Northern Laurentia Research Articles

A Palaeozoic Northwest Passage: incursion of Caledonian, Baltican and Siberian terranes into eastern Panthalassa, and the early evolution of the North American Cordillera

Abstract Palaeozoic to early Mesozoic terranes of the North American Cordillera mostly originated from three distinct regions in Palaeozoic time: the western peri-Laurentian margin, western (Asian) Panthalassa, and the northern Caledonides–Siberia. A review of geological history, fossil and provenance data for the Caledonian–Siberian terranes suggests that they probably occupied an intermediate position between northern Baltica, northeastern Laurentia and Siberia, in proximity to the northern Caledonides, in early Palaeozoic time. Dispersion of these terranes and their westward incursion into eastern Panthalassa are interpreted to result from development of a Caribbean- or Scotia-style subduction system between northern Laurentia and Siberia in mid-Palaeozoic time, termed here the Northwest Passage. Westward propagation of a narrow subduction zone coupled with a global change in plate motion, related to the collision of Gondwana with Laurentia–Baltica, are proposed to have led to initiation of subduction along the western passive margin of Laurentia and development of the peri-Laurentian terranes as a set of rifted continental fragments, superimposed arcs and marginal ocean basin(s) in mid- to late Palaeozoic time. Diachronous orogenic activity from Late Silurian in Arctic Canada, to Early Devonian in north Yukon and adjacent Alaska, Middle Devonian in southeastern British Columbia, and Late Devonian–Early Mississippian in the western USA records progressive development of the Northwest Passage and southward propagation of subduction along western Laurentia.

Late Paleoproterozoic to mid-Neoproterozoic history of northern Laurentia: An overview of central Rodinia

At the assumed core of reconstructed Rodinia, northern Laurentia, comprising the Canadian Shield (including Greenland) and its buried western and northern extensions beneath Phanerozoic cover, is considered in two parts. On its southeast side, the Grenville Province is part of a major late Mesoproterozoic orogen inferred to be related to the assembly of the Rodinia supercontinent. The remainder of northern Laurentia experienced a long period of crustal stability beginning at ca. 1.8 Ga, recorded for the most part by intracontinental and passive-margin sedimentation along with mafic igneous activity, mainly in the mid-Mesoproterozoic, related to limited extension. To the south, this stable part of northern Laurentia is overprinted by late Paleoproterozoic orogeny (Yavapai, Mazatzal, and Central Plains orogens) and early Mesoproterozoic magmatism (granite-rhyolite provinces). Deformed and metamorphosed rocks related to these form a major part of the interior of the Grenville orogen, where mid- to late Mesoproterozoic orogenic events led to accretion of terranes to Laurentia and subsequent terminal collision. The first manifestation of Rodinia breakup began in the west at ca. 0.8 Ga, some 0.2 Ga earlier than along the southeast side; the age of initial rifting along the northern margin is not known.

