Archean Tectonics and Magmatism

Abstract

The Earth began to form ∼4.56 Ga, and probably was entirely molten about 4.45 Ga and likely also before. Felsic igneous rocks of unknown provenance were in the crust by 4.3 Ga (clastic zircons to 4.3 Ga in <3.0 Ga quartzites, and relict zircons to 4.0 Ga in migmatites stabilized <3.6 Ga). Lunar analogy requires that the entire surface of the Earth was recycled by impact melting and brecciation before 3.9 Ga, and greatly modified until 3.8 Ga, but no certain relics of this history have been identified. When the fog clears and Archean time proper begins, at ∼3.6 Ga, the oldest granite-and-greenstone terrains are beginning to form. The granite-and-greenstone terrains that dominate the upper crust, formed from ∼3.6 to 2.6 Ga, record magmatic and tectonic processes that have no close younger analogues. They indicate heat loss by the Archean Earth primarily by voluminous magmatism. The upper mantle was 200°C, perhaps 300°C, hotter than at present. Magmatism in the granite-and-greenstone terrains began with regional volcanic plains, erupted over and around discontinuous basements of recently stabilized migmatitic tonalites of unknown provenance. Although the oldest stratified rocks are quartzites or felsic volcanic rocks in a number of regions, the oldest voluminous rocks typically are basalts and ultramafic lavas that were erupted at liquidus temperatures. These typically are succeeded by felsic volcanic rocks and often by repeated intercalations of mafic and felsic rocks. The stratiform rocks are preserved in “greenstone belts”—anastomosing networks of upright synforms formed by crowding aside by, and sinking between, large rising diapiric, elliptical composite batholiths. The belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions, not by relics of linear volcanic features. Little deformation generally preceded the diapirism, and metamorphism was primarily of contact type. The regionally uniform spatial density and accordant crustal level of the diapiric batholiths, their contacts primarily against the oldest strata of the synforms, their general age at 10 to 20 m.y. younger than most of the flanking stratiform rocks except the younger felsic volcanic 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. The rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high within it. Cooling of deeper crust and subjacent mantle, and thus cratonization, followed. 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. Plate-tectonic processes were not then operating. The distinctive array of petrologic, structural, and stratigraphic features that characterize Phanerozoic subduction and collision systems-ophiolites, magmatic arcs, accretionary wedges, forearc basins, etc.-have no viable analogues in Archean terrains. Purported Archean plate-tectonic indicators consist merely of rock types that 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, and they often depositionally overlie felsic basement rocks and commonly are intercalated with sedimentary and felsic-volcanic rocks. Archean graywackes are coherent strata derived from nearby volcanic rocks during the diapiric-granite histories of their regions, and they lack the setting and disruption that characterize modern accretionary wedges. The lithologic, structural, and stratigraphic assemblages that characterize Proterozoic and Phanerozoic rifted and reassembled margins similarly have no Archean analogues, and no evidence has been found for Archean rifting, rotation, and reassembly of continental plates. Plate-tectonic rifting and convergence were operating by ∼2.0 Ga, perhaps earlier, and were in an essentially modern mode by ∼0.8 Ga. The nature of the transition from the granite-and-greenstone mode at 2.6 Ga to plate mode by 2.0 Ga is poorly defined, but 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.