Structural Evolution Of The Northeastern Superior Province (NESP) Over 1 Billion Years, With Emphasis On Greenstone Belts And Their Relation With Enclosing Granitoids

Greenstone belts (GB) impart for <10%\ of bedrock exposure in the NESP, which is predominantly composed of granitoids. Bimodal GB and enclosing granitoids were emplaced as sequential magmatic pulses spanning over 1 billion years from early-Archean (3.8 Ga), through mid-Archean (2.88-2.74 Ga), to late-Archean (2.73-2.70 Ga). Early- to mid-Archean GB consist of tholeiitic basalts and komatiites interlayered with mafic to ultramafic sub-volcanic bodies, which dominate relative to calcalkaline felsic volcaniclastic rocks (assemblage 1). Late-Archean GB are characterized by higher volumes of calc-alkaline felsic volcaniclastic rocks and paragneisses relative to tholeiitic and calc-alkaline mafic volcanic rocks (assemblage 2). Rock assemblages 1 and 2 are both variously and upwardly interlayered with iron formations, paragneisses, carbonaceous rocks and conglomerates (assemblage 3). Volcano-plutonic rocks of the NESP are affected by four phases of ductile deformation (D1-D4). Early-Archean (3.8 Ga) Nuvvuagittuq belt differs from its younger counterparts by preserving a rock assemblage 1 affected by early foliation and isoclinal folding (S1/F1), as well as being surrounded by a early-Archean mylonitic tonalitic gneisses. Rock assemblages 1 are refolded and transposed into larger, E-W isoclinal folds (F2) and have strongly to moderately plunging linear fabrics (L2). The well preserved and more voluminous mid-Archean GB are engulfed in coeval tonalite-trondhjemite (TT) plutonic suites (2.85-2.74 Ga), which preserve evidence of syn-magmatic deformational features (TT infilled shear-zones branching along strike into F1 folds and tectonic or magmatic S1). These are oriented E-W to WNW-ESE and associated with strongly to moderately plunging linear fabrics (L1). This involves N-S to NNE-SSW compression and corresponds to D2 in the NESP evolution history. Late-Archean GB are surrounded by coeval granite-granodiorite (GG), enderbite-opdalite-charnockite (EOC) or diatexite (D) plutonic suites (2.74-2.69 Ga), whose emplacement coincides with D3. D3 is characterized by a pervasive NW-SE to NNW-SSE striking foliation (S3) which is axial planar to regional folds (F3) and contains axis-parallel, strongly plunging lineations (L3). Mylonitic shear zones (S3), with kinematic indicators of dextral strike-slip movement, occur as corridors striking NNW-SSE to N-S. These shear zones have GG/EOC/D infillings which preserve magmatic textures and grade out along strike into GG/EOC/D plutons. The geometry between strike-slip mylonitic shear zones and low-angle (10-30°), coeval and subvertical plutonic fabrics is consistent with emplacement under a transcurrent regime involving NNE-SSW to NE-SW compression, and where late-Archean, rock assemblages 2/3 were emplaced in syn-magmatic and syn-tectonic pull-apart basins. In early- to mid-Archean rock assemblages 1 and TT plutonic suites, D3 fabrics superposed D1/D2 fabrics, producing fold interference patterns. Peak regional metamorphism recorded in rock assemblages reached granulite facies concomitantly with syn-D3 EOC magmatism. Locally, late-Archean assemblages 2/3 differ by preserving low-grade, but polydeformed, primary structures. Thus, syn-D3 volcano-sedimentation was still operative at a late- to post-metamorphic stage in the evolution of the NESP. Late-D3 to D4 phase of deformation (<2.685-2.64 Ga) is associated with the formation of oblique shear zones and controlled the emplacement of late GG-D and syenite-carbonatite.