Some key issues in reconstructions of Proterozoic supercontinents

Supercontinents containing most of the earth's continental crust are considered to have existed at least twice in Proterozoic time. The younger one, Rodinia, formed at ∼1.0 Ga by accretion and collision of fragments produced by breakup of the older supercontinent, Columbia, which was assembled by global-scale 2.0–1.8 Ga collisional events. Little consensus has been reached regarding configurations of these supercontinents because of some unresolved issues concerning continental fits. One of these issues concerns how Siberia was related to Laurentia. Previous reconstructions that consider the Aldan Shield of Siberia as a continuation of the Wyoming Province of Laurentia have been largely abandoned in favor of models connecting Siberia to northern Laurentia, but it remains controversial which part of Siberia is contiguous with northern Laurentia. Also at issue is the western Laurentia–Australia–East Antarctica connection. Most Rodinia reconstructions place Australia, together with East Antarctica, adjacent to either western Canada (the SWEAT hypothesis) or the western United States (the AUSWUS hypothesis). However, recent studies combining paleomagnetic and isotopic age data have called into question the validity of SWEAT, AUSWUS and other variants. Another issue is the position of North China in Rodinia/Columbia. Limited paleomagnetic data seem to be consistent with the Paleo-Mesoproterozoic North China–Siberia/Baltica connection, whereas geological data support the recently proposed Archean to Mesoproterozoic North China–India connection. Controversial issues have also been raised about the timing and history of the amalgamation and fragmentation of South America and West Africa. Both geological and paleomagnetic data suggest that South America (São Francisco and Amazonia Cratons) and West Africa (Congo and West African Cratons) coalesced into a single landmass along the 2.1–2.0 Ga Transamazonian/Eburnean orogens. However, whether they were divorced and then re-married to form part of Gondwana, or remained largely coherent from their amalgamation at 2.1–2.0 Ga until their incorporation into Gondwana is unclear. Also little known is the position of Amazonia–West Africa in the proposed supercontinents, with some workers believing that they existed as a separate landmass, whereas others place Amazonia–West Africa adjacent to Baltica. In summary, although geological and paleomagnetic data are supportive of the existence of Proterozoic supercontinents Rodinia and Columbia, they are insufficient to determine their exact geometries.

Mafic intrusions in southwestern Siberia and implications for a Neoproterozoic connection with Laurentia

Geochemical and geochronological studies reveal three distinct groups of Neoproterozoic to Early Paleozoic mafic intrusions in the Biryusa metamorphic massif of southwestern Siberia. Group 1 includes mafic sills that intruded Neoproterozoic passive-margin sediments, and were sourced from mantle-derived melts contaminated by crustal components. One sill yielded a plagioclase 40Ar– 39Ar age of 741 ± 4 Ma, similar to ages of sills in the adjacent Sharyzhalgai metamorphic massif (all ages are ±2 σ or 95%). Group 2 is represented by north-trending dolerite dykes, one of which furnished a 40Ar– 39Ar age of 612 ± 6 Ma. Group 2 dykes may have formed by passive asthenospheric upwelling and decompression during lithospheric stretching within the Neoproterozoic passive margin, a suggestion supported by the occurrence of coeval carbonatites and alkaline ultramafic rocks in the adjacent Sharyzhalgai massif. Group 3 consists of NW-trending dolerite dykes produced by melting of a lithospheric mantle source metasomatised by subduction-related fluids. Zircons from one Group 3 dyke provided a SHRIMP U–Pb zircon age of 511 ± 5 Ma, similar to the age of regional metamorphism during Early Paleozoic orogenesis. Our results show that mafic intrusions, broadly similar in age to the Franklin intrusions of northern Laurentia, exist along the southwestern margin of Siberia. Paleomagnetic data permit a reconstruction in which southern Siberia is located opposite northern Canada, but at a sufficient distance to accommodate other crustal blocks (e.g. northern Alaska, Chukchi Peninsula), and in which Group 1 intrusions could be distal products of the mid-Neoproterozoic Franklin mantle plume that affected northern Laurentia.

A probable late Neoproterozoic age for the Kennedy Channel and Ella Bay formations, northeastern Ellesmere Island and its implications for passive margin history of the Canadian Arctic

The Kennedy Channel and Ella Bay formations are the two oldest stratigraphic units exposed in the Franklinian margin sedimentary sequence in the Canadian Arctic Islands. An Early Cambrian age had previously been accepted by the occurrence of trilobites and small shelly fossils in the type section of the Kennedy Channel Formation. Reinvestigation of the area around the type section shows that several large strike-slip faults cut the succession and that the olenelloid trilobites are from an infaulted slice of a younger unit, the Lower Cambrian Kane Basin Formation. Thus, there is no unambiguous paleontological evidence for the age of either the Kennedy Channel or Ella Bay formations. However, the abundance of stromatolites, absence of trace fossils, and separation from overlying Lower Cambrian clastics by a regional angular unconformity indicate a probable late Neoproterozoic age for these two formations. The Ella Bay Formation likely correlates with the Portfjeld Formation in North Greenland, the Spiral Creek Formation in East Greenland, and the Risky Formation of the Mackenzie Mountains in northwestern Canada. The passive margin that existed in northern Laurentia during the early Paleozoic was, therefore, established in the late Neoproterozoic, and the onset of rifting must have preceded this, rather than occurring in the Early Cambrian as some authors have suggested.

