No abstract available.

Modern-style plate tectonics can be tracked into the geologic past with petrotectonic assemblages and other platetectonic indicators. These indicators suggest that modern plate tectonics were operational, at least in some places on the planet, by 3.0 Ga, or even earlier, and that they became widespread by 2.7 Ga. The scarcity of complete ophiolites before 1 Ga may be explained by thicker oceanic crust and preservation of only the upper, basaltic unit. The apparent absence of blueschists and ultrahigh-pressure metamorphic rocks before ca. 1 Ga may reflect steeper subduction geotherms and slower rates of uplift at convergent margins. It is unlikely that plate tectonics began on Earth as a single global “event” at a distinct time, but rather it is probable that it began locally and progressively became more widespread from the early to the late Archean.

Abstract Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

In this review we summarize the major lithological and geochemical characteristics of the Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt, Nuuk region, southern West Greenland. In addition, the geological characteristics of the Ivisaartoq greenstone belt are compared with those of other Archean greenstone belts in the area. The Ivisaartoq greenstone belt is the largest Mesoarchean supracrustal lithotectonic assemblage in the Nuuk region. The belt contains well-preserved primary magmatic structures including pillow lavas, volcanic breccias, and cumulate (picrite) layers. It also includes variably deformed gabbroic to dioritic dikes, actinolite schists, serpentinites, siliciclastic sediments, and minor cherts. The Ivisaartoq rocks underwent at least two stages of postmagmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. The trace element systematics of the least altered rocks are consistent with a subduction zone geodynamic setting. On the basis of lithological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc/backarc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean suprasubduction zone oceanic crust. The Sm-Nd isotope system appears to have remained relatively undisturbed in picrites, tholeiitic pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (ɛ Nd = +4.2 to +5.0) than gabbros, diorites, and tholeiitic basalts (ɛ Nd = +0.3 to +3.1), consistent with a strongly depleted mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (>90%) of Ca-plagioclase and are interpreted as anorthositic cumulates of the lower oceanic crust brought to the surface by upwelling gabbroic magmas. Alternatively, the inclusions may represent the xenoliths from older (>3075 Ma) anorthositic crust onto which the Ivisaartoq magmas were emplaced as an autochthonous sequence. However, no geological evidence has been found for such older anorthositic crust in the region. The anorthositic cumulates have significantly higher initial ɛ Nd values (+4.8 to +6.0) than the surrounding gabbroic matrix (+2.3 to +2.8), suggesting two different mantle sources for these rocks.

The distribution of lamprophyre-hosted Neoarchean diamond deposits in the southern Superior Province, Canada, corresponds to that of coeval giant (>100 t Au) examples of “orogenic” gold deposits, which are typically associated with quartz-carbonate veining. This common association in the southern Superior Province, and at Yellowknife in the Slave craton, suggests that the occurrence of both diamonds and gold was promoted by the same geodynamic factors. A previously proposed subduction diamond model invokes flat subduction of buoyant oceanic plateau crust as the only means of entraining diamonds in shoshonitic lamprophyres, which are generally derived from relatively shallow mantle depths. Computer modeling of the thermal evolution and dehydration processes associated with this tectonic scenario clarifies observations made in present-day flat subduction settings and suggests many factors that should enhance the hydrothermal mineralization systems responsible for Archean and post-Archean orogenic gold deposits. The case for links among mantle plume–derived oceanic plateaus, crustal growth, and anomalously large gold deposits is strengthened by the newly recognized association of oxidized granites, lamprophyres, and diamonds at 3.1 Ga in the Kaapvaal craton and by evidence for similar recurring gold-diamond ± lamprophyre associations throughout the geologic record, including the Mother Lode deposits of California.

Considerable geochemical evidence supports initiation of plate tectonics on Earth shortly after the end of the Hadean. Nb/Th and Th/U of mafic-ultramafic rocks from the depleted upper mantle began to change from 7 to 18.2 and 4.2 to 2.6 (respectively) at 3.6 Ga. This signals the appearance of subduction-altered slabs in general mantle circulation from subduction initiated by 3.9 Ga. Juvenile crustal rocks began to show derivation from progressively depleted mantle with typical igneous ɛ Nd : ɛ Hf = 1:2 after 3.6 Ga. Cratons with stable mantle keels that have subduction imprints began to appear by at least 3.5 Ga. These changes all suggest that extraction of continental crust by plate tectonic processes was progressively depleting the mantle from 3.6 Ga onwards. Neoarchean subduction appears largely analogous to present subduction except in being able to produce large cratons with thick mantle keels. The earliest Eoarchean juvenile rocks and Hadean zircons have isotopic compositions that reflect the integrated effects of separation of an early enriched reservoir and fractionation of Ca-silicate and Mg-silicate perovskite from the terrestrial magma oceans associated with Earth accretion and Moon formation, superposed on subsequent crustal processes. Hadean zircons most likely were derived from a continent-absent, mafic to ultramafic protocrust that was multiply remelted between 4.4 and 4.0 Ga under wet conditions to produce evolved felsic rocks. If the protocrust was produced by global mantle overturn at ca. 4.4 Ga, then the transition to plate tectonics resulted from radioactive decay-driven mantle heating. Alternatively, if the protocrust was produced by typical mantle convection, then the transition to plate tectonics resulted from cooling to the extent that large lithospheric plates stabilized.

The early continental crust is dominated by high-grade gneisses with the composition of sodic granites (the tonalite-trondhjemite-granodiorite or TTG suite) that date as far back as >3800 Ma. These are considered by many to be formed in subduction zones, and so have a critical role in the discussion about when plate tectonics may have begun. Trace elements can be used to learn about the identity of minerals in the source rocks during melting, but only indirectly to infer tectonic environments. The integrated results from experimental petrology and major and trace element geochemistry of the TTG suite indicate that most of them formed by melting of garnet amphibolites of broadly basaltic composition. This can explain low Nb/Ta coupled with high Zr/Sm, as well as low concentrations of HREEs. Melting of eclogite probably increased in importance in the Late Archean, as shown by an increase in Nb/Ta. Melting of garnet amphibolite can be achieved either in subduction zones at appropriate geotherms or in the lower reaches of thick basaltic crust. Water contents must be much higher than in the original basalts, indicating hydrothermal alteration at near-surface conditions. Thus, the thick crust scenario requires volcanic piling to deeply bury hydrothermally altered basalts, and also delamination of underlying thick cumulates. If subduction occurred in the Archean (implying the operation of plate tectonics), then with a slightly higher average mantle temperature, subduction geotherms would have been disproportionately hotter than today. However, there is no evidence for large volumes of TTG gneisses formed by melting of garnet-free amphibolites in the Early Archean, which constrains the average mantle temperature to be only marginally hotter than on modern Earth. In the Early Archean, melting of more magnesian volcanics and cumulates produced melts and prevented the production of voluminous continental crust with low-SiO 2 >55 wt% SiO 2 . Given current trace element evidence, the most likely scenario for Archean tectonics is a slightly modified plate tectonics with only marginally hotter average geotherms, but substantially hotter subduction geotherms, enabling melting of garnet amphibolites in the Late Archean.

The Steep Rock Group, Little Falls assemblage, Finlayson greenstone belt, and Lumby Lake greenstone belt form a once-continuous Mesoarchean terrane at the southern margin of Wabigoon subprovince, Canadian Shield. Synchronous with eruptive activity, oceanic plateau basalts in this region were intruded by tonalitic batholiths, which fed intermediate to felsic volcanic cones and associated mass-flow sediment aprons. A sedimentary succession capping the volcanic pile records stream-induced channel incision into upraised basalt and tonalite, feeding sediment to a delta complex. Base-level rise caused the incised channels to backfill, stromatolitic carbonates to develop in shallow-marine areas, and finally, deposition of iron formation to dominate. A Mesoarchean terrane was also examined in the Wallace Lake area of Manitoba. Here a kilometer-thick, transgressive succession records a transition from fluvial to delta-front to prodelta turbiditic environments, with eventual drowning and siliciclastic sediment starvation leading to dominance by carbonate and iron formation deposition. Sedimentary rocks that cap the oceanic Cretaceous to Holocene Kerguelen Plateau are a direct analogue for the depositional environments, sediment source rocks, and drowning events that define the stratigraphy of the Mesoarchean successions studied here. The Steep Rock–Lumby oceanic plateau formed and actively grew during an interval of 230 m.y. This sequence produces serious problems for Mesoarchean tectonic models in which increased heat dissipation produces fast spreading or numerous spreading ridges. A Mesoarchean, mantle plume–dominated heat dissipation process is in much better agreement with the data presented here.

