On the Pseudomorphing of Melt-filled Pores During the Crystallization of Migmatites

Abstract

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 melt-filled 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 one. 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 reaction-controlled 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 H2O on grain boundaries.