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

摘要

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 (Sr-87/Sr-86= 0.7056 to 0.7064, epsilon Nd = -1.9 to -2.3 and Pb-206/Pb-204 = 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 (tip 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 (Sr-87/Sr-86= 0.7101 to 0.7122, epsilon Nd = -2 to -7, and Pb-206/Pb-204 = 18.74 to 19.10); (2) ol-normative Horingbaai-type with trace element patterns and initial isotope ratios similar to E-MORB (Sr-87/Sr-86= 0.7037 to 0.7050, epsilon Nd = +2 to +6, and Pb-206/Pb-204 = 18.00 to 18.83); and (3) ne-normative magmas, with moderate to high incompatible trace element abundances and intermediate initial isotope ratios (Sr-87/Sr-86= 0.7055 to 0.7059, epsilon Nd = 0 to -2, and Pb-206/Pb-204 = 18.30 to 18.61). The contrast in magmatic intensity between the low- and high-flux margin segments corresponds with systematic compositional differences in contemporary dike swarms. This is supported by geophysical evidence for a change in both the thickness and the average seismic velocity of intruded lover crust at the continent-ocean boundary in the two regions. The greater diversity of magma types and the sharp rise in MgO contents of dikes in northwestern Namibia (maximum 8 weight % MgO for False Bay versus 16 weight % MgO for HOD) suggest that elevated temperatures and active upwelling of a mantle plume caused the enhanced melt production in the north. Dolerite compositions from the False Bay swarm and other localities south of the Orange River are consistent with a passive upwelling model without plume enhancement.