Northwestern Laurentia Research Articles

Paleoproterozoic magmatic and metamorphic events link Yangtze to northwest Laurentia in the Nuna supercontinent

Zircons from granitic gneisses in North Vietnam have magmatic cores dated at 2.28–2.19 Ga, and constitute the first reported evidence of continental crust with these ages in the Yangtze Block of the South China Craton. Overgrowths on zircon rims indicate two periods of metamorphism at 1.97–1.95 Ga and ∼1.83 Ga. These events, along with a previously reported ∼2.36 Ga metamorphic overgrowth on ∼2.9 Ga crystallized zircons from the same region, suggest a sequence of events similar to that recorded for the northwestern region of Laurentia and possibly Siberia, which are associated with assembly of the Nuna supercontinent. These include the 2.4–2.3 Ga Arrowsmith Orogen and a range of events in the interval 2.32–1.80 Ga, including accretionary magmatism in northwestern Laurentia and Siberia (2.32–2.07 Ga), the Thelon orogeny (2.02–1.96 Ga) and the 1.85–1.80 Ga collision between the Superior and Hearne–Rae cratons during the Trans-Hudson Orogen in Laurentia, and the Akitkan Orogen in Siberia (2.03–1.86 Ga). Subsequent attempted breakup of Nuna may be represented by ca. 1.80 to 1.59 Ga consanguineous extension related sedimentation and magmatism in the southwestern Yangtze Block and northwestern Laurentia. These correlations favor location of the Yangtze Block adjacent to northwest Laurentia, and possibly Siberia, within the Nuna supercontinent.

Characterization of the Paleoproterozoic Hottah terrane, Wopmay Orogen using multi-isotopic (U-Pb, Hf and O) detrital zircon analyses: An evaluation of linkages to northwest Laurentian Paleoproterozoic domains

The Paleoproterozoic Wopmay orogen is modeled as a simple arc-continent collision involving the Archean Slave craton and an enigmatic continental arc terrane termed the Hottah terrane. Hottah is a composite terrane that includes a juvenile 2.6-2.1 Ga basement and a younger 1.93-1.90 Ga continental arc. The extent and crustal age structure of the terrane is poorly known which limits interpretations as to its origin, and eventual role in the development of the orogen. Multi-isotopic (U-Pb, Hf, O) analyses of detrital zircon from rocks of the Hottah terrane and its cover sequence are used to indirectly evaluate its age and crustal characteristics. The data confirm that ca 2.6-2.1 Ga crust is an important component of the terrane but Hf isotopic data indicate mostly non-juvenile compositions consistent with recycled Neoarchean crust sources. Ages of detrital zircon are distinct from those expected of the Slave craton but are similar to adjacent 2.0-1.90 Ga Paleoproterozoic terranes of northwestern Laurentia. The similarity to terranes such as the Ksituan/Buffalo Head and Taltson-Thelon magmatic zones, south and east of the Wopmay Orogen indicate that Hottah could represent a terrane that initially evolved adjacent to the earliest assembled parts of the Nuna continent between 2.0 and 1.9 Ga, and was rifted prior to the collision with the Slave margin that produced the Calderian orogeny.

Late Paleoproterozoic to Mesoproterozoic rift successions in SW China: Implication for the Yangtze Block–North Australia–Northwest Laurentia connection in the Columbia supercontinent

The late Paleoproterozoic to Mesoproterozoic Dongchuan Group in the southwestern Yangtze Block formed in a series of fault-controlled, rift-related basins associated with the fragmentation of the Columbia supercontinent. The Dongchuan Group is composed, from the base upward, of the Yinmin, Luoxue, Etouchang and Luzhijiang formations. The Yinmin Formation has a basal layer of conglomerate and coarse sandstone, up to 2m thick, marking initiation of rifting. It is a laterally discontinuous layer, indicative of lateral variations in the accommodation space created adjacent to juvenile normal faults in the earliest stages of rifting. Fluvial to intertidal facies sedimentary rocks of the Yinmin Formation are characterized by fining-upward patterns from sandstone to interbedded carbonate and mudstone, representing syn-rifting sedimentation. Enlargement of the basin and establishment of a carbonate platform during sedimentation of the Luoxue Formation suggest that faulting and volcanism were significantly reduced, marking a transition from syn-rifting subsidence to post-rifting subsidence. Black carbonaceous shale of the Etouchang Formation likely formed in a continental slope and deep ocean basin, generated by slow thermal subsidence of heated subcrustal materials during the post-rifting stage. Subsequent shoaling led to re-establishment of a rimmed carbonate shelf, in which massive argillaceous dolostone of the Luzhijiang Formation started to accumulate. Two major igneous events marked the ca. 1710-Ma rifting climax and 1685–1660-Ma waning phase of rifting. The rift sequence in the southwestern Yangtze Block correlates with the late Paleoproterozoic to early Mesoproterozoic sequence in the Mount Isa Basin, North Australia. These successions have identical zircon age patterns and similar igneous and sedimentary assemblages. In addition, the rifting-related 1735–1663Ma Hornby Bay and ~1640–1600Ma Wernecke successions in northwestern Laurentia show similarities with those synchronous successions in the Yangtze Block and North Australia. This correlation suggests that the Yangtze Block was likely connected to northern Australia and northwestern Laurentia during the assembly of Columbia supercontinent but drifted away from these continents before ca. 1590Ma and showed tectonic evolution different from them after that.

Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic

Two supercontinents have been proposed for the latter half of the Precambrian: Columbia (or Nuna) from ca. 1.9 to 1.3Ga, and Rodinia from ca. 1.1 to 0.75Ga. In both supercontinents, Laurentia and Australia are regarded as probable neighbours, although their relative positions are contentious. Here we use detrital zircons ages from unit PR1 of the lower Fifteenmile group in Yukon, Canada, to demonstrate that northeastern Australia and northwestern Laurentia were firmly connected in the Mesoproterozoic. The zircon ages define a near-unimodal population with a peak at 1499±3Ma, which lies in an interval of magmatic quiescence on Laurentia, known as the North American magmatic gap (NAMG), and abundant magmatism in Australia. Sediment compositions and textures suggest the sediment was derived from a proximal metaplutonic source. We suggest that the Williams and Naraku batholiths in the Mt. Isa inlier in northeastern Australia, with crystallization ages ranging from 1493±8Ma to 1508±4Ma, are the most probable sources of sediment for the PR1 basin. The plutons were exhumed between 1460 and 1420Ma, and likely formed an active, eroding highland in the Australian part of Columbia. Sediment derived from these plutons was carried eastward by a short, direct river system and deposited into the PR1 marine basin. Formation of the PR1 basin coincides with the formation of the southern Cordilleran Belt-Purcell, Hess Canyon, and Trampas basins. These basins, formed on the western margin of Laurentia, also have detrital zircon populations that fall into the NAMG, suggesting that sediment was derived from a non-Laurentian westerly source. The PR1 basin is herein correlated with the Belt-Purcell, Hess Canyon, and Trampas basins to the south, and together these basins record the onset of Columbia breakup along the length of the western margin of Laurentia from as far north as Yukon to as far south as Arizona.

Provenance and tectonic setting of the Paleo- to Mesoproterozoic Dongchuan Group in the southwestern Yangtze Block, South China: Implication for the breakup of the supercontinent Columbia

Abstract The Paleoproterozoic to Mesoproterozoic (1742–1503 Ma) Dongchuan Group in the southwestern Yangtze Block is a rift-related sedimentary sequence that was associated with the breakup of the supercontinent Columbia and is particularly important for the possible linkage between the Yangtze Block and other continents in Columbia. The Dongchuan Group consists of the Yinmin, Luoxue, Etouchang and Luzhijiang formations from the base upward. Sandstones from the Yinmin Formation are mainly arkose containing dominant K-feldspar with subordinate plagioclase and quartz. Abundant feldspar and high Qm/Q ratios (0.94–1) are indicative of plutonic sources. These sandstones have high La/Sc (3.06 to 4.32), low Sc/Th (0.74 to 1.15) and Co/Th (0.85 to 1.52) and highly evolved Nd isotopes (eNd(t) = − 6.2 to − 8.2), consistent with an old, felsic igneous source. Detrital zircons of this formation have two major age groups at 2602–2887 Ma and 2224–2392 Ma. Siltstones of the Etouchang Formation have detrital zircons with a prominent age peak at ~ 2560 Ma and several subordinate peaks at ~ 2180 Ma, ~ 2100 Ma and ~ 1900 Ma. They have high Sc/Th (1.00–7.08), Co/Th (0.13 to 6.31) and eNd(t) (− 2.1 to − 6.7), significantly different from the Yinmin Formation. The Yinmin Formation is interpreted to deposit during the initial stage of extensional rifting receiving detritus of granites and TTG mainly from uplifted shoulder. The Etouchang Formation more likely formed in a passive margin with sedimentary material largely from cratonic sources. Paleoproterozoic to Mesoproterozoic rift basins in the southwestern Yangtze Block, north Australia and northwestern Laurentia have remarkably similar provenance and tectonic setting in their lower part (1742–1596 Ma), but significantly different since the onset of the Etouchang Formation (ca. 1596 Ma). Therefore, the southwestern Yangtze Block was likely connected with the north Australia and northwestern Laurentia in Columbia and drifted away from these continents on or before ca. 1596 Ma.

The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia

The Wernecke igneous clasts consist of blocks of plutonic and volcanic rock that range up to hundreds of metres in size. These clasts occur exclusively within zones of hydrothermal breccia (Wernecke Breccia) which are widespread in central and northern Yukon. The breccia zones are hosted by the Wernecke Supergroup and have been dated by U–Pb titanite at 1599Ma. Four U–Pb zircon ages on the Wernecke igneous clasts (1714–1706Ma) demonstrate that the clasts are older than the Wernecke Supergroup (<1.64Ga) and indicate that the clasts were not derived from dykes within the Wernecke Supergroup. Instead, the clasts were derived from an obducted terrane named Bonnetia. Geochemical characteristics of the Wernecke igneous clasts infer that Bonnetia formed as a volcanic arc with a component of within-plate magmatism. Neodymium mantle depletion ages of 2080–2760Ma suggest that the arc was built on older continental crust. Consequently, Bonnetia may have been a volcanic arc, possibly built on a rifted fragment of Laurentia, on another continental fragment, or possibly on the leading edge of another continent. The subsequent event of breccia-formation may represent a hydrothermal response to obduction-caused tectonic loading of the crust. The characterization of Bonnetia as a volcanic arc complex that underwent obduction requires that northwestern Laurentia was flanked by an ocean basin in the late Paleoproterozoic.

Laurentian origin for the North Slope of Alaska: Implications for the tectonic evolution of the Arctic

The composite Arctic Alaska-Chukotka terrane plays a central role in tectonic reconstructions of the Arctic. An exotic, non-Laurentian origin of Arctic Alaska-Chukotka has been proposed based on paleobiogeographic faunal affi nities and various geochronological constraints from the southwestern portions of the terrane. Here, we report early Paleozoic trilobite and conodont taxa that support a Laurentian origin for the North Slope subterrane of Arctic Alaska, as well as new Neoproterozoic-Cambrian detrital zircon geochronological data, which are both consistent with a Laurentian origin and profoundly different from those derived from similar-aged strata in the southwestern subterranes of Arctic Alaska-Chukotka. The North Slope subterrane is accordingly interpreted as allochthonous with respect to northwestern Laurentia, but it most likely originated farther east along the Canadian Arctic or Atlantic margins. These data demonstrate that construction of the com- posite Arctic Alaska-Chukotka terrane resulted from juxtaposition of the exotic southwestern fragments of the terrane against the northern margin of Laurentia during protracted Devonian(?)-Carboniferous tectonism.

The 1750 Ma Magmatic Event of the West African Craton (Anti-Atlas, Morocco)

The Precambrian inliers of the Anti Atlas belt located at the southern part of Morocco contain numerous mafic dyke and sill swarms thought to represent the erosional remnants of Proterozoic Large Igneous Provinces (LIPs). As part of a continuing research campaign to date and characterize these LIPs, four dykes have been dated by the U–Pb TIMS (Thermal Ionization Mass Spectrometry) method on baddeleyite and zircon: ca. 1758Ma in the Tagragra of Akka inlier, 1741±10Ma in the Tafeltast-Kerdous inlier, 1746.8±3.7Ma in the Iguerda-Taïfast and 1734±5Ma in the Zenaga inlier. The age matches (and chemical features akin to continental flood basalts) confirm a widespread intraplate magmatic event at 1750Ma, herein termed the Tagragra of Akka LIP, that represents an important new magmatic ‘barcode’ line for the West African Craton (WAC). Contemporaneous 1750Ma LIP magmatism is reported in other crustal blocks including northwestern Laurentia and Siberia; together with the newly defined Tagragra of Akka event of the Anti Atlas region of the WAC this widespread 1750Ma magmatism is postulated to collectively constitute a single huge LIP associated with a breakup attempt of Nuna (Columbia) shortly after its final assembly at ca. 1.8Ga.

Chapter 17 Biogeographical distribution patterns in Early Palaeozoic Rostroconchia (Mollusca)

Abstract The geographical distribution of rostroconch taxa of the orders Ribeirioida and Conocardiida, mostly across the equatorial continents and oceans, is compiled for six presumed diversity acmes in the early Early Ordovician ( c. 485 Ma), the late Early–early Middle Ordovician ( c. 475 Ma), the Late Ordovician ( c. 455 Ma), the Early Silurian ( c. 435 Ma), the late Early–early Late Silurian ( c. 425 Ma) and the early Early Devonian ( c. 415 Ma), based on our present, uneven knowledge. Rostroconchs show distribution patterns which enable a provisional separation of biogeographical provinces at least from the Silurian onward, comparable with Late Palaeozoic rostroconch distributional patterns. The distributions of tabulate corals, trilobites and bivalves appear roughly comparable, but not the nektic and planktic groups. A restriction to low latitudes (‘tropical’ realms) is clear for members of Ribeiria and Eopteria in the Early Ordovician, similar to proven patterns for hippocardiid rostroconchs from the Late Silurian onward until the Middle Permian. Preliminary rostroconch provinces or subprovinces, respectively, are currently discernable in the Silurian and Devonian for northwestern Laurentia, south-central Laurentia/Baltica and the north-central margin of Gondwana (i.e. Perunica, Bohemia). Rostroconch distributional data for SE Asia, China, Kazakhstan, Siberia and Australia are sparse and require further studies.

Detrital zircon geochronology and provenance of Devono-Mississippian strata in the northern Canadian Cordilleran miogeoclineThis article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.Northwest Territories Geoscience Office Contribution 0047. Geological Survey of Canada Contribution 20100432.

U–Pb ages have been determined on detrital zircons from the Upper Devonian Imperial Formation and Upper Devonian – Lower Carboniferous Tuttle Formation of the northern Canadian Cordilleran miogeocline using laser ablation – multicollector – inductively coupled plasma – mass spectrometry. The results provide insights into mid-Paleozoic sediment dispersal in, and paleogeography of, the northern Canadian Cordillera. The Imperial Formation yielded a wide range of detrital zircon dates; one sample yielded dominant peaks at 1130, 1660, and 1860 Ma, with smaller mid-Paleozoic (∼430 Ma), Neoproterozoic, and Archean populations. The easternmost Imperial Formation sample yielded predominantly late Neoproterozoic – Cambrian zircons between 500 and 700 Ma, with lesser Mesoproterozoic and older populations. The age spectra suggest that the samples were largely derived from an extensive region of northwestern Laurentia, including the Canadian Shield, igneous and sedimentary provinces of Canada’s Arctic Islands, and possibly the northern Yukon. The presence of late Neoproterozoic – Cambrian zircon, absent from the Laurentian magmatic record, indicate that a number of grains were likely derived from an exotic source region, possibly including Baltica, Siberia, or Arctic Alaska – Chukotka. In contrast, zircon grains from the Tuttle Formation show a well-defined middle Paleoproterozoic population with dominant relative probability peaks between 1850 and 1950 Ma. Additional populations in the Tuttle Formation are mid-Paleozoic (∼430 Ma), Mesoproterozoic (1000–1600 Ma), and earlier Paleoproterozoic and Archean ages (&gt;2000 Ma). These data lend support to the hypothesis that the influx of sediments of northerly derivation that supplied the northern miogeocline in Late Devonian time underwent an abrupt shift to a source of predominantly Laurentian affinity by the Mississippian.

Detrital zircon geochronology of the western Ellesmerian clastic wedge, northwestern Canada: Insights on Arctic tectonics and the evolution of the northern Cordilleran miogeocline

Detrital zircon provenance investigations of mid-Paleozoic sandstone from the western Ellesmerian clastic wedge and Cordilleran miogeocline in northern Yukon and Northwest Territories, northwestern Canada, provide critical new data on the source of foreland basin sedimentation attributed to terrane accretion and plate convergence along the ancestral Arctic margin of North America. Late Devonian and early Mississippian clastic wedge strata yield “exotic” ca. 360–390, 430–460, 530–680, and 1500–1600 Ma detrital zircon populations that are consistent with source rocks that originated near the Caledonian and Timanian orogenic belts. Specifically, the Pearya and Arctic Alaska–Chukotka terranes, the landmass of Crockerland, and Caledonian rocks in eastern Greenland are the inferred sources for exotic detrital zircons in clastic wedge strata. Progressive recycling of Ellesmerian foreland basin sediments into the continental margin environment along northwestern Laurentia is indicated by the presence of ca. 360–430 Ma and 1500–1600 Ma detrital zircons in post-tectonic, middle to late Mississippian miogeoclinal strata in Yukon. Provenance data from these Mississippian samples record a dramatic shift in the source of the Cordilleran miogeocline, since Caledonian and Baltican (Timanide) detrital zircon signatures are not recognized in pre–Late Devonian sedimentary rocks in western Canada. Devonian strata of the Alexander terrane and Yreka subterrane (eastern Klamath terrane) have Caledonian and Baltican detrital zircon age signatures similar to Ellesmerian clastic wedge sandstones, implying that several Cordilleran terranes originated in the paleo-Arctic realm. Speculative correlations suggest that the Arctic Alaska–Chukotka terrane was located to the west of Crockerland and the Canadian Arctic Islands in pre-Cretaceous time, prior to opening of the Amerasian basin. Rifting models for the western Arctic Ocean featuring counterclockwise rotation of the Arctic Alaska–Chukotka terrane away from the Canadian Arctic Islands may need reevaluation.

U–Pb geochronology of the Western Channel Diabase, northwestern Laurentia: Implications for a large 1.59 Ga magmatic province, Laurentia's APWP and paleocontinental reconstructions of Laurentia, Baltica and Gawler craton of southern Australia

