Aeromagnetic mapping and reconnaissance geochemistry of the Early Cretaceous Henties Bay-Outjo dike swarm, Etendeka Igneous Province, Namibia

摘要

An interpretation of high-resolution aeromagnetic data, backed up by Landsat ETM+ images and field observations, reveals a major NE-trending regional dike swarm in west-central Namibia which we name the Henties Bay-Outjo dike swarm (HOD). The HOD is some 100km wide and extends at least 500km from the continental margin, thus ranking among the regionally important dike swarms on the South Atlantic margins. Field relations and radiometric dates indicate Early Cretaceous emplacement ages for the dikes, contemporary with Etendeka Group flood basalts and with the Damaraland intrusive complexes that occur in the same area. The orientation and distribution of dikes within the HOD suggest a strong influence by Damara Belt structures within the first 100km from the coast. Farther inland, the dikes are more discordant to the Damara Belt and finally the swarm leaves the Damara Belt entirely and crosses into the Angola craton, where dikes fan out to the north and extend for at least another 200km. Geochemical analysis of about 100 dikes distributed throughout the HOD reveals a compositional spectrum ranging from basalt to rhyolite, with the dominant composition being tholeiitic, low-Ti basalt. The basaltic dikes show some compositional diversity, but most can be assigned to known compositional subtypes of the Etendeka Group and are thus likely to represent feeder dikes to now-eroded lava fields. The silicic dikes have compositional variations (metaluminous to peraluminous, 64-76 wt% SiO_2) matching the range found in the Early Cretaceous Damaraland intrusive complexes, and they only marginally overlap with felsic volcanic units of the Etendeka. These dikes are probably related to the silicic magma systems of the Damaraland complexes. We interpret the HOD as the failed arm of a triple junction centered at the shelf edge off Walvis Bay. Late Cretaceous magmatism in Namibia is plume-related, but we believe the triple junction did not result from domal uplift above a plume. The triple junction coincides with the intersection of three Pan-African orogenic belts: the inland Damara Belt and the coast-parallel Gariep/Dom Felic-iano Belt and Kaoko/Ribeira Belt. Mesozoic opening of the South Atlantic propagated northward from the Cape, and when rifting reached the inherited Proterozoic triple junction, extension and magmatism affected all three belts initially but the Damara Belt became inactive shortly thereafter and continental separation followed the coast-parallel belts.