Migmatites:A Mixture of Igneous and Metamorphic Rocks

Metamorphism transforms pre-existing rocks into distinctly new rocks (metamorphic) as the result of high temperatures and high pressures. Metamorphic rocks, when raised to temperatures and pressures at or near their melting point, form a migmatite— a rock that has almost melted and characteristically produces a fantastic display of mixed igneous and metamorphic rocks. A migmatite, intermediate between metamorphic and igneous rocks, is a mixed rock in which at least one part is igneous.

This migmatite, a mixture of igneous and metamorphic rocks, is actually a glacial erratic deposited by a retreating glacier at the end of the Ice Age in the Rocky Mountain National Park.  This migmatite is along the shore of Sprague Lake.  Photo date Aug, 2004, by S. Veatch.

High temperatures cause partial melting with segregation of granitic melt bands that form swirled banding. This banding reveals that the light colored minerals (felsic) have undergone melting and flow while the dark colored minerals (mafic) have not yet reached their melting point and have been contorted by flow. If a rock undergoes extensive metamorphism and light and dark minerals have been segregated, it is gneiss. If a rock undergoes partial melting with segregation of granitic melt bands it is a migmatite. To find out if a rock is a migmatite, carefully look at the felsic layers, if they have completely melted and re- crystallized, the rock is a migmatite. If re-melting has not taken place, it’s a gneiss.



The swirled banding of light-colored felsic minerals and dark-colored mafic minerals seen in this boulder are characteristic of a migmatite. Photo date Aug, 2004, by S. Veatch.