Metamorphic history of the Coast Mountains orogen, western British Columbia and southeastern Alaska

Abstract

The northern Coast Mountains are underlain by the Coast Plutonic Complex, which separates the Intermontane and Insular superterranes. Syndeformational contact and regional metamorphism associated with compressional and transpressional tectonic events occurred from about 101 to 50 Ma, followed by contact metamorphism adjacent to late Oligocene and Miocene plutons and stocks. Metamorphic events comprise pre-101 Ma regional metamorphism (M R 1 ), 101-90 Ma contact metamorphism (M C 2 ) and related regional metamorphism (M R 2 ), 90-85 Ma regional metamorphism (M R 3 ) that formed during thrusting that inverted metamorphic isograds, and 72-50 Ma contact and regional metamorphism (M C - R 4 ) in the central part of the orogen. Middle to Late Cretaceous moderate- to high-pressure metamorphic assemblages in the western metamorphic belt record tectonic burial and subsequent exhumation. Late Cretaceous to Tertiary sillimanite-zone metamorphic assemblages in the central pluton gneiss and the adjacent edge of the western metamorphic belt record emplacement and subsequent exhumation of plutons in the Paleogene Coast Mountains batholith, including Coast Plutonic Complex sill plutons. The final metamorphic event was ca. 25-20 Ma contact metamorphism (M C 5 ) associated with gabbroic and granitic plutons.