Southern Superior Province Research Articles

Testing the TTG–Metabasite Connection in the Southern Superior Province: an Integrated Geochemical, Isotopic, and Petrogenetic Modelling Approach

Abstract Archean cratons are dominated by tonalite–trondhjemite–granodiorite (TTG) suites, the products of crustal differentiation that formed early continental crust. These rocks may have been primarily generated by partial melting of hydrated basaltic crust in a variety of settings including subduction zones or the deep lithosphere. Sources are mainly inferred from examination of TTGs alone, as potential source rocks are rarely exposed. In the southern Superior Province, Canada, the Kapuskasing Uplift exposes an important crustal cross-section with upper- to middle-crustal TTGs and lower-crustal metabasites, which show evidence of having produced trondhjemitic anatectic melt. Here, we test the hypothesis that these metabasites were the source of the Mesoarchean to Neoarchean TTGs in the Kapuskasing Uplift by combining phase equilibrium and melt trace element modelling with whole-rock and zircon Lu–Hf isotope analysis and geochronology (garnet Lu–Hf and zircon U–Pb) of metabasic samples. By comparison of the results with existing data from TTGs in the Kapuskasing Uplift, we determined that the metabasites are plausible source rocks for the TTGs. The Lu–Hf systematics of the metabasites and TTGs are the most robust evidence of a genetic connection. Modelling results support an increase in TTG source depth over time. New geochronological data constrain partial melting of metabasite and crystallization of anatectic melt between ca. 2685 Ma and ca. 2600 Ma, coeval with crystallization of only the youngest TTGs. Overall, these results indicate a complex history of intracrustal differentiation in the Kapuskasing Uplift, with partial melting of two isotopically distinct lower-crustal metabasic sources at different times and depths.

Non-zero I/(Ca + Mg) recorded in Archean and Paleoproterozoic shallow marine Ca-carbonate sediments

Marine oxygenation during the Archean and Paleoproterozoic is poorly constrained. Iodine in carbonate is a possible oxygen indicator. We obtained molar ratios of iodine to calcium-magnesium [I/(Ca + Mg)] for 2.93–2.31 Ga from shallow marine Ca-carbonate (limestone, dolostone) platform sediments at several locations in the southern Superior Province in Canada and the USA. Maximum I/(Ca + Mg) ratios obtained are 0.13, 0.26, 0.05 and 0.22 μmol/mol, for 2.93, 2.86, 2.80 and 2.31 Ga samples, respectively. These non-zero carbonate I/(Ca + Mg) ratios indicate localized oxygen in Archean and Paleoproterozoic shallow marine environments. These data support an early origin for biological oxygen-production and, specifically, the idea that mid-late Archean oxygenation was sufficient to facilitate Ca-carbonate precipitation in shallow marine ‘oxygen oases’ adjacent to anoxic iron-rich seas

Anatomy of a craton: Isotopic heterogeneity across an Archean crustal cross-section

The formation of granitoids, predominantly tonalite-trondhjemite-granodiorite (TTG) suites was a key process in the differentiation of Earth’s earliest crust. However, the relative importance of intracrustal reworking versus input from external sources remains unclear. U–Pb and Hf isotopic data collected from zircon in Archean granitoids frequently support reworking and partial melting of a common crustal reservoir over time to produce subsequent generations of melt. A less frequently observed isotopic signature, in a single region, implies the production of different generations of TTGs from unrelated sources. These end-member scenarios of granitoid generation have quite different implications for Archean crustal evolution. Here, we investigate TTGs from across a crustal cross-section known as the Kapuskasing Uplift in the Neoarchean Wawa terrane of the southern Superior Province. We report U–Pb and Hf isotopic data from zircon in TTGs from the lower, middle, and upper crust to evaluate their sources. Results indicate three groups of TTGs: a ca. 2885 Ma upper-crustal TTG with chondritic εHf, ca. 2835–2820 Ma lower-crustal TTGs with subchondritic εHf, and ca. 2750–2685 Ma TTGs with suprachondritic εHf emplaced at middle- to upper-crustal levels. Each group was derived from an isotopically distinct source. We propose that the Mesoarchean rocks exposed in the Kapuskasing Uplift represent a crustal fragment rifted from the margin of existing Mesoarchean terranes in the Superior Province. Subsequent Neoarchean juvenile magmatism dominates the upper and middle crust resulting in isotopic stratification of the Kapuskasing Uplift, as only the lower crust contains evidence of all three isotopic sources. Inferences about Archean crustal evolution in general rely on upper- to middle-crustal rocks (i.e., greenstone belts and grey gneiss), as these represent the majority of exposed Archean crust worldwide. The potential for the lower crust to be decoupled from overlying rocks implies that some parts of the Archean geological record may be preserved only at the deepest crustal levels—particularly rocks related to the most ancient sources.

