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1. Zircon geochronology and geochemistry of the Ward Hunt pluton, Pearya terrane, Canadian High Arctic: Insights into its age, origin, and circum-Arctic Timanide connections

   https://doi.org/https://doi.org/10.1007/s41063-020-00078-9  

   Malone, S.J.; McClelland, W.C. (December 2020, Arktos)

   null (Ed.)

   The northern margin of the Neoproterozoic Timanide Orogen is truncated by Paleozoic deformation of the Caledonian Orogen. Evidence for dispersion of terranes affected by the Timanide Orogen is documented through contemporaneous tectonothermal activity, and by detrital zircon in sedimentary rock from across the Arctic Ocean margins. However, directly tying these terranes to the Caledonide realm is hindered by the paucity of appropriate events in proximal terranes. The Ward Hunt Pluton, a previously undated syenite–monzodiorite intrusion located on Ward Hunt Island, northern Pearya terrane, yields a crystallization age of 542 ± 2 Ma. Trace-element data from the igneous zircon suggest that the pluton intruded older metasedimentary rocks of the terrane as part of a volcanic arc system, indicated by juvenile Hf isotopic signatures and traceelement data. The data support links between the Pearya terrane and other Neoproterozoic–Cambrian arc systems, such as those proposed in Arctic Alaska-Chukota and the Alexander terrane. 

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2. Evidence of Carboniferous arc magmatism preserved in the Chicxulub impact structure

   https://doi.org/10.1130/B35831.1  

   Ross, Catherine H.; Stockli, Daniel F.; Rasmussen, Cornelia; Gulick, Sean P.S.; de Graaff, Sietze J.; Claeys, Philippe; Zhao, Jiawei; Xiao, Long; Pickersgill, Annemarie E.; Schmieder, Martin; et al (April 2021, GSA Bulletin)

   null (Ed.)

   Determining the nature and age of the 200-km-wide Chicxulub impact target rock is an essential step in advancing our understanding of the Maya Block basement. Few age constraints exist for the northern Maya Block crust, specifically the basement underlying the 66 Ma, 200 km-wide Chicxulub impact structure. The International Ocean Discovery Program-International Continental Scientific Drilling Program Expedition 364 core recovered a continuous section of basement rocks from the Chicxulub target rocks, which provides a unique opportunity to illuminate the pre-impact tectonic evolution of a terrane key to the development of the Gulf of Mexico. Sparse published ages for the Maya Block point to Mesoproterozoic, Ediacaran, Ordovician to Devonian crust are consistent with plate reconstruction models. In contrast, granitic basement recovered from the Chicxulub peak ring during Expedition 364 yielded new zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) concordant dates clustering around 334 ± 2.3 Ma. Zircon rare earth element (REE) chemistry is consistent with the granitoids having formed in a continental arc setting. Inherited zircon grains fall into three groups: 400−435 Ma, 500−635 Ma, and 940−1400 Ma, which are consistent with the incorporation of Peri-Gondwanan, Pan-African, and Grenvillian crust, respectively. Carboniferous U-Pb ages, trace element compositions, and inherited zircon grains indicate a pre-collisional continental volcanic arc located along the Maya Block’s northern margin before NW Gondwana collided with Laurentia. The existence of a continental arc along NW Gondwana suggests southward-directed subduction of Rheic oceanic crust beneath the Maya Block and is similar to evidence for a continental arc along the northern margin of Gondwana that is documented in the Suwannee terrane, Florida, USA, and Coahuila Block of NE México. 

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3. P–T–t Path of Unusual Garnet–Kyanite–Staurolite–Amphibole Schists, Ellesmere Island, Canada — Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

   https://doi.org/https://doi.org/10.1093/petrology/egac068  

   Kosminska, Karolina; Gilotti, Jane A.:; Coble, Matthew A.; Thomas, Jay B (July 2022, Journal of petrology)

   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, 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 Petersen Bay Assemblage. 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 Petersen Bay Assemblage are low grade; they never saw the heat and were not directly involved in Pearya accretion. 

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4. P–T –t Path of Unusual Garnet–Kyanite–Staurolite–Amphibole Schists, Ellesmere Island, Canada—Quartz Inclusion in Garnet Barometry and Monazite Petrochronology

   https://doi.org/10.1093/petrology/egac068  

   Kośmińska, Karolina; Gilotti, Jane A; McClelland, William C; Coble, Matthew A; Thomas, Jay B (August 2022, Journal of Petrology)

   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. 

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5. A detrital zircon test of large-scale terrane displacement along the Arctic margin of North America

   https://doi.org/10.1130/G48336.1  

   Gibson, Timothy M.; Faehnrich, Karol; Busch, James F.; McClelland, William C.; Schmitz, Mark D.; Strauss, Justin V. (January 2021, Geology)

   null (Ed.)

   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. 

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