U–Pb baddeleyite ages of 1592 ± 3 and 1590 ± 4 Ma are reported for paleomagnetic sites in sheets and dykes of Western Channel Diabase (WCD) that intrude Proterozoic rocks of the flat-lying Hornby Bay Group in the Hornby Bay basin and the deformed volcanic-plutonic Great Bear Magmatic Zone of Wopmay orogen of northwestern Laurentia. A published WCD paleomagnetic pole at 9°N, 115°W ( A 95 = 6°) has been demonstrated primary. The new ages indicate that the WCD pole falls midway in time between poles for the 1.74 Ga Cleaver dykes and 1.48–1.42 Ga Elsonian-aged plutons, filling an important gap in the Proterozoic apparent polar wander path (APWP) for Laurentia. The WCD pole can be compared with poles reported from similar-aged magmatic units on other cratons in order to test paleocontinental reconstructions. A comparison of the Laurentian WCD pole with primary ca. 1.63 Ga and ca. 1.575 Ga poles for Baltica, along with an earlier comparison of precisely dated 1.27–1.255 Ga poles for Laurentia and Baltica, suggests that the two cratonic blocks drifted as a single entity with Baltica adjacent to eastern Greenland during the ca. 1.59–1.27 Ga interval. On the basis of less well constrained ca. 1.84–1.83 Ga poles from Laurentia and Baltica, it is possible that this reconstruction existed as early as ca. 1.83 Ga. The WCD is the same age as Wernecke breccias of the Wernecke and Ogilvie Mountains of northwestern Laurentia and bimodal Gawler Range Volcanics (GRV) and related Olympic Dam breccias of the Gawler craton. It has been proposed by others that the Gawler craton lay adjacent to northwestern Laurentia at 1.59 Ga, with the Olympic Dam and Wernecke breccias forming a large hydrothermal province. The primary WCD pole provides an opportunity to test Laurentia–Gawler craton reconstructions at 1.59 Ga. A paleopole has been reported for the GRV, although its primary or secondary nature is open to interpretation. If primary, or if acquired as an overprint during the later stages of 1.60–1.58 Ga Hiltaba-GRV magmatism, then a position for the Gawler craton adjacent to northwestern Laurentia is permitted. If the GRV pole is a later secondary overprint then a reliable comparison with Laurentian poles cannot be made.

Insights into the metamorphic evolution of the Belt–Purcell basin; evidence from Lu–Hf garnet geochronology

We have determined Lu–Hf garnet ages from spatially separated garnet bearing localities in northern Idaho. The Lu–Hf ages are diverse and reflect a progression of Mesoproterozoic metamorphic events. The oldest Lu–Hf garnet age determined in this study is 1463 ± 24 Ma for garnet within a kyanite schist exposed in the core-zone of the Boehls Butte metamorphic complex. A garnet schist from the Priest River complex yields a well-defined age of 1379 ± 8 Ma. A garnet–staurolite schist, a garnet–mica schist, and a gem-grade Idaho star garnet sample all from the general vicinity of Clarkia, Idaho, yield ages of 1064 ± 10, 1081 ± 20, and 1078 ± 17 Ma, respectively. A garnet amphibolite, also collected from the Boehls Butte metamorphic complex, yields an age of 1151 ± 41 Ma for garnet porphyroblasts and 137 ± 25 Ma for diffuse overgrowths. The Hf in all of the Proterozoic garnets is extraordinarily radiogenic with εHf values ranging from +1210 to +10546, which attest to both their high Lu/Hf ratios and their great antiquity. All of the samples analyzed in this study are from the rocks of the Mesoproterozoic Belt–Purcell Supergroup or its basement. These data provide evidence of polymetamorphism in northwestern Laurentia during the Mesoproterozoic.

Phylogenetic and biogeographic analysis of deiphonine trilobites

Cladistic parsimony analysis of the subfamily Deiphoninae Raymond, 1913 was conducted to produce a hypothesis of relationship for the group. The genera Deiphon Barrande, 1850 and Onycopyge Woodward, 1880 are found to be monophyletic, while the genus Sphaerocoryphe Angelin, 1854, as it was previously defined, is paraphyletic. A modified Brooks Parsimony Analysis using the phylogenetic hypothesis reveals patterns of biogeography, in particular, vicariance and geodispersal, during the Ordovician-Silurian. The analysis yields three major conclusions about deiphonine biogeography: Eastern Laurentia and Baltica were close enough during the late Ordovician to exchange taxa via sea level rise and fall; chance dispersal occurred between Northwestern Laurentia and Australia; and deiphonine trilobites likely originated in Baltica or Eastern Laurentia.

Correlations and reconstruction models for the 2500-1500 Ma evolution of the Mawson Continent

Abstract Continental lithosphere formed and reworked during the Palaeoproterozoic era is a major component of pre-1070 Ma Australia and the East Antarctic Shield. Within this lithosphere, the Mawson Continent encompasses the Gawler–Adélie Craton in southern Australia and Antarctica, and crust of the Miller Range, Transantarctic Mountains, which are interpreted to have assembled during c . 1730–1690 Ma tectonism of the Kimban–Nimrod–Strangways orogenies. Recent geochronology has strengthened correlations between the Mawson Continent and Shackleton Range (Antarctica), but the potential for Meso- to Neoproterozoic rifting and/or accretion events prevent any confident extension of the Mawson Continent to include the Shackleton Range. Proposed later addition ( c . 1600–1550 Ma) of the Coompana Block and its Antarctic extension provides the final component of the Mawson Continent. A new model proposed for the late Archaean to early Mesoproterozoic evolution of the Mawson Continent highlights important timelines in the tectonic evolution of the Australian lithosphere. The Gawler–Adélie Craton and adjacent Curnamona Province are interpreted to share correlatable timelines with the North Australian Craton at c . 2500–2430 Ma, c . 2000 Ma, 1865–1850 Ma, 1730–1690 Ma and 1600–1550 Ma. These common timelines are used to suggest the Gawler–Adélie Craton and North Australian Craton formed a contiguous continental terrain during the entirety of the Palaeoproterozoic. Revised palaeomagnetic constraints for global correlation of proto-Australia highlight an apparently static relationship with northwestern Laurentia during the c . 1730–1590 Ma time period. These data have important implications for many previously proposed reconstruction models and are used as a primary constraint in the configuration of the reconstruction model proposed herein. This palaeomagnetic link strengthens previous correlations between the Wernecke region of northwestern Laurentia and terrains in the eastern margin of proto-Australia.

Cambrian–Ordovician trilobite family Missisquoiidae Hupé, 1955: Systematic revision and palaeogeographical considerations based on cladistic analysis

The missisquoiids are among the most important trilobites in the Cambrian–Ordovician boundary interval of Laurentia and Gondwana. This study deals with a systematic review of the family Missisquoiidae based on a cladistic analysis and explores their palaeobiogeographical history. A total of 22 missisquoiid species were selected for cladistic analysis. The cladistic results demonstrate that the family Missisquoiidae includes Parakoldinioidia, Pseudokoldinioidia, Tangshanaspis, and Tasmanocephalus; Lunacrania and Hardyia are included with reservation. The well-known genus Missisquoia is treated as a junior synonym of Parakoldinioidia. Based on the distribution of the missisquoiids, six palaeogeographical areas are recognised: Sino-Korea, Yangtze, Australia, southern Laurentia, northwestern Laurentia, and northeastern Laurentia. Palaeogeographical analyses show that the missisquoiids originated in Gondwana and continued to expand their geographical range within the continent and eventually expanded into the Laurentia. Optimisation results of geographical area transitions demonstrate that there were transitions between the two continents, and evolution of the Laurentian missisquoiids appears to have been strongly influenced by the Gondwanan counterparts. We suggest that dispersal of non-fossilised egg stage probably transported by oceanic currents may have been responsible for these inter-continental transitions.

Queen Maud block: A newly recognized Paleoproterozoic (2.4–2.5 Ga) terrane in northwest Laurentia

The Queen Maud block of Arctic Canada is central to understanding the Proterozoic tectonic history of northwestern Laurentia, but its crustal history is largely unknown. Results of an in situ U-Pb zircon, monazite, and whole-rock Sm-Nd study through the central and eastern Queen Maud block indicate: (1) widespread 2.46–2.50 Ga magmatism derived from Neoarchean source rocks, (2) an extensive NE-trending 2.44–2.39 Ga sedimentary belt characterized by 2.45–2.50 Ga detritus, and (3) regional ca. 2.39 Ga granulite metamorphism. There is no evidence of metamorphic or magmatic activity at 1.9–2.0 Ga, concurrent with orogenesis in the adjacent Taltson-Thelon belt. We propose that the eastern Queen Maud block was the site of an incipient continental rift ca. 2.5 Ga. Exhumation of 2.46–2.50 Ga granitoids produced in the early stages of rifting provided detritus to a short-lived basin that underwent granulite metamorphism ca. 2.39 Ga.

