Abstract

Some possible chemical and morphological criteria to distinguish different migmatization mechanisms are considered and examples of the different processes discussed. Leucosomes formed by anatexis are likely to contain K-feldspar and should have a more sodic plagioclase than the restite rocks. Metamorphic segregation can cause a slight fractionation of albite into the leucosome only. Anatectic leucosomes might form irregular bodies or might agmatize the restite; close-spaced planar veins are improbable. Leucosome veins formed by hydrothermal processes are likely to be coarse grained or pegmatitic, whereas water-rich anatectic melts may freeze readily to aplites.

The criteria developed are applied to the Skagit Gneiss migmatites, and although the temperature of metamorphism was high (<700 °C), an anatectic model is rejected, as in earlier studies. Much of the migmatization that accompanied regional deformation and metamorphism was due to metamorphic segregation. Many other leucosomes are believed to be related to an extensive suite of plagioclase pegmatite dikes, probably of hydrothermal origin, that were emplaced at a late stage. These dikes are of uniform composition (plagioclase ≃ An20), irrespective of the host rock, and the plagioclases of some feldspathized schists tend toward this composition.

Recent fluid-inclusion studies have shown that high-grade rocks have a mixed CO2-H2O fluid. Therefore, partial melting is unlikely to begin until temperatures well in excess of the water-saturated granite solidus are reached. Melts so formed might readily coalesce and migrate out of the source rock without producing an extensive migmatite zone. Many migmatite terranes may have formed by predominantly hydrothermal processes, notably metamorphic segregation, and it is possible that this might represent a particular type of tectonic environment.

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