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USGS Geology in the Parks

Geologic story of the North Cascades button North Cascades National Park:
Orthogneisses and Migmatites

Skagit gneiss with dike
Banded Skagit gneiss with cross-cutting dike of younger granite orthogneiss.

Orthogneiss of at least two different ages makes up much of the Skagit Gneiss Complex. Some magma intruded while rocks of the Metamorphic Core Domain were being squeezed and probably folded. While the magma crystallized or soon thereafter, the squeezing (or flattening) aligned the minerals and the rock became foliated orthogneiss. Twenty million years or so later, much of the squeezing had ceased. When new magma invaded, the newly crystallized granitic rocks developed much less foliation than their predecessors. But because they were subjected to an episode of stretching (see Shifting the Pieces), these younger granitic rocks exhibit strong lineation; stretched minerals look like pencils, or on broken surfaces of the rock, scattered parallel dashes. (For more information on foliation and lineation, visit Diablo Dam.)

Most of the older orthogneiss has the composition of tonalite. Most of the younger orthogneiss has a composition closer to granite. From isotopic dating of zircons, geologists know that the older magmas solidified about 65-90 million years old (Cretaceous) and the younger ones about 45 million years old (Eocene). The older orthogneiss bodies in the Skagit Gneiss Complex are stitching plutons. The younger ones embroider the already stitched-together terrane mosaic.

Sketch of migmatite in Skagit Gneiss
In many areas, the complex sequence of invading magma and deformation left a confusing mixture of rocks. Gneiss was cut by light-colored dikes or sills which were then all squeezed and deformed. This deformation was followed by intrusion of still more dikes and further squeezing and stretching. Even more dikes may have intruded after that. Geologists call such rocks migmatites (from the Greek for mixed rocks), but they are hard pressed to describe them.

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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