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Gorgonum Chaos is part of the Eridania paleolake in Terra Sirenum and displays a number of prominent light‐toned morphological features that bear a record of the regional climatic conditions throughout most of Martian history. Based on an intergrated analysis of orbital data, we mapped a 1,500 km2 area in the southeast of Gorgonum Chaos. Morphologic, spectroscopic, and stratigraphic analyses were used to determine age and composition of the main geological units in the area. We identified four major geological units with decreasing content of hydrated minerals from the oldest to the youngest units, which were completely free of hydrated minerals. In the study area, phyllosilicate‐rich Noachian units compose the majority of the basin floor. Deposits enriched with evaporites were formed around the Noachian/Hesperian transition and erosion created prominent inverted morphologies. Loess‐like material without significant amounts of hydrated minerals was deposited until the late Hesperian. The youngest unit is an Amazonian layer free of hydrated minerals that originated from volcanic activities. This succession of minerals reflects the transition from more humid climatic conditions with the ability to sustain liquid water on the planet's surface during the Noachian to the hyper‐arid Amazonian environment we observe currently on Mars. Plain Language Summary: Gorgonum Chaos is part of the Eridania basin, which is a former lake system located at the southern hemisphere of Mars. The landforms observed in this area and their variable brightness suggest dramatically changing climatic conditions during the history of Mars. In an area of 1,500 km2 in size, we analyzed different landforms, their spectral characteristics, and their temporal sequences to determine the age and composition of the geological units. We found four major geologic units whose content of hydrated minerals decreased dramatically from oldest to youngest. The oldest unit with a high content of clay minerals is about 3.7 billion years old and formed the former lake bottom. This is followed by a younger unit with a mineral composition that indicates desiccation of the lake and erosion by wind. These, in turn, were largely covered by materials deposited by wind, which show very little evidence of liquid water. The last and youngest unit is volcanic in origin and completely devoid of minerals indicative of water. This succession of minerals reflects the evolution of the Martian surface, which was capable to sustain liquid water in the early stages of the planet and lost this ability during a drastic climate change. Key Points: We produced a geological map of southeastern Gorgonum ChaosWe find a succession from phyllosilicates to olivine in aeolian depositsThe presence of water decreases over time [ABSTRACT FROM AUTHOR]

Highlights • Summary of internal lithological variations of coarse-crystalline mafic bodies from Archean greenstone belts in the Eastern Goldfields Superterrane (Australia) • Comparison of the world famous Golden Mile Dolerite (1700 t Au) to a number of other mineralised and unmineralised bodies. • Comparison of coarse-crystalline mafic bodies to ponded lavas from the literature. • Present an emplacement model for differentiated mafic sills. Abstract Coarsely crystalline mafic rocks, commonly referred to as dolerite and diabase, make up a substantial proportion of Archean greenstone belts and are hosts to major orogenic gold deposits, yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence. We present results from drill core logging of several late Archean coarsely crystalline packages from the Eastern Goldfields Superterrane of the Yilgarn Craton, Western Australia, including the Golden Mile Dolerite. These large mafic bodies comprise mostly medium- to coarsely crystalline mafic rocks, but also include a variety of other products of differentiation. The bodies range from ∼75 to 460 m thick and extend for tens of kilometers, are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy. As such, this study excludes mafic dykes and extremely thick and internally complex bodies such as the Bushveld Complex. From base to top, the internal stratigraphy generally consists of a lower unit of aphyric basalt, dolerite/gabbro and peridotite ± pyroxenite, then a voluminous middle dolerite/gabbro, and an upper quartz dolerite/gabbro, and a topmost aphyric basalt. Granophyric veins and domains are common in the upper third of the mafic body. Contacts are gradational between all lithofacies, with the exception of granophric veins, which have sharp contacts. Within this overall stratigraphic framework there is variability in the relative thickness of lithofacies, number of lithofacies, as well as repetition of some lithofacies (several examples also include basalt in the interior). In well exposed examples, the key evidence for distinguishing intrusions from extrusions is provided by the top contact. However, the top contact from these examples is ambiguous. To overcome this gap, we review the character of well-studied thick ponded lavas from the literature. Most aspects of the internal character are not unique to intrusions, although the maximum groundmass grain size (excluding granophyric veins) of the Archean examples (>3 mm) is significantly coarser than that for any known ponded lavas (≪1 mm). The ability to distinguish intrusions from extrusions has important implications for reconstructing Archean greenstone stratigraphy and mineral exploration. [ABSTRACT FROM AUTHOR]

Highlights • Major, trace and 142–143Nd of mafic rocks from WAC. • 143Nd errochron age of 3300 Ma. • No 142Nd anomaly in any of the mafic samples analysed. • Trace element and 142–143Nd data unable to distinguish geodynamical regime. Abstract Determining the extent of mantle mixing during the Hadean-Archean is an integral factor in our understanding of early geodynamic processes as well as the speed and degree of mantle homogenisation. If a dynamic convecting mantle was the predominant regime it would be expected that early formed isotopic anomalies would be quickly homogenised, whilst in a stagnant lid regime these anomalies might last for hundreds of millions of years. Archean terranes are dominated by felsic tonalite, trondhjemite and granodiorite rocks, as such these rocks have been the main rock type studied, even though they are not formed by primary mantle melts. Therefore, in this study, we have focused on mafic rock samples as tracers of primary mantle melting, from the relatively unknown West African craton. The mafic rocks have undergone metamorphism to granulite facies with the majority of samples retrogressed to amphibolite facies. Major element compositions reveal that the protolith was tholeiitic basalt. The majority of trace elements display limited mobility, as primary correlations with Zr are still present. Trace element patterns are typical for Archean mafic rocks, with Nb-Ta and Ti depletions. The meta-basalts yield a 147Sm-143Nd errorchron with an age of 3300 ± 300 Ma, with initial εNd of +4.4. The Nd compositions indicate that the meta-basalts have experienced minimal crustal assimilation and retain primary geochemical attributes. A positive initial εNd for the meta-basalts suggests that the magmatic protolith evolved from an already differentiated source. Nevertheless, 142Nd compositions are within error of modern day terrestrial, as such, the protolith for these samples either did not experience any early differentiation, or that mantle convection had homogenised any anomalous signal. These results highlight the lack of large (>±8 µ142Nd) 142Nd anomalies detected from outside the North Atlantic and Superior Cratons, suggesting that Archean mantle dynamics was complex, with different processes occurring on different parts of Earth, similar to today. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Structural signatures for 2.45 Ga rifting were preserved. • Rifting utilized pre-existing structures of the Archaean basement. • Pull-apart structures controlled deposition and structural evolution of supracrustal belts. • Segmentation of the Archaean crust explained. • Active mode of rifting favoured. Abstract Constraints from restoration of minor rift-related faults preserved from subsequent Palaeoproterozoic overprint along the southern margin of the Peräpohja Belt, northern Fennoscandia indicate local NE-SW extension directions prevailed at the time of 2.44 Ga rifting of the Archaean continent. This palaeostress field is attributed to pull-apart setting of the Peräpohja Belt at a left overstepping zone of major sinistral N-S trending deformation zones. Consequently, the variably overprinted NW-SE trending structures of the Peräpohja Belt, including the NW-SE central graben were originally generated as normal faults. The proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step-over zone between strike-slip faults. By contrast, the faults bounding E-W to ENE-WSW trending basement horsts are the result of reactivation of older basement structures underlying the pull-apart basin. Based on the results, we infer that the final break-up of the Archaean continent in northern Fennoscandia utilized the 2.44 Ga rift-related NW-SE trending crustal anisotropy (parallel with dyke swarms), with pre-existing Archaean N-S trending structures reactivated as second-order shear zones in-between. The voluminous syn-rift 2.5–2.4 Ga and later pulses of mafic magmatism (2.32–1.98 Ga) are considered indicative of active-type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the Superior, Hearne and Karelian (-Kola) cratons. The presented model supports the continuity of the Archaean craton beyond the Caledonian Orogen towards north-west and explains its segmentation, suggesting a model for the localization of the Palaeoproterozoic supracrustal belts of Northern Fennoscandia. Moreover, this paper provides an approach to understand the basement-cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits. [ABSTRACT FROM AUTHOR]

The early history of life harbours many unresolved evolutionary questions, none more important than the genomic origin and cellular evolution of eukaryotes. An issue central to eukaryote origin concerns the position of eukaryotes in the tree of life and the relationship of the host lineage that acquired the mitochondrion some two billion years ago to lineages of modern-day archaea. Recent analyses indicate that the host lineage branches within the Archaea, prompting the search for novel archaeal lineages that can improve our understanding of the cellular evolution of eukaryotes. Here we give a brief review of the studies on Archaea, the tree of life and the cellular evolution of eukaryotes, which is followed by an overview of recent progress fueled by new genomic technologies and recent status of archaeal research in China. Future directions for the study of early evolution are considered. [ABSTRACT FROM AUTHOR]

Integration of existing isotopic and geological data allows a reconsideration of the distribution and age of the Río de la Plata Craton within South America. The reinterpretation increases the area of the craton to about 2,400,000 km2 with implications for the tectonic map of South America and for global reconstruction of palaeocontinents. Four areas previously considered as separate cratons (Luís Alves, Curitiba, Tebicuary, and Paranapanema) are interpreted as part of the same Río de la Plata Craton. The craton is organized into six provinces and domains: Buenos Aires-Piedra Alta, Taquarembó, Tebicuary, Luís Alves, Encantadas, and Nico Pérez. The term ‘Transplatense’ is proposed to replace ‘Trans-Amazonian’ for Rhyacian events that occurred within the Río de la Plata Craton. The craton is formed not only by dominant Rhyacian rocks and local Archaean rocks, but also by Statherian and Mesoproterozoic rocks. The domains are all partially to totally covered by Phanerozoic basins (Paraná, Chacoparanense, Claromecó, Salado, Balcarce, and Colorado) which makes their investigation difficult. The Ediacaran-Cambrian collisions of the Brasilian orogen generated tectonic mixtures of orogenic and cratonic zones. This is more evident in the eastern margin of the craton, which behaved as a metacraton. [ABSTRACT FROM AUTHOR]

Abstract U-Pb and Lu-Hf isotopic studies were performed in igneous zircon crystals from orthogneisses, metagranodiorites and a gabbro of the Tumucumaque Complex, in Vila Nova region, south of Amapá state, Brazil. The complex is located in the southeastern region of the Guyana Shield, northeastern portion of the Amazonian Craton. The Tumucumaque Complex is represented by tonalitic to granodioritic gneisses and amphibolites, with subordinate metagranites and intrusive gabbros, metamorphosed in conditions of middle to Upper Amphibolite facies. The complex represent the basement of the metavolcano-sedimentary associations of the Vila Nova and Serra do Navio regions, and occur as NW-SE elongated domes. The contact of infra- and supracrustal rocks is defined by N45°-55°W-trending high-angle ductile shear zones. In this contribution we provide U-Pb zircon ages by LA-MC-ICPMS which attest for the presence of a broad area of Archean rocks in the Guyana Shield. In addition, we present stratigraphic relationships and structural data between the Tumucumaque Complex and the Rhyacian Vila Nova Greenstone Belt. The orthogneisses yielded zircon igneous ages of 2.851 ± 37 Ma and 2.852 ± 32 Ma and the metagranodiorites of 2.822 ± 33 Ma and 2.812 ± 17 Ma. Intrusive gabbro present age of 2.671 ± 46 Ma. The Lu-Hf data suggest an enriched mantle source and reworking of Paleo-archean crustal rocks. The results obtained suggest a magmatic orogenic episode during the Meso-archean (2.85–2.81 Ga) and a later Neo-archean (2.67 Ga) event, probably anorogenic. Highlights • The geochronological results suggest a broad Archean area in the study area. • Were identified two magmatic episodes, an Meso-archean and a Neo-archean. • The Lu-Hf data suggest an enriched mantle source and reworking of Paleo-archean crustal rocks. [ABSTRACT FROM AUTHOR]

