Magma accumulation and segregation during regional-scale folding: The Holland’s dome granite injection complex, Damara belt, Namibia

• The geology of a regional-scale, mid-crustal granite injection complex is described.
• Leucogranites intrude as high-angle sets of intersecting, interconnected sheets.
• Magma accumulation and melt segregation are related to fold amplification.
• Multiple sheeting records episodic segregation of small melt batches.
• Incremental, deformation-driven melt segregation leads to fractionation of magmas.

The regional-scale, upright fold of the Holland’s dome in the Damara belt of central Namibia contains a kilometre-scale network of intrusive, highly fractionated uraniferous leucogranites. Three broadly orthogonal and intersecting sets of leucogranite sheets that intruded parallel and at right angles to the axial plane of the first-order fold can be distinguished. The granites are internally sheeted and illustrate the growth of the injection complex through the successive addition of thousands of smaller magma batches. Spatial and timing relationships point to a stepwise evolution of the injection complex. Early dilatancy-driven segregation and accumulation of granitic magmas in the core of the fold, above a basal detachment, was followed by compaction-driven segregation of a melt phase during fold tightening. The intersecting leucogranite sets provide a suitably organized permeability structure for melt segregation, while the successive injection of magma batches ensures compatibility between regional strain rates during folding and the rates of magma segregation. The three-dimensional network of melt-bearing structures further assisted regional shortening past the lock-up of the fold. The Holland’s dome injection complex illustrates the geometric complexity of magma transfer pathways and the significance of regional-scale folding for the accumulation, segregation and fractionation of granitic magmas in suprasolidus crust.