MELT SEGREGATION FROM MIGMATITES: ROLE OF COMPACTION BANDS

The process of extraction of magma from migmatite terranes is a key process in the formation of granitic bodies and therefore an essential process in the evolution of the crust. A common and puzzling feature of migmatite terranes is the presence of regularly spaced leucosomes oriented parallel to the axial planar orientation of folds. Current models and experiments predict that leucosomes should form at high angles to these planes rather than parallel to them, and suggest that our understanding of melt extraction is incomplete. Here, we propose that axial planar leucosomes and their regular spacing are a result of ductile compaction bands: instabilities that arise in compressive environments as a result of compaction of porous media with non-linear rheology and interstitial fluids. These instabilities form at high angles to the maximum compressive stress and their spacing is controlled by the compaction length.