Abstract Lead isotopes are a powerful tool in studying Archaean granitoids and continental growth, because 207Pb is a sensitive indicator of involvement of lead from old Archaean crustal sources. This paper aims to demonstrate the significance and promote the use of lead isotopes by reviewing the theory, concepts, and traditions for interpretations of Pb isotope data as well as by introducing a global model based on crust-mantle recycling for discriminating late Archaean convergent margin settings of granitoids. Case plots of K-feldspar and whole-rock data of ca. 2.7 Ga granitoids from several Archaean cratons show that lead isotope systematics, when interpreted in the correct geological context, can be used for resolving many questions on crustal evolution from magmatic and tectonothermal history of individual intrusions to large-scale tectonics and supercontinent correlations. Highlights • Pb isotope systematics of granitoids is a powerful tool for Archaean granitoid petrology and plate tectonics • Pb–Pb diagrams can detect later events from low-grade Pb redistribution to high-grade U depletion • A model for 2.7 Ga granitoids reveals a global pattern reflecting recycling of Pb via mantle • The Pb isotope pattern reflects evolution of convergent margins from oceanic to continental [ABSTRACT FROM AUTHOR]

This study uses field observations and new U–Pb ages of detrital zircon grains from three samples to question the stratigraphic position of the Firgoun and Niamey siliciclastic sediments, presumed to be Neoproterozoic in age. Sharing several lithological similarities with the Late Cryogenian “Triad” of the Taoudenni, Gourma, and Volta basins, the uppermost siliciclastic sediments of the Firgoun and Niamey areas were likely also deposited during this period. This is corroborated by matrix-supported diamictites with faceted or striated pebbles as well as by structures resembling cryoturbation processes. However, the detrital zircon U–Pb age record that we present here for the lowermost deposits of Firgoun and Niamey provides mainly Paleoproterozoic ages, and very few Archean ages, altogether in a range from 1822 ± 9 to 3392 ± 9 Ma. Therefore, the new data only show that the Firgoun and Niamey sediments were deposited before about 1800 Ma. Nevertheless, the U–Th–Pb zircon age data allows examining the possible provenance of the sediments. We show that the latter was likely in the westerly close vicinity of the studied areas. The Archean zircons are likely inherited, and possibly originating from a more westerly source. The nearby source of the Niamey and Firgoun sediments suggests that a high topographic relief was still existing in the south-central part of the West African Craton in the Mid Neoproterozoic. [ABSTRACT FROM AUTHOR]

The lithosphere beneath the Archaean Dharwar craton was investigated with an east–west oriented, 280 km long profile (from Yellapura to Sindhanur) with 22 magnetotelluric stations. Regional strike directions, estimated were −5° and 13° for the crust and the lithospheric mantle respectively. Our results indicate in western Dharwar craton, presence of low resistivity zones in the crust besides two significant upper mantle conductive features within the highly resistive Archaean lithosphere. We analyze the available geophysical data that include heat flow, seismic tomography and magnetotellurics (MT) from the Dharwar craton. Our inference supports to the existence of a thick lithosphere. A thickness of more than 200 km is estimated for the lithosphere beneath the Dharwar craton by our magnetotelluric model. The study has brought out the presence of lithospheric upper mantle conductive features in the depth range of 100–200 km bounded to the west part of the magnetotelluric profile. Significant variations in conductivity are seen on either side of the Chitradurga shear zone. The conductive feature in the depth range 120–150 km is related with kimberlite melts and the conductive nature in the depth range 160–200 km is explained by refertilization process, as craton passed over the Marion (ca. 90 Ma) hotspot. [ABSTRACT FROM AUTHOR]

Although Archean gneisses of the Teton Range crop out over an area of only 50 x 15 km, they provide an important record of the Archean history of the Wyoming Province. The northern and southern parts of the Teton Range record different Archean histories. The northern Teton Range preserves evidence of 2.69-2.68 Ga high-pressure granulite metamorphism (>12 kbar, ~900 °C) followed by tectonic assembly with isotopically juvenile quartzofeldspathic metasedimentary rocks under high-pressure amphibolite-facies conditions (~7 kbar, 675 °C) and intrusion of extensive leucogranites. Together, these events record one of the oldest continent-continent collisional orogenies on Earth. Geochemical, thermobarometric, and geochronological data from the gneisses of the southern Teton Range show that this part of the uplift records a geologic history that is distinct from the northern part. It contains a variety of quartzofeldspathic gneisses, including a 2.80 Ga granodioritic orthogneiss and the 2.69-268 Ga Rendezvous Gabbro. None of these preserves evidence of the granulite metamorphism seen in the northern Teton Range. Instead, they have affinities with rocks elsewhere in the Wyoming Province. The boundary between the northern and southern areas is occupied by the Moran deformation zone, a broad zone of high strain along which the northern and southern areas were assembled at ca. 2.62 Ga under moderate pressures and temperatures (T = 540-600 °C and P < 5.0 kbar). The final Archean event of the Teton Range was the emplacement at 2.55 Ga of the Mount Owen batholith, a peraluminous leucogranite that intrudes the Moran deformation zone. The rocks of the northern Teton Range record events that are not present elsewhere in the Wyoming Province. We propose that they formed at 2.70-2.67 Ga some place distal to the Wyoming Province and that they were accreted from the west against the Wyoming Province along the Moran deformation zone at ca. 2.62 Ga. This date is coeval with deformation and metamorphism in the southern accreted terranes and indicates that at this time, accretion was taking place along both the southern margin and western margins of the Wyoming Province. [ABSTRACT FROM AUTHOR]

This work examines the global distribution of Archaean and modern igneous rock's compositions, without relying on preconceptions about the link between rock compositions and tectonic sites (in contrast with “geotectonic” diagrams). Rather, Archaean and modern geochemical patterns are interpreted and compared in terms of source and melting conditions. Mafic rocks on the modern Earth show a clear chemical separation between arc and non-arc rocks. This points to the first order difference between wet (arc) and dry (mid-ocean ridges and hotspots) mantle melting. Dry melts are further separated in depleted (MORB) and enriched (OIB) sources. This three-fold pattern is a clear image of the ridge/subduction/plume system that dominates modern tectonics. In contrast, Archaean mafic and ultramafic rocks are clustered in an intermediate position, between the three main modern types. This suggests that the Archaean mantle had lesser amounts of clearly depleted or enriched portions; that true subductions were rare; and that the distinction between oceanic plateaus and ridges may have been less significant. Modern granitic rocks dominantly belong to two groups: arc-related granitoids, petrologically connected to arc basalts; and collision granitoids, related to felsic sources. In contrast, the Archaean record is dominated by the TTG suite that derives from an alkali-rich mafic source (i.e. altered basalt). The geochemical diversity of the TTG suite points to a great range of melting depths, from ca. 5 to > 20 kbar. This reveals the absence of large sedimentary accumulations, again the paucity of modern-like arc situations, and the importance played by reworking of an earlier basaltic shell, in a range of settings (including some proto-subduction mechanisms). Nonetheless, granitoids in each individual region show a progressive transition towards more modern-looking associations of arc-like and peraluminous granites. Collectively, the geochemical evidence suggests an Archaean Earth with somewhat different tectonic systems. In particular, the familiar distinction between collision, arcs, ridges and hotspots seems to blur in the Archaean. Rather, the large-scale geochemical pattern reveals a long-lived, altered and periodically resurfaced basaltic crust. This protocrust is reworked, through a range of processes occurring at various depths that correspond to a progressive stabilization of burial systems and the establishment of true subductions. A punctuated onset of global plate tectonics is unlikely to have occurred, but rather short-term episodes of proto-subduction in the late Archaean evolved over time into longer-term, more stable style of plate tectonics as mantle temperature decayed. [ABSTRACT FROM AUTHOR]

Framing of the Archaean greenstone belts of the Kursk Block (KB) of the East Sarmatia preserves rocks of the TTG association: those do not form massifs with distinct boundaries, but occur as fields gradually transiting into gneisses and migmatites. According to Sm-Nd isotope-geochemical data, the TTG are characterized by positive values of εNd(2960) = +0.3+1.6 and protolith model ages of Nd(DM) = 3100-3200 Ma. Magmatic protoliths of the Kursk Block TTG were formed about 2960 Ma by melting from a juvenile basite source. These age estimates are significantly younger than heterochronous (3.19, 3.13 and 3.07 Ga) TTGs of the Middle Dnieper granite-greenstone terrane. On the other hand, the similarity of εNd(T) implies a single source of their protoliths. Consequently, the KB TTGs, apparently, are a result of transformation of an older sial crust preserved within the Middle Dnieper Block. [ABSTRACT FROM AUTHOR]

Orogenic style gold mineralizations in the Archean Hattu Schist belt (E Finland) are present in all major host rock lithologies including epiclastic sedimentary and volcanogenic rocks, as well as felsic intrusives. The gold mineralizations occur as dissemination in altered wall rocks and within hydrothermal quartz veins. Hydrothermal tourmalines are often associated with the gold mineralizations occurring in the quartz veins, as alteration minerals in contacts of the metasedimentary host rocks and quartz veins, but also within mineral assemblages of the metasedimentary and magmatic rocks. We characterize and compare the major, trace and rare earth element chemistry of these tourmalines in order to evaluate their suitability as a petrogenetic tool for tracing the fluid sources of the hydrothermal system. By comparing the chemistry of tourmalines from gold mineralization with those from metamorphic and magmatic host rocks, we test whether tourmaline composition can be used to identify the source rocks of the gold transporting hydrothermal fluids. All analyzed tourmalines belong to the alkali super group and plot along the schorl-dravite join. The hydrothermal tourmalines have Li, Sr and V concentrations comparable to metamorphic tourmalines and clearly distinct from magmatic tourmalines. Because the concentrations of Ni, Pb, Cr, Mn, Ga, Zn and Sn show a wide overlap between magmatic, metamorphic and hydrothermal tourmalines, these elements do not permit to discriminate between different source rocks. The co-variations of many elements in hydrothermal tourmalines, when plotted against Li, show more similarity to metamorphic tourmalines and the whole-rock compositions of metasedimentary and metavolcanic host rocks than to their magmatic counterparts. This indicates that the major and trace element composition of hydrothermal tourmalines in the Hattu Schist Belt is predominantly controlled by the host rocks and local fluid-rock interactions, and does not reflect the distal fluid sources. The effect of local fluid-rock interaction is also manifested by the REE patterns of tourmalines. The magmatic tourmalines have distinct LREE enriched patterns resembling the whole-rock REE patterns of granitic intrusives, while the metamorphic and hydrothermal tourmalines have flat or weakly fractionated patterns similar to metavolcanic and metasedimentary host rocks. Taken together, the tourmaline data suggest that hydrothermal tourmalines associated to gold mineralizing fluids are most likely genetically related to metamorphic rock sources without important contributions of magmatic fluids, and that local fluid-rock interaction exerted a major control on tourmaline chemistry. [ABSTRACT FROM AUTHOR]

Chromite deposits associated with layered anorthosite complexes in the Archaean high-grade terranes are rare in the world. The late Archaean Sittampundi Layered Magmatic Complex, Tamil Nadu, India is one of the few such deposits in the world where layers of Fe-Al rich chromites are associated with extremely calcic (An>95) anorthosite. ‘Frozen in’ magmatic mineralogy of the chromitite and the enclosing anorthosite suggest successive crystallization of chromite + clinopyroxene and chromite + clinopyroxene + anorthite from a hydrous Al-rich basaltic melt that was emplaced in a suprasubduction zone setting. Intense deformation and upper amphibolite facies metamorphism at ∼2.45 Ga converted the magmatic assemblages to hitherto unreported hornblende + gedrite + Mg-Al rich spinel ± chlorite bearing assemblages. During metamorphic reconstitution, chromite was pseudomorphically replaced by green spinel in the domains rich in secondary amphiboles. Mass-balance calculation and algebraic analyses of the observed mineralogy suggest that a number of chemical species including chromium became mobile during the formation of spinel pseudomorph in response to infiltration driven metamorphism. Aluminium became mobile in the length scale of chromite grain but remained immobile in the length scale of a thin section. [ABSTRACT FROM AUTHOR]

