Abstract

Numerous Mesoproterozoic alkaline intrusions belonging to the Pilanesberg Alkaline Province are present within the Transvaal sub-basin of the Kaapvaal Craton. The Pilanesberg Complex is the best-known example; it represents one of the world’s largest alkaline complexes, and is associated with a northwest-southeast trending dyke swarm that extends from Botswana to the southwest of Johannesburg. This paper documents the results of a petrological and geochemical study of a thin mafic sill (here referred to as an alkaline igneous body, AIB), which intrudes the ca. 2 200 Ma Silverton Formation close to the southernmost part of the Pilanesberg dyke swarm. The AIB has only been observed in cores from a borehole drilled close to Carletonville. It is hypocrystalline, containing randomly oriented elongated skeletal kaersutite crystals and 6 to 8 mm varioles mainly composed of radially oriented acicular plagioclase. These two textures are related to undercooling, probably linked to the limited thickness (70 cm) of the AIB coupled with a probable shallow emplacement depth. Ar-Ar dating of the kaersutite gives an age of ca. 1 400 Ma, similar to the age of Pilanesberg Complex. However, the AIB is an alkaline basaltic andesite and is thus notably less differentiated than the Pilanesberg Complex and some of its associated dykes, such as the Maanhaarrand dyke, for which we provide whole-rock geochemical data. Literature data indicate that the Pilanesberg dyke swarm also contains mafic hypabyssal rocks suggesting a link between the dyke swarm and the AIB. The AIB is characterized by strongly negative εNd and εHf, that cannot be related to crustal contamination, as shown by positive Ti and P anomalies, and the absence of negative Nb-Ta anomalies in mantle-normalised trace element diagrams. The AIB magma is interpreted to have been derived from a long-lived enriched, probably lithospheric mantle reservoir. The AIB thus provides important information on the magma source of the Pilanesberg Alkaline Province.

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