Alkaline Rocks and Carbonatites of the World

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Umat Hill (Bokkos Plug)


Occurrence number: 
Longitude: 8.98, Latitude: 9.37

Rising some 60-70 m above surrounding plains of basalt, Umat Hill consists of a plug of phonolitic rocks with basalts on the lower western flank. The rocks were described as trachyte and basalt by Wright (1969), but later chemistry (Wright, 1976; Irving and Price, 1981) indicated that they range from phonotephrite and tephriphonolite to phonolite (Le Maitre et al., 1989). They are flow banded with anorthoclase phenocrysts, which are exceptionally up to 10 cm in length (Joyce and Game, 1952), and microphenocrysts of zoned aegirine-augite in a groundmass of alkali feldspar, aegirine, magnetite and zeolite. Xenocrysts of andesine, with rims of alkali feldspar, are present and there is a rich assemblage of xenoliths including both mantle and possible cognate types such as syenite and diorite (Wright, 1969). The Umat plug is one of the few occurrences in the world in which mantle xenoliths are found in such felsic rocks. These xenoliths are olivine-rich, Cr-diopside, spinel lherzolite with some pyroxenites. Analyses of rocks with trace element data, including rare earths, will be found in Irving and Price (1981), and Dautria et al. (1983) give analyses of xenolith mineral phases from which the nature of the upper mantle is considered in the light of temperature and pressure estimates. A review of the mineralogy of mantle xenoliths is given by Wright (1976).

DAUTRIA, J.M., GIROD, M. and RAHAMAN, O. 1983. The upper mantle beneath eastern Nigeria: inferences from ultramafic xenoliths in Jos and Biu volcanics. Journal of African Earth Sciences, 1: 331-8.IRVING, A.J. and PRICE, R.C. 1981. Geochemistry and evolution of lherzolite-bearing phonolitic lavas from Nigeria, Australia, East Germany and New Zealand. Geochimica et Cosmochimica Acta, 45: 1309-20.JOYCE, J.R.F. and GAME, P.M. 1952. Note on anorthoclase from Nigeria. Bulletin of the British Museum (Natural History), Mineralogy, 1: 73-93.LE MAITRE, R.W., BATEMAN, P., DUDEK, A., KELLER, J., LAMEYRE, J., LE BAS, M.J., SABINE, P.A., SCHMID, R., SORENSEN, H., STRECKEISEN, A., WOOLLEY, A.R. and ZANETTIN, B. 1989. A classification of igneous rocks and glossary of terms: recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell, Oxford. 193 pp.WRIGHT, J.B. 1969. Olivine nodules and related inclusions in trachyte from the Jos Plateau, Nigeria. Mineralogical Magazine, 37: 370-4.WRIGHT, J.B. 1976. Volcanic rocks in Nigeria. In C.A. Kogbe (ed) Geology of Nigeria, 93-142. Elizabethan Publishing, Lagos.
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