Prolonged (>100 Myr) magmatic and thermal evolution in the world’s largest massif-type anorthosite complex (Kunene Complex, Angola and Namibia)

Proterozoic massif-type anorthosites are large plutons, predominantly composed of plagioclase, emplaced between 2.7 and 0.5 Ga. The Mesoproterozoic Kunene Complex is the largest massif-type anorthosite complex in the world, with an estimated area of ∼ 42,500 km², emplaced in southern Angola and northern Namibia. Recent geochronological studies on the main lithologies indicate ages between 1.50 and1.36 Ga, with the anorthosite dating between 1.43 and 1.37 Ga.

The anorthosite suite of the Kunene Complex locally hosts irregular pegmatoidal enclaves (a few meters-long, one meter-wide), primarily composed of large grains of orthopyroxene, clinopyroxene, Fe-Ti oxides, apatite, and plagioclase, with minor quartz, zircon, titanite and sulphide. The mineralogy and pegmatitic texture suggest that these enclaves represent evolved residual melts, occurring during the final stages of the liquid line of descent of parental magmas to the anorthosite. However, a U-Pb zircon date at ∼1.50 Ga obtained from an enclave is 60 Myr older than the oldest age measured on the Kunene anorthosite so far.

In this study, we provide new U-Pb dates and mineral trace element chemistry for zircon, apatite, and titanite in a set of enclaves and their direct host anorthosites. Samples were collected in two quarries in the central region of the complex in Angola, where these enclaves are well exposed.

Zircon dates from anorthosites and hosted enclaves range between 1.52 and 1.40 Ga. This establishes the beginning of the Kunene magmatism at around 1.5 Ga, testifies to coeval crystallisation of enclaves and host anorthosite, and indicates a prolonged zircon resetting due to the magmatism extending more than 130 Myr.

Both the subhedral cm-scale apatite observed in the enclaves and the smaller grains (max 200 µm) show textural features suggesting they are primary phases. Their trace element signature (relative enrichment in light rare earth elements) agrees with a magmatic origin. With no Pb loss after crystallisation, the igneous age would have been preserved. However, no ages at 1.5 Ga were documented for apatite, as they range between 1.41 and 1.35 Ga and overlap with the youngest zircon dates. We attribute these ages to partial resetting of the parent-daughter system during prolonged thermal activity and fluid circulation triggered by the long-lived Kunene magmatism, which resulted in apparent or mixed apatite ages.

Titanite in the enclaves crystallised as a secondary phase, appearing as clusters of minute anhedral grains closely associated with other alteration minerals. The U-Pb dates for titanite range from 1.41 to 1.37 Ga, overlapping with those for apatite. Titanite records the greenschist facies assemblages observed in the enclaves, providing key evidence of fluid-rock interactions during the post-magmatic stage.

The prolonged magmatic history of the Kunene Complex testifies to extended interaction between crystallisation processes, thermal reworking, and fluid-induced alteration. The new findings indicate that the enclave and host anorthosite are coeval, place the beginning of the anorthosite magmatism at 1.5 Ga, with metasomatic and thermal overprint at 1.42–1.35 Ga. The new data refine the temporal framework of the Kunene Complex emplacement and provide new fascinating insights into the magma dynamics of massif-type anorthosites.