Reconstructing the redox state of the mantle is critical in discussing the evolution of atmospheric composition through time. Kimberlite-borne mantle eclogite xenoliths, commonly interpreted as representing former oceanic crust, may record the chemical and physical state of Archaean and Proterozoic convecting mantle sources that generated their magmatic protoliths. However, their message is generally obscured by a range of primary (igneous differentiation) and secondary processes (seawater alteration, metamorphism, metasomatism). Here, we report the Fe 3+ /ΣFe ratio and δ 18 O in garnet from in a suite of well-characterised mantle eclogite and pyroxenite xenoliths hosted in the Lace kimberlite (Kaapvaal craton), which originated as ca. 3 Ga-old ocean floor. Fe 3+ /ΣFe in garnet (0.01 to 0.063, median 0.02; n = 16 ) shows a negative correlation with jadeite content in clinopyroxene, suggesting increased partitioning of Fe 3+ into clinopyroxene in the presence of monovalent cations with which it can form coupled substitutions. Jadeite-corrected Fe 3+ /ΣFe in garnet shows a broad negative trend with Eu ⁎ , consistent with incompatible behaviour of Fe 3+ during olivine-plagioclase accumulation in the protoliths. This trend is partially obscured by increasing Fe 3+ partitioning into garnet along a conductive cratonic geotherm. In contrast, NMORB-normalised Nd/Yb – a proxy of partial melt loss from subducting oceanic crust (<1) and metasomatism by LREE-enriched liquids (>1) – shows no obvious correlation with Fe 3+ /ΣFe, nor does garnet δ 18 O VSMOW (5.14 to 6.21‰) point to significant seawater alteration. Median bulk-rock Fe 3+ /ΣFe is roughly estimated at 0.025. This observation agrees with V/Sc systematics, which collectively point to a reduced Archaean convecting mantle source to the igneous protoliths of these eclogites compared to the modern MORB source. Oxygen fugacites ( f O 2 ) relative to the fayalite–magnetite–quartz buffer (FMQ) range from Δ log ⁡ f O 2 = FMQ-1.3 to FMQ-4.6. At those reducing conditions, the solubility of carbon in the fluids released by dehydration is higher than in fluids closer to FMQ. The implication is that Archean processes of C transport and deposition would have differed from those known in modern-style subduction zones, and diamond would have formed from methane-rich fluids. In addition, such reducing material could drive redox melting or freezing upon deep recycling and migration of CH 4 -bearing fluids into the ambient mantle. [ABSTRACT FROM AUTHOR]

Higher mantle potential temperatures characterized the early Earth, resulting in thicker, more mafic oceanic crust entering subduction systems. This change in the composition of subducted slabs, combined with the enhanced temperature contrast between the slab and ambient mantle, would have altered the buoyancy forces driving subduction in the early Earth. Here we investigate this “compositional effect” through a combination of petrologic and thermal modeling. Specifically, we construct density profiles for sinking slabs under modern and early Earth conditions based on a range of mafic crust and mantle compositions. Slab and mantle densities are then determined from mineral assemblages calculated using the thermodynamic modeling program Perple_X along slab geotherms estimated from an analytic thermal model. Consistent with previous studies, we find that modern MORB compositions are typically less dense than the ambient mantle in the basalt barrier zone, located immediately beneath the mantle transition zone. By contrast, possible early Earth oceanic crust compositions are denser than ambient mantle at all depths down to 1000 km. This compositional effect results in slabs that would have more readily penetrated the transition zone, promoting single-layered convection and effective mantle mixing in the early Earth. [ABSTRACT FROM AUTHOR]

Oxygen isotopes in marine cherts have been used to infer hot oceans during the Archean with temperatures between 60 °C (333 K) and 80 °C (353 K). Such climates are challenging for the early Earth warmed by the faint young Sun. The interpretation of the data has therefore been controversial. 1D climate modeling inferred that such hot climates would require very high levels of CO 2 (2–6 bars). Previous carbon cycle modeling concluded that such stable hot climates were impossible and that the carbon cycle should lead to cold climates during the Hadean and the Archean. Here, we revisit the climate and carbon cycle of the early Earth at 3.8 Ga using a 3D climate-carbon model. We find that CO 2 partial pressures of around 1 bar could have produced hot climates given a low land fraction and cloud feedback effects. However, such high CO 2 partial pressures should not have been stable because of the weathering of terrestrial and oceanic basalts, producing an efficient stabilizing feedback. Moreover, the weathering of impact ejecta during the Late Heavy Bombardment (LHB) would have strongly reduced the CO 2 partial pressure leading to cold climates and potentially snowball Earth events after large impacts. Our results therefore favor cold or temperate climates with global mean temperatures between around 8 °C (281 K) and 30 °C (303 K) and with 0.1–0.36 bar of CO 2 for the late Hadean and early Archean. Finally, our model suggests that the carbon cycle was efficient for preserving clement conditions on the early Earth without necessarily requiring any other greenhouse gas or warming process. [ABSTRACT FROM AUTHOR]

