ALBERT

Description: Ridge subduction is an inescapable plate tectonic process, but has only been documented in modern circum-Pacific environments and not yet been recognised from suture zones associated with supercontinent assembly, likely because its imprint is obliterated by later collision. The formation of the Pan-African Damara Belt of central Namibia involved northwards subduction of the Khomas Sea underneath the Congo Craton, prior to final suturing of the Congo and Kalahari Cratons. The accretionary history of the Belt is preserved in the Southern and Southern Marginal Zones, which consist of turbiditic metasedimentary and intercalcated mafic rocks with MORB affinity. Two localities in the Kuiseb and Gaub canyons reveal that aluminous metapelites contain a fabric-defining assemblage of fine-grained muscovite, chlorite, biotite, quartz and graphite that is overprinted by randomly-oriented porphyroblasts and poikiloblasts of garnet, staurolite, kyanite and biotite. Associated metamafic rocks consist of hornblende, chlorite, epidote, rutile and quartz, with actinolite cores preserved in amphibole porphyroblasts. Metamorphic conditions for the fabric-defining assemblage are estimated at _10 kbar and 540–560 ◦C, whereas peak metamorphism likely occurred at 10–10.5 kbar and 600 ◦C. Consequently, these rocks preserve a two-stage prograde metamorphic history where initial tectonic burial was followed by relatively rapid, near-isobaric heating without attendant deformation to peak metamorphic conditions. We propose that initial burial occurred through subduction and underplating to the accretionary prism, before ridge subduction and opening of a slab window heated the rocks to peak metamorphic conditions. The exceptional preservation of the tectono-thermal imprint of the accretionary orogenic stage is due to the relatively soft, largely aborted collision that characterised the Damara orogeny, which can be attributed to the confined extent of the Khomas Sea. This article is protected by copyright. All rights reserved.