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1 Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa
2 College of Oceanography,Oregon State University,Corvallis, USA
Mesozoic igneous rocks of northwestern Namibia comprise continental flood basalts and quartz latites of the Paraná–Etendeka province, and a suite of central sub-volcanic intrusions, the Damaraland complexes. Conventional K–Ar ages published for these rocks range from 377–88 Ma, with the majority of samples yielding ages of 150–110 Ma. This large spread of ages is interpreted as being due to disturbance of the K–Ar system by low-temperature alteration, which is ubiquitous among the volcanic rocks. Rb–Sr and 40Ar/39Ar dating methods, which are less susceptible to such disturbance, yield a narrower range of ages (132–124 Ma, with one outlier at 149 Ma). New Rb–Sr internal isochron ages of 129.8 ± 3.8 Ma, 126.8 ± 1.3 Ma and 126.1 ± 7.3 Ma have been determined for an Etendeka quartz latite, and nepheline syenite intrusions in the Messum and Okorusu complexes, respectively. In addition, 40Ar/39Ar release spectrum ages of 137.0 ± 0.7 Ma, 135.0 ± 0.7 Ma and 126.9 ± 0.6 Ma have been determined for samples from the Paresis (comendite), Cape Cross (nepheline syenite) and Okorusu (nepheline syenite) complexes, respectively.
A 13 Ma period between 137 and 124 Ma is considered to be the best estimate of the timing of Mesozoic igneous activity in northwestern Namibia. The continental flood basalt volcanism was probably limited to the early part of this period and is inferred to have been relatively short-lived (1–2 Ma). Magmatic activity among the Damaraland complexes was contemporaneous with the onset of flood basalt volcanism, but continued for approximately 15 Ma and terminated after the onset of sea floor spreading in this area (130–125 Ma). Both the flood basalt volcanism and the magmatic activity of the Damaraland complexes are thought to have occurred in response to rifting across the upwelling Tristan mantle plume. The linear distribution of intrusions in Damaraland is interpreted as being due to magmatism focused along a structural discontinuity between the Pan-African Damara terrain to the south and Proterozoic cratonic basement to the north.
Key Words: igneous rocks • intrusions • age • volcanic rocks • continental drift
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