Alkaline Rocks and Carbonatites of the World

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Ofoud (Enfoud) And Meugueur-Meugueur


Occurrence number: 
Longitude: 8.72, Latitude: 18.82

The largest of the Air complexes, covering 900 km2, the structure of Ofoud is controlled by a peripheral ring fault (Black et al., 1967). According to the mapping of Husch and Moreau (1982) much of the central area of the complex is occupied by basic rocks with peripheral granites, syenites and microsyenites. The basic rocks range from gabbros to anorthosites, the latter consisting of plagioclase and interstitial amphibole, with relict cores of pyroxene, titanomagnetite, with biotite coronas, minor olivine and rare, primary clinopyroxene. At some localities interstitial quartz and alkali feldspar are present. Gabbros and anorthositic gabbros occur towards the apparent stratigraphic base of the basic series (Husch and Moreau, 1982) and contain olivine, clinopyroxene, titanomagnetite, amphibole and biotite interstitial to the plagioclase. Little petrographic information seems to be available on the granites and syenites but sodic amphibole, ferrohastingsite-biotite and biotite granites are found (Black et al., 1967). Extending to the northeast from Ofoud is the Elabag intrusion which is 12x6 km and consists of two phases of hastingsite-biotite granite and a small body of alaskite. Centred on Ofoud, but extending to Tamgak and Taghmeurt, is the Meugueur-Meugueur ring-dyke which Moreau et al. (1986) claim is the world's biggest. The dyke is 65 km in diameter and averages 250 m in thickness. It is composed of a troctolitic gabbro and contains inclusions of leucogabbro, anorthosite and dunite. Moreau et al. (1995) discuss the emplacement of the dyke which, because of its inward dip and magmatic foliation parallel to the contacts, they suggest is a cone-sheet. Rock analyses are given by Husch and Moreau (1982).

A Rb-Sr whole rock and mineral age determination of 430(15 Ma is reported by Bowden et al. (1976), and Moreau et al. (1994) give a Rb-Sr isochron age based on four granites and a syenite from Ofoud of 409(12 Ma.
BOWDEN, P., VAN BREEMEN, O., HUTCHINSON, J. and TURNER, D.C. 1976. Palaeozoic and Mesozoic age trends for some ring complexes in Niger and Nigeria. Nature, London, 259: 297-9.HUSCH, J.M. and MOREAU, C. 1982. Geology and major element geochemistry of anorthositic rocks associated with Paleozoic hypabyssal ring complexes, Air massif, Niger, West Africa. Journal of Volcanology and Geothermal Research, 14: 47-66.MOREAU, C., BROWN, W.L., DEMAIFFE, D., DUPONT, P.-L. and ROCCI, G. 1986. Un des plus grands ring-dykes du monde: le Meugueur-Meugueur, massif de l'Aïr, République du Niger. Comptes Rendus de l’Académie des Sciences, Paris, 302 (II): 223-6.MOREAU, C., DEMAIFFE, D., BELLION, Y. and BOULLIER, A.-M. 1994. A tectonic model for the location of palaeozoic ring complexes in Air (Niger, West Africa). Tectonophysics, 234: 129-46.MOREAU, C., OHNENSTETTER, D., DIOT, H., DEMAIFFE, D. and BROWN, W.L. 1995. Emplacement of the Meugueur-Meugueur cone-sheet (Niger, West Africa), one of the world’s largest igneous ring-structures. In G. Baer and A. Heimann (eds) Physics and chemistry of dykes. 41-9. Balkema, Rotterdam.
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