Alkaline Rocks and Carbonatites of the World

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Cape Cross


Occurrence number: 
Longitude: 13.97, Latitude: -21.8

Much of the Cape Cross Peninsula is occupied by Karoo lavas and intrusive rocks or obscured by recent deposits, including evaporites, but Gevers (1933) described a small area of alkaline rocks intruding the Karoo that is probably part of a large complex extending beneath the sea. The alkaline rocks form a small hill, which is partly covered by wind-blown sand, and include syenites and tinguaite. The most abundant rock type, and the youngest, is a sodalite microsyenite consisting of perthite, a greenish amphibole, some of which has cores of a colourless pyroxene, a little blue amphibole (?arfvedsonite) and aegirine-augite, biotite, abundant interstitial sodalite, cancrinite and occasional euhedra of nepheline, apatite and titanite. A syenite which forms a small dyke comprises alkali feldspar, some displaying microcline cross-hatching, abundant biotite, a greenish amphibole with occasional patches of blue amphibole, a few needles of aegirine, plentiful titanite and apatite and minor sodalite. These two rock types intrude a tinguaite of similar mineralogy in which the sodalite forms large irregular aggregates. Gevers (1933) gives an analysis of the sodalite microsyenite. There is an oxygen isotope analysis in Harris (1995).

40Ar/36Ar dating of nepheline syenite gave an age of 140.2(1.0 Ma (Milner et al., 1995).
GEVERS, T.W. 1933. Kaoko-eruptives and alkali-rocks at Cape Cross, S.W. Africa. Transactions and Proceedings of the Geological Society of South Africa, 35: 85-96.HARRIS, C. 1995. Oxygen isotope geochemistry of the Mesozoic anorogenic complexes of Damaraland, northwest Namibia: evidence for crustal contamination and its effect on silica saturation. Contributions to Mineralogy and Petrology, 122: 308-21.MILNER, S.C., LE ROEX, A.P. and O’CONNOR, J.M. 1995. Age of Mesozoic igneous rocks in northwestern Namibia, and their relationship to continental breakup. Journal of the Geological Society of London, 152: 97-104.
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