Archean geology

Archean, Paleoproterozoic, and Mesoproterozoic rocks, assemblages, and structures differ greatly both from each other and from modern ones, and lack evidence for subduction and seafloor spreading such as is widespread in Phanerozoic terrains. Most specialists nevertheless apply non-actualistic plate-tectonic explanations to the ancient terrains and do not consider alternatives. This report evaluates popular concepts with multidisciplinary information, and proposes options. The key is fractionation by ca. 4.45 Ga of the hot young Earth into core, severely depleted mantle, and thick mafic protocrust, followed by still-continuing re-enrichment of upper mantle from the top. This is opposite to the popular assumption that silicate Earth is still slowly and unidirectionally fractionating. The protocrust contained most material from which all subsequent crust was derived, either directly, or indirectly after downward recycling. Tonalite, trondhjemite, and granodiorite (TTG), dominant components of Archean crust, were derived mostly by partial melting of protocrust. Dense restitic protocrust delaminated and sank into hot, weak dunite mantle, which, displaced upward, enabled further partial melting of protocrust. Sinkers enriched the upper mantle, in part maintaining coherence as distinct dense rocks, and in part yielding melts that metasomatized depleted-mantle dunite to more pyroxenic and garnetiferous rocks. Not until ca. 3.6 Ga was TTG crust cool enough to allow mafic and ultramafic lavas, from both protocrust and re-enriched mantle, to erupt to the surface, and then to sag as synclinal keels between rising diapiric batholiths; simultaneously upper crust deformed ductily, then brittly, above slowly flowing hot lower TTG crust. Paleoproterozoic and Mesoproterozoic orogens appear to be largely ensialic, developed from very thick basin-filling sedimentary and volcanic rocks on thinned Archean or Paleoproterozoic crust and remaining mafic protocrust, above moderately re-enriched mantle. Subduction, and perhaps the continent/ocean lithospheric dichotomy, began ca. 850 Ma – although fully modern plate- tectonic processes began only in Ordovician time – and continued to enrich the cooling mantle in excess of partial melts that contributed to new crust. “ Plumes ” from deep mantle do not operate in the modern Earth and did not operate in Precambrian time.

The Carswell structure in the western Athabasca Basin, northern Saskatchewan (Canada), has previously been interpreted as an eroded impact structure with a minimum diameter of ~36 km, the outer margin of which is broadly defined by an outer ring of sediments composed of the only algal reefs observed in the Athabasca Group, the Carswell Formation. This ring surrounds an 18-km-wide uplifted basement core composed of gneiss units of Archean to Paleoproterozoic age that display shatter cones, planar deformation features (PDFs), pseudotachylyte veins, and impact melts and breccias (Cluff melt sheet, Cluff breccias) indicating that pressures and temperatures locally exceeded 60 GPa and 1500 °C. Detailed analysis of the basement–Athabasca Group contact from field and drill core samples indicates that shock features are not present in the Athabasca Group sediments in direct contact with the highly shocked basement gneisses. This pattern is inconsistent with a post–Athabasca Group age for the impact. Moreover, our study has revealed PDF-bearing quartz grains in basal units of the Athabasca Group 14 km south of the southern edge of the basement core (outside the estimated outer ring). The new proposed model suggests that the impact event is of pre-Athabasca (Proterozoic) age and that it produced a multiring structure that controlled the paleogeography of the Athabasca Group units in the western part of the basin. The model is well supported by basin analysis and gravity data. The Carswell Formation is the result of algal reefs building on peak-ring–related seamounts at the end of Athabasca Group deposition. The overturned bedding observed locally adjacent to the basement core is interpreted as the result of gravity-driven readjustment of the central uplift.

    Accepted 29 September 2009.

The magmatic and tectonic processes of the pre–2.5 Ga hot, young Earth differed profoundly from those of the modern planet. The ancient rocks differ strikingly in individual and collective composition, occurrence, association, and structure from modern rocks. Widespread forcing of Archean geology into plate-tectonic frameworks reflects unwarranted faith in uniformitarianism and in inappropriate chemical discriminants, and disregard for the lack of features that characterize plate interactions. Archean crust records extreme and prolonged internal mobility and was far too weak and mobile to behave as rigid plates, required, by definition, for plate tectonics. None of the geologic indicators of subduction, arc magmatism, and continental sundering, separation, and convergence have been documented. No Archean oceanic crust or mantle has been recognized, and the only known basement to supracrustal rocks, including the thick basalts, high-Mg basalts, and ultramafi c lavas that typify greenstone successions, consists of tonalite-trondhjemite-granodiorite (TTG) migmatites and gneisses. A thick global melabasaltic protocrust likely formed by ca. 4.45 Ga, and from it TTG suites were extracted by partial melting over the next 2 b.y. Delamination of the increasingly dense restitic protocrust enabled rise of lighter and hotter depleted mantle and hence more melting. The oldest known crustal materials are zircons, which scatter in age back to 4.4 Ga and are recycled in migmatites whose final crystallization was after 3.8 Ga, and in ancient sediments. Earth may have had a dense greenhouse atmosphere, not a hydrosphere, before 3.6 Ga, for the oldest proved supracrustal rocks are of that age, and older felsic crust may have been too hot to permit rise of dense melts. Rigid plates of lithosphere did not stabilize until a billion years after that and then were mostly small and local.

Dense lavas erupted atop mobile felsic crust after 3.6 Ga produced a density inversion that was partly righted by sinking of the volcanic rocks and rising of the subjacent TTG. In some places, the early dense rocks retained cohesion and sank as synclinal keels between rising domiform diapiric batholiths. In others, the early dense rocks sank deep into mobile TTG crust, and only later in Archean time was the felsic substrate strong enough to enable dome-and-keel style. The TTG substrate rose slowly, with variable amounts of partial melting to generate more-fractionated melts and with additions of new TTG from the underlying protocrust, for hundreds of millions of years. The mantle beneath preserved cratons generated ultramafic melts that required a temperature ~300°C hotter than modern asthenosphere ca. 3.5 Ga. Severe and prolonged lateral deformation was superimposed on large parts of some cratons during the era of volcanism and diapirism, obscuring dome-and-keel geology over broad tracts. Lower crust was at high temperature for prolonged periods and fl owed pervasively, coupled discontinuously to the upper crust to produce lateral deformation therein.

Rifting, separation, rotation, and collision of internally more rigid lithosphere fragments began ca. 2.1 Ga, but may have been dominantly intracontinental deformation, quite distinct from modern plate tectonics. The products of this regime differ greatly from those of Phanerozoic plate tectonics, and reflect a transitional era of erratically stiffening lithosphere. An early-depleted upper mantle has been progressively re-enriched, by delamination and subduction of crustal materials, while new “juvenile” crust derived from it has become progressively more depleted, during Proterozoic and Phanerozoic time.

The Middle Marker – horizon H1 of the Hooggenoeg Formation – is the oldest sedimentary horizon in the Barberton greenstone belt and one of the oldest sedimentary horizons on Earth. Herein, we describe a range of carbonaceous microstructures in this unit which bear resemblance to phototrophic microbial biofilms, biose-dimentary structures, and interpreted microfossils in contemporaneous greenstone belts from the Early Archaean. Post-depositional iron-rich fluid cycling through these sediments has resulted in the precipitation of pseudo-laminated structures, which also bear resemblance, at the micron-scale, to certain microbial mat-like structures, although are certainly abiogenic. Poor preservation of multiple putative microbial horizons due to coarse volcaniclastic sedimentation and synsedimentary fragmentation by hydrothermal fluid also makes a conclusive assessment of biogenicity challenging. Nonetheless, several laminated morphologies within volca-niclastic sandstones and siltstones and coarse-grained volcaniclastic sandstones are recognisable as syngenetic photosynthetic microbial biofilms and microbially induced sedimentary structures; therefore, the Middle Marker preserves the oldest evidence for life in the Barberton greenstone belt. Among these biosignatures are fine, crinkly, micro-tufted, laminated microbial mats, pseudo-tufted laminations and wisp-like carbonaceous fragments interpreted as either partially formed biofilms or their erosional products. In the same sediments, lenti-cular objects, which have previously been interpreted as bona fide microfossils, are rare but recurrent finds whose biogenicity we question. The Middle Marker preserves an ancient record of epibenthic microbial communities flourishing, struggling and perishing in parallel with a waning volcanic cycle, an environment upon which they depended and through which they endured. Direct comparisons can be made between environment-level characters of the Middle Marker and other Early Archaean cherts, suggesting that shallow-water, plat-formal, volcanogenic-hydrothermal biocoenoses were major microbial habitats throughout the Archaean.

Le camp minier de Matagami, situé dans le nord de la ceinture archéenne de roches vertes de l’Abitibi au Québec, abrite 19 gisements de sulfures massifs volcanogènes (SMV) connus dont dix ont été exploités depuis 1963. Ces gisements sont de type bimodal-mafique puisqu’associés à des niveaux de roches volcaniques felsiques dans une séquence dominée par les laves mafiques sous-marines. Malgré la richesse minérale de la région, les roches ont été peu étudiées d’un point de vue volcanologique. Pourtant, la taille, la teneur, la forme et la position des gisements de SMV sont contrôlées en partie par l’architecture volcanique. Ailleurs en Abitibi et dans le monde, le lien spatial entre la position des centres éruptifs des unités volcaniques, les conduits hydrothermaux, les failles synvolcaniques et les gisements a été établi, mais ce genre de travaux n’a jamais été réalisé dans le cas de Matagami. Plus largement, la géologie régionale de Matagami n’a pas été mise à jour depuis les années 1980.
Au niveau de la géologie régionale, les travaux réalisés avec le MRN définissent deux grands domaines (« Nord » et « Sud ») séparés par une zone de cisaillement ENE. Le Domaine Nord montre des évidences de compression D2 orientée N-S. Le Domaine Sud est affecté par une compression faible à modérée D2, mais est principalement caractérisé par des plis P1 orientés N-E. La superposition des déformations D1 et D2 crée une géométrie de « dôme et bassin » dans la Plaine Centrale, ce qui a des implications importantes pour l’exploration. Le Domaine Sud contient les secteurs suivants : le Flanc Nord, le Flanc Sud, le Camp Ouest et la Plaine Centrale. La majorité des gisements découverts à Matagami se situent sur le Flanc Sud et le Flanc Nord, alors que les deux autres secteurs représentent des zones intéressantes pour l’exploration régionale.
La séquence stratigraphique régionale contient trois groupes : le Groupe du lac Watson à la base, suivi du Groupe de Wabassee, puis du Groupe de Daniel. Le Groupe du lac Watson (~2726 Ma) contient principalement des laves felsiques tholéiitiques, surmontées par un niveau repère, la Tuffite Clé. Le Groupe de Wabassee (2726-2725 Ma) est principalement composé de laves mafiques à intermédiaires, avec localement des unités felsiques, d’affinité tholéiitique à transitionnelle. Finalement, le Groupe de Daniel (2723 Ma), nouvellement proposé et présent seulement dans la Plaine Centrale, est caractérisé par des roches volcaniques mafiques à felsiques d’affinité transitionnelle à calco-alcaline. La mise en carte de ce dernier groupe montre une géométrie de dômes et bassins dans la Plaine Centrale, avec des pendages faibles. Ces âges correspondent à l’assemblage de Deloro (2730-2724 Ma, Ayer et al., 2002). La datation par méthode U-Pb sur zircons d’une rhyolite de type Watson sous le gisement Caber dans le Camp Ouest permet de la corréler avec la Rhyolite du lac Watson du Flanc Sud, ce qui indique – avec la géochimie – que le même niveau stratigraphique favorable, celui de la Tuffite Clé, est présent également dans le Camp Ouest.
Des travaux plus détaillés ont eu lieu sur le Flanc Sud afin d’en reconstruire l’architecture volcanique. En termes méthodologiques, la géochimie des éléments majeurs et en traces a d’abord servi à la caractérisation des unités présentes, permettant ainsi la création d’une colonne stratigraphique complète. Une fois les unités identifiées, leurs variations de faciès ont été observées au sein de 19 forages et de quelques affleurements. Finalement, afin de contraindre temporellement la mise en place des unités, quatre unités felsiques du Flanc Sud ont été datées par la méthode U-Pb sur zircons.
Sur le Flanc Sud, ces travaux ont montré que deux unités effusives mafiques à intermédiaires importantes étaient présentes dans le Groupe de Wabassee, soit l’Andésite Inférieure et le Basalte Supérieur. La connaissance précise de la stratigraphie du Flanc Sud a permis de corréler des niveaux exhalatifs (« tuffites »), augmentant le potentiel économique de certains secteurs. Par ailleurs, la Rhyolite de Dumagami dans le centre du Flanc Sud a été divisée en deux: la Dacite de Dumagami-O surmontée par la Rhyolite de Dumagami-O.
Les travaux de volcanologie ont montré que les unités volcaniques felsiques du Flanc Sud se mettent en place selon le modèle de coulée de lave de type lobes-hyaloclastite. Dans le cas de la Rhyolite de Bracemac, il s’agirait d’une seule coulée, mais le cas de la Rhyolite du lac Watson, plusieurs coulées sont juxtaposées et superposées, ce qui explique son grand volume total. Le modèle de coulées de type lobes-hyaloclastite implique une présence plus importante de roches fragmentaires qu’au préalablement supposé, principalement au sommet et en bordure des coulées, ce qui a des implications pour la mise en place des gisements de SMV par remplacement sous le fond marin, notamment à Bracemac-McLeod. Finalement, la géochronologie montre que la durée du volcanisme sur le Flanc Sud est de l’ordre 5 Ma au maximum, et que les gisements se sont mis en place à l’intérieur de 2,5 Ma (probablement moins).
Ces différentes approches ont permis la création d’un modèle présentant la reconstruction volcanologique, métallogénique et géochronologique du Flanc Sud, où les principales étapes sont (1) la mise en place de la Rhyolite du lac Watson à partir de plusieurs centres effusifs; (2) la déposition de la Tuffite Clé et la mise en place de plusieurs lentilles de sulfures massifs; (3) la mise en place de la Rhyolite de Bracemac et de la Rhyolite de Dumagami-P; (4) la mise en place de l’Andésite Inférieure et de la Rhyolite/Dacite de Dumagami-O; et (5) la mise en place du Basalte Supérieur et des dernières minéralisations.
Les travaux ont également permis d’établir le lien entre la volcanologie et la minéralisation sur le Flanc Sud. Trois modèles sont proposés pour les SMV: (1) un modèle classique de mise en place de monticules de sulfures sur le fond marin, applicable aux gisements de Mattagami Lake, Orchan Ouest, Orchan, Bell Allard et Isle Dieu; (2) un modèle de lentilles tabulaires, témoins de la circulation de fluides au sein des roches fragmentaires (remplacement sous le fond marin), applicable aux gisements de Bracemac et de McLeod; (3) un modèle de remplacement le long de failles synvolcaniques, créant des lentilles discordantes, applicable aux gisements de Persévérance. Ainsi, au sein d’un même camp minier, plusieurs modèles de formation de lentilles de sulfures peuvent être considérés, selon les faciès volcaniques retrouvés dans la séquence.
Finalement, à un niveau plus régional, les travaux de géochimie ont permis de proposer un modèle pétrogénétique et tectonique suivant la théorie actualiste. Les roches de la région de Matagami se seraient tout d’abord formées en contexte d’arrière-arc océanique (groupes du lac Watson et de Wabassee). Au cours du temps, un changement de pendage dans la plaque subductée aurait fait s’approcher l’arc (Groupe de Daniel).

This review evaluates and rejects the currently dominant dogmas of geodynamics and geochemistry, which are based on 1950s–1970s assumptions of a slowly differentiating Earth. Evidence is presented for evolution of mantle, crust, and early Moho that began with fractionation of most crustal components, synchronously with planetary accretion, into mafic protocrust by ~4.5 Ga.We know little about Hadean crustal geology (N3.9 Ga) except that felsic rocks were then forming, but analogy with Venus, and dating from the Moon, indicate great shallow disruption by large and small impact structures, including huge fractionated impact-melt constructs, throughout that era.

The mantle sample and Archean (b3.9 Ga) crustal geology integratewell. The shallow mantle was extremely depleted by early removal of thick mafic protocrust, which was the primary source of the tonalite, trondhjemite, and granodiorite (TTG) that dominate preserved Archean crust to its base, and of the thick mafic volcanic rocks erupted on that crust. Lower TTG crust, kept mobile by its high radioactivity and by insulating upper crust, rose diapirically into the upper crust as dense volcanic rocks sagged synformally. The mobile lower crust simultaneously flowed laterally to maintain subhorizontal base and surface, and dragged overlying brittler granite-and-greenstone upper crust. Petrologically required garnet-rich residual protocrust incrementally delaminated, sank through low-density high-mantle magnesian dunite, and progressively re-enriched upper mantle, mostly metasomatically. Archean and earliest Proterozoic craton stabilization and development of final Mohos followed regionally complete early delamination of residual protocrust, variously between ~2.9 and 2.2 Ga. Where some protocrust remained, Proterozoic basins, filled thickly by sedimentary and volcanic rocks, developed on Archean crust, beneath which delamination of later residual protocrust continued top-down enrichment of upper mantle. That reenrichment enabled modern-style plate tectonics after ~600 Ma, with a transition regime beginning ~850 Ma.

The Archean‐Proterozoic boundary is placed at 2500 Ma and marks possibly the most dramatic change in Earth’s history. The Great Oxidation Event (GOE) took place between 2.45 and 2.32 Ga as part of an Archean‐Proterozoic transition rather than sharp boundary and postdates the currently established boundary. Apart from geological proxies of atmospheric oxygenation, such as banded iron formation (BIF) abundance and paleosol mineralogy, isotope chemostratigraphy provides the most powerful tool for studying the GOE and establishing the boundary. Sulfur isotopes, and especially mass‐independent fractionation of sulfur, are the best studied proxy, indicating the formation of an ozone layer at ca. 2.33 Ga. Cr and Mo isotopes are more sensitive indicators of surface environment oxygenation, reflecting the redox state of surface seawater. We discuss three different proposals for the placement of the Archean‐Proterozoic boundary and a corresponding Global Stratotype Section and Point: (i) to keep it at 2500 Ma, aided by prominent BIF units, Mo abundance, and Mo isotopes; (ii) to place it at the base of the second Huronian glaciation (ca. 2.35–2.40 Ga), thought to represent a “snowball” event; and (iii) to use the termination of the mass‐independent fractionation of sulfur and the
increase in the δ34S amplitude of sulfides as the main criteria.

Assuming that the period 2.75-2.65 Ga corresponds to a single, but global geodynamic event, we investigate through numerical experiments the mechanisms that could have led to the profound continental reworking that occurred at that time. Although the extent of the crisis at the Earth’s surface pledges in favour of the involvement of mantle plumes, our numerical experiments suggest that the thermal impact of mantle plumes is unlikely to explain both the amplitude and timing of the thermal anomaly, as observed in the Superior Province (Canada) and the Yilgarn Craton (Australia). Similarly, moderate crustal thickening can not lead to significant reworking of the continental crust within the observed time constraint. Crustal thickening with a factor >1.5 is also unlikely because it is not consistent with the moderate metamorphic grade observed at the surface of many Archaean cratons. Burial of a radiogenic crust under a 10 km thick greenstone cover also falls short of explaining, not so much the amplitude and the extent, but the timing of the thermal anomaly. In contrast, the combination of the thermal anomaly related to the greenstone blanketing effect with the heat transfer from a plume head spreading at the top of the thermal boundary layer can adequately explain the amplitude, the timing and the extent of the 2.75-2.65 Ga crisis.
Our favoured model involves a global re-arrangement of convection cells in the deep mantle and formation of multiple mantle plumes. The greenstones emplaced at the surface and the plumes that spread in the thermal boundary layer contributed to heat the crust from both above and below. This produced massive crustal partial-melting that reached its climax ca. 40 Myr after the emplacement of the plumes and associated greenstone cover rocks. This led to gravitational instabilities in the crust, as dense greenstone cover rocks began to sink into the thermally softened crust and, granite domes rose in response. The extraction of heat-producing elements toward the upper part of the crust has contributed to the cooling and stabilization of the cratons. This succession of events, which is not incompatible with plate-tectonic processes, may have profoundly changed the nature of the crust exposed at the surface and could explain the contrasting geochemical signatures of Archaean and post-Archaean shales.

Planetary climate and surface chemistry are tightly coupled; atmospheric composition affects the transfer of solar and infrared radiation, and therefore climate, whereas the components of climate, including temperature and precipitation, strongly affect the chemical composition of the surface environment and the geochemical cycling of elements through it. An element of climatic, environmental and biological importance is sulfur. The modern biogeochemical sulfur cycle has been extensively studied and is relatively well understood. How the sulfur cycle may have differed early in the evolution of Earth and Mars, when the surface was anoxic and biological activity was geochemically unimportant, is less well understood and much more poorly constrained by data.

Microthermometry and Raman spectroscopy techniques are routinely use to constrain ore-fluids d18O and molar proportions of anhydrous gas species (CO2, CH4, N2). However, these methods remain imprecise concerning the ore-fluids composition and source. Synchrotron radiation X-ray fluorescence allows access to major and trace element concentrations (Cl, Br and K, Ca, Fe, Cu, Zn, As, Rb, Sr) of single fluid inclusion. In this paper, we present the results of the combination of these routine and newly developed techniques in order to document the fluids composition and source associated with a Mesoarchaean lode gold deposit (Warrawoona Syncline, Western Australia). Fluid inclusion analyses show that quartz veins preserved records of three fluid inclusion populations. Early fluids inclusions, related to quartz veins precipitation, are characterized by a moderate to high Br/Cl ratio relative to modern seawater, CO2 ± CH4 ± N2, low to moderate salinities and significant base metal (Fe, Cu, Zn) and metalloid (As) concentrations. Late fluid inclusions trapped in secondary aqueous fluid inclusions are divided into two populations with distinct compositions. The first population consists of moderately saline aqueous brines, with a Br/Cl ratio close to modern seawater and a low concentration of base metals and metalloids. The second population is a fluid of low to moderate salinity, with a low Br/Cl ratio relative to modern seawater and significant enrichment in Fe, Zn, Sr and Rb. These three fluid inclusion populations point to three contrasting sources: (1) a carbonic fluid of mixed metamorphic and magmatic origin associated with the gold-bearing quartz precipitation; (2) a secondary aqueous fluid with seawater affinity; and (3) a surface-derived secondary aqueous fluid modified through interaction with felsic lithologies, before being flushed into the syncline. Primary carbonic fluids present similar characteristics than those ascribed to Mesoarchaean lode gold deposits. This suggests similar mineralization processes for mid- and Mesoarchaean lode gold deposits despite contrasting fluid–rock interaction histories. However, in regard to the protracted history documented in the Warrawoona Syncline, we question the robustness of the epigenetic crustal continuum model, as ore-fluid characteristics equally support an epigenetic or a polyphased mineralization process.

The timing of the onset of an orogeny is commonly constrained indirectly, because early orogenic structures are rarely exposed, or are overprinted. Establishing the onset of an Archean orogeny is considerably more challenging, because of the more fragmented geological record and the general lack of consensus about Archean geodynamics.
We combine existing tectono-stratigraphic data with new structural and geophysical datasets to establish the onset of the Neoarchean Yilgarn Orogeny (Yilgarn Craton, Western Australia). We show that the surface of the c. 2960–2750 Ma deep-marine Yilgarn greenstone sequence was uplifted, eroded and unconformably overlain by a c. 2730 Ma, syntectonic clastic sequence, deposited in shallow marine to subaerial conditions, and derived from the erosion of the underlying greenstones. This c. 2730 Ma regional unconformity predates the oldest-known Yilgarn structures, therefore its tectonic significance is so far unknown.
At around the same time, at deeper crustal levels, the c. 2728 Ma Yarraquin pluton was being emplaced along an active, large-scale shear zone network. Our meso- and microstructural analysis shows that the bulk of the fabric in the granite and its country rocks developed during pluton emplacement, and was largely assisted by magma-present shearing. Overall, these structures reflect an important event of syn-emplacement crustal shortening. The regional unconformity and the syndeformational emplacement of the Yarraquin pluton are both expressions of a c. 2730 Ma regional deformation event associated with significant crustal thickening, marking the onset of the Neoarchean Yilgarn Orogeny.

Common models for modern calcite precipitation in and around caves, soils, springs and streams involve CO2 supplied by thick, high pCO2 biogenic soils which were probably thin or non-existent before vascular plants. Indeed plant-influenced chemical weathering might have caused accelerated terrestrial carbonate production from the Devonian onwards. However terrestrial carbonates have also been documented from the Archaean, Proterozoic, Cambrian, Ordovician and Silurian. Mechanisms which could have caused non-marine carbonates to precipitate without organic-rich soils are described, and some geological events likely to have influenced non-marine carbonate precipitation up to the origin of vascular plants are highlighted. As organisms have evolved, so have the petrographic characteristics of non-marine carbonates; some examples of this are also given here .

In this study we present U–Pb and Hf isotope data combined with O isotopes in zircon from Neoarchean granitoids and gneisses of the southern São Francisco craton in Brazil. The basement rocks record three distinct magmatic events: Rio das Velhas I (2920–2850 Ma), Rio das Velhas II (2800–2760 Ma) and Mamona (2750–2680 Ma). The three sampled metamorphic complexes (Bação, Bonfim and Belo Horizonte) have distinct ε Hf vs. time arrays, indicating that they grew as separate terranes. Paleoarchean crust is identified as a source which has been incorporated into younger magmatic rocks via melting and mixing with younger juvenile material, assimilation and/or source contamination processes. The continental crust in the southern São Francisco craton underwent a change in magmatic composition from medium-to high-K granitoids in the latest stages, indicating a progressive HFSE enrichment of the sources that underwent anatexis in the different stages and possibly shallowing of the melting depth. Oxygen isotope data shows a secular trend towards high δ 18 O (up to 7.79‰) indicating the involvement of metasediments in the petrogenesis of the high potassium granitoids during the Mamona event. In addition, low δ 18 O values (down to 2.50‰) throughout the Meso-and Neoarchean emphasize the importance of meteoritic fluids in intra-crustal magmatism. We used hafnium isotope modelling from a compilation of detrital zircon compositions to constrain crustal growth rates and geodynamics from 3.50 to 2.65 Ga. The modelling points to a change in geodynamic process in the southern São Francisco craton at 2.9 Ga, from a regime dominated by net crustal growth in the Paleoarchean to a Neoarchean regime marked by crustal reworking. The reworking processes account for the wide variety of granitoid magmatism and are attributed to the onset of continental collision.

The North China craton and the Yangtze craton (South China) both contain Archean rocks in eastern China. Unlike the North China craton, where Archean rocks are widespread, in the Yangtze craton the exposed Archean rocks are only known in the Kongling terrain (360 km2). Zircon U-Pb ages and Lu-Hf isotopic compositions of three granodioritic-trondhjemitic gneisses and three metasedimentary rocks from the Kongling terrain were analyzed by LA-ICP-MS and LA-MC-ICP-MS. Igneous zircons in one trondhjemitic gneiss in the north of the Kongling terrain have an age of 3302 +/- 7 (1 sigma) Ma. Evidence from cathodoluminescence imaging, variations in Th/U and degree of U-Pb age discordance suggest that apparently younger zircons in the same population are variably disturbed 3302 Ma grains. Thus, this trondhjemitic gneiss is the oldest known rock in South China and predates the earlier reported similar to 2900 Ma granitoid magmatism by 400 Ma. Zircon cores from one granodioritic gneiss in the north of the Kongling terrain also give a concordant age group at 3200 to 3300 Ma. Regardless as inherited or not, these cores crystallized from a magma indistinguishable in age with the trondhjemite. Concordant U-Pb ages for igneous zircons in one granodioritic gneiss in the south of the Kongling terrain yielded a weighted average (206)pb/(207)pb age of 2981 +/- 13 Ma (2 sigma, MSWD=9.7, n=21). The zircon age and initial Hf isotopic compositions are similar to those of widespread granitoid gneisses from the north of the Kongling terrain (2903-2947 Ma), and indicate that the south and north of the Kongling terrain are correlative. The results also reinforce that magmatism of the whole Kongling terrain mainly occurred at 2900 Ma. Available Hf isotopic data from the Kongling terrain show that juvenile crustal additions occurred mainly between 3150 and 3800 Ma with a significant peak at 3300 to 3500 Ma. The similar to 3300 Ma zircons from the trondhjemitic gneiss have Hf crust formation ages of 3450 to 3730 Ma, some of which have nearly chondritic epsilon(Hf) (t). The whole-rock depleted mantle Nd model age of this rock is 3400 Ma, close to its age of magmatism and consistent with the Hf model age. Its epsilon(Nd) value at 3300 Ma is nearly chondritic (1.26). These lines of evidence suggest that the 3300 Ma trondhjemite represent juvenile crust additions to the pre-existing continental crust.

Mélanges characterize Phanerozoic convergent plate boundaries, but have rarely been reported from Archean orogens. In this paper, we document a Neoarchean ophiolitic mélange in the Eastern Hebei Province of the North China Craton. The Zunhua ophiolitic mélange is composed of a structural mixture of metapelites, ortho- and para-gneisses, and magnetite-quartzite mixed with exotic tectonic mafic blocks of metabasalts, metagabbroic rocks, and metadiabases, along with ultramafic blocks of serpentinized peridotites and podiform chromitites. The Zunhua ophiolitic mélange shows typical “block in matrix” structures. All units of the mélange have been intruded by granitic dikes and quartz veins that clearly cross-cut the foliation of blocks and matrix of the mélange. Laser-ablation–inductively coupled plasma–mass spectrometry zircon U-Pb dating of detrital zircons from the meta-sedimentary mélange matrix and intruding granitic dikes constrains the formation time of the Zunhua mélange to be be...

Mélanges characterize Phanerozoic con-vergent plate boundaries, but have rarely been reported from Archean orogens. In this paper, we document a Neoarchean ophiolitic mélange in the Eastern Hebei Province of the North China Craton. The Zunhua ophiolitic mélange is composed of a structural mixture of metapelites, ortho-and para-gneisses, and magnetite-quartzite mixed with exotic tec-tonic mafic blocks of metabasalts, metagab-broic rocks, and metadiabases, along with ultramafic blocks of serpentinized perido-tites and podiform chromitites. The Zunhua ophio litic mélange shows typical "block in matrix" structures. All units of the mélange have been intruded by granitic dikes and quartz veins that clearly cross-cut the fo-liation of blocks and matrix of the mélange. Laser-ablation-inductively coupled plasma-mass spectrometry zircon U-Pb dating of detrital zircons from the meta-sedimentary mélange matrix and intruding granitic dikes constrains the formation time of the Zunhua mélange to be between 2.52 and 2.46 Ga. Metamorphic rims on zircons from meta-sedimentary mélange matrix have ages of 2467 ± 27 Ma, confirming metamorphism of the mélange occurred at ca. 2.47 Ga. High-precision (scale 1:20 and 1:50) litho-structural mapping, along with detailed structural observations along several transects documents the internal fabrics and kinematics of the mélange, revealing a northwest to southeast directed transportation. The asymmetric structures in the mélange with folding and faulting events in the Zunhua mélange record kinematic information and are similar to the tectonic style of an accretionary wedge. Field relationships and geochemical analysis of various mafic blocks show that these blocks formed in an arc-related subduction tectonic environment. We suggest that the Zunhua mélange marks the suture zone of a Neoarchean arc-continent collisional event in the Central Orogenic Belt of the North China Craton. Combined with our previous studies , we demonstrate that a ca. 2.5 Ga tectonic suture exists between an arc/accretionary prism terrane in the Central Orogenic Belt and the Eastern Block of the North China Craton. We correlate this segment of the su-ture with other similar zones along strike, for >1000 km, including sections of the ca. 2.5 Ga in Dengfeng greenstone belt in the southern margin of the Central Orogenic Belt, and the ca. 2.5 Ga Zanhuang ophiolitic mélange in the center of the orogen. These relationships demonstrate that tectonic processes in the late Archean included subduction/accretion at convergent margins, and the horizontal movement of plates, in a style similar to modern day accretionary convergent margins.

The present contribution reviews bulk-rock geochemical data for mid-Archaean (ca. 3075–2840 Ma) metavolcanic rocks from the North Atlantic Craton of southwest Greenland. The data set includes the most recent high quality major and trace element geochemical analyses for ten different supracrustal/greenstone belts in the region. When distilling the data set to only include the least altered metavolcanic rocks, by filtering out obviously altered samples, mafic/ultramafic cumulate rocks, late-stage intrusive sheets (dolerites) and migmatites, the remaining data (N = 427) reveal two fundamentally distinct geochemical suites. The contrasting trends that emerge from the filtered geochemical data set, which best represents the melt compositions for these mid-Archaean metavolcanic rocks are: (1) tholeiitic (mainly basaltic) versus (2) calc-alkaline (mainly andesitic). These two rock suites are effectively separated by their La/Sm ratios (below or above three, respectively). It is demonstrated by geochemical modelling that the two contrasting suites cannot be related by either fractional crystallization or crustal assimilation processes, despite occurring within the same metavolcanic sequences. The tholeiitic basaltic rocks were directly mantle-derived, whereas the petrogenesis of the calc-alkaline andesitic rocks involve a significant (>50%) felsic component. The felsic contribution in the calc-alkaline suite could either represent slab-melt metasomatism of their mantle source, mafic-felsic magma mixing, or very large degrees of partial melting of mafic lower crust. At face value, the occurrence of andesites, and the negative Nb-Ta-Ti-anomalies of both suites, is consistent with a subduction zone setting for the origin of these metavolcanic rocks. However, the latter geochemical feature is inherent to processes involving crustal partial melts, and therefore independent lines of evidence are needed to substantiate the hypothesis that plate tectonic processes were already operating by the mid-Archaean.

There are growing indications that life began in a radioactive beach environment. A geologic framework for the origin or support of life in a Hadean heavy mineral placer beach has been developed, based on the unique chemical properties of the lower-electronic actinides, which act as nuclear fissile and fertile fuels, radiolytic energy sources, oligomer catalysts, and coordinating ions (along with mineralogically associated lanthanides) for prototypical prebiotic homonuclear and dinuclear metalloenzymes. A four-factor nuclear reactor model was constructed to estimate how much uranium would have been required to initiate a sustainable fission reaction within a placer beach sand 4.3 billion years ago. It was calculated that about 1-8 weight percent of the sand would have to have been uraninite, depending on the weight percent, uranium enrichment, and quantity of neutron poisons present within the remaining placer minerals.

"The secular cooling of the Earth's mantle and the growth of the continental crust together imply changes in the isostatic balance between continents and oceans, in the oceanic bathymetry and in the area of emerged continental crust. The evolution of these variables is of fundamental importance to the geochemical coupling of mantle, continental crust, atmosphere and ocean. To explore this further, we developed a model that evaluates the area of emerged continental crust as a function of mantle temperature, continental area and hypsometry.

In this paper, we investigate the continental freeboard predicted using different models for the cooling of the Earth. We show that constancy of the continental freeboard (± 200 m) is possible throughout the history of the planet as long as the potential temperature of the upper mantle was never more than 110–210 °C hotter than present. Such numbers imply either a very limited cooling of the planet or, most likely, a change in continental freeboard since the Archaean. During the Archaean a greater radiogenic crustal heat production and a greater mantle heat flow would have reduced the strength of the continental lithosphere, thus limiting crustal thickening due to mountain building processes and the maximum elevation in the Earth's topography [Rey, P. F., Coltice, N., Neoarchean strengthening of the lithosphere and the coupling of the Earth's geochemical reservoirs, Geology 36, 635–638 (2008)]. Taking this into account, we show that the continents were mostly flooded until the end of the Archaean and that only 2–3% of the Earth's area consisted of emerged continental crust by around 2.5 Ga. These results are consistent with widespread Archaean submarine continental flood basalts, and with the appearance and strengthening of the geochemical fingerprint of felsic sources in the sedimentary record from not, vert, similar 2.5 Ga. The progressive emergence of the continents as shown by our models from the late-Archaean onward had major implications for the Earth's environment, particularly by contributing to the rise of atmospheric oxygen and to the geochemical coupling between the Earth's deep and surface reservoirs."

The Valentines Formation consists of oligoclase gneisses, pyroxenites, granites with chordate perthites (mesoperthites) and magnetitic-augitic quartzites (valentinesites). The unit is developed both in in the southern Nico Pérez Terrane (Valentines-Las Palmas) and in the Isla Cristalina de Rivera (Zapucay-Vichadero), with continuity in southernmost Brazil (Rio Grande do Sul). Its banded structure, mineralogy and geochemistry show that the valentinesites are banded iron formations (BIF) with a 28-35% Fe, metamorphosed in granulite facies. At least two metamorphic events are recognized by U-Pb SHRIMP dating of zircon, one at 2060-2080 Ma and an earlier one at 2170-2115 Ma, both assignable to the Transamazonian Cycle. Archean ages up to 2619 ± 8 Ma may represent a third metamorphic event or correspond to inherited zircons in metagranites. The age of sedimentation is still unknown but may be Neoarchean or lower Paleoproterozoic (Siderian-Rhyacian). From a mining point of view, measured resources are more than 750 million tonnes (Mt) of valentinesite and potential resources of 2,000 Mt. Half of these resources are located in the area of Las Palmas, and the other half in the Valentines area.

The La China Complex (LCC) comprises amphibolite-facies, orthoderived metamorphic rocks. The most common lithologies are tonalitic to granitic gneisses, amphibolites and metagabbros. The LCC occurs as a N30E-trending belt between the Tupambaé shear zone (strike: N65E) in the north and the town of Minas in the south, with a total length of 180 km. To the W, the LCC is separated from granulites of the Valentines Formation by the Sierra de Sosa Shear Zone (=Cueva del Tigre Shear Zone). The eastern boundary is more diffuse, but it ultimately is represented by the Sierra Ballena Shear Zone. The evolution of the LCC is typical of a TTG complex, recording several metamorphic events. Tonalites crystallized at 3.4 Ga and were affected by two main metamorphic events: the Uruguayan Cycle between 3.1-3.0 Ga (M1) and the Jequié Cycle at 2.7-2.8 Ga (M2). Additionaly, a third and last, less intense tectonothermal event is evidenced at 1.25 Ga (Grenvillian sensu lato). Two new U-Pb LA-ICP MS ages are presented: one foliated metagranite with a concordant, crystallization age of 2.787 ± 6 Ma and a amphibolitic gneiss yielding a concordant age of 2.718 ± 8 Ma, also interpreted as the crystallization of the rock. Both ages represent magmatism associated to the Jequié Cycle in Uruguay. Reported ages of detrital zircons from Meso- to Neoproterozoic sandstones include a few ages of 3.55 and 3.6 Ga, which suggest the existence of even older, still undiscovered rocks in the Nico Pérez Terrane.

The ca. 2720 Ma Neoarchean Bad Vermilion Lake (BVL) greenstone belt, in the western Superior Province, Canada, is composed of a suite of tholeiitic to calc-alkaline basalts to rhyolites, volcaniclastic rocks, gab-bros, and Timiskaming-type siliciclastic sedimentary rocks. The greenstone belt was intruded by Neoarchean granitic rocks, and underwent greenschist facies metamorphism and intense deformation, resulting in mobilization of many elements (e.g., Rb, Ba, Sr, K, U, Pb). The high-field strength element and rare earth element systematics of the volcanic and volcaniclastic rocks, and gabbros are consistent with subduction zone geochemical signatures, suggesting that the BVL greenstone belt formed in a magmatic arc setting. On the basis of lithological associations and trace element systematics, the BVL greenstone belt is defined as a fragment of a Neoarchean subduction-related ophiolite. Three rhyolite samples from the belt have yielded 2722 ± 18 Ma, 2706 ± 13 Ma and 2710 ± 28 Ma U-Pb zircon ages, representing the approximate age of the arc volcanism in the study area and development of a subduction zone between the western Wabigoon terrane to the north and the Wawa-Abitibi terrane to the south. The intrusion of the ca. 2664 ± 15 Ma late-to post-tectonic, potassic Ottertail Lake granite marks the end of tectonic accretion in the study area. Both the volcanic rocks and gabbros display large ranges of Nd (143 Nd/ 144 Nd = 0.511600-0.512849; e Nd (2720 Ma) = + 0.8 to + 4.0), Pb (206 Pb/ 204 Pb = 13.80-60.67) and Sr (87 Sr/ 86 Sr = 0.701481-1.01154) isotopic compositions, suggesting that these isotope systems were variably affected by post-magmatic element mobility. Neither the Sm-Nd (2921 ± 200 Ma) nor Rb-Sr (2130 ± 610 Ma) system has yielded reliable regression (isochron) ages, reflecting the open-system behavior of these systems during metamorphism. Despite large uncertainties, Pb-Pb regression ages yielded by all rock types (2661 ± 60 Ma), and basalts and gabbros (2725 ± 83 Ma) agree with the zircon U-Pb ages of the rhyolites, suggesting that the U-Pb system was the most robust among all three systems.