Anorthosite first occurrence in Nigeria and relevance to Younger Granite genesis

SUMMARY. Plagioclase crystal fragments and a block of anorthosite occur in small basaltic dykes on the Jos Plateau. The anorthosite is unusual, for the feldspar is labradoritic, but the interstitial mineralogy isgranitic rather than basic, displaying ranophyric ntergrowths inplaces. The anortho-site is probably not an intermediate stage in fractionation from a basic parent magma towards granite, since over 95 ~ of this large province is granitic and the volume of basic magma required would be prohibitively large. Nor did it develop as an intermediate stage in fractionation from a salic parent magma, e.g. syenite, because the plagioclase is too calcic to have crystallized from a syenitic melt. Basic to intermediate dykes are numerous though volumetrically insignificant in the Younger Granite province. The anorthosite may be related to the evolution of small volumes of basaltic magma, which had little to do with Younger Granite magmatism. ANORTHOSITIC intrusions occur in the ring complexes of the 'Younger Granite' province of Air in Niger. Black (I965) regarded them as part of a postulated differentia-tion sequence from basaltic magma through to peralkaline granites: plagioclase cumulates removed alumina from the melt, thus raising the alkali:alumina ratio, so that later liquids evolved through alkali syenites to aegirine- and riebeckite-bearin