Petrogenesis of the 723 Ma Coronation sills, Amundsen basin, Arctic Canada: implications for the break-up of Rodinia

The Coronation mafic sills and associated Natkusiak flood basalts in Arctic Canada are part of the ∼723 Ma Franklin igneous events, a flood basalt and dyke swarm province that extends across the southern Arctic of North America and Greenland. Two representative sills, 70 and 25 m thick, respectively outcrop near Coronation Gulf in Nunavut. They are composed of clinopyroxene, plagioclase, olivine and Fe–Ti oxides, and have tholeiitic composition similar to the Natkusiak basalts. Their εNd values range between +4.2 and +5.9 and have (La/Yb)n ratios of ∼1.5–2. These rocks are mantle derived and were affected by minor crustal contamination. Compositional variations indicate that the Coronation sills have experienced extensive fractionation including flow differentiation during emplacement and show increased differentiation from, west to east, across the province, a feature similar to the Mackenzie dyke swarm. However, the Mackenzie dyke swarm and the Franklin igneous events have distinct chemical composition. The Franklin rocks are related to a mantle plume located near Victoria Island at the center of the dyke swarm and could represent an offshoot from the superplume related to the break-up of Rodinia. The absence of such dykes and sills in Siberia argues against locating Siberia against northern Laurentia in a Rodinia reconstruction.

The Late Cambrian Spice ( 13C) Event and the Sauk II-SAUK III Regression: New Evidence from Laurentian Basins in Utah, Iowa, and Newfoundland

Carbon isotope data from Upper Cambrian sections in three Laurentian basins in northern Utah, central Iowa, and western Newfoundland record a large positive d 13 C excursion (SPICE event) of up to 1 5‰. Peak d 13 C ratios are well dated by trilobite collections to the middle of the Steptoean Stage (Dunderbergia Zone) and occur during maximum regression associated with formation of the Sauk II- Sauk III subsequence boundary on the North American craton. Max- imum regression was marked by an influx of quartz sand into carbon- ate-platform settings in all three widely separated basins. In northern Utah, this quartz sand formed a thick sequence known as the Worm Creek Quartzite, which marks a conspicuous interruption of carbonate deposition during the Middle to Late Cambrian in the region. In west- ern Newfoundland, the thickness of the quartz sand unit is much re- duced but still marks a brief shutdown of the carbonate factory that is unique to the Cambrian shelf succession of the area. In the central Iowa area of the cratonic interior, an upward-shallowing carbonate succession culminates in cross-stratified trilobite grainstones at the peak of the SPICE in Dunderbergia Zone time, and the lowest point on the relative-sea-level curve is associated with the occurrence of coarse quartz sand derived from the encroaching shoreface. Although it is difficult to determine precisely the departure from baseline d 13 C that marks the beginning of the SPICE excursion in the stratigraphic successions analyzed, our results are consistent with a rise and subsequent fall in d 13 C tracking a major regressive-transgressive event recorded across northern Laurentia. The correlation of a major d 13 C excursion with regression is similar to that described for the Late Ordovician, for which the pattern has been attributed to either in- creased carbonate relative to terrigenous weathering rates as ice sheets covered up organic-matter-containing silicates at high latitudes or high productivity and organic-carbon burial driven by oceanic overturn. The lack of known Steptoean-age ice sheets that could have affected the ratio of carbonate to silicate weathering rates suggests that organic- carbon burial was the likely cause of the SPICE event. We suggest that increased weathering and erosion rates during relative sea-level fall (Sauk II-III) increased the burial fraction of organic carbon in an expanded region of fine-grained siliciclastic deposits in shelf and upper slope environments during the Steptoean.

