Stop 7: Migmatite
In the Okanogan Highlands, the down-faulted grabens are mostly filled by volcanic and sedimentary rocks from the upper crust. Up-faulted blocks bordering the grabens are rocks of the metamorphic core complexes, which consist of metamorphic and granitic rock from deep in the crust that domed upward and became revealed at the earth's surface by erosion. At this stop, we are in a metamorphic core complex zone between the Republic graben and the Toroda graben. The rocks here are a combination of light-colored granitic rock and dark-colored metamorphic rock. The two rock types are so intimately interlayered and mixed together that this type of rock is called migmatite, which means, “mixed rock.” The metamorphic rock here is dark-colored schist formed mostly of black mica (biotite), recrystallized from mud and silt deposited on an ocean floor.
The light-colored granitic rock, which varies from medium-grained granite to a white, coarse-grained type called pegmatite, formed from partial melting of the metamorphic rock. The granitic rock, especially the coarse white pegmatite, contains red garnets.
Participants take a close look at the granitic rock with garnets.