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1 Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada
2 2-Yukon Geology Program, P.O. Box 2703 (F-3), Whitehorse, Yukon Y1A 2C6, Canada
3 Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canada
4 Mineral Resources Division, Geological Survey of Canada, 9860 West Saanich Road, Sidney, British Columbia V8L 4B2, 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 SiO2 contents, high Mg#'s (MgO/ (MgO+FeO*), Al2O3/TiO2, and Zr(Hf)/middle rare earth element (REE) ratios; low TiO2, 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.
Key Words: boninite • geochemistry • magmatism • continental crust • Yukon-Tanana terrane • arcs
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