POLYPHASE DEFORMATION ALONG THE WANN RIVER SHEAR ZONE: IMPLICATIONS FOR THE EVOLUTION OF THE YUKON-TANANA TERRANE IN THE NORTHERN CANADIAN CORDILLERA

The Yukon-Tanana terrane of the northern Canadian Cordillera is a composite peri-cratonic terrane comprising segments with distinct protoliths and/or tectono-metamorphic histories separated by shear zones. In northwestern British Columbia, the Wann River shear zone separates the Carboniferous and older Florence Range suite, composed of metamorphosed passive margin rocks, from the pre-Carboniferous Boundary Ranges suite that includes arc-related meta-igneous and metasedimentary rocks. Based on sheared early Jurassic plutons and post-metamorphic cooling ages, the shear zone has been previously interpreted to have been active between 185 and 170 Ma and subsequently folded. We investigated the ~4 km thick Wann River shear zone in two locations: (1) a klippe on White Moose Mountain where it is sub-horizontal and (2) north of Llewellyn glacier ~40 km to the SSE where it is sub-vertical. Although initial field mapping was consistent with previous interpretations of a top-to-the-SE/sinistral sense of shear, quartz crystallographic <c> axis preferred orientations (CPO) analyses and SHRIMP U/Pb geochronology on variably deformed intrusive rocks reveal a significantly more complex deformation history. At White Moose Mountain, seven quartz-rich units yield high-temperature (~550-700 °C) top-to-the-SE CPO fabrics. A 191 ± 2 Ma tightly folded granitic dyke cross-cuts the shear foliation and contains a similar high-temperature top-to-the-SE CPO fabric, indicating that the shear zone has been active mostly prior to, but also after its emplacement. North of Llewellyn glacier, eleven samples yield moderate-temperature (~400-550 °C) dextral CPO fabrics, which, after unfolding of the shear zone, indicate a top-to-the-NW sense of shear contrasting with that documented at White Moose Mountain. Shearing at Llewellyn glacier is constrained between 185 ± 2 Ma and 180 ± 2 Ma based on the crystallization ages of pre- and post-kinematic orthogneisses. Moreover, the 180 ± 2 Ma orthogneiss and metapelites in this area record evidence of a low-temperature (~300-400 °C) sinistral overprint. Our study demonstrates that the Florence and Boundary Ranges suites must have been juxtaposed before 191 Ma along the Wann River shear zone, and that this tectonic boundary was reactivated at least twice at 185-180 Ma and after 180 Ma.