The geochemistry of Neoarchean (ca. 2700 Ma) tholeiitic basalts, transitional to alkaline basalts, and gabbros, Wawa Subprovince, Canada: Implications for petrogenetic and geodynamic processes

Abstract

Neoarchean (ca. 2700 Ma) Wawa greenstone belts, Superior Province, contain two lithologically and geochemically distinct volcanic associations: (1) a plume-derived oceanic plateau association; and (2) a subduction-derived oceanic island arc association. The oceanic plateau association is composed of three groups: (1) voluminous tholeiitic basalts with near-flat REE patterns; (2) volumetrically minor moderately LREE-depleted tholeiitic basalts and Al-undepleted komatiites; and (3) volumetrically minor moderately to strongly LREE-enriched transitional to alkaline basalts and Al-depleted komatiites. This geochemical diversity is comparable to those of Phanerozoic oceanic plateaus and ocean islands. The oceanic island arc association is characterized mainly by variably LREE-enriched and Nb-depleted (relative to Th and La) tholeiitic to calc-alkaline basalts, andesites, dacites and rhyolites, with minor adakites, magnesian andesites, and Nb-enriched basalts. All these lavas are intruded by subduction-derived late- to post-tectonic gabbros, tonalites, trondhjemites, granodiorites, and monzonites. Within the Wawa subprovince the Schreiber–Hemlo greenstone belt is dominated by plume-derived oceanic plateau volcanic rocks (60–80%), whereas the Winston Lake and Manitouwadge greenstone belts are composed dominantly of subduction-derived (70–90%) volcanic rocks. New major and trace element, and Sm–Nd isotope data are presented for volcanic rocks from the Schreiber–Hemlo, Winston Lake, and Manitouwadge greenstone belts, to address their petrogenetic origin. In addition, new major and trace element data are reported for subduction-derived gabbros from the Schreiber–Hemlo greenstone belt. The geochemical diversity of the oceanic plateau association is consistent with a compositionally heterogeneous mantle plume source. Depleted initial Nd isotopic compositions ( ɛ Nd = +1.7 to +2.2) indicate that transitional to alkaline basalts were derived from a long-term LREE-depleted mantle source. It is shown that the origin of transitional to alkaline lavas can be explained by mixing of LREE-depleted and LREE-enriched melts in an upwelling mantle plume. Depleted to enriched LREE patterns in the plume-derived association are consistent with variable degrees of melting that took place over a range of temperatures and pressures as the plume ascended towards the surface. On the basis of field relationships and geochemical data, it is suggested that the geodynamic evolution of the Schreiber–Hemlo greenstone belt began with the initiation of a subduction zone at the margin of an oceanic plateau at about 2725 Ma, forming an intra-oceanic island arc. In response to plate reorganization and subducted slab rollback, the island arc underwent extension leading to the opening of a backarc basin. Hydrothermal alteration of backarc oceanic crust resulted in the formation of volcanogenic massive sulphide (VMS) deposits in the Winston Lake and Manitouwadge greenstone belts.