Archean geology

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.

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 .

"Zircon U–Pb–Lu–Hf–O isotopic compositions of two granitic gneisses from the Kongling Terrain in the Yangtze Craton, South China were determined by SIMS, LA-ICP-MS and LA-MC-ICP-MS. Whole rocks of the two samples were analyzed for major and trace element compositions. The SIMS and LA-ICP-MS data reveal similar five zircon age groups of 3.4, 3.3, 2.9, 2.7, and 2.0 Ga for both gneisses. Three groups (magmatic Group A, metamorphic Group B, and overgrowth Group C) of the 3.4 Ga zircons were identified based on their CL images. These three groups have indistinguishable ages and Th/U ratios. Groups A and B show identical 176Hf/177Hf (t), although Group C was too thin to be analyzed by LA-ICP-MS. Taken together, zircons from the two samples with 98–102% age concordance give weighted average SIMS ages of 3434.3 ± 9.6 Ma (2σ, MSWD = 13, n = 8) for Group A, 3446.0 ± 8.8 Ma (2σ, MSWD = 10.7, n = 15) for Group B, and 3479 ± 26 Ma (2σ, MSWD = 0.49, n = 2) for Group C. Groups A and B together yield an upper intercept age of 3457 ± 14 Ma (2σ, MSWD = 0.85, n = 23). The LA-ICP-MS data yield weighted average ages of 3442 ± 19 Ma (2σ, MSWD = 0.17, n = 7) for Group A and 3435 ± 11 Ma (2σ, MSWD = 0.44, n = 16) for Group B. They yield an upper intercept age of 3443 ± 13 Ma (2σ, MSWD = 0.63, n = 23). These SIMS and LA-ICP-MS ages are consistent. We propose that the above SIMS and LA-ICP-MS ages of Groups A and B are the best estimates of the granitic magmatism and the subsequent metamorphism. The metamorphism must have occurred after the granitic magmatism within a few tens of million years, as constrained by their age errors. Accordingly, these two granitic gneisses represent the oldest rocks currently known in South China. They predate the previously reported 3300-Ma-old trondhjemitic gneiss from the Kongling Terrain by 150 Ma.

The 3.4 Ga zircons show near chondritic ɛHf (t) (−0.7 ± 1.0, 2σ, MSWD = 1.14, n = 8), which is below the coeval value of the depleted mantle. This suggests that the granitic magma contained materials of pre-existing continental crust. Their higher-than-mantle δ18O values (6.1–6.4‰) imply that such materials must have been interacted with surface water. Crust formation ages (TDM2) of the 3.4 Ga zircons vary from 3.9 to 3.6 Ga with a weighted average of 3703 ± 27 Ma (2σ, MSWD = 1.05, n = 7). Our results support previous studies that the Yangtze Craton may have contained the continental crust as old as 3.8 Ga.

Among the younger age groups, the 3.3 Ga zircons exhibit 176Hf/177Hf (t) and δ18O values similar to the 3.4 Ga zircons, suggesting that they were altered from the 3.4 Ga zircons. The 2.9 and 2.7 Ga zircons in both samples are rare and magmatic. Their 176Hf/177Hf (t) ratios are distinct from the 3.4 Ga zircons, indicating different sources. These two age groups are consistent with the 2.9 Ga trondhjemitic–tonalitic–granodioritic and the 2.7 Ga A-type granitic magmatism in the Kongling Terrain. The 2.0 Ga metamorphic zircons, regardless of being concordant or discordant, have 176Hf/177Hf (t) ratios overlapping those of the 2.7 Ga zircons, suggesting a common source. In contrast, δ18O of the 2.0 Ga zircons is strongly variable and positively correlated with age concordance. The low δ18O (down to 3.1‰) requires interaction with hydrothermal fluid. These results suggest that at least some of the 2.0 Ga zircons were likely to have been altered from the 2.7 Ga zircons by hydrothermal fluid."

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.

This paper explores the secular evolution of the height of an isostatically balanced collisional mountain belt in the context of ongoing convergence. We show that until the Neo- archean, continents were unable to sustain topography >2500 m. During the Neoarchean the continental lithosphere evolved through a rheological threshold, allowing for the devel- opment of significant topography. The consequence of the strengthening of the continental lithosphere is fundamental for the coupling of the Earth’s geochemical reservoirs. The Neo- archean was a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values. We propose that during the Neoarchean the exogenic envelopes, which were until then coupled to the mantle through hydrothermal processes and volcanism, also became coupled to the continental crust through relief- generating tectonics processes and erosion, hence changing the balance between mantle versus crustal interaction with the exogenic Earth.

