STRUCTURAL CHARACTERIZATION AND TECTONIC SETTING OF THE NEMISCAU SUBPROVINCE, ARCHEAN SUPERIOR PROVINCE, CANADA: PRELIMINARY RESULTS AND INTERPRETATIONS

The Archean Nemiscau subprovince is dominated by high-grade metasedimentary rocks that crop out in the central part of the Superior Province in Quebec. To the north and the south, it is bounded by the La Grande and the Opatica subprovinces, which are composed of mafic volcanic rocks and intermediate-felsic plutonic rocks. Heterogeneously deformed and partially migmatized metasedimentary rocks and felsic intrusives (belonging to the Rupert and Champion complexes) form the innermost part of the Nemisceau subprovince in the study area whereas mafic-to-ultramafic volcanic and intrusive rocks predominate along its northern (Lac des Montagnes belt) and southern (Colomb-Chaboullié belt) borders.

E-W and NE-SW-trending strike-slip to oblique shear zones separate the Nemiscau subprovince from adjacent plutonic rocks along both margins. Two generations of structures are visible in the study area, D_n and D_n+1. D_n structures are marked by an E-W trending, variably-dipping regional foliation. The related mineral and stretching lineations show a similar orientation and a down-dip plunging trend. D_n folds are close to tight. They show E-W-trending axial surfaces and shallow-to-intermediate plunges. D_n+1 structures trend E-W and NE-SW and associated with a subvertical crenulation cleavage/foliation that is more intensely-developed along shear zones. Moderately-plunging mineral and/or stretching lineations are present.

The metamorphic paragenesis involves orthopyroxene in the innermost part of the Nemisceau subprovince, suggesting granulites facies metamorphic conditions. Migmatized amphibolites are spatially related to highly-deformed and felsic plutonic rocks showing upper amphibolite metamorphic minerals assemblages. Regional metamorphism seems to decrease, from granulite to amphibolite, from the inner to the outermost parts of the Nemisceau subprovince in the study area.

The Nemiscau subprovince has been recently interpreted as the vestiges of an accretionary prism or of a back-arc basin. However, the innermost, granulitic part of the subprovince shows a well-developed dome-and-basin geometry that could be related to gravitationally-driven and/or compressional/extensional tectonic settings, indicating that a sagduction-diapirism origin cannot be excluded.