Continent‐scale sediment dispersal for the Proterozoic Baraboo Interval quartzites in the Laurentian midcontinent

AbstractThe well‐known Baraboo Interval quartzites in the Laurentian midcontinent are a succession of mature red siliciclastic strata, locally more than 1.5 km thick, and covering an area >300,000 km2, much if not all of which was deposited after the Mazatzal orogeny (<1.63 Ga), and before the Baraboo Orogeny (~1.47 Ga). Ca. 1,300 new LA‐ICPMS detrital zircon ages from six Baraboo Interval inliers in Wisconsin and the Sioux Quartzite in South Dakota, USA attest to significant distal provenances not previously recognized. The principal ~1.8–1.9 Ga and >2.5 Ga age peaks represent local, southern Superior province sediment source areas. Yet more than 30% of our dataset includes ages between 1.9 and 2.5 Ga which are essentially absent from the region. We interpret these non‐locally derived zircons to have been transported ~2,500 km southeast to the Columbian supercontinental margin. The subsequent accretion of the Mazatzal terrane caused a major drainage reorganization, and transport of sediment for the uppermost Baraboo interval strata was north rather than south‐directed.

Structural and geochronological constraints on orogenic gold mineralization in the western Wabigoon subprovince, Canada

Some of the world’s largest orogenic gold deposits are hosted within granite-greenstone subprovinces of the Superior Province of Canada. The known gold endowment of the western Wabigoon subprovince is much less than that of the Abitibi subprovince, despite both Neoarchean subprovinces sharing similar structural and metallogenic histories. Further, the relationship between regional deformation and orogenic gold mineralizing events in the western Wabigoon subprovince is poorly constrained in comparison with other greenstone belts within the Superior Province. New structural mapping of the Dryden area of the western Wabigoon subprovince indicates that it experienced a N–S shortening event (D1), circa 2705–2695 Ma, which reactivated, or initiated the formation of, the E–W-trending Wabigoon and Mosher Bay – Washeibemaga deformation zones (Wdz and MBWdz, respectively). Following the D1 event, after 2695 Ma, a protracted period of NNW–SSE shortening (D2) localized transpression along the Manitou Dinorwic deformation zone (MDdz), which is spatially associated with orogenic gold systems, including the Goldlund deposit (∼1.7 Moz Au) and Kenwest prospect (∼0.3 Moz Au). During D2, the MDdz experienced NW-over-SE sinistral transpression, while the Wdz and MBWdz experienced N-over-S dextral-sense movement. U–Pb geochronology of vein-hosted xenotime from the Goldlund deposit and Kenwest prospect indicate that the main hydrothermal event, and likely gold mineralization, occurred syn- to late-D2 deformation around 2664 ± 8.3 Ma. A second population of xenotime, and overgrowths, recorded a younger hydrothermal overprint between ∼2590 and ∼2580 Ma. These ages are comparable with other absolute geochronology data on hydrothermal events in the gold-rich Abitibi subprovince. The structural evolution and timing of orogenic hydrothermal events in the western Wabigoon subprovince are broadly similar to the Abitibi subprovince. Therefore, we propose that the structural evolution and timing of individual greenstone belts is not the primary controlling factor on their bulk gold endowment. Nevertheless, orogenic gold deposits are spatially associated with major D2 deformation zones and linked to widespread hydrothermal event(s), spanning 2680–2660 Ma across the southern Superior Province.

Diversification of Archean tonalite-trondhjemite-granodiorite suites in a mushy middle crust

Abstract Tonalite-trondhjemite-granodiorite (TTG) suites are the dominant component of Earth's first continents, but their origins are debated. The trace element concentrations of these rocks are conventionally linked to their source depths and inferred geodynamic settings with the implicit assumption that TTG compositions are source-controlled. Alternatively, their variable compositions may be caused by fractional crystallization in TTG plutons after emplacement and less clearly linked to source depth. Most TTGs in Archean mid-crustal exposures are the dominant component of igneous gray gneiss complexes; the processes that influence the evolution of TTG magmas in this setting are poorly understood. We present a petrological–geochemical model that explains the coexistence of TTGs in the middle crust with low-pressure and high-pressure geochemical trends, irrespective of tectonic setting or depth of the TTG source. We propose that mid-crustal TTGs were long-lived crystal mushes with compositions controlled by the separation of early-crystallizing plagioclase and melt. Using phase equilibrium modeling, we demonstrate that a suite of TTGs from the southern Superior Province in Canada represents variably plagioclase-rich and melt-rich fractions from a common parent magma. The behavior of plagioclase may have a strong influence on the geochemical trends of TTGs, including the degree of rare earth element fractionation. Our results suggest that trace element compositions of TTGs may not primarily reflect the depth of the source and cannot be used alone to infer Archean geodynamic settings.

Mapping structural complexity using geophysics: A new geostatistical approach applied to greenstone belts of the southern Superior Province, Canada

With the increasing size and complexity of geoscientific databases, statistical tools and machine learning algorithms are needed to highlight subtle patterns within dense data clouds and reduce human-related bias in geological mapping. This study presents a new tool that maps structural complexity using circular variance and spherical dispersion in two Archean greenstone belts near Dryden and Timmins in the Superior Province, Ontario. Structural complexity quantitatively highlights areas where structures are folded, juxtaposed, and/or parallel. Bedding measurements and autodetected aeromagnetic lineaments are used as inputs to test if structural complexity can refine or add confidence to existing geological interpretations. Linear, low variance trends frequently delineate the location and extent of deformation zones, which likely reflect parallel transposed fabrics in high strain corridors. Areas of high variance spatially correlate with fold traces. Sharp boundaries between high and low variance anomalies are interpreted as faults and/or shear zones that juxtapose structural blocks. The near continuous and representative nature of autodetected magnetic lineaments was effective in capturing structural complexity at a regional scale; however, spherical dispersion captured the highest resolution of structural complexity at local scales (<1:30000). Key considerations when performing structural complexity analysis are scale, data density, and optimization of input parameters (e.g., neighborhood radius, lineament length, minimum neighborhood populations). Gold deposits are found <~1700 m from low (< μ - 1σ) and < ~1500 m from high circular variance anomalies (> μ + 1σ). Additionally, the gold grade of large deposits (> 1 Moz) increases with proximity to high variance anomalies. When paired with expert knowledge, these techniques will increase repeatability in future exploration endeavors, making exploration a more rigorous process with increased confidence.

Geophysical insights into Paleoproterozoic tectonics along the southern margin of the Superior Province, central Upper Peninsula, Michigan, USA

The southern margin of the Archean Superior Province in the central Upper Peninsula of Michigan was a nexus for key Paleoproterozoic tectonic events involved in the ~2.1 Ga rifting of proposed Archean supercraton Superia and subsequent assembly of Laurentia. Interpretations of the region’s tectonic history have historically been hampered by extensive Pleistocene glacial and Paleozoic sedimentary cover and a lack of appropriate geophysical data. These rifting and orogenic events formed geologic effects that are readily mappable with modern geophysical methods. New aeromagnetic and gravity data provide a critical means of mapping and interpreting the complex geological framework through cover, allowing development of significantly richer geographical and process-based perspectives on all these tectonic events. Interpretations of Precambrian contacts and structure are here, for the first time, carried >30 km eastward under Paleozoic cover. Effects of ~2.1 Ga rifting are strongly expressed geophysically, including the Dickinson Group, perhaps a unique record of the progression of rift-related sedimentation and magmatism, shown here to be a geographically extensive and largely concealed tectonic feature of the southern Superior Province. The geophysical evidence for plausible ~2.1 Ga rift-related intrusive magmatism includes a previously unrecognized swarm of northeast-striking mafic dikes cutting Archean rocks and gravity lows produced by granites. Effects of the ~1.87–1.83 Ga Penokean orogeny include gravity and magnetic gradients and pattern breaks along the Niagara fault zone suture, abundant evidence for thin-skinned thrusting and folding in the Menominee iron district, and speculative emplacement of an allochthonous sedimentary sequence in the Calumet trough. Numerous east–west trending structures imaged geophysically likely originated, or were significantly reactivated by, post-Penokean deformation. Metamorphic events at ~1.76 Ga and ~1.65 Ga may correspond to orogenies involving younger, outboard Paleoproterozoic crustal provinces recognized in southern Laurentia. For example, the previously unrecognized West Branch fault, separating the Dickinson Group from Archean rocks, is shown to be a major structure in the region, and is a proposed expression of ~1.76 Ga thick-skinned deformation. Oblique disruptions of crudely east–west striking structures have robust geophysical expressions and are speculatively connected to transpressive deformation at ~1.65 Ga. These new geophysical observations and interpretations collectively help illuminate a critical period in the tectonic evolution of Laurentia, as it transitioned from a disparate array of Archean cratons to a more coherent, growing continent.

Insight into Archean crustal growth and mantle evolution from multi-isotope U-Pb and Lu-Hf analysis of detrital zircon grains from the Abitibi and Pontiac subprovinces, Canada