Models of Rodinia assembly and fragmentation

Abstract Amongst existing palaeogeographic models of the Rodinia supercontinent, or portions thereof, arguments have focused upon geological relations or palaeomagnetic results, but rarely both. A new model of Rodinia is proposed, integrating the most recent palaeomagnetic data with current stratigraphic, geochronological and tectonic constraints from around the world. This new model differs from its predecessors in five major aspects: cratonic Australia is positioned in the recently proposed AUSMEX fit against Laurentia; East Gondwanaland is divided among several blocks; the Congo-São Francisco and India-Rayner Cratons are positioned independently from Rodinia; Siberia is reconstructed against northern Laurentia, although in a different position than in all previous models; and Kalahari-Dronning Maud Land is connected with Western Australia. The proposed Rodinia palaeogeography is meant to serve as a working hypothesis for future refinements.

U-Pb zircon age constraint for late Neoproterozoic rifting and initiation of the lower Paleozoic passive margin of western Laurentia

A concordant U–Pb zircon age of 569.6 ± 5.3 Ma from synrift volcanic rocks of the Hamill Group, southeastern Canadian Cordillera, provides the first direct U–Pb geochronologic constraint on timing of latest Neoproterozoic rifting along western Laurentia. This age confirms a previous estimate of 575 ± 25 Ma for timing of continental breakup, as derived from the analysis of tectonic subsidence in lower Paleozoic miogeoclinal strata of the North American Cordillera. It also corresponds to the timing of passive margin deposition in the "underlying" Windermere Supergroup of the northern Cordillera, as determined by chemostratigraphic correlations. These timing relationships imply a different breakup history for the northern, as compared to the southern, Cordillera. We propose a model that attempts to explain this paradox of Cordilleran geology. The earlier Neoproterozoic (Windermere-age) rifting event probably records breakup of a continental mass from northern Laurentia followed by development of a passive margin. Accordingly, the Windermere Supergroup of the southern Canadian Cordillera was deposited in an intracontinental rift. The second Neoproterozoic rifting (Hamill–Gog) is interpreted to indicate continental breakup and establishment of a passive margin along western Laurentia.

Evidence for Extensive Proterozoic Remobilization of the Aldan Shield and Implication for Proterozoic Plate Tectonic Reconstructions of Siberia and Laurentia

Baddeleyite LA-ICP-MS U-Pb geochronology, major and trace element geochemistry and Nd-Pb isotope analysis of Late Paleoproterozoic mafic dykes from the northern Yangtze block, South China craton are used to investigate their petrogenesis and tectonic implications. The dykes mainly consist of fine-grained dolerite. Baddeleyite U-Pb dating indicates that the dolerites formed at 1866 ± 21 Ma. These rocks are alkaline in composition with high TiO2 contents (2.21–3.17 wt%) and display LREE enrichments ((La/Yb)N = 6.0–6.4) with slight positive Eu anomalies (Eu/Eu* = 1.06–1.14). On primitive mantle normalized multi-element diagrams they generally show convex upward curves with weak negative Nb-Ta anomalies. All of the samples have similar Nd-Pb isotopic compositions with negative εNd(t) values ranging from −4.3 to −3.2, (206Pb/204Pb)i from 14.91 to 15.31, (207Pb/204Pb)i from 14.99 to 15.08, and (208Pb/204Pb)i from 34.75 to 35.58. Geochemical features such as EM1-like isotopic compositions suggest the dolerites were derived from partial melting of metasomatised subcontinental lithospheric mantle. The mafic dykes are inferred to have formed in a post-orogenic extensional setting. We suggest that the northern Yangtze block experienced a tectonic switch from convergence to extension at ~1.87 Ga. A comparison of Paleoproterozoic magmatic, metamorphic, and sedimentary events in the Yangtze block with the southern Siberian craton and northern Laurentia supports a Paleoproterozoic position of the Yangtze block adjacent to southern Siberia and northern Laurentia within the Columbia supercontinent.

