Petrologic, structural, and age relations of serpentinite, amphibolite, and blueschist in the Shuksan Suite of the Iron Mountain–Gee Point area, North Cascades, Washington

Abstract

Part of the Shuksan blueschist terrane, near Iron Mountain and Gee Point, North Cascades, Washington, has associated serpentinite, amphibolite, barroisite schist, blueschist, rare eclogite, and blackwall-type metasomatic rock. Field, petrographic, and microprobe observations indicate that the amphibolite and barroisite schists were metamorphosed in contact with peridotite and suggest that the peridotite may have been a heat source. The serpentinite and associated rocks are structurally concordant with the regional blueschists and have been overprinted by blueschist metamorphism. Isotopic dating gives metamorphic ages of 148 ± 5 to 164 ± 6 m.y. for the amphibolite and barroisite schist and 129 ± 5 m.y. for nearby regional Shuksan blueschists. The origin of the serpentinite + amphibolite + blueschist assemblage is interpreted to be the result of sequential events in a subduction zone. As subduction began, oceanic crustal materials underwent high-temperature metamorphism along the hot ultramafic hanging wall and were converted to amphibolites; materials that were subducted later came in contact with a cooler hanging wall and recrystallized as blueschist. This hypothesis may be applicable to the origin of similar rock associations in the Franciscan terrane and other orogenic belts.