Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks

The grain- and outcrop-scale distribution of melt has been mapped in anatec tic rocks from regional and contact metamorphic environments and used to in fer melt movement paths. At the grain scale, anatectic melt is pervasively distributed in the grain boundaries and in small pools; consequently, most melt is located parallel to the principal fabric in the rock, typically a f oliation. Short, branched arrays of linked, melt-bearing grain boundaries c onnect melt-depleted parts of the matrix to diffuse zones of melt accumulat ion (protoleucosomes), where magmatic flow and alignment of euhedral crysta ls grown from the melt developed. The distribution of melt (leucosome) and residual rocks (normally melanocra tic) in outcrop provides different, but complementary, information. The res idual rocks show where the melt came from, and the leucosomes preserve some of the channels through which the melt moved, or sites where it pooled. Di fferent stages of the melt segregation process are recorded in the leucosom e-melanosome arrays. Regions where melting and segregation had just begun w hen crystallization occurred are characterized by short arrays of this, bra nching leucosomes with little melanosome. A more advanced stage of melting and segregation is marked by the development of residual rocks around exten sive, branched leucosome arrays, generally oriented along the foliation or melting layer. Places where melting had stopped, or slowed down, before cry stallization began are marked by a high ratio of melanosome to leucosome; b ecause most of the melt has drained away, very few leucosomes remain to mar k the melt escape path - this is common in melt-depleted granulite terranes . Many migmatites contain abundant leucosomes oriented parallel to the foli ation; mostly, these represent places where foliation planes dilated and me lt drained from the matrix via the branched grain boundary and larger branc hed melt channel (leucosome) arrays collected. Melt collected in the foliat ion planes was partially, or fully, expelled later, when discordant leucoso mes formed. Leucosomes (or veins) oriented at high angles to the foliation/ layering formed last and commonly lack melanocratic borders; hence they wer e not involved in draining the matrix of the melting layer. Discordant leuc osomes represent the channels through which melt flowed out of the melting layer.