|Title||Evidence for a widespread mafic cover sequence and its implications for continental growth in the Northeastern Superior Province|
|Author||Maurice, C; David, J; Bédard, J H; Francis, D|
|Source||Superior Province: the nature and evolution of the Archean continental lithosphere; by Stott, G (ed.); Mueller, W (ed.); Precambrian Research vol. 168, issue 1-2, 2009 p. 45-65, https://doi.org/10.1016/j.precamres.2008.04.010|
|Alt Series||Earth Sciences Sector, Contribution Series 20080319|
|Alt Series||Quebec Department of Natural Resources and Wildlife, Contribution 2007-8430-04|
|Alt Series||GEOTOP, Contribution 2008-0026|
|Meeting||GAC-MAC Joint Annual Meeting 2006, GAC Precambrian Geology Section symposium; Montreal, QC; CA; May 2006|
|Media||paper; on-line; digital|
|File format||html; pdf (Adobe® Reader®)|
|NTS||23M; 24D; 24E; 24L; 24M; 25C; 25D; 25E; 33O; 33P; 34A; 34B; 34C; 34F; 34G; 34H; 34I; 34J; 34K; 34L; 34M/01; 34N; 34O; 34P; 35A; 35B; 35C; 35F; 35G; 35H|
|Lat/Long WENS|| -79.0000 -69.0000 61.5000 55.0000|
|Subjects||igneous and metamorphic petrology; geochronology; tectonics; geochemistry; Archean; igneous rocks; volcanic rocks; greenstone belts; tholeiites; crustal movements; crustal studies; crustal evolution;
alteration; magmatism; tectonic interpretations; tectonic environments; tectonic evolution; uranium lead dating; uranium lead dates; radiometric dating; Superior Province; Precambrian|
|Illustrations||location maps; plots; tables|
|Program||Fonds québecois de la recherche sur la nature et les
|Program||NSERC Natural Sciences and Engineering Research Council of
|Abstract||Archean greenstone belts of the Northeastern Superior Province (NESP) were emplaced over a 160 Ma period (2.88-2.72 Ga), spanning a major episode of crustal reworking in which early
tonalite-trondhjemite plutonism evolved to dominant granite-granodiorite and pyroxene-bearing felsic plutonism. The numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites: |
that have chemical compositions typical of many Archean basalts, with 4-10 wt.% MgO, 9-15 wt.% Fe2O3t, 0.4-1.2 wt.% TiO2 and La/Sm ratios between 1 and 3.
(2) Fe-tholeiites that have similar MgO contents and La/Sm ratios, but markedly higher
Fe2O3t (11-20 wt.%), TiO2 (1.0-2.6 wt.%), and Gd/Yb ratios that may reflect derivation from a distinct garnet-bearing mantle source.
(3) Light rare earth element (LREE)-enriched rocks that have the chemical characteristics typical of
'calc-alkaline' mafic magmas, with higher SiO2, Al2O3 and La/Sm ratios, but lower Fe2O3t and TiO2 than the tholeiitic suites at a given MgO content.
The presence of unconformities within single greenstone belts, numerous inheritance ages in
volcanic rocks replicating those of older volcanic-plutonic assemblages, and the recognition of widespread, geographically separate, yet coeval (2.78 Ga) extrusive assemblages containing the distinctive Fe-tholeiites, do not support a plate tectonic
model for a large portion of the NESP. The correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence. Nd isotopic evidence for the
presence of evolved crust in some syn-volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust.
The amalgamation of two isotopically distinct terranes at ca. 2.76-2.74 Ga
created a proto-cratonic mass that consisted of a composite tonalite-trondhjemite-greenstone crust. Underplating by mafic mantle melts and widespread insulation after 2.75 Ga increased temperatures sufficiently to produce extensive melting of this
crust to generate pyroxene-bearing felsic rocks and granodiorite to granite partial melts. The isotopically enriched character of mantle-derived magmas after 2.75 Ga may reflect more extensive contamination by a felsic crust affected by widespread
partial melting. Furthermore, a decrease of Nb/Th ratios in the LREE-enriched mafic rocks younger than 2.75 Ga reflects a change in the nature of the crustal contaminant, from tonalite-trondhjemite prior to 2.75 Ga, to granite-granodiorite