Magmatism and continental breakup at the west margin of southern Africa: A geochemical comparison of dolerite dikes from northwestern Namibia and the Western Cape

Abstract

This paper presents a comparative geochemical study of Early Cretaceous mafic dike swarms along the rifted volcanic margin of southern Africa from northwestern Namibia to the Cape Peninsula. These dikes are under study to provide a record of magma compositions and conditions of mantle melting at margin segments with high and low magma flux. Our focus is on localities representing the two ends of the spectrum: the Henties Bay-Outjo dike swarm (HOD) in the high magma-flux Etendeka Province of Namibia, and the False Bay dike swarm from the low-flux margin segment in the Western Cape Province, RSA. At False Bay and other localities examined in the south, dikes are low-Ti, tholeiitic dolerites representing a single magma type. Whole-rock MgO contents and olivine compositions in these dikes reach a maximum of 8 weight % and Fo 70 , respectively. Differentiated dikes (MgO = 6 to 2 weight %) show correlations of trace element and isotopic ratios indicating crustal assimilation, but the least-evolved dikes have uniform incompatible trace element ratios and initial Sr-Nd-Pb isotope compositions ( 87 Sr/ 86 Sr= 0.7056 to 0.7064, Nd = -1.9 to -2.3 and 206 Pb/ 204 Pb = 18.53 to 18.59) suggesting a lithospheric mantle source or a mixture of enriched lithosphere and asthenospheric sources. The dolerites from the HOD swarm in Namibia are mainly low-Ti tholeiites like their counterparts in the south but with much higher Mg-contents (up to 16 weight % MgO in whole rock and Fo 86-90 in olivine). The compositional diversity is also greater in the HOD dikes and at least three magma types are recognized: (1) qz-or ol-normative Tafelberg type, with "spiky" mantle-normalized trace element patterns and initial isotope ratios consistent with a lithospheric mantle source and minor crustal input ( 87 Sr/ 86 Sr= 0.7101 to 0.7122, Nd = -2 to -7, and 206 Pb/ 204 Pb = 18.74 to 19.10); (2) ol-normative Horingbaai-type with trace element patterns and initial isotope ratios similar to E-MORB ( 87 Sr/ 86 Sr= 0.7037 to 0.7050, Nd = +2 to +6, and 206 Pb/ 204 Pb = 18.00 to 18.83); and (3) ne-normative magmas, with moderate to high incompatible trace element abundances and intermediate initial isotope ratios ( 87 Sr/ 86 Sr= 0.7055 to 0.7059, Nd = 0 to -2, and 206 Pb/ 204 Pb = 18.30 to 18.61).