The geochemical evolution of the alkaline and carbonatite complexes of the Damaraland igneous province, South West Africa

Prins, Petre (1978)

Thesis (PhD.) -- University of Stellenbosch, 1978.

Thesis

ENGLISH ABSTRACT: The DamaraZand igneous province &n northern South West Africa contains basalt/granite, alkaline, peralkaline and carbonatite complexes. The geochemical features of the Okorusu, Kalkfeld, Ondurakorwne and Osongombo carbonatite complexes and the peralkaline Etaneno complex form the main thesis of this study . X-ray fluorescence electron microprobe and wet chemical methods were used to establish the behaviour of major and trace elements (Nb , Zr, Y, Sr, Rb, Ba, Ce , Nd, La, CZ and F) during the formation of these complexes. The two largest carbonatite complexes, namely Okorusu and Kalkfeld, consist of confocal rings of syenite and nepheline syenite into which smaller plugs of carbonatite were emplaced. An early phase of Na- Fe fenitization occurred at both complexes and resulted in the formation of aegirine granites and syenites at Kalkfeld and a massive pyroxene fenite at Okorusu. At Okorusu this initial fenitization was followed by extensive brecciation and feldspathization. Syenitic rocks of Kalkfeld also underwent K-metasomatism. Concentrations of hematite ore (Kalkfeld, Osongombo, Okorusu),rare earth minerals (Ondurakorume), apatite (Ondurakorume ) and fluorite (Okorusu) are of potential or actual economic importance. Plots of oxides and trace elements versus differentiation index, K/Rb plots and plots on diagrams such as Qz-Ne- Kp and Sio2- Az2o3- Na 2o+x2o suggest that f ractionation of feldspar and nepheline (also plagioclase at Etaneno) occurred during the evolution of the alkaline magmas at these complexes. The alkaline rocks of Okorusu are much more Na- and volatile-rich than similar rocks of Kalkfeld and Ondurakorume. These high concentrations might be connected with the relatively high REE contents of the Okorusu alkaline rocks and the relatively low REE contents of their associated carbonatitic rocks. Most of the carbonatitic rocks from the Kalkfeld group of complexes are highly enriched in the rare earth elements. Major and trace element con= tents show that the partly fenitized granite ring at Kalkfeld does not, as previously thought, belong to the alkaline suite but that it represents wallrocks of Salem granite. It is suggested that at both Kalkfeld and Okorusu sodium loss during fenitization drove the composition of the magmas from the ijolitic cotectic in the system Na20-AZ2o3- Fe 2o3- sio2, into the oxide field. This resulted in an increase in the precipitation of magnetite as indicated by the increase in modal magnetite in the rocks with lower Na2o contents. The fenitized rocks at Kalkfeld and Okorusu display trace element (Zr, Sr, Rb, Ba, F and Cl ) as well as REE contents which are similar to those of their associated alkaline rocks while the concentration of these elements in feldspathiz ed rocks corresponds to that of their associated carbonatitic rocks. The early phase of Na- metasomatism is thus considered to have been caused by fluids escaping from the crystallizing silicate magmas while the later phase of feldspcthization apparently resulted from a K- co2- rich fluid in equilibrium with the carbonate magma. A similar distinction between Na and K- metasomatism is suggested by an investigation of the chemical trends displayed by analysed fenites from other alkaline/carbonatite complexes. The carbonatite plugs of Kalkfeld and Ondurakorwne consist of multiple intrusions that are of magmatic origin. Their major element contents vary regularly with the relative age of the intrusions within each complex, as can be expected if fractionation processes were operative. Leucocratic globules occur in monchiquite dykes at Okorusu and in highly altered micaceous rocks at Ondurakorume and Kalkfeld. These globules consist mainly of calcite, sodalite and/or analcite and lesser amounts of K- feldspar and phlogopite . They become more siliceous as they increase in size. Their shape and texture suggest that they could have formed by a process of liquid irnmiscibility. This feature , together with the different chemical trends (SrO/CaO and CaO + MgO versus a differentiation parameter) exhibited by the alkaline and carbonatitic rocks, strongly indicate that the two magma types responsible for the alkaline/carbonatite complexes, followed individual fractionation paths after their separation as two immiscible liquids. Chemical variation diagrams of the available analyses of Damaraland igneous rocks (e . g. total alkalies versus silica), indicate that a hypersthene normative tholeiitic magma was parental to the basic comp l exes whereas the carbonatitic complexes originated through the differentiation of a nephelinitic magma. The major element chemistry of the basic complexes is analogous to that of islands in the Atlantic Ocean. This, together with the f act that their ages correspond to that of the breakup of Gondwanaland, suggest a similar origin for the tholeiitic magmatism on the continent and the nearby magmatic activity of the pristine oceanic ridge. The Damaraland igneous province exhibits a change in composition from the margin of the continent towards the interior. Basic complexes are found near the coast whereas further inland the complexes become alkalic and/or carbonatitic . A similar pattern is shown to exist for the predominantly alkaline/carbonatite complexes of Angola; including clusters of kimberlite pipes. In fact, the Damaraland province is just one of a series of parallel linear igneous provinces along the southwest coast of Africa. Their position can be correlated with unique fracture zones which offset the Mid Atlantic ridge and their petrologic variation is tentatively ascribed to warp axes parallel to the continental margin. The same relationship seems to exist between plate tectonic rupturing of a continent and linear zones of igneous activity in Greenland, India, North and South America.

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