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.