The timing of assembly and tectonic origins of terranes in the northern Cordillera of Alaska, British Columbia, and the Pacific Northwest are debated. Stikinia, a long-lived arc terrane, has an enigmatic regional Mesozoic accretionary history and its tectonic origins remain unconstrained. Zircon U-Pb geochronology and Lu-Hf isotopic data on Triassic-Jurassic sedimentary and igneous rocks from central Stikinia shed light on the terrane-scale effects of a latest Triassic-Early Jurassic collision between Stikinia and pericratonic Yukon-Tanana terrane. Main age peaks from central Stikinia are 250-160 Ma, reflecting ongoing Mesozoic arc-related igneous activity within Stikinia. Comparison of isotopic evolution and unconformity development between central Stikinia and northern Stikinia (Whitehorse trough) provide new constraints on regional latest Triassic-earliest Jurassic deformation. We attribute the shortening-related deformation to variable along-strike interactions during end-on collision with the Yukon-Tanana terrane, with significant crustal thickening at the northern apex of Stikinia that did not persist farther south. A small pre-Devonian zircon population is significant, as the oldest exposed rocks in Stikinia are Early Devonian. Pre-Devonian age peaks differ from those of the northern Yukon-Tanana terrane, but resemble zircons from southern Wrangellia. These zircons are likely multi-cyclic, derived from crust that originated in the Arctic region near the northern end of the Caledonide orogeny. We suggest that Stikinia was an independent crustal block prior to latest Triassic onset of collision with Yukon-Tanana terrane. The ongoing, end-on collision in turn promoted oroclinal assembly of the peri-Laurentian terranes. GEORGE ET AL.

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.

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Wrangellia, an exotic arc terrane to North America, is interpreted to have been constructed near the margin of the Paleo-Arctic and Paleo-Pacific during middle-late Paleozoic time, before finally accreting to the western margin of North America during Late Jurassic to Early Cretaceous time. Utilizing the detrital zircon record of Paleozoic sedimentary rocks and Cretaceous basin fill we can provide further insight into the magmatic and depositional evolution of southern Wrangellia. 1422 U-Pb LA-ICPMS analyses from five samples of the Fourth Lake Formation in the Carboniferous Buttle Lake Group were performed. 1055 U-Pb LA-ICPMS analyses from four samples of the Comox formation within the Cretaceous Nanaimo Group were acquired in order to provide a broader sampling of the Lower Mesozoic-Paleozoic rocks of Vancouver Island. U-Pb analyses within the Fourth Lake Formation reveal prominent Carboniferous age peaks (344, 339, 336, 331, and 317 Ma), with minor pre-400 Ma grains from adjacent terranes of Paleo-Arctic origin. Paleozoic detrital zircons exhibit juvenile, with ƐHf (t) values between +15 and +5. U-Pb analyses of Nanaimo Group sedimentary rocks reveal dominant peak ages at 341, 195, 167, and 86 Ma. All major populations yield juvenile epsilon ƐHf (t) values in the range of +15 to +6. The detrital zircon U-Pb geochronologic and Hf isotope data in this study suggest that sediment from the Fourth Lake Formation was derived mainly from bimodal magmatism within the Paleozoic southern Wrangellia arc system as well as minor 6 contributions of recycled detritus from the adjacent Alexander Terrane. Hf isotope data from the Comox Formation indicate that Triassic and Jurassic igneous rocks of the Bonanza Arc, and Late Jurassic-Early Cretaceous sources from the central Coast Mountains Batholith (CMB), are highly juvenile. This new geochronologic and geochemical data set contributes to a new tectonic model for the Paleozoic Southern Wrangellia Arc system from Late Devonian to Early Permian time and reveals, during Cretaceous time, very locally derived detritus was deposited in sedimentary basins along the inboard margin of Wrangellia.