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 signatures of boninites: high MgO, Cr, Ni, and Co contents, intermediate Si[O.sub.2] contents, high Mg#'s (MgO/ (MgO+FeO*), [Al.sub.2][O.sub.3]/Ti[O.sub.2], and Zr(Hf)/middle rare earth element (REE) ratios; low Ti[O.sub.2], REE, and high-field-strength element contents; and U-shaped primitive mantle-normalized trace element patterns. However, unlike most modern and ancient boninitic rocks that are typically associated with intraoceanic realms, those from the Finlayson Lake region are part of a mid-Paleozoic continental margin arc-backarc magmatic system. We propose a model in which the boninitic rocks from the Finlayson Lake region formed as a result of spreading ridge propagation into an arc built on composite basement of oceanic and continental crust. In the oceanic segment, upwelling asthenosphere induced melting of a subducted-slab metasomatized refractory mantle source to form boninitic magmatism. In the continental sector, upwelling asthenospheric mantle, and/or the melts derived thereof, induced crustal melting, which explains the large volume of temporally equivalent felsic volcanic and intrusive rocks. Keywords: boninite, geochemistry, magmatism, continental crust, Yukon-Tanana terrane, arcs.