Fenitizing Processes Induced by Ferrocarbonatite Magmatism at Swartbooisdrif, NW Namibia

The southeastern margin of the anorthositic Kunene Intrusive Complex, NW Namibia, has been subsequently invaded by Mesoproterozoic syenite, nepheline syenite and ferrocarbonatite dykes along NE- and SE-trending faults. The first generation of carbonatite intrusions frequently contains fenitized anorthositic wall-rock fragments set in a ferrocarbonatite matrix; later, subordinate veins of massive ferrocarbonatite are almost xenolith-free and cut through the main carbonatite dykes. A mantle source for both carbonatite generations is constrained by their respective oxygen and carbon isotope compositions of ankerite (δ18OSMOW 8·91–9·73‰; δ13CPDB −6·98 to −6·76‰). Na-rich fluids, released from the melt parental to the ferrocarbonatites, caused the fenitization of both the incorporated anorthosite xenoliths and the bordering anorthosite, syenite and nepheline syenite. This process is mainly characterized by the progressive transformation of Ca-rich plagioclase, K-feldspar and nepheline into albite and/or sodalite. The changing mineral modes indicate that the fenitizing fluids were sodium-rich and strongly Si-deficient solutions, which also contained significant amounts of Sr, Ba, Nb and the light rare earth elements. On the basis of mineral equilibria studies, it is possible to reconstruct the temperature conditions for carbonatite emplacement (c. 830 ± 200°C) and recrystallization (c. 480 ± 130°C), and for the metasomatic formation of sodalite (c. 700 ± 70°C).