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1. The Ordovician Thores volcanicn island arc of the Pearya Terrane from northern Ellesmere Island formed on Precambrian continental crust

   https://doi.org/https://doi.org/10.1016/j.lithos.2021.105999  

   Majka, J.; Kosminska, K.; Bazarnik, J.; McClelland, W.C. (April 2021, Lithos)

   null (Ed.)

   Ion microprobe U–Pb zircon dating of intermediate to felsic rocks coupled with bulk-rock geochemistry analyses and compared to previously published data shows that the Thores Suite of the Pearya Terrane of northern Ellesmere Island (Arctic Canada) represents an Early Ordovician (c. 490–470 Ma) suite formed in an island arc setting. Interestingly, three out of five dated samples contain abundant xenocrystic zircon that have ages spanning from c. 2690 Ma to c. 520 Ma. The vast majority of xenocrystic zircon are Precambrian in age and typical of Laurentia. The youngest well-pronounced age cluster around 580–570 Ma is inferred to be an expression of the Timanide Orogen, traditionally ascribed to Baltica. This geochronological dataset provides new insight on the origin of the Thores Suite of the Pearya Terrane, which was traditionally thought to be formed due to the M'Clintock orogenic event and commonly treated as independent from Caledonian tectonism. We suggest that the Thores island arc formed on a sliver of continental crust within the Iapetus Ocean. The timing of igneous activity recorded by the Thores Suite is consistent with other island arcs and subduction-related metamorphic units that occur within the Caledonides of northern Scandinavia and Svalbard. Hence, we suggest that the Thores volcanic island arc was closely associated with age equivalent arcs developed within the northern Iapetus Ocean. Its juxtaposition with the other successions of the Pearya Terrane is explained by a large-scale, left lateral, strike-slip system operating along the northeastern margins of Baltica and Laurentia, coeval with the main collision between the two continents. This strike-slip system was responsible for the juxtaposition of multiple terranes with contrasting Precambrian histories that can be traced in the present day High Arctic, e.g. in southwest Svalbard and the Pearya Terrane. 

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2. 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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3. U-Pb detrital zircon analysis of sedimentary rocks of the southeastern New England Avalon terrane in the US Appalachians: Evidence for a separate crustal block

   Kuiper, Yvette D; Murray, Daniel P; Crowley, James L (October 2021, Geological Society of America data repository)

   Kuiper, Yvette D; Murphy, J Brendan; Nance, R Damian; Strachan, Rob A; Thompson, Margaret D (Ed.)

   The Avalon terrane of southeastern New England is a composite terrane, in which various crustal blocks may have different origins and/or tectonic histories. The northern part (west and north of Boston, Massachusetts) correlates well with Avalonian terranes in Newfoundland, Nova Scotia and New Brunswick, Canada, based on rock types and ages, U–Pb detrital zircon signatures of metasedimentary rocks, and Sm–Nd isotope geochemistry data. In the south, fewer data exist, in part because of poorer rock exposure, and the origins and histories of the rocks are less well constrained. We conducted U–Pb laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) analysis on zircon from seven metasedimentary rock samples from multiple previously interpreted subterranes, in order to constrain their origins. Two samples of Neoproterozoic Plainfield Formation quartzite from the previously interpreted Hope Valley subterrane in the southwestern part of the southeastern New England Avalon terrane and two from the Neoproterozoic Blackstone Group quartzite from the adjacent Esmond-Dedham subterrane to the east have Tonian youngest detrital zircon age populations. One sample of Cambrian North Attleboro Formation quartzite of the Esmond-Dedham subterrane yielded an Ediacaran youngest detrital zircon age population. Detrital zircon populations of all five samples include abundant Mesoproterozoic zircon and smaller Paleoproterozoic and Archean populations, and are similar to those of the northern part of the southeastern New England Avalon terrane and the Avalonian terranes in Canada. These are interpreted as having a Baltican/Amazonian affinity based primarily on published U-Pb and Lu-Hf detrital zircon data. Based on U-Pb detrital zircon data, there is no significant difference between the Hope Valley and Esmond-Dedham subterranes. Detrital zircon of two samples of the Price Neck and Newport Neck formations of the Neoproterozoic Newport Group in southern Rhode Island is characterized by large ~647–643 and ~745–733 Ma age populations and minor zircon up to ~3.1 Ga. This signature is most consistent with a northwest African affinity. The Newport Group may thus represent a subterrane, terrane or other crustal block with a different origin and history than the southeastern New England Avalon terrane to the northwest. The boundary of this Newport Block may be restricted to the boundaries of the Newport Group, or it may extend as far north as Weymouth, MA, as far northwest as (but not including) the North Attleboro Formation quartzite and associated rocks in North Attleboro, MA, and as far west as Warwick, RI, where eastern exposures of the Blackstone Group quartzite exist. The Newport Block may have amalgamated with the Amazonian/Baltican part of the Avalon terrane prior to mid-Paleozoic amalgamation with Laurentia, or have arrived as a separate terrane after accretion of the Avalon terrane. Alternatively, it may have arrived during the formation of Pangea and been stranded after the breakup of Pangea, as has been proposed previously for rocks of the Georges Bank in offshore Massachusetts. If the latter is correct, then the boundary between the Newport Block and the southeastern New England Avalon terrane is the Pangean suture zone. 

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4. Open-system Evolution of a Crustal-scale Magma Column, Klamath Mountains, California

   https://doi.org/10.1093/petrology/egab065  

   Barnes, Calvin G; Coint, Nolwenn; Barnes, Melanie A; Chamberlain, Kevin R; Cottle, John M; Rämö, O Tapani; Strickland, Ariel; Valley, John W (December 2021, Journal of Petrology)

   Abstract This study addresses the question of how and where arc magmas obtain their chemical and isotopic characteristics. The Wooley Creek batholith and Slinkard pluton are a tilted, mid- to upper-crustal part of a vertically extensive, late-Jurassic, arc-related magmatic system in the Klamath Mountains, northern California. The main stage of the system is divided into an older lower zone (c. 159 Ma) emplaced as multiple sheet-like bodies, a younger upper zone (c. 158–156 Ma), which is gradationally zoned upward from mafic tonalite to granite, and a complex central zone, which represents the transition between the lower and upper zones. Xenoliths are common and locally abundant in the lower and central zones and preserve a ghost stratigraphy of the three host terranes. Bulk-rock Nd isotope data along with ages and Hf and oxygen isotope data on zircons were used to assess the location and timing of differentiation and assimilation. Xenoliths display a wide range of εNd (whole-rock) and εHf (zircon), ranges that correlate with rocks in the host terranes. Among individual pluton samples, zircon Hf and oxygen isotope data display ranges too large to represent uniform magma compositions, and very few data are consistent with uncontaminated mantle-derived magma. In addition, zoning of Zr and Hf in augite and hornblende indicates that zircon crystallized at temperatures near or below 800 °C; these temperatures are lower than emplacement temperatures. Therefore, the diversity of zircon isotope compositions reflects in situ crystallization from heterogeneous magmas. On the basis of these and published data, the system is interpreted to reflect initial MASH-zone differentiation, which resulted in elevated δ18O and lowered εHf in the magmas prior to zircon crystallization. Further differentiation, and particularly assimilation–fractional crystallization, occurred at the level of emplacement on a piecemeal (local) basis as individual magma batches interacted with partial melts from host-rock xenoliths. This piecemeal assimilation was accompanied by zircon crystallization, resulting in the heterogeneous isotopic signatures. Magmatism ended with late-stage emplacement of isotopically evolved granitic magmas (c. 156 Ma) whose compositions primarily reflect reworking of the deep-crustal MASH environment. 

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5. A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America

   https://doi.org/10.1130/B35981.1  

   LaMaskin, Todd A.; Rivas, Jonathan A.; Barbeau, David L.; Schwartz, Joshua J.; Russell, John A.; Chapman, Alan D. (July 2021, GSA Bulletin)

   Abstract Differing interpretations of geophysical and geologic data have led to debate regarding continent-scale plate configuration, subduction polarity, and timing of collisional events on the western North American plate margin in pre–mid-Cretaceous time. One set of models involves collision and accretion of far-traveled “exotic” terranes against the continental margin along a west-dipping subduction zone, whereas a second set of models involves long-lived, east-dipping subduction under the continental margin and a fringing or “endemic” origin for many Mesozoic terranes on the western North American plate margin. Here, we present new detrital zircon U-Pb ages from clastic rocks of the Rattlesnake Creek and Western Klamath terranes in the Klamath Mountains of northern California and southern Oregon that provide a test of these contrasting models. Our data show that portions of the Rattlesnake Creek terrane cover sequence (Salt Creek assemblage) are no older than ca. 170–161 Ma (Middle–early Late Jurassic) and contain 62–83% Precambrian detrital zircon grains. Turbidite sandstone samples of the Galice Formation are no older than ca. 158–153 Ma (middle Late Jurassic) and contain 15–55% Precambrian detrital zircon grains. Based on a comparison of our data to published magmatic and detrital ages representing provenance scenarios predicted by the exotic and endemic models (a crucial geologic test), we show that our samples were likely sourced from the previously accreted, older terranes of the Klamath Mountains and Sierra Nevada, as well as active-arc sources, with some degree of contribution from recycled sources in the continental interior. Our observations are inconsistent with paleogeographic reconstructions that are based on exotic, intra-oceanic arcs formed far offshore of North America. In contrast, the incorporation of recycled detritus from older terranes of the Klamath Mountains and Sierra Nevada, as well as North America, into the Rattlesnake Creek and Western Klamath terranes prior to Late Jurassic deformation adds substantial support to endemic models. Our results suggest that during long-lived, east-dipping subduction, the opening and subsequent closing of the marginal Galice/Josephine basin occurred as a result of in situ extension and subsequent contraction. Our results show that tectonic models invoking exotic, intra-oceanic archipelagos composed of Cordilleran arc terranes fail a crucial geologic test of the terranes’ proposed exotic origin and support the occurrence of east-dipping, pre–mid-Cretaceous subduction beneath the North American continental margin. 

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