Melt segregation and migration

Abstract

The processes by which melt separates from its associated solids are relevant to a wide range of geological problems. How is melt extracted during anatexis to form migmatites or granites? What drives melt segregation, especially at low melt fractions? What volume fraction of melt can be extracted? How do residual liquids percolate through accumulating crystal piles? How does melt reach high‐level plutons? These and related problems were discussed in a special session, “Melt Segregation and Migration in Partially Molten Rocks,” of the Geological Association of Canada‐Mineralogical Association of Canada (GAC‐MAC) annual meeting in Toronto, May 27–29. This session, sponsored by the MAC, attracted contributions on a wide range of field, structural, geochemical, theoretical, and geophysical studies.All of the field‐based studies brought out the close temporal and spatial association between shear zones and melt segregation. Sally Goodman of the University of Aberdeen showed that migmatization in the Crinan subgroup of the Scottish Dalradian began with metamorphic differentiation and progressed to extensive melting in shear zones associated with the influx of fluids. Although some melt pockets remained, rocks in the shear zone now consist largely of recrystallized restite material. Shearing at the time of melting apparently drove melt segregation, and the possibility that the expelled melt formed the nearby granite bodies is being investigated.