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

Published online by Cambridge University Press:  01 May 2009

Anton P. Le Roex ,
Ronald T. Watkins  and
Arch M. Reid
Anton P. Le Roex
Affiliation:
Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa
Ronald T. Watkins
Affiliation:
Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa
Arch M. Reid
Affiliation:
Department of Geological Sciences, University of Cape Town, Rondebosch, 7700, South Africa
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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.

Type
Articles
Information
Geological Magazine , Volume 133 , Issue 6 , November 1996 , pp. 645 - 670
Copyright
Copyright © Cambridge University Press 1996

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References

Albee, A. L. & Ray, L. 1970. Correction factors for electron probe microanalysis of silicates, oxides, carbonates, phosphates, and sulphates. Analytical Chemistry 42, 1408.Google Scholar
Bence, A. E. & Albee, L. 1968. Empirical correction factors for the electron microanalysis of silicates and oxides. Journal of Petrology 76, 382–403.Google Scholar
Clague, D. A. 1987. Hawaiian alkaline volcanism. In Alkaline Igneous Rocks (eds Fitton, J. G. and Upton, B. G. J.), pp. 227–52. Geological Society Special Publication no. 30.Google Scholar
Foley, S. F. 1988. The genesis of continental basic alkaline magmas – an interpretation in terms of redox melting. Journal of Petrology, Special Lithosphere Issue, 139–61.Google Scholar
Frey, F. A., Green, D. A. & Roy, S. D. 1978. Integrated models of basalt petrogenesis: A study of quartz tholeiites to olivine melilitites from south eastern Australia utilising geochemical and experimental petrological data. Journal of Petrology 19, 463–513.CrossRefGoogle Scholar
Frey, F. A., Walker, N., Stakes, D., Hart, S. R. & Nielsen, R. 1993. Geochemical characteristics of basaltic glasses from the AMAR and FAMOUS axial valleys, Mid-Atlantic Ridge (36°–37°N): Petrogenetic implications. Earth and Planetary Science Letters 115, 117–36.Google Scholar
Huppert, H. E., Sparks, R. S. J., Wilson, R. J., & Hallworth, M. A. 1986. Cooling and crystallization at an inclined plane. Earth and Planetary Science Letters 79, 319–28.CrossRefGoogle Scholar
Irvine, T. N., Keith, D. W. & Todd, S. G. 1983. The J-M platinum-palladium reef of the Stillwater Complex, Montana: II. Origin by double-diffusive convective magma mixing and implication for the Bushveld Complex. Economic Geology 78, 1287–1334.Google Scholar
Korn, H. & Martin, H. 1939. Junge vulkano-plutone in Sudwestafrika. Geologische Rundschau 30, 631–6.Google Scholar
Lanyon, R. & le Roex, A. P. 1995. Petrography, mineralogy and classification of lamprophyric intrusives, Okenyenya igneous complex, northwestern Namibia. South African Journal of Geology 98, 140–56.Google Scholar
le Roex, A. P., Erlank, A. J., & Needham, H. D. 1981. Geochemical and mineralogical evidence for the occurrence of at least three distinct magma types in the FAMOUS region. Contributions to Mineralogy and Petrology 77, 24–37.Google Scholar
le Roex, A. P. & Watkins, R. T. 1990. Analysis of rare earth elements in geological samples by gradiention chromatography: an alternative to ICP and INAA. Chemical Geology 88, 151–62.Google Scholar
Marsh, B. D. 1989. On convective style and vigor in sheet-like magma chambers. Journal of Petrology 30, 479–530.CrossRefGoogle Scholar
Martin, H., Mathias, M. & Simpson, E. S. W. 1960. The Damaraland sub-volcanic ring complexes in South West Africa. Report of the International Geological Congress XXI 13, 156–74.Google Scholar
Milner, S. C. & le Roex, A. P. 1996. Isotope characteristics of the Okenyenya igneous complex, northwestern Namibia: constraints on the composition of the early Tristan plume and the origin of the EMI mantle component. Earth and Planetary Science Letters 141, 277–91.CrossRefGoogle Scholar
Milner, S. C., le Roex, A. P & Watkins, R. T. 1993. Rb-Sr age determinations of rocks from the Okenyenya igneous complex, north-western Namibia. Geological Magazine 130, 335–43.Google Scholar
Milner, S. C., le Roex, A. P. & O’Connor, J. 1995. Age of Mesozoic igneous rocks in northwestern Namibia, and their relationship to continental breakup. Journal of the Geological Society, London 152, 97–104.Google Scholar
Norrish, K., & Hutton, J. T. 1969. An accurate X-ray spectragraphic method for the analysis of a wide variety of geological samples. Geochimica et Cosmochimica Acta 33, 431–53.CrossRefGoogle Scholar
Raedeke, L. D. & McCallum, I. S. 1984. Investigations in the Stillwater Complex: Part II. Petrology and Petrogenesis of the Ultramafic Series. Journal of Petrology 25, 395–420.CrossRefGoogle Scholar
SACS, 1980. Stratigraphy of South Africa. Part I. Lithostratigraphy of the Republic of S.A., South West Africa/Namibia and the Republics of Bophuthatswana, Transkei and Venda. Handbook of the Geological Survey, South Africa no. 8.Google Scholar
Simpson, E. S. W. 1954. The Okonjeje igneous complex, South West Africa. Transactions of the Geological Society of South Africa 57, 125–72.Google Scholar
Sun, S.-S. & McDonough, W. F. 1989. Chemical and isotope systematics of oceanic basalts: implications for mantle composition and processes. In Magmatism in the Ocean Basins (eds Saunders, A. D. and Norry, M. J.), pp. 313–45. Geological Society of London, Special Publication no. 42.Google Scholar
Tait, S. R. & Kerr, R. C. 1987. Experimental modelling of interstitial melt convection in cumulus piles. In Origins of Igneous layering (ed. Parsons, I.), pp. 569–87. D. Reidel Publishing Company.CrossRefGoogle Scholar
Watkins, R. T. & le Roex, A. P. 1991. Petrology and structure of syenite intrusions of the Okenyenya igneous complex. Communications of the Geological Survey Namibia 7, 55–70.Google Scholar
Watkins, R. T. & le Roex, A. P. 1994. A revised geology of the Okenyenya igneous complex: Structure and history of emplacement. Communications of the Geological Survey Namibia 9, 13–21.Google Scholar
Watkins, R. T., McDougall, I. & le Roex, A. P. 1994. K–Ar ages of the Brandberg and Okenyenya igneous complexes, north-western Namibia. Geologische Rundschau 83, 348–56.Google Scholar
Wilson, J. R. & Larsen, S. B. 1985. Two-dimensional study of a layered intrusion – the Hyllingen Series, Norway. Geological Magazine 122, 97–124.Google Scholar
Wilson, J. R., Menuge, J. F., Pederson, S. & Engell-Sorensen, O. 1987. The southern part of the Fongen–Hyllingen layered mafic complex, Norway. Emplacement and crystallization of compositionally stratified magma. In Origins of Igneous Layering (ed. Parsons, I.), pp. 145–84. D. Reidel Publishing Company.Google Scholar
Wyllie, P. J. 1987. Discussion of recent papers on carbonated peridotite, bearing on mantle metasomatism and magmatism. Earth and Planetary Science Letters 82, 391–7.Google Scholar