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Archean crustal evolution of the northwestern Superior craton margin: U–Pb zircon results from the Split Lake Block1 Christian O. Böhm, Larry M. Heaman, and M. Timothy Corkery
Abstract: The Split Lake Block forms a partly retrogressed, granulite-grade basement segment located at the northwestern margin of the Superior Province in Manitoba. Unlike other segments along the craton margin, the effects of Proterozoic tectonism are relatively minor in the Split Lake Block, making it amenable to establishing firm temporal constraints for the Archean magmatic and metamorphic history of the northwestern Superior Province margin. Consequently, samples from the main lithological units within the Split Lake Block were selected for precise singlegrain U–Pb zircon geochronology. Heterogeneous zircon populations isolated from representative enderbite, tonalite, and granodiorite samples reveal a complex growth history with pre-2.8 Ga protolith ages (e.g., 2841 ± 2 Ma tonalite), possibly as old as 3.35 Ga as indicated in a granodiorite sample. The youngest Archean granitic magmatism identified in the eastern Split Lake Block is represented by the 2708 ± 3 Ma Gull Lake granite. A U–Pb zircon age of 2695+−41 Ma obtained for leucosome in mafic granulite is interpreted to reflect the timing of granulite-grade metamorphism in the Split Lake Block, supported by polyphase zircon growth and (or) lead loss at ca. 2.7 Ga in the enderbite sample. A younger phase of metamorphic zircon growth at ca. 2.62 Ga is documented in the tonalite and granodiorite zircon populations. The 2.70–2.71 Ga crust formation, the occurrence of ca. 2695 Ma high-grade metamorphism, and broadly contemporaneous Paleoproterozoic mafic dykes in both the Split Lake Block and Pikwitonei Granulite Domain imply a common evolution of these high-grade terrains along the northwestern Superior craton margin since the late Archean. Résumé : Le bloc de Split Lake représente un segment de socle de faciès des granulites, partiellement rétrogradé, exposé sur la marge nord-ouest de la Province du Supérieur, au Manitoba. Contrairement à ce que l’on observe dans les autres segments localisés le long de la marge du craton, ici les effets du tectonisme protérozoïque sont relativement peu importants dans le bloc de Split Lake, ce qui autorise l’établissement de contraintes chronologiques fermes pour reconstituer l’histoire magmatique et métamorphique de l’Archéen, de la partie nord-ouest de la marge de la Province du Supérieur. En raison de ces conditions, les échantillons des principales unités lithologiques à l’intérieur du bloc de Split Lake ont été sélectionnés pour effectuer des déterminations géochronologiques U–Pb précises sur grains uniques de zircon. Les populations hétérogènes de zircons extraits d’échantillons représentatifs d’enderbite, tonalite et granodiorite suggèrent une croissance évolutive complexe, avec des âges de protolites plus vieux que 2,8 Ga (exemple, 2841 ± 2 Ma pour la tonalite), et, peut-être aussi anciens que 3,35 Ga, tel que le laisse présager un échantillon de granodiorite. Le magmatisme archéen le plus jeune, identifié dans la partie est du bloc de Split Lake, est représenté par le granite de Gull Lake âgé de 2708 ± 3 Ma. Un âge U–Pb sur zircon de 2695+−41 Ma fourni par un leucosome dans une granulite mafique, est interprété comme l’âge de l’événement métamorphique de faciès des granulites dans le bloc de Split Lake, cette interprétation est confortée par la recristallisation de zircon polyphasée et (ou) la perte de plomb dans l’échantillon d’enderbite, vers 2,7 Ga. Une phase plus jeune de cristallisation du zircon, vers 2,62 Ga, est documentée par les populations de zircons extraits de la tonalite et de la granodiorite. La formation d’une croûte âgée de 2,70–2,71 Ga, l’apparition d’un événement métamorphique de degré élevé vers 2695 Ma et l’existence de dykes mafiques paléoprotérozoïques quasi contemporains, observés à la fois dans le bloc de Split Lake et dans le Domaine à granulites de Pikwitonei, impliquent que ces terrains de degré métamorphique élevé, localisés le long de la marge nordouest du craton du Supérieur, ont depuis l’Archéen tardif une histoire commune. [Traduit par la Rédaction]
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Introduction Received October 22, 1998. Accepted August 17, 1999. C.O. Böhm2 and L.M. Heaman. Dept. of Earth and Atmospheric Sciences, 1–26 Earth Sciences Building., University of Alberta, Edmonton AB T6G 2E3, Canada. M.T. Corkery. Manitoba Energy and Mines, Geological Services, 360–1395 Ellice Ave., Winnipeg MB R3G 3P2, Canada. 1 2
Lithoprobe Publication 1116. Corresponding author (e-mail:
[email protected]).
Can. J. Earth Sci. 36: 1973–1987 (1999)
The northwestern margin of the Superior craton consists of one of the largest and best-preserved high-grade terrains on Earth, potentially harbouring valuable clues to the nature of the generally inaccessible lower crust, yet fundamental information pertaining to the origin of this high-grade terrain is generally lacking. As shown in Fig. 1, this important terrain has been subdivided, based primarily on differences in structural trends, into two crustal segments separated by the Aiken River deformation zone, the Pikwitonei Granulite Domain, and the Split Lake Block (Fig. 1). Most of the infor© 1999 NRC Canada
1974 Fig. 1. Simplified geological map of the northwestern Superior Province margin in Manitoba (inset) showing the main geological domains (divided by broken lines) and structural boundaries (solid lines). GLD, Gods Lake Domain; PGD, Pikwitonei Granulite Domain. The rectangular frame marks the location of the geological map shown in Fig. 2.
Can. J. Earth Sci. Vol. 36, 1999
crust forming events as well as the number, nature, and timing of high-grade metamorphic episodes recorded in the Split Lake Block. These objectives are approached using primarily high precision U–Pb zircon geochronology for a variety of Archean lithologies in the Split Lake Block. Unlike the Pikwitonei Granulite Domain and the Thompson Nickel Belt, there have been no U–Pb geochronological studies in the Split Lake Block, even though there is ample evidence from field-based studies for multiple episodes of metamorphism, deformation, and mafic dyke emplacement (Corkery 1985). Both its peripheral location and its well-exposed lithological assembly, including a variety of Archean gneisses, favour the Split Lake Block as a case study for investigating the Archean tectonic history of the northwestern Superior craton margin. A better understanding of the temporal, structural, and tectonic evolution of the Split Lake Block as part of the northwestern Superior margin would provide an important basis for comparing Western Superior high-grade terrains in general and evaluating the overall nature of the Western Superior middle to lower crust imaged through Lithoprobe seismic profiling (e.g., Lucas et al. 1993). The results from this study have broader implications for the tectonic significance of Archean high-grade terrains worldwide and their tectonic significance in the late Archean.
Regional geology
mation available for this high-grade terrain is derived from the Pikwitonei Granulite Domain where pressures up to 9 kbar (1 kbar = 100 MPa) and temperatures up to 880°C have been reported from the Sipiwesk Lake area (Arima and Barnett 1984). Both crustal segments consist predominantly of pristine and variably retrogressed granulite-facies assemblages and are separated from the Kisseynew Domain (Trans-Hudson Orogen) by the Superior Boundary Zone (e.g., Syme et al. 1993), a heterogeneous transition zone that includes the Thompson Nickel Belt (Peredery et al. 1982; Bleeker 1990) and possibly the Orr Lake Block (Lenton and Corkery 1981). To establish the tectono-metamorphic history of the northwestern margin of the Superior craton, the present study focuses on the distribution and timing of the main Archean
In the area of the Split Lake Block, domains of Archean Superior Province and Proterozoic Trans-Hudson Orogen affinity have been identified during regional mapping programs (Corkery 1985; Corkery and Lenton 1990). Boundaries between both the major geologic provinces and internal domains are marked by major deformation zones. Subdivision into geologic domains in the region (Fig. 1) was based on contiguous areas with comparable lithologies and metamorphic and structural histories. The high degree of preservation of intrusive, structural, and metamorphic features in this region of the Superior margin is unique. Prior to this study, the lack of a geochronologic framework, including both the ages of lithological units and, equally important, the relationships and timing of deformational events, has precluded any reasonable understanding of the tectonic history of the area. To place the Split Lake Block in a regional geological context, the following paragraphs introduce the general geology of the adjacent domains (see Fig. 1). Orr Lake Block The nature and location of the Superior craton margin and its basement components in the area northwest of the Split Lake Block (Fig. 1) are poorly constrained. The main lithologies are generally considered to follow a northeast structural trend between Assean Lake and Waskaiowaka Lake and in the Orr Lake Block area (Lenton and Corkery 1981). Felsic gneisses in the southwestern half of the Orr Lake Block are similar to basement in the Pikwitonei Granulite Domain (see below) in that both crustal segments show evidence for at least one older (presumably Archean) granulite-facies metamorphism (M1) followed by an amphibolite facies overprint (M2). © 1999 NRC Canada
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In contrast, gneisses and granitoids in the northeast segment of the Orr Lake Block are situated north of metasediments and metavolcanic rocks on Assean Lake. These supracrustal gneisses on Assean Lake have historically been interpreted as being Paleoproterozoic, and therefore either belong to the Thompson Group sediments (Bleeker and Macek 1987, 1988) or to the Kisseynew Basin metasediments (e.g., Zwanzig 1990). However, since there are no age data available for the metasediments on Assean Lake, there is currently no proof for a Proterozoic age of these metasediments. Thompson Nickel Belt Southwest of the Split Lake Block, the highly deformed, northeast-trending Thompson Nickel Belt is in fault contact with the metasediments of the Kisseynew Basin (TransHudson Orogen). Although the exact location of the Superior – Trans-Hudson boundary is better constrained along the western margin of the Thompson Nickel Belt, the extreme degree of deformation and metamorphism associated with Early Proterozoic collisional events between 1809 and 1720 Ma (Machado et al. 1990, and references therein; Bleeker 1990) have overprinted or completely destroyed much of the earlier geological history. The Thompson Nickel Belt is dominated by a complex assembly of orthogneisses that preserve middle to late Archean and Paleoproterozoic magmatic and metamorphic events (Machado 1990) and a sequence of supracrustal rocks of presumed Paleoproterozoic age (Bleeker and Macek 1987, 1988; Machado et al. 1988) that were deposited unconformably on Archean migmatites. The simplified stratigraphic sequence from oldest to youngest includes the following: (1) basal quartzo-feldspathic sediments, which were locally derived from Archean gneisses; (2) marine carbonate, greywacke sandstone, and iron formation; and (3) a mafic volcanic sequence that includes komatiites (Bleeker 1990) and was intruded by numerous ultramafic dykes and sills. These nickeliferous Proterozoic supracrustal rocks formed isoclinally folded keels that were subsequently intruded by granitoids and deformed during terminal collision of the Trans-Hudson Orogen accreted arcs (Bleeker 1990). Pikwitonei Granulite Domain The Pikwitonei Granulite Domain hosts well-preserved granulite-facies mineral assemblages that occur in a variety of rock types (e.g., granodiorite, basalt, etc.). The vast majority of the Pikwitonei Granulite Domain consists of metaigneous units of tonalitic to granodioritic composition, but traces of supracrustal rocks such as banded iron formation, pillow basalt, and paragneiss have been reported (e.g., Weber 1983). Based on field relationships, petrography, and preliminary U–Pb geochronology, there is an indication of at least two high-grade Archean deformational and (or) metamorphic episodes in the Pikwitonei Granulite Domain (Weber and Scoates 1978; Heaman et al. 1986a). For example, at least two distinct metamorphic zircon growth events at ca. 2685 and 2640 Ma are recorded in the Cauchon Lake area (Heaman et al. 1986a; Mezger et al. 1990) with initiation of granulite conditions possibly occurring as early as 2695 ± 2 Ma, the age of an orthopyroxene-bearing granitic dyke. Late Archean granitic pegmatites as young as 2598 ±
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2 Ma have also been reported from the Cauchon Lake area (Mezger et al. 1990). However, linking these ages of metamorphic zircon growth to specific metamorphic assemblages recognized in the field or discrete tectonic events has proven difficult. Estimates of peak pressure–temperature conditions during granulite-facies metamorphism are about 6.7– 7.3 kbar and 730–770°C in the Cauchon Lake area and about 7.0–7.8 kbar and 780–840°C in the Natawahunan Lake area (Mezger et al. 1990). The youngest igneous activity in the Pikwitonei Granulite Domain is the intrusion of Paleoproterozoic mafic dykes (formerly known as the Molson dyke swarm; Scoates and Macek 1978). At least two ages of mafic dyke emplacement at 2092 Ma (Halls and Heaman 1997) and 1883 Ma (Heaman et al. 1986b) are currently recognized in the Pikwitonei Granulite Domain.
Geology of the Split Lake Block The Split Lake Block (e.g., Corkery 1985) forms a lensoid tectonic slice of reworked Superior craton margin that is bounded by cataclastic to mylonitic deformation zones. Based on field evidence, the Split Lake Block consists dominantly of metaigneous protoliths that have a range in composition from gabbro to granite, but tonalite and granodiorite predominate. There are no reliable age determinations for these metaigneous units, but there is clear field evidence for more than one age of magmatism, as an intrusive contact can be seen between the Gull Lake granite and host hornblende– biotite gneiss. These Archean units have been subjected to at least two, and possibly three, distinct Archean metamorphic events: two older granulite-grade events (M1a and M1b) followed by an M2 amphibolite-grade overprint (Corkery 1985). The M1a event is considered to be hornblende granulite-grade and is only recognized in gneissic xenoliths within a tonalite unit and by the occurrence of M1b leucosomes that crosscut an earlier fabric developed in some mafic units (Corkery 1985). Mineral assemblages formed during the pyroxene granulite M1b event are only locally preserved, but are considered to be of regional extent based on the widespread presence of a granoblastic texture throughout the Split Lake Block. In the following discussion, both of these high-grade metamorphic events are referred to as M1 for simplification. In addition to these high-grade metamorphic episodes, locally the Split Lake Block is overprinted by regional upper greenschist-facies retrogression (M3) that is interpreted to be linked to Paleoproterozoic tectonism and terminal collision involving accretion of crustal slices of the Trans-Hudson Orogen onto the Superior craton margin and the emplacement of Paleoproterozoic granitoid plutons (e.g., the 1825 Ma Fox Lake granite; Heaman and Corkery 1996) and possible Molson-type mafic dykes (Heaman et al. 1986b; Heaman and Corkery 1996). One of these dykes has a U–Pb age of 2072 ± 3 Ma, possibly reflecting the time of Proterozoic rifting of the proto-Superior craton (Heaman and Corkery 1996). In general, the Split Lake Block is one of the few regions along the craton margin where a detailed record of the Archean tectono-metamorphic history is well preserved and only affected in a minor way by Paleoproterozoic collisional events related to the Trans-Hudson orogeny (Corkery 1985; Böhm et al. 1997). There are currently no pressure–tempera© 1999 NRC Canada
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Fig. 2. Geological map of the Split Lake Block showing the distribution of main lithologies and locations of geochronology sampling sites.
ture estimates for high-grade metamorphic conditions in the Split Lake Block. However, medium- to coarse-grained granoblastic gneisses containing both hypersthene and diopside and retrograde equivalents, similar in character to rocks documented in the Pikwitonei Granulite Domain, dominate the Split Lake Block (Corkery 1985). The Split Lake Block is bounded by two discrete linear belts of cataclastic rocks and mylonites, the Aiken River and Assean Lake deformation zones (Fig. 2). The timing of polyphase deformation and movement along the major deformation zones is poorly constrained. It is possible that some of these deformation zones, such as the Aiken River shear zone, which separates the Split Lake Block from the Pikwitonei Granulite Domain, may represent ancient (i.e., Archean) structures (Corkery 1985; Ch.O. Böhm, unpublished data 1996) that have been reactivated during Proterozoic tectonism (e.g., Bleeker 1990; Bickford et al. 1990; Machado 1990). The northern boundary of the Split Lake Block, the Assean Lake deformation zone, is dominated by layered and laminated mylonites and cataclastic gneiss up to 500 m thick in the Assean Lake area and 1100 m thick to the east at Gull Rapids (Corkery 1985). Both regional magnetic and gravity signatures indicate continuity of this structure between Assean Lake and Gull Rapids. There is little indication of postdeformational recrystallization. A minimum age for the latest dextral movement in the Assean Lake area is indicated by the progressive deformation and mylonitisation of the northeast margin of the Fox Lake granite (1825 Ma; Heaman and Corkery 1996). Compared to the Assean Lake deformation zone, the Aiken River shear zone, which forms the southern boundary of the Split Lake Block, does not contain the wide continuous mylonite zones nor the well-developed, southeast-trending fanned linear structures. Rather a strong pancake flattening in recrys-
tallized phyllonitic rocks predominates in the Aiken River shear zone southwest of Split Lake.
Samples Based on detailed mapping and sampling campaigns in the Split Lake Block area (Haugh 1965; Corkery 1985; Corkery and Lenton 1990; Böhm 1997a, 1997b), five representative samples were selected along the Split Lake – Nelson River – Stephens Lake transect (Fig. 2). Four of these were collected along the Nelson River in the central domain of the Split Lake Block, including a leucosome segregation within a mafic granulite (36-75-80), a tonalite gneiss (36-75-99), an enderbite (36-75-102), and a sample from the Gull Lake granite (36-75-112). In addition, a granodiorite gneiss paleosome sample (36-77-127) was collected from central Split Lake. Extensive portions of the Split Lake Block show M2 amphibolite-facies and M3 greenschist-facies retrograde metamorphism. However, due to the static nature of these events, the pseudomorphed textures indicative of the granulite-facies metamorphic events are ubiquitous (Corkery 1985). The rock names given for each sample reflect the present mineralogical composition at the sample site. All samples labelled with medium-grade metamorphic rock names also have an associated granulitefacies representative. Mafic granulite gneiss and retrograde equivalents form the oldest rock units in the Split Lake Block. In the least deformed zones, mafic granulite gneiss is associated with relict igneous layering and rarely with structures interpreted to be pillowed basalt. Leucosome in some outcrops forms irregular lenses of plagioclase–diopside–hypersthene-bearing pegmatite interpreted as mobilizate formed during M1 granulitefacies metamorphism. Consequently, the leucosome sample © 1999 NRC Canada
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36-75-80 was collected to constrain the timing of the M1 metamorphic event. The tonalite gneiss (36-75-99; felsic granulite in unretrogressed rocks) is closely associated with the mafic granulite and may represent more felsic units in the protolith association. The enderbite (36-75-102) is an early intrusive phase with euhedral hypersthene that may be magmatic and may be coeval with M1 metamorphism. The Gull Lake granite (36-75-112) underlies much of the eastern part of the Split Lake Block (Hubregtse 1975; Corkery 1985). It contains a large proportion (50 to 75%) of large, almost completely assimilated rafts of older migmatite. Pyroxene and pseudomorphs of pyroxene are restricted to nebulitic zones of highly assimilated older gneisses. A representative sample of biotite granite was collected at a site where there was a least amount of assimilated older material visible. The biotite granite sample does not contain any pyroxene, and therefore was not named a charnockite. The granodiorite gneiss paleosome sample (36-77-127) forms part of a northeast-trending felsic gneiss belt through central Split Lake. Specimens from this location contain two distinct types of hornblende aggregate that form pseudomorphs after M1b garnet and diopside (Corkery 1985). The timing of this amphibolite facies event is equivocal and has been interpreted as Archean or Proterozoic (Corkery 1985). The granodiorite gneiss paleosome phase was sampled to get an estimate for the primary age of the granodiorite gneiss. Based on field relationships and petrographic observations described above, all investigated samples are presumably of Archean age. Additional sample descriptions are given in the appendix and sample locations are shown in Fig. 2.
Analytical techniques A total of between 5 and 15 kg of sample was pulverised using a jaw crusher and Bico disk mill. Isolation of zircon involved standard mineral separation techniques including a Wilfley Table, heavy liquid (methylene iodide), and magnetic (Frantz Isodynamic Separator) steps. Prior to final selection, single grains, single-grain fragments, and multigrain fractions of visibly different zircon types were carefully examined with a high magnification binocular microscope, and grains with inclusions, fractures, or turbidity were generally avoided. Most zircon fractions were abraded prior to dissolution (Krogh 1982). Hand drawings and photomicrographs were used to document morphological features and internal structures and to estimate the percentage of zircon removed during abrasion. All fractions were prepared and analysed at the University of Alberta Radiogenic Isotope Facility. Following washing in 4N HNO3, H2O, and acetone, the zircon fractions were weighed to better than ±0.5 µg using a Mettler UM2 ultra micro-balance. The zircon fractions were placed into 10 mL TFE Teflon dissolution vessels with a mixture of HF and HNO3 (10:1) and a mixed 205Pb–235U tracer solution. Uranium and lead were separated and measured using standard anion exchange chromatography (Krogh 1973) closely following the procedure outlined in Heaman and Machado (1992). Repeat analysis of total procedure blanks during this study indicated 3–4 pg and
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