Archean magmatism and deformation were not products of plate tectonics

Abstract

The granite-and-greenstone terrains that dominate upper crust formed from about 3.6 to about 2.6 Ga, and record magmatic and tectonic processes very different from those of a younger time. They indicate heat loss by the Archean Earth primarily by voluminous magmatism from a mantle much hotter than that of the present. Plate-tectonic processes were not then operating. The distinctive array of petrologic, structural and stratigraphic features that characterize Phanerozoic convergent-plate systems—ophiolites, magmatic arcs, accretionary wedges, fore-arc basins, etc.—have no viable analogues in Archean terrains. Purported Archean plate-tectonic indicators consist merely of rock types that even superficially resemble actual Phanerozoic indicators only when considered in isolation from their association and structure. Archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite-type successions, they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic-volcanic rocks, and they require a mantle about 200°C hotter than now. Archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions, and they lack the setting and disruption that characterize modern accretionary wedges.The lithologic, structural and stratigraphic assemblages that typify Proterozoic and Phanerozoic rifted and reassembled margins also lack Archean analogues, and no evidence has been found for Archean rifting, rotation, and reassembly of continental plates.Conversely, characteristic Archean assemblages lack modern equivalents in any tectonic setting. Ultramafic lavas, erupted at liquidus temperatures, are voluminous. Granite-and-greenstone terrains have no modern analogues. Greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by, and sinking between, large rising diapiric, elliptical composite batholiths. The batholiths include both the products of new crustal melts and variably remobilized mid-crustal gneisses. The greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions, and are not relics of successively formed volcanic chains. Little deformation generally preceded the diapirism, and metamorphism was primarily of contact type. The regionally uniform areal density and accordant crustal level of the diapiric batholiths, their contacts primarily against the oldest strata of the synforms, their general age 10–20 million years younger than most of the flanking stratiform rocks, and considerations of high Archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating. Diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust, producing the orientations of the batholiths. Rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it, resulting in cooling of the deeper crust and subjacent mantle, and thus cratonization. The upper crust, containing the granite-and-greenstone aggregates, was decoupled from the gneissic middle crust, which underwent flattening and extension subparallel to the elongation of the shallow batholiths. This deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle.The early Earth was probably wholly molten. The surface of the Earth, like that of the Moon, must have been wholly recycled by impacts before 3.9 Ga and heavily modified by them until 3.8 Ga. Zircons as old as 4.2 Ga have been found as clastic grains in much younger Archean quartzites, and polycyclic migmatites, last partly melted and reconstituted under hydrous conditions only after 3.6 Ga, contain relict zircons as old as 4.0 Ga. The lithologies of the early Earth protoliths in which these zircons formed have not been established, but impact melts and breccias must be represented, and magma-ocean fractionates may be. The nature of the transition in tectonic style into the granite-and-greenstone mode is unknown.Plate tectonic rifting and convergence were operating by about 2.0 Ga and were in an essentially modern mode by about 0.8 Ga. The nature of the transition from the granite-and-greenstone mode at about 2.6 Ga to plate mode by about 2.0 Ga has yet to be defined. The change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense, but hydrated, delaminated Archean crust sank into it.