Mineralogy and origin of a silico carbonatite mega xenolith in the alvikites of the Dicker Willem Complex (Namibia)

The Eocene (49 Ma) Dicker Willem carbonatite complex represents an “Inselberg” (5 km²) forming a subvolcanic intrusion in southern Namibia. It consists of several stages of carbonatitic intrusions including, alvikite, sövite, nepheline sövite as well as carbonatite breccia. Associated silicate rocks (ijolite and nepheline syenite) are rare but found as xenoliths of various sizes embedded in carbonatite (sövite and alvikite). To decipher the genetic relation between carbonatites and silicate rocks and the formation of nepheline sövite (which is seen as a transitional lithology), we investigated a center-margin profile within a silicate-rich carbonatite xenolith (Ø 7 m) enclosed in fine-grained calcite-carbonatite (alvikite). Detailed observations show systematic mineralogical and mineral chemical variations depending on the contact proximity. Multiple element diffusions and transformation reactions can be traced. Mineralogically, feldspathoids are increasingly replaced by K-feldspar. Secondary biotite crystallized at the expense of feldspar and clinopyroxene close to the contact. Clinopyroxene compositions develop from a Ca (Mg, Fe) dominated to a Na (Fe) bearing variety. Excess iron (Fe³⁺) not being incorporated in biotite crystallized as hematite. With closer proximity to the host rock the dominant mineral within the xenolith becomes calcite (up to 95 modal%). These besides other indications suggest that a magmatic replacement of a silicate xenolith resulted in the successive transformation into a carbonatite, that is barely distinguishable from a coarse grained sövite. This process thus reflects the origin of the nepheline sövites rather than the liquid immiscibility that was thought to follow the genetic link between silicate rock and carbonatite at depth.