Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 14.55, Latitude: -21.13

This is a 20 km diameter, circular granite pluton rising to over 2500 m, which is surrounded by a collar of Karoo sediments and lavas that are folded downwards at the contact and metamorphosed and tourmalinised. Sheet-like remnants of the roof, considered to be fragments of a large caldera, are preserved at high topographic levels. The granite is, for the most part, hornblende-bearing with a little biotite and tourmaline, but in places a finer grained variety of granite contains microcline and stellate clusters of arfvedsonite. Dyke rocks include aplite and arfvedsonite-biotite aplite, which were called brandbergite by Chudoba (1930), who gives a rock analysis. Granite from an area described by von Knorring (1986) consists of microcline perthite, quartz, prismatic riebeckite and uraniferous pyrochlore. A highly radioactive variety of granite contains rosettes of aegirine with riebeckite partly replaced by astrophyllite. Pegmatitic varieties contain abundant pyrochlore and zircon together with monazite and fluorite. An area of arfvedsonite-aegirine granite at the southwestern margin of the complex, known as the Amis intrusion, is described by Schmitt et al. (2000), who give feldspar, pyroxene, amphibole, biotite and rock analyses, including Sr and Nd isotope data. Earlier work on this complex is summarised by Martin et al. (1960). Five rock analyses and oxygen isotope data are given by Harris (1995).

K-Ar ages on seven minerals from five rocks ranged from 135.5(1.4 to 125.4(1.4 Ma (Watkins et al., 1994). 40Ar-39Ar dating of biotite, amphibole and astrophyllite ranged from 132 to 130 ma (Schmitt et al., 2000).
CHUDOBA, K. 1930. "Brandbergit", ein neues aplitsches Gestein aus dem Brandberg (SW-Afrika). Centralblatt fur Mineralogie, Geologie und Palaontologie, Abteilung, A: 389-95.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.MARTIN, H., MATHIAS, M. and SIMPSON, E.S.W. 1960. The Damaraland sub-volcanic ring complexes in South West Africa. Report, International Geological Congress, Norden, 13: 156-74.KNORRING, O. VON 1986. A note on tin-tantalum pegmatites in the Damara orogen and alkali rocks associated with the Brandberg complex. Communications of the Geological Survey of South West Africa/Namibia, 2: 63-4.SCHMITT, A.K., EMMERMANN, R., TRUMBULL, R.B., BUHN, B. and HENJES-KUNST, F. 2000. Petrogenesis and 40Ar-39Ar geochronology of the Brandberg complex, Namibia: evidence for a major mantle contribution in metaluminous and peralkaline granites. Journal of Petrology, 41: 1207-39. WATKINS, R.T., MCDOUGALL, I. and LE ROEX, A.P. 1994. K-Ar ages of the Brandberg and Okenyenya igneous complexes, north-western Namibia. Geologische Rundschau, 83: 348-56.
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