A layer of alkalic gneiss, 7 to 20 m thick, extends almost continuously for 60 km in the Precambrian Dahomeyan basement complex of southeastern Ghana. Leucocratic nepheline–syenitic gneiss predominates, whereas mafic nepheline syenitic gneiss and rare feldspar-poor nepheline gneiss occur locally. Silica-saturated leucocratic gneiss is intercalated with the undersaturated rocks throughout the layer, but oversaturated peralkaline types are rare. The alkalic rocks are chemically similar to phonolite, alkali trachyte, comendite, mugearite, and ijolite; the leucocratic rocks plot in the experimentally determined low-temperature region for salic magmatic liquids. The alkalic gneiss has been derived by the isochemical metamorphism of pre-existing alkaline igneous rocks; the parental alkalic rocks may have been of volcanic origin.

Regionally associated with the alkalic rocks are mafic gneiss and amphibolite similar in chemical composition to basanite, alkali basalt, and hawaiite. Mafic nepheline gneiss, chemically similar to mugearite, forms a geochemical link between the mafic gneiss and amphibolite and the leucocratic alkaline gneiss. Petrochemical trends on variation diagrams and close field relations suggest that the parent igneous rocks of the alkalic gneiss were derived by fractionation from mafic magmas that were parental to the mafic gneiss and amphibolite.

The alkalic gneiss, mafic gneiss, and amphibolite have undergone at least two stages of regional metamorphic recrystallization that were separated by a period of strong penetrative deformation. The latest recrystallization was in the epidote-amphibolite facies and probably was associated with the Pan-African thermotectonic event.

First Page Preview

First page PDF preview
You do not currently have access to this article.