GEOCHEMICAL EVOLUTION OF THE OKENYENYA SUB-VOLCANIC RING COMPLEX, NORTHWESTERN NAMIBIA

The Okenyenya gabbro-syenite complex, one of a number of intrusive ign eous complexes of late-Mesozoic age in northwestern Namibia, was empla ced at the time of opening of the South Atlantic Ocean. The 5-km-diame ter complex comprises a wide variety of rock types that can be subdivi ded into two contrasting magmatic suites, one tholeiitic and the other alkaline, which were emplaced in close proximity over a time-span of similar to 5 Ma. The tholeiitic suite of rocks includes picritic gabbr o, 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, mineral ogical and bulk rock geochemical data show that the earliest, saucer-s haped, intrusion of olivine gabbro-quartz monzodiorite rocks can be su bdivided into an Inner Zone and an Outer Zone (each comprising three d istinct intrusive units). The individual units can be readily distingu ished 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 eac h 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 oliv ine control. Incompatible trace element ratios (e.g. Zr/Nb = 12.5 +/- 1.3) suggest that the picritic gabbro magmas were derived from a disti nct source region compared to that giving rise to the tholeiitic olivi ne 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 eart h element variations, can be subdivided into four distinct intrusive b odies, interpreted as remnant ma,oma chambers, each having experienced variable degrees of crystal accumulation. In places, magma chamber pr ocesses have given rise to centimetre-scale rhythmic layering. Incompa tible trace element ratios (e.g. Zr/Nb = 4.4 +/- 1.2) serve to disting uish the source region of the alkaline gabbro magmas from those giving rise to the tholeiitic suite of magmas. Younger rocks of both the tho leiitic and alkaline suites show strong evidence of the effects of ext ensive crystal fractionation. The quartz syenite is characterized by a strong negative Eu anomaly indicative of substantial feldspar fractio nation and also shows evidence for direct contamination by earlier gab bro, whereas the syenite shows evidence for feldspar accumulation. Bot h syenites have geochemical characteristics suggesting consanguinity w ith the Outer Zone rocks of the olivine gabbro-quartz monzodiorite int rusion. In contrast, the essexite and nepheline syenite compositions a re qualitatively consistent with derivation from one of the alkaline g abbro 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 xenolithi c material, including mantle nodules. The alternation between tholeiit ic and alkaline magmatism evident within the Okenyenya complex is simi lar to that characteristic of the evolution of many ocean island volca noes.