Where plates converge, one-sided subduction generates two contrasting thermal environments in the subduction zone (low dT/dP ) and in the arc and subduction zone backarc or orogenic hinterland (high dT/dP ). This duality of thermal regimes is the hallmark of modern plate tectonics, which is imprinted in the ancient rock record as penecontemporaneous metamorphic belts of two contrasting types, one characterized by higher-pressure–lower-temperature metamorphism and the other characterized by higher-temperature–lower-pressure metamorphism. Granulite facies ultrahigh-temperature metamorphism (G-UHTM) is documented in the rock record predominantly from the Neoarchean to the Cambrian, although it may be inferred at depth in some younger Phanerozoic orogenic systems. Medium-temperature eclogite–high-pressure granulite metamorphism (E-HPGM) also is first recognized in the Neoarchean, although well-characterized examples are rare in the Neoarchean-to-Paleoproterozoic transition, and occurs at intervals throughout the Proterozoic and Paleozoic rock record. The first appearance of E-HPGM belts in the rock record registers a change in geodynamics that generated sites of lower heat flow than previously seen, inferred to be associated with subduction-to-collision orogenesis. The appearance of coeval G-UHTM belts in the rock record registers contemporary sites of high heat flow, inferred to be similar to modern arcs, abd backarcs, or orogenic hinterlands, where more extreme temperatures were imposed on crustal rocks than previously recorded. Blueschists first became evident in the Neoproterozoic rock record, and lawsonite blueschists, low-temperature eclogites (high-pressure metamorphism, HPM), and ultrahigh-pressure metamorphism (UHPM) characterized by coesite or diamond are predominantly Phanerozoic phenomena. HPM-UHPM registers low to intermediate apparent thermal gradients typically associated with modern subduction zones and the eduction of deeply subducted lithosphere, including the eduction of continental crust subducted during the early stage of the collision process in subduction-to-collision orogenesis. During the Phanerozoic, most UHPM belts have developed by closure of relatively short-lived ocean basins that opened due to rearrangement of the continental lithosphere within a continent-dominated hemisphere as Eurasia was formed from Rodinian orphans and joined with Gondwana in Pangea, and then due to successive closure of the Paleo-Tethys and Neo-Tethys Oceans as the East Gondwanan sector of Pangea began to fragment and disperse. The occurrence of both G-UHTM and E-HPGM belts since the Neoarchean manifests the onset of a “Proterozoic plate tectonics regime,” which evolved during a Neoproterozoic transition to the “modern plate tectonics regime” characterized by HPM-UHPM. The “Proterozoic plate tectonics regime” may have begun locally during the Mesoarchean to Neoarchean and may only have become global during the Neoarchean-to-Paleoproterozoic transition. The age distribution of metamorphic belts that record extreme conditions of metamorphism is not uniform. Extreme metamorphism occurs at times of amalgamation of continental lithosphere into supercratons (Mesoarchean to Neoarchean) and supercontinents (Paleoproterozoic to Phanerozoic), and along sutures due to the internal rearrangement of continental lithosphere within a continent-dominated hemisphere during the life of a supercontinent.

