Abstract
Oxygen isotope data are presented for silicate minerals separated from a variety of rock types from the 130 Ma Okenyenya igneous complex of northwestern Namibia. These rock types include a tholeiitic suite ranging from olivine gabbro to quartz monzodiorite and syenite, and an alkaline suite including gabbro, syenite, nepheline syenite, and essexite. In general, the difference in δ18O values between coexisting feldspar, pyroxene, and biotite are consistent with equilibrium at high temperatures and preclude significant interaction with magmatic or external fluids.The δ18O values of pyroxene can be used to model magmatic processes and show a systematic increase from 7.1–9.0‰ along a transect through a tholeiitic olivine gabbro-quartz monzodioritic body, corresponding to a systematic decrease in Mg#. Pyroxene δ18O values also show a strong correlation with initial strontium and neodymium isotope ratios, with the alkaline suite having lower δ18O values, lower initial strontium-isotope ratios, and higher initial neodymium isotope ratios than the tholeiitic suite. Simple oxygen mass balance calculations suggest that the extremes of isotope composition resulted from at least 60% contamination by material of similar isotope composition to the nearby southern Etendeka quartz latites. Assimilation/crystal fractionation models of oxygen, strontium, and neodymium isotope data suggest that the rate of assimilation was high and may have approached simple mixing. The contaminated tholeiitic olivine gabbro-quartz monzodioritic body was the earliest to crystallize in the complex and this was followed by relatively uncontaminated silica-undersaturated magmas.
Published Version
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