HR: 1340h
AN: V13D-1590    [Abstracts]
TI: Sr-Nd-Pb-Hf isotope geochemistry of mafic alkaline igneous rocks from the Gibeon Dicker Willem lineament, southern Namibia
AU: * Janney, P E
EM: pjanney@asu.edu
AF: Arizona State University, School of Earth and Space Exploration P.O. Box 871404, Tempe, AZ 85287-1404, United States
AB: The western margin of southern Africa is intersected by three NE-SW trending, age-progressive lineaments composed of alkaline igneous rocks of Late Cretaceous to Early Tertiary age. The northernmost of these is the "Gibeon-Dicker Willem" lineament of southern Namibia. The Gibeon kimberlite province (300 km inland; 72-77 Ma; Davies et al., JPet., 2001) and the Dicker Willem carbonatite complex (125 km inland; 49 Ma; Reid et al., Geol. Mag. 1990) are the two best known localities in this lineament, but it encompasses at least four other dated localities: the Blue Hills intrusive complex (75 Ma, Kurzlaukis et al., Chem. Geol. 1999) and Gross Brukkaros carbonatite complex (77 Ma, Reid et al., 1990) both located just south of and presumably related to the Gibeon kimberlite province, and the Schwarzeberg olivine nephelinites (30 Ma; Spriggs et al., PhD thesis, 1988) and Klinghardt Mountain phonolites (46 Ma; J.S. Marsh, pers. commun.), both located about 50 km inland. This lineament also includes an undated nephelinite plug at Dreyer Rucken, located within 15 km of the coast. Reid et al. (1990) proposed that the lineament may have resulted from the activity of the Vema and/or Discovery hotspots, currently located in the South Atlantic. New elemental and Sr-Nd-Pb-Hf isotope data are presented for samples from Blue Hills, Schwarzeberg and Dreyer Rucken. Like the Western Cape (WC) and Namaqualand- Bushmanland-Warmbad (NBW) lineaments further south (Janney et al., JPet. 2002 and 7IKC abstract #127 2003; Rogers et al., Lithos 1992), the isotope compositions of alkaline magmas from the Gibeon-Dicker Willem lineament extend between mild EM1 and strong HIMU compositions. However, unlike the WC and NBW lineaments, there is no clear transition from HIMU compositions near the coast to EM1 compositions in the interior. Samples from Dicker Willem and Blue Hills in the interior display extremely radiogenic ²⁰⁶Pb/²⁰⁴Pb_i values of 19.9 to 21.2 (Cooper and Reid, CMP 2000, this study), whereas the Gibeon kimberlites have the least radiogenic values (18.2 to 19.0; Spriggs, PhD thesis 1988). The near-coastal Dreyer Rucken and Schwarzeberg nephelinite samples are isotopically intermediate (²⁰⁶Pb/²⁰⁴Pb_i = 19.57 to 19.75). Although mixing of lithospheric and sublithospheric sources appears to have contributed to the isotopic heterogeneity displayed by the Gibeon-Dicker Willem lineament, a major role for extreme fractionation of parent-daughter elements (especially U+Th/Pb) during very low-degree partial melting is also inferred.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 3615 Intra-plate processes (1033, 8415)
DE: 3621 Mantle processes (1038)
DE: 9305 Africa
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting