Alkaline Rocks and Carbonatites of the World

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Nkonglong (Mont De L'Elephant) And Lolodorf

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Occurrence number: 
029-00-034
Country: 
Cameroon
Location: 
Longitude: 10.48, Latitude: 3.08

Eight kilometres south-southwest of Nkonglong, at a locality referred to by Champetier de Ribes and Reyre (1959) as 'Mont de l'Elephant', is a conformable, 5 km long lens of nepheline-bearing gneisses depicted on the 1:500,000 geological map (Champetier de Ribes, 1959) as ' 'syenite microclinique a amphibole'. The rock is coarse, with grains up to 3 mm diameter, foliated and gneissose with alternating feldspar, pyroxene and amphibole layers. It is composed of microcline, albite, up to 25% nepheline, aegirine and amphibole with accessory biotite, fluorite, cancrinite, carbonate, lavenite and chevkinite. The amphibole is mboziite, two analyses and optical data for which are given by Kornprobst et al. (1976), who also give analyses of a pyroxene and a whole rock. Peralkaline syenites are also described by Champetier de Ribes and Reyre (1959) from the area northeast of Lolodorf, which is 75 km east-northeast of Nkonglong. These gneisses are dominantly composed of alkali feldspar with prisms of aegirine-augite.

Age: 
A K-Ar age determination on amphibole gave 529(15 Ma (Kornprobst et al., 1976), which may reflect a thermal event later than the time of formation of the nepheline gneiss. A Rb-Sr whole rock isochron on pyroxene-bearing gneisses gave 2890(45 Ma (Lasserre and Soba, 1976).
References: 
CHAMPETIER DE RIBES, G. and REYRE, D. 1959. Notice explicative sur la feuille Yaoundé-Ouest. Carte Géologique de Reconnaissance à l'échelle du 1/500,000. (O21) République du Cameroun. Paris.KORNPROBST, J., CANTAGREL, J.-M., FABRIES, J., LASSERRE, M., ROLLET, M. and SOBA, D. 1976. Existence, au Cameroun, d'un magmatisme alcalin panafricain ou plus ancien; la syénite néphélinique à mboziite de Nkonglong - comparaison avec les roches alcalines connues dans la même région. Bulletin de la Société Géologique de France, 18: 1295-305.LASSERRE, M. and SOBA, D. 1976. Age liberien des granodiorites et des gneiss a pyroxene du Cameroun meridional. Bulletin du Bureau de Recherches Géologiques et Minières. Paris, IV: 17-32.
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith