Permo-Triassic magmatism in the Damaraland Igneous Province, NW Namibia: The Ondurakorume alkaline‑carbonatite complex

The four spatially associated igneous complexes of Kalkfeld, Ondurakorume, Osongombo and Etaneno are sit-
uated within the Damaraland Igneous Province (northwestern Namibia), which formed in response to the rifting
of the South Atlantic during the early Cretaceous. Spatially-resolved LA-ICP MS U-Pb age dating on zircon and
titanite confirms the Cretaceous age for Etaneno (mean of 139.2 ± 6.7 Ma), while Triassic and Permian
emplacement ages are indicated for nepheline syenites from Kalkfeld (249.6 ± 3.2 and 249.4 ± 2.9 Ma) and
Ondurakorume (272.1 ± 1.5 Ma). Furthermore, apatite ages for nepheline syenites from Etaneno (mean of 122.8
± 3.8 Ma) and Kalkfeld (217.4 ± 24.5 Ma), and for carbonatites of Ondurakorume (248.1 ± 4.8 Ma) broadly
agree with the zircon and titanite ages, while apatite from basement marbles yields a presumably metamorphic
age of 479.6 ± 2.6 Ma and 465.1 ± 7.0 Ma.
Detailed petrographic analysis of syenites, nepheline syenites, carbonatites, silicocarbonatites and fenites from
Ondurakorume reveals variable interaction processes between alkaline-silicate rocks and carbonatites. Syenites
and nepheline syenites contain interstitial calcite with burbankite or carbocernaite inclusions (as commonly
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found in calcite carbonatites) and baddeleyite-zircon replacement textures. In some carbonatites and in silico-
carbonatites, local contamination with (nepheline) syenites and granitic basement caused elevated Si activity,
triggering enhanced formation of clinopyroxene, amphibole and mica.
Compositional variations in the released fenitizing fluids are indicated by clinopyroxene compositions that
vary from nearly end-member aegirine (Aeg$_{69-91}$Di$_{0-10}$Hed$_{0-7}$) in proximal fenites to less sodic aegirine-augite
(Aeg$_{54-96}$Di$_{0-17}$Hed$_{0-16}$) in more distal fenite samples, with the latter containing additional sodic amphibole.
Compared to clinopyroxene in nepheline syenites and carbonatites, clinopyroxene in fenites shows elevated Ti
contents (mostly >0.05 apfu Ti) that are highest in distal fenites (up to 0.22 apfu Ti), suggesting Ti mobility.
These changes suggest either a compositional evolution during fluid-rock interaction or two different fluid
sources (carbonatites and (nepheline) syenites, respectively).