Abstract
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.
Highlights
Wrangellia is one of the most outboard of the major northern Cordilleran terranes
During collection of the Fourth Lake Formation, in the southern Cowichan uplift, field relations of the Comox Formation unconformably overlaying the Fourth Lake Formation and observations of clasts resembling the Fourth Lake Formation prompted sample collection (Figures 4(c) and 4(d)). These observations lead to the hypothesis that the Comox Formation was likely to yield detrital zircon grains that would provide insights into the Paleozoic and Early Mesozoic evolution of southern Wrangellia
This study presents the first robust detrital zircon analysis of Paleozoic strata from southern Wrangellia
Summary
Wrangellia is one of the most outboard of the major northern Cordilleran terranes It occupies two separate regions: southcentral Alaska and southwestern Yukon (northern Wrangellia) and Haida Gwaii and Vancouver Island (southern Wrangellia) (Figure 1 inset). It was originally defined as a coherent terrane based on characteristic thick piles of Triassic flood basalts, the Nikolai Greenstone of northern. Paleozoic sequences of southern Wrangellia comprise the Upper Devonian to Lower Permian Sicker and Buttle Lake Groups [3,4,5,6,7] Those in northern Wrangellia comprise the Carboniferous to Lower Permian Skolai Group, including the Station Creek and Hasen Creek formations [8,9,10,11,12,13]
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