Geodynamic processes, continental growth, and mantle evolution recorded in late Archean greenstone belts of the southern Superior Province, Canada

Abstract

An episodic continental growth model, by lateral spread of subduction–accretion complexes, is proposed for the late Archean southern Superior Province, based on structural relationships and high precision trace element data for the 2.7 Ga Wawa and Abitibi subprovinces. Two principal volcanic associations are recognized: (1) tholeiitic basalt–komatiite; and (2) tholeiitic to calc-alkaline bimodal basalt–rhyolite. Tholeiitic basalts of the former association are characterized by near-flat REE patterns and complex Th–U–Nb–LREE systematics; rare transitional to alkaline basalts and Al-depleted komatiites have fractionated REE patterns and OIB-like trace element signatures. This is interpreted as representing an oceanic plateau derived from a heterogeneous mantle plume. The latter bimodal association has fractionated REE patterns and negative Nb, Ta, P, and Ti anomalies typical of arc magmas. Turbidites plot on mixing hyperbolae between mafic and felsic end members. This association is interpreted as an arc–trench system. Tonalite plutons derived from partial melting of subducted oceanic slabs intrude the subduction–accretion complex as the magmatic arc axis migrated towards the trench. In the Abitibi belt, boninite-series low-Ti tholeiite flows stratigraphically intercalated with the basalt–komatiite association signifying coeval eruption of plume and arc lavas. Plateau tholeiites are characterized by Nb/U, Th/U, Nb/Th, Nb/La, Th/La, and La/Sm pm ratios that span the primitive mantle values. Arc associations have Nb/U, Nb/Th, and Nb/La ratios that are systematically lower than the primitive mantle ratios, but have La/Sm pm ratios that are systematically higher than the primitive mantle ratio. The variability of these ratios in oceanic plateau basalts is interpreted in terms of recycling variable quantities of oceanic crust processed through a subduction zone (high Nb/U, Nb/Th), and complementary arc crust (low Nb/U, Nb/Th) into the source of the plumes. In the Wawa and Abitibi greenstone belts of the southern Superior Province, eruption of voluminous ocean plateau and island arc volcanic sequences, deposition of kilometer-thick siliciclastic trench turbidites, and intrusion of the supracrustal units by syn- to post-kinematic TTG plutons occurred in the relatively short period of time of 2.75–2.65 Ga. All these events were accompanied by contemporaneous intense poly-phase deformation, regional greenschist to amphibolite facies metamorphism, and terrane accretion along a north–northwest-dipping subduction zone. Collectively, these events are interpreted as a part of a late Archean supercontinent cycle, involving accretion of oceanic plateaus, island arcs, continental fragments, closure of ocean basins, rifting of magmatic arcs, and plume arc interactions.