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