Browsing by Author "Trumbull, R B"

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    Geochemistry of the early Cretaceous Koegel Fontein anorogenic igneous complex, South Africa
    (2011) Curtis, C G; Trumbull, R B; de Beer, C H; Harris, C; Reid, D L; Romer R L
    The Koegel Fontein Igneous Complex in the Western Cape Province of South Africa is a high-level granite-syenite intrusive suite with minor plugs and dykes of felsic and mafic composition. Age dating and field relations show that the felsic rocks intruded as two sequential series, beginning with syenites and the Kerskloof suite of felsic dykes (andesite-trachyandesite, trachyte-rhyolite) at about 144 Ma, followed by the Rietpoort granite pluton and quartz-porphyry dykes at about 134 Ma. The intmsion of the mafic dykes and plugs took place between the two felsic series. The early Cretaceous age of the Koegel Fontein associates its formation with Gondwana breakup and establishment of the volcanic rifted margin in southwest Africa. This paper reports results from the first geochemical saidy of the Koegel Fontein Complex. Two groups of mafic units are di.stinguished based on mineralogy, field characteristics and chemical composition. The tholeiitic group comprises basalt and basaltic andésites, the alkaline group ranges from basanite-tephrite to phono-tephrite and basaltic trachy-andesite. Distinctions of incompatible element concentrations (K, P, Ti) and ratios (Y/Nb, Zr/Nb) indicate the presence of two .separate magma series. Regional dolerite dykes (Gariep dykes and others) associated with contemporary magmatism along the continental margin occur in and near the complex. These regional dolerites are much more homogeneous compositionally than the mafic dykes of Koegel Fontein. Compositionally, they closely resemble the less-differentiated tholeiite dykes. The felsic units at Koegel Fontein show geochemical and isotopic evidence for two magma series, which correspond to the two intrusive episodes suggested by geochronology and field relations. The first series comprises the volumetrically minor intrusions of syenites, the Kerskloof dykes and the Rooivleitjie granite, with high values (eNd(t) from -0.4 to -1.8) indicating a strong mantle input. Similarities of trace element signatures suggest an affinity with the alkaline mafic units. The second series of felsic magmas makes up the bulk of the Koegel Fontein complex, with the large Rietpoort granite pluton and quartz-porphyry dykes (eNd(t) = -4.8 to -6.9). These rocks may be related to the tholeiitic mafic series but their isotopic composition indicates a large crustal component. The granitic rocks at Koegel Fontein have geochemical characteristics regarded as typical for anorogenic (A-type) granites, including high contents of Y, Nb, U and Th. In this and in many other features, the Koegel Fontein complex resembles the contemporary intrusive complexes in the Damaraland of northwest Namibia. Like the Damaraland granites, the Koegel Fontein magmas are of mixed origin, and their isotopic composition reflect the proportion of mantle vs. crustal source contributions.
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    Petrogenesis of the False Bay Dyke Swarm Cape Peninsula South Africa: Evidence for basement assimilation
    (Geological Society of South Africa, 2011) Backeberg, N R; Reid, D L; Trumbull, R B; Romer, R L
    The False Bay Dyke Swarm represents igneous activity associated with the opening of the South Atlantic during the early Cretaceous, and can be considered a distal component of the Etendeka-Paraná Large Igneous Province. In contrast to the intense magmatism in Namibia and Brazil, with the Paraná-Etendeka continental flood basalts and huge dyke swarms, the False Bay swarm is considered to be the product of a relatively low magma-flux in a passive rift setting. Previous work suggested that the False Bay dykes consist of a single magma type, with a marked differentiation from olivine-tholeiite basalt to ferro-tholeiitic andesite, accompanied by crustal contamination. This study uses new trace element and radiogenic Sr and Nd isotope analyses to better constrain the processes of magma evolution in the dykes. Combined assimilation − crystal fractionation (AFC) modelling suggests a first stage of nearly closed system fractionation of a gabbroic assemblage (olivine + clinopyroxene + plagioclase + magnetite), accompanied by progressively larger amounts of crustal assimilation at intermediate and late stage. The AFC models show that the exposed country rocks, including Cape Granites and Malmesbury Group metasediments, are compositionally unfavorable for producing the observed assimilation trends in the dykes. Instead, a more suitable crustal assimilant would be Mesoproterozoic granitic gneisses similar to those exposed in the neighbouring Namaqua Province, which may underly the Cape Peninsula.