MELTING AND DIFFUSION-PROCESSES IN CLOSED-SYSTEM MIGMATIZATION

Estimated variations in mineral concentrations across leucosomes suggest that leucosomes are generated during anatexis by a diffusive exchange between the leucosome and the mesosome, and not by the migration of melt from the mesosome. However, the presence of melt is a precondition for the diffusive exchange to take place. Initially a crack is formed due to shear stress. The formation of a crack allows a diffusive exchange to take place through the melt, which causes melting of minerals situated near the crack. The diffusive exchange of material is less efficient in the mesosome where the melt is isolated at grain corners and edges. The microcline enrichment of some granitic leucosomes is thought to be due to the diffusive depletion of the mesosome caused by growth of alkali feldspar during the consolidation of the migmatite. In general, it seems unnecessary to invoke concentrations of water in the leucosome or the intrusion of external fluids or magmas for migmatite formation.