Rare earth element geochemistry of an anorthosite-diorite suite, Namaqua mobile belt, South Africa

Abstract

A swarm of small dyke-like plutons, ranging in composition from anorthosite through diorite and norite, to hypersthenite and glimmerite, occurs in the western part of the Namaqua mobile belt, South Africa. Known as the Koperberg Suite, these rocks host important magmatic sulphide deposits. Zircon ages of approximately 1100 Ma suggest that the suite was emplaced after the peak of the regional high-grade metamorphism, dated at 1200 Ma. A postulated comagmatic origin for the various rock types is confirmed by the REE distributions of anorthosite, leucodiorite, diorite, hypersthenite and glimmerite. The Koperberg Suite resembles the well-known massif anorthosite complexes, typified by the Adirondack Suite, in most properties except for the dispersed mode of occurrence. It is now shown that the similarity also holds for the REE spectra. Thus, the predominant anorthosite and leucodiorite of the Koperberg Suite are comparable to similar rocks of the Adirondacks and the Burwash area, Ontario, albeit more enriched in LREE. More mafic rock types such as norite, mica-diorite and glimmerite are substantially enriched in LREE compared to mafic differentiates of massif anorthosites. Apatite is abundant in the mafic rocks and stores large proportions of the lanthanides. The occurrence of apatite- and mica-rich assemblages indicates source magmas enriched in K, REE and P. A few intrusives exhibit excessively enriched REE patterns which are attributed to late hydrothermal alteration or metasomatism. The REE data for syenite and two-pyroxene assemblages from the Copper District argue against a genetic relationship to the Koperberg Suite in spite of close spatial association.