Subduction Zone
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And some of the top layers of basalt too. The oceanic crust starts with pillow basalts on top. Those are basalts that were exposed to sea water on eruption. They tend to have structures or "pillows" telling us this. The structures are rounded in shape with a skin of very finely crystalized basalt where it directly met seawater and cooled almost instantly with a coarser, but still fine grained, interior. Below that are a sheeted dike complex and under that gabbros. Gabbro is the oceanic crust equivalent of granite. It is a coarse grained mafic (high in MAgnessum and FerrIC or iron based, minerals).So the Olympics is basically stuff scraped off the top of the Juan de Fuca plate as the rest of the plate subducts, as well as part of the North American plate folding at the edge from the force of the subduction going underneath.
Geology of Olympic - Olympic National Park (U.S. National Park Service).
file:///C:/Users/DOUGH/Downloads/4.13%20White%20and%20Klein%20Oceanic%20crust.pdf