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

[摘要] 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 peralkalineEtaneno 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- Fefenitization occurred at both complexes and resulted in the formationof aegirine granites and syenites at Kalkfeld and a massive pyroxenefenite at Okorusu. At Okorusu this initial fenitization was followedby extensive brecciation and feldspathization. Syenitic rocks ofKalkfeld also underwent K-metasomatism. Concentrations of hematiteore (Kalkfeld, Osongombo, Okorusu),rare earth minerals (Ondurakorume), apatite (Ondurakorume ) and fluorite (Okorusu) are of potentialor actual economic importance.Plots of oxides and trace elements versus differentiation index, K/Rbplots and plots on diagrams such as Qz-Ne- Kp and Sio2- Az2o3- Na 2o+x2osuggest that f ractionation of feldspar and nepheline (also plagioclaseat Etaneno) occurred during the evolution of the alkaline magmas atthese complexes. The alkaline rocks of Okorusu are much more Na- andvolatile-rich than similar rocks of Kalkfeld and Ondurakorume. These high concentrations might be connected with the relatively highREE contents of the Okorusu alkaline rocks and the relatively lowREE contents of their associated carbonatitic rocks. Most of thecarbonatitic rocks from the Kalkfeld group of complexes are highlyenriched in the rare earth elements. Major and trace element con=tents show that the partly fenitized granite ring at Kalkfeld doesnot, as previously thought, belong to the alkaline suite but thatit represents wallrocks of Salem granite. It is suggested thatat both Kalkfeld and Okorusu sodium loss during fenitization drovethe composition of the magmas from the ijolitic cotectic in the system Na20-AZ2o3- Fe 2o3- sio2, into the oxide field. This resulted inan increase in the precipitation of magnetite as indicated by theincrease 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 oftheir associated carbonatitic rocks. The early phase of Na- metasomatism is thus considered to have been caused by fluids escaping fromthe crystallizing silicate magmas while the later phase of feldspcthization apparently resulted from a K- co2- rich fluid in equilibrium withthe 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 elementcontents vary regularly with the relative age of the intrusions within each complex, as can be expected if fractionation processes wereoperative. Leucocratic globules occur in monchiquite dykes at Okorusu and inhighly altered micaceous rocks at Ondurakorume and Kalkfeld.These globules consist mainly of calcite, sodalite and/or analciteand lesser amounts of K- feldspar and phlogopite . They becomemore siliceous as they increase in size. Their shape and texturesuggest 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) exhibitedby the alkaline and carbonatitic rocks, strongly indicate that thetwo magma types responsible for the alkaline/carbonatite complexes,followed individual fractionation paths after their separation astwo immiscible liquids.Chemical variation diagrams of the available analyses of Damaralandigneous rocks (e . g. total alkalies versus silica), indicate that ahypersthene normative tholeiitic magma was parental to the basiccomp l exes whereas the carbonatitic complexes originated throughthe differentiation of a nephelinitic magma.The major element chemistry of the basic complexes is analogous tothat of islands in the Atlantic Ocean. This, together with thef act that their ages correspond to that of the breakup of Gondwanaland, suggest a similar origin for the tholeiitic magmatism on thecontinent and the nearby magmatic activity of the pristine oceanicridge. 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 inlandthe complexes become alkalic and/or carbonatitic . A similar patternis shown to exist for the predominantly alkaline/carbonatite complexesof Angola; including clusters of kimberlite pipes. In fact, the Damaraland province is just one of a series of parallel linearigneous provinces along the southwest coast of Africa. Theirposition 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 inGreenland, India, North and South America.

[发布机构] Stellenbosch University