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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 cratonal provenance (mostly 1.0–2.0 Ga, with resolved concentrations of 1.49–1.61 Ga zircon) is also exhibited by adjacent Early Devonian lower greenschist units of the Yreka subterrane (Duzel phyllite and Moffett Creek Formation). Additional results from temporally equivalent arc-derived sedimentary units (Sissel Gulch graywacke and Gazelle Formation) yield strongly unimodal zircon age distributions of early Paleozoic zircon. The results indicate that the Yreka subterrane formed at an Ordovician–Silurian–Early Devonian convergent margin near a Mesoproterozoic-Paleoproterozoic craton and Ediacaran crust. Appreciable 1.49–1.61 Ga zircon within the Yreka subterrane is compatible with a recent biogeographic analysis that indicates a non-Laurentian origin for the eastern Klamath terrane. Additional new data reveal that key early Paleozoic convergent-margin rocks within the northern Sierran-Klamath and Alexander terranes share similar arc and cratonal provenance, including 1.49–1.61 Ga zircon. We hypothesize that the rocks from all three areas are dispersed tectonic fragments that were derived from the same convergent margin and were independently transported to western North America. Of the orogenic source regions indicated by previous paleomagnetic and biogeographic analysis, the detrital zircon provenance favors western Baltica over eastern Australia.

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