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Terranes of the North Cascades Terranes of the North Cascades: Chelan Mountains Terrane

Location of Chelan Mtn. Terrane rocks
Location of Chelan Mountains Terrane rocks shown in green.

Summary: Metamorphosed deep ocean deposits and metamorphosed volcanic arc rocks and sediments. Includes a metamorphosed arc-root pluton and overlying arc rocks that are 220 million years old (Triassic).

The Chelan Mountains terrane contains both metamorphosed oceanic and arc rocks. The ocean-born rocks characteristically contain metamorphosed basalts; they are now amphibolite (hornblende and plagioclase rock) or hornblende schist. Some are metamorphosed cherts called metachert, mica-quartz schist, or metaquartzite. Metamorphosed sandstones and shales are also present, now as mica schists. In addition, the ocean-born rocks contain marble derived from marine animals, but no fossils are preserved. Scattered small pods of metamorphosed ultramafic rocks indicate that bits of mantle were incorporated into the ocean floor rocks at some stage in the development of the Chelan Mountains terrane, such as when the oceanic plate riding on the mantle swept into a subduction zone prior to metamorphism or when pieces of uplifted mantle, crunched up at the edge of an earlier subduction zone slid into the ocean deeps. The ocean-born metamorphic rocks have various local names such as the Napeequa Schist and Twisp Valley Schist.

 

View of Mount Johannesburg, made mostly of the Cascade River Schist.
View of Mount Johannesburg, made mostly of the Cascade River Schist.

In the Chelan Mountains terrane, rocks born of a volcanic arc are mostly metamorphosed sandstone and conglomerate. The sandstones are now mica schists. The metamorphosed conglomerates metaconglomerate look like schists, but many still retain their characteristic cobbly look on some surfaces. The metamorphosed arc rocks also include hornblende schist and amphibolites from basaltic volcanic rocks, along with white mica schists derived from explosively erupted volcanic ash. These rocks are known all together as the Cascade River Schist.

Especially impressive in the Chelan Mountains terrane are remnants of the plutonic root of the old volcanic arc, known to geologists as the Marblemount pluton or plutons, which form high ridges in a long southeast-trending belt (see "MM" and "MD" on geologic map). This rock forms the imposing west ridge of Lookout Mountain east of Marblemount, makes up Le Conte Mountain and Old Guard Peak on the South Fork of the Cascade River, and holds up imposing peaks to the southeast. We know from radiometric dating that the volcanic arc fed by the magma that formed the Marblemount plutons thrived about 220 million years ago (Triassic). For an explanation of radiometric dating, click here.

Metaconglomerate in the Cascade River Schist contains boulders of the Marblemount plutons, indicating that streams were eroding the cooled, solid pluton when the arc rocks were deposited.
Many rocks of the Chelan Mountains terrane have been so thoroughly metamorphosed and laced with younger granitic igneous material, some as young as 45 million years old (Eocene), that we include them in the Skagit Gneiss Complex, described in the section on metamorphism.

Visit Ruby Mountain to see more examples of the ocean-born rocks of the Chelan Mountains terrane
Visit the Nason Terrane
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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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