Migmatite from the Precambrian of Minnesota, USA. (public display, Nebraska State Museum of Natural History, Lincoln, Nebraska, USA)

Migmatites are very high grade metamorphic rocks that form by partial melting of gneisses. The felsic minerals have melted, cooled, and recrystallized - they form the light-colored, crystalline-textured bands in the rock. The mafic minerals have higher melting temperatures and still retain their metamorphic foliated character.

The rock seen here is an Archean-aged migmatite from Minnesota's Morton Gneiss. Archean high-grade metamorphics in the Minnesota River Valley area of southern Minnesota, such as the Morton Gneiss and the Montevideo Gneiss, have been isotopically dated to ~3.4 to 3.5 Ga.

Brief description from Goldich et al. (1980):

The Morton Gneiss of southwestern Minnesota is a migmatitic hybrid rock. The paleosome consists of tonalitic to granodioritic gneisses and amphibolites; the neosome, of a variety of granitic gneisses.

Geologic unit & age: Morton Gneiss Complex, Archean, ~3.524 Ga

Locality: unrecorded/undisclosed site in Minnesota, USA

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