THE MANTLE SOURCE FOR OLIVINE NEPHELINITE, BASANITE, AND ALKALINE OLIVINE BASALT AT FORT-SELKIRK, YUKON, CANADA

Olivine nephelinite, basanite, and transitional alkaline basalt lavas of the Quaternary Fort Selkirk volcanic complex in the central Yukon represent three distinct alkaline magma series which have evolved along diverging paths. They cannot be related by low-pressure crystal-liquid fractionation, and systematic isotopic differences make it difficult to derive them by variable degrees of melting of a common mantle source. Field evidence requires, however, that these three magma series are intimately related in time and space, and they share a number of anomalous chemical characteristics including low Ca/Na ratios with respect to the majority of terrestrial equivalents. When the effects of differential olivine fractionation are ignored, the compositional spectrum of the Fort Selkirk lavas approximates a binary mixing line between transitional alkaline basalt and olivine nephelinite. A population gap along this mixing line, located between the compositions of the nephelinite and basanite lavas, coincides with the compositions of amphibole and/or amphibole-garnet-clinopyroxene assemblages observed in mantle xenoliths. This compositional gap may represent a thermal divide separating two minimum-melt compositions in a mantle source consisting of a lherzolite host cut by amphibole-garnet-clinopyroxenite veins. The olivine nephelinite endmember may have been derived by early melting in the amphibole-garnet-clinopyroxenite veins, whereas the transitional alkaline basalt would represent more extensive melting of the host lherzolite. © 1990 Oxford University Press.