Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 8.67, Latitude: 19.05

35 km in diameter, Tamgak is cut by the Meugueur-Meugueur ring-dyke (No. 121-00-004) and is in contact on its southern margin with the Ofoud complex (No. 121-00-004). Early rhyolites and welded tuffs, which overlie basement rocks, are preserved in a screen between an outer ring-dyke of quartz syenite and an inner massif of peralkaline granites. Pre-dating the granites are anorthosites and microdiorites which occur as small bodies along the eastern margin and to the north. Dykes of microsyenite and hastingsite-biotite granophyre cut the basement rocks. The peralkaline granites contain phenocrysts of aegirine and/or riebeckite or ferrohastingsite. The granophyres are peralkaline also with riebeckite and ferrohastingsitic; a little biotite, zircon, titanite, apatite and opaques are present. Porphyries contain phenocrysts of perthite and quartz in a matrix of orthoclase, quartz, arfvedsonite and a little aegirine. The rhyolites are similarly peralkaline with phenocrysts of perthite and ferrohastingsite in a matrix of quartz, orthoclase and aegirine.

A Rb-Sr whole-rock isochron gave 455(19 Ma (Karche and Vachette, 1978) and Moreau et al. (1994) give a Rb-Sr isochron age, based on six rocks, of 414(9 Ma.
BLACK, R., JAUJOU, M. and PELLATON, C. 1967. Notice explicative sur la carte geologique de l'Air à l'échelle du 1/500 000. Direction des Mines et de la Geologie. Republique du Niger, 1-57.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.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.
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith