LINKING PERCOLATIVE MELT TRANSPORT TO CHANNELED MELT TRANSPORT IN THE CRUST

Transfer of melt from source to final emplacement site initiates at the grain scale but must become channeled to be effective. However, the nature of the linkages between grain-scale percolative flow and channeled flow remains poorly understood. In an attempt to understand this intermediate stage of melt movement in crustal rocks, we present observations at different scales from regionally-developed migmatites from the Armorican Massif of France, part of the Variscides orogenic belt. Within the high-grade core of the southern Brittany metamorphic belt, stromatic migmatites show interconnected networks of leucosomes over several orders of magnitude. We see mm- to cm-scale concordant leucosomes, probably conduits for melt flow, that merge in apparent microstructural and compositional continuity with cm- to dm-scale syn-kinematic discordant leucosomes. These discordant leucosomes occur in discordant shear surfaces, inter-boudin and mullion partitions, fold noses, and parallel to axial surfaces of late folds. The concordant and discordant leucosomes merge in apparent microstructural continuity with a network of m-scale crosscutting sheets that typically extend over at least several tens of meters, and probably represent conduits that fed granites tens to hundreds of meters in thickness exposed at shallower levels in the crust. Leucosome in layers parallel to compositional layering/metamorphic fabric and in syn-anatectic structural sites, and the preferred orientation of the sheets indicate that pre-existing and syn-anatectic structures enabled melt movement at a variety of scales. The distribution of cordierite, a peritectic product of melting during decompression, is instructive. Cordierite occurs in structures such as mullion partitions to suggest that sites of melting were also controlled, at least in part, by deformation. Our observations have implications for understanding melt transfer in the crust, the rheology of melt-bearing crustal regions, and the geochemical signatures in upper crustal granites.