4 Some Remarks on Melting and Extreme Metamorphism of Crustal Rocks

Abstract: Typically melting occurs during decompression in ultra-high-pressure terranes, along the evolution from peak pressure to peak temperature in medium-temperature eclogite-high-pressure granulite terranes and by simple prograde heating in granulite facies and ultra-high-temperature (UHT) metamorphic terranes. The source of heat must be due to one or more processes among thickening and radiogenic heating, viscous dissipation, and heat from asthenospheric mantle. Melt-bearing rocks become porous at a few vol % melt initiating an advective flow regime. As the melt volume approaches and exceeds the melt connectivity transition ( ~ 7 vol % melt), melt may be lost from the system in the first of several melt-loss events. In migmatites and residual granulites, a variety of microstructures indicates the for-mer presence of melt at the grain scale and leucosome networks at outcrop scale record melt extraction pathways. This evidence supports a model of focused melt flow by dilatant shear failure at low melt volume as the crust weakens with increasing melt production. Melt ascent is initiated as ductile fractures but continues in dykes that propagate as brittle fractures. Crustal rocks undergo melting via a sequence of reactions beginning with minimal melt pro-duction at the wet solidus (generally < 1 vol % melt, unless there is influx of H2O-rich fluid)