Palaeoarchaean cherts preserve the most ancient direct traces of life, but this palaeobiological testament is rarely assimilated into ecosystem or biome models. Trace and rare earth element plus yttrium (REE + Y) compositions reliably decode the palaeodepositional settings of these cherts, and thus constrain the environments within which early microbial life flourished. Herein, we present systematic comparisons between bulk inductively coupled plasma mass spectrometry (ICP-MS) of four fossiliferous cherts from the Barberton greenstone belt, South Africa (the 3.472 Ga Middle Marker horizon, 3.45 Ga Hooggenoeg H5c chert, 3.334 Ga Footbridge Chert, and~3.33 Ga Josefsdal Chert), and in situ laser ablation (LA) ICP-MS transects through microbial laminations therein. Normalised bulk ICP-MS analyses generally exhibit fractionated REE + Y patterns typical of anoxic hydrogenous sedimentation, supporting previous assertions that the Palaeoarchaean habitable realm was a hydrothermally influenced ocean. Suppressed La, Eu and Y anomalies, together with supra-chondritic Y/Ho ratios, however, indicate restriction from the open ocean and influences from non-marine waters. In situ LA ICP-MS transects through fossiliferous layers yield flat, light REE-enriched REE + Y patterns and chondritic Y/Ho ratios indicating major contributions from terrigenous, riverine fluids, i.e. continental weathering. Resurgences of marine chemistry (increased Y/Ho ratios, La and Y anomalies) occur within microbial laminations themselves. Combined, these results evidence the presence of emergent, volcanic landmasses in the Palaeoarchaean, and highlight the importance of epicontinental basins atop these landmasses as loci for microbial biomes up to 250 Ma before large-scale terrestrial ecosystems. Increased riverine weathering of mafic-felsic continental material , together with periodic seawater recharge into these basins, generated disequilibrium redox conditions under which microbial life flourished. Emergent landmasses may thus have catalysed the flourishing of widespread productive photosynthetic biomes. Charting the relative dominance of biomes through time could illuminate microbial evolutionary trajectories through the lens of environmental reconstruction. Furthermore, we advocate the use of correlated bulk and in situ geochemical approaches in reconstructing ancient environments, since signals relating to small-scale palaeoenvironmental fluctuation can evidently be masked by bulk rock chemistry.

40Ar/39Ar age spectra with progressively increasing step ages are well known for metamorphic hornblende and have been classically interpreted by partial loss of radiogenic argon by diffusion processes during younger thermo-tectonic reworking. Application of a number of numerical modelling tools based on diffusion theory and that assume thermally activated loss of radiogenic 40Ar by solid-state volume diffusion suggests that staircaseshaped age spectra of Neoarchaean tschermakitic hornblende from the Lapland-Kola Orogen are due to argon losses of 40–50% during reheating to 450 ± 25 ˚C in Palaeoproterozoic time. However, in hornblende samples that yielded staircase-type age spectra, biotite occurs in the matrix, as well as intimately and abundantly intergrown with the amphibole along grain boundaries, cleavages, fractures and other defects. Drilling of 1.5 mm diameter discs from carefully selected hornblende grains in petrographic thin sections permitted to minimise the effects of contaminant biotite inclusions and/or compositional zoning of the amphibole. 40Ar/39Ar laser probe step-heating of drilled biotite-free hornblende discs yielded flat age spectra, suggesting absence of thermally activated radiogenic 40Ar loss. This would imply unrealistically contrasting temperature histories for neighbouring grains. Apparent-loss age spectra, thus, result from differential gas release of hornblende and an included, earlier degassing minor contamination of much younger biotite, which had apparently not been completely eliminated from the amphibole separate, despite careful handpicking. This is confirmed by the Ca/K ratio spectra − a proxy for 37ArCa/39ArK − of hornblende that are flat for drilled biotite-free hornblende grains, but initially low for hornblende separates. A drilled disc and a separate of hornblende from a biotite-free amphibolite did not yield apparent loss spectra, but flat age and Ca/K ratio spectra, confirming the interpretation of the role of biotite.

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.

