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Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion. 

Abstract Detrital zircon U-Pb geochronology is one of the most common methods used to constrain the provenance of ancient sedimentary systems. Yet, its efficacy for precisely constraining paleogeographic reconstructions is often complicated by geological, analytical, and statistical uncertainties. To test the utility of this technique for reconstructing complex, margin-parallel terrane displacements, we compiled new and previously published U-Pb detrital zircon data (n = 7924; 70 samples) from Neoproterozoic–Cambrian marine sandstone-bearing units across the Porcupine shear zone of northern Yukon and Alaska, which separates the North Slope subterrane of Arctic Alaska from northwestern Laurentia (Yukon block). Contrasting tectonic models for the North Slope subterrane indicate it originated either near its current position as an autochthonous continuation of the Yukon block or from a position adjacent to the northeastern Laurentian margin prior to >1000 km of Paleozoic–Mesozoic translation. Our statistical results demonstrate that zircon U-Pb age distributions from the North Slope subterrane are consistently distinct from the Yukon block, thereby supporting a model of continent-scale strike-slip displacement along the Arctic margin of North America. Further examination of this dataset highlights important pitfalls associated with common methodological approaches using small sample sizes and reveals challenges in relying solely on detrital zircon age spectra for testing models of terranes displaced along the same continental margin from which they originated. Nevertheless, large-n detrital zircon datasets interpreted within a robust geologic framework can be effective for evaluating translation across complex tectonic boundaries. 

Abstract The origin and displacement history of terranes emplaced along the northern margin of North America remain contentious. One of these terranes is the North Slope subterrane of the Arctic Alaska-Chukotka microplate, which is separated from the northwestern margin of Laurentia (Yukon block) by the Porcupine Shear Zone of Alaska and Yukon. Here, we present new field observations, geological mapping, detrital zircon U-Pb geochronology, and sedimentary/igneous geochemistry to elucidate the stratigraphic architecture of deformed pre-Mississippian rocks exposed within the Porcupine Shear Zone, which we distinguish herein as the newly defined Ch’oodeenjìk succession. The oldest rocks in the Ch’oodeenjìk succession consist of siliciclastic strata of the Lahchah and Sunaghun formations (new names), which yield detrital zircon U-Pb age populations of ca. 1050-1250, 1350-1450, 1600-1650, and 2500-2800 Ma (n =800). This succession is overlain by chert-bearing dolostone and limestone of the Caribou Bar formation (new name) that contains vase-shaped microfossils and yields carbonate carbon (δ13Ccarb) and strontium (87Sr/86Sr) isotopic data that range from ca. -3‰ to +3‰ and 0.70636 to 0.70714, respectively. These data suggest that Lahchah, Sunaghun, and Caribou Bar formations are late Tonian in age. These Neoproterozoic rocks are intruded by Late Devonian (Frasnian-Famennian) felsic plutons and mafic dikes, one of which yielded a sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG) U-Pb age of 380 ± 4 Ma. Neoproterozoic strata of the Ch’oodeenjìk succession are also unconformably overlain by Upper Devonian-Carboniferous (?) siliciclastic rocks of the Darcy Creek formation (new name), which yields detrital zircon populations of ca. 365–385, 420-470 and 625-835 Ma, in addition to Proterozoic age populations similar to the underlying Tonian strata. Together, these new stratigraphic, geochronological, geochemical, and micropaleontological data indicate that pre-Mississippian rocks exposed within the Porcupine Shear Zone most likely represent a peri-Laurentian crustal fragment that differs from the adjacent Yukon block and North Slope subterrane; thus, the Porcupine Shear Zone represents a fundamental tectonic boundary separating autochthonous Laurentia from various accreted peri-Laurentian crustal fragments.