Abstract The Palaeozoic geodynamic environment of northern California is considered to be one of offshore island arcs bounded by marginal seas. The petrological and geochemical features of the eastern Klamath and northern Sierra Nevada island-arc sequences are important indicators of the geodynamic evolution of the Cordillera. The eastern Klamath pre-Early Devonian island-arc sequence consists of pillowed basalts andesites and boninites, overlain by low-K rhyolites, both suites erupted in a submarine environment. These volcanics, characterized by almost flat REE patterns and high ϵ Nd represent a primitive intraoceanic island arc. The mid-Palaeozoic volcanic succession of northern Sierra Nevada is formed of calc-alkaline rhyolites conformably overlain by either calc-alkaline (in the north) or low-K tholeiite (in the south) basalts and andesites. The volcanic episode ended in the Early Carboniferous with a differentiated shoshonitic suite. All the massive or pillowed flows are interbedded with volcaniclastic sediments, suggesting that they were erupted partly in a shallow-water environment. The geochemical features of the volcanics, moderate to high LREE enrichment and low ϵ Nd values, are evidence for the involvement of a crustal component in the magma genesis. The mid-Palaeozoic volcanic sequence therefore represents a continent-based island arc. This microcontinent could be the Sonomia microplate (excluding the eastern Klamath plate). Thus, these two northern Californian Palaeozoic sequences cannot be considered as lateral counterparts but could represent the remnants of oceanic and continental island arcs as observed in the present-day East Pacific areas along the Eurasiatic margin.