New geological mapping and geochronology in the northeast Yilgarn Craton has changed our geological understanding of this region. The Yilgarn Craton had previously been divided into a series of terranes, with the easternmost Eastern Goldfields Superterrane separated from the Youanmi Terrane, which forms the core of the protocraton, by the Ida Fault zone. The Eastern Goldfields Superterrane was subdivided into the western Kalgoorlie, central Kurnalpi, and eastern Burtville terranes, with the latter, easternmost terrane the focus of the new field mapping and geochronology. Four main episodes of greenstone crustal growth have been recognised in the northeast Yilgarn Craton: ca 2970–2910 Ma, ca 2815–2800 Ma, 2775–2735 Ma, and ca 2715–2630 Ma. Rather than a single Burtville Terrane, as previously proposed, the distribution of greenstone magmatism reveals a previously unrecognised young (<2720 Ma) Yamarna Terrane in the northeast corner of the craton. The Yamarna Terrane is separated from the older (>2735 Ma) redefined Burtville Terrane by the Yamarna Shear Zone, which is now regarded as a terrane boundary. The correlation of lithologies and ages of magmatism in the northeast Yilgarn Craton with the rest of the craton indicates that the Burtville Terrane has affinities with the Youanmi Terrane that forms the nucleus of the craton, whereas the Yamarna Terrane has affinities with the Kalgoorlie Terrane in the west of the Eastern Goldfields Superterrane. The Burtville and Youanmi terranes shared a common history from ca 2970 Ma until ca 2720 Ma, when regional extension accommodated deposition of the Kambalda Sequence in the Kalgoorlie Terrane. It appears that extension also occurred along the Yamarna Shear Zone after ca 2720 Ma, accommodating the deposition of greenstones in the Yamarna Terrane. Like the Kalgoorlie and Kurnalpi Terranes, the Yamarna Terrane contains inherited zircon and local older rocks. This suggests that the ca 2720 Ma extension did not result in widespread rifting and the formation of extensive oceanic crust. Rather, there was thinning of older crust that extended right across the current Yilgarn Craton.

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.

Stresses acting on cold, thick and negatively buoyant oceanic lithosphere are thought to be crucial to the initiation of subduction and the operation of plate tectonics, which characterizes the present-day geodynamics of the Earth. Because the Earth’s interior was hotter in the Archaean eon, the oceanic crust may have been thicker, thereby making the oceanic lithosphere more buoyant than at present, and whether subduction and plate tectonics occurred during this time is ambiguous, both in the geological record and in geodynamic models. Here we show that because the oceanic crust was thick and buoyant, early continents may have produced intra-lithospheric gravitational stresses large enough to drive their gravitational spreading, to initiate subduction at their margins and to trigger episodes of subduction. Our model predicts the co-occurrence of deep to progressively shallower mafic volcanics and arc magmatism within continents in a self-consistent geodynamic framework, explaining the enigmatic multimodal volcanism and tectonic record of Archaean cratons. Moreover, our model predicts a petrological stratification and tectonic structure of the sub-continental lithospheric mantle, two predictions that are consistent with xenolith and seismic studies, respectively, and consistent with the existence of a mid-lithospheric seismic discontinuity. The slow gravitational collapse of early continents could have kick-started transient episodes of plate tectonics until, as the Earth’s interior cooled and oceanic lithosphere became heavier, plate tectonics became self-sustaining.

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.

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.

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 origin of the iron oxides in Archean and Paleoproterozoic Banded Iron Formations (BIFs) is still a debated question. We report low and high temperature magnetic properties, susceptibility and saturation magnetization results joined with scanning microscope observations within a 35 m section of the Late Archean Boolgeeda Iron Formation of the Hamersley Group, Western Australia. With the exception of two volcanoclastic intervals characterized by low susceptibility and magnetization, nearly pure magnetite is identified as the main magnetic carrier in all iron-rich layers including hematite-rich jasper beds. Two populations of magnetically distinct magnetites are reported from a 2 m-thick interval within the section. Each population shows a specific Verwey transition temperature: one around 120-124 K and the other in the range of 105-110 K. This temperature difference is interpreted to reflect two distinct stoichiometry and likely two episodes of crystallization. The 120-124K transition is attributed to nearly pure stoichiometric magnetite, SEM and microprobe observations suggest that the lower temperature transition is related to chemically impure silician magnetite. Microbial-induced partial substitution of iron by silicon is suggested here. This is supported by an increase in Total Organic Carbon (TOC) in the same interval.

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.

Metamorphic hornblende frequently yields spectra with progressively increasing 40Ar/39Ar age steps, often interpreted as caused by partial resetting due to thermally activated radiogenic argon loss by solid-state diffusion. Yet, in many cases rising Ca/K ratio spectra for such samples imply the presence of minor inclusions of K-contaminant minerals. To avoid parts of grains with mineral inclusions or compositional zoning we drilled tiny discs from thin sections under a petrographic microscope. Laser step-heating of drilled biotite-free hornblende discs yielded flat age and ratio spectra. In contrast, furnace step-heated hornblende separates from the same samples produced apparent loss age spectra. Moreover, biotite- free samples yielded flat spectra by laser and furnace dating. Consequently, apparent loss spectra result from degassing of included substantially younger biotite before its hornblende host during laboratory step-heating; c. 2640 Ma hornblende ages constrain the Murmansk Terrane’s cooling.

Archean greenstone belts and their possible inclusion of fragments of ophiolites is an important
research subject, since it is correlated with the nature of early oceanic crust, and can yield information
on the nature of early planetary lithospheres, the origin of TTG (tonalite-trondhjemitegranodiorite)
continental crust, the formation of early cratons and continents, and is related to when
plate tectonics started in the Earth’s evolutionary history. This article briefly reviews the North China
craton’s Archean ophiolite argument and proposes further studies aimed at understanding the generation
of greenstone belts and Archean ophiolites, and suggests some key scientific questions that remain
to be answered.

The 3.46 Ga Marble Bar Chert Member of the East Pilbara
Craton, Western Australia, is one of the earliest and best preserved sedimentary successions on Earth. Here, we
interpret the finely laminated thin-bedded cherts, mixed
conglomeratic beds, chert breccia beds and chert folded
beds of the Marble Bar Chert Member as the product
of low-density turbidity currents, high-density turbidity
currents, mass transport complexes and slumps, respectively.
Integrated into a channel-levee depositional model, the
Marble Bar Chert Member constitutes the oldest documented
deep-sea fan on Earth, with thin-bedded cherts, breccia
beds and slumps composing the outer levee facies tracts,
and scours and conglomeratic beds representing the channel
systems.

Large basaltic provinces up to 15 km thick are common in Archean cratons. Many of these flood basalts erupted through continental crust but remained at sea level. While common in the Archean, subaqueous Continental Flood Basalts (CFBs) are rare to absent in the post-Archean. Here we show that gravity-driven lower crustal flow may have contributed to maintaining Archean CFBs close to sea level. Our numerical experiments reveal that the characteristic time to remove the thickness anomaly associated with a CFB decreases with increasing Moho temperature (TM) from 500 million years (Myr) for TM ≈ 320 °C to 1 Myr for TM ≈ 900 °C. This strong dependency offers the opportunity to assess, from the subsidence history of CFBs, whether continental geotherms were significantly hotter in the Archean. In particular, we show that the subsidence history of the ~2.7 billion-year-old upper Fortescue group in the East Pilbara Craton, Western Australia, requires Moho temperatures much greater than 700 °C. Applied to eight other unambiguous subaqueous Archean CFBs, our results indicate Moho temperatures well in excess of 650 °C at the time of eruption. We suggest that the decrease in the relative abundance of subaqueous CFBs over Earth’s history could reflect the secular cooling of the continental lithosphere due to the decrease in radiogenic heat production.

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.

Tidalites are preserved within a metavolcano-sedimentary succession of the Late Archaean BababudanGroup (Dharwar Supergroup) along the western boundary of the NNW-SSE trending Chitradurga green-stone belt, West Dharwar Craton, southern India. These may represent the oldest record of tidal processesin peninsular India. Millimetre thick sand–mud alternations, bidirectional cross-strata, mudstone-drapedsandy foresets, reactivation surfaces indicating time–velocity asymmetry, sigmoidal cross-strata andmudstone draped flaser/lenticular bedding are displayed by the sandstone mudstone heterolithic faciesin the upper part of the Bababudan succession, together implying a tidal depositional system. Thick–thinpairs of rhythmic foreset bundles correspond to neap–spring tidal cycles within a semidiurnal tidal sys-tem. Development of the studied coastal sediments suggests formation of stable platform along thewestern margin of the late Archaean Chitradurga greenstone belt.

The Matagami mining camp in the northern Abitibi greenstone belt of Canada contains 19 known Archean volcanogenic massive sulfide (VMS) deposits, 11 of which have collectively produced 46.5 Mt of zinc-rich ore to date. The VMS deposits occur in three NW-SE– to WNW-ESE–oriented trends called the north flank, the south flank, and the west camp, which are composed of a felsic to mafic volcanic sequence cut by mafic to intermediate, synvolcanic sills and dikes. In order to clarify stratigraphic relationships between the south flank and the west camp, and to constrain the temporal evolution of volcanic activity, six new high-precision U-Pb zircon ages have been obtained. These data show that the total duration of felsic volcanism in the south flank was no more than 2.5 m.y., with the rhyolites extruded in the following order: Watson Rhyolite (2725.9 ± 0.8 Ma), Bracemac Rhyolite (2725.8 ± 0.7 Ma), Dumagami Rhyolite at the Persévérance Mine (2725.4 ± 0.7 Ma), and Dumagami Rhyolite in the Orchan West VMS deposit area (2724.9 ± 0.7 Ma). A hiatus in effusive volcanism is represented by the Key Tuffite, an important marker horizon in the camp. The hiatus probably lasted on the order of 0.5 m.y. Significantly, the rhyolite from the footwall of the Caber VMS deposit in the west camp has an age of 2725.9 ± 1.2 Ma, identical to that of the Watson Rhyolite on the south flank.

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.

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).

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.