The 3.0 to 2.9 Ga Lumby Lake belt of the Superior province is composed of plume-related komatiite-tholeiite sequences and calc-alkalic volcanic units, and tonalite-trondhjemite-granodiorite units formed by coeval magmatism; they are all intercalated throughout a 100 m.y. interval. These observations are inconsistent with plateau-accretion models of crustal growth. Sporadic subduction of plume-modified ocean spreading centers, followed by plume impingement beneath a northern Superior province cratonic nucleus, more readily accounts for the long duration of coexisting plume- and arc-type volcanism. The recognition of such complex geodynamic settings in the Archean has important consequences for crustal-growth models.