Geology of the Shuksan Suite, North Cascades, Washington, U.S.A.

The Shuksan Metamorphic Suite (Misch 1966) and correlative Easton Schist (Smith 1903) occur on the western flank of the North Cascades of Washington as large fault-hounded fragments (10s of kms long) imbricated with other rock units in a north-south belt extending more than 180 km. The Shuksan Suite is dominated by greenschist, blueschist, quartzose carbonaceous phyllite, and quartzofeldspathic semi-schist. A Jurassic, oceanic, near-arc tectonic site of deposition is hypothesized. Metamorphism began with production of Late Jurassic high pressure amphibolites in a contact aureole localized near peridotite, and was followed by regional blueschist facies metamorphism that lasted into the Early Cretaceous. Phase assemblages of the regional metamorphism include (in addition to quartz, chlorite, phengite, and sphene) lawsonite + albite in pelitic and psammitic schists, actinolite + albite + epidote + pumpellyite in greenschists, and crossite + albite + epidote + iron oxide in blueschists. The protolith rock type markedly controlled the development of the metamorphic index minerals. Temperatures of regional metamorphism ranged from approximately 330 to 400°C; pressures, not easily estimated, were perhaps 7 to 9 kilobars. Deformation during the blueschist metamorphism was directed approximately normal to the continental margin, as evidenced by the regional pattern of shear lineations. Uplift and imbrication of the Shuksan Suite with other units occurred by fault motion parallel to the continental margin. The metamorphic and uplift events are correlated with periods of high angle and low angle convergence respectively between the Farallon and North American plates.