U-Pb and Hf Analyses of Detrital Zircons from Paleozoic and Cretaceous Strata on Vancouver Island, British Columbia: Constraints on the Paleozoic Tectonic Evolution of Southern Wrangellia

Abstract

Wrangellia is a late Paleozoic arc terrane that occupies two distinct coastal regions of western Canada and Alaska. The Skolai arc of northern Wrangellia in south-central Alaska and Yukon has been linked to the older, adjacent Alexander terrane by shared Late Devonian rift-related gabbros and also by Late Pennsylvanian postcollisional plutons. Late Devonian to Early Permian Sicker arc rocks of southern Wrangellia are exposed in uplifts on Vancouver Island, southwestern British Columbia, surrounded by younger strata and lacking physical connections to other terranes. Utilizing the detrital zircon record of Paleozoic and Cretaceous sedimentary rocks, we provide insight into the magmatic and depositional evolution of southern Wrangellia and its relationships to both northern Wrangellia and the Alexander terrane. 1422 U-Pb LA-ICPMS analyses from the Fourth Lake Formation (Mississippian–Permian) reveal syndepositional Carboniferous age peaks (344, 339, 336, 331, and 317 Ma), sourced from the Sicker arc of southern Wrangellia. These populations overlap in part known ages of volcanism, but the Middle Mississippian cumulative peak (337 Ma) documents a previously unrecognized magmatic episode. Paleozoic detrital zircons exhibit intermediate to juvenile ƐHft values between +15 and +5, indicating that southern Wrangellia was not strictly built on primitive oceanic crust, but instead on transitional crust with a small evolved component. The Fourth Lake samples yielded 49 grains (3.4% of the total grains analyzed) with ages between 2802 Ma and 442 Ma, and with corresponding ƐHft values ranging from +13 to -20. In age—ƐHft space, these grains fall within the Alexander terrane array. They were probably derived from sedimentary rocks in the basement of the Sicker arc. By analogy with northern Wrangellia, this basement incorporated rifted fragments of the Alexander terrane margin as the combined Sicker-Skolai arc system advanced ocean-ward due to slab rollback in Late Devonian to Early Mississippian time. Ultimately, data from detrital zircons preserved in the Fourth Lake Formation provides significant information allowing for an updated tectonic model of Paleozoic Wrangellia.

Triassic–Jurassic Accretionary History and Tectonic Origin of Stikinia From U‐Pb Geochronology and Lu‐Hf Isotope Analysis, British Columbia

“…provenance, along with igneous and tectonic constraints, support models whereby the Alexander terrane originated in the northernmost Caledonides of the Arctic region (Beranek et al, 2012;2013a, 2013b. The Alexander terrane migrated into northern Panthalassa in the mid-Paleozoic, and Wrangellia was built as an intraoceanic arc on an Alexander terrane substrate, before both terranes accreted to the North American margin in the Jurassic-Cretaceous (Alberts et al, 2021;Colpron & Nelson, 2009, 2011a, 2011bGehrels et al, 1996;Nelson et al, 2013;White et al, 2016;Yokelson et al, 2015). A number of other terranes with suspected Arctic basement domains are strung along the length of the Cordillera.…”

“…Shared Devonian gabbros and Pennsylvanian plutons in the St. Elias Mountains of southwest Yukon and adjacent Alaska, indicate mid-to late Paleozoic connections between northern Alexander and Wrangellia (Beranek et al, 2014;Gardner et al, 1988;Israel et al, 2014). New detrital zircon data from Carboniferous sedimentary rocks (Fourth Lake Formation) on southern Vancouver Island include 49 zircons with pre-Late Devonian ages of 2802-942 Ma, 714-682 Ma, 586-536 Ma, and 490-440 Ma (Alberts et al, 2021). These are similar to igneous ages and detrital populations in the Alexander terrane, suggesting that the basement of southern Wrangellia also incorporated crustal fragments of paleo-Arctic affinity (Alberts et al, 2021).…”

“…Wrangellia includes arc-related Upper Devonian to Lower Permian volcanic and sedimentary rocks, Upper Triassic non-arc basalts, Triassic-Jurassic mixed sedimentary and volcanic arc strata and coeval plutons, all unconformably overlain by Cretaceous clastic overlap strata (Figure 3; Alberts et al, 2021;Isachsen, 1987;Massey, 1995aMassey, , 1995bSutherland Brown et al, 1986). The Triassic Nikolai and Karmutsen flood basalts first led to the definition of Wrangellia as a terrane (Coney et al, 1980;Jones et al, 1977) and subsequently fostered its popular conception as an oceanic plateau; however, both older and younger successions are of arc affinity.…”

“…New detrital zircon data from Carboniferous sedimentary rocks (Fourth Lake Formation) on southern Vancouver Island include 49 zircons with pre-Late Devonian ages of 2802-942 Ma, 714-682 Ma, 586-536 Ma, and 490-440 Ma (Alberts et al, 2021). These are similar to igneous ages and detrital populations in the Alexander terrane, suggesting that the basement of southern Wrangellia also incorporated crustal fragments of paleo-Arctic affinity (Alberts et al, 2021). Like Alexander, Wrangellia shows no direct ties to or affinity with the more inboard Intermontane terranes until Jurassic-Cretaceous suturing, though data herein may indicate early connections.…”

“…380-280 Ma zircons in both the Carboniferous Fourth Lake Formation and the Cretaceous Comox Formation of Wrangellia. They interpret the Devonian-Permian zircons to be derived from the underlying Sicker Group, a Late Devonian-Permian arc sequence (Alberts et al, 2021;Ruks, 2015). Results from Stikinia also strongly resemble the Permian upper Mystic assemblage from the Farewell terrane of Alaska (Malkowski et al, 2014).…”