Fractionation modes and trace element characteristics of Phalaborwa-type magmas: insights from the bimodal Otjisazu carbonatite complex, Namibia
✍ Scribed by B. Bühn
- Book ID
- 104354072
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 129 KB
- Volume
- 32
- Category
- Article
- ISSN
- 1464-343X
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✦ Synopsis
Dicker Willem, southern Namibia. Coarser-scale 'spinifex' textures have been noted in epizonal dolomite-carbonatite at Nooitgedacht and Marinkas-Quellen, South Africa and in rare earth-rich dolomite-carbonatite at Kangankunde, Malawi. These 'spinifex' textures could be re-interpreted as classically pegmatitic, constrained by the concept of delayed nucleation in a volatile-enriched low viscosity fractionate, producing crystal morphologies equivalent to those found in granite-pegmatites.
The Mesoproterozoic Nooitgedacht carbonatite, Kaapvaal Craton, South Africa (27'30' 25"30') consists of plugs of pyroxenite and ijolite, magnetite pyroxenite, 10 cm to 2 m wide cone sheets of 'phoscorite', cone sheets of magnetite-apatite-calcite carbonatite followed by dolomite and ankerite carbonatite, and then intrusion of nepheline syenite. There are also radial dykes of dolomite carbonatite and a newly discovered plug of mixed dolomite-calcite carbonatite. The mixed dolomitecalcite carbonatite plug may represent the products of delayed nucleation at the top of the same magma chamber. lt consists of large, centimetre diameter ferroan dolomite crystals as an original magmatic phase (according to C and 0 isotope ratios), but the composition of the calcite matrix suggests that it is secondary. Nevertheless, some textures are well preserved and 6 cm long 'spinifex-like' calcite crystals suggest emplacement of a volatile-rich, liquid magma, forming a carbonate pegmatite in the magma-chamber cupola.
However, if 'spinifex-textured' pegmatitic crystallisation, effectively quenching the carbonate phases of liquid and vapour, does not take place in shallow carbonate magma chambers, then other, more destructive, processes could occur. This can be demonstrated at Gross &ukkaros, central Namibia, and to some extent at Dicker Willem in southern Namibia, where outcrops of carbonatite-tuffisite dykes, breccia-pipes and country rock collapse structures, point to magma decarbonisation with explosive release of volatiles from sub-volcanic levels.