Petrogenesis of the Dunite Peak ophiolite , south-central Yukon and the 1 distinction of upper plate and lower plate settings : a new hypothesis for 2 the late Paleozoic – early Mesozoic tectonic evolution of the Northern 3 Cordillera 4

16 Upper plate and lower plate settings within subduction zones have distinct geological signatures. Identifying 17 and discriminating between these settings is crucial to the study of accretionary orogens. We apply this 18 distinction to the Northern Cordillera in Yukon, British Columbia and Alaska, and focus on the 19 identification of upper plate and lower plate domains during the late Paleozoic to early Mesozoic evolution 20 of the allochthonous Yukon-Tanana terrane, the west Laurentian margin and the intervening Slide Mountain 21 Ocean. We present new data from the Dunite Peak ophiolite in south-central Yukon, previously interpreted 22 as ocean plate stratigraphy that was obducted from the subducting Slide Mountain Ocean (i.e. lower plate). 23 Whole-rock geochemical and Sm-Nd isotopic analyses, and U-Pb zircon geochronology indicate that the 24 Dunite Peak ophiolite formed in an intra-oceanic suprasubduction zone setting (i.e. upper plate) with 25 2 magmatism at 265 ± 4 Ma. We propose that the Dunite Peak ophiolite correlates with other mid-Permian 26 suprasubduction zone ophiolites of the Slide Mountain terrane, collectively defining the previously 27 unrecognized mid-Permian Dunite Peak intra-oceanic arc. This intra-oceanic arc was active from ~280 to 28 260 Ma, located within the Slide Mountain Ocean, between the Yukon-Tanana terrane and west Laurentia. 29 Existence of this arc is incompatible with previous models which proposed that accretion of the Yukon30 Tanana terrane to Laurentia was facilitated by Permian subduction of Slide Mountain Ocean beneath the 31 Yukon-Tanana terrane. Our results, combined with existing datasets suggest that during the midto late 32 Permian, Yukon-Tanana terrane subducted eastward beneath the Dunite Peak intra-oceanic arc. Subsequent 33 collision and accretion of the Yukon-Tanana – Dunite Peak composite terrane with Laurentia must have 34 occurred after the Middle Triassic. 35 36