Subaerial basalt flows of the Neohelikian Nauyat Formation from northwestern Baffin Island, Northwest Territories, constitute an approximately 360 m thick unit that overlies an Archean–Aphebian metamorphic basement. The lavas have undergone a low-grade regional metamorphism that affected the abundances of Na, K, and related trace elements. The basalts are continental tholeiites possessing some characteristics of mid-ocean-ridge basalts. They underwent fractional crystallization of clinopyroxene, plagioclase, and olivine. Mantle-normalized patterns show an enrichment of the lithophile elements, including Th and light rare-earth elements, relative to the high-field-strength elements and a distinct depletion of Nb and Sr. The parental magma of the basalts was derived either from oceanic-type mantle and subsequently affected by lower crustal contamination or from the subcontinental lithospheric mantle. The Nauyat basalts are probably related to the initial stage of the opening of the Poseidon (Proto-Arctic) Ocean.