Pseudomorphs of melt-filled pores, recognized by their generally cuspate shape, are used as diagnostic for the former presence of partial melt. They are commonly observed in migmatites from the mid-to deep crust although they occur in the smaller pores in migmatites from shallower levels (1^2 kbar). The pseudomorphing of meltfilled pores is controlled by the kinetics of nucleation and is a consequence of the greater supersaturation required for nucleation in a small pore compared with a larger

more »

... ne. We examine three migmatites in detail: a contact metamorphosed cherty band from an iron formation; an Archaean regional granulite from an accretionary prism; and an amphibolite-facies sample from the roots of an Archaean mountain chain. The greater undercooling required for nucleation in progressively smaller pores is recorded by the composition of plagioclase pseudomorphs. A study of dihedral angles at the corners of pseudomorphed pores demonstrates that melt^solid textural equilibrium was probably attained only in the contact aureole.The regional granulite preserves an almost unmodified reaction-controlled melt distribution, with little evidence for either melt^solid textural equilibration or solid^solid re-equilibration, whereas the reactioncontrolled melt distribution in the regional amphibolite-facies example has been modified by a partial approach to solid^solid textural equilibrium. It is not clear whether the differences in dihedral angle population are due to differences in uplift and exhumation rates or due to the presence of H 2 O on grain boundaries.