Rocks in the eastern Klamath Mountains were affected by polyphase tectonism. The older Callahan tectonic event, of Late Ordovician–Early Silurian age, produced high-pressure and epizonal metamorphism and is expressed in the Yreka-Callahan nappes by intrafolial folds. The main tectonic event occurred during Late Devonian and Early Mississippian time and is interpreted here to be coeval with the Antler orogeny, defined in the Roberts Mountain allochthon to the east in Nevada. It resulted in the amalgamation of lower Paleozoic Trinity ophiolite and the pre-Devonian to Early Devonian intra-oceanic island arc with flysch-type sediments deposited in a continent margin (Yreka-Callahan assemblage), and a metamorphic sequence of amphibolite, marble, and quartz-mica-schist of the Central Metamorphic Belt. The magmatic evolution of Paleozoic and lower Mesozoic rocks in the eastern Klamath Mountains is characterized by the predominance of convergent plate boundary magmatism. Nevertheless, lower Paleozoic pillow basalts associated with coralline limestone occur as olistoliths in the Yreka-Callahan assemblage. They display either T mid-ocean ridge basalt (MORB) affinities with flat rare earth element (REE) patterns or mild alkalic affinities marked by higher contents in TiO 2 and HFS elements and LREE-enriched patterns. The Ashgillian calc-alkaline differentiated suite, characterized by LREE-enriched patterns and low ɛ Nd (T) + 3.5, was erupted through continental crust in an Andean margin setting. The pre-Devonian to Early Devonian volcanic rocks that show tholeiitic affinities with very depleted to somewhat enriched LREE patterns and high ∊ Nd (T) values represent the remnants of an intra-oceanic immature island arc. After the Late Devonian(?)–Early Mississippian tectonic event, island-arc magmatism resumed with the eruption of depleted low-K tholeiitic basalt and andesite, characterized by depleted LREE patterns during Late Devonian(?) or Carboniferous-Permian(?) time (Gregg Ranch and Copper Peak sections). A major pulse in volcanism occurred during Middle to Late Permian and is now represented by the voluminous sequence of volcanogenic debris and flows of the Nosoni, Dekkas, and Bully Hill Formations. This volcanism, formed of predominantly low-K tholeiitic basalt and andesite, characterized by flat REE patterns and high ɛ Nd (T) + 4 to 8, with minor calc-alkaline, alkaline (enriched LREE patterns), and boninitic (Mg-Cr-rich) basalts, was erupted through a thickened crust. At the end of the Permian island-arc volcanism, uplift and faulting occurred in the arc. The magmatic affinities of the middle to late Paleozoic volcanic rocks in the eastern Klamath Mountains suggest an evolution through time from an immature to a mature island-arc setting. The Permian-Triassic history of the Blue Mountains (Seven Devils Group; Huntington Formation) presents many similarities with that of the eastern Klamath terrane. In both areas, evidence for Permian volcanism is similar with flat REE patterns and high ɛ Nd (T) + 6.7 to 8.4. Triassic volcanic rocks display low-K tholeiitic affinities (flat REE patterns) with minor calc-alkaline flows in the uppermost levels of the volcanic pile. Apparently, compressive structures related to the Sonoma orogeny are absent in both areas. Thus the eastern Klamath and Blue Mountains could have formed part of the same arc in Permian-Triassic time.