No abstract

Migmatites in the Damara Belt in Namibia preserve relationships between anatectic leucosomes and intrusive leucogranite sheets illustrating the consecutive stages of partial melting, local melt segregation and far-field mobilization of melts over hundreds of metres and out of the anatectic region. Initial melting and localized segregation of melts are controlled by gradients in fluid pressure created by pervasively developed, shallowly dipping dilatant fractures. Subsequent melt transfer out of these initial sites of melting and melt storage occurs along subvertical, disc-shaped leucogranite sheets that intersect the leucosomes. The leucogranite sheets propagate as isolated, melt-filled hydrofractures, driven by the pressure differential along the subvertical fractures. The collapse of wall rocks into former melt-bearing fractures, and the presence of residual peritectic phases of the melting reaction trapped in wall rocks testify to the efficient extraction of melt by the mobile hydrofractures during their ascent. This process of melt drainage into and transport by hydrofractures leaves almost no trace of the ascent conduits. This study also shows that melt transport and the stability of melt pathways in strongly layered mid-crustal levels characterized by low deviatoric stresses are determined by the presence of pre-existing anisotropies and the progressively evolving structure in deforming orogens.