Magma genesis along an arc-transform transition zone

The Wrangell volcanic belt lies in a transpressional zone that results from the oblique convergence between the NE Pacific and North American plates. Transitional lavas predominate in the SE transform regime, whereas a calc-alkaline arc lies to the NW. The geochemical evolution of the St. Clare Creek field (Yukon) reflects changes in mode of crustal interaction and nature of mantle source components. The Wolverine centre (18-16 Ma) consists of alkaline and voluminous transitional lavas overlain by calc-alkaline lavas. The Klutlan centre (16-10 Ma) comprises calc-alkaline and Fe-rich transitional lavas followed by voluminous Fe-poor transitional basalts and more felsic transitional lavas. Primitive Fe-rich Wolverine transitional and alkaline lavas have higher Nb/Y ratios relative to Klutlan lavas, suggesting an enriched lithospheric mantle source with high $ sp{87}$Sr/$ sp{86}$Sr (0.7042) and low K/Nb ratios. Younger Fe-poor transitional lavas have high K/Nb ratios similar to calc-alkaline lavas. The Klutlan calc-alkaline lavas have high K/Nb ratios and low $ sp{87}$Sr/$ sp{86}$Sr ratios ($ sim$0.7035) derived from a slab component. Depleted Klutlan basalts have low La/Yb, low $ sp{87}$Sr/$ sp{86}$Sr (0.7031) and high $ sp{143}$Nd/$ sp{144}$Nd (0.5131) ratios reflecting an origin in the depleted asthenospheric mantle. Mixing between enriched, slab-derived, and depleted mantle components accounts for the trace element and isotopic composition of primitive transitional magmas, whereas different degrees of partial melting of depleted mantle are required to explain the range of Fe contents. Crustal assimilation accompanied the differentiation of early evolved Wolverine alkaline and late Klutlan transitional lavas. Selective contamination resulted in K-enrichment of evolved intervening transitional lavas