Western Canadian Shield Research Articles

Exploiting radiation damage control on apatite (U–Th)/He dates in cratonic regions

Apatites from four pairs of samples of Precambrian basement from the western Canadian shield were analyzed by (U–Th)/He thermochronometry to test for the influence of radiation damage on apatite (U–Th)/He dates in this cratonic region. Recent studies have demonstrated that the accumulation of radiation damage increases the apatite He retentivity, so that apatites with a span of effective U concentrations, eU, that experienced the same thermal history may be characterized by a range of closure temperatures. In this investigation, each sample pair consisted of a mafic dike cross-cutting felsic gneisses from a single outcrop or nearby outcrops that contained apatites with a span of eU. The apatites yielded (U–Th)/He dates from 846 to 123 Ma, and were positively correlated with eU within each sample pair. These results can be explained using a model that tracks the evolution of He mobility in response to the accumulation of radiation damage. When coupled with regional geological constraints, the data appear to require partial to complete He loss due to burial and reheating in Phanerozoic time. New apatite fission- track dates and length data were obtained for five of these samples. The apatite fission- track dates are Proterozoic regardless of apatite eU. Thermal history simulations indicate that the apatite fission-track data are compatible with the (U–Th)/He results, although the thermal histories are not identical in detail and the fission-track results alone do not require Phanerozoic heating. Together the data are consistent with burial of this region by ≥ 1 km of Phanerozoic strata that were subsequently denuded, thus pointing toward significant Phanerozoic deposition in the North American cratonic interior hundreds of kilometers east of where previously documented. The results suggest that exploiting radiation damage control on apatite (U–Th)/He dates through investigation of surface sample apatites with a span of closure temperatures can impose tighter restrictions on thermal histories in low relief cratonic landscapes than was previously possible.

The role of heterogeneous strain in the development and preservation of a polymetamorphic record in high‐P granulites, western Canadian Shield

AbstractMafic rocks in the Chipman domain of the Athabasca granulite terrane, western Canadian Shield, provide the first well‐documented record of two distinct high‐P granulite facies events in the same domain in this region. Textural relations and the results of petrological modelling (NCFMASHT system) of mafic granulites are interpreted in terms of a three‐stage tectonometamorphic history. Stage 1 involved development of the assemblage Grt + Cpx + Qtz ± Pl (M1) from a primary Opx‐bearing igneous precursor at conditions of 1.3 GPa, 850–900 °C. Field and microstructural observations suggest that M1 developed synchronously with an early S1 gneissic fabric. Stage 2 is characterized by heterogeneous deformation (D2) and synkinematic partial retrogression of the peak assemblage to an amphibole‐bearing assemblage (M2). Stage 3 involved a third phase of deformation and a return to granulite facies conditions marked by the prograde breakdown of amphibole (Amph2) to produce matrix garnet (Grt3a) and the coronitic assemblage Cpx3b + Opx3b + Ilm3b + Pl3b (M3b) at 1.0 GPa, 800–900 °C. M1 and M3b are correlated with 2.55 and 1.9 Ga metamorphic generations of zircon, respectively, which were dated in a separate study. Heterogeneous strain played a crucial role in both the development and preservation of these rare examples of multiple granulite facies events within single samples. Without this fortuitous set of circumstances, the apparent reaction history could have incorrectly led to an interpretation involving a single‐cycle high‐grade event. The detailed P–T–t–D history constructed for these rocks provides the best evidence to date that much of the east Lake Athabasca region experienced long‐term lower crustal residence from 2.55 to 1.9 Ga, and thus the region represents a rare window into the reactivation and ultimate stabilization processes of cratonic lithosphere.

High-resolution dating of granite petrogenesis and deformation in a lower crustal shear zone: Athabasca granulite terrane, western Canadian Shield

The Athabasca granulite terrane represents > 20,000 km 2 of continental lower crust exposed in the hanging wall of the ca. 1.85 Ga thrust–sense Legs Lake shear zone in the western Canadian Shield. The terrane and the thrust zone are cut by the Grease River shear zone. The structure hosts a syn-tectonic granite dike with a high-precision ID-TIMS zircon 207Pb/ 206Pb crystallization age of 1922.6 ± 0.4 Ma (MSWD = 3.1, 2 σ). Detailed textural analysis and in situ electron probe microanalysis (EPMA) of monazite in the dike indicate three discrete episodes of strain accumulation, each corresponding to syn-kinematic growth of monazite. The results provide evidence for a previously unrecognized > 100 m.y. episodic record of sub-horizontal NW–SE shortening and dextral shear strain along the Grease River shear zone. High-Ca domains in ca. 1.92–1.90 Ga monazite are linked to protracted high- T strain (ca. 650–700 °C) at lower crustal depths (ca. 1.0 GPa) and correlated with indentation of the Slave craton along the Great Slave Lake shear zone. Syn-kinematic overgrowths of low-Ca monazite + apatite are interpreted as products of deformation-enhanced dissolution and re-precipitation of monazite at 1846 ± 9 Ma (MSWD = 1.1, 2 σ). The third generation of syn-kinematic monazite yield ca. 1.8 Ga ages. The dataset constrains, for the first time, coeval movement along the strike–slip Grease River shear zone and the thrust–sense Legs Lake shear zone during ca. 1.85 Ga dextral transpression and uplift of continental lower crust to middle crustal levels (ca. 0.5 GPa). Circa 1.8 Ga syn-kinematic monazite domains in dynamically recrystallized feldspars support brittle–ductile reactivation along the Grease River shear zone at shallower crustal levels, coincident with intra-continental strike–slip strain during culmination of the Trans-Hudson orogeny.

Stabilization and reactivation of cratonic lithosphere from the lower crustal record in the western Canadian shield

New U–Pb geochronology for an extensive exposure of high-pressure granulites in the East Lake Athabasca region of the western Canadian shield is consistent with a history characterized by 2.55 Ga stabilization of cratonic lithosphere, 650 million years of lower crustal residence and cratonic stability, and 1.9 Ga reactivation of the craton during lithospheric attenuation and asthenospheric upwelling. High precision single-grain and fragment zircon data define distinctive discordia arrays between 2.55 and 1.9 Ga. U–Pb ion microprobe spot analyses yield a similar range of U–Pb dates with no obvious correlation between date and cathodoluminescence zonation. We attribute the complex U–Pb zircon systematics to growth of the primary populations during a 2.55 Ga high-pressure granulite facies event (~1.3 GPa, 850°C) recorded by the dominant mineral assemblage of the mafic granulite gneisses, with subsequent zircon recrystallization and minor secondary zircon growth during a second high-pressure granulite facies event (1.0 GPa, ~800°C) at 1.9 Ga. The occurrence of two discrete granulite facies metamorphic events in the lower crust, separated by an interval of 650 million years that included isobaric cooling for at least some of this time, suggests that the rocks resided at lower crustal depths until 1.9 Ga. We infer that this phase of lower crustal residence and little tectonic activity is coincident with an extended period of cratonic stability. Detailed structural and thermochronological datasets indicate that multistage unroofing of the lower crustal rocks occurred in the following 200 million years. Extended lower crustal residence would logically be the history inferred for lower crust in most cratonic regions, but the unusual aspect of the history in the East Lake Athabasca region is the subsequent lithospheric reactivation that initiated transport of the lower crust to the surface. We suggest that a weakened strength profile related to the 1.9 Ga heating left the lithosphere susceptible to far-field tectonic stresses from bounding orogens that drove the lower crustal exhumation. An ultimate return to cratonic stability is responsible for the preservation of this extensive lower crustal exposure since 1.7 Ga.

Formation of eclogite, and reaction during exhumation to mid‐crustal levels, Snowbird tectonic zone, western Canadian Shield

AbstractA re‐evaluation of the P–T history of eclogite within the East Athabasca granulite terrane of the Snowbird tectonic zone, northern Saskatchewan, Canada was undertaken. Using calculated pseudosections in combination with new garnet–clinopyroxene and zircon and rutile trace element thermometry, peak metamorphic conditions are constrained to ∼16 kbar and 750 °C, followed by near‐isothermal decompression to ∼10 kbar. Associated with the eclogite are two types of occurrences of sapphirine‐bearing rocks preserving a rich variety of reaction textures that allow examination of the retrograde history below 10 kbar. The first occurs as a 1–2 m zone adjacent to the eclogite body with a peak assemblage of garnet–kyanite–quartz interpreted to have formed during the eclogite facies metamorphism. Rims of orthopyroxene and plagioclase developed around garnet, and sapphirine–plagioclase and spinel–plagioclase symplectites developed around kyanite. The second variety of sapphirine‐bearing rocks occurs in kyanite veins within the eclogite. The veins involve orthopyroxene, garnet and plagioclase layers spatially organized around a central kyanite layer that are interpreted to have formed following the eclogite facies metamorphism. The layering has itself been modified, with, in particular, kyanite being replaced by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites, as well as the kyanite being replaced by sillimanite. Petrological modelling in the CFMAS system examining chemical potential gradients between kyanite and surrounding quartz indicates that these vein textures probably formed during further essentially isothermal decompression, ultimately reaching ∼7 kbar and 750 °C. These results indicate that the final reaction in these rocks occurred at mid‐crustal levels at upper amphibolite facies conditions. Previous geochronological and thermochronological constraints bracket the time interval of decompression to <5–10 Myr, indicating that ∼25 km of exhumation took place during this interval. This corresponds to minimum unroofing rates of ∼2–5 mm year−1 following eclogite facies metamorphism, after which the rocks resided at mid‐crustal levels for 80–100 Myr.

Multistage exhumation and juxtaposition of lower continental crust in the western Canadian Shield: Linking high‐resolution U‐Pb and 40Ar/39Ar thermochronometry with pressure‐temperature‐deformation paths

New U‐Pb (titanite, apatite, rutile) and 40Ar/39Ar (hornblende, muscovite, biotite) data are linked with existing pressure‐temperature‐deformation paths to impose detailed temporal constraints on the juxtaposition and unroofing of domains in an extensive (>20,000 km2) region of exhumed lower crust in the East Lake Athabasca region of the western Canadian Shield. In the ∼200 m.y. between circa 1.9 Ga high‐pressure granulite facies metamorphism and circa 1.7 Ga unconformable deposition of Athabasca basin sediments on the exhumed rocks, our analysis reveals at least three distinct phases of unroofing at different rates. Specifically, we distinguish (1) an early phase of extensional unroofing in the Chipman domain at rates of 1.5–2.0 km/m.y. from ∼1.0 GPa to ∼0.8 GPa associated with mafic magmatism and metamorphism, (2) an episode of regional contractional uplift and associated unroofing along the Legs Lake shear zone at circa 1850 Ma from 0.7–0.8 GPa to 0.4–0.5 GPa, and (3) a final period of extensional unroofing at rates of 0.2–0.3 km/m.y. from 0.4 to 0.5 GPa that culminated in transport of current exposures to near‐surface conditions. The cooling patterns and retrograde assemblages are consistent with pauses between exhumational phases. The apparent convergence of disparate higher temperature histories in several deep crustal domains at circa 1.89–1.88 Ga implies their juxtaposition at 0.7–0.8 GPa conditions. Regional east directed thrusting of the deep crustal domains as a coherent unit at 1.85 Ga juxtaposed the granulites with middle crustal Hearne domain rocks at 0.4–0.5 GPa conditions. The detailed exhumational history allows correlation with changing regional tectonic regimes associated with the amalgamation of Laurentia. The temporal and spatial heterogeneity of exhumation patterns in the East Lake Athabasca region may be a common feature of the unroofing histories of lower crustal rocks.

Dating metamorphic reactions and fluid flow: application to exhumation of high‐P granulites in a crustal‐scale shear zone, western Canadian Shield

AbstractThe Legs Lake shear zone is a crustal‐scale thrust fault system in the western Canadian Shield that juxtaposes high‐pressure (1.0+ GPa) granulite facies rocks against shallow crustal (< 0.5 GPa) amphibolite facies rocks. Hangingwall decompression is characterized by breakdown of the peak assemblage Grt + Sil + Kfs + Pl + Qtz into the assemblage Grt + Crd + Bt ± Sil + Pl + Qtz. Similar felsic granulite occurs throughout the region, but retrograde cordierite is restricted to the immediate hangingwall of the shear zone. Textural observations, petrological analysis using P–T/P–MH2O phase diagram sections, and in situ electron microprobe monazite geochronology suggest that decompression from peak conditions of 1.1 GPa, c. 800 °C involved several distinct stages under first dry and then hydrated conditions. Retrograde re‐equilibration occurred at 0.5–0.4 GPa, 550–650 °C. Morphology, X‐ray maps, and microprobe dates indicate several distinct monazite generations. Populations 1 and 2 are relatively high yttrium (Y) monazite that grew at 2.55–2.50 Ga and correspond to an early granulite facies event. Population 3 represents episodic growth of low Y monazite between 2.50 and 2.15 Ga whose general significance is still unclear. Population 4 reflects low Y monazite growth at 1.9 Ga, which corresponds to the youngest period of high‐pressure metamorphism. Finally, population 5 is restricted to the hydrous retrograded granulite and represents high Y monazite growth at 1.85 Ga that is linked directly to the synkinematic garnet‐consuming hydration reaction (KFMASH): Grt + Kfs + H2O = Bt + Sil + Qtz. Two samples yield weighted mean microprobe dates for this population of 1853 ± 15 and 1851 ± 9 Ma, respectively. Subsequent xenotime growth correlates with the reaction: Grt + Sil + Qtz + H2O = Crd. We suggest that the shear zone acted as a channel for fluid produced by dehydration of metasediments in the underthrust domain.

Low long-term erosion rates and extreme continental stability documented by ancient (U-Th)/He dates

Zircon and apatite crystals from the western Canadian shield yield (U-Th)/He dates that are the oldest yet reported for terrestrial rocks. Zircon dates from 1.73 to 1.58 Ga are consistent with independent geological and thermochronological constraints, and indicate that the rocks were at temperatures ≤180 °C and crustal depths ≤7–10 km since ca. 1.7 Ga. Apatite dates from 0.95 to 0.55 Ga suggest residence of rocks at temperatures ≤40–50 °C and crustal depths ≤1.5–2 km for ∼1.0–0.6 b.y., when interpreted using conventionally accepted apatite He diffusion kinetics and considering the proposed effect of radiation damage on apatite He retentivity. Our analysis implies long-term integrated unroofing rates of ≤2.5 μm/yr since ca. 1.7 Ga. These rates are significantly lower than the long-term rates suggested by previous thermochronological data sets in continental interiors, but are within the range of short-term erosional estimates. The results are consistent with the extreme stability of this region since the Proterozoic.

Late Neoarchean thick-skinned thrusting and Paleoproterozoic reworking in the MacQuoid supracrustal belt and Cross Bay plutonic complex, western Churchill Province, Nunavut, Canada

In the western Churchill Province, Canadian Shield, Neoarchean supracrustal and plutonic rocks, intruded by Paleoproterozoic mafic dykes and granitic rocks, comprise the MacQuoid supracrustal belt and the structurally overlying Cross Bay plutonic complex. They form part of the northwestern Hearne subdomain that occupies an intermediate position between the continental Rae domain to the north and west, and the oceanic central Hearne subdomain to the south and east. New geological mapping and supporting geoscience are compatible with the presence of 2550–2500 Ma, southeast-directed, mid-crustal, thick-skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt. The structural contact between the MacQuoid supracrustal belt and the Cross Bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks. The ∼2190 Ma MacQuoid mafic dyke swarm cuts across Neoarchean deformation fabrics, but records ∼1.9 Ga, deep-crustal, regional metamorphism that affected both the supracrustal belt and the plutonic complex. Other Paleoproterozoic deformation events that occurred at ∼1850–1810 Ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere, related to the Trans-Hudson orogen.

Heat flow, thermal regime, and elastic thickness of the lithosphere in the Trans-Hudson Orogen

Heat flow studies on the exposed part of the Trans-Hudson Orogen (THO) in northern Manitoba and Saskatchewan allow constraints on crustal composition and lithosphere structure. The average of all heat flow values in the THO is the same as in other geological provinces of the Canadian Shield. However, where juvenile crust is exposed, heat flow is on average lower than in the Superior and Grenville provinces (37 vs. 41 mW m–2). Heat flow increases towards the surrounding Archean provinces, Rae–Hearne to the west, Sask to the south, and Superior to the east. There are strong differences in heat flow within and between the belts of the THO. The poor correlation between heat flow and heat production in the rocks exposed at the surface implies that these differences involve a large fraction of the crustal column. One new heat flow determination confirms the existence of a ``cold spot'' around the town of Lynn Lake in the northern part of the THO. Heat flow data in the Kisseynew and Glennie domains remain sparse, but they indicate that this low heat flow region extends as far south as the Flin Flon – Snow Lake Belt. The Lynn Lake Belt is underlain by poorly radiogenic rocks, possibly Kisseynew-type crust with oceanic basement. Northward increase in heat flow along the Thompson Belt is consistent with the view that the belt is thrust over Kisseynew-type basement only in the south. Heat flow increases southward in the Paleozoic basin because of higher heat production in basement rocks, probably from the Sask craton. We used the low heat flow regions to obtain an upper bound of 15 mW m–2 for the mantle heat flow in the THO. The effective elastic thickness of the lithosphere can be determined from the coherence between the topography and the Bouguer gravity. The effective elastic thickness is high (>40 km) thoughout the Canadian Shield and is highest in the central part of the shield, in particular in the Lynn Lake region. There seems to be a negative correlation between elastic thickness and heat flow in the central and western Canadian Shield. This indicates that, even in stable continents, the elastic thickness is largely controlled by the lithospheric temperatures that depend strongly on crustal heat generation and hence crustal structure.

Reconstruction of a large deep-crustal terrane: Implications for the Snowbird tectonic zone and early growth of Laurentia

Research Article| May 01, 2005 Reconstruction of a large deep-crustal terrane: Implications for the Snowbird tectonic zone and early growth of Laurentia K.H. Mahan; K.H. Mahan 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Search for other works by this author on: GSW Google Scholar M.L. Williams M.L. Williams 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Search for other works by this author on: GSW Google Scholar Author and Article Information K.H. Mahan 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA M.L. Williams 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA Publisher: Geological Society of America Received: 11 Oct 2004 Revision Received: 22 Dec 2004 Accepted: 23 Dec 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2005) 33 (5): 385–388. https://doi.org/10.1130/G21273.1 Article history Received: 11 Oct 2004 Revision Received: 22 Dec 2004 Accepted: 23 Dec 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation K.H. Mahan, M.L. Williams; Reconstruction of a large deep-crustal terrane: Implications for the Snowbird tectonic zone and early growth of Laurentia. Geology 2005;; 33 (5): 385–388. doi: https://doi.org/10.1130/G21273.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The ∼2800-km-long Snowbird tectonic zone is a well-recognized but still enigmatic feature in the western Canadian Shield. It has been interpreted as a Paleoproterozoic continental suture or an Archean strike-slip fault system, but here we suggest that the distinctive geometry of the central Snowbird tectonic zone is primarily due to the interaction of crosscutting Paleoproterozoic intracontinental thrust and strike-slip shear zones having a length of hundreds of kilometers. First, a major zone of thrust-sense shearing, coeval with early continent-continent collision between the Superior and western Churchill provinces, accommodated uplift of a large exposure of granulite facies lower continental crust. Younger strike-slip shear zones, perhaps analogous to Asian fault systems behind the Himalayan orogen, offset the thrust zone. Thus, the current geometry and distribution of deep-crustal rocks in this region represent a relatively late stage in the tectonic evolution of the western Churchill province rather than an accretionary one. Earlier structures oriented at a high angle to the Snowbird tectonic zone may record the fundamental accretionary history in this part of Laurentia. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Eclogites of the Snowbird tectonic zone: petrological and U-Pb geochronological evidence for Paleoproterozoic high-pressure metamorphism in the western Canadian Shield

Eclogite occurs within the southern domain of the East Athabasca mylonite triangle in northern Saskatchewan. Situated at the boundary between the Archean Rae and Hearne Provinces of the western Canadian Shield, the East Athabasca mylonite triangle is a fundamental exposure of the ~3,000-km-long Snowbird tectonic zone. The eclogite occurs in association with a variety of lower crustal high-pressure granulites that record a complex metamorphic history from 2.6 to 1.9 Ga. Temperatures of the eclogite facies metamorphism are constrained by garnet-clinopyroxene exchange thermometry at 920–1,000 °C. Minimum pressure conditions are recorded by the jadeite+quartz=albite geobarometer at 1.8–2.0 GPa. A near-isothermal decompression path to granulite facies conditions is inferred from retrograde reaction textures involving the formation of granulite facies assemblages such as orthopyroxene-plagioclase and pargasite-plagioclase. U-Pb IDTIMS zircon geochronology of the eclogite yields a weighted mean 207Pb/206Pb date of 1,904.0±0.3 Ma, which we interpret as the time of peak eclogite facies metamorphism. SHRIMP in situ analyses of metamorphic zircons included within omphacitic clinopyroxene support this interpretation with a weighted mean 207Pb/206Pb date of 1,905±19 Ma. Inclusion suites of high-pressure phases and the petrographic setting of zircon are a direct link between zircon growth and eclogite facies metamorphism. Zircon from one eclogite sample has older cores that are 2.54 Ga, which is a minimum age for the emplacement or earliest metamorphism of the gabbroic protolith. U-Pb rutile data indicate slow cooling at ~1°C/Ma below ~500 °C from 1.88 to 1.85 Ga. The formation and exhumation of the eclogites at ca.1.9 Ga has important implications for the tectonic significance of the Snowbird tectonic zone during the Paleoproterozoic. The eclogites described here are consistent with transport of continental crust to mantle depths during the Paleoproterozoic, followed by rapid buoyancy-driven exhumation to normal lower crustal depths.

Contractional uplift of deep crustal rocks along the Legs Lake shear zone, western Churchill Province, Canadian Shield

The Legs Lake shear zone juxtaposes high-pressure (1.0+ GPa) granulite- and eclogite-facies rocks with low-pressure (~0.5 GPa) amphibolite- to granulite-facies rocks, and thus may represent an important crustal-scale, exhumation-related structure in the western Canadian Shield. Field mapping and structural and petrologic analysis document the deformation and metamorphic history of rocks within and adjacent to the shear zone. At least two important phases of deformation are recorded: (1) early oblique thrusting (D2) that placed high-grade rocks over lower grade rocks, and (2) more discrete and lower grade brittle–ductile normal faulting (D3) that may represent the later part of the exhumation history. The northwest-dipping shear zone consists of 5–8 km of mylonite in map view, which bounds the southeast margin of the East Athabasca mylonite triangle. High-pressure granulite-facies metamorphism (~750–850 °C, 1.0–1.2 GPa) occurred in the East Athabasca mylonite triangle at ca. 1900 Ma, prior to D2 juxtaposition with the adjacent low-pressure Hearne domain. Thermobarometry from Grt–Crd–Sil–Bt–Qtz metapelites in the Hearne domain suggests peak conditions reached 600–700 °C and 0.45–0.5 GPa, which are interpreted to have occurred late during D2. Published and preliminary U–Pb isotope dilution - thermal ionization mass spectrometry (ID-TIMS) zircon geochronology and electron microprobe monazite geochronology suggest that deformation in the Legs Lake shear zone coincided with the ca. 1830–1810 Ma terminal collision in the Trans-Hudson orogeny. Extensional faulting during D3 most likely occurred after ca. 1780 Ma. A multi-stage process of exhumation involving both thrust and normal-sense shearing, may serve as a model for the exhumation of other regionally extensive deep-crustal exposures.

Magnetotelluric and teleseismic study across the Snowbird Tectonic Zone, Canadian Shield: A Neoarchean mantle suture?

The Snowbird tectonic zone (STZ) is a fundamental boundary within Canada's Western Churchill Province, one of the world's largest yet poorly‐known fragments of Archean crust. Geophysical data from a collocated magnetotelluric and teleseismic transect across the northeastern segment of the STZ provide an image of its subsurface geometry and indicate that it may have been previously mislocated. The model suggests that (1) the STZ has played a major role in the Neoarchean assembly and Paleoproterozoic reworking of the western Canadian Shield, (2) it was reactivated in a manner comparable to other crustal‐scale features such as the Kapuskasing zone of the Superior Province, Canada, and the Redbank thrust of the Arunta block, central Australia, and (3) it juxtaposes mantle blocks with contrasting geophysical properties, revealing a lithosphere‐scale overlap of the leading edges of the Rae and Hearne domains. The STZ thus records plate interactions in the Neoarchean comparable in scale with that of modern orogenic belts.

Petrogenetic interpretation from gamma-ray spectrometry and geological data: the Arch Lake zoned peraluminous granite intrusion, Western Canadian Shield

Airborne gamma-ray spectrometry data acquired over the Arch Lake granite batholith in the Western Canadian Shield display compositional patterns not previously identified by geological field mapping. An integrated analysis of gamma-ray spectrometric, geochemical and mineralogical data was conducted to map and analyse these patterns, and to obtain insight in their petrogenetic significance. Enhancement and cluster analysis of the K, eTh and eU grids reveal that the Arch Lake granite is zoned in a complex manner with a Th-rich centre and Th-rich, locally Urich outer margins. The Th-rich centre broadly corresponds to highs in interpolated patterns of Zr and TiO2, reflecting a regional mineralogical association between biotite, monazite, zircon and Fe-oxides. The negative trend of this Th-rich mineralogical association with SiO2 is not consistent with normally-zoned plutons formed by differentiation of a single magmatic pulse. Instead, two spectrally distinct K-eTh zones suggest that the Arch Lake batholith is a composite intrusion. Gamma-ray spectrometry surveys can be considered a useful tool for mapping subtle compositional variations in granites. Integrated analysis on the basis of geochemical data and mineralogical observations, however, is required to constrain the possible petrogenetic interpretations.

Proterozoic tectonic accretion and growth of western Laurentia: results from Lithoprobe studies in northern Alberta

The western Canadian Shield of northern Alberta is composed of a series of continental slivers that were accreted to the margin of the Archean Rae hinterland ca. 1.9–2.0 Ga., preserving a unique record of continental evolution for the interval 2.1–2.3 Ga. This part of Laurentia owes its preservation to the accretionary style of tectonic assembly south of the Great Slave Lake shear zone, which contrasts with indentation–escape processes that dominate the Paleoproterozoic record farther north. The Buffalo Head and Chinchaga domains form the central core of this region, comprising a collage of ca. 2325–2045 Ma crustal elements formed on an Archean microcontinental edifice, and similar age crust is preserved as basement to the Taltson magmatic zone. The distribution of magmatic ages and inferred collision and subduction zone polarity are used to indicate closure of intervening oceanic basins that led to magmatism and emplacement of continental margin arc and collisional belts that formed from ca. 1998 to1900 Ma. Lithoprobe crustal seismic profiles complement the existing geochronologic and geologic databases for northern Alberta and elucidate the nature of late stages of the accretionary process. Crustal-scale imbrication occurred along shallow eastward-dipping shear zones, resulting in stacking of arc slivers that flanked the western Buffalo Head terrane. The seismic data suggest that strain is concentrated along the margins of these crustal slivers, with sparse evidence for internal penetrative deformation during assembly. Post-collisional mafic magmatism consisted of widespread intrusive sheets, spectacularly imaged as regionally continuous subhorizontal reflections, which are estimated to extend over a region of ca. 120 000 km2. The form of such mid-crustal magmatism, as subhorizontal sheets (versus vertical dykes), is interpreted to represent a style of magma emplacement within a confined block, for which a tectonic free face is unavailable.

Isotopic provenance of the lower Muskwa assemblage (Mesoproterozoic, Rocky Mountains, British Columbia): new clues to correlation and source areas

The Proterozoic Muskwa assemblage is a thick (6 km) sequence of essentially unmetamorphosed, predominantly fine-grained siliciclastic and carbonate strata carried within thrust sheets of the Canadian Rocky Mountains of northeast British Columbia. It is inferred to be widespread in the subsurface of the autochthon to the east on the basis of seismic reflection profiles. The age and stratigraphic affinities of this succession are uncertain, although it is known to be older than 779 Ma based on the age of cross-cutting dikes. We report U-Pb SHRIMP ages for detrital zircons from four sandstone units and Sm-Nd data for interbedded shales that refine stratigraphic ages and correlation. The provenance characteristics of the Muskwa assemblage offer another piece in the puzzle of correctly assembling Rodinia. 207Pb/ 206Pb ages determined by SHRIMP analysis of 94 detrital zircon grains from four sandstone units range from ca. 3075–1766 Ma with a pronounced concentration of ages at ca. 1850 Ma and a paucity of grains in the interval 1920–2000 Ma. The youngest, most concordant analysis constrains the maximum age of the Muskwa assemblage to 1766±14 Ma (99.5% concordant). Sm-Nd analysis of associated mudrocks result in a range of ε Nd values that overlap with western Canadian Shield sources but result in T DM model ages of 2.30–2.45 and 2.18–2.33 Ga, using Goldstein et al. (1984) or DePaolo (1981) models, respectively. The pattern of U-Pb ages is similar to that of the western Canadian Shield but precludes a significant contribution from Proterozoic orogens in the northwest Canadian Shield, specifically the Thelon, Taltson and Calderian phase of Wopmay orogen. The abundance of grains with young ages (1850–1770 Ma) implies derivation from similarly aged rocks of the Rimbey granite belt, Hearne Province and possibly the internides of the TransHudson Orogen. The provenance characteristics of the Muskwa are compatible with sediment derived from unroofing and erosion of the orogenic topography formed by the collisional shortening of the Hearne Province during terminal assembly of the Canadian Shield ca. 1.8 Ga. The provenance characteristics of the Muskwa assemblage are distinct from those considered typical of Succession B strata, which are characterized by detrital zircons and Sm-Nd signatures derived from the Grenville Province of eastern North America. Hence, the Muskwa assemblage is assigned to Succession A, although it cannot be demonstrated which, if any, of the Succession A successions in western Canada are direct correlatives. Regional considerations suggest that the most likely correlation is with cratonic sand sheets of the Amundsen, Thelon and Athabasca basins that developed immediately after assembly of the core of Laurentia.

Patterns of pervasive shear strain near the boundaries of the La Ronge Domain, inner Trans-Hudson Orogen, western Canadian Shield

The Saskatchewan segment of juvenile Paleoproterozoic crust, inner Trans-Hudson Orogen (Reindeer Zone), is widely regarded as a NW-dipping stack of litho-structural domains. The metavolcanic-metasedimentary La Ronge Domain (LRD), whose least deformed portion has the geological map pattern of an Archean greenstone belt, is underlain by metasedimentary rocks of the Kisseynew Domain and overlain by migmatitic rocks of the Rottenstone Domain. In easily accessible parts of LRD boundary walls, the horizontal patterns of tangential (boundary-parallel) strain accumulated under peak metamorphic conditions are investigated. Utilizing the strain patterns and recently published reflection seismic profiles, we estimated the direction and derived the sense of tangential shear-strain components on various horizontal scales. In all parts of boundary walls investigated, the dip shear has the sense of ductile thrusts, but the strike-shear sense varies with position, on scales ranging from 100 m to 10 km. The regional persistence of thrust shear in both LRD walls accords with tectonic scenarios of protracted horizontal convergence of juvenile crust, after initial collision between the Archean Superior and Hearne continents. The patterns of strike shear, on the other hand, are difficult to reconcile with indentation hypotheses or counter-clockwise oroclinal rotation.

Matrix mosaics, brittle deformation, and elongate porphyroclasts: granulite facies microstructures in the Striding–Athabasca mylonite zone, western Canada

The Late Archean Striding–Athabasca mylonite zone, western Canadian Shield, is a 400-km-long, linked system of granulite facies mylonite belts developed in the continental crust. The granulite facies mylonites (800–1000°C at 0.8–1.1 GPa) developed in subsets of the anhydrous mineral assemblage clinopyroxene–orthopyroxene–garnet–plagioclase–quartz±hornblende. Comparison of the microstructures of the Striding–Athabasca mylonites with published descriptions of natural and experimental deformation of geological and analogue materials at elevated homologous temperatures provides some insight into the role played by different processes in the development of the Striding–Athabasca mylonites. In addition to dislocation creep and dynamic recrystallisation, extensive mass transfer occurred contemporaneously with brittle fracture and cataclasis during granulite facies metamorphism. The microstructure and extreme phase dispersal in anhydrous polymineralic matrix mosaics is indicative of efficient grain-scale and aggregate-scale diffusion, grain boundary mobility, and mass transfer, at relatively slow strain rates. Despite their annealed appearance, the granoblastic matrix mosaics developed synkinematically; deformation-induced dilation may enhance metamorphic reactions where products are more voluminous than reactants. In the porphyroclast population, highly elongate (>20:1) monocrystalline orthopyroxenes appear to be fragments of dismembered, kinked parent grains, rather than stretched porphyroclasts. Granulite facies mylonites should not be treated as direct analogues of greenschist facies mylonites. In particular, it is essential to evaluate the potential positive feedback between structural and metamorphic processes in highly strained, high-temperature shear zone rocks.

Tectonic entrapment and its role in the evolution of continental lithosphere: An example from the Precambrian of western Canada

New geophysical data acquired over the buried crystalline basement of western Canada provide constraints on the history of tectonic assembly of the western Canadian Shield in the interval 1.75–1.85 Ga. Specifically, these data provide new perspectives on the evolution of an Archean continental fragment (Heame province) that was trapped in a tectonic “vise” between coeval orogenic belts that dipped beneath the Hearne province. The Trans‐Hudson orogen developed along the southeastern margin of the Hearne province as a series of ocean floor, oceanic arc, and arc marginal basins were telescoped and thrust obliquely beneath the Hearne. Along the northwest edge of the Hearne, collapse and subduction of a narrow marginal basin, now marked by the subsurface extension of the Snowbird Tectonic Zone, led to formation of magmatic arc and collision of older Proterozoic terranes. The Hearne province itself is characterized by regional granulite‐grade metamorphism and evidence of extensive and pervasive partial melting of the crust. The internal character of the Hearne province seen on crustal seismic reflection profiles is that of a crustal‐scale structural fan with reflection fabrics that verge toward the bounding orogens. The deformation of the Hearne is predominantly of Paleoproterozoic age and constitutes a thorough reworking of this formerly Archean crustal domain over a distance of more than 600 km across strike. Entrapment and thermal weakening of the Hearne resulted from mechanical coupling of inferred buoyant subduction‐collision zones and removal of or modification of Archean lithospheric mantle that may have originally formed the keel to the Hearne. Long‐period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day Hearne province, which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination/convective removal of thickened lithosphere in the Proterozoic. Tectonic entrapment, as illustrated by the Proterozoic structural and thermal evolution of the Hearne province crust and subcontinental mantle, may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents.