The tectonic significance of early Paleozoic convergent-margin rocks of the Alexander and Sierran-Klamath terranes is poorly understood. New phengite ⁴⁰Ar/³⁹Ar and Rb-Sr results from the schist of Skookum Gulch of the Yreka subterrane in the Klamath Mountains (454 ± 10 Ma) confirm that blueschists are the oldest known subduction-zone rocks of the western North American Cordillera. The blueschists are juxtaposed with kilometer-scale tectonic blocks of ca. 565 Ma tonalite. Detrital zircons from the blueschists require close proximity to a diverse source of cratonal or derivative supracrustal rocks and preclude formation within an isolated intra-oceanic setting. This strong...