Coast Plutonic Complex

The Coast Range Arc was a large volcanic arc system, extending from northern Washington through British Columbia and the Alaska Panhandle to southwestern Yukon. The Coast Range Arc lies along the western margin of the North American Plate in the Pacific Northwest of western North America. Although taking its name from the Coast Mountains, this term is a geologic grouping rather than a geographic one, and the Coast Range Arc extended south into the High Cascades of the Cascade Range, past the Fraser River which is the northward limit of the Cascade Range proper.

The Coast Range Arc formed as a result of subduction of the Kula and pre-existing Farallon Plates. It is most famous for being the largest granitic outcropping in North America, which then it is usually referred to as the Coast Plutonic Complex or the Coast Mountains Batholith. It is a coast-parallel continental volcanic arc similar to the Andes of South America and the largest continential volcanic arc fossil in the world.

Volcanism in the arc began during the Late Cretaceous period 100 million years ago based on andesitic composition of the Early Cretaceous volcanic sections and their close temporal and spatial association with masses of felsic intrusive igneous rock with phaneritic texture called tonalite. The basement of the Coast Range Arc was likely Early Cretaceous and Late Jurassic intrusions. Stratigraphic and field relations in the arc suggest that the Coast Range Arc was created on Stikinia, a geologic feature that formed in an older volcanic arc environment during the Paleozoic and Mesozoic periods.

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