Pattern and source of unusually high-temperature metamorphism in an Eocene forearc recorded by the Pacific Rim Terrane, British Columbia, Canada

The Pacific Rim Terrane is of forearc affinity and one of the most recent crustal elements accreted to the North American Cordillera in western Canada. Two units, the Leech River Complex and Pandora Peak Unit, within the terrane were subject to high-temperature, medium-pressure metamorphism. Biotite, garnet and staurolite isograds occur concentrically in the Leech River Complex, centred on the Leech River shear zone at its southern boundary. A local thermal overprint in the Pandora Peak Unit is characterized by replacement of prehnite-pumpellyite and lawsonite-bearing assemblages with muscovite + chlorite. Pseudosection models (Perple_X), and thermometry using garnet-biotite Fe-Mg exchange and Raman spectroscopy of carbonaceous material (RSCM) show a thermal gradient at ~3.8 kbar from ~230°C in the north to ~600°C in the south. Isotherms are continuous across the Leech River–Pandora Peak boundary. The small-volume, interfoliated intrusions of Eocene age occurring throughout the terrane show no spatial relation to the isotherms. Elevated forearc metamorphism is due to the subcretion at ~51 Ma of nascent oceanic crust (and related spreading ridge or hotspot) of the underlying Siletz-Crescent terrane along the south-bounding Leech River shear zone. Our re-evaluation of the metamorphic history requires revision of the role of magmatism as a source of heat transport in forearc metamorphism and the tectonic assembly in this setting.