Fenitizing Processes Induced by Ferrocarbonatite Magmatism at Swartbooisdrif, NW Namibia

The southeastern margin of the anorthositic Kunene IntrusiveComplex, NW Namibia, has been subsequently invaded by Mesoproterozoicsyenite, nepheline syenite and ferrocarbonatite dykes alongNE- and SE-trending faults. The first generation of carbonatiteintrusions frequently contains fenitized anorthositic wall-rockfragments set in a ferrocarbonatite matrix; later, subordinateveins of massive ferrocarbonatite are almost xenolith-free andcut through the main carbonatite dykes. A mantle source forboth carbonatite generations is constrained by their respectiveoxygen and carbon isotope compositions of ankerite (¹⁸O_SMOW8·91–9·73; ¹³C_PDB –6·98 to–6·76). Na-rich fluids, released from the meltparental to the ferrocarbonatites, caused the fenitization ofboth the incorporated anorthosite xenoliths and the borderinganorthosite, syenite and nepheline syenite. This process ismainly characterized by the progressive transformation of Ca-richplagioclase, K-feldspar and nepheline into albite and/or sodalite.The changing mineral modes indicate that the fenitizing fluidswere sodium-rich and strongly Si-deficient solutions, whichalso contained significant amounts of Sr, Ba, Nb and the lightrare earth elements. On the basis of mineral equilibria studies,it is possible to reconstruct the temperature conditions forcarbonatite emplacement (c. 830 ± 200°C) and recrystallization(c. 480 ± 130°C), and for the metasomatic formationof sodalite (c. 700 ± 70°C). KEY WORDS: anorthosite; fenitization; ferrocarbonatite; sodalite; stable isotopes