The Mesoproterozoic event within the Rehoboth Basement Inlier of Namibia: review and new aspects of stratigraphy, geochemistry, structure and plate tectonic setting

The Mesoproterozoic domain of the Rehoboth Basement Inlier (RBI) in central Namibia is composed of sedimentary and volcanic rocks which have been intruded by coeval granites of batholithic dimension as well as numerous quartz-feldspar porphyries and mafic dykes. Review of the geochemical, petrographical and geochronological data in combination with field observations and the analysis of high-resolution geophysical magnetic and radiometric data suggests Mesoproterozoic evolution of this area in three stages. The development of a first basin is recorded by clastic fluvial and limnic sedimentary rocks of the Billstein formation with only minor if any felsic pyroclastic activity. Intrusive porphyry dykes constrain a minimum age of 1210 +/− 7 Ma. The second stage (Nuckopf, Langberg formations, Kartatsaus member), lasting from at least 1.23 Ga to 1.09 Ga is marked by major felsic igneous activity and strong vertical tectonics: ignimbrite deposits as well as rhyolitic flows occur throughout the RBI and suggest several volcanic eruption centres. The calcalkaline Gamsberg Granite Suite represents the plutonic equivalent to this volcanism; geochemically it shows affinity to arc-related igneous rocks. Contemporaneous sediments range from polymict unsorted debris flow deposits to arkoses and cross-bedded feldspathic sandstones deposited in grabens. An intra- or backarc region of an active continental margin is suggested as possible plate-tectonic setting for stage two. The initiation of the third stage (Opdam formation, Kupferberg, Idaberg members) is marked by regional outpouring of basalts deposited subaerial or in shallow water. High Ti-content and flat REE-patterns in the tholeiites suggest their emplacement in an extensional setting, whereas high Th/Ta and La/Nb ratios, low Ce/Pb values and negative anomalies for Nb-Ta require a subduction-style enrichment of the mantle source. These contrasting features are best explained by extensional collapse related to continent-collision further north: Docking of continents led to slab detachment, allowing the interaction between the asthenospheric mantle and the mantle wedge enriched during the subduction process. At regional scale, the mafic rocks are considered to be part of the Umkondo igneous province. Tectono-metamorphism of the Mesoproterozoic domain took place only during the Pan-African Damara orogeny. It is marked by intense south-eastward directed thrusting within an anastomizing network of shear zones which constitute the South Damara shear zone. Open to tight southeasterly verging folds form second order deformation structures. The metamorphism varies from medium grade in the north to very low grade in the south.