The Kunene Complex (KC) of SW Angola, covering an area of 15,000 km2, 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 1371 ± 2.5 Ma, implying a Mesoproterozoic 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 143Nd/144Nd 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 143Nd/144Nd 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.
Mesoproterozoic Sm-Nd and U-Pb ages for the Kunene Anorthosite Complex of SW Angola. Prec. Research, 133, 187-206
SINIGOI, SILVANO;
2004-01-01
Abstract
The Kunene Complex (KC) of SW Angola, covering an area of 15,000 km2, 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 1371 ± 2.5 Ma, implying a Mesoproterozoic 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 143Nd/144Nd 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 143Nd/144Nd 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.Pubblicazioni consigliate
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