Biogeographic analyses of the Ediacara biota: a conflict with paleotectonic reconstructions

Paleotectonic reconstructions for the late Proterozoic have differed over the timing of the Cordilleran rifting between Laurentia and the East Gondwana cratons. Parsimony Analysis of Endemism (PAE) and phenetic clustering of the “Ediacara biota” were carried out, for comparison with competing paleotectonic hypotheses. All analyses show a common pattern of similarities among biotas. The biotas of Charnwood Forest and Newfoundland consistently group together, while the south Australian biota is closest to those of Baltica, northern Laurentia, and Siberia. The biota of southwest North America, though still poorly known, strikingly resembles that of Namibia—not that of northwestern Canada. This pattern is not obviously due to facies-related or preservational bias and is very different from Cambrian biogeographic patterns. The overall pattern is most consistent with the hypothesis that the western margin of Laurentia was in close contact with East Gondwana, with rifting taking place either during or just before the Vendian. This arrangement has been previously proposed as a paleotectonic hypothesis; however, most recent paleomagnetic and structural studies support the alternate hypothesis that this rifting took place more than 100 million years before the Vendian. Resolving this contradiction will require much more data on both organismal distribution and cratonal position.

U–Pb geochronology of Riphean sandstone and gabbro from southeast Siberia and its bearing on the Laurentia–Siberia connection

Thirty-one detrital zircons from the mid–late Riphean Mayamkan Formation sandstone (Uy Group) of the Sette–Daban fold belt, southeast Siberia yielded SHRIMP 207Pb/ 206Pb ages ranging between 1500 and 1050 Ma. Other grains yielded ages between 2.7 and 1.8 Ga. There is no known source region for the Mesoproterozoic zircons in Siberia; however, this range of ages closely matches those of detrital zircons from Neoproterozoic sandstones from northwest Canada, which are considered to have been derived from the Grenville Province of southeast Laurentia (all directions cited are with reference to present-day coordinates). These data suggest a formerly close connection between southeast Siberia and northwest Laurentia prior to their separation in the Neoproterozoic. However, two gabbro sills which intrude the Riphean sedimentary succession of the Sette–Daban fold belt are dated here at 1005±4 Ma and 974±7 Ma (U–Pb baddeleyite), an unknown age in northern Laurentia and unlike the widespread and well characterized 723 Ma Franklin and 1267 Ma Mackenzie mafic magmatic events. These somewhat incongruous results cast uncertainty on existing continental reconstructions, which link Siberia to Laurentia from about 1900 to 700 Ma. Our data can be reconciled with existing data by proposing an alternative continental configuration based on former continuity of the following tectonic entities: Archean Tungus Province (Siberia) with Archean Slave Province (Laurentia); Paleoproterozoic Angara fold belt (Siberia) with Paleoproterozoic Wopmay orogen and Great Bear magmatic zone (Laurentia); and Paleoproterozoic Akitkan fold belt (Siberia) with Paleoproterozoic Thelon–Taltson magmatic zone (Laurentia). Our reconstruction also considers the proposed northern extension of the Grenville orogen to be a potential source for Mesoproterozoic detrital zircons from the Mayamkan Formation. Such an orientation also is required to explain the apparent absence of Franklin and/or Mackenzie mafic magmatic rocks and the lack of distinctive Neoproterozoic lithofacies in the Sette–Daban fold belt. An additional conclusion of our study is that the lowermost Uy Group can be no younger than ca. 1010 Ma because it is intruded by a diabase sill dated at 1005±4 Ma. Previous work indicated that the Uy Group and underlying Lakhanda Group are of late Riphean age (1000–650 Ma). The youngest detrital zircon from the Mayamkan Formation provides a maximum U–Pb age of 1070±40 Ma for the upper Uy Group.

Lower Devonian aulacopleuroidean trilobites from Oklahoma

New aulacopleuroidean trilobites from the Lochkovian of Oklahoma include the otarionineCyphaspis carrollinew species from the Haragan Formation, and the brachymetopidCordania wessmaninew species from the overlying Bois d'Arc Formation.Cyphaspis carrolliis the first record of the genus from the North American Devonian. It is a highly plesiomorphic species, dissimilar to contemporaries from Europe, but closely related to Silurian species from Northern Laurentia and England.Cordania wessmanihad previously been interpreted as a possible sexual dimorph ofCordania falcataWhittington, 1960, but new material and information shows that the forms occur separately with no stratigraphic overlap. New information on trilobite occurrence in the Haragan and Bois d'Arc Formations does not support previous hypotheses of trilobite sexual dimorphism, but rather indicates the presence of distinct, stratigraphically successive faunas.

Silurian Odontopleurinae (Trilobita) from the Cape Phillips Formation, Arctic Canada

Odontopleurids are a diverse component of rich silicified trilobite faunas recovered from the Wenlock and Ludlow of the Cape Phillips Formation, central Canadian Arctic. Odontopleurinae and Acidaspidinae are common, but Ceratocephalinae and Koneprusiinae are also represented. This work treats all of the Odontopleurinae, with the exception of the genusAcanthalominaPrantl and Přibyl, 1949.New species ofKettneraspisPrantl and Přibyl, 1949, include the upper SheinwoodianK. wrightae,the lower HomerianK. lindoei,and the GorstianK. caldwelli.Rare specimens assigned toOdontopleuraEmmrich, 1839, andRadiaspisRichter and Richter, 1917, occur in the Sheinwoodian of the central Arctic.Edgecombeaspis(type speciesE. johansonaenew species) is proposed for an odontopleurine clade endemic to Laurentia, and in the Silurian restricted to northern Laurentia. Cladistic analysis yields a hypothesis of ingroup structure that is in general calibrated with stratigraphic sequence. An exception is a group of Telychian species from the Mackenzie Mountains, whose stratigraphic sequence was used to support a previous hypothesis of an ancestral-descendant lineage. The cladistic result indicates that the stratigraphic pattern is the inverse of the phylogenetic pattern: the stratigraphically lowest species in the proposed lineage is the most derived, and the highest is most primitive. In addition to the type, new species ofEdgecombeaspisinclude the mid-SheinwoodianE. jahansiand the lower HomerianE. soehni. Edgecombeaspisapparently became extinct in the Homerian.The speciesKettneraspis lenzi(Chatterton and Perry, 1983) andRadiaspiscf.R. norfordi(Chatterton and Perry, 1983) occur in the central Arctic, and further strengthen previous correlations with strata in the central Mackenzie Mountains, Northwest Territories.

The “Encrinurus” variolaris plexus (Trilobita, Silurian): Relationships of Llandovery species

Phylogenetically basal, Llandovery parts of the diverse variolaris plexus of encrinurine trilobites havebeen assigned to a grade group, Nucleurus.. Parsimony analysis of 24 exoskeletal characters for adequately known species of Nucleurus and allied post-Llandovery taxa provides a phylogenetic scheme for reclassifying the base of the variolaris plexus. Nucleurus Ramsköld,, 1986 is redefined with Aeronian-Telychian core and Rhuddanian-Aeronian stem groups ( Nucleurus s.s. and s.l., respectively). Monophyletic groups in the shortest cladograms include Elsarella nov. gen. (Llandovery, east Baltic region) and an unnamed clade (Llandovery-Ludlow?, Baltic region and Japan). Billevittia nov. gen. (Telychian, northern Laurentia) is the closest relative of a diverse post-Llandovery radiation. Previously established east Baltic Llandovery species of the variolaris plexus are revised and illustrated. Most type specimens have been further prepared to show new details, and additional information is provided by new material. New species are Billevittia adraini and Encrinurus ralfi..

ACRITARCHSA REVIEW

ACRITARCHSA REVIEW

Cool climate bauxite

The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.