The Archean mantle was probably warmer than the modern one. Continental plates underlain by such a warmer mantle would have experienced less subsidence than modern ones following extension because extension would have led to widespread melting of the underlying mantle and the generation of large volumes of mafic rock. A 200 °C increase in mantle temperature leads to the production of nearly 12 km of melt beneath a continental plate extended by a factor of 2, and the resulting thinned plate rides with its upper surface little below sea level. The thick, submarine, mafic-to-ultramafic volcanic successions on continental crust that characterize many Archean regions could therefore have resulted from extension of continental plates above warm mantle. Long-term subsidence of passive margins is driven by thermal relaxation of the stretched continental plate (cf. McKenzie). With a warmer mantle, the relaxation is smaller. For a continental plate stretched by a factor of 2, underlain by a 200 °C warmer mantle than at present, the cooling-driven subsidence drops from 2.3 km to 1.1 km. The combined initial and thermal subsidence declines by more than 40%, and by even more than this if initial continental crustal thicknesses were lower. The greatly reduced subsidence results in a concomitant decline in accommodation space for passive-margin sediments and may explain the scarcity of passive-margin sequences in the Archean record. The formation of diamonds in the Archean requires geotherms similar to modern ones, which in turn probably reflect the presence of cool mantle roots beneath the continents. Stretching of continents underlain by cool mantle roots would yield passive margins similar to modern ones. Thus, development of significant passive margins may have occurred only through rifting of continents underlain by cool mantle roots. Furthermore, the widespread subcontinental melting associated with rifting of continents devoid of roots may have been a significant contributor to development of the roots themselves.

Today, plate tectonics is the dominant tectonic style on Earth, but in a hotter Earth tectonics may have looked different due to the presence of more melting and associated compositional buoyancy as well as the presence of a weaker mantle and lithosphere. Here we review the geodynamic constraints on plate tectonics and proposed alternatives throughout Earth’s history. Observations suggest a 100–300 °C mantle potential temperature decrease since the Archean. The use of this range by theoretical studies, parameterized convection studies, and numerical simulations puts a number of constraints on the viability of the different tectonic styles. The ability to sufficiently cool early Earth with its high radiogenic heat production forms one of the major constraints on the success of any type of tectonics. The viability of plate tectonics is mainly limited by the availability of sufficient driving forces and lithospheric strength. Proposed alternative mechanisms include local or global magma oceans, diapirism, independent dynamics of crust and underlying mantle, and large-scale mantle overturns. Transformation of basaltic crust into dense eclogite is an important driving mechanism, regardless of the governing tectonic style.

The Abitibi-Opatica terrane is defined to include the Abitibi granite-greenstone Subprovince and the Opatica granite-gneiss domain in southeastern Superior Province, Canada. We combine the geological, structural, geochronological, and geochemical knowledge base for the region, with new geochemical data for suites of granitic rocks, in order to establish a testable model for the geodynamic setting and the tectonomagmatic evolution of the Late Archean crust. The geochemistry of TTG orthogneiss and plutons are correlated, petrogenetically and temporally, with the published data and interpretations for the volcanic stratigraphy. The geochemistry of later granodiorite plutons is correlated with the crustal melting signatures of the youngest volcanic assemblage. Putting the geochemical data and interpretations into a framework with data on crustal structure, crustal thickness, and geochronology allows us to define the precollisional tectonomagmatic history of the Abitibi-Opatica terrane. A geodynamic-tectonic model is proposed, involving subduction of an ocean basin beneath an existing, magmatically active and partially differentiated oceanic plateau. The geochemical signature of “plume-arc interaction” is attributed to subduction that was initiated under the magmatically active oceanic plateau, in the presence of a still-active plume. The proposed plate tectonic model explains the presence of plume-type and subduction-type signatures in the volcanic stratigraphy, and in the TTG gneiss and plutons, and requires a single period of plate convergence and subduction that lasted for ~35 million years, ending in a tectonic collision event, ca. 2700 Ma. We propose that the interstratification of plume-type and subduction-type lavas, and the concomitant emplacement of TTG plutons with slab-melting characteristics, might be explained by the formation of a slab window in the subplateau subduction zone.