Mafic granulite xenoliths are hosted by garnetiferous charnockites in the Archaean to Palaeoproterozoic Bumbuger Complex of northwestern Mongolia, one of the exotic basement terranes in the Central Asian Orogenic Belt. These rocks crystallized at ca. 1850 Ma under granulite-facies conditions (800 ± 27 °C, 6.8 ± 0.6 kbar) in the lower crust and were partly retrogressed to amphibolite-facies during ascent to higher crustal levels as a result of strong deformation resulting in northwest-trending isoclinal folds. The mafic xenoliths are likely derived from gabbroic protoliths, and geochemical, Hf-in-zircon and Nd whole-rock isotopic data suggest these rocks to have originated from parental melts of an enriched mantle source or from a depleted mantle with minor contamination by crustal material. The host garnetiferous charnockites crystallized from a strongly inhomogeneous melt predominantly derived from an Archaean to early Palaeoproterozoic lower crustal source, as also evidenced by partly recrystallized xenocrystic zircons, and minor contributions of mantle-derived material. The above data are compatible with a model whereby mantle-derived gabbroic melts under- and intraplate the lower crust, causing high-grade metamorphism, migmatization and anatexis, and the resulting ascending melts are mixtures of mantle and crustal sources. We speculate that the late Palaeoproterozoic events in the Baidarik Block may be the result of ca. 1.90–1.85 Ma accretion and collision processes that led to formation of the Columbia supercontinent. In the Neoproterozoic this block drifted toward Siberia and was incorporated into the Central Asian Orogenic Belt. [ABSTRACT FROM AUTHOR]

The Archaean granitoid pluton of the Singhbhum craton in E India is overlain by Archaean to Palaeoproterozoic metasediments. These sediments are still poorly known and their stratigraphy is under debate. Several scattered, most probably Meso- to Neoarchaean, conglomerates are present in the state of Jharkhand that differ so much in characteristics that they are probably not related to each other. The sedimentology of a series of conglomerate patches and layers near Bhurkuli has been investigated, including the characteristics of the clasts. It is deduced on the basis of these characteristics and the sedimentological context that the Bhurkuli conglomerates represent the channel facies of a river system that differed from the types of fluvial systems that exist nowadays. [ABSTRACT FROM AUTHOR]

Archaea play an important role in global carbon and nitrogen cycles. Archaeal lipids, such as isoprenoid glycerol diakyl glycerol tetraethers (iGDGTs), are important biomarkers tracing change in archaeal community structure and biogeochemical processes in the natural environments. In this research, the spatial distributions of archaeal lipids in the surface sediments of the Jiulong River (JR) and the Jiulong River estuary (JRE) were examined. GDGT-0 (containing zero cyclopentyl ring) and crenarchaeol were the most abundant iGDGTs in the JR and JRE. From the rivers to the estuary, the total iGDGTs, GDGT-0, crenarchaeol and archaeol concentrations showed significant spatial variation; in particular, GDGT-0 and archaeol in the river may be predominantly derived in situ from methanogens, whereas crenarchaeol in the estuary mainly derived in situ from Thaumarchaeota. We inferred that archaeal community was dominated by methanogens in the Jiulong River and by Thaumarchaeota in the Jiulong River estuary, which are consistent with change in archaeal community structure observed in other estuarine environments. [ABSTRACT FROM AUTHOR]

The Anshan-Benxi iron producing area, which is located at the northeastern margin of the North China Craton, is the main distribution area of Archaean BIFs in China. In their eastern part, including the Gongchangling and Waitoushan deposits, BIFs mainly are hosted in the Archaean middle Anshan Group. Amphibolites are widely distributed in the iron-bearing rock series, reflecting the tectonic setting of BIFs. Amphibolites not only have MORB-like compositional characteristics, but also have island arc-like ones, and they are consistent with back-arc basin basalts (BABB). In the study area, the protolith of amphibolites belongs to Okinawa-type BABB; it indicates that tectonic setting of BIFs is the intra-continental back-arc basin. In the study area, the formation of sedimentary basins for BIFs had been associated with oceanic plate subduction. Amphibolites from Gongchangling deposit are characterized by relative enrichments in LILE and LREE, and depletions in HFSE. This indicates that they had a relatively large influence of subduction in their formation. Amphibolites from Waitoushan deposit are characterized by relative enrichments in LILE without conspicuous depletions in HFSE, indicating relatively low subduction rates. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We processed five thermal infrared (TIR) bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) “at-sensor radiance” (Level 1B) data to derive few new indices to delineate variation in quartz, feldspar and mafic minerals in three different variants of granitoid; commonly occurring quartz bearing intrusive rock group. In this regard, three indices named as quartz-bearing rock index (QRI), feldspar-bearing rock index (FRI) and mafic-bearing rock index(MRI) were proposed.QRI index was derived using band 10, band 12 and band 13((band 10/band 12) ∗ (band 13/band 12)) of ASTER radiance image. MRI index was derived using band 12, band 13 and band 14 ((band 12/band 13) ∗ (band 13/band 14)) of ASTER radiance bands while band 10, band 11 were combined to derive FRI ((band 10/band 11) index. Three indices were combined in false colour composite image (FCC) and three-dimensional scatter plot to delineate granite, alkali granite and mafic rich granodioritic gneiss from each other as these granitoids had variable abundances of quartz, feldspar and mafic minerals. QRI and MRI were compared with the corresponding quartz and mafic indices proposed by Ninomiya (2005). It was observed from the respective ratio images and their regression plots that MRI and Ninomiya's mafic index (NMI) were complementary to each other. On the other hand, QRI image was better in enhancing quartz enrichment in alkali granites than Ninomiya's quartz index (NQI).However, QRI index was comparable with the quartz index proposed by Rockwall and Hofstra (2008) in terms of delineating quartz enrichment in alkali granite. Mutual exclusive nature of mafic minerals and quartz in granitoids was also evident from the negative correlation between MRI and QRI indices of the granitoids. On the other hand, FRI and QRI were negatively correlated with low regression value. This was resulted due to the combined effect of inverse relation of abundance of two dominant feldspars with quartz in different granitoids. In granitoids, abundance of plagioclase is known to increase with decreasing quartz content in granodiorite and tonalite although alkali feldspar bearing granites are characterised with high silica content. Results of discrimination of granitoids using proposed indices were validated based on deriving emissivity spectra of rocks and comparing them with ASTER TIR band resampled laboratory spectra of respective granitoids in addition to use geological map of the study area. Emissivity spectra of granitoids were derived from emissivity image (derived using emissivity normalisation method) after geospatially tagging it with QRI–FRI–MRI image composite; which was used to delineate exposures of granitoids. Further, we also found that the QRI, MRI and FRI indices had poor temperature dependence; when these indices were compared with relative surface temperature image derived from radiance bands using emissivity normalisation algorithm. Therefore, proposed indices can be implemented for delineating mineralogical variations of granitoids irrespective of surface temperature condition. Hence, proposed indices may be used successfully to delineate different granitic intrusions and relating their mineralogical variations with metallogeny. [ABSTRACT FROM AUTHOR]

