Terranes of the North Cascades: Easton Terrane

Location of Easton Terrane rocks shown in green.

Summary: Formed from deep-ocean sand and mud and underlying ocean-floor basalt about 150 million years ago (Jurassic). Metamorphosed at especially high pressure to the Darrington Phyllite and Shuksan Greenschist respectively.

The uppermost terrane in the Western Domain, the Easton terrane, was originally oceanic basalt and overlying deep-ocean mud and sand. The basalt became what has long been called the Shuksan Greenschist; and the overlying sediments became what is known as the Darrington Phyllite. The metamorphosed basalt is not everyday greenschist. Shuksan Greenschist locally contains some unusual blue amphiboles, and the phyllite contains the uncommon mineral lawsonite.

A geologist rests on Mount Sefrit with a view of Mount Shuksan, carved from Shuksan Greenschist.

We know from experiments and geologic relations elsewhere in the world that rocks with these minerals (called blueschists if the blue amphibole is abundant) form only where rocks are buried deeply (to mantle depths, in fact) in a relatively cool environment and then regurgitated relatively rapidly. These conditions are most easily met where plates collide in a subduction zone. If the rocks had remained buried for a long time (tens of millions of years) they would have become hot enough for more ordinary green amphiboles and other minerals of high temperature metamorphic rocks to have formed. Blueschists are a sort of geologic Baked Alaska.

Ned Brown (professor of geology at Western Washington University and an ardent fan of the Shuksan Greenschist) and his colleagues have determined that the original ocean-floor basalt and overlying mud and sand may have formed about 150 million years ago (Jurassic). They were metamorphosed some 30 million years later, in the Cretaceous Period.