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TitleEvidence for a widespread mafic cover sequence and its implications for continental growth in the Northeastern Superior Province
AuthorMaurice, C; David, J; Bédard, J H; Francis, D
SourceSuperior 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,
Alt SeriesEarth Sciences Sector, Contribution Series 20080319
Alt SeriesQuebec Department of Natural Resources and Wildlife, Contribution 2007-8430-04
Alt SeriesGEOTOP, Contribution 2008-0026
PublisherElsevier BV
MeetingGAC-MAC Joint Annual Meeting 2006, GAC Precambrian Geology Section symposium; Montreal, QC; CA; May 2006
Mediapaper; on-line; digital
File formathtml; pdf (Adobe® Reader®)
NTS23M; 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
Arearivière Arnaud
Lat/Long WENS -79.0000 -69.0000 61.5000 55.0000
Subjectsigneous 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
Illustrationslocation maps; plots; tables
ProgramFonds québecois de la recherche sur la nature et les technologies
ProgramNSERC Natural Sciences and Engineering Research Council of Canada
AbstractArchean 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:
(1) Mg-tholeiites 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 afterwards.

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