Boninitic magmatism in a continental margin setting, Yukon-Tanana terrane,southeastern Yukon, Canada

Mid-Paleozoic mafic rocks in the Finlayson Lake region of the Yukon-Tanana terrane, southeastern Yukon, Canada, have the diagnostic geochemical signat ures of boninites: high MgO, Cr, Ni, and Co contents, intermediate SiO2 con tents, high Mg#'s (MgO/ (MgO+FeO*), Al2O3/TiO2, and Zr(Hf)/middle rare eart h element (REE) ratios; low TiO2, REE, and high-field-strength element cont ents; and U-shaped primitive mantle-normalized trace element patterns. Howe ver, unlike most modern and ancient boninitic rocks that are typically asso ciated with intraoceanic realms, those from the Finlayson Lake region are p art of a mid-Paleozoic continental margin arc-backarc magmatic system. We p ropose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an are built on comp osite basement of oceanic and continental crust. In the oceanic segment, up welling asthenosphere induced melting of a subducted-slab metasomatized ref ractory mantle source to form boninitic magmatism. In the continental secto r, upwelling asthenospheric mantle, and/or the melts derived thereof, induc ed crustal melting, which explains the large volume of temporally equivalen t felsic volcanic and intrusive rocks.