Partial-melt topology in statically and dynamically recrystallized granite

Microstructures of the Murray granite pluton (central Ontario, Canada) show evidence of both static and dynamic crystallization subsequent to partial melting. Backscattered electron analyses reveal interstitial K-feldspar and plagioclase at triple junctions of strain-free, isometric quartz grains. The geometry of the quartz-feldspar boundaries mimics the original topology of the quartz-melt contacts during crystallization. This conclusion is suggested by the occurrence of both rounded and planar faces of quartz grains, and by low (27°) dihedral angles of quartz-quartz-feldspar boundaries, similar to dihedral angles in experimentally crystallized quartz-quartz-silicic melt systems. In contrast, feldspar seams in deformed granites have high axial ratios, are usually elongated perpendicular to the foliation plane, and are located preferentially along individual grain boundaries. Quartz grains are dynamically recrystallized and occasionally transected by feldspar seams, indicating that fracturing occurred in the presence of melt during crystal-plastic deformation of quartz. The subparallel orientation in quartz grains of intragranular, feldspar-bearing fractures and interstitial feldspar seams suggests that these features originated as intragranular and intergranular fractures, respectively. Partial-melt topology was therefore controlled by intergranular and, occasionally, by intragranular fracturing.