Archean mantle plumes: Evidence from greenstone belt geochemistry

Two main Archean greenstone associations and one minor one are recognized as possible mantle plume-generated sequences. The mafic-plain association, which may represent oceanic plateau remnants, comprises predominantly pillow basalts, variable amounts of komatiite, and small amounts of chert and banded iron formation (BIF). The platform association, which may represent plume-generated magmas erupted through continental crust, overlies tonalitic basement and comprises tonalite conglomerates, quartz arenites, carbonates and BIF, overlain by komatiites and basalts. A third but minor association, comprising andesitic to felsic arc rocks overlain by komatiitebasalt sequences, may represent arcs that rifted in response to mantle plumes. Basalts and komatiites from these greenstone associations have Th/Ta and La/Yb ratios similar to those of oceanic plateau basalts with plume affinities. If we employ these lithologic and geochemical screens, we find that ∼35% of Late Archean (3.0-2.5 Ga) and 80% of Early Archean (>3.0 Ga) greenstones may have plume affinities. Plume-related greenstones contaminated by continental crust acquire a "pseudosubduction" zone geochemical signature (Ta and Nb depletion and Th enrichment). For this reason, estimated percentages should be considered to be minimums. High Th/Ta ratios may reflect crustal contamination or recycling of continental crust within mantle plumes. Low Th/Ta and high La/Yb ratios may reflect melting at different depths within mantle plumes or recycling of oceanic crust within plumes. The relative abundance of plume-related greenstones in the Archean may reflect the warm, less easily subducted Archean oceanic lithosphere. However, the variety of Archean plume-generated assemblage types, combined with their abundance, may suggest instead that mantle plumes were more widespread in the Archean than subsequently.