Geochemistry of the early Cretaceous Koegel Fontein anorogenic igneous complex, South Africa

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dc.contributor.author Curtis, C G
dc.contributor.author Trumbull, R B
dc.contributor.author de Beer, C H
dc.contributor.author Harris, C
dc.contributor.author Reid, D L
dc.contributor.author Romer R L
dc.date.accessioned 2018-04-09T12:16:06Z
dc.date.available 2018-04-09T12:16:06Z
dc.date.issued 2011
dc.identifier http://dx.doi.org/10.2113/gssajg.114.3-4.353
dc.identifier.citation Curtis, C. G., Trumbull, R. B., De Beer, C. H., Harris, C., Reid, D. L., & Romer, R. L. (2011). Geochemistry of the early Cretaceous Koegel Fontein anorogenic igneous complex, South Africa. South African Journal of Geology, 114(3-4), 353-378.
dc.identifier.uri http://hdl.handle.net/11427/27777
dc.description.abstract 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.
dc.source SOUTH AFRICAN JOURNAL OF GEOLOGY
dc.source.uri http://sajg.geoscienceworld.org/
dc.title Geochemistry of the early Cretaceous Koegel Fontein anorogenic igneous complex, South Africa
dc.date.updated 2017-10-27T12:22:03Z
dc.publisher.institution University of Cape Town
dc.publisher.faculty Faculty of Science en_ZA
dc.publisher.department Department of Geological Sciences en_ZA
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