Genetic and Province-Scale Relationships of Alkaline Leucogranite-Foid Syenite in the Haruchas Dual Complex (South Namibia)

The Haruchas Dual Complex in southern Namibia is part of the Richtersveld Suite and formed by two stocks that comprise coexisting silica-oversaturated alkaline granite and silica-undersaturated foid syenite, respectively. This contribution integrates mapping, petrography, whole-rock and mineral geochemistry, and U-Pb geochronology to constrain the magmatic evolution, crustal assimilation, and post-magmatic alteration of the complex. Three magmatic stages are distinguished: (1) emplacement of the main intrusive bodies between 914–891 Ma; (2) intrusion of a second magma pulse with similar magma composition at 866 Ma; and (3) the intrusion of a dike suite at ∼815 Ma. These intrusion ages confirm the Haruchas Dual Complex as the northernmost and oldest intrusion of the Richtersveld Suite and the coeval emplacement of the two stocks. Geochemical data indicate that the parental magmas were mantle-derived and generated in a within-plate extensional setting, evolving through variable degrees of fractional crystallization and crustal contamination. The southern stock, dominated by leucogranite, records stronger crustal assimilation, whereas the northern stock, composed of varieties of foid syenite, and tawite, exhibits minor contamination, reflecting spatial and temporal heterogeneity in magma evolution. Post-magmatic alteration overprinted the central portion of the northern stock in two stages: an earlier autometasomatic stage related to Na-rich orthomagmatic fluids and characterized by aegirine growth, followed by a later hydrothermal stage associated with trachyte dike emplacement and characterized by replacement of feldspathoids by natrolite and cancrinite. Combined, geochronological and geochemical data indicate a southward migration of the magmatic center within the Richtersveld Suite during early Rodinia-related crustal thinning.