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Petrogenesis of the False Bay dyke swarm, Cape Peninsula, South Africa N R Backeberg 1 , D L Reid 1 , R B Trumbull 3 , R L Romer 4 1. University of Cape Town, South Africa, nils.backeberg@gmail.com 2. University of Cape Town, South Africa, david.reid@uct.ac.za 3. Deutsches GeoForschungsZentrum, 14473 Potsdam, Germany, bobby@gfz-potsdam.de 4. Deutsches GeoForschungsZentrum, 14473 Potsdam, Germany, romer@gfz-potsdam.de ABSTRACT The False Bay dyke swarm is the southern NW-SE trending end-member of the Cretaceous dolerite dyke intrusions on the western African margin, associated with rifting of Gondwana and opening of the South Atlantic Ocean. This southern dyke swarm has been associated with a low-flux, passive rifting end member compared to its northern high-flux, active rifting counterpart: the Henties Bay-Outjo dyke swarm in Namibia. The contrast in basaltic magma types and magma flux between north and south has been related to different tectonic settings (i.e. magma sources) along the current coast line during Cretaceous rifting. Further, the False Bay dyke swarm is characterised by olivine-tholeiites with quite extreme differentiation to ferro-tholeiites within a monogenetic magma system. The finer details of the differentiation process identified through trace element analysis and major element modelling are presented here. Both crystal fractionation and assimilation trends are identified for the False Bay dolerites during magma evolution. Crustal assimilation processes are only identified in samples with less than 5 wt% MgO. This observation is consistent with trace elements and Sr and Nd radiogenic isotopes, while initial differentiation (greater than 5wt% MgO) is characterised by pure fractional crystallisation. Initial variation in conserved trace element ratios, prior to assimilation, is not correlated to differentiation and is assumed as a result of variations within the enriched source rock. KEYWORDS: dolerite dykes, trace element ratios, Sr and Nd isotopes, Gondwana rifting, differentiation 4
Evolution of Proterozoic mantle lithosphere: petrological and geophysical evidence from the Kaapvaal – Namaqua boundary D. Bell 1,3 , U. Weckmann 2 ,P. Janney 1 , M. de Wit 3 1. SESE, Arizona State University, Tempe, AZ, USA, David.R.Bell@asu.edu 2. GeoForschungsZentrum Potsdam, Germany, uweck@gfz-potsdam.de 3. AEON, University of Cape Town, South Africa, Maarten.deWit@uct.ac.za ABSTRACT The subcontinental lithospheric mantle (SCLM) beneath Proterozoic and Archean crustal provinces differs in composition, temperature, and thickness. To shed light on the origins of these differences we combined petrological data from xenocrysts, megacrysts, and xenoliths in kimberlites with a lithospheric resistivity model derived from a new high-resolution magnetotelluric (MT) traverse, to examine the transition between Archean and Proterozoic lithosphere at the SW margin of the Kaapvaal Craton. Highly resistive SCLM persists up to 100 km west of the Brakbos fault zone, confirming inferences from garnet xenocrysts of craton-like mantle beneath Proterozoic crust (Kobussen et al. 2008). Xenolith thermobarometry also suggests craton-like mantle temperatures in this region prior to Mesozoic disturbance (Bell et al. 2003; Janney et al. 2010). Surface and body-wave seismic tomography studies reveal craton-like high-velocity domains within the SCLM beneath the Namaqua Belt. Average whole-rock Mg#, Al 2 O 3 contents, and Os isotope model ages of Kaapvaal and Namaqua mantle xenoliths differ, but many individual off-craton xenoliths overlap the cratonic population and suggest complex multistage history. Clinopyroxene megacrysts from the Pofadder kimberlites suggest interaction of deep melts with mantle lithosphere similar to that beneath the Archean craton at Kimberley. The MT study reveals prominent vertical zones of low resistivity within the high-resistivity SCLM in the boundary region. These may be metasomatized zones that formed during the focused ascent of melts generated beneath the SCLM and have been inferred from xenocryst compositions (Begg et al. 2009). Some kimberlite eruptions occur earlier in the boundary region than would be predicted from their broader regional eruptive patterns, trace elements suggest some have mixed source regions, and evolved megacrysts indicate local magma infiltration to shallow levels. These results support models of (1) melt-refertilized Archean SCLM beneath Proterozoic crust, and (2) the focusing of mantle–derived magmas into ancient shear zones that serve as repeating loci of tectonic reactivation. This model allows for tectonic obduction of relatively young Namaqua arc crust across subducting Archean SCLM during arc-continent collision, as is presently observed in the Banda Arc (Fichtner et al., 2010) [335 words] KEYWORDS: mantle, lithosphere, kimberlite, conductivity, megacryst, xenolith, subduction-accretion. 5
Page 1 and 2: Getting Deeper into the System Inka
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Topographic imprint of Geomorpholog
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The Mineralogy and PGE Geochemistry
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A proposal for the determination of
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An Investigation of the Trace Eleme
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High-precision steering and pointin
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Low Enthalpy Geothermal Energy puri
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Mineralogical, geochemical and stru
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Petrophysical evaluation of the Alb
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Towards a magmatic reaction model f
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Denudation rates and geomorphic evo
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Structural mapping and analysis of
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Monitoring the 2-D urban heat islan
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Statistical Downscaling of Climate
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New insights on the role of a mantl
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Palaeoceanographic changes identifi
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The development and evaluation of s
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Records of Late Quaternary environm
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