Lu-Hf laser ablation – multi-collector – inductively coupled plasma – mass spectrometry (LA-MC-ICP-MS) analysis was conducted on ~1800 detrital zircon grains from successor basins of the Archean Abitibi and Pontiac subprovinces of Ontario and Quebec, Canada, and paired with previous U-Pb LA-MC-ICP-MS analyses of the same grains. Results are used to constrain the isotopic character of magmatic source domains of the zircon grains to establish the sedimentary provenance of the ~2690–2670 Ma successor basins, to provide constraints on terrane configurations and amalgamations at the time of basin formation, and to assess their significance for the record of crust-mantle growth in the region. The majority of results (95%) yield ɛHf values of + 1 to + 10 for ~2850–2675 Ma zircon, and clusters along compositions of the Archean depleted mantle (DM), which is based on projections of modern MORB compositions. Subordinate results, comprising ~2% of the data set, yielded values (ɛHf > +10) corresponding to extremely depleted mantle compositions, reflecting anomalously depleted sources in the ~2950–2670 Ma age range. The remaining 3% correspond to chondritic uniform reservoir (CHUR)-like to negative ɛHf values that reflect primitive sources and/or evolved magmas in zircon that crystallized in the ~3250–3050 Ma and ~2950–2670 Ma age ranges. While Neoarchean grains dominate the data set (~88%), approximately 12% are Mesoarchean. The Lu-Hf data collected on these zircon grains, when compared with published isotopic results, preserve signatures indicative of derivation from exotic crustal domains juxtaposed during ~2690–2670 Ma amalgamation of the southern Superior Province. Since depleted compositions are characteristic of Neoarchean and Mesoarchean zircon groups in the southern Superior Province, and sources include local and distal domains that were likely separated by many 100s of kilometers prior to amalgamation, it is inferred that a depleted upper mantle reservoir was not only well-established, but prevalent in the mantle below each of these areas during their construction. Based on the predominant Hf isotope signatures in the detrital zircon results and predicted isotopic trends produced by probable geodynamic mechanisms, crustal growth by direct differentiation from a depleted mantle reservoir is likely to have been moderated by subduction-accretion processes.

Regional and local controls on Archean rock-hosted cobalt mineralization at the McAra deposit, southern Superior Province, Ontario, Canada

The McAra deposit is in eastern Ontario, Canada, and is hosted in an Archean inlier to the Paleoproterozoic Huronian basin. It is currently estimated to contain ∼2.4 million pounds of cobalt at an average grade of 1.25%. New drill data show the mineralized zone comprises glaucodot–cobaltite veins and breccias that transect a mafic–siliciclastic volcanogenic massive sulfide (VMS) deposit. The high cobalt grade and host stratigraphy at the McAra deposit contrast with five-element (Ag–Co–Ni–Bi–As) deposits at the Cobalt and Gowganda camps in the region that produced high-grade silver and by-product cobalt from veins spatially associated with Nipissing Gabbro intrusions. However, geochemical data from recent core samples alongside fluid inclusion and mineralogical data suggest the cobalt zone at McAra and the five-element veins share a similar metal assemblage and were deposited from similar fluids. The mafic–siliciclastic VMS deposit at McAra contains anomalous amounts of cobalt, suggesting the Archean host stratigraphy was the source for the high-grade cobalt zone. Basin brines in the Paleoproterozoic are interpreted to have leached cobalt from Archean rocks and then redeposited it through oxidation–reduction reactions along synvolcanic faults that controlled earlier VMS deposit formation. High-resolution aeromagnetic data show that McAra is immediately adjacent to a mafic dike that transects the Huronian basin along a northwest-striking, crustal-scale fault system. These data, alongside observations from field mapping, also suggest the deposit is on the margin of a sub-basin that contains an 80 km2 Nipissing sill that may have originally overlain the deposit area and been a hydrologic seal during mineralization. The new deposit- and regional-scale data and interpretations are used to create a model for the McAra deposit and provide evidence for why it is cobalt-rich relative to other five-element veins. The model and data can be used to guide exploration for additional cobalt-rich deposits in the region and similar settings globally.

DEEP CRUSTAL STRUCTURE AND MODEL OF NEORCHAEAN EVOLUTION OF THE NORTH AMERICAN CRATON

The structure of the Archaean crust of the North America has been studied based on the synthesis of geolo‐ gical and geophysical data, including seismic sections along LITHOPROBE Geotransects, magnetic and gravity anomaly maps, and seismic tomography data. The authors rely on the experience gained in the Russian Program of the deep geological and geophysical studies of the East European Craton. The juvenile Neoarchaean crust, containing the frag‐ ments of reworked Meso‐ and Paleoarchaean rocks, forms an asymmetric round‐oval‐shaped domain, wherein the geophysical, structural, and metamorphic parameters display a concentric zoning pattern. The Central zone occupies the Hudson Bay basin. The Internal zone (the northeastern and northern Superior Province) is mainly composed of the granulite facies of metaplutonic, metavolcanic and metasedimentary rocks. The External zone encompasses the southern Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of sou‐ thern Superior Province. The crust development resulted from rifting and a partial disruption of the continental crust, short‐term opening of the linear oceans, successive northward subduction and accretion of the ancient continental and juvenile Neoarchaean oceanic and island‐arc terranes between ~2.78 and ~2.70 Ga. Subsequent events in the epicontinental environment, including formation of the metasedimentary belts, granulite facies metamorphism and intense ore formation processes, took place within the range from ~2.71 to ~2.63 Ga. The SCLM morphology within the limits of the Archaean North American Craton can be represented as a flattened overturned cone with a vertical axis (down to a depth of ~350 km). The Hudson Bay basin is located right above the lithospheric keel. A number of the main features of the structure and evolution of the Archaean crust of the North American Craton, primarily the oval‐ concentric zoning, the important role of high‐temperature magmatic and metamorphic processes and mainly in‐ tracontinental magmatism and sedimentation, indicates the leading role of the mantle‐plume type processes. The Neoarchaean evolution of the North American craton represents the plate‐tectonic processes initiated by a super‐ plume. The Neoarchaean North American Craton is one of a series of similar phenomena that occurred ~2.75 Ga ago in a number of continental regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58 Ga; (2) mainly intracontinental development; (3) the prevalence of oval‐shaped synformal tectonic structures of different ranks with some form of concentric zoning; (4) high‐temperature magmatism (usually with the participation of enderbite‐charnockites and gabbro‐anorthosites) and metamorphism of the granulite facies; (5) a frequently repeated combination of high‐grade (granulite and high‐ temperature amphibolites facies) and low‐ or moderate‐grade (greenschist and epidote‐amphibolite facies) meta‐ morphic rocks; (6) the lower‐crust granulite‐basaltic layer that had formed and was deformed at the final stage of endogenic activity; (7) a thick lithosphere (the lithospheric keel reaches a depth of 250–350 km).

Mica composition as a vector to gold mineralization: Deciphering hydrothermal and metamorphic effects in the Malartic district, Quebec

Canadian Malartic, with a total endowment of 16.3 Moz Au, is an important example of a large-tonnage, low-grade Archean gold deposit (current reserves of 204 Mt @ 1.08 g/t Au). It is located in the southern Superior Province in contact with, and immediately south of the east-west trending Cadillac-Larder Lake fault zone, which delineates the boundary between the Pontiac and Abitibi subprovinces. The deposit is hosted by Pontiac Group metaturbidites, Piché Group mafic-ultramafic metavolcanics and by porphyritic quartz monzodiorite to granodiorite that intruded these lithologies. The metamorphic grade increases southward from upper greenschist facies, which characterizes the immediate metamorphic environment of the Canadian Malartic deposit, to mid-amphibolite facies; the garnet isograd is crossed 1.5 km south of the deposit, and the staurolite isograd about one kilometer further south. Textural, structural and geochronological observations are consistent with a syn-kinematic (D2), early- to syn-peak metamorphic (M2) timing for the main ore-forming event.Mineralization in the metasedimentary rocks and porphyritic intrusions consists of disseminated native gold and minor gold-tellurides within stockworks of quartz-biotite-microcline-carbonate±pyrite veinlets (v2) and associated pervasive microcline-albite-biotite±(white mica)-carbonate-pyrite alteration. A zonal distribution of alteration is centered on structures that acted as preferential pathways for the hydrothermal fluids (e.g., Sladen Fault, NW-SE deformation zones). In the metasedimentary rocks, the proximal potassic alteration zone is dominated by microcline-albite±quartz with variable proportions of phlogopite (±white mica), carbonate minerals (calcite-ankerite±Fe-dolomite), quartz, pyrite and rutile. It grades outwards into a distal potassic-sericitic alteration zone characterized by relatively abundant Mg-rich biotite and phengitic white mica, as well as microcline, albite, quartz, calcite, pyrite and rutile. The zonal distribution of alteration features was due in part to a decrease in the total activity of sulfur species (∑aS) and oxygen fugacity (fO2) away from the main hydrothermal corridors, which is manifested by systematic changes in Fe-sulfide and (Fe-Ti)-oxide mineralogy.The effects of sulfidation (∑aS), oxidation (fO2) as well as K+, Fe2+ and H+ activity on biotite and white mica compositions were assessed from silicate-oxide-sulfide equilibria. Increasing ∑aS-fO2 conditions proximal to the hydrothermal fluid pathways caused pyrite to be stabilized over biotite, and iron sequestration in the sulfide phase in turn promoted the stability of magnesian biotite. Increasing sulfur metasomatism towards the hydrothermal centers was associated with systematic increases in biotite Mg# [Mg/(Fetotal+Mg)] and fluorine concentration, in agreement with the Fe-F avoidance principle. Alteration was also associated with a decrease in Al in biotite and white mica, coincident with increases in Si and Fe + Mg concentrations. These compositional trends were controlled by a Tschermak exchange reaction and are consistent with a gradual decrease in aK+ and/or pH upon progressive rock-buffering of a mildly alkaline, potassium-rich ore-forming fluid.Multicomponent phase equilibria (pseudosections), constrained for specified bulk rock compositions typical of non-mineralized Pontiac Group metasedimentary rocks, are used to model the expected composition of mica solid solutions under varying metamorphic P-T conditions. The thermodynamic modelling of biotite and white mica compositions suggests that the district-scale compositional trends documented for biotite and white mica along the metamorphic gradient, including the progression of the Tschermak substitution towards more aluminous compositions southwards, were likely the product of increasing metamorphic P-T conditions, rather than a distal effect of hydrothermal alteration.Mica mineral chemistry is clearly a sensitive indicator of hydrothermal and metamorphic processes at Canadian Malartic. Our results show that biotite and white mica compositions provide valuable tools that can be used to define hydrothermal fluid pathways in and around the deposit. These factors should be particularly useful for mapping the zonation of alteration features that characterize the footprint of major gold deposits in metamorphic terranes where micas typically display a spatial distribution ideal for defining mineral-chemical vectors to ore.

The composite North American Craton, Superior Province: Deep crustal structure and mantle-plume model of Neoarchaean evolution

This paper aims to synthesize geological and geophysical data on Superior Province and, in general, Archean crust of the composite North America Craton, relying on experience with deep geological and geophysical studies of composite East European Craton. By onset of the Neoarchean, Archean provinces of the North American Craton were parts of the single stable continent. Intensely reworked in the Neoarchean, crustal rocks were combined into a round-oval domain, wherein distribution of geophysical, structural, and metamorphic parameters displays concentric zoning. The Central zone occupies Hudson Bay basin located immediately above the lithospheric keel. The Internal zone is mainly composed of granulite facies plutonic, volcanic and sedimentary rocks. The External zone encompasses southern part of the Superior Province together with Hearne and Rae Provinces. This paper presents 3D crustal models of southern Superior Province obtained through synthesis of geological maps with a new geological interpretation of seismic sections along LITHOPROBE Geotransects and of the eastern Abitibi Belt based on higher resolution reflection seismic images. Neoarchean evolution of the North American craton represents plate-tectonic processes initiated by a superplume. It is only one of series of similar phenomena recorded ∼2.75Ga ago in a number of regions. The most important features, repeated to a certain degree in tectonic units of this type, are: (1) synchronous formation between 2.79 and 2.58Ga; (2) frequently repeated combination of high-grade (granulite and high-temperature amphibolite facies) and low- or moderate-grade (greenschist and epidote–amphibolite facies) metamorphic rocks; (3) lower crustal granulite-basaltic layer (reflectivity zone displayed in seismic images of crust) formed or transformed at the final stage of endogenic activity.

Constraints on the geodynamic evolution of the southern Superior Province: U-Pb LA-ICP-MS analysis of detrital zircon in successor basins of the Archean Abitibi and Pontiac subprovinces of Ontario and Quebec, Canada

To investigate the tectonic development of the southern Superior Province, new U-Pb laser ablation inductively-coupled mass-spectrometry analysis of detrital zircon was conducted on graywacke samples from successor basins of the Abitibi and Pontiac subprovinces. The three successor basin successions investigated are the 2690–2685Ma Porcupine assemblage and the 2679–2669Ma Timiskaming assemblage of the Abitibi subprovince, and the ∼2682Ma sedimentary rocks of the Pontiac subprovince. Predominately Neoarchean zircon grains (80–95% of the total zircon population) reflect local sources in the Abitibi and Pontiac subprovinces. The successor basins were deposited within 100Ma of the majority of the detrital zircon grains primary crystallization ages, which is consistent with patterns observed at modern convergent margin settings. Porcupine assemblage samples contain ∼5% Mesoarchean zircon, while Pontiac subprovince and Timiskaming assemblage samples contain ∼18% and ∼13%, respectively. No local Mesoarchean sources exist in the Abitibi or Pontiac subprovinces, suggesting input from an adjacent hinterland. The higher proportion of Mesoarchean zircon in the Timiskaming assemblage relative to the Porcupine assemblage suggests that detritus from the hinterland was more prevalent during the later stages of collision, probably as a result of progressive uplift and denudation of the hinterland. The high proportion of Mesoarchean zircon in the Pontiac subprovince sedimentary rocks may reflect an additional Mesoarchean source. Comparison to previously published geochronology data for the Superior Province indicates that the hinterland was to the northwest and is, in part, composed of the Winnipeg River, Marmion, and Opatica subprovinces. Patterns of contractional deformation, successor basin formation, and felsic plutonism progressively young to the south in the southern Superior Province and are interpreted in terms of accretionary processes. Detrital transport across subprovince or terrane boundaries, progressive hinterland emergence, and foreland-directed propagation of the deformational front recorded by successor basins is similar to that observed in modern-style collisional or accretionary orogens.

Crustal and lithospheric scale structures of the Precambrian Superior–Grenville margin

Abstract The lithosphere beneath the margin of the Archean Superior and Proterozoic Grenville provinces was investigated with a northwest–southeast oriented, 650-km-long profile of 40 magnetotelluric stations. Dominant geoelectric strike azimuths of N45°E and N85°E were defined for the crust and the lithospheric mantle respectively. A 2-D isotropic resistivity model derived using the crustal strike images resistive Laurentian margin rocks dipping southeast to the base of the crust, bounded to the northwest by the Grenville Front, and to the southeast by the Central Metasedimentary Belt Boundary Zone. The observation is in contrast to conductive mid to lower crust elsewhere in the region. A 2-D isotropic resistivity model determined using the lithospheric mantle strike azimuth reveals an extremely resistive region in the upper 100 km of the mantle lithosphere of the northern Grenville Province. The geometry of this body, which includes a well-defined base and southeast dip, suggests that it is Superior lithosphere. A sub-vertical conductor, located approximately 50 km along strike from the Mesozoic Kirkland Lake and Cobalt kimberlite fields, is interpreted to be due to re-fertilization of an older mantle scar. The resistivity model includes a horizontal conductor at 160 km depth beneath the southern Superior Province that is possibly the resistivity signature of the lithospheric–asthenospheric boundary.

Detrital Zircon Geochronology and Provenance of the Paleoproterozoic Huron (∼2.4–2.2 Ga) and Animikie (∼2.2–1.8 Ga) Basins, Southern Superior Province

The Archean nucleus of Laurentia, the Superior province, formed between 2.7 and 2.6 Ga. There is little record of sediment deposition on the craton for 100 m.yr. until deposition of the Huronian clastic sedimentary rocks (4–12 km thick) began along the south-central margin of the Superior province between 2.5 and 2.2 Ga. Huronian detrital zircon ages ( zircon SHRIMP 207Pb/206Pb ages from six samples) are dominantly 2.6–2.7 Ga, with a small population aged 3.0–3.5 Ga in the middle of the Huron section. The mainly younger, and western, Animikie basin (6–12 km thick; 2.21–0.8 Ga) contains a wide range of nearshore and deep-water clastic rocks, including banded iron formations and the 1.85 Ga Sudbury “impactite” deposits with overlying turbidites. Animikie detrital zircon ages ( laser ablation ICP-MS 207Pb/206Pb zircon ages from 10 samples) contain a complex spectrum, with ages ranging from 1.8 to 3.8 Ga: the basal Chocolay (Denham, East Branch Arkose) samples contain zircons ranging in age from 2.1 to 3.7 Ga, with a dominant population at ∼2.6 Ga; the basal Animikie (Menominee) quartzites contain zircons with ages between 2.1 and 3.8 Ga, with peaks variably at 2.1, 2.3, 2.6, and 3.0 Ga; the upper Animikie (Baraga) turbidites contain zircons with ages between 1.8 and 3.6 Ga, with a dominant peak at 1.85 Ga. The Huron and Animikie basins were intensely deformed by Penokean orogenic (1.85 Ga) accretion from the south. Deposition into the Huron basin was from the north and west for 200 m.yr.; then the depocenter shifted to the west at ∼2.2 Ga, with the Animikie basin receiving clastic debris from all directions, including from terranes (e.g., Marshfield and the Wisconsin Magmatic Terrane) approaching from the south. Combined Huron and Animikie detrital zircon results record a migrating east-to-west depocenter, perhaps in conjunction with the dynamics of the central Marathon large igneous province mantle plume and dike swarm (∼2.07 Ga), over a period of 500 m.yr.

Lower-Crustal Xenoliths from Jurassic Kimberlite Diatremes, Upper Michigan (USA): Evidence for Proterozoic Orogenesis and Plume Magmatism in the Lower Crust of the Southern Superior Province

Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/204Pb vs 207Pb/204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/204Pb vs 206Pb/204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a unique peraluminous composition. It has the lowest eNd and eHf values of the suite. Its isotopic compositions indicate that it is significantly older than the other granulites. Broken zircon cores encased by younger overgrowths suggest that this granulite includes a large component of pre-existing sedimentary rocks. Two distinct populations of zircons from S69-5 were dated by sensitive high-resolution ion microprobe. Abundant rounded zircons yield ages of 1104 ± 42 (2σ) Ma, which coincide with the Mid-Continent Rift flood basalt eruptions. Their morphology is similar to those found in lower-crustal rocks that have undergone granulite-facies metamorphism and thus they are considered to represent the age of Group 2 granulites. Also present are less abundant elongate zircon grains that yield a mean age of 1387 ± 32 (2σ) Ma. Their elongate shapes indicate growth from a melt or fluid, possibly associated with 1·3–1·5 Ga anorogenic granite magmatism exposed in the shallow crust to the south in Wisconsin, or related to an initial encroachment of the Keweenawan plume upon the lower crust. Older ages recognized in zircon cores are less well constrained but may be related to tectono-magmatic events in the southern Superior craton. Within the studied suite only S69-5 was recognized as a remnant of the Late Archean lower crust into which the Group 1 and 2 mafic granulite precursor basalts were intruded. Collectively, the data show that the lower crust beneath northern Michigan formed in Archean times and underwent a variety of tectono-magmatic processes throughout the Proterozoic, including orogenesis, partial melting and mafic magmatic underplating in response to upwelling mantle plumes.

Vanished diamondiferous cratonic root beneath the Southern Superior province: evidence from diamond inclusions in the Wawa metaconglomerate

We studied diamonds from a 2.697–2.700 Ga Wawa metaconglomerate (Southern Superior craton) and identified mineral inclusions of high-Cr, low-Ca pyrope garnet, low-Ti Mg-chromite, olivine (Fo93), and orthopyroxene (En94). The diamonds have δ13C of −2.5 to −4.0 ‰ and derive from the spinel-garnet and garnet facia of harzburgite. Geothermobarometry on non-touching, coexisting garnet-olivine and garnet-orthopyroxene pairs constrains the maximum geothermal gradient of 41 mW/m2 for the Neoarchean and a minimum lithosphere thickness of 190 km. The depleted harzburgitic paragenesis equilibrated at a relatively cold geotherm suggests the presence of a pre-2.7 Ga diamondiferous cratonic root beneath the northern Wawa terrane or the Opatica terrane of the Southern Superior craton, i.e., beneath terranes identified as sources for the metaconglomerate diamonds. Geophysical surveys, geothermal data, and petrology of mantle xenoliths emplaced in the Proterozoic-Mesozoic trace evolution of the mantle thermal regime and composition from the Archean to present. The root was thinned down to 150 km by the Jurassic, when the mantle was heated to 41–42 mW/m2. The diamondiferous root destruction was accompanied by more significant heating and was complete by 1.1 Ga in areas adjacent to the Midcontinent Rift. The geometry of the current high-velocity root and spatial correlations with boundaries of crustal terranes that docked to the nuclei of the Superior protocraton in the Neoarchean suggest that the root destruction in the Southern Superior may have been associated with tectonic erosion, craton amalgamation, and ensuing ingress of asthenospheric fluids.

Mantle plume – volcanic arc interaction: consequences for magmatism, metallogeny, and cratonization in the Abitibi and Wawa subprovinces, CanadaThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

The Abitibi and Wawa subprovinces of the southern Superior Province differ in terms of the extent of pre-existing 2750 Ma sialic crust and relationships between mantle plume type (tholeiitic basalt – komatiite) and arc type (tholeiite to calc-alkaline basalt – andesite – dacite – rhyolite) volcanic successions but evolved in close proximity to each other. Isotopic data, evidence from the Kapuskasing uplift, continuation of major structures associated with large gold deposits from the Abitibi into the Wawa subprovince and the related occurrence of diamonds in lamprophyric rocks in both subprovinces point to a common evolution prior to and during orogeny. Differences preserved in supracrustal sequences of the two subprovinces suggest that the main petrogenetic controls on orogenic gold deposits and lamprophyre-hosted diamond deposits lay in the lower crust and upper mantle. Similar processes must also have been active where gold and diamonds are associated on other Archean cratons, such as the Slave and possibly the Kaapvaal craton. Based on evidence preserved in the Abitibi–Wawa orogen, rapid crustal growth at ∼2.7 Ga was linked to the interplay between plate tectonics and mantle plumes. Key indicators in the model developed for the Abitibi–Wawa arc are the co-existence of plume-related rock types, modern-style adakites, major gold deposits, and lamprophyre-hosted diamond occurrences, at least in cases where shoshonitic host magmas can ascend rapidly through the crust. All of these indicators are now identified on the Kaapvaal craton by 3.1 Ga and many recur together in Paleoproterzoic and younger terranes, suggesting a common mechanism for rapid crustal growth through much of Earth’s history. The variety of granitoid types found within the Abitibi–Wawa orogen demonstrates that local tectonic factors, rather than a hotter average upper mantle, were important in controlling the type of magmas formed. Based on the geodynamic history of the subprovince, mantle plume interaction with an existing volcanic arc and the subduction of oceanic plateau crust played an important role in the formation of magmas similar to Cenozoic adakites. Flat subduction beneath a mantle wedge was probably responsible for the generation of the adakites and also promoted diamond stability at shallow depths while enhancing the reservoirs for subsequent orogenic gold deposits. The history of magmatism and mineralization in the Abitibi and Wawa subprovinces precludes an early or gradual development of a cratonic keel, which instead must have coupled with crust during the latest stages of orogeny.

In Situ U/Pb Granulite‐Hosted Zircon Dates, Kapuskasing Structural Zone, Ontario: A Late Archean Large Igneous Province (LIP) as a Substrate for Juvenile Crust

Abstract A geochronological study of the Kapuskasing structural zone (KSZ), a section of Late Archean lower crust within the southern Superior Province, Canadian Shield, was undertaken to test a plate tectonic model for the Late Archean Abitibi‐Opatica terrane as a modified large igneous province (LIP). New geochronological data allow us to interpret (1) construction of the LIP in an environment where inherited zircons were derived from an older sialic crust and preserved in the granulite‐grade lower crust and (2) high‐grade metamorphism in the lower crust related to crustal refinement of the LIP. U/Pb and Pb/Pb dates were obtained by LA‐ICP‐MS with beams of 8–30 μm. The study yielded 21 interpretable zircon dates spanning 500 m.yr. from one mafic granulite and two garnetiferous metatonalites. Nine of the dates are significantly older than the oldest known magmatic event recorded in the upper crust of the Abitibi‐Opatica terrane and indicate >2760‐Ma proto‐crustal material in the KSZ lower crust. The rema...