Clockwise, low-[formula omitted] metamorphism of the Aus granulite terrain, southern Namibia, during the Mesoproterozoic Namaqua Orogeny

Abstract

The Aus granulite terrain forms part of the Mesoproterozoic Namaqua Metamorphic Complex (NMC) of southern Africa. The terrain consists of pre- to syn-tectonic granitoid gneisses containing subordinate aluminous metasediments, metapsammites, mafic granulite and calc-silicate, all of which have been metamorphosed and migmatised to varying degrees. Pseudosection modelling of garnet–sillimanite–cordierite–biotite aluminous metapelitic samples constrain peak metamorphic conditions at 5.5kbar and 825°C. Pseudomorphs of sillimanite after andalusite constrain early prograde conditions of ∼550–600°C at below 4kbar, whereas the replacement of sillimanite by garnet–cordierite and subsequent partial replacement of garnet by cordierite at near-peak conditions indicates a clockwise trajectory for this segment of the P–T path. Subsequent near-isobaric cooling below the solidus resulted in secondary, texturally distinct sillimanite partially replacing cordierite. U–Pb zircon dating of pre-, syn- and post-tectonic granitoid intrusions indicates that granitoid plutonism occurred at c. 1120–1085Ma, whereas metamorphic overgrowths on these zircons constrains metamorphism to have occurred from c. 1065 to 1045Ma. A post-tectonic granite dyke intruded after the terrain cooled to subsolidus temperature has an age of 1004±6Ma. The timing and style of metamorphism is similar to that recorded in other parts of the NMC, suggesting that the various crustal fragments that constitute the NMC are not as allochtonous as previously suggested. The emplacement of voluminous granitoid magmas into this terrain triggered initial andalusite grade metamorphism through advective heating, with further slow burial of this juvenile crust leading to low-P–high-T peak metamorphic conditions. Magmatism and metamorphism appear to form a continuum, implying the presence of a long-lived (>50–70Ma), crustal-scale thermal anomaly as the cause of both. Such a primary anomaly most likely involves lithospheric delamination, although direct evidence for mantle involvement, such as contemporaneous mafic magmatism, does not occur in the Aus terrain.