Anorthosite—First Occurrence in Nigeria and Relevance to Younger Granite Genesis

J. B. Wright
Department of Earth Sciences, The Open University, Walton Hall, Bletchley, Milton Keynes, Bucks

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 is granitic rather than basic, displaying granophyric intergrowths in places. The anorthosite 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.

Clay Minerals; June 1975 v. 40; no. 310; p. 193-196; DOI: 10.1180/minmag.1975.040.310.08
© 1975, The Mineralogical Society
Mineralogical Society (