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

Abstract

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 apply this distinction to the Northern Cordillera in Yukon, British Columbia and Alaska, and focus 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 2 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 ~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 which proposed that accretion of the Yukon-Tanana terrane to Laurentia was facilitated by Permian subduction of Slide Mountain Ocean beneath the Yukon-Tanana terrane. Our results, combined with existing datasets suggest that during the mid-to late Permian, Yukon-Tanana terrane 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.

Structural evolution of a crustal-scale shear zone through a decreasing temperature regime: The Yukon River shear zone, Yukon-Tanana terrane, Northern Cordillera

“…Ar/ 39 Ar thermochronometry suggests that top-WNW thrusting on the YRSZ occurred during or after the Early to Middle Jurassic (e.g., Joyce et al, 2015). The kinematics and timing constraints derived from the YRSZ are not easily reconciled with current models for the Northern Cordillera (e.g., Berman et al, 2007;Nelson et al, 2013;Staples et al, 2016;Parsons et al, 2018;van Staal et al, 2018) and are considered further in our discussion. The spatial trends in deformation temperature, strain geometry and intensity, grain size, and differential stress displayed by top-ESE deformation on the YRSZ represent an excellent example of progressive deformation and strain localization within an exhuming (i.e., cooling) and narrowing shear zone (e.g., Sibson, 1977;Ramsay, 1980;Law, 1990;Handy et al, 2007;Faulkner et al, 2010).…”

“…Current popular and widely cited tectonic models for the Northern Cordillera suggest that following Parsons et al | The Yukon River shear zone, Yukon-Tanana terrane, Northern Cordillera RESEARCH initiation of the Devonian Ecstall arc along the west margin of Laurentia, backarc rifting resulted in separation of YTT from Laurentia. Separation occurred sometime during the Late Devonian-Early Mississippian accompanied by formation of the intervening Slide Mountain Ocean (Colpron et al, 2006(Colpron et al, , 2007Nelson et al, , 2013Parsons et al, 2018). Several arc cycles were subsequently built upon YTT between the Late Devonian and Permian during east-dipping subduction of the Panthalassa Ocean beneath the western margin of YTT.…”

“…Polyphase amphibolite-to eclogite-facies metamorphism recorded by the YTT continental basement occurred during the mid-Permian to Middle Triassic, latest Triassic-Early Jurassic, and Late Jurassic-Early Cretaceous (e.g., Dusel-Bacon et al, 1995;Creaser et al, 1997;Berman et al, 2007;Staples, 2014;Petrie et al, 2016;Staples et al, 2016;Clark, 2017;Gilotti et al, 2017;Morneau et al, 2017). Each of these metamorphic events was followed by rapid cooling (e.g., Dusel-Bacon et al, 2002;Knight et al, 2013;Staples et al, 2016;Parsons et al, 2018).…”

“…Beranek and Mortensen (2011) proposed that YTT collided with and accreted back onto Laurentia during the mid to Late Permian "Klondike orogeny," facilitated by west-dipping subduction of the intervening Slide Mountain ocean beneath YTT. However, recent models have proposed that YTT collided with an intra-oceanic arc during the Late Permian (the Dunite Peak intraoceanic arc- Parsons et al, 2018;or YTTjvan Staal et al, 2018) and did not accrete to Laurentia until after the Middle Triassic, and probably during the Late Triassic-Early Jurassic (Hansen et al, 1991;Stevens et al, 1996;Plint and Gordon, 1997;Hansen and Dusel-Bacon 1998;Parsons et al, 2018).…”

The 3D Geophysical Investigation of a Middle Cretaceous to Paleocene Regional Décollement in the Cordillera of Northern Canada and Alaska

“…The terrane is generally accepted to have formed upon extended basement of the ancestral North American margin, rifted away in the early Mississippian (Colpron et al, ; Murphy et al, , and references therein), and to have reaccreted to the North American margin during Late Paleozoic to Triassic closure of the Slide Mountain Ocean along a west dipping subduction zone (Beranek & Mortensen, ; Cook et al, ; Cook & Erdmer, ; Mortensen, ; Tempelman‐Kluit, ). Models with more nuance have recently been published (Parsons et al, ; van Staal et al, ). In association with an extensive Mesozoic volcanic arc (Logan & Mihalynuk, ), Yukon‐Tanana terrane was intruded by primarily coarse‐grained monzogranite to granodiorites (Ryan et al, , ) of the Late Triassic to Early Jurassic Minto and Long Lake suites (circa 204–196 and 190–180 Ma, respectively, Joyce et al, ).…”