GRAIN-BOUNDARY MELT DISTRIBUTION AND DEFORMATION MECHANISMS IN MELT-BEARING CRUSTAL ROCKS

In melt-bearing rocks, the grain-scale spatial arrangement of melt and solid phases exerts a strong control over which of several possible deformation mechanisms might accommodate strain. To investigate this control, we have constrained the shape and distribution of inferred grain-boundary melt in anatectic migmatites from shallow contact aureoles surrounding late-kinematic Devonian plutons in central Maine. To do this we use microstructural criteria for the identification of former melt, such as cuspate outlines of mineral aggregates inferred to mimic melt-solid dihedral angles, rational faces of grains inferred to have grown in contact with melt, and compositional zoning patterns in feldspar. In concordant quartz-rich leucosomes from migmatites in the inner aureoles, feldspars form elongate grains with cuspate boundaries with sub-equant, rounded grains of quartz. This microstructure suggests crystallization of feldspar from a grain-boundary melt located between quartz grains that remained largely solid, except for inferred magmatic overgrowths on quartz added during crystallization of the melt . Aggregates of cuspate feldspar locally link together to form extensive strings between rounded grains of quartz, suggesting melt connectivity over distances much greater than the grain size of the solid matrix. In addition, cuspate feldspar grains and aggregates show a shape-preferred orientation (SPO). In some cases, this SPO is parallel to shear bands and an associated macroscopic foliation, whereas in other cases, the feldspar SPO shows no clear relationship to other structural elements. This observation suggests different controls on grain-scale melt distribution. In some cases, melt distribution is controlled by pre-existing or syn-anatectic structures, whereas in other cases, melt distribution may be controlled by the instantaneous stresses acting on the melt-bearing rock. In either case, the interconnected grain-boundary melt inferred in these migmatites suggests that grain-boundary diffusion-enhanced deformation mechanisms will be favored in crustal rocks containing melt.