Evolution of ptygmatic folds in migmatites from the type area (S. Finland)

It is proposed that ptygmatic folds in migmatites form during leucosome–melanosome differentiation. Volume loss (by depletion of quartz and feldspar) takes place in the interlimb domains of folds, leaving a residual melanosome. Buckling leucosomes increase in volume due to this process and are protected from substantial thinning. Concurrent with tectonic segregation, albeit at a lower rate, the leucosome expands by replacing the melanosome. This process is induced by fluid activity at the leucosome boundary, as suggested by the occurrence of secondary offshoot veinlets. Segregation and growth causes the most typical aspect of ptygmatic folds: preservation of an overall buckle shape during the stage when the folds become isoclinal and devoid of interlimb matrix.