Alkaline Rocks and Carbonatites of the World

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Iskou (Askout, Ashkout, Achkout)


Occurrence number: 
Longitude: 8.87, Latitude: 18.07

Emplaced in Precambrian gneisses and granites this complex has an essentially concentric structure and consists of gabbros and syenites, of which the latter are the more abundant. Leucogabbros form inner and outer arcuate intrusions and are characterised by the presence of xenoliths, from 1 to 10 m in diameter, of anorthosite, leuconorite and leucotroctolite. They comprise plagioclase (An52-47), ortho- and clinopyroxene, olivine, opaques, biotite and a little quartz. Rather less abundant monzogabbros contain a lower proportion of plagioclase. At least five distinct syenite units have been mapped by Leger (1985) who distinguishes three mineralogical types. The first includes syenites and granosyenites with hornblende and biotite, zoned oligoclase, perthite and some ferroaugite and fayalitic olivine. The second type includes both syenites with ferroaugite, fayalite and oligoclase (An20), and syenites with ferrohedenbergite, fayalite and perthite with only rare amphibole and biotite. Rocks of the third type, located in the outer part of the complex, are peralkaline syenites and granites. They comprise perthite, altered fayalite, pyroxene zoned from ferrohedenbergite cores to more sodic margins, and amphiboles including ferrobarroisite, ferrorichterite, riebeckite and arfvedsonite. The chemistry of the rocks is discussed by Leger (1985) but analyses are not given. Sr isotope data are in Moreau (1987) and Karche and Vachette (1978).

A whole-rock Rb-Sr isochron gave 427(20 Ma (Karche and Vachette, 1978).
KARCHE, J.-P. and VACHETTE, M. 1978. Age et migration de l'activité magmatique dans les complexes paléozoïques du Niger. Consequences. Bulletin de la Société Géologique de France, 20: 941-53.LEGER, J.-M. 1985. Géologie et évolution magmatique du complexe plutonique d'Iskou (Aïr, Niger). Journal of African Earth Sciences, 3: 89-96.MOREAU, C. 1987. Les complexes anorogéniques à suite anorthositique de l'Aïr (Niger): contribution à l'étude des anorthosites. Bulletin de la Société Géologique de France, 8(3): 115-22.
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