Correlation of Settler Schist with Darrington Phyllite and Shuksan Greenschist and its tectonic implications, Coast and Cascade mountains, British Columbia and Washington

Amphibolite-facies Settler Schist in the southeastern Coast Mountains of British Columbia has long been correlated with Chiwaukum Schist of the Cascade metamorphic core, North Cascade Mountains, northwestern Washington. The additional correlation proposed here of Settler Schist with Darrington Phyllite and Shuksan Greenschist (and blueschist) of the Northwest Cascade System in Washington is based on along-strike near-continuity of outcrop areas, a similar protolith composition range, the same structural position relative to the Shuksan fault zone, and distinctive irregular structures in variably metamorphosed sandstone and pelite of both Darrington Phyllite and Settler Schist. If this correlation is valid, then the record of Early Cretaceous; subduction-related blueschist metamorphism of Shuksan–Darrington rocks was destroyed in Settler Schist by overprinting by early Late Cretaceous Barrovian metamorphism; only some distinctive, premetamorphic structures remain. The implication is that within the southeastern Coast Mountains, a cryptic record of subduction is overprinted by Barrovian metamorphism.