Geochemical evolution of the Okenyenya sub-volcanic ring complex, northwestern Namibia

Abstract

The Okenyenya gabbro–syenite complex, one of a number of intrusive igneous complexes of late-Mesozoic age in northwestern Namibia, was emplaced at the time of opening of the South Atlantic Ocean. The 5-km-diameter complex comprises a wide variety of rock types that can be subdivided into two contrasting magmatic suites, one tholeiitic and the other alkaline, which were emplaced in close proximity over a time-span of ~5 Ma. The tholeiitic suite of rocks includes picritic gabbro, olivine gabbro through quartz monzodiorite and syenite, whereas the alkaline suite includes alkaline gabbro, essexite, nepheline syenite and a range of lamprophyric rock types. Detailed petrographic, mineralogical and bulk rock geochemical data show that the earliest, saucer-shaped, intrusion of olivine gabbro–quartz monzodiorite rocks can be subdivided into an Inner Zone and an Outer Zone (each comprising three distinct intrusive units). The individual units can be readily distinguished on the basis of bulk rock geochemical variations, together with cryptic and modal mineralogical variations. An unusual feature of the intrusive body is that bulk rock and mineral compositions become more evolved with apparent depth, within the body as a whole and within each unit. Compositional variation within the individual intrusive units requires a complex interplay between in situ crystallization, variable expulsion of interstitial melt, magma recharge, and re-equilibration of primocrysts with trapped interstitial melt. Cross-cutting dykes of picritic gabbro (MgO = 13–21 %) have compositions consistent with olivine control. Incompatible trace element ratios (e.g. Zr/Nb= 12.5 ± 1.3) suggest that the picritic gabbro magmas were derived from a distinct source region compared to that giving rise to the tholeiitic olivine gabbros (Zr/Nb = 6.8 ± 1.1).Alkaline gabbro occupies the central region of the complex and, on the basis of major, trace and rare earth element variations, can be subdivided into four distinct intrusive bodies, interpreted as remnant magma chambers, each having experienced variable degrees of crystal accumulation. In places, magma chamber processes have given rise to centimetre-scale rhythmic layering. Incompatible trace element ratios (e.g. Zr/Nb = 4.4 ± 1.2) serve to distinguish the source region of the alkaline gabbro magmas from those giving rise to the tholeiitic suite of magmas. Younger rocks of both the tholeiitic and alkaline suites show strong evidence of the effects of extensive crystal fractionation. The quartz syenite is characterized by a strong negative Eu anomaly indicative of substantial feldspar fractionation and also shows evidence for direct contamination by earlier gabbro, whereas the syenite shows evidence for feldspar accumulation. Both syenites have geochemical characteristics suggesting consanguinity with the Outer Zone rocks of the olivine gabbro–quartz monzodiorite intrusion. In contrast, the essexite and nepheline syenite compositions are qualitatively consistent with derivation from one of the alkaline gabbro magmas by extensive fractionation of plagioclase, clinopyroxene, olivine and amphibole. The final stage of magmatism is represented by a suite of alkaline and ultramafic lamprophyres emplaced as dykes and diatremes, the latter carrying a variety of megacrystic and xenolithic material, including mantle nodules. The alternation between tholeiitic and alkaline magmatism evident within the Okenyenya complex is similar to that characteristic of the evolution of many ocean island volcanoes.