Studies of preservation have allowed environmental conditions at numerous localities in space and time to be inferred. However, in the search for sufficient vertebrate specimens to provide a large dataset, details of specimen taxonomy, geographic origin and stratigraphic level are often ignored or not collected. Any preservational variation identified as a result is therefore from a broad range rather than a specific group, locality or time. The effect of this limitation, mainly regarding the loss of crucial information on environmental processes in ancient settings and the biology of long extinct organisms, is demonstrated using specimens of Ichthyopterygia (Reptilia). Taphonomic analysis of a dataset of 173 specimens infers a consistent reduction of skeletal condition from peripheral to medial within a carcass and a taphonomic pathway involving minimal floating and prolonged residence on the sediment surface. The same dataset divided into three sub-datasets, from the Middle Triassic Besano Formation, Lower Jurassic Blue Lias Formation and Lower Jurassic Posidonia Shale Formation, highlight preservational variation, in the extent of loss of articulation and completeness that follows an environmental gradient related to proximity to major landmasses. [ABSTRACT FROM AUTHOR]

The formation mechanisms for early Archaean continental crust are controversial. Continental crust may have accumulated via horizontal accretion in modern-style subduction zones or via vertical accretion above upper mantle upwelling zones. However, the characteristics of the continental crust changes at the transition between the Archaean and Proterozoic eons, suggesting that continental crust did not form in subduction zones until at least the late Archaean. Here I use seismic receiver function data to analyse the bulk properties of continental crust in Western Australia, which formed and stabilized over a billion years in the Archaean. I find that the bulk seismic properties of the crust cluster spatially, with similar clusters confined within the boundaries of tectonic terranes. I use local Archaean crustal growth models to show that both plume and subduction processes may have had a role in creating crust throughout the Archaean. A correlation between crustal age and the bulk seismic properties of the crust reveals a trend: from about 3.5 Gyr ago (Ga) to the end of the Archaean, the crust gradually thickened and simultaneously became more evolved in composition. I propose that this trend reflects the transition between crust dominantly formed above mantle plumes, to crust formed in subduction zones-a transition that may reflect secular cooling of Earth's mantle. [ABSTRACT FROM AUTHOR]

Stable continental cratons are the oldest geologic features on the planet. They have survived 3.8 to 2.5 billion years of Earth's evolution. The key to the preservation of cratons lies in their strong and thick lithospheric roots, which are neutrally or positively buoyant with respect to surrounding mantle. Most of these Archaean-aged cratonic roots are thought to have remained stable since their formation and to be too viscous to be affected by mantle convection. Here we use a combination of gravity, topography, crustal structure and seismic tomography data to show that the deepest part of the craton root beneath the North American Superior Province has shifted about 850 km to the west-southwest relative to the centre of the craton. We use numerical model simulations to show that this shift could have been caused by basal drag induced by mantle flow, implying that mantle flow can alter craton structure. Our observations contradict the conventional view of cratons as static, non-evolving geologic features. We conclude that there could be significant interaction between deep continental roots and the convecting mantle. [ABSTRACT FROM AUTHOR]

Mudrocks are a primary archive of Earth’s history from the Archean eon to recent times, and their source-to-sink production and deposition play a central role in long-term ocean chemistry and climate regulation. Using original and published stratigraphic data from all 704 of Earth’s known alluvial formations from the Archean eon (3.5 billion years ago) to the Carboniferous period (0.3 billion years ago), we prove contentions of an upsurge in the proportion of mud retained on land coeval with vegetation evolution. We constrain the onset of the upsurge to the Ordovician-Silurian and show that alluvium deposited after land plant evolution contains a proportion of mudrock that is, on average, 1.4 orders of magnitude greater than the proportion contained in alluvium from the preceding 90% of Earth’s history. We attribute this shift to the ways in which vegetation revolutionized mud production and sediment flux from continental interiors. [ABSTRACT FROM AUTHOR]

This paper addresses the relationships between relic amphibole-eclogite facies (AE) eclogites and their host units, Archaean amphibolites, enveloped by Archaean tonalite–trondhjemite–granodiorite (TTG) gneisses, in the Kuru-Vaara study area in the northern Belomorian Province. According to observational constraints, the crystallization of the relic peak omphacite + Mg-garnet ± kyanite assemblage and the subsequent replacement of omphacite by clinopyroxene–plagioclase symplectite occurred before the earliest deformational, metamorphic, and migmatization events that are recorded in the amphibolites. The amphibolites and their TTG hosts have a shared deformational and metamorphic history that is composed of the Archaean and Palaeoproterozoic periods. This history favours the conclusion that the AE metamorphism recorded in the relic eclogites within the amphibolites occurred during the Mesoarchaean to Neoarchaean periods. The deformation and metamorphism of the amphibolite facies of the second period resulted from the Lapland–Kola collisional orogeny at 1.91–1.93 Ga, which led to eclogite–high-pressure granulite (E–HPG) facies conditions in the lowermost portions of the over-thickened crust in Belomorian Province (the southwestern foreland of the Lapland–Kola collisional orogen). The Palaeoproterozoic E–HPG overprint was reported from the Palaeoproterozoic Gridino mafic dikes. Although the ages of the oldest low Th/U zircons are close to the time of the Lapland–Kola collision, the low Th/U 1.9–1.8 Ga zircons reflect a zircon response to regional fluid infiltration in the eclogites during slow exhumation following the Lapland–Kola orogeny and do not record any metamorphic event. Contrary to the Palaeoproterozoic E–HPG overprint, the areal occurrence of the 2.7–2.8 Ga AE eclogites with mid-ocean ridge basalt-like chemistry and their paragenetic link with the TTG gneisses suggest a tectonic regime that involves subduction. This research favours concepts suggesting that the modern-style plate tectonics has operated in some places, at least since the late Mesoarchaean. [ABSTRACT FROM AUTHOR]

