Triassic emplacement age of the Kalkfeld complex, NW Namibia: implications for carbonatite magmatism and its relationship to the Tristan Plume

摘要

Rb-Sr whole-rock and mineral isotope data from nepheline syenite, tinguaite, and carbonatite samples of the Kalkfeld Complex within the Damaraland Alkaline Province, NW Namibia, indicate a date of 242 +/- 6.5 Ma. This is interpreted as the age of final magmatic crystallization in the complex. The geological position of the complex and the spatially close relationship to the Lower Cretaceous Etaneno Alkaline Complex document a repeated channeling of small-scale alkaline to carbonatite melt fractions along crustal fractures that served as pathways for the mantle-derived melts. This is in line with Triassic extensional tectonic activity described for the nearby Omaruru Lineament-Waterberg Fault system. The emplacement of the Kalkfeld Complex more than 100 Ma prior to the Parana-Etendeka event and the emplacement of the Early Cretaceous Damaraland intrusive complexes excludes a genetic relationship to the Tristan Plume. The initial epsilon Sr-epsilon Nd pairs of the Kalkfeld rocks are typical of younger African carbonatites and suggest a melt source, in which EM I and HIMU represent dominant components.