John A. Percival

Archean crust as revealed in the Kapuskasing uplift, Superior province, Canada

In the central Superior province of the Canadian Shield, a 120-km-wide transition from the low-grade Michipicoten greenstone belt to the high-grade Kapuskasing structural zone represents an oblique section through some 20 km of Archean crust, uplifted along a northwest-dipping thrust fault. The restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries: (1) 0 to 25 km, a high-grade heterogeneous gneissic assemblage, in part older than the upper supracrustal succession.

Heat production in an Archean crustal profile and implications for heat flow and mobilization of heat-producing elements

We have measured concentrations of heat producing elements (Th, U, and K) in 58 samples representative of the main lithologies in a 100 km transect of the Superior Province of the Canadian Shield, from the Michipicoten (Wawa) greenstone belt, near Wawa, Ontario, through a domal gneiss terrane of amphibolite grade, to the granulite belt of the Kapuskasing Structural Zone, near Foleyet. This transect has been interpreted as an oblique cross section through some 25 km of crust, uplifted along a major thrust fault, and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of Archean age. Mean heat production values for these terranes, based on aereal distribution of major rock types and calculated from their Th, U, and K concentrations are: Michipicoten greenstone belt = 0.72 μW m−3; Wawa domal gneiss terrane (amphibolite grade) = 1.37 μW m−3; Kapuskasing granulites = 0.44 μW m−3. Among the silicic plutonic rocks (tonalites, granites, and their derivative gneisses), the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite, sphene, zircon, and apatite. We interpret these variations as primary (pre-metamorphic). The relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late-stage Th-, U-, and K-rich granitoid plutons. These may have been derived from the underlying Kapuskasing granulite terrane, leaving it slightly depleted in heat producing elements. Transport of Th, U, and K, therefore, could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids. This conclusion is supported by the lack of a statistically significant difference in heat production between tonalites, tonalite gneisses and mafic rocks of amphibolite versus granulite grade. The pre-metamorphic radioactivity profile for this crustal section is likely to have been uniformly low, with a mean heat production value less than 1 μW m−3. This result is distinctly different from measured profiles in more silicic terranes, which show decreasing heat production with depth. This implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes, and may be an important factor in selective reactivation of lithologically different terranes, possibly resulting in preferential stabilization of basic terranes in the geological record. Our results indicate that a previously determined apparently linear heat flow-heat production relationship for the Kapuskasing area does not relate to the distribution of heat production with depth. Low, but significant heat production, 0.4–0.5 μW m−3, continues to lower crustal depths with no correlation to the depth parameter from the linear relationship. This low heat production may be a minimum average granulite heat production and suggests that, in general, heat flow through the Moho is 8–10 mW m−2 lower than the reduced heat flow calculated from the heat flow-heat production regression.

Minto block, Superior province: Missing link in deciphering assembly of the craton at 2.7 Ga

Plate-tectonic models of the Superior province are rooted in the granite-greenstone and metasedimentary belts of the southern part of the craton. North-striking domains of the Minto block in the northeastern part of the province evolved at similar times, in 3.1-2.8, 2.725, and 2.69 Ga events, requiring expansion of models of late Archean assembly to accommodate Minto geology. Western and eastern protocratons (∼3.1-2.8 Ga) rifted at ∼2.79 Ga to produce an ocean basin that was mostly consumed by subduction at 2.725 Ga. The Leaf River plutonic suite of cale-alkalic hornblende + biotite ± orthopyroxene ±clinopyroxene granodiorite represents magmatic arcs built on the protocratons, whereas the intervening Goudalie domain—containing fault-bounded fragments of rifted continental crust, rift volcanics, primitive oceanic crust, 2724 Ma island-arc rocks, and a <2718 Ma back-arc assemblage—marks the suture. Terminal collision at ∼2.7 Ga led to thickening and crustally derived granitoid magmatism. The southern Superior province also experienced vigorous activity between 2.725 and 2.69 Ga as island arcs, oceanic plateaus, continental fragments, and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites. A northern proto-Superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west-northwest-directed subduction; orthogonal convergence in the east produced wide plutonic arcs, in contrast to terrane-accretion tectonics in the more oblique regime along the southern margin.

Late Archean Quetico accretionary complex, Superior province, Canada

The 1200-km-long Quetico subprovince consists of monotonous metagraywacke, with derived migmatite and granite, in thrust and/or transcurrent fault contact with the adjacent Wabigoon and Wawa metavolcanic subprovinces. Imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10-100-km-wide, stratigraphically north-facing pile. Thermal relaxation in the 20 m.y. following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low-pressure metamorphism.

Crustal growth through successive arc magmatism: reconnaissance U–Pb SHRIMP data from the northeastern Superior Province, Canada

Abstract North of the La Grande subprovince in northern Quebec, the Superior Province is dominated by plutonic rocks of the Bienville subprovince and Minto block, which have been further subdivided into domains on the basis of reconnaissance geological mapping and geochronology, and projected along strike using aeromagnetic anomalies. To test the validity of the extrapolations, U–Pb ages were obtained through SHRIMP analyses of zircon from 18 small archival samples selected from a suite of ∼2000 specimens collected during a helicopter reconnaissance survey conducted in the 1960s. The results reveal a history of successive arc magmatism between 3.07 and 2.70 Ga, followed by metamorphism, emplacement of crustally-derived granites (2.702–2.685 Ga) and late hydrothermal events (ca. 2.65 Ga). In the northeastern Minto block, the Douglas Harbour domain, previously extrapolated from dated localities to the northeast, is substantiated through an age of 2.87 Ga. The older tonalitic crust is punctured by 2.734–2.725 Ga granodioritic plutons of the Leaf River suite that locally carry 3.01 Ga inherited zircon. The northern Utsalik domain is made up of pyroxene and hornblende-bearing Leaf River plutons with consistent 2.725–2.723 Ga ages and local inheritance of 2.82 and 2.77 Ga. However, the southern extension of Utsalik domain contains pyroxene-bearing granodiorites both older (2.762 Ga) and younger (2.692 Ga) than the Leaf River suite. A tonalitic rock from the southern Goudalie domain with an age of 2.833 Ga supports a linkage to 2.84–2.83 Ga units of the Qalluviartuuq domain along strike to the north. Tikkerutuk domain represents a north-trending, 50-km wide calc-alkaline magmatic arc with ages between 2.71 and 2.70 Ga, that extends southward into the Bienville subprovince on the basis of aeromagnetic anomalies and geochronology. Zircon inheritance ages indicate that the Tikkerutuk arc was built upon antecedent arcs of 2.84, 2.77 and 2.725 Ga. The Inukjuak domain in the west consists mainly of granite, dated in one locality at 2.72 Ga.

Geochemical evolution of the Minto block: a 2.7 Ga continental magmatic arc built on the Superior proto-craton

Abstract A 400 km-long by 100 km-wide geological transect across the Minto block of northeastern Superior Province reveals five plutonic suites and minor amphibolite- to granulite-grade supracrustal remnants. The oldest regionally significant plutonic rocks include 3.0±0.1 Ga foliated to gneissic tonalite characterized by high Na K ratios, high Al contents, light REE enrichment, and initial geNd= +1.8. The Leaf River plutonic suite is the most widespread, with crystallization ages of about 2725 ± 5 Ma across the transect. The suite comprises massive to foliated pyroxene- and hornblende-bearing granodiorite, tonalite, granite, diorite, and minor gabbro-pyroxenite and syn-plutonic mafic dykes. The rocks are I-type, calc-alkaline, have variable Na K ratios, moderate to high Ba and Sr contents, and REE patterns with variable light REE enrichment depletion of heavy REEs with increasing silica, and neutral, negative, or positive Eu anomalies. Initial ϵNd values for the suite range from −0.5 to +1.3 (mean = +0.4), and initial 87 Sr 86 Sr ratios (ISr) are ∼0.7020. The diatexite plutonic suite (2713 Ma) occurs within a 150 km-wide zone, and consists of orthopyroxene±garnet granodiorite containing 25–50% inclusions of granulite-grade mafic, supracrustal, and tonalitic inclusions. The variable major- and trace-element chemistry of the suite, with generally high Ni and Cr contents, reflects heterogeneous source materials and the presence of non-liquid components. Initial ϵNd values are from +0.1 to −1.7. Monzogranite and orthopyroxene granite dykes, plugs and plutonic masses, both ∼2690 Ma, comprise the last important plutonic suites. Both biotite monzogranite and orthopyroxene granite are light REE-enriched, but the former have negative Eu anomalies, and the latter positive. Initial ϵNd values range from −0.7 to −3.2 for monzogranite (ISr=0.7016 to 0.7067), and +0.7 to −0.2 for orthopyroxene granite (ISr=0.701 to 0.702). The initial ϵNd and ISr values of the ∼ 2.7 Ga plutonic suites are lower and higher, respectively, than estimatesf of the contemporaneous depleted mantle, suggesting significant and wide-scale reworking of older, tight REE-enriched, high Rb SR lithosphere. Early tonalite orthogneiss from Goudalie domain may be derived by partial melting of mafic crust at high pressures. The lithological and geochemical diversity of the 2725 Ma Leaf River plutonic suite indicate a range of parental magma compositions and petrogenetic processes, but fractional crystallization of mantle-derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism. Crustal-inclusion-charged granodioritic diatexite (2713 Ma) was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions. Late biotite monzogranite was generated by partial melting of older, sialic crust, and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral. We infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth. The ∼ 2.7 Ga magmatism in the Minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern Superior Province. The northwest-directed subduction regime inferred from the southern Superior Province arc terranes may also have been responsible for the magmatism that acted on 3 Ga lithosphere in the northern Superior Province. Melting within the mantle wedge beneath the proto-cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs.

Allochthonous 2.78 Ga oceanic plateau slivers in a 2.72 Ga continental arc sequence: Vizien greenstone belt, northeastern Superior Province, Canada

Embedded within the vast granitoid terrane of the Minto block of northeastern Superior Province are Late Archean greenstone belts of the Goudalie domain that preserve a long-lived record of continent-ocean interaction. The Vizien greenstone belt is one such belt and it contains four fault-bounded structural panels. The 2786 Ma mafic-ultramafic sequence is an allochthonous package of pillowed basaltic andesite, komatiite and volcaniclastic rocks cut by peridotite and gabbro sills. The mafic rocks are LREE-depleted tholeiites which have primitive mantle (PRIM)-normalized abundances of Th La > Nb, and a range of ϵNd values from −0.1 to +1.7. The ~ 2722 Ma lac Serindac bimodal, subaerial tholeiitic volcanic sequence contains andesite (locally with tonalite xenoliths), basalt, gabbro sills, lenses of quartz-rich sedimentary rocks and a thick, upper rhyolite sequence. The lac Serindac tholeiites are LREE-enriched, have PRIM-normalized Th > La > Nb, high Zr (to 300 ppm) and Ti contents, and low ϵNd values from +0.8 in basalt to −1.4 in rhyolite. The < 2718 Ma basement-cover sequence comprises 2.94 Ga tonalitic gneiss unconformably overlain by clastic sediments and a thin upper sequence of 2700 Ma gabbro, siliceous high-Mg basalt (SHMB) and andesite. The SHMB are characterised by LREE depletion and ϵNd values of +2.6, whereas the andesite is LREE-enriched and has ϵNd values of −0.3. The 2786 Ma mafic-ultramafic sequence is interpreted as a sliver of plume-related oceanic plateau crust. The 2724 lac Lintelle sequence represents a continental arc formed on the eastern protocraton. The ~ 2722 Ma lac Serindac volcanic sequence represents late continental rift deposits. The various 2.8-2.7 Ga supracrustal sequences were accreted, deformed and metamorphosed to mid-amphibolite facies during late-stage assembly of the Minto block between 2.718 and 2.693 Ga.

Origin of deep crystal reflections: seismic profiling across high-grade metamorphic terranes in Canada☆

Abstract In an attempt to better understand the origin of deep crustal reflections LITHOPROBE has sponsored or co-sponsored Seismic reflection surveys across tracts of high-grade metamorphic rock in the Archean Superior craton, the Proterozoic Grenville orogen and the Phanerozoic Cordilleran orogen. Common to these three diverse terranes are near-surface zones of prominent Seismic reflectivity that are typically associated with velocity discontinuities at highly strained contacts between gneissic rocks of varying lithology. At some locations the reflective layering resulted from transposition and rearrangement of previously layered rocks (stratified assemblages, sills, etc.), whereas in other regions it was generated by extreme attenuation, stretching and ductile flow of weakly layered or irregularly organized rocks. It seems likely that compositionally layered gneissic rock is a common source of reflections in the deep crust, with reflections originating at lithological boundaries and zones of mylonite.

Mafic to felsic magmatism and crustal recycling in the Obonga Lake greenstone belt, western Superior Province: evidence from geochemistry, Nd isotopes and U–Pb geochronology

Abstract The central Wabigoon Subprovince of the western Superior Province contains Mesoarchean granitoid and supracrustal rocks (3.01–2.83 Ga) and 2.78–2.69 Ga granitoid plutons and supracrustal sequences. It is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement. The Obonga Lake greenstone belt contains two distinct assemblages: (1) a Ta>Nb>Th. They have eNd values of +0.7 to +2.4. A dacitic unit in the northern assemblage has high La/Yb, high Sr/Y, low Nb, Y and heavy REE and is interpreted to represent a mantle-modified slab melt similar to adakites in Cenozoic arcs. The associated enriched basalts may represent melts from the mantle wedge modified by slab melt (adakitic) metasomatism, or they may represent an enriched (OIB-like) asthenospheric source. Rocks of the southern assemblage mostly show Th and light REE enrichment and negative Nb and Ta anomalies. Low eNd values (down to −0.9) can be modelled through contamination of a mafic liquid by 3.3–3.2 Ga sialic crust, although rocks of that age have not been observed. Suggested Mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years. The southern assemblage volcanism may have occurred in a continental arc to continental back-arc system where crustal recycling played an important role.

Thin thrust sheet formation of the Kapuskasing structural zone revealed by Lithoprobe seismic reflection data

Regional and high-resolution seismic reflection data across the Kapuskasing structural zone in Ontario, Canada, image at least three significant thrust faults that are low angle, merge into a flat detachment on the west, and together were responsible for the uplift of amphibolite and granulite facies rocks. Their geometry resembles a ramp-and-flat style of deformation that results in a thin upper plate above the 10-12 km (about 4.0 s) detachment. Northwest-southeast horizontal shortening is estimated to be at least 55 km. This large amount of shortening implies that much of the Superior province was detached during the formation of the Kapuskasing structural zone.