LONG-LIVED MANTLE-PLUME INFLUENCE ON AN ARCHEAN PROTOCONTINENT - GEOCHEMICAL EVIDENCE FROM THE 3 GA LUMBY LAKE GREENSTONE-BELT, ONTARIO, CANADA

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 volca nic units, and tonalite-trondhjemite-granodiorite units formed by coev al 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 pr ovince cratonic nucleus, more readily accounts for the long duration o f coexisting plume- and are-type volcanism. The recognition of such co mplex geodynamic settings in the Archean has important consequences fo r crustal-growth models.