The Protopangaea-Palaeopangaea model for the Precambrian continental crust predicts quasi-integrity reflecting a dominant Lid Tectonics defined by a palaeomagnetic record showing prolonged near-static polar behaviour during very long time intervals (~2.7–2.2, 1.5–1.2, and 0.75–0.6 Ga). Intervening times show polar loops radiating from the geometric centre of the crust explaining the anomalous Precambrian magnetic inclination bias. The crustal lid was a symmetrical crescent-shaped body confined to a single hemisphere on the globe comparable in form to the Phanerozoic supercontinent (Neo)Pangaea. There were two major transitions in the tectonic regime when prolonged near-static motion was succeeded by widespread tectonic-magmatic activity, and each coincided with the major isotopic/geochemical signatures in the Precambrian record. The first comprised a ~90° reconfiguration of crust and mantle at ~2.2 Ga terminating the long 2.7–2.2 Ga static interval; the second was the largest continental break-up event in geological history and is constrained to the Ediacaran Period at ~0.6 Ga by multiple isotopic and geochemical signatures and the subsidence history of marine passive margins. Break-up of the lid at ~0.6 Ga defines a transition from dominant Lid Tectonics to dominant Plate Tectonics and is the primary cause of contrasts between the Precambrian and Phanerozoic aeons of geological times. The long interval of minimal continental motion in the mid-Proterozoic correlates with extensive emplacement of anorogenic anorthosite-rapakivi plutons unique to these times, and high-level emplacement was probably caused by blanketing of the mantle and comprehensive thermal weakening of the crust. Continental velocities were low during the two Proterozoic intervals characterized by profound glaciation (~2.4–2.2 and ~0.75–0.6 Ga) when partial or complete magmatic shutdown is likely to have reduced volcanic greenhouse gas production. Specific implications of Protopangaea-Palaeopangaea include: (i) support for recent evidence that 60–70% of the present continental crust had accreted by ~2.5 Ga; (ii) recognition from spatially constrained mineral provinces that sub-crustal lithosphere was already chemically differentiated by mid-Archaean times; (iii) strong axial alignment of younger greenstone belts, major Palaeoproterozoic shear zones, and later Meso–Neoproterozoic mobile/orogenic belts; (iv) concentration of anorogenic magmatism and progressive contraction of activity towards the orogenic margin subsequently to become the focus of Grenville (~1.1 Ga) orogenesis; and (v) late Neoproterozoic arc magmatism/tectonics at the instep margin of the continental crescent persisting until the Ediacaran continental break-up. [ABSTRACT FROM AUTHOR]

Fifty-five new multi-dimensional diagrams, mostly based on log-ratio transformations, were used to decipher tectonic settings for 17 cases of Precambrian belts in Canada, the USA, Poland, Finland, Jordan, Democratic Republic of Congo and Zambia, China, and India. The results of different sets of diagrams for basic-ultrabasic, intermediate, and acid magmas were generally internally consistent. Possible reasons for some inconsistencies may be related to the use of a sample group of mixed ages because of their uncertainties, extreme element mobility caused by metamorphism especially of high-grade type, analytical data quality, different petrogenetic processes for basic to acid magmas such as mantleversuscrustal origin, and some deficiencies that still exist in the multi-dimensional diagrams. To partly overcome the last problem, one new multi-dimensional diagram is proposed and used to discriminate mid-ocean ridge and oceanic plateau settings. [ABSTRACT FROM AUTHOR]

New data on the U-Pb (SHRIMP II) age are obtained for aluminous gneisses of the Kola Group of the Baltic shield. Gneisses are typically ascribed to metasedimentary rocks with detrital zircons. Our work interprets the isotopic data based on the magmatic (tonalite) origin of gneisses and zircon that was established from study of the morphology and geochemistry of zircons. The age of crystallization of the protolith is 2.9 Ga. The existence of two stages of Archean granulite metamorphism is confirmed: an early stage (2.9 Ga) and later granulite metamorphism (2707-2656 Ma). [ABSTRACT FROM AUTHOR]

Interacting, diverse microbe-sediment systems exist in natural environments today but have not yet been recognized in the oldest records of life on Earth (older than 3.3 Ga) because of lack of distinctive biomarker molecules and patchy preservation of microbial paleocommunities. In an in-situ outcrop- to microbial-scale study, we have differentiated probable phototrophic, chemolithotrophic, and chemo-organotrophic fossil microbial signatures in a nearshore volcanogenic sedimentary setting in 3.33 Ga rocks of the Josefsdal Chert, Barberton greenstone belt, South Africa, while demonstrating the importance of contemporaneous hydrothermal activity. Hydrothermal fluids, as a nutrient source, strongly controlled the development and distribution of the microbial communities and, as a silicifying agent, contributed to their rapid fossilization. We thus show that intricate microbe-sediment systems are deep-rooted in time and that at least some early life may indeed have been thermophilic. [ABSTRACT FROM AUTHOR]

We report zircon ages and Hf isotopic compositions for the Nkandla Granite of the Ilangwe Granitoid/Gneiss Suite exposed to the southeast of Nkandla in northern KwaZulu-Natal, South Africa. The Ilangwe Granitoid/Gneiss Suite is intrusive into the Ilangwe Greenstone Belt along the southeastern margin of the Kaapvaal Craton. Zircons obtained from the Nkandla Granite yielded an upper intercept age of 3254 ± 8 Ma