The early syn-tectonic mafic to felsic Oamikaub Ring Complex (Damara orogen, Namibia) revisited – No role for a Pacific-type subduction?

Abstract

The Damara orogen of Namibia is an essentially ensialic orogen that was formed in the Neoproterozoic between ∼ 750 and ∼ 450 Ma. Most igneous rocks have negative εNd values and radiogenic Pb isotope compositions reflecting their crustal provenance. Mafic rocks are rare. The ∼ 564 Ma-old Oamikaub Ring Complex as part of the Goas Intrusive Suite belongs to a suite of mafic (ultramafic) to intermediate rocks that mark the onset of collision tectonics in the Damara Belt of Namibia. Therefore, they are particularly suitable to constrain the contribution of mantle material during the early stages of the Damara orogeny. The Oamikaub Ring Complex contains gabbros, gabbro-diorites, diorites, granodiorites and granites. All gabbros, gabbro-diorites and diorites are moderately enriched in most incompatible elements including REE but depleted in Rb, Ba and Sr. HFSE are depleted relative to primitive mantle (Zr/Hf: 28.3–40.5, Nb/Ta: 7.88–19.2) as a result of accumulation processes and the presence of enriched interstitial melts. The Sr and Nd isotopic compositions are variable (initial ⁸⁷Sr/⁸⁶Sr: 0.7058 to 0.7095; initial εNd: −1.9 to −11.8). The granodiorites are also enriched in incompatible elements showing similar HFSE abundances as the gabbros, gabbro-diorites, and diorites (Zr/Hf: 29.4–37.3; Nb/Ta: 8.25–19.7) and more evolved initial ⁸⁷Sr/⁸⁶Sr (0.7078 to 0.7103) and initial εNd (−10.7 to −14.1). Granites show lower Zr/Hf (18.7–28.7) but broadly similar Nb/Ta (4.25–14.6). Strontium and Nd isotopes are distinct relative to the other rock types (initial εNd: −18.2 to −18.8; initial ⁸⁷Sr/⁸⁶Sr: 0.7117–0.7128) implying an ancient lower crustal source. There are two competing models for the Damara orogen; a “Pacific-type” model involving subduction of oceanic crust and an ensialic model with subduction of mainly extended continental crust. Our new data from the Oamikaub Ring Complex, especially from the gabbros, gabbro-diorites and diorites, lack depleted isotope signatures but bear a strong crustal flavour for all rock types making a common subduction zone environment involving the subduction of oceanic crust unlikely. The rocks of the Oamikaub Ring Complex have isotope signatures similar to “lower crust” that are explained by “flat” subduction involving ancient continental mantle lithosphere and continental crust without melting of depleted asthenospheric mantle.