The Wrangellia terrane of North America contains a large volume of Middle to Late Triassic oceanic flood basalts which were emplaced on top of a preexisting island arc. Nd-, Sr-, and Pb-isotopic compositions reflect derivation from a plume source with ε_Nd(T)∼ +6 to + 7, ⁸⁷Sr/⁸⁶Sr_i∼0⋅7034, and ²⁰⁶Pb/²⁰⁴Pb_i∼19⋅0. Major and trace element compositions suggest the Wrangellia flood basalts (WFB) formed through relatively small degrees of partial melting at greater depths than estimated for other oceanic plateaux such as Ontong Java. It appears that the WFB did not form in a rifting environment, and that preexisting arc lithosphere limited the ascent and decompression melting of the source plume. Rocks from the preexisting arc are strongly depleted in high field strength elements (HFSEs) relative to large ion lithophile elements (LILEs), but the WFB are not. Assimilation of arc lithospheric mantle or crust was therefore generally minor. However, some contamination by arc components is evident, particularly in basalts erupted in the early stages of volcanism. Minor isotopic shifts, to lower ε_Nd(T) and ²⁰⁶Pb/²⁰⁴Pb_i and higher ⁸⁷Sr/⁸⁶Sr_i, are accompanied by shifts in trace element ratios towards more arclike signatures, e.g. low Nb/Th and Nb/La. Arc contamination is greatest in the most evolved basalts, indicating that assimilation was coupled with fractional crystallization. A comparison of the WFB with other continental and oceanic flood basalts reveals that continental flood basalts generally form through smaller degrees of melting than oceanic flood basalts and that the contribution of material from the crust and litho-spheric mantle is significantly greater.