The magmatic evolution of the Northeastern Superior Province, from
Archean to Proterozoic times

    I had the opportunity between 1999 and 2003 to map the rocks the Northeastern Superior Province (NESP, figure 1) with the Ministère des Ressources naturelles du Québec (MRN).  My involvement in this mapping program, combined with the large amount of data collected, offers a unique opportunity to pursue a PhD study to better understand the genesis and the assembly of this part of the Canadian Shield.

    I propose to examine the chemical and isotopic evolution of Archean greenstone belts, enclosing TTGs and Proterozoic mafic dykes of the NESP, using both my own and MRN datasets.  The results of this PhD project will contribute to the resolution of many of the outstanding issues in Archean geology and to the understanding of the chemical changes that have occurred in the Earth between Archean, Proterozoic and modern times.

The Archean times

    The granite-greenstone terranes of the ancient Earth differed from modern analogues in being formed with higher mantle temperatures and having volcano-plutonic rocks with distinct chemical compositions.  As such, the compositions of Archean greenstones do not resemble any modern basalt, while the voluminous Archean felsic plutonic rocks (tonalite-trondhjemite-granodiorite – TTG) have no modern counterparts.  Although these basic differences are well established, the origin of the rocks that make up Precambrian shields is still disputed.  There is little consensus about:

• The rate at which the continental crust formed (Taylor and McLennan, 1985; Nelson and DePaolo, 1985; Campbell, 2003)

• The relationship between Archean volcanic belts and enclosing felsic plutons (Peschler et al. 2004)

• The mode of emplacement of Archean greenstone belts (Ayer et al., 2002; Scott et al., 2002; Thurston, 2002)

• The composition of the mantle (Herzberg, 1993; Francis, 2003)

• The dominant tectonic regime (Bédard et al., 2003; De Wit, 1998; Hamilton, 2003)

• The nature of the Archean-Proterozoic transition (Breuer and Spohn, 1995; Griffin et al., 2003)

 
Geology of the Northeastern Superior Province

    The NESP was formerly described as being composed mostly of granulite-grade granitoids (Stevenson, 1968; Herd, 1978).  Recent mapping by governmental surveys (Geological Survey of Canada - GSC and Ministère des Ressources naturelles - MRN) have, however, shown that the NESP is comprised of dominant neo- to paleoarchean plutonic suites and minor volcano-sedimentary belts that were affected by low pressure amphibolite to granulite metamorphism (3-8.5 kbar and 560-950°C; Percival and Skulski, 2000; Bédard, 2003; Leclerc, 2004).

    The magmatic and tectono-metamorphic evolution of the NESP spans nearly 2 billion years (3.8 – 1.9 Ga, as determined from ~220 U-Pb zircon ages by provincial and federal governmental surveys).  The temporal architecture of the NESP can be summarized as follows:

1. The oldest rocks outcrop in a Paleoarchean belt on the eastern coast of Hudson Bay (ca. 3.8-3.6 Ga; David et al., 2002).
2. Mesoarchean (ca. 3.1-2.9 Ga) protocratonic nuclei either occur as tonalite ‘rafts’ within younger units or as remnants identified in inherited zircon cores and old Sm-Nd model ages in younger rocks.
3. Voluminous tonalite-trondhjémite bodies and sparse mafic volcanic belts are emplaced between 2.89 and 2.75 Ga.
4. The intrusion of granite-granodiorite plutons between 2.73 and 2.72 Ga marks the onset of potassic magmatism.  These units appear to characterize a major episode of intracrustal melting, coeval with charnockite-type magmatism that begins in the northern part of the NESP and gradually extends to the remaining of the area until 2.69 Ga.
5. The emplacement of monzonite, granite, granodiorite, diatexite and pegmatite bodies mark the recycling of older lithologies at ca. 2.69-2.67 Ga.  This is also the time of the emplacement of carbonatite dykes in the northwestern part of the area.
6. Small alkaline and carbonatite intrusions were emplaced locally between ca. 2.68-2.62 Ga, associated with important hydrothermal activity along brittle faults.
7. The intrusion of numerous mafic dyke swarms (ca. 2.5 to 2.0 Ga) marks the beginning of Proterozoic times, while the intrusion of mafic-ultramafic alkaline lamprophyres and carbonatites marks the end of the known magmatic activity within the NESP (ca. 1.94 Ga).
8. Coevally to the intrusion of mafic dyke swarms, rifting of the Archean craton occurs at ca. 2.2 Ga and is followed by the deposition of ca. 2.17 to 1.87 Ga volcano-sedimentary belts that surround the Archean rocks of the NESP (figure 2).

Proposal

    Despite the recent progress in understanding the timing of the geological events, few detailed geochemical studies have been conducted in this area.  The NESP is not included in current tectonomagmatic evolution models for the Superior Province because of a lack of knowledge, despite the fact that it constitutes ~20% of the Superior Province.  New data collected across this enormous territory during regional mapping projects between 1998 and 2003 has changed this situation, offering the possibility of integrating the evolution of the NESP into that of the Superior Province as a whole.  This PhD project will not only contribute to the understanding of the Earth’s magmatic evolution through time, but also help to reconstruct the evolution of the Superior craton.

Archean rocks
    A comparison of volcanic rocks from five greenstone belts of the northern part of the NESP with ages between ca. 2.82 and 2.74 Ga* will characterize the mafic volcanism during the early Neoarchean era (figure 3).  Sections have been sampled across each of these belts and ~275 samples have now been analyzed for major and trace elements.  These greenstone belts consist of amphibolite-facies rocks of both volcanic and sedimentary origin, but are small and dismembered compared to the Archean greenstone belts of the Southern Superior Province.  The degree of deformation is quite variable within individual belts and even though primary textures are locally preserved, stratigraphic tops were rarely observed and relationships among many of the rock assemblages remain uncertain.

    The basaltic rocks from these assemblages are divided in three chemical groups: Mg-tholeiites, Fe-tholeiites and calc-alkaline basalts, the latter of which occur only in the youngest belt (ca. 2740 Ma).  And while Mg-tholeiites are found in belts of ca. 2820 Ma, Fe-tholeiites compositions are found in belts 45 Ma younger (ca. 2775 Ma).  If these basaltic melts were produced from a similar mantle source, the evolution from Mg- to Fe-tholeiites compositions could be accounted for by the fractionation of a gabbroic assemblage under low fO₂ conditions.  If correct, this hypothesis implies that the emplacement of ca. 2775 Ma greenstone belts was autochthonous and that the northern part of the NESP was stable before this time.  Ca. 2820 Ma inherited zircons found in ca. 2775 Ma felsic volcaniclastic rocks imply that older felsic material was deposited earlier in the stratigraphic sequence.  Sm-Nd isotopic analysis will be acquired at GEOTOP under the supervision of J. David and R. Stevenson and will be used to characterize the mantle sources of both the Mg- and Fe-tholeiite magma types, constrain the degree of crustal interaction in volcanic and plutonic felsic rocks, as well as trace the distribution of older Mesoarchean cratonic nuclei in the northern part of the NESP.

*U-Pb ages acquired by J. David

Proterozoic rocks
    Rifting of the Archean craton at ca. 2.2 Ga was followed by the deposition of Proterozoic (ca. 2.17 to 1.87 Ga) volcano-sedimentary rocks that now surround the NESP (figure 2).  These belts include the Labrador Trough to the east, the Cape Smith foldbelt to the north and the Ottawa and Belcher islands and associated coastal continental basalts (the Richmond Gulf and Nastapoka groups) to the west.  The many mafic dyke swarms that crosscut the Archean basement likely fed the basaltic rocks of the belts.  These dykes provide a unique opportunity to examine the feeder systems associated with the surrounding mobile belts and will characterize their mantle source beneath the NESP in Proterozoic times.  You can download a presentation I made in August 2005 and its accompanying text that discuss the implications of the trace element chemistry of these dykes.

    Only four U-Pb ages were determined for the proterozoic dykes in the NESP, out of which only one has been obtained for the southern part of the NESP (Buchan et al., 1998).  Further isotopic dating is required to constrain the exact timing of intrusion of the many still unknown swarms.  An effort is currently being made in collaboration with J. David in order to recover baddaleyite and/or zircon crystals for U-Pb dating.  Also, both Sm-Nd and Rb-Sr isotopes will be acquired to constrain the mantle sources of the dykes.

References
Ayer, J., Amelin, Y., Cofu, F., Kamo, S., Ketchum, J., Kwok, K. and Trowell, N., 2002 : Evolution of the southern Abitibi greenstone belt based on U-Pb geochronology: autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation, Precambrian Research 115, 63-95.

Barker, F. (1979). Trondhjemite: definition, environment and hypotheses of origin. In: Barker, F. (ed.) Trondhjemites, Dacites, and Related Rocks. Amsterdam: Elsevier, pp. 1–12.

Bédard, J. H., Brouillette, P., Madore, L., Berclaz, A., 2003 - Archean cratonization and deformation in the northern Superior Province, Canada : an evaluation of plate tectonic versus vertical tectonic models. Precambrian Research; 127, pages 61-87.

Bédard, J. H., 2003 - Evidence for regional-scale, pluton-driven, high-grade metamorphism in the Archean Minto Block, Northern Superior Province, Canada. Journal of Geology; 111, pages 183-205.

Breuer, D. and T. Spohn 1995. Large Possible Flush Instability in Mantle Convection at the Archean-Proterozoic Transition. Nature, 378, 608-610

Buchan, K. L. - Mortensen, J. K. - Card, K. D. - Percival, J. A., 1998 - Paleomagnetism and U-Pb geochronology of diabase dyke swarms of Minto block, Superior Province, Quebec, Canada. Canadian Journal of Earth Sciences; 35, pages 1054-1069.

Campbell, I.H., 2003- Constraints on continental growth models from Nb/U ratios in the 3.5 Ga Barberton and other Archaean basalt-komatiite suites, American Journal of Science 303, 319-351

David, J. - Parent, M - Stevenson, R. - Nadeau, P. - Godin, L., 2002 - La séquence supracrustale de Porpoise Cove, région d'Inukjuak; un exemple unique de croûte paléo-archéenne (ca 3.8 Ga) dans la Province du Supérieur. Ministère des Ressources naturelles, Québec; DV2002-10, page 17.

de Wit, M.J., On Archean granites, greenstones, cratons and tectonics: does the evidence demand a verdict? Precambrian Res. 91 (1998), pp. 181–226

Francis, D., 2003: Cratonic Mantle roots, remnants of a more chondritic Archean Mantle? Lithos 71, 135-152

Griffin, W.L., O’Reilly, S.Y., Abe, N., Aulbach, S., Davies, R.M., Pearson, N.J., Doyle, B.J. and Kivi, K., 2003: The origin and evolution of Archean lithospheric mantle, Precambrian Research 127, 19-41.

Hamilton, W.B. 2003: An alternative earth; GSA Today 13, 4-12.

Herd, R. K., 1978 - Notes on metamorphism in New Quebec. In: Metamorphism in the Canadian Shield. Geological Survey of Canada; Paper 78-10, pages 79-83.

Herzberg, C. T. (1993). Lithosphere peridotites of the Kaapvaal craton. Earth and Planetary Science Letters 120, 13–29

Leclerc, F., 2004 – Évolution tectonostratigraphique et métamorphique de la ceinture volcano-sédimentaire de Qalluviartuuq-Payne. Thèse de maîtrise, Université du Québec à Montréal.

Martin, H. (1994). The Archean grey gneisses and the genesis of continental crust. In: Condie, K. C. (ed.) Archean Crustal Evolution. Amsterdam: Elsevier, pp. 205–259

Moyen, J.-F. and Martin, H., 2002 : Secular changes in tonalite-trondhjemite-granodiorite composition as markers of the progressive cooling of  Earth, Geology 30, 319-322.

Percival, J. A., Skulski, T., 2000 - Tectonothermal evolution of the Northern Minto Block, Superior Province, Quebec, Canada. The Canadian Mineralogist; volume 38, pages 345-378.

Peschler, A.P., Benn, K., Roest, W.R., 2004. Insights on Archean continental geodynamics from gravity modelling of granite-greenstone terranes. Journal of Geodynamics, 38: 185-207

Rapp, P. R., Watson, E. B., Miller, C. F. (1991). Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites. Precambrian Research 51, 1–25

Stevensen, I. M., 1968, A geological reconnaissance of Leaf River map-area, New Quebec and Northwest Territories. Geological Survey of Canada; memoir 356, 112 pages.

Taylor, S.R. and McLennan, S.M., 1985 - The Continental Crust; Its composition and evolution; an examination of the geochemical record preserved in sedimentary rocks. Blackwell, Oxford. 312.

Thurston, P.C., 2002 : Autochthonous development of Superior Province greenstone belts? Precambrian Research 115, 11-36