Carbonate-bearing eruptives between the Great Karas mountains and the Bremen igneous complex, South West Africa

Abstract

Abstract by author:

The bodies contain between 10 and 20 per cent CO- and about 25 per cent SiO2 and are obviously not typical carbonatite. Biotite, pyroxene, amphibole and ilmenite form both phenocrysts and fine-grained crystals in a groundmass of ankerite. Minute ankeritised lath-shaped crystals (either melilite or feldspar originally), are almost invariably present in the lamprophyric carbonate rock. Interstitial quartz and feldspar occur sporadically, whereas inclusions of wall-rock, where present, are usually abraded and rounded. Fenitisation has been observed at two localities, where quartz and feldspar in the wall-rock have been replaced by soda amphibole

Fluidisation appears to provide a satisfactory mechanism for the emplacement of these bodies. Strong evidence in favour of this interpretation are the intrusive contacts, abraded and rounded inclusions, non-dilational veins in the wall-rock, accretionary pisolites, upward and downward movement of inclusions in the bodies, carbonated inclusions and matrices. and the absence of contact or pyrometamorphic effects

Chemically the carbonate rocks bear similarities to kimberlite and olivine-melilitite, but are most akin to alnöite. Carbonatite, olivine-melilitite, kimberlite, alnöite, damkjernite and the Garub rocks are all considered to have the same magmatic affinities. It is tentatively suggested that the Garub suite is genetically related to an unexposed alkaline complex of the Fen type, and that the composition of the carbonate-bearing rocks approaches that of the parent magma of the plutonic complexes along the Kuboos lineament