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Terranes of the North Cascades Terranes of the North Cascades: Swakane terrane

Location of Swakane Terrane rocks
Location of Swakane Terrane rocks shown in green.

Summary: Possibly metamorphosed volcanic arc rocks or an unusual sandstone that might be older than 570 million years (Precambrian).

The Swakane terrane, made up of the Swakane Biotite Gneiss, is one of the region’s most significant geologic mysteries. The best place to see it is along the Columbia River between Wenatchee and Chelan, just south of the area described in this website. Only a small part of the gneiss shows up on the geologic map shown in this website.

So far, the evidence for the origin and age of the Swakane terrane is inconclusive. The mineralogy and chemical composition of most Swakane Biotite Gneiss suggest that it could have begun either as a sandstone or as a volcanic rock, but not likely both, because it is monotonously uniform overall. Interlayered volcanic and sedimentary rocks, even when thoroughly metamorphosed, are more varied in look and composition.

 

Location of the Swakane terrane (SWT), a mystery terrane of the North Cascades.
Location of the Swakane terrane (SWT), a mystery terrane of the North Cascades.

Many years ago, pioneering geochronologist James Mattinson and colleague Professor Clifford Hopson separated zircon crystals from samples of the Swakane gneiss. Analyses of radioactive uranium and lead isotopes (see Newhalem field trip stop) in these crystals indicate that the zircons are very old, perhaps as old as 1,600 million years (older Precambrian). If the original rock was volcanic, then the crystals formed when the volcanoes were erupting. But if the rock was a sandstone, then the zircons are sand grains, older than the rock which contains them. The zircons crystals may have been eroded from an older continent. (Geologists know that the Swakane gneiss was metamorphosed with the rest of the Metamorphic Core Domain about 90 million years ago, and is thus older than that). Even though both the Swakane and surrounding rocks of the Chelan Mountains terrane are thoroughly metamorphosed and much of their original relationship is obscured, some evidence indicates the Chelan Mountains terrane was thrust over the Swakane. If the Swakane Gneiss is Precambrian in age, it is indeed an exotic terrane, one located many miles west of the nearest Precambrian rocks exposed on the older parts of North America. The Swakane terrane has probably traveled far.

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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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