Journal of Petrology | Volume 45 | Number 1 | Pages 59-105 | 2004
© Oxford University Press 2004; all rights reserved
Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 1: Introduction, Mafic Lavas and Re-evaluation of Mantle Source Components
1 ADVANCED CENTER FOR QUEENSLAND UNIVERSITY ISOTOPE RESEARCH EXCELLENCE (ACQUIRE), THE UNIVERSITY OF QUEENSLAND, ST. LUCIA, QLD. 4072, AUSTRALIA
2 DEPARTMENT OF GEOLOGY, RHODES UNIVERSITY, GRAHAMSTOWN 6140, SOUTH AFRICA
3 PANALYTICAL, LELYWEG 1, 7602EA ALMELO, THE NETHERLANDS
4 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7700, SOUTH AFRICA
5 RESEARCH SCHOOL OF EARTH SCIENCES, THE AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
* Corresponding author. E-mail: ewart{at}cust.caloundra.net
The bimodal NW Etendeka province is located at the continental end of the Tristan plume trace in coastal Namibia. It comprises a high-Ti (Khumib type) and three low-Ti basalt (Tafelberg, Kuidas and Esmeralda types) suites, with, at stratigraphically higher level, interstratified high-Ti latites (three units) and quartz latites (five units), and one low-Ti quartz latite. Khumib basalts are enriched in high field strength elements and light rare earth elements relative to low-Ti types and exhibit trace element affinities with Tristan da Cunha lavas. The unradiogenic 206Pb/204Pb ratios of Khumib basalts are distinctive, most plotting to the left of the 132 Ma Geochron, together with elevated 207Pb/204Pb ratios, and Sr–Nd isotopic compositions plotting in the lower 143Nd/144Nd part of mantle array (EM1-like). The low-Ti basalts have less coherent trace element patterns and variable, radiogenic initial Sr (
0·707–0·717) and Pb isotope compositions, implying crustal contamination. Four samples, however, have less radiogenic Pb and Sr that we suggest approximate their uncontaminated source. All basalt types, but particularly the low-Ti types, contain samples with trace element characteristics (e.g. Nb/Nb*) suggesting metasediment input, considered source-related. Radiogenic isotope compositions of these samples require long-term isolation of the source in the mantle and depletions (relative to unmodified sediment) in certain elements (e.g. Cs, Pb, U), which are possibly subduction-related. A geodynamic model is proposed in which the emerging Tristan plume entrained subducted material in the Transition Zone region, and further entrained asthenosphere during plume head expansion. Mixing calculations suggest that the main features of the Etendeka basalt types can be explained without sub-continental lithospheric mantle input. Crustal contamination is evident in most low-Ti basalts, but is distinct from the incorporation of a metasedimentary source component at mantle depths.
KEY WORDS: Etendeka flood basalts; LOMU–EM1 Pb isotope signatures; recycled crust; three-component magma mixing; Tristan plume
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