MELT SEGREGATION AND DEFORMATION INTERACTIONS IN CRUSTAL ROCKS: CONSTRAINTS FROM MIGMATITES IN THREE CONTACT AUREOLES, MAINE, USA

Deformation affects melt movement and mechanisms of crustal differentiation, and the presence of melt has implications for tectonic evolution of orogens. However, the protracted nature of deformation and recrystallization associated with regional migmatites means they preserve imperfect records of melt segregation and migration. To minimize this problem, we are studying migmatites in aureoles of three Acadian plutons. In order of increasing emplacement depth and duration of anatexis these are: Moxie mafic complex; Onawa granodiorite pluton; and Flagstaff Lake igneous complex. These show different relative timing of emplacement and regional deformation. Due to this range of conditions, the behavior of rocks inferred to have contained different amounts of melt, and affected by different intensity and duration of imposed stresses can be constrained. Macroscopically, low-melt-fraction rocks show mm-thick layer-parallel leucosomes bounded by thin melanosomes, interpreted to have formed by grain-scale flow of melt from less to more competent layers. With increasing melt-fraction, layer-parallel leucosomes are thicker and link to cross-cutting leucosomes, the spatial distribution of which is affected by deformation. Microscopically, layer-parallel leucosomes and mesosomes in low-melt-fraction rocks show cuspate feldspar grains at triple junctions of rounded quartz interpreted to represent pseudomorphs after inter-granular melt pockets in a framework of solid quartz grains. Locally, these pockets link to form connected networks extending over many grain diameters. These networks show preferred orientations related to macroscopic structures. With increasing amount of cuspate feldspar, the extent of connectivity increases, but pockets maintain preferred orientation. These observations suggest melt segregation by grain-scale flow through an interconnected grain-boundary network, the geometry of which was controlled by imposed differential stress. Cryptic bright vein-like networks observed in SEM-cathodoluminescence of rounded quartz suggest that intra-granular melt-bearing fractures also accommodated grain-scale melt movement. The coarser-grained and more melt-like composition of cross-cutting leucosomes indicates these represent conduits for bulk melt flow. Our observations suggest melt-enhanced grain-boundary sliding and melt-enhanced cataclastic flow may have been important grain-scale deformation mechanisms during anatexis.