Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: -8.67, Latitude: 25.98

This is an elliptical intrusion of 7x4.5 km which has a positive relief of 500 m; it is emplaced into Precambrian granites and migmatites of the Hassi-el-Fogra series (Bonin and Lameyre, 1978). Nordmarkite forms the core of the intrusion and is surrounded by an annular body of ditroite. Nordmarkite also forms arcuate intrusions along the western and, particularly, the eastern margins with an intrusion of foyaite lying between the nordmarkite and ditroite in the northeast. All rock types are cut by dykes of riebeckite granite. The nordmarkites consist of microperthite, aegirine, arfvedsonite, biotite and either nepheline or quartz. The foyaites comprise microperthite, nepheline, cancrinite, aegirine and arfvedsonite with the ditroites being distinguished by the presence of sodalite and biotite, instead of arfvedsonite.

A Rb-Sr isochron based on three whole rocks, biotite and feldspar gave an age of 2021(48 Ma (Lameyre and Lasserre, 1967). However, this result is discussed by Cahen et al. (1984) who re-calculate the result as 2091(31 Ma but point out that the data are not acceptable. They conclude that the data can only be taken as reflecting the age of the biotite and thus a younger limit to the age of the rock. This appears to be the oldest date for a silica undersaturated alkaline complex in North Africa.
BONIN, B. and LAMEYRE, J. 1978. Réflexions sur la position et l'origine des complexes magmatiques anorogéniques. Bulletin de la Société Géologique de France, 20: 45-59.CAHEN, L., SNELLING, N.J., DELHAL, J. and VAIL, J.R. 1984. The geochronology and evolution of Africa. Clarendon Press, Oxford. 512 pp.LAMEYRE, J. and LASSERRE, M. 1967. Étude géochronologique des syénites alcalines et néphéliniques du massif annulaire de Hassi-el-Fogra (Mauritanie du Nord). Comptes Rendus Hebdomadaires des l'Académie des Sciences, Paris, 265D: 733-6.
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