The stabilization of self-organised leucogranite networks—Implications for melt segregation and far-field melt transfer in the continental crust

Abstract

Net-structured leucocratic vein systems are ubiquitous in high-grade and migmatite terrains and are interpreted to represent interconnected former magma-bearing structures that accommodated the migration of granitic magmas out of their anatectic source regions. Net-structured leucogranite vein systems in high-grade gneisses from the Damara Belt in Namibia describe a size-based hierarchy and spatial distribution that indicates self-organisation of the magma transfer system. The internal geometry and connectivity of the net-structured leucogranites are controlled by variations in melt pressure, whereas regional strains control the orientation of the leucogranites. Foliation-parallel (stromatic) and shear-band-hosted granite veins accommodate transport at relatively low melt volumes and account for the metre-scale segregation of melt close to the anatectic source. During times of higher melt supply, the formation of extensional fractures that cross-cut stromatic and shear-band-hosted melt sheets at high angles is promoted. Extensional fracturing and associated dilatancy create gradients in magmastatic head that cause melt to be drained from pervasive stromatic and shear-band-hosted vein networks into spaced dilatant fractures. This has the potential to initiate buoyancy-driven far-field melt transfer along suitably orientated extensional fractures as vertical dykes or self-propagating hydrofractures. Our results suggest that melt transfer systems are able to adjust to variations in melt supply by forming conduits that accommodate varying melt volumes and will tend to develop towards a state of self-organisation.