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U‐Pb Zircon Geochronology From the Northern Cordillera, Central Yukon, With Implications for Its Tectonic Assembly
Journal article   Open access   Peer reviewed

U‐Pb Zircon Geochronology From the Northern Cordillera, Central Yukon, With Implications for Its Tectonic Assembly

Andrew J Parsons, William C McClelland, Alexandre Zagorevski, James J Ryan, Mark J Coleman, Nathan Cleven and Cees R Staal
Tectonics (Washington, D.C.), Vol.41(2), e2021TC006918
02/2022
DOI: 10.1029/2021TC006918
url
https://doi.org/10.1029/2021TC006918View
Published (Version of record) Open Access

Abstract

The tectonic assembly of the Northern Cordillera is currently disputed and directly impacts Paleozoic‐to‐recent paleogeographic and plate tectonic reconstructions of North America. In this study, we present new U‐Pb zircon geochronology from the allochthonous Yukon‐Tanana terrane and the parautochthonous Cassiar terrane of the Northern Cordillera from south‐central Yukon, Canada. Our data provide new constraints for the assembly of the Northern Cordillera in this region. Metasedimentary samples from the Ingenika Group (Cassiar terrane) and the Snowcap and Finlayson assemblages (Yukon‐Tanana terrane) yielded detrital zircon age spectra that are comparable to known northwest Laurentia age spectra. One of our samples from the Snowcap assemblage yields a detrital zircon age spectrum that is anomalous for northwest Laurentia, but comparable to Early Paleozoic strata deposited in the Nevada‐Idaho‐Utah region. Zircon rim growth and Pb‐loss recorded by detrital zircon in the Snowcap assemblage and Ingenika Group samples record metamorphism at 370 ± 4 Ma and between 171 ± 5 and 135 ± 3 Ma. Late Devonian metamorphism and magmatism possibly corresponds to rifting of the Snowcap assemblage from the Laurentian margin. Middle Jurassic‐Early Cretaceous metamorphic zircon rim ages from the Cassiar terrane record metamorphism during collisions between the Intermontane superterrane (including the Yukon‐Tanana terrane) and Laurentia (Early to Middle Jurassic), and between the Insular superterrane and Laurentia (Middle to Late Jurassic). Our study suggests that collision between the Yukon‐Tanana terrane and the Laurentian margin began no earlier than ∼205 Ma. Plain Language Summary The Northern Cordillera (northwestern North America) represents an amalgamation of continental, volcanic, and oceanic crustal blocks (terranes), which collided with North America (Laurentia) over the last 380 million years. Constraining the timing and sequence of these collisional events is crucial for our understanding of the tectonic and paleogeographic evolution of North America and the natural resources and hazards that it provides. In Yukon, Canada, the most widely accepted models propose that the earliest Cordilleran collisional event occurred between the Yukon‐Tanana terrane and Laurentia, ∼270 to 250 million years ago. However, in this study, we present new data which suggest that collision between the Yukon‐Tanana terrane and North America took place sometime between 205 and 170 million years ago. Our findings also suggest that the Yukon‐Tanana terrane represents a collage of smaller crustal blocks, one of which originated from western North America. Importantly, the present‐day structurally overlapping relationships of Cordilleran terranes means that our new interpretation of the Yukon‐Tanana‐Laurentia collision directly influences the interpretation of all younger and older collisional events in Northern Cordillera. Our study adds to a growing body of evidence that argues that the current and most widely accepted models and terrane definitions for the Northern Cordillera require revision. Key Points Cassiar terrane in south‐central Yukon records Late Devonian and Middle Jurassic‐Early Cretaceous metamorphism of the Laurentian margin Middle Jurassic‐Early Cretaceous metamorphism records collision between Yukon‐Tanana terrane and Laurentia, which began no earlier than ∼205 Ma Yukon‐Tanana terrane contains metasedimentary rocks sourced from Idaho‐Nevada‐Utah region
 Cassiar geochronology metamorphism Northern Cordillera Snowcap Yukon‐Tanana

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