Open-system evolution versus source control in basaltic magmas: Matachewan–Hearst dike swarm, Superior Province, Canada

Abstract

The 2.45 Ga Matachewan–Hearst dike swarm was emplaced over 250 000 km2 in diverse granitoid–greenstone and metasedimentary terranes of the Superior Province of Canada. The Fe-rich tholeiitic dikes host large, uniform plagioclase megacrysts and display significant trace-element variations, e.g., (La/Sm)N = 0.62–2.23, not correlated to terrane lithologies.Fractional crystallization alone cannot produce these variations or simultaneously account for both major- and trace-element abundances. Combined periodic replenishment–fractional crystallization (RFC) in shallow magma chambers is consistent with major- and trace-element concentrations and with field evidence for periodic magma injection within the dikes. RFC cannot, however, produce the observed variation in incompatible-trace-element ratios, e.g., (La/Sm)N. Models invoking mixed mantle sources are unsuccessful at reproducing trace-element trends. Combined assimilation–fractional crystallization (AFC) models, assuming depleted parental magmas and using crustal rock data from xenoliths and from the Kapuskasing Structural Zone, can accommodate the trace-element variations, including the light-rare-earth-element enrichment and the observed relative depletions of the high-field-strength elements. The AFC process apparently took place in the lower crustal regions from where evolved magmas were periodically transported to shallow chambers dominated by RFC.