Study of strain and partial-melt transfer in a banded migmatite

An attempt is made to analyse in detail the strain and area change around the tip of a foliation-parallel leucosome. Strain is determined in the mesosomes above and below the leucosome and the melanosome, in both the XZ (foliation and lineation parallel) and YZ (foliation parallel, lineation perpendicular) planes, using the following methods: March analysis of biotite orientation fabric; Fry method of zircon position fabric; and Rfφ′ method using cordierite grain outlines. In addition, March analysis of the orientation of sillimanite inclusions within the cordierite grains helps confirm the last method. Area change is measured by determining the population density of zircon around the leucosome in the XZ plane. The amount of material lost can account for the area gain in the leucosome and we suggest the sample, at the observed scale, was ‘closed’ in terms of melt movement. The results are used to construct tentative strain paths for the various components of the migmatite. Evidence suggests the leucosome was filled by true melt. Fluid deformation of the leucosome melt is demonstrated by the orientations of zircons.