Protracted emplacement and multi-stage evolution of the Kunene Complex (Angola and Namibia) revealed by multi-mineral U-Pb geochronology

The Mesoproterozoic Kunene Complex of Angola and Namibia is the world’s largest massif-type anorthosite complex, and represents an exceptional area for studying the large-scale magmatic and thermal record of long-lived Proterozoic anorthosites. We examined evolved pegmatoidal enclaves and their host anorthosites from the central part of the complex to clarify their crystallisation sequence and investigate the magma emplacement dynamics and post-crystallisation thermal and metasomatic history. In combination with detailed petrography and mineral mapping, zircon, apatite and titanite were analysed for U-Pb isotope and trace element concentrations. Zircon dates for both the enclaves and the host anorthosites are ca. 1500 Ma, indicating contemporaneous crystallisation, but some zircon grains underwent coupled dissolution-reprecipitation, likely driven by high-temperature fluids related to protracted regional Kunene magmatism. The oldest age for one Kunene anorthosite, at 1510 ± 4 Ma, attests to a total duration for the magmatism reaching 150 Myr. Apatite also crystallised at ca. 1500 Ma as it mostly preserves magmatic textures and igneous trace element compositions, but records resetting ages of ca. 1400 Ma, corresponding with a subset of zircon dates and the crystallisation ages of secondary titanite, which may reflect a major Kunene magmatic pulse at this time. Overall, zircon age variability in single samples, combined with trace element decoupling, suggest localised post-crystallisation disturbance of the magmatic system. This study shows the contribution that multiple geochronological datasets from large igneous bodies can provide to explain prolonged histories of magma recharge, thermal rejuvenation and fluid-rock interaction.