Title: Amphiboles from the Younger Granites of Nigeria. Part I. Chemical classification.
Abstract: Summary. The Younger Granites of Nigeria contain varied assemblages of iron-rich ferromagnesian minerals in which amphiboles are an important component. Eighteen of these amphiboles (separated from rocks belonging to nine complexes) have been chemically analysed and the results are presented and discussed. The amphiboles are shown to fall into two groups, a lime-alkali group (comprised of ferrohastingsite of general formula NaCa2Fe4Fe'A12Si6022(OH)~ ) and an alkali group (of general formula (Na, K, Ca)2.5_a.0(Fe, Fe)5(A1, Si)aO2~(OH)s). The alkali amphiboles are characterized by their extremely low magnesia content, by the presence of Li and F (both in significant amounts), and are of specialinterest because of the unusual amount of ZnO they contain. Optical properties of the amphiboles have been determined but it has not been found possible to use these properties to determine accurately the chemical composition of the alkali amphiboles. It is suggested that, according to current classifications of amphiboles, the analysed alkali amphiboles should be called riebeckitic arfvedsonites or arfvedsonites. The relationship between the hastingsites and the alkali amphiboles is not yet clear. ~ THOUGH a great deal of research has been carried out on the granitic ring-complexes of Northern Nigeria, no systematic study has been made of variations in the chemical composition, optical properties, and structure of the different groups of their ferromagnesian minerals, and in particular of the large group of iron-rich alkali and calcalkali amphiboles, which are associated with fayalite, pyroxene, and biotite. This paper deals mainly with the alkali amphiboles riebeckitic arfvedsonite and affvedsonite--bu t a small number of calc-alkali amphiboles of hastingsite composition are discussed briefly (the latter will be dealt with more fully in a future paper). The Younger Granites occur throughout an area of 25 000 square miles in the Northern Region of Nigeria. About forty of these complexes have been found and a large number mapped in detail ; some are intrusive into the Pre-Cambrian basement complex and most are of uncertain age. 1 Many show a partial or, less commonly, a complete ring structure and may have a number of intrusive phases present, belonging both to 1 Darnley e~ al. (1962) give a mean age of 159~ 25 m.y. to fergusonites from two different localities in the Jos area of Northern Nigeria.
Publication Year: 1962
Publication Date: 1962-01-01
Language: en
Type: article
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