Lithostratigraphic, structural and geochronological framework of the Namibe region, SW Angola: Insights into the late Eburnean Orogeny in the Congo Shield

Abstract

The Namibe region in southwestern Angola represents the southernmost extent of the Southwestern Congo Shield; however, its tectonothermal evolution is still poorly understood. To better constrain the timing and deformation style of the outcropping rocks in this region, we integrated (i) 1:250,000-scale lithostratigraphic and structural geological mapping, (ii) SHRIMP U–Pb zircon dating of metamorphic rocks, and (iii) 40Ar/39Ar muscovite cooling ages. The study area comprises the Epupa Complex of ortho- and paragneisses, overlain by the Namibe Group, which is composed of supracrustal metasediments. The sharp lithological contrast, absence of orthogneisses, and the preservation of basal marbles suggest that the protoliths of the Namibe Group were unconformably deposited on the Epupa Complex. Both units experienced polyphase ductile deformation during the Eburnean event. Two penetrative deformation phases (D1 and D2) are overprinted by two later, less pervasive phases (D3 and D4). A subhorizontal D2 shear zone links sinistral transpression in the upper crust with lateral flow in a partially molten lower crust. Peak metamorphic conditions led to widespread anatexis. SHRIMP U–Pb zircon ages from migmatitic leucosomes, granitic–tonalitic orthogneisses, and detrital to metamorphic zircons cluster tightly between 1.82 and 1.80 Ga, while 40Ar/39Ar muscovite ages indicate cooling at ∼1.80 Ga. This implies that the thermal peak, melt segregation, granite emplacement, and cooling occurred within ≤10 Ma. The near-synchronicity of high-grade metamorphism and magmatism, together with the crustal-scale D2 shear architecture, supports a model of hot, thickened crust undergoing vertical strain transfer during late Eburnean sinistral transpression. This study provides the first precise 1.82–1.80 Ga age bracket for the late Eburnean event in southwestern Angola, highlighting a well-preserved subhorizontal attachment zone that couples upper crustal shear with deep crustal flow.