North American Shield Research Articles

Tectonic history of the North American shield recorded in uranium deposits in the Beaverlodge area, northern Saskatchewan, Canada

Paragenetic and structural relationships and geochronology of minerals in uranium deposits in the Beaverlodge area, Northwestern Saskatchewan, Canada, reveal six periods of uranium mineralization associated with multi-stage deformation during the Proterozoic. The Saint Louis fault and the Main Ore shear zone are northeast striking, southeast-dipping, oblique-normal and dextral fault systems that have mylonite-dominated footwalls. These faults display evidence of exhumation and episodic structural reactivation at progressively shallower crustal levels, accompanied by fluid flow, hydrothermal alteration, and uranium mineralization.New geochronologic data on uranium mineralization and alteration events record over 2.3Gyr of protracted tectonic evolution of the North American shield. 207Pb/206Pb ages of 2293±17Ma and 2289±20Ma date two minor uranium mineralization stages associated with cataclasite rocks and early tensional veins, respectively, coincident with late Arrowsmith orogenic exhumation. Following emplacement of the Gunnar granite at 2321±3Ma, albite metasomatic alteration of the granite is associated with a third moderate uranium mineralizing event. During the late Paleoproterozoic, exhumation along major faults caused the deformation style to change from dominantly brittle–ductile to brittle at shallower structural levels. Massive brecciation of pre-existing rocks is associated with the fourth and most significant uranium-mineralizing event at 1848±5Ma, coincident with fault reactivation during tectonic shortening and regional metamorphism related to the Trans-Hudson Orogen or more likely post-peak compressional uplift during terminal collision of the Taltson-Thelon Orogen. Subsequently, formation of the Martin Lake Basin and intrusion of mafic dikes are related to the fifth minor uranium mineralizing event at 1812±15Ma reflecting rifting related to late stage Trans-Hudson Orogen. Late mineralized veins during the Mesoproterozoic crustal growth of the Nuna supercontinent are associated with the sixth mineralizing event at 1620±4Ma, coincident with the Mazatzal Orogen and the formation of the unconformity-related uranium mineralization in the proximal Athabasca Basin. Post-mineralization alteration and resetting events of uraninite further record various major orogenic events that have affected the North American shield.

Impact of melting of the Laurentide Ice Sheet on sediments from the upper continental slope off southeastern Canada: evidence from Sm–Nd isotopesThis article is one of a series of papers published in this Special Issue on the themePolar Climate Stability Network.

We present new Sm–Nd isotope data for sediments from a core located on the continental slope of the St. Pierre Bank of Canada’s east coast. The Nd analyses indicate that the sediments were derived from two principal sources: the North American Shield that yields an average early Proterozoic isotopic signature and a younger Proterozoic signature attributed to Appalachian crustal sources. The Appalachian-sourced sediments predominated during the last glacial maximum (LGM) and were associated with low sedimentation rates (<30 cm/ka), with the exception of a strong North American Shield signature present in a detrital carbonate layer that corresponds to Heinrich Layer 1 (H1). The dominance of the Appalachian signature decreased subsequent to H1. The Appalachian signatures closely follow the distribution of sediments interpreted as locally derived glacial tills, while the North American Shield signatures follow the distribution of hemipelagic mud that was likely deposited by the Labrador Current. The Nd data are consistent with the persistence of the Wisconsinan Ice Sheet coverage of Newfoundland and the Grand Banks after the LGM, although the coverage began to wane prior to 12.5 ka as evidenced by the increasing influence of the Labrador Current. However, an increase in the Appalachian isotope signature at the close of the Younger Dryas event likely indicates the final melting of the ice sheet covering the Grand Banks and the Avalon Peninsula, and the initiation of the Labrador Current’s modern circulation pathway.

Glacial/interglacial instabilities of the Western Boundary Under Current during the last 365 kyr from Sm/Nd ratios of the sedimentary clay-size fractions at ODP site 646 (Labrador Sea)

We present 40 Sm–Nd isotope measurements of the clay-size (< 2 μm) fractions of sediments from the Southern Greenland rise (ODP-646) that span the last 365 kyr. These data track changes in the relative supply of fine particles carried into the deep Labrador Sea by the Western Boundary Under Current (WBUC) back to the fourth glacial–interglacial cycles. Earlier studies revealed three general sources of particles to the core site: (i) Precambrian crustal material from Canada, Greenland, and/or Scandinavia ( North American Shield — NAS), (ii) Palaeozoic or younger crustal material from East Greenland, NW Europe, and/or western Scandinavia ( Young Crust — YC) and (iii) volcanic material from Iceland and the Mid-Atlantic Ridge (MAR). Clay-size fractions from glacial sediments have the lowest Nd isotopic ratios. Supplies of young crustal particles were similar during glacial oxygen isotope stages (OIS) 2, 6, and 10. In contrast the mean volcanic contributions decreased relative to old craton material from OIS 10 to OIS 6 and then from OIS 6 to OIS 2. The glacial OIS 8 interval displays a mean Sm/Nd ratio similar to those of interglacials OIS 1, 5, and 9. Compared with other interglacials, OIS 7 was marked by a higher YC contribution but a similar ∼ 30% MAR supply. The overall NAS contribution dropped by a factor of 2 during each glacial/interglacial transition, with the MAR contribution broadly replacing it during interglacials. To decipher between higher supplies and/or dilution, particle fluxes from each end member were estimated. Glacial NAS fluxes were systematically higher than interglacial fluxes. During the time interval examined, fine particle supplies to the Labrador Sea were strongly controlled by proximal ice-margin erosion and thus echoed the glacial stage intensity. In contrast, the WBUC-carried MAR supplies from the eastern basins did not change significantly throughout the last 365 kyr, except for a marked increase in surface-sediments that suggests unique modern conditions. Distal WBUC-controlled inputs from the Northern and NE North Atlantic seem to have been less variable than proximal supplies linked with glacial erosion rate.

Nd and Pb isotope signatures of the clay‐size fraction of Labrador Sea sediments during the Holocene: Implications for the inception of the modern deep circulation pattern

Nd and Pb isotopes were measured on the fine fraction of one sediment core drilled off southern Greenland. This work aims to reconstruct the evolution of deep circulation patterns in the North Atlantic during the Holocene on the basis of sediment supply variations. For the last 12 kyr, three sources have contributed to the sediment mixture: the North American Shield, the Pan‐African and Variscan crusts, and the Mid‐Atlantic Ridge. Clay isotope signatures indicate two mixtures of sediment sources. The first mixture (12.2–6.5 ka) is composed of material derived from the North American shield and from a “young” crustal source. From 6.5 ka onward the mixture is characterized by a young crustal component and by a volcanic component characteristic of the Mid‐Atlantic Ridge. Since the significant decrease in proximal deglacial supplies, the evolution of the relative contributions of the sediment sources suggests major changes in the relative contributions of the deep water masses carried by the Western Boundary Undercurrent over the past 8.4 kyr. The progressive intensification of the Western Boundary Undercurrent was initially associated mainly with the transport of the Northeast Atlantic Deep Water mass until 6.5 ka and with the Denmark Strait Overflow Water thereafter. The establishment of the modern circulation at 3 ka suggests a reduced influence of the Denmark Strait Overflow Water, synchronous with the full appearance of the Labrador Seawater mass. Our isotopic data set emphasizes several changes in the relative contribution of the two major components of North Atlantic Deep Water throughout the Holocene.

Deep circulation changes in the Labrador Sea since the Last Glacial Maximum: New constraints from Sm‐Nd data on sediments

Sm‐Nd concentrations and Nd isotopes were investigated in the fine fraction of two Labrador Sea cores to reconstruct the deep circulation patterns through changes in sedimentary supply since the last glacial stage. Three sources are involved: the North American Shield, Palaeozoic rocks from northeastern Greenland, and mid‐Atlantic volcanism. The variable input of these sources provides constraints on the relative sedimentary supply, in conjunction with inception of deep currents. During the last glacial stage a persistent but sluggish current occurred inside the Labrador Basin. An increasing discharge of volcanic material driven by the North East Atlantic Deep Water is documented since 14.3 kyr, signaling the setup of a modern‐like deep circulation pattern throughout the Labrador, Irminger, and Iceland basins. During the last deglacial stage the isotopic record was punctually influenced by erosion processes related mainly to ice‐sheet instabilities, especially 11.4, 10.2, and 9.2 kyr ago.

Palaeozoic Palaeogeography in Hercynian and Appalachian Belts by Sm-Nd Isotopic Studies of Shales

The Sm-Nd isotopic studies of shales give important informations about the crustal growth and the provenance of the sediments. Because shales integrate the diversity of eroded rocks and because the Sm-Nd isotopic system is left undisturbed by surface processes, the Nd isotopic initial composition reflects an average of the different mechanically eroded sources. This composition reflects a balance between juvenile inputs from the mantle by volcanism and reworked inputs from the continental crust. However, the Sm-Nd isotopic signal is a biased average in which pre-existing sediments and young mountains appear to be over represented. The Nd isotopic initial composition can be expressed by depleted mantle model ages (TDM). Assuming that similar TDM values can characterize a common tectonic domain, comparisons between data from several areas can give new palaeogeographical constraints. First, we test this assumption for the Hercynian belt and we expand it to the Appalachian belt in eastern Canada. Our data from the Hercynian belt concern Sm-Nd isotopic data on 46 shales from the Montagne Noire (southern French Massif Central). The sedimentary record, which comprises one of the most complete Palaeozoic sequences in Europe, was sampled from Early Cambrian to Early Jurassic (560 to 180 Ma). This sequence is interrupted by two hiatuses. The first is situated in Middle Ordovician (490 to 450 Ma) and the second during Carboniferous (330 to 300 Ma). We analysed volcanogenic materials and lavas in order to identify possible source materials. During the studied time span, most of the TDM's are around 1.6 Ga. We observe some rapid TDM changes toward older ages at 520 and 270 Ma and toward younger ages at 300 Ma. These variations are related to different sedimentary settings and to the volcanic event (300 Ma) detected in a continental basin. Our results indicate that in any case, younger TDM do not reflect large Palaeozoic juvenile inputs related to orogenic processes in the south of the French Massif Central. We compare our data with others published data from Brittany (Medio North Armorican Domain) and from Spain (Central Iberian Domain and Cantabrian Domain). The similar TI~M calculated for these areas from Early Ordovician to Early Carboniferous (490 to 330 Ma) suggest that these three areas belonged to the same palaeogeographic entity at least for this time span. This contradicts models that propose there is an oceanic suture between Brittany and Spain and between the two Spanish zones. Conversely, our observations are in agreement with palaeontological data indicating that Brittany and Spain were adjoining during this time span. On the other hand, because the Montagne Noire has younger TI~M from 490 to 400 Ma compare to Brittany and Spain, we propose that this area probably undergone a different tectonic evolution during this time span. The Appalachian belt was created by several accretion events of exotic terranes to the North American shield (Laurentia) during the Palaeozoic. The origin, the timing and the link between the accreted terranes are a matter to debate. In order to give new constraints, we analysed shales from the Dunnage zone, which represent eroded products from the Laurentia, and shales from accreted terranes: the Avalon and Meguma zones. For the Dunnage zone, we observe a rapid TDM shift toward younger ages near 440 Ma. This result is interpreted as an evidence for the accretion of Avalon to the Laurentia around that time.

The Origin of the Early Proterozoic Kuandian Complex: Evidence from Geochemistry

AbstractThe Kuandian Complex is scarcely preserved Early Proterozoic volcanic suite, formed 2.3–2.4 Ga ago. It is located in an Early Proterozoic mobile belt bounded by the Archaean Rangrim and Ryonggang Blocks of the northeastern Sino–Korean Cratoa The Complex is mainly made up of amphibolites, gneisses, leucoleptite, leptite and layered granite. Petrological and geochemical studies show that the protoliths of the Complex are mainly associations of bimodal volcanics and anorogenic granites. The Kuandian amphibolites are depleted in Nb, Ta, P and Ti, and enriched in LILE, e.g. K, Rb and Cs, with pronounced depletion of Sr relative to Nd and Pb; La/Nb ratios are higher than 1 (1.75 to 5.18). The trace element patterns of the amphibolites are similar to continental flood basalts formed by the Gondwana break–up, such as those in South Karoo and Tasmania, which shows continental contamination. Nd values ranging from 0.70 to 1.94 of the Kuandian amphibolites and the relationships between Nb/Yb and La/Yb suggest that contamination of basaltic magma happened in the mantle, rather than along the conduit. Isotope ratios of 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 143Nd/144Nd and 87Sr/86Sr indicate that the magma was derived from a contaminated mantle source like DMM or a mixture of DMM and EM2. The Kuandian Complex has Dupal anomaly, as is the case with some continental basalts in the south hemisphere, e.g. in South Karoo and Tasmania. Petrochemical modelling proposes that the Kuandian gneiss, granite, and amphibolite came from the same parental magma, being products of strong fractional crystallization. Protoliths of the Kuandian Complex were formed in extensional tectonic setting during the transition from continental crust to oceanic crust. The formation of the Kuandian Complex indicates that 2.3 or 2.4 Ga ago tectonic evolution of the Sino–Korean craton was different from that of other well–studied Precambrian cratons, e.g. the North American shield, European platform and Australian continent in that strong volcanic eruption resulted in its accretioa Besides, the contaminated magma source with a Dupal anomaly for the Complex indicates that crust–mantle convection whose scale was similar to that of the present plate tectonics had occurred at least before the formation of the Kuandian Complex (2.3–2.4 Ga B.P.).

SmNd signature of modern and late Quaternary sediments from the northwest North Atlantic: Implications for deep current changes since the Last Glacial Maximum

Nd isotopes and SmNd concentrations have been investigated in the clay size fraction of surface and late Quaternary sediments of the northern North Atlantic. Four sources of Nd have been identified in the surface sediments: (1) a radiogenic Nd typical of the Iceland volcanism; (2) a “Pan-African” Nd issued from eastern Greenland; (3) a “Variscan” Nd derived from the Arctic Ocean; (4) an old unradiogenic Nd related to the North American Shield. In the Iceland and Irminger basins, the major sources of Nd are the Variscan Nd derived from the Arctic Ocean and the Iceland Nd, except in the western Irminger Basin where the Pan-African Nd originating from eastern Greenland is dominant and labels sedimentary supplies linked to the Denmark Strait Overflow Water. Nd derived from Icelandic volcanism is never clearly dominant, even on the Reykjanes Ridge, and the Nd derived from the American Shield becomes dominant only in the Labrador Sea. Sediments lying in the pathway of the Western Boundary Undercurrent have Nd isotopic compositions which are shifted towards more radiogenic values indicating an input of material from the eastern basins, carried by this deep current. Late Quaternary sediments were cored from the southern Greenland Rise (core 90-013-013) and eastern Labrador Rise (Orphan Knoll; core 91-045-094). In core 90-013-013, ϵNd values during the Last Glacial Maximum are as low as −20, but subsequently rose to −9 in the modern sediments, indicating that the sedimentary material during the Last Glacial Maximum was dominated by unradiogenic Nd from the North American Shield. This may be related to a lesser outflow of the Western Boundary Undercurrent. In core 91-045-094, located along the Labrador Current trajectory, a dominant supply of North American Shield Nd is recorded, perhaps originating at least partially from the Hudson Strait. The SmNd data are compatible with smectite vs. illite studies at the same sites which suggest a continuous but more modest Western Boundary Undercurrent supply during Last Glacial Maximum.

Precambrian geological evolution

Geologists from Brazil, the three Guianas, Venezuela, the northern Andean countries, North America, and Europe are participating in a new project of the International Geological Correlation Program, concentrating on the Precambrian geological evolution of the Amazonian region. Most of the major Precambrian rock units of the region are currently being studied by geologists in the various countries that have exposures of the Guiana and Guaporé shields, which make up the Amazonian craton. The craton is notable because of the great expanses of crust that formed in the Early Proterozoic. There are granite‐greenstone terranes as extensive as those of the Archean provinces of the North American shield and high‐grade gneiss and granulite terranes of both Archean and Proterozoic age. These provide opportunities to compare continental formation and evolution of Proterozoic age with their more ancient counterparts in other shields. Most of the craton was established in the Early Proterozoic and was affected by the Trans‐Amazonian orogeny; about half of this was then covered by Middle Proterozoic intracratonic sedimentray basins, associated felsic volcanics, mafic intrusives, and epizonal granitoid rocks. The felsic magmatic rocks of the Middle Proterozoic are particularly extensive and well exposed: This must rank as one of the prime regions in the world to study such rocks. Alkaline complexes with associated carbonatites occur in Brazil, Guyana, and Venezuela. All of the rock types mentioned have important ore deposits associated with them, and one of the goals of the project is to develop further understanding of the mineral potential and metallogenesis of the region.

Rôles respectifs de l'advection et de la décantation sur la ride médio-atlantique (40° à 50°N)

Coring at 45 sites in the North Atlantic permitted to determine flux velocity and chemical composition of pelagic sediments. Piston cores were used to carry out a comparative study between the post-glacial period (10,000 yr. B.P. until present) and last glacial period (75,000–10,000 yr. B.P.). Special attention has been paid to the Mid-Atlantic Ridges-Azores-Iceland area, where an enrichment of chemical elements was observed with regards to “regional ocean ground noise”, perceived on an isolated seamount of the abyssal plain. This “ground noise” characterized by the presence of Pb, Rb, U, Th, illite and chlorite, is associated to a settling vertical flux since the surface brought by surface currents and wind transport since the North American shield: sediments, here, are continental soil erosion products. The ridge is clearly enriched by Ba, Br, Fe, Ti, Mn, Cu, Ni, Co and As. Ba and Br are mostly associated with planktonic carbonates. Part of Ba may be linked to the ridge's activity. Basalt weathering on the ridge supplies a part of Fe in excess. Hydrothermal activity may account for Mn, Cu, Co, Ni and As enrichment. Most of the excess observed may be explained by intrusion of advective inputs from erosion Icelandic products (Fe, Ti, Cu, Ta, Sc and smectites), probably transported by Norwegian bottom currents. Advective flux (Icelandic-Faeroan basaltic materials) and flux linked to submarine ridge activity represent ≅ 30% of inorganic sedimentation. Vertical flux (North American continental-derived terrigenous materials) represent ≅ 70% of inorganic sedimentation. These percentages are very similar to those which were calculated for the Pacific.

Palaeomagnetism of the Svecofennian Loftahammar gabbro and some Jotnian dolerites in the Swedish part of the Baltic Shield

The Svecofennian Loftahammar gabbro (RbSr isochron age 1,694 · 10 6 year) and Jotnian dolerites (KAr isochron age 1,245 · 10 6 year) in Sweden were found to have palaeomagnetic pole positions in close agreement with poles from other Svecofennian rocks in Sweden and Jotnian dolerites in Finland, respectively. These results support Neuvonen's apparent polar wandering path during the period from 1,900 · 10 6 to 1,300 · 10 6 year. A reconstruction of the position of the Baltic Shield during the (1,900-1,200) · 10 6-year interval, based on available pole positions, indicates that the Baltic shield was close to Greenland and the North American Shield and reveals a contiguity of North Atlantic geologic-geochronological zones until 1,200 · 10 6 year ago.

The evolution of the atlantic mega-undation: Causing the American continental drift

Mega-undations are the largest class of deformations of the earth's surface, with diameters of many thousands of kilometers. They result from deepseated mass-circuits, probably in the inner mantle. Though their potential amplitude may be in the order of 100 km, their actual height is not more than some dozens of meters, because the arching up is partially compensated by sideward spreading of the earth's upper layers under gravity. By this process isostatic balance is more or less preserved. If mega-undations occur in continental areas (such as Gondwana and Laurasia) new oceanic basins will open up on their top, with mid-oceanic ridges marking their crest; meanwhile relatively deep parts of the outer mantle rise to the oceanfloor. The overlying structural levels drift sidewards under gravity, towards the adjacent mega-undatory downwarps — like the spreading of a tilted stack of books. The highest unit i.e., the crust, advances farthest flowing (gliding) over the next deeper one (the “asthenosphere”). The deeper levels of the outer mantle (the “sclerosphere”) are also affected by these sliding movements, but their sideward displacement becomes less and less, and finally the lateral transport may become zero at the base of the outer mantle. This mechanism of continental drift occurs by means of the potential energy, which is present in every mass particle. The wandering continental shield will show characteristic geotectonic phenomena at all sides; at the frontal side or “stem” geosynclinal subsidence (followed by orogenesis); at the rear-side or “stern” extension phenomena; along the “starboard” systems of right-lateral strike-slip faults will occur, whereas along the “port” left-lateral transcurrency is to be found. These expectations (prognosis) of the working hypothesis are tested by the geotectonic analysis of a case history (diagnosis), namely the evolution of the Atlantic Mega-Undation and the corresponding drift-movements of the North American Shield. This geotectonic study provides strong converging evidence in support of the soundness of the undation theory; also in its application to the largest features of the earth known as mega-undations.