Subprovince Boundary Research Articles

Early depositional and magmatic history of the Beardmore-Geraldton Belt: Formation of a transitional accretionary belt along the Wabigoon-Quetico Subprovince boundary in the Archean Superior Craton, Canada

The Beardmore-Geraldton Belt (BGB) is a major gold camp along the boundary between the granite-greenstone eastern Wabigoon subprovince and the metasedimentary Quetico subprovince of the Archean Superior Province, Canada. New detrital zircon geochronology suggests that the metasedimentary rocks of the BGB and northern Quetico subprovince were derived from the erosion of the eastern Wabigoon subprovince that is dominated by ca. 2900–2680 Ma rocks but also comprises crust as old as > 3200 Ma. Uplift of the eastern Wabigoon subprovince as a result of convergence and compression along its southern margin exposed those rocks to erosion and shed sediments in the Quetico basin to the south. Sedimentation in the Quetico basin is bracketed between ca. 2700 Ma, the age of the youngest detrital zircon population, and 2694.0 ± 1.0 Ma, the crystallization age of cross-cutting feldspar-quartz porphyries (FQP). Continued convergence facilitated the thrust-imbrication of the metavolcanic rocks of the eastern Wabigoon subprovince with the metasedimentary rocks of the Quetico basin resulting in the assembly of the BGB. This resulted in crustal thickening, the introduction of hydrated mafic rocks at the base of the crust, and partial melting of these rocks to form TTG suite melts at depths >50 km, as suggested by the geochemical characteristics of the FQP intrusions. Imbrication and thrusting ended with the emplacement of the stitching, 2690 ± 1 Ma Croll Lake Stock, which has a high transition elements and ferromagnesian oxides signature of sanukitoid suite rocks. It formed during delamination of the lower crust or slab break-off from magmas generated by mixing of hot, mantle material with TTG suite melts at depths > 50 km. The early geological evolution of the Geraldton gold camp is broadly similar to that of other major Archean gold camps associated with major faults and/or terrane boundaries, where deposition of fluvial conglomerate and turbiditic sandstones in fault-bounded basins overlying older volcanic rocks is accompanied by magmatic activity and the upward migration of gold-bearing hydrothermal fluids, while differences between the Geraldton and other camps may be explained by craton-scale variations in geodynamic processes.

Impact of ocean barriers, topography, and glaciation on the phylogeography of the catfish Trichomycterus areolatus (Teleostei: Trichomycteridae) in Chile

We examined the role of several earth history events on the phylogeographic distribution of the catfish Trichomycterus areolatus in Chile using the cytochrome b gene. We explored three biogeographic hypotheses: that sea level changes have resulted in the isolation of populations by drainages; that glaciation has impacted genetic diversity; and that ichthyological subprovince boundaries correspond to phylogeographic breaks in our focal species. We found seven well-supported clades within T. areolatus with high levels of genetic divergence. The strongest signal in our data was for an important role of sea level changes structuring populations. Five of the seven clades mapped cleanly to the geographic landscape and breaks corresponded closely to areas of narrowest continental shelf. In addition, few haplotypes were shared between rivers within clades, suggesting that only limited local movement of individuals has occurred. There was no relationship between the levels of genetic diversity and the proportion of individual drainages covered by glaciers during the last glacial maximum. Two phylogeographic breaks within T. areolatus did match the two previously identified faunal boundaries, but we found three additional breaks, which suggests that faunal breaks have only limited utility in explaining phylogeographic patterns. These results imply that the narrow continental shelf coupled with sea level changes had a strong influence on the obligate freshwater fishes in Chile.

Emplacement and deformation features of syntectonic leucocratic veins from Rainy Lake zone (Western Superior Province, Canada)

Networks of leucocratic veins are widespread in the Rice Bay–Redgut Bay domain of the Rainy Lake zone at the Wabigoon–Quetico subprovince boundary (Archean Superior Province, Canada). Vein geometries and their cross-cutting relationships with host rocks are indicative of syntectonic emplacement within a regional late Archean transpressional deformation event (Kenoran orogeny). The relative folding and boudinage of the veins compared to their host rocks indicate that they were emplaced syntectonically with the D 2 deformation. Semi-quantitative strain and vorticity analyses confirm the syn-D 2 timing of the veins. The observed structural features are interpreted as being formed during magma emplacement along dilatant fractures and subsequent solid-state deformation under overall ductile deformation conditions. The structures related to emplacement are better preserved in domains of low post-emplacement strain and veins intruded into competent host rocks, whereas the folds and boudins related to post-emplacement deformation are well developed in veins intruded into relatively incompetent host rocks. In domains of high strain, the primary intrusive structures are modified or even locally obliterated due to increasing deformation. It is envisaged that regional kinematics and the rheological state of host rocks are the main controls on the style of vein emplacement and deformation. The variety of observed structures is considered the result of deformation and lithological heterogeneity rather than several distinct magmatic events.

Assessment of the relationship between emplacement of the Algoman plutons and regional deformation in the Rainy Lake region, Ontario

The Algoman suite of granites and granodiorites, located within the Rainy Lake region of the Superior Province, intruded the Wabigoon–Quetico subprovince boundary about 2686 Ma. Deformation occurred during the late Archean collision between the Wabigoon and Wawa volcanic arcs. The Algoman plutons are the youngest rocks in the region. Their apparent lack of macroscopic deformation fabrics has led previous researchers to use their age as a constraint for the cessation of regional deformation. However, contrasting competence between the relatively rigid granites and surrounding volcanic and metasedimentary rocks suggests that obvious deformation fabrics may not be observed in the plutons even if they were present during deformation. A field, anisotropy of magnetic susceptibility (AMS), gravity inversion model, and microstructural study were undertaken on two neighboring Algoman plutons to determine their emplacement history relative to the regional deformation. There is a steep macroscopic foliation in the adjacent rocks and in parts of the plutons. The magnetic foliation is very strong and roughly coincident with the field foliations. The gravity inversion models indicate that the plutons are relatively shallow (1 km and 4.5 km at their deepest points), and their shapes are consistent with emplacement during dextral transpression. Quartz microstructures, such as undulose extinction and subgrain formation, indicate that there was some solid-state deformation accommodated by dislocation creep mechanisms. The field, AMS, gravity, and microstructural evidence indicate that the plutons were most likely emplaced syntectonically. Thus, the pluton age does not provide a conclusive constraint to the cessation of regional deformation.

The Quetico Intrusions of Western Superior Province: Neo-Archean examples of Alaskan/Ural-type mafic–ultramafic intrusions

The Quetico Intrusions are a series of ultramafic–mafic igneous rocks that occur along the northern Quetico subprovince boundary. These intrusions show lithological zoning from cores of olivine-bearing ultramafic rocks to margins of hornblendite and gabbroic rocks, and they are commonly accompanied by Cu–Ni–platinum group element (PGE) sulphide mineralization. We identified three additional Quetico Intrusions in the centre and southern part of the Quetico subprovince, bringing the entire Quetico Intrusion array to be over 125 km in length across the subprovince. The Samuels Lake intrusion in the centre of the subprovince has an U–Pb age of 2688 + 6/−5 Ma, which is similar to the age of ∼2690 Ma for the Quetico Intrusions along the northern subprovince boundary, suggesting that all the Quetico Intrusions are likely to be contemporaneous. The parental magmas of the intrusions were Mg-rich based on the high Mg of olivine and clinopyroxene, and they were hydrous as evidenced by abundant hornblende, early crystallization of clinopyroxene, and the common occurrence of mafic pegmatite. The bulk rocks show arc geochemical signatures, including a high concentration of large ion-lithophile elements combined with a low concentration of high field-strength elements. The mineral compositions of pyroxene and hornblende also plot in the field of arc igneous rocks. The data suggest the derivation of the parental magmas from a mantle wedge or a refractory mantle that was previously metasomatized in a subduction setting. The Quetico Intrusions display many similarities with Alaskan/Ural-type zoned mafic–ultramafic intrusions along major sutures of Phanerozoic orogenic belts. Common features include their geodynamic settings, internal lithological zoning, mineralogy and mineral chemistry, and common association with PGE mineralization. The striking similarities between the two indicate that the late Archean Quetico Intrusions likely formed in a tectonic setting similar to that of the Alaskan/Ural-type intrusions. We propose that the parental magmas for the Quetico Intrusions formed during the very early stage of accretion of the Wawa arc to the Wabigoon subprovince. Oblique accretion of the Wawa arc likely steepened the subduction angle, which allowed upwelling of hot asthenospheric mantle. This resulted in high degrees of partial melting in the mantle wedge to produce the Quetico Intrusions.

U–Pb ages constraining structural development of an Archean terrane boundary in the Lake of the Woods area, western Superior Province, Canada

Layered gneisses in the Winnipeg River subprovince contain magmatic zircon with U–Pb ages of 3317 ± 9 and 3055 ± 4 Ma at Tannis Lake, and ~3170 and 3255 ± 5 Ma at Cedar Lake, indicating widespread occurrence of Mesoarchean crust. This is in contrast to the well-documented Neoarchean age of the western Wabigoon subprovince. Further geochronology using both SHRIMP (sensitive high resolution ion microprobe) and ID-TIMS (isotope dilution thermal ionization mass spectrometry), combined with structural observations, in the Kenora area and Lake of the Woods greenstone belt show the effects of juxtaposition of these two terranes. Isoclinally folded gneiss north of the subprovince boundary zone near Kenora gives a magmatic age of 2882 ± 2 Ma with 3051 ± 6 Ma inheritance. Ages of syntectonic dykes show that asymmetric refolding of these gneisses occurred between 2717 ± 2 and about 2713 ± 1 Ma. Subsequent regional vertical flattening and horizontal extension are dated at 2708 ± 2 Ma by syntectonic tonalite sheets. These events are broadly coeval with deposition of orogenic sediments in the Warclub Group and a first stage of regional folding (age brackets of 2716–2709 Ma) in the Lake of the Woods greenstone belt to the south. A second stage of folding and regional faulting in the greenstone belt occurred about 2695 ± 4 Ma and is approximately coeval with open upright folding in the Winnipeg River subprovince. These observations are consistent with overthrusting and collapse of a Mesoarchean continental terrane by a juvenile Neoarchean arc terrane over the time span 2717–2695 Ma.

Structural testing of tectonic hypotheses by field-based analysis of distributed tangential shear: examples from major high-strain zones in the Grenville Province and other parts of the southern Canadian Shield

The Grenville Province and other parts of the Canadian Shield contain major (>100 km long) high-strain zones, also called shear belts or ductile shear zones, that are hosted by heterogeneously deformed gneisses and schists. In well-exposed segments of three nontabular zones whose dip angle is known locally, at the erosion level and (or) in the shallow subsurface, we investigate the tangential shear strain (better called the tangential unit shear or TUS) without assuming that mineral-shape lineations, common varieties of stretching lineations, are effectively parallel to the local TUS direction. Employing a graphic technique that copes with the geometric conditions of general triaxial strain, we approximate the actual direction and find the sense of local TUS in parts of (i) the Parry Sound shear zone, Grenville Province; (ii) the South Range shear zone, Southern Province; and (iii) the Uchi – English River subprovince boundary zone, Superior Province. Information thus obtained for individual high-strain zones in Ontario confirms the validity of published hypotheses: (i) 1020–970 Ma, normal-sense distributed shearing in the Grenvillian thrust stack; (ii) northwest- directed thrusting of Huronian rocks over Archean basement; and (iii) north-directed thrusting of English River metasediments and associated migmatites onto the Uchi granite–greenstone terrain, under peak metamorphic conditions.

The Aubry Pegmatites: Exploration for Highly Evolved Lithium-Cesium-Tantalum Pegmatites in Northern Ontario

Research Article| January 01, 2005 The Aubry Pegmatites: Exploration for Highly Evolved Lithium-Cesium-Tantalum Pegmatites in Northern Ontario P.M. DIMMELL; P.M. DIMMELL Linear Gold Corp., 56 Carpasian Road, St. John’s, NL A1B 2R2 Search for other works by this author on: GSW Google Scholar J.A. MORGAN J.A. MORGAN Consulting Geologist 28a Frank’s Road, Conception Bay South, NL A1X 6W8 Search for other works by this author on: GSW Google Scholar Author and Article Information P.M. DIMMELL Linear Gold Corp., 56 Carpasian Road, St. John’s, NL A1B 2R2 J.A. MORGAN Consulting Geologist 28a Frank’s Road, Conception Bay South, NL A1X 6W8 Publisher: Canadian Institute of Mining, Metallurgy and Petroleum Received: 23 May 2003 Accepted: 07 Jul 2004 First Online: 02 Mar 2017 © 2006 Canadian Institute of Mining, Metallurgy and Petroleum Exploration and Mining Geology (2005) 14 (1-4): 45–59. https://doi.org/10.2113/gsemg.14.1-4.45 Article history Received: 23 May 2003 Accepted: 07 Jul 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 P.M. DIMMELL, J.A. MORGAN; The Aubry Pegmatites: Exploration for Highly Evolved Lithium-Cesium-Tantalum Pegmatites in Northern Ontario. Exploration and Mining Geology 2005;; 14 (1-4): 45–59. doi: https://doi.org/10.2113/gsemg.14.1-4.45 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 SocietyExploration and Mining Geology Search Advanced Search Abstract The Aubry pegmatites are located on the Seymour Lake property, 15 km north of Lake Nipigon in northwestern Ontario. The property lies within the Caribou Lake-Marshall Lake greenstone belt along the English River-Wabigoon subprovince boundary of the Superior province of the Canadian Shield. The property was optioned in March 2001 by Linear Resources (now Linear Gold Corp.), who carried out gridding, prospecting, geological mapping, soil and lithogeochemical sampling, trenching, channel sampling, and a total of 1,866 m of diamond drilling in 32 holes. The Aubry pegmatites are classified by Breaks et al. (2003) as complex type, spodumene-subtype, lithium-cesium-tantalum pegmatites (Černý, 1989). These pegmatites generally form under high-pressure–low-temperature conditions, display complex internal zoning, have relatively low Nb/Ta ratios in the ore-forming assemblages, and contain significantly elevated tantalum values (Černý, 1989). Other examples of spodumene-subtype pegmatites include the formerly producing Hugo pegmatite in South Dakota, and the currently producing Greenbushes pegmatite in Australia. The Aubry pegmatites occur as an undulating system of stacked sheets that dip moderately to steeply eastward over a 1-km strike length. The main portion of the North Aubry pegmatite ranges in true thickness from 10 to 28 m over a strike length of 125 m, whereas the thickest portion of the South Aubry has an estimated true thickness of 15 to 16 m over a minimum strike length of 95 m. The North Aubry pegmatite remains open downdip and along strike to the north, whereas the South Aubry pegmatite remains open downdip and along strike in both directions. The Aubry pegmatites are among the most highly evolved in Ontario, with the southern end of the North Aubry (trench 5/6 area) being the most highly fractionated (Breaks et al., 2002). Channel samples from the trench NA-5 area of the North Aubry pegmatite produced values up to 2.49 wt.% Ta2O5 over 4 m. Coarse manganotantalite crystals, up to 5 cm in diameter, from this trench contain up to 84.9 wt.% Ta2O5 (Breaks et al., 2002). Drill hole SL02-31 in the trench NA-6 area gave values of 0.046 wt.% Ta2O5 over 17.7 m, including 1 m @ 0.464 wt.% Ta2O5. Further exploration will focus on defining the grade of the North Aubry zone, extending and defining both known pegmatites, and identifying additional Ta-rich zones within them. Lithogeochemical and enzyme leach soil geochemical data suggest the presence of multiple undiscovered pegmatites on the property. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geoelectric response of Archean lithosphere in the western Superior Province, central Canada

Magnetotelluric (MT) soundings were made at more than 230 sites in the western Superior Province, in central Canada, as part of the Lithoprobe Western Superior Transect investigations. The Superior Province forms the nucleus of the North American continent. Its western part contains a regional pattern of east–west trending 100–200 km wide granite–greenstone and metasedimentary belts or subprovinces. The objectives of the MT study are to define the electrical resistivity structure of the crust of the subprovinces and their boundaries, and to determine if there exists a signature in the upper mantle related to the formation of the craton. The exceptionally large MT database for the western Superior Province provides a three-dimensional sampling of the resistivity structure. In this paper, the primary MT responses from the data set, including geoelectric strike, apparent resistivity and phase, are used to define first-order geoelectrical structures in the region. Apparent resistivity responses indicate the ca 3.0 Ga North Caribou terrane contains extremely resistive crust. Crust to the south, accreted at ca. 2.7 Ga, is significantly more conductive, with resistivities similar to in the southeast Superior Province. The results suggest that the supracrustal component of the North Caribou terrane provides minimal enhancement of the crustal conductivity. At the southern margin of the former North Caribou superterrane, induction arrows define a conductivity anomaly that extends in an east–west direction for at least 600 km and causes a reversal of arrows in a region 100–200 km wide. This anomaly is interpreted to be caused by a conductive component of the 2.7 Ga metasedimentary rocks of the English River subprovince. The observations demonstrate the ability of electromagnetic methods to map some Neoarchean continental margins. In the southwest Superior Province, geoelectric strike azimuths for periods corresponding to signal penetration into the upper mantle are aligned with the trend of subprovince boundaries and are interpreted to be related to fossil structure in the mantle lithosphere. This result provides support to the interpretation that the lithospheric mantle of the western Superior Province has remained intact since the 2.7 Ga accretion of the subprovinces. The MT responses from the western Superior Province have thus defined a number of large-scale resistivity structures providing information on tectonic structure and history, and confirming the value of electromagnetic imaging of Archean lithosphere.

Seismic tomographic images of the cratonic upper mantle beneath the Western Superior Province of the Canadian Shield—a remnant Archean slab?

Abstract Knowledge of the velocity structure of the upper mantle beneath the Western Superior Province (WSP) is key to understanding better the accretionary processes active during the Archean. To that end, teleseismic P- and S-wave travel times recorded as part of the Teleseismic Western Superior Transect (TWST) were inverted for their respective seismic velocities. This experiment involved 17 portable broadband stations arrayed in northern Ontario, Canada, so as to cross-cut the strike of many subprovinces as well as the boundary with the Proterozoic Trans-Hudson Orogen to the north. The 5-month deployment yielded 1423 P-wave and 651 S-wave high-quality residuals for inversion. The resulting tomographic images reveal three apparently-robust velocity anomalies represented by: (i) a dipping tabular high-velocity anomaly; (ii) a relatively shallow low-velocity anomaly directly above the positive anomaly; and (iii) a deep low-velocity body. The first anomaly may be interpreted in the Western Superior context as a 30–50 km thick eclogite/dunite layer representing remnant subducted oceanic lithosphere. The presence of such a body within the cratonic root would suggest its origin at around 2.7 Ga and the apparent SE–NW strike is noticeably oblique to the main EW trend of the subprovince boundaries. The low-velocity anomalies may be related to processes that occurred at the edges of the descending slab or they may be expressions of later upwelling material. The presence of a thick cratonic root (≈300 km) may also be revealed by the tomographic images. Overall, these travel time results are considered compatible with late Archean structures at depth resembling those of modern subduction tectonics, though other possibilities (e.g. small-scale convection cells) exist.

Latitudinal gradients in benthic community composition in Western Atlantic estuaries

Summary AimThe community composition of benthic macroinvertebrates from 295 estuarine sites was examined in order to either confirm or challenge established boundaries of zoogeographical provinces. We also investigated the postulate that, while local distributions were determined by natural habitat characteristics such as salinity, sediment type and depth, distributions on a large geographical scale would be correlated with temperature. LocationThe Atlantic coast of the United States (on a latitudinal gradient from 42° to 25°N). MethodsUsing the descriptive techniques of cluster analysis and nonmetric multi‐dimensional scaling, we determined the similarities in benthic community composition between sites within 1° latitudinal bands and compared the biotic ordinations to natural habitat characteristics such as salinity, sediment type and depth. We then evaluated the overall community composition within each 1° latitudinal band and established whether or not similarities existed between adjacent 1° latitudinal bands. In this manner, we were able to confirm that a latitudinal gradient existed in estuarine benthic community composition along the western Atlantic coast. This latitudinal gradient was demarcated by biogeographical boundaries at (1) Biscayne Bay, Florida, (2) the southern end of the Indian River Lagoon, Florida, (3) St John's River, Florida, (4) Cape Island, South Carolina, (5) the mouth of the Cape Fear River, North Carolina and (6) Cape Cod, Massachusetts, with a subprovince boundary just north of Cape May at Wildwood, New Jersey. ResultsThe major divisions approximated widely recognized biogeographical boundaries. Average summer water temperatures correlated better than sediment type, depth, or salinity with the latitudinal groupings of sites determined by cluster analysis. ConclusionsBased on this relationship, we speculate on the potential impacts of current global climate change scenarios on the distribution of benthic macroinvertebrates along the western Atlantic coast.

Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks

Fifty million years of Archean evolution is recorded in the Manitouwadge greenstone belt and the Wawa-Quetico boundary region, from ca. 2720 Ma volcanism and subvolcanic plutonism associated with massive sulphide deposits and hydrothermal alteration zones, to 2689-2687 Ma and 2680-2677 Ma synkinematic plutonism. In the greenstone belt, greywackes were deposited after 2693 Ma, post-dating local volcanism by at least 25 Ma, and requiring that the volcanic-sedimentary contact is an unconformity or a fault. In migmatitic greywackes in the Quetico subprovince, detrital zircons limit the depositional age to <2690 Ma, permitting correlation of greywackes across the Wawa-Quetico subprovince boundary. Upward-facing inclined F2 folds that deform the volcanic-sedimentary contact are bracketed by the 2687 ± 2 Ma Loken Lake pluton, which shows strong D2 fabrics, and by 2680+4-3 Ma foliated granite which cuts D2 fabrics. Dextral transpression producing regional F3 folds and the overall east-west trends of the Wawa-Quetico boundary region post-dated the 2680 ± 2 Ma Nama Creek pluton. Field relationships and isotopic ages support correlation of greywackes across the subprovince boundary, and demonstrate that most or all of the ductile deformation post-dated sedimentation. Ductile structures, especially those associated with dextral transpression, are not directly related to juxtaposition of the Wawa and Quetico subprovinces, as these were already contiguous, either through sedimentation on a volcanic substrate or as a result of earlier cryptic structures. Our results imply that the belt-like configuration of the subprovinces, emphasized in accretionary models of the Superior Province, is a relatively late feature that overprints older, tectonically significant structures.

Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks

Geological evolution of the Manitouwadge greenstone belt and Wawa-Quetico subprovince boundary, Superior Province, Ontario, constrained by U-Pb zircon dates of supracrustal and plutonic rocks

Seismic reflections from subvertical diabase dikes in an Archean terrane

A 20 km seismic reflection profile in the Superior province of northwestern Ontario imaged polydeformed Archean terrane, including the Manitouwadge greenstone belt and the adjacent Wawa-Quetico subprovince boundary. In addition to reflectivity from Archean lithostructural elements, we recognize discrete reflections that correlate with undeformed subvertical Proterozoic dikes. Moderately to steeply inclined linear events in the stacked section are related to Matachewan dikes that intersect the seismic profile at 50°–75°. The reflections project to the surface at points corresponding to dike-profile intersections and have apparent dips in the seismic section consistent with the intersection angles in map view. Subhorizontal, linear to curvilinear reflections originate from Marathon dikes that are subparallel to the seismic line. On the basis of forward modeling studies, the events occur at two-way traveltimes for horizontally propagating waves. We conclude that a number of dipping and subhorizontal events represent refracted waves propagated along horizontal ray paths near the surface and reflected from subvertical dikes. In many other seismic profiles in crystalline crust, reflections originating from subvertical dikes have likely been overlooked or misinterpreted, either because the presence of dikes was not recognized, or because their seismic response was not evaluated.

A magnetotectonic study correlating late Archean deformation in northwestern Ontario

Late Archean lamprophyre dikes crop out on both sides of the Quetico–Shebandowan subprovince boundary. They post date F1 folds in late Archean rocks (2690 Ma) but their intrusion overlapped with the waning phase of S1 development. S1 developed to varying degrees or is absent in some dikes. Nevertheless, the dikes show a cryptic tectonic fabric in the ferromagnetic minerals revealed by anisotropy of magnetic susceptibility (AMS). This is parallel to S1 in the host metamorphic rocks. Thus, the S1 fabric-forming episode may be correlated from one subprovince to the other using the dike swarm as a chronological marker.

The mesothermal gold-lamprophyre association: significance for an accretionary geodynamic setting, supercontinent cycles, and metallogenic processes

Archean shoshonitic lamprophyres are cotemporal and cospatial with gold mineralization in the Superior Province of Canada, both being emplaced along translithospheric structures that demark subprovince boundaries. By analogy with geochemically similar Phanerozoic counterparts, the dikes are a product of specific plate interactions rather than a deep asthenosphere plume-initiated event, and their onset in the late-Archean at ∼ 2.7 Ga signifies that Phanerozoic style plate-tectonics was operating at this time. Fresh shonshonitic dikes are characterized by normal background gold contents of 3.9 ± 8.1 ppb (lσ), close to the value of 3.0 ppb for the bulk continental crust, and average abundances of As, Sb, Bi, W, TI, B, Cu, Pb, Zn, and Mo are also close to their values in bulk continental crust. Thus, fresh lamprophyres are not intrinsically enriched either in Au or elements affiliated with gold in mesothermal deposits, and accordingly do not constitute a special source rock. Platinum group element contents (Ir = 0.4 ± 0.58 ppb; Pt = 5.9 ± 26.5, Pd = 5.5 ± 1.8), in conjunction with Cu, Au, and Ni abundances, define approximately flat patterns on primitive mantle-normalized diagrams, consistent with derivation of the alkaline magmas from a depleted mantle source variably enriched by incompatible elements. Comparable abundances and ratios of Pd/Au, Os/Ir, and Ru/Ir in Archean lamprophyres, Archean komatiites, and Gorgona komatiites signify that the Archean and Phanerozoic upper mantle had similar noble metal contents, such that the prolific greenstone belt Au-Ag vein deposits cannot be explained by secular variations in upper mantle Au abundance alone. The lack of covariation between Au and light rare earth elements in lamprophyres rules out mantle metasomatism as a process generating intrinsically Au-rich magmas. Emplacement of the lamprophyres was diachronous from north (2710 Ma) to south (2670 Ma) in the Superior Province, as was the gold mineralization. Both were related to late transpressional tectonics during successive accretions of individual subprovinces. Alkaline magmatism and gold mineralization are temporally and spatially related because they share a common geodynamic setting, but they are otherwise the products of distinct processes. Much of Archean time was devoid of shoshonites and mesothermal gold deposits. The first widespread inception of this duality at 2.71–2.65 Ga in the Superior and Slave Provinces, Canada, and in India and Australia, may reflect one of the first supercontinent aggregations involving accretionary, “Cordilleran style” tectonics. Giant mesothermal gold provinces and shoshonites recur through time in the Palaeozoic and Mesozoic in this geodynamic setting.

Archean uplift of a subprovince boundary in the Canadian Shield, revealed by magnetic fabrics

A high strain zone, reported elsewhere as a subprovince or tectonic-terrane boundary, shows evidence of uplift during a penetrative D 1 event. Some outcrops reveal this by conventional L-S petrofabric observations. However, all outcrops permit measurements of the anisotropy of low field magnetic susceptibility (AMS) which here records the preferred crystallographic orientation of chlorite, biotite and amphibole. Furthermore, anisotropy of anhysteretic magnetic remanence (AARM) reflects the preferred orientation of ferromagnetic grains (magnetite and pyrrhotite). All three types of fabric show that the rocks were shortened in a north-south direction, perpendicular to the terrane boundary, and stretched upwards in a nearly coaxial strain history. A few anomalous magnetic fabrics may indicate some late D 1 subhorizontal E-W extension, compatible with transpression along the boundary. We conclude that this boundary is a zone of high strain associated with the uplift of the high-grade plutonic rocks of the Winnipeg River subprovince, with only slight evidence of minor components of late transpression during the penetrative D 1 fabrics.

Late Archaean thrusting in the northwestern Pontiac Subprovince, Canadian Shield

Abstract In the northwestern Archaean Pontiac Subprovince, south-verging structures, such as drag folds with subhorizontal axes, stretched objects, down-dip mineral lineations, low-angle thrusts and allochthonous metavolcanic slabs were mainly produced by thrusting in response to N-S regional compression. Deformation of the dominantly metasedimentary sequences occurred under Barrovian-type metamorphism, and the horizontal shortening was accompanied by downdip stretching in the regional foliation (S1). Two nappes are assumed to be present, their boundary being marked by an abrupt change in structural style and metamorphic facies. Two phases of close to isoclinal folding are evident in the upper nappe but are absent from the lower nappe. In the upper nappe, D2 gives and E-W tectonic trend to the rock units. F2 folds range from upright to recumbent, but are most typically moderately southward inclined folds that plunge gently towards the east or west. Besides mostly retrograde recrystallization within D2 fault zones, S2 is a crenulation cleavage that did not develop sufficiently to obliterate the regional schistosity (S1) and its associated stretching and mineral lineations. This contrasts with the regionally penetrative nature of the main deformation event (D1), and suggests a narrowing, or partitioning of high-strain zones with time, possibly due to a decrease in the regional temperature. Syn-orogenic sedimentation in the northwestern Pontiac Subprovince was probably enhanced by tectonically controlled uplift resulting from early regional Kenoran compression. The sedimentary basin was subsequently deformed by further N-S compression which led to internal stacking nappe-type displacements, Barrovian metamorphism and thrusting of the southern Abitibi over the Pontiac Subprovince. The common style of folding and the same orientation of various structural elements such as cleavage and lineation in the transition zone between the Abitibi and Pontiac subprovinces indicate continuity in deformation style. Regional-scale transcurrent displacement along the northern subprovince boundary appears as a late, localized response to regional compression.

Deformation across the western Quetico subprovince and adjacent boundarl regions in Minnesota

North- to northwest-directed crustal shortening across the western Quetico subprovince and its boundary regions produced a complex deformation sequence within the Quetico belt and resulted in concentrated zones of dextral ductile shear in the boundary regions within the adjacent greenstone–granite terranes. In this paper, we review and introduce new data on the regional deformation features and their geometries and discuss the history of generation of these features. We attribute the deformation sequence to differential partitioning of shortening and shear strains during dextral transpression associated with oblique convergence and accretion along the southern margin of the Superior Province.The turbiditic wacke in the western Quetico subprovince, now typically amphibolite-facies schist and migmatite, underwent an early deformation stage that included recumbent folding (F1) and the generation of an S1 bedding-parallel foliation. This event is most evident along the northern and southern boundaries of the subprovince, but it is also recognized in the lower grade metasedimentary rocks in the adjacent Wawa and Wabigoon subprovinces. In these subprovinces, F1 folding may have been associated with higher level thrusting and allochthonous emplacement of greenstone units. Despite our F1 designation of this event, it it unlikely that this deformation was synchronous across the subprovinces.Widespread upright folding of the overturned limbs of F1 folds produced moderately to gently plunging F2 folds with east–west-trending axial planes. F, folds, with an associated L, stretching lineation subparallel to fold hinges, are well developed along the southern and northern margins of the Quetico subprovince and in the metasediments of the adjacent Wawa subprovince. During this event, ductile dextral shear was concentrated in steeply dipping east–west-trending shear zones in the Wawa subprovince and in the region of the Rainy Lake – Seine River fault along the Quetico–Wabigoon subprovince boundary. In the northern Wawa subprovince, shear was strongly concentrated in relatively incompetent, steeply dipping metasedimentary and tuffaceous units interlayered with more competent greenstone units. Concentrated zones of ductile shear are not evident within the Quetico subprovince away from its boundary regions. However, emplacement of syntectonic plutons in the central Quetico reoriented F2 folds which were then refolded by large regional F3 folds during continued regional shortening.

Deformation at an Archean subprovince boundary, northern Minnesota

Structural analysis in the northern margin of the Quetico subprovince (part of the Archean Superior Province of the Canadian Shield) in Minnesota reveals that the main deformation involved polyphase folding (F1 recumbent and nappe-like, and F2 upright, east–west trending, and tight to isoclinal) during regional ductile transpression and amphibolite-facies metamorphism. A younger deformation, developed during the latter stages of regional transpression, resulted in the generation of localized ultraphyllonites along the steeply dipping Rainy Lake – Seine River fault (RLSRF), the major fault separating the Quetico subprovince from the Rainy Lake wrench zone (a wedge-shaped block between the Quetico and Wabigoon subprovinces). The transpression involved north–south shortening and east–west dextral shear. The presence of shear zones in amphibolite-facies wall rock south of the fault and in lower grade ultraphyllonites within the RLSRF suggests that localization of shear occurred by work and (or) reaction softening, possibly enhanced by the influx of fluids during regional cooling. The youngest structures in the wall rock are conjugate brittle faults oriented similarly to the youngest ductile shear structures in the RLSRF, indicating that the zone of transpression widened following the stage of strain localization, possibly due to work hardening during continued regional cooling. Widening of the zone of deformation was accompanied by an increase in the relative intensity of the north–south shortening component of transpression, revealed by chloritized necks of boudinaged quartz ribbons, quartz and calcite microfabrics, and flattening strains. Protracted ductile flow and localized greenschist-facies conditions in the RLSRF, which occurred during widening of the zone of deformation and rotation of the kinematic frame (to produce north–south shortening structures), are best explained by an influx of fluid phases.Mesostructures and quartz microfabrics in late tectonic (but synkinematic) peraluminous leucogranitoid intrusions and host schist 10 km south of the RLSRF record north–south shortening, but not east–west dextral shear, and further support late north–south shortening across the RLSRF.Tectonic settings for the RLSRF include (i) a suture between distinct lithotectonic terranes or (ii) a zone of localization of deformation within the northern margin of the Quetico subprovince following collision between the Quetico and Wabigoon terranes.