Mesoproterozoic Sm-Nd and U-Pb ages for the Kunene anorthosite Complex of SW Angola

The Kunene Complex (KC) of SW Angola, covering an area of 15,000km(2), is one of the largest anorthosite bodies in the world. Although considered by several authors as a layered complex, field evidences demonstrate that it is a massive anorthosite. In the study area, a Northern and a Southern KC were distinguished on the basis of a clear difference in anorthite contents. The boundary between them corresponds to a belt of coeval granitoids. Analyses of two multigrain zircon populations and several single-grains (evaporation and vapour dissolution methods) from a late-stage mangerite vein, inferred to be cogenetic with the KC, yielded a concordant age of 13714 +/- 2.5 Ma, implying a Meso-proterozoic age for the KC. Because the anorthosites are unmetamorphosed, cooling after emplacement was also constrained by Sm-Nd mineral isochrons of 1470 +/- 25 Ma, for a troctolite near Quihita, and 1319 +/- 28 Ma, for a hyperporphyritic "megacrystic" anorthosite dyke, considered to represent a quenched plagioclase crystal-mush, sampled near Lufinda. The discrepancy between the two data can be interpreted in two ways: (1) the Sm-Nd isochrons are geologically meaningful, and thus the KC is constituted by various plutons with different Mesoproterozoic ages; (2) most likely, the Sm-Nd systematic was cryptically perturbed at mineral-scale. and thus only the U-Pb age of the mangerite vein yields to a reliable intrusion age of the KC. The initial Nd-143/Nd-144 of the Quihita sample (0.510734 +/- 28), as well as the high anorthite content of Northern KC, suggests that parent magma of KC derived from a mantle-source, and thus that the emplacement of KC anorthosites reflect an episode of continental accretion rather than a within-continent differentiation. The initial Nd-143/Nd-144 of the Lufinda dyke (0.510766 +/- 24) implies instead a minor addition of crustal material into the parental magma. Collectively, the emplacement of the Kunene Complex requires an extensional setting and a significant thermal anomaly at the margin of the Congo Craton during the early Kibaran cycle. (C) 2004 Elsevier B.V. All rights reserved.