Melt-dominated mid-crust in the 575–500 Ma Damara Orogen: Field constraints on an interconnected migmatite–granite system

Thomas Lloyd Jones, Alex Otto, Ed Becker, Andy Wilde

Modern thermomechanical models emphasise the role of partial melting and granite magmatism on the structural and tectonic evolution of orogenic belts. Partially molten mid-crust can trigger a change from localised deformation to distributed ductile flow, while also accommodating chemical differentiation of the crust. In the southern Central Zone of Namibia’s Damara Orogen, it is unclear whether high-temperature granulite-facies rocks consisted of relatively isolated zones of crustal melting within a predominantly rigid crust, or contained sufficiently widespread partially molten rocks such as to be capable of modifying mechanical behaviour during deformation. Here, we use extensive cross-cutting field relationships, remote sensing, and whole-rock geochemistry to investigate the composition, physical state, and magmatic architecture in the cores of two dome structures. The outcrop area is dominated by metatexite and diatexite migmatites, larger interconnected leucogranite dyke networks and pluton-like accumulations, and subordinate entrained metasedimentary rafts and schlieren. Migmatites and leucogranites intrude into and cross-cut the overlying ca. 870-590 Ma Damara Supergroup along a diffuse and intrusive contact. Geochemical data records similar major, trace, and REE-element patterns between leucosomes in the underlying migmatites and leucogranite dykes intruding into the overlying Damara Supergroup. Overlying leucogranite dykes are more evolved than underlying leucosomes, consistent with differentiation during magma extraction and ascent. Partially molten crust therefore formed a volumetrically dominant interconnected domain throughout deeper structural levels in the cores of these dome structures, providing a foundation for broader discussion on deformation mechanisms and the potential for distributed ductile flow in the Damaran mid-crust.