ABSTRACT This contribution attempts to recount our collective progress in understanding the Archean–Hadean Earth system over the past 50 yr. Many realms of the geological sciences (geochemistry, petrology, geophysics, structural geology, geobiology, planetary science, and more) have made substantive contributions to this effort. These contributions have changed our understanding of the Archean–Hadean Earth in five major areas: (1) the expanse of Archean–Hadean time; (2) tectonics and lithospheric evolution, particularly possible analogs for the sites of modern, primary crust production and mantle differentiation (e.g., magmatic arcs, ocean ridges, and large igneous provinces); (3) evolution of the atmosphere-hydrosphere system, and its impact on the evolution of Earth’s endogenic and exogenic systems; (4) the history of liquid water, particularly at the ocean scale; and (5) the origin and development of the biosphere and its impact on the geologic record. We also emphasize that much of the progress made in understanding the evolution of early Earth systems over the past 50 yr has been fueled by important technological advances in analytical geochemistry, such as the advent of ion probes for U-Pb zircon geochronology, inductively coupled plasma–mass spectrometry for trace-element and Hf isotopic analyses, Raman spectroscopy in organic geochemistry, and molecular reconstructions in biology. Within this context, we specifically review progress in our understanding of the Eoarchean history of southern West Greenland as an example of the value of continuous integration of careful geologic observation and mapping with evolving technology, which have combined to further open this window into Earth’s earliest systems.

Late Paleozoic granitic rocks have been identified in two drill holes south of the proposed Alleghanian suture separating the Suwannee terrane from terranes to the north. These granites are the only known expression of magmatism on the accreted Gondwanan plate during the Alleghanian collision of Laurentia and Gondwana, an event that represents the final episode in the assembly of Pangea. Zircons from samples of granite from drill holes in southwestern Georgia and northern Florida were analyzed by ion probe, yielding igneous crystallization ages of 294 ± 6 and 296 ± 4 Ma (2σ), respectively. Xenocrystic zircons with ages of ca. 1.0–1.2 Ga and ca. 560 Ma were also obtained from one sample. The Proterozoic ages of these grains are consistent with previous proposals of a Gondwanan origin for the Suwannee terrane, particularly those proposals involving the Orinoquian or Sunsas-Rondonian provinces of South America. The major- and trace-element geochemical compositions of the granites are inconsistent with generation in a subduction environment. Instead, the data support models of the Alleghanian event in the Southern Appalachians involving an oblique collision between Laurentia and Gondwana followed by a late-stage, postorogenic episode of magmatism related to a postcollisional, lithospheric collapse event that may have included delamination.

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.

Quartzofeldspathic gneisses and metamorphic mafic rocks are the dominant lithologies in the Indian Creek and Pony–Middle Mountain Metamorphic Suites of the Tobacco Root Mountains. Field relations, geochemical discriminant analysis, and isotopic systematics indicate that these rocks derive from a bimodal volcanic suite ca. 3.3 Ga. The quartzofeldspathic gneisses contain sodic rocks of the tonalite-trond-hjemite-granodiorite suite as well as potassic varieties. This suite of rocks most likely contains some lithologies derived from sedimentary or volcaniclastic sources, and there is evidence that alkali metasomatism occurred prior to or during subsequent major tectonothermal events. The entire suite of gneisses and metamorphic mafic rocks has geochemical characteristics that are indicative of an active continental arc setting, with deposition most likely in an extensional, backarc setting similar to the Mesozoic through Tertiary rocks of the eastern Sierra Nevada Mountains or Mojave Desert. The formation of these rocks represents an early, distinct stage of crustal evolution that preceded the (unconformable?) deposition of one or more platform-type sedimentary sequences (e.g., marbles, pelitic schists, quartzites, banded iron formations). All primary lithologic contacts and textures or structures indicative of possible protoliths have been largely obliterated due to transposition during Archean and Paleoproterozoic (ca. 2.4 and ca. 1.8 Ga) deformation and metamorphism.

U-Pb analyses of zircons from gneisses, anatectic leucosome, metasedimentary rocks, and a younger (metamorphosed) mafic dike from the Tobacco Root Mountains of southwestern Montana document a Precambrian history that extends from at least 3.90–1.77 Ga. The oldest U-Pb age reported here (3.8 Ga) is from a detrital zircon from a quartzite within the Spuhler Peak Metamorphic Suite, although younger ages of clearly detrital grains suggest the protolith was deposited subsequent to 3.2 Ga. Alternatively, a Pb-Pb age of ca. 2.45 Ga from a single subhedral zircon from this quartzite suggests the quartzite, and perhaps other Spuhler Peak Metamorphic Suite lithologies, may have formed in the Proterozoic. An Archean age, however, seems most compatible with the Archean Sm-Nd model ages of mafic and metasedimentary components of the Spuhler Peak Metamorphic Suite and the age distribution of zircons from the quartzite, which is very similar to the age distribution present in Archean quartzites in the region. The Spuhler Peak Metamorphic Suite lies in tectonic contact with volumetrically dominant, Archean, quartzofeldspathic gneisses and intercalated metasedimentary rocks. The protoliths of these gneisses were apparently emplaced 3.2–3.4 Ga, and are interpreted to be the basement upon which the intercalated (meta)sedimentary rocks were deposited. U-Pb analyses of zircons from anatectic leucosome near the boundary between the gneisses and the Spuhler Peak Metamorphic Suite, however, yield a significant population of 1.77 Ga grains, which are interpreted to have crystallized from the leucosome. All other grains are Archean (to 3.48 Ga) and interpreted to derive from the metasedimentary source of the leucosome. In addition, U-Pb analyses of zircons extracted from a granulite facies mafic dike that cuts across Archean gneissic banding indicate the dike was intruded at 2.06 Ga, but reached granulite facies at 1.76 Ga. Structural, petrologic, and geochronologic data suggest all lithologies experienced granulite facies metamorphism at ca. 1.77 Ga and that the Spuhler Peak Metamorphic Suite was tectonically emplaced after 2.06 Ga, but before 1.77 Ga. This Paleoproterozoic tectonic activity is most likely a result of burial during terrane collision (e.g., the juxtaposition of the Wyoming and Hearne provinces) and/or to postcollisional mafic underplating.

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