Upper-plate and lower-plate settings within subduction zones have distinct geological signatures. Identifying and discriminating between these settings is crucial to the study of accretionary orogens. We applied this distinction to the Northern Cordillera in Yukon, British Columbia, and Alaska, and we focused on the identification of upper-plate and lower-plate domains during the late Paleozoic to early Mesozoic evolution of the allochthonous Yukon-Tanana terrane, the west Laurentian margin, and the intervening Slide Mountain Ocean. We present new data from the Dunite Peak ophiolite in south-central Yukon, previously interpreted as ocean plate stratigraphy that was obducted from the subducting Slide Mountain Ocean (i.e., lower plate). Whole-rock geochemical and Sm-Nd isotopic analyses, and U-Pb zircon geochronology indicate that the Dunite Peak ophiolite formed in an intra-oceanic suprasubduction-zone setting (i.e., upper plate) with magmatism at 265 ± 4 Ma. We propose that the Dunite Peak ophiolite correlates with other mid-Permian suprasubduction-zone ophiolites of the Slide Mountain terrane, collectively defining the previously unrecognized mid-Permian “Dunite Peak intra-oceanic arc.” This intra-oceanic arc was active from ca. 280 to 260 Ma, located within the Slide Mountain Ocean, between the Yukon-Tanana terrane and west Laurentia. Existence of this arc is incompatible with previous models proposing that accretion of the Yukon-Tanana terrane to Laurentia was facilitated by Permian subduction of the Slide Mountain Ocean beneath the Yukon-Tanana terrane. Our results, combined with existing data sets, suggest that during the mid- to Late Permian (Late Permian = Guadalupian to Lopingian, 272 Ma to 252 Ma), the Yukon-Tanana terrane was subducted eastward beneath the Dunite Peak intra-oceanic arc. Subsequent collision and accretion of the Yukon-Tanana–Dunite Peak composite terrane with Laurentia must have occurred after the Middle Triassic.

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