South Africa - Inkaba.org
South Africa - Inkaba.org
South Africa - Inkaba.org
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Low-δ 18 O magmas of the Cretaceous Koegel Fontein Complex,<br />
<strong>South</strong> <strong>Africa</strong><br />
Catherine Curtis 1 , Chris Harris 1 , Coenraad de Beer 2 , David Reid 1 , Robert B. Trumbull 3 ,<br />
Rolf Romer 3<br />
1. Department of Geological Sciences, University of Cape Town, Rondebosch, <strong>South</strong> <strong>Africa</strong><br />
2. Council for Geoscience, Bellville, <strong>South</strong> <strong>Africa</strong><br />
3. GeoForschungsZentrum Potsdam, Telegrafenberg, Potsdam, Germany<br />
ABSTRACT<br />
Koegel Fontein is a relatively unstudied, Cretaceous, large, silicic complex associated with the opening of the<br />
<strong>South</strong> Atlantic. This paper reports O,- H-, Nd-, Pb- and Sr-isotope data for the complex. O- and H-isotope ratios for<br />
Koegel Fontein are unusually low (δ 18 O 6‰) for quartz and quartz<br />
phenocrysts and low δ 18 O values for feldspar separates have been hydrothermally altered. Koegel Fontein has the<br />
lowest known magmatic δ 18 O values in <strong>South</strong>ern <strong>Africa</strong>. O- and Sr-isotope assimilation models and the initial Srisotope,<br />
Pb-isotope and εNd results rule out assimilation as a possible mechanism for the generation of the lowδ<br />
18 O magmas.<br />
We suggest that the source of the low-δ 18 O magmas is low-δ 18 O crust, hydrothermally altered before the<br />
emplacement of the Koegel Fontein Complex. Hydrothermal alteration did occur synchronously with the<br />
emplacement of the complex. The hydrothermal fluid responsible was meteoric in origin with a calculated δ 18 O<br />
value of -9.9‰.<br />
KEYWORDS: Koegel Fontein, Cretaceous, Silicic Complex, Stable Isotopes, Radiogenic Isotopes, Low d18O<br />
Magmas<br />
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