Modes of crustal growth and construction for the southwestern Congo Craton in the Mesoproterozoic

A major contribution to the crustal growth and construction of the Congo Craton was the addition and preservation of the ≤ 45 000 km² Kunene AMCG Complex (KC), which straddles the international border between Angola and Namibia. KC magmatism encompasses dominantly juvenile anorthositic rocks (anorthosite, leuco-gabbro, -norite, -troctolite) and A-type granitoids (Red Granite Suite) of mixed crustal and juvenile signature. High-precision U-Pb dates of zircon and baddeleyite from the exposed western parts of the KC (~15 000 km²) in between 1500 and 1360 Ma indicate that both the anorthosites and Red Granites were pulsed and exceptionally long-lived. The remaining eastern portion of the KC can only be imaged using potential field geophysical methods as it is covered by a thin (≤ 300 m) cover of Cenozoic Kalahari sediments. Field mapping and recent remote sensing in the exposed part of the complex, together with airborne geophysics of the entire KC, indicate that the anorthosites were emplaced in up to 12 layered or massive batholiths, which are elliptical in a NNE-SSW or E-W direction. They are commonly separated by relatively thin and elongated KC granitoid bodies and are in tectonic or intrusive contact with Paleoproterozoic basement rocks.

Regional horizontal contraction in the Angolan portion of the KC is dated by U-Pb in zircon and Ar-Ar in micas at 1400-1370 Ma. Contraction formed N-S to NE-SW-striking, cm- to km-wide, discrete, syn- to post-magmatic thrust zones mainly localised in KC granitoids. The shear zones are parallel to magmatic foliation in the granitoids and magmatically layered anorthosites. A compilation of crystallisation ages (n = 60) suggests that the regional shortening triggered the magmatism that formed ~ 60% of the exposed KC by mobilising magmas from deep crustal mush zones. In contrast, the southern part of the KC in Namibia exhibits E-W- to ENE-WSW-striking magmatic layering, gneissic foliations and shear zones formed at amphibolite to greenschist facies conditions. These are compatible with north-directed ductile to brittle thrusting over the Angolan KC. Northward thrusting post-dates KC emplacement and is broadly constrained in between 1360 and 1330 Ma by Ar-Ar dating of micas. Airborne aeromagnetic and satellite gravimetric data indicate that the southern KC is parallel to and overlies a crustal and continental-scale geophysical lineament, which is interpreted as the relic of a linear Mesoproterozoic orogenic belt extending to the Kibaran Belt of Central Africa. The orogenic activity was terminated by 1127 Ma, which is the oldest age of a suite of mafic dykes crosscutting post-KC and undeformed capping siliciclastic units. U-Pb dates of detrital zircon and Hf-in-zircon data for these siliciclastic rocks overlap with those of the KC granitoids, indicating local recycling of KC rocks between 1360 and 1127 Ma.

Our results highlight that the 1500-1360 Ma period of the Congo Craton was a time of significant crustal growth in the form of voluminous Kunene Complex magmatism. The assembly of the entire KC magmatic edifice was facilitated by syn- to post-magmatic contractional deformation that juxtaposed distinct crustal domains during two newly defined Mesoproterozoic orogenic events.