Evidence of 1.7- to 1.8-Ga Collisional Arc in the Kaoko Belt, NW Namibia

Abstract

Detailed structural and isotopic analyses in the Hoanib and Ugab River Valleys indicate the existence of an exotic 1.7- to 1.8-Ga terrane in the Pan-African Kaoko Belt. This crustal block, called as Mudorib Complex, is imbricated between autochthonous and para-autochthonous rocks of Congo Craton, Kaoko Basin, and Western Kaoko Batholith units during the main tectono-thermal phase of Kaoko Belt collision around 580 Ma, involving the Rio de La Plata, Congo and Kahalari paleoplates. This terrain is positioned between the 1.9-Ga Pruwes Complex units of SW edge of the Congo Craton and the 0.58- to 0.55-Ga Amspoort Suite granitoids of the Western Kaoko Batholith. It is coincident with a regional positive aeromagnetic anomaly trending from NNW in the Ugab region to the Namibia-Angola border. Internally, Mudorib Complex consists in 1.73- to 1.81-Ga tonalitic–trondhjemitic–dioritic–granodioritic sequence of gneisses associated with cogenetic gabbroic and anothositic-gneisses in the core zone of this Pan-African structure. Field relationship and U–Pb zircon and Sm–Nd whole-rock isotope data combined with geochemical information suggest the existence of two rock associations in the Mudorib Complex, namely late Paleoproterozoic tonalitic–trondhjemitic–dioritic-gneisses with island-arc affinity and tholeiitic metabasites of juvenile origin, showing Nd model age of 1.73–2.17 Ga and eNd(t) of −2.05–+4.3. This 1.8- to 1.7-Ga complex is also intruded by granitic dykes formed at 1.49–1.50 Ga with Nd model age of 1.75–2.34 Ga during stable tectonic conditions. In addition to widespread Pan-African tectono-metamorphic events, a secondary metamorphic event of ~1.3 Ga is also recognized in the Mudorib rocks, which may be associated with accretion process of the complex to the Paleoproterozoic to Archean nucleus of the Kaoko Belt in the Hoanib River Valley.