Compaction-driven melt segregation in migmatites

A common and puzzling feature of migmatite terranes is the presence of synkinematic leucosomes oriented perpendicular to the maximum principal compressive stress axis, s1, particularly leucosomes oriented parallel to the axial plane of folds. These are parallel to planes of compaction, but experiments predict that leucosomes should form in dilational bands oriented instead at small angles to s1. This discrepancy suggests that our understanding of melt extraction is incomplete. We propose that leucosomes at high angles to s1 are results of ductile compaction instabilities that arise in compressive environments as a result of compaction of porous media with nonlinear rheology and interstitial fluids. These instabilities form at high angles to s1 and their spacing can be regular, controlled by the compaction length. INTRODUCTION Anatectic terranes are characterized by complex networks of leucosomes interpreted to form by segregation of melt from the interstices of partially molten surroundings and thought to represent the roots of channels that extract magma from its source. Leucosomes have varied shapes and orientations, commonly in complex relationships. Some form sheets that are subparallel to dilational shear zones, at small angles to the maximum principal compressive stress axis, s1, as predicted by experiments (e.g., Butler, 2010; Holtzman et al., 2003; Katz et al., 2006; Kohlstedt and Holtzman, 2009; Rabinowicz and Vigneresse, 2004). Many others form sheets at high angles to these, and are parallel to the axial plane of folds or to contractional shear zones. These are interpreted to form at high angles to the inferred s1 (Fig. 1) (see Hand and Dirks, 1992; Rabinowicz and Vigneresse, 2004; Vernon and Paterson, 2001; Weinberg and Mark, 2008). At a larger scale, it is common in magmatic arcs to have elongated sheeted plutons also at right angles to the contractional direction (Ingram and Hutton, 1994; Miller and Paterson, 2001). This is puzzling because this orientation is perpendicular to mode 1 tension fractures, and at a high angle to melt segregations in dilational bands in experiments (cf. Figs. 2A and 2B). Vernon and Paterson (2001) proposed a number of explanations for the origin leucosomes parallel to axial plane of folds, including (1) occasional switches between s1 and s3 during deformation, (2) melt injection into axial planar foliation, (3) localization of later folds by the anisotropy represented by leucosomes, (4) preferential melting along the axial plane due to preferential fluid diffusion or influx, or (5) increased strain along these planes (Hand and Dirks, 1992). Alternatively, Weinberg and Mark (2008) proposed that axial planar leucosomes result from the self-organization of the melt network during folding so that volume is reduced through melt transfer along axial planar leucosomes, causing