Before melt will segregate from its restite and source, there must be both a driving force and space into which the melt can move. The continental crust is anisotropic and in most cases is subject to deviatoric stresses during melting. Under these conditions, melt segregation begins when an interconnected network of melt forms and the source becomes permeable. Melt is driven, by pressure gradients, from its restite toward dilatant sites set up in the heterogeneously deforming source. The source is continually drained of melt and so need never contain more melt than that required to maintain permeability (<5%). Thus, the source remains coherent and ductile even with large degrees of partial melting. Successive melt increments derived from the same source record progressively higher degrees of partial melting. Further geochemical complexity may arise if melt segregation occurs before chemical equilibrium is reached between melt and solid, and disequilibrium compositions are preserved. Special conditions of rapid melting, or the absence of applied stresses, are necessary in the continental crust for the volume of melt to accumulate in the source to the point (>26%) where the solid matrix breaks down and diapirism can occur.

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