The nature of mantle sources for recent alkaline basalts across the northern Canadian Cordillera

The Stikine Volcanic Belt is a lineament of Tertiary to Recent alkaline volcanic centres that cross disparate oceanic and continental terranes of the northern Canadian Cordillera, and extend onto the North America craton. This volcanic lineament thus offers an unique opportunity to probe the lithospheric mantle beneath these terranes and investigate the relative roles of lithospheric and asthenospheric mantle sources in the generation of alkaline lavas.; Alkaline lavas in twelve volcanic centres of the Stikine Volcanic Belt were sampled and studied for major and trace elements and Sr-Nd-Pb isotopic compositions. The primitive lavas of each volcanic centre define binary arrays between two compositional end-members, olivine nephelinite (NEPH) and hypersthene-normative olivine basalt (Hy-NORM). The NEPH end-member is characterized by large enrichments in incompatible trace elements with respect to primitive mantle, but is depleted in term of its isotopic composition. The presence of amphibole in its source suggests that this end-member is derived from the lithospheric mantle. The Hy-NORM end-member has lower incompatible trace element contents, but is still relatively enriched in incompatible elements compared to primitive mantle. Although this end-member has always more radiogenic Sr and Pb, and less radiogenic Nd isotopic ratios than the NEPH end-member, its isotopic signature varies with the tectonic belt in which it erupted, indicating a significant role for the lithospheric mantle in the derivation of the Hy-NORM basalts.; The Canadian Cordilleran, and other continental Hy-NORM basalts, have low Ca and high Na contents compared to their equivalents at oceanic hot spots such as Hawaii or associated with mid-ocean ridges. A comparison with experimental melts of mantle peridotite indicates that these characteristics reflect smaller degrees of partial melting (<10%) in continental regime. The range of observed lava compositions is explained by the melting of two distinct lithospheric components, a NEPH end-member representing the melting of amphibole-rich veins, and a Hy-NORM end-member resulting from small degrees of partial melting of their host garnet lherzolite. The high heat flow values reported in the northern Canadian Cordillera are consistent with a model in which ongoing melting and thinning of the lithospheric mantle is responsible for generating the mafic alkaline magmas.