Petrology and isotope geochemistry of the Mesoproterozoic anorthosite and related rocks of the Kunene Intrusive Complex, NW Namibia

Abstract

The intrusion of the large (c. 18.000 km 2) anorthosite body of the Kunene Intrusive Complex, Angola/Namibia, which is associated with minor marginal granite bodies and syenite/syenodiorite dykes, marked the beginning of Mesoproterozoic igneous activity in NW Namibia. Anorthosites display an igneous mineral assemblage of dominating plagioclase, together with minor olivine, orthopyroxene or clinopyroxene, Fe–Ti oxides, whereas biotite and pargasite are late-magmatic phases. Apatite is a common accessory phase. In regionally associated felsic rocks K-feldspar, plagioclase, clinopyroxene and/or hastingsite crystallized first, followed by interstitial quartz, hastingsite, Fe–Ti oxides, and titanite. The general geochemical characteristics for the anorthositic rocks of the Kunene Intrusive Complex (i.e. high Al 2O 3, CaO, Sr and Eu, intermediate to high Mg numbers of 0.37–0.74, positive ɛNd (T) values of +3.0 to +1.0, and initial Sr ratios of 0.7028–0.7041) are in excellent agreement with their derivation from fractionated basaltic liquids. The δ 18O values obtained for plagioclase (5.88 ± 0.19‰ δ 18O) support their development from mantle-derived magmas. Relic crustal xenoliths of calc-silicate rocks and Sr and Nd isotope data, however, suggest that the anorthosite magma underwent minor crustal contamination, which was most prominent during the early intrusion stage. A U–Pb–zircon age of 1376 ± 2 Ma was obtained for a felsic rock associated with the 1385 ± 25 Ma anorthosites, which suggests that the two suites were emplaced during the same igneous event. The geochemical and isotope signatures of the granites, syenites, and syenodiorites provide convincing evidence against consanguinity of the anorthositic and the felsic rock suites, i.e.: (1) a compositional gap exists between the major and trace element contents of the two suites, (2) trace element and Sr, and Nd isotopic data of the felsic rocks point to a crustal source ( ɛNd: +2.0 to −0.4, initial Sr ratio: 0.7024–0.7063), (3) the REE patterns of the felsic rocks do not exhibit ubiquitous negative Eu anomalies, which would be expected from fractionation products of melts that previously formed extensive plagioclase cumulates, and (4) δ 18O values of magmatic feldspar from the felsic rock suite fall in a restricted range (7.20–7.92‰) that is about 1.6‰ higher than the average igneous plagioclase δ 18O of the anorthosites and suggests a formation of the felsic melts by anatexis of the lower crust.