Early and Middle Proterozoic evolution of Yukon, Canada

This paper provides a comprehensive synthesis of virtually all units and events of Early and Middle Proterozoic age in the Yukon, spanning ~1 Ga. Early and Middle Proterozoic time was dominated by a series of extensional-basin-forming events punctuated by orogenesis, magmatism, and hydrothermal activity. Basinal deposits include the Wernecke Supergroup (&gt;1.71 Ga), Pinguicula Group (~1.38 Ga), and Mackenzie Mountains Supergroup (1.00–0.78 Ga). Igneous rocks include the Bonnet Plume River Intrusions (1.71 Ga), Slab volcanics (≥1.6 Ga), Hart River sills and volcanics (1.38 Ga), and Bear River (Mackenzie) dykes (1.27 Ga). A voluminous hydrothermal event generated the widespread Wernecke breccias at 1.60 Ga. The Racklan orogeny deformed the Wernecke Supergroup prior to emplacement of the Wernecke Breccia. The Corn Creek orogeny deformed Mackenzie Mountains Supergroup and older rocks prior to deposition of the Windermere Supergroup (&lt;0.78 Ga). Long intervals with scanty rock records extended for as much as 300 Ma and appear to represent periods of crustal stability and subaerial conditions. By the time of Windermere rifting (&lt;0.78 Ga), the supracrust of northwestern Laurentia was a mature, largely denuded orogenic belt with a composite sedimentary–metamorphic–igneous character. New isotopic data include Nd depleted mantle model ages for the Wernecke Supergroup (2.28–2.69 Ga) and Wernecke Breccia (2.36–2.96 Ga), a U–Pb zircon age for a Hart River sill 1381.9+5.3-3.7 (Ma), detrital U–Pb zircon ages from the basal Pinguicula Group (1841–3078 Ma), detrital muscovite ages from the Mackenzie Mountains Supergroup (1037–2473 Ma), and muscovite 40Ar/39Ar cooling ages from the Wernecke Supergroup (788 ± 8 and 980 ± 4 Ma).

Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia

Northwestern Laurentia, after cratonization at about 1.85 Ga, underwent a series of tectonic and magmatic events during the Proterozoic that were followed by separation of Laurentia from another landmass, probably Australia. The oldest magmatic event produced the Bonnet Plume River Intrusions (BPRI), which intruded the Wernecke Supergroup as short dikes and small stocks. The BPRI are hydrothermally altered tholeiitic diorites, gabbros, and subordinate anorthositic and syenitic rocks, with trace element signatures consistent with a rift origin. Depleted mantle model ages range from 2.29 to 2.57 Ga and εNdvalues range from +0.7 to –1.7. An increasing crustal component is apparent in rocks with more evolved compositions. Four U–Pb zircon ages (1705.9 ± 0.7, 1709.4 ± 1.4, 1711.1 ± 5.1, and 1713.6 ± 12.7 Ma) indicate a Paleoproterozoic age for the BPRI. These dates constrain the age of the Wernecke Supergroup to [Formula: see text] ca. 1710 Ma, making it the oldest supracrustal succession in western Laurentia, e.g., &gt;240 Ma older than the Belt Supergroup of southeastern British Columbia and the northwestern United States. The Wernecke Supergroup was deposited in the first rift basin to open along the western margin of Laurentia, but was later inverted by the pre-1.6 Ga Racklan Orogeny, an event possibly influenced by transmission of compression from the Yavapai and Mazatzal orogenies in southern Laurentia. The Neoproterozoic southwestern United States – east Antarctica (SWEAT) reconstruction, which places Australia next to northwestern Laurentia, is supported by linkages between Paleoproterozoic and Mesoproterozoic geological features in northwestern Canada and Australia.

Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia

Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia