Timing of post–Karoo alkaline volcanism in southern Namibia

Abstract

New radiometric age data are reported for alkaline centres in southern Namibia, and are discussed together with published age data in terms of models put forward to account for post-Karoo (Mesozoic-Recent) alkaline magmatism within the African plate. Agreement between K-Ar and Rb-Sr ages indicate emplacement of the Dicker Willem carbonatite in southern Namibia at 49 + 1 Ma. Alkaline rocks associated with the Gross Brukkaros volcano show a discordant radiometric age pattern, but the best estimate for the age of this complex is 77 + 2 Ma, similar to that obtained for the neighbouring Gibeon carbonatite-kimberlite province. The Dicker Willem carbonatite is therefore younger than the Luderitz alkaline province (133 ± 2 Ma), and the Gross Brukkaros volcano, but is older than the Klinghardt phonolite field (29-37 Ma). The new age data argue against a distinct periodicity in alkaline igneous activity in southern Africa, thereby ruling out possible controls by episodic marginal upwarping of the subcontinent. Although the available age data do not appear to be consistent with the passage of one or even two hotspots under southern Namibia, it is argued that the surface expression of hotspots under continents may be so large and overlapping that within-plate magmatism attributed to these thermal anomalies need not necessarily be confined to narrow linear belts or show an age progression. The role of hotspots in continental alkaline magmatism is most likely one of melt generation, while local crustal structure probably controls the distribution and timing of eruption. Major tectonic boundaries in the Precambrian basement underlying southern Namibia seem to have controlled the development of Tertiary alkaline centres in that region.

Volcanological features of a low-viscosity melt: the carbonatitic Gross Brukkaros Volcanic Field, Namibia

“…Rb/Sr age determinations on phlogopites of Blue Hills rocks geochemically similar to Gross Brukkaros carbonatites reveal an Upper Cretaceous isochron age of 75.1B0.6 Ma, which probably also represents the age of the activity around Gross Brukkaros ( 87 Sr/ 86 Sr initial ratiop0.7035; C. B. Smith in: Kurszlaukis 1994). This is comparable to a K-Ar age of 77B2 Ma obtained by Reid et al (1990).…”

The African landscape through space and time

“…(b) Early Neogene paleogeography of Africa. Red triangles/shading = dated volcanic craters/flows [ Thorpe and Smith , ; Ebinger et al , ; Reid et al , ; Woolley , ; Liégeois et al , ; Permenter and Oppenheimer , ; McDougall and Brown , ]; warping of Cenomanian surface represented by stippled pattern <0 km, black line = 1 km, and hatching >2 km [ Sahagian , ]; gray shading = >10 mGal long‐wavelength free‐air gravity anomaly; brown lobes at coastline = major deltas showing age of onset of clastic deposition [ Sestini , ; Nyagah , ; Salman and Abdula , ; Séranne and Nzé Abeigne , ; Anka and Séranne , ; Reijers , ]; numbered red arrows = radiometrically dated marine deposits with average uplift rate given in mm yr −1 [ Partridge and Maud , ; Odada , ; Partridge , ; Van der Wateren and Dunai , ; Roberts and Brink , ; Elmejdoub and Jedoui , ; Guiraud et al , ].…”

“…Observed and calculated uplift histories for six topographic swells. Thick black lines = cumulative uplift history at swell center; solid circles = present‐day height at swell center; other symbols = independent geologic estimates of uplift/denudation; red bars/lines = magmatic activity [ Reid et al , ; Woolley , ; Liégeois et al , ; Permenter and Oppenheimer , ; McDougall and Brown , ]. (a, c) Blue bars = periods of marine limestone deposition [ Goudarzi , ; Swezey , ].…”

The Hoggar swell and volcanism: Reactivation of the Precambrian Tuareg shield during Alpine convergence and West African Cenozoic volcanism

“…In the Namaqualand-Bushmanland and Cape province of western South Africa, numerous small melilitite (with carbonatite-and kimberlite-type) plugs have ages of 77-54 Ma (Duncan et al, 1978;Moore and Verwoerd, 1985;Verwoerd et al, 1990). In Namibia, they are 50-48 Ma (Reid et al, 1990). Rare younger phonolite and melilite occur in southwestern Namibia (37-35 Ma; Kröner, 1973).…”