Pete Hollings

Trace element systematics of Mg-, to Fe-tholeiitic basalt suites of the Superior Province: implications for Archean mantle reservoirs and greenstone belt genesis

Magnesian-, to Fe-rich tholeiitic basalts with near flat REE patterns, locally associated with komatiites, are the prevalent rock type in several areally extensive volcanic sequences of the 2.9–2.7 Ga Archean greenstones belts of the Superior Province. In each of five greenstone belt sequences in the Uchi, Wabigoon, Wawa and Abitibi subprovinces, there is a continuous trend in compositions from high to low Mg# (total range, 65 to 21) accompanied by decreasing Cr and Ni contents, but increasing contents of Fe, Th, Nb, Zr, Hf, REE and Y. The total range of La/Smn is 0.60 to 1.40 and HREE patterns are generally flat. Collectively the basalts have a total range of Nb/Lapm from 0.60 to 1.95, and Th/Lapm from 0.61 to 2.36; in three of the sequences there is a negative correlation of Nb/Lapm with La/Smn. Alteration, crustal contamination, thermal assimilation of altered ocean crust, melting processes, and fractional crystallization can all be ruled out as the cause of the spectrum of Nb and Th anomalies. Rather, the anomalies are interpreted in terms of a heterogeneous multi-component mantle plume: positive anomalies represent a residual slab component from oceanic crust processed through a subduction zone and recycled into the mantle, whereas the negative anomalies reflect a recycled slab-derived lithosphere component. These components were likely reactivated in a mantle plume, and/or entrained in a plume. Consequently, greenstone belt formation was preceded by ocean floor spreading, arc formation and recycling into the mantle. These tholeiitic basalt sequences are directly analogous to basalts of Phanerozoic ocean islands or ocean plateaus, such as Iceland and Ontong Java, many of which also possess ranges of Nb/Lapm and Th/Lapm interpreted as mixing of different mantle components. In two of the belts there is clear evidence for intercalated coeval plume and calc-alkaline arc magmatism. Some Mg- to Fe-tholeiitic basalts in these belts have pronounced negative Nb-anomalies where Nb/Lapm<0.6; compositionally these basalts are similar to Phanerozoic primitive arc, or back-arc tholeiites. Jamming of plume-related plateaus into primitive arcs may govern the transition to calc-alkaline sequences.

An Archean arc basalt–Nb-enriched basalt–adakite association: the 2.7 Ga Confederation assemblage of the Birch–Uchi greenstone belt, Superior Province

Abstract The Uchi subprovince of the Archean Superior Province is a series of greenstone belts extending 600 km east–west along the southern margin of the North Caribou Terrane protocontinent. The 2.7 Ga Confederation tectonostratigraphic assemblage of the Birch–Uchi greenstone belt, northwest Ontario, is dominated by volcanic suites of mafic, intermediate and felsic composition. Tholeiitic basalts range compositionally from Mg# 59–26 evolving continuously to greater REE contents (La=2–19 ppm; Th/Lapm˜1), with small negative Nb anomalies. Primitive tholeiites are similar to modern intraoceanic arc basalts, whereas evolved members extend to greater concentrations of Ti, Zr, V, Sc, and Y, and lower Ti/Zr, but higher Ti/Sc and Ti/V ratios characteristic of back arc basalts. Calc-alkaline basalts to dacites are characterised by more fractionated REE (La/Ybn=1–8), high Th/Nbpm ratios and deeper negative Nb anomalies; they plot with modern oceanic arc basalts and some may qualify as high magnesium andesites. The two suites are interpreted as a paired arc–back arc sequence. A third group of Nb-enriched basalts (NEB; Nb=9–18 ppm) extend to extremely high TiO2, Ta, P2O5, Sc and V contents, with strongly fractionated REE and ratios of Nb/Ta and Zr/Hf greater than primitive mantle values whereas Zr/Sm ratios are lower. The most abundant rhyolitic suite has extremely enriched but flat trace element patterns and is interpreted as strongly fractionated tholeiitic basalt liquids. A second group are compositionally similar to Cenozoic adakites and Archean high-Al, high-La/Ybn tonalites; they possess Yb ≤ 0.4 ppm, Y ≤ 6 ppm and Sc ≤ 8 ppm, with La/Ybn of 19–30 and Zr/Sm of 50–59. They are interpreted as melts of ocean lithosphere basaltic crust in a hot shallow subduction zone. Adakites are associated with NEB in Cenozoic arcs where there is shallow subduction of young and/or hot ocean lithosphere, often with oblique subduction. Slab melt adakites erupt, or metasomatise sub-arc mantle peridotite to generate an HFSE-enriched source that subsequently melts during induced mantle convection. The Archean adakite–NEB association erupted during development of the tholeiitic to calc-alkaline arc and its associated back arc. Their coexistence in the Confederation assemblage of the Birch–Uchi greenstone belt implies convergent margin processes similar to those in Cenozoic arcs.

Komatiite–basalt–rhyolite volcanic associations in Northern Superior Province greenstone belts: significance of plume-arc interaction in the generation of the proto continental Superior Province

Abstract Archean greenstone belts from the northern Superior Province, ranging in age from 3.0 to 2.9 Ga, comprise an association of komatiite–tholeiite sequences, some of which were erupted onto continental crust, intercalated with felsic volcanic rocks. Komatiites from the Uchi subprovince are characterised by variable LREE enrichment and negative Nb anomalies consistent with contamination of Munro-type Al-undepleted komatiites by a felsic melt. The majority of komatiites display variable HREE patterns and positive normalised V–Sc anomalies. Tholeiitic flows intercalated with the komatiites typically have flat rare earth element (REE) patterns with variable Nb anomalies and high Th/Ce ratios, consistent with minor degrees of contamination by sialic crust. Two suites of felsic volcanic rock, Type 1 and Type 2, have been recognised intercalated with the komatiite–tholeiite sequences. Both types display LREE enriched patterns and negative Nb and Ti anomalies, but Type 1 has strongly fractionated HREE patterns, whereas Type 2 HREE are generally flat and have elevated Ni and Cr contents. The predominant Type 1 rhyolite is directly comparable to southern Superior Province felsic volcanic rocks associated with oceanic arc sequences and inferred to be the products of oceanic slab melting. Type 2 rhyolite geochemical signatures may result from mixing of Type 1 rhyolites with tholeiitic magmas, or a contribution from mantle wedge sources located above the garnet stability field. The intercalation of komatiites and arc-type felsic volcanic rocks is best explained by interaction between an ascending mantle plume and adjacent subduction zones.

Trace element and Sm–Nd systematics of volcanic and intrusive rocks from the 3 Ga Lumby Lake Greenstone belt, Superior Province: evidence for Archean plume–arc interaction

Abstract The metavolcanic Lumby Lake belt comprises mafic tholeiites intercalated with thin felsic pyroclastic units. Al-undepleted komatiites are present towards the top of the stratigraphy. Identification of Al-depleted pyroclastic komatiites associated with chemical and siliciclastic sedimentary rocks indicates the upper portion of the 3 Ga Lumby Lake stratigraphy is directly comparable to a stratigraphic sequence developed on a paleoregolith in the nearby Steep Rock greenstone belt. The lower portion of the Lumby Lake sequence therefore represents a rarely preserved association of komatiite–tholeiite and calc alkaline volcanism developed prior to rifting episodes identified in ∼3 Ga terranes of the northern Superior Province. Al-undepleted komatiites are characterised by elevated MgO (18–24 wt.%) and Ni (600–1500 ppm) contents, in conjunction with variable LREE depletion (La/Sm n =0.5–0.8). Intercalated spinifex textured komatiitic basalts possess lower MgO (10–11 wt.%) and Ni (150–180 ppm) and flat to weakly enriched LREE (La/Sm n =0.9–1.1). Pyroclastic Al-depleted komatiites (Al 2 O 3 /TiO 2 =4–5) are strongly LREE enriched (La/Yb n =6.7–10.5) with variable HFSE anomalies. Compositionally uniform tholeiites with variably depleted to enriched LREE (La/Sm n =0.8–1.2) and minor HFSE anomalies dominate the stratigraphy of the belt. A distinct subset of tholeiites, occurring towards the centre of the belt, is characterised by low Al 2 O 3 /TiO 2 ratios, LREE enrichment and the absence of HFSE anomalies. Minor intermediate (SiO 2 =53–64 wt.%) volcanic rocks with pronounced REE fractionation (La/Yb n =0.8–1.1), high Al 2 O 3 /TiO 2 and Zr/Y ratios also occur throughout the belt. Two distinct subtypes of felsic pyroclastic rocks are recognised intercalated sporadically throughout the stratigraphy. Both types display pronounced LREE enrichment (La/Sm n =3.9–6.1) but Type 1 has strongly fractionated HREE patterns (Gd/Yb n =1.5–4.6) whereas, Type 2 HREE patterns are generally flat (Gd/Yb n =1.7–1.9). Intrusive counterparts to both subtypes have been identified within the Marmion Lake batholith to the south of the greenstone belt. The felsic subtypes may be derived from similar parental magmas by variable degrees of hornblende involvement during fractionation processes. Sm/Nd isotope systematics from a range of rock compositions have yielded e Nd values of +2 to +5, typical of the range for Archean volcanic and intrusive rocks. Komatiite–tholeiite associations within Archean terranes are interpreted as the result of plume related magmatism likely in a geodynamic setting comparable to Phanerozoic oceanic plateaux. In contrast intermediate and felsic rocks are typical of calc alkaline suites generally attributed to Archean subduction related environments. The coeval eruption of the two magma suites in the Lumby Lake belt is best interpreted as the result of the sporadic subduction of plume-modified oceanic spreading centres over 10s of m.y. and the eventual impingement of a mantle plume upon an active subduction zone. This process can also account for the young age of basement material relative to overlying rift sequences in 3 Ga terranes of the Superior Province.

Variability of Nb/U and Th/La in 3.0 to 2.7 Ga Superior Province ocean plateau basalts: implications for the timing of continental growth and lithosphere recycling

Extensive volcanic sequences of tholeiitic basalts, having near-flat REE patterns and spatially associated with komatiites, occur in many Archean Superior Province greenstone belts; they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume. Basalts from the 2.9–3.0 Ga Lumby Lake greenstone belt, carefully screened for minimum alteration, have variable Nb/U ratios of 36 to 58. Least-altered basalts from the Abitibi and Wawa greenstone belts also have variable Nb/U ratios of 25 to 50 and 28 to 42, respectively, compared to an average value of 47 for modern ocean basalts. In the Abitibi suite Nb/U correlates positively with Nb/Lapm but negatively with Th/Lapm. Alteration can be ruled out as the cause of Nb/U variation, as there are no correlations of Nb/U with LOI or Eu/Eu*, and Nb/U correlates with Nb/Th in all three suites of basalts. Numerous lines of evidence indicate that crustal contamination can be eliminated as the cause of Nb/U variability, especially for samples with Nb/U > 36. High Nb/U ratios can be explained by recycling ocean crust processed through a subduction zone (high Nb/U and Nb/Lapm, low Th/Lapm) into the mantle source of the basalts, whereas low Nb/U ratios can be accounted for by recycling complementary subarc mantle lithosphere, or continental crust (low Nb/U and Nb/Lapm, high Th/Lapm) into the mantle source. For the entire population of basalts, Th/Lapm ratios generally <1 may result from recycling the residue of slab-derived Archean-type tonalites having high Th/La ratios. Nb/U ratios as high as the 47 ± 10 range of modern ocean basalts have been found in a 2.7 Ga volcanic belt of the Yilgarn craton. Taken together, the results signify very early growth of the continental crust, rather than episodic growth over several Ga.

3D analysis of inverted extensional fault systems, southern Bristol Channel basin, UK

Abstract East-West trending inverted extensional fault systems offset Triassic to Lower Jurassic strata close to the southern margin of the Bristol Channel basin along the north Somerset coast. Field mapping, using exceptionally detailed aerial photographs, has revealed a three phase tectonic evolution. (i) North South orientated stretching, resulting in a well developed extensional fault system. The faults are segmented, linked by relay ramps and horsetail toward their tips. (ii) North-South oriented compression, resulting in partial inversion of the extensional fault system, with the development of hangingwall buttress anticlines and zones of intense folding. (iii) North-South orientated compression, resulting in NW-SE trending dextral and NE-SW trending sinistral strike-slip faults. Comparison of hangingwall buttress anticlines exposed in North Somerset with similar larger scale structures observed on seismic profiles from the Bristol Channel shows that both are directly analogous. Correlation with regional data on the tectonic evolution southwest England, and the Bristol Channel, indicates that extension occurred during the Jurassic-Lower Cretaceous, and contractional inversion and strike-slip deformation during the Tertiary.

Archean Nb-enriched basalts in the northern Superior Province

Geochemical data from the ∼2.9-Ga Northern Pickle assemblage of the Pickle Lake greenstone belt in the northern Superior Province distinguish three distinct volcanic suites. Suite I, from the north of the assemblage, comprises tholeiitic basalts with unfractionated rare earth elements (REE). Suite II basalts are light REE (LREE)-enriched with negative mantle-normalized Nb anomalies relative to Th and La, similar to the CA suite of Sajona et al. [J. Petrol. 37 (1996) 693]. Basalts from Suite III are also LREE-enriched but lack negative Nb anomalies. Normalized Nb abundances are either similar to or higher than Th and La, comparable to Nb-enriched basalts (NEB) from Phanerozoic arcs. Variable high field strength elements and heavy REE (HREE) systematics support the subdivision of Suite III basalts into two spatially distinct suites. Suite IIIa is interpreted to have melted at relatively shallow depths in the presence of Nb–Ti-bearing silicates whereas Suite IIIb melted in the presence of garnet. Variations in the geochemistry of the three suites can be accounted for by interaction among primitive mantle, adakite melts, and subduction-modified mantle. The association of the three suites is interpreted to be the result of rifting in a back-arc environment, characterized by Suite I tholeiites, permitting asthenospheric upwelling of subduction-modified NEB and associated arc-like volcanic rocks (Suites II and III). The association of arc-like basalts (Suite II) with NEB (Suite III) in the Northern Pickle assemblage extends the known occurrence of Archean NEB beyond the ∼2.7-Ga age of previously recognized examples.

Quantification of oxygen consumption and sulphate release rates for waste rock piles using kinetic cells: Cluff lake uranium mine, northern Saskatchewan, Canada

Oxidation rates of low sulphide (< 0.5 wt.%) gneissic waste rock from the Cluff lake U mine, northern Saskatchewan, Canada were determined using 3 independent methods: O 2 consumption rates in kinetic cells, SO 4 measurements of kinetic cell effluent and humidity cell SO 4 release rates. The O 2 consumption measurements demonstrated that the oxidation of pyrite was strongly dependent on grain size and moderately dependent on water content, temperature and microbiology. Oxygen consumption rates were highest at water contents of 5-10 wt.% (12-25% saturation). Measured SO 4 release rates (3.1-91 mg SO 4 kg -1 wk -1 ) for the kinetic cells were comparable to rates calculated from the O 2 consumption values (6.9-70 mg SO 4 kg -1 wk -1 ). Sulphate release rates determined from humidity cells were generally higher than those obtained from the kinetic cells, ranging from 6 to 64 mg SO 4 kg -1 wk -1 for the coarsest and finest fraction, respectively. These differences were attributed to sample heterogeneity.

Long-lived mantle-plume influence on an Archean protocontinent: Geochemical evidence from the 3 Ga Lumby Lake greenstone belt, Ontario, Canada

The 3.0 to 2.9 Ga Lumby Lake belt of the Superior province is composed of plume-related komatiite-tholeiite sequences and calc-alkalic volcanic units, and tonalite-trondhjemite-granodiorite units formed by coeval magmatism; they are all intercalated throughout a 100 m.y. interval. These observations are inconsistent with plateau-accretion models of crustal growth. Sporadic subduction of plume-modified ocean spreading centers, followed by plume impingement beneath a northern Superior province cratonic nucleus, more readily accounts for the long duration of coexisting plume- and arc-type volcanism. The recognition of such complex geodynamic settings in the Archean has important consequences for crustal-growth models.

Paleoproterozoic arc magmatism imposed on an older backarc basin: Implications for the tectonic evolution of the Trans-Hudson orogen, Canada

Compositional data from 72 granitoids and associated mafic rocks in the Paleoproterozoic Kisseynew domain, Trans-Hudson orogen, Canada, were used to evaluate the tectonic setting of magma generation between 1840 and 1820 Ma. These data thus provide insights into the development of the Trans-Hudson orogen just prior to amphibolite-grade metamorphism and the final stages of continental collision. The granitoid rocks are hornblende- or orthopyroxene-bearing, medium- to high-K calc-alkalic, and weakly peraluminous. The trends on Harker variation diagrams are consistent with crystal-fractionation processes, although the range in values is related to the temporal and spatial distribution of samples. The extent of LREE (light rare earth element) enrichment increases with increasing SiO 2 , as does variability in HREE (heavy rare earth element) fractionation and magnitude of Eu anomalies. Most samples have (Gd/Yb) n < 10 (suite Ia), although five have (Gd/Yb) n = 17-24, suggesting the presence of garnet residua (suite Ib). The range in rock types, calc-alkalic character, and negative Nb and Ti anomalies are all characteristic of arc environments. Magma generation within the Trans-Hudson orogen, including the Kisseynew domain, between 1.84 and 1.82 Ga was related to interaction between the Sask craton and the Flin Flon-Glennie complex. A decrease in subduction angle likely resulted from northward-directed subduction of more buoyant crust, with associated changes in thermal conditions leading to melting over a wide area. Most magmas formed in a slab-dehydration-dominated regime, although localized slab melts may have been generated. Ultimately subduction ceased, and the slab foundered into the mantle, potentially resulting in the generation of alkalic and potassic magmas within the Rae and Hearne cratons.