al. (1986), and of Pb, Nd and Sr isotopes and U, Pb, Sm, Nd, Rb and Sr concentrations in Cohen et al. (1984), while Hutchison and Dawson (1970) give Rb, Sr and 87Sr/86Sr isotope data for xenoliths and host rocks. Harzburgite, clinopyroxene dunite and wehrlite xenoliths containing glass have been described from the Olmani cone 8 km southeast of Arusha (Jones et al., 1983). They are well rounded, up to 30 cm across, and set in ankaramitic agglutinate. Peridotites are described by Jones et al. (1982b) and Rudnick et al. (1993), the latter paper, in particular, including a very full set of major and chemical element data. Brief descriptions of the Lemagur and Loljoro tuff rings and Cameron's Crater are given by Dawson (1964a). The tuff rings are of basaltic fragments with a thin outer layer of calcite-rich tuffs at Lemagur and a calcite matrix to the pyroclastics at Loljoro. Cameron's Crater is a 1 km diameter maar of agglomerates and breccias cemented by calcite. The small conical hills of Mutelelu and Loiwilokwin consist of calcite tuffs containing blocks of basalt and fragments of orthopyroxene.

K-Ar dating of an olivine basalt from Monduli gave 2.09(0.06 and 2.15(0.05 Ma (Evans et al., 1971).
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. 1964a. Carbonate tuff cones in northern Tanganyika. Geological Magazine, 101: 129-37.DAWSON, J.B. 1964b. Carbonatitic volcanic ashes in northern Tanganyika. Bulletin Volcanologique, 27: 81-91.DAWSON, J.B. and PICKERING, R. 1964. Monduli. Geological Survey of Tanganyika, Quarter Degree Sheet, 54.DAWSON, J.B. and SMITH, J.V. 1973. Alkalic pyroxenite xenoliths from the Lashaine volcano, northern Tanzania. Journal of Petrology, 14: 113-31.DAWSON, J.B., POWELL, D.G. and REID, A.M. 1970. Ultrabasic xenoliths and lava from the Lashaine volcano, northern Tanzania. Journal of Petrology, 11: 519-48.EVANS, A.L., FAIRHEAD, J.D. and MITCHELL, J.G. 1971. Potassium-argon ages from the volcanic province of northern Tanzania. Nature, Physical Science, 229: 19-20.HENJES-KUNST, F. and ALTHERR, R. 1992. Metamorphic petrology of xenoliths from Kenya and northern Tanzania and implications for geotherms and lithospheric structures. Journal of Petrology, 33: 1125-56.HUTCHISON, R. and DAWSON, J.B. 1970. Rb, Sr and 87Sr/86Sr in ultrabasic xenoliths and host-rocks, Lashaine volcano, Tanzania. Earth and Planetary Science Letters, 9: 87-92.JONES, A.P., SMITH, J.V., DAWSON, J.B. and HANSEN, E.C. 1982a. Metamorphism, partial melting, and K metasomatism of garnet-scapolite-kyanite granulite xenoliths from Lashaine, Tanzania. Journal of Geology, 91: 143-65.JONES, A.P., DAWSON, J.B. and SMITH, J.V. 1982b. Peridotites from the Olmani scoria cone, N. Tanzania. (Abstract) Terra Cognita, 2: 221.JONES, A.P., SMITH, J.V. and DAWSON, J.B. 1983. Glasses in mantle xenoliths from Olmani, Tanzania. Journal of Geology, 91: 167-78.PIKE, J.E.N., MEYER, C.E. and WILSHIRE, H.G. 1980. Petrography and chemical composition of a suite of ultramafic xenoliths from Lashaine, Tanzania. Journal of Geology, 88: 343-52.PORCELLI, D.R., O'NIONS, R.K. and O'REILLY, S.Y. 1986. Helium and strontium isotopes in ultramafic xenoliths. Chemical Geology, 54: 237-49.REID, A.M., DONALDSON, C.H., BROWN, R.W., RIDLEY, W.I. and DAWSON, J.B. 1975b. Mineral chemistry of peridotite xenoliths from the Lashaine volcano, Tanzania. Physics and Chemistry of the Earth, 9: 525-43.RHODES, J.M. and DAWSON, J.B. 1975. Major and trace element chemistry of peridotite inclusions from the Lashaine volcano, Tanzania. Physics and Chemistry of the Earth, 9: 545-57.RIDLEY, W.I. and DAWSON, J.B. 1975. Lithophile trace element data bearing on the origin of peridotite xenoliths, ankaramite and carbonatite from Lashaine volcano, N. Tanzania. Physics and Chemistry of the Earth, 9: 559-69.RUDNICK, R.L., MCDONOUGH. W.F. and CHAPPELL, B.W. 1993. Carbonatite metasomatism in the northern Tanzanian mantle: petrographic and geochemical characteristics. Earth and Planetary Science Letters, 114: 463-75.WILKINSON, P., DOWNIE, C., CATTERMOLE, P.J. and MITCHELL, J.G. 1983. Arusha. Geological Survey of Tanzania, Quarter Degree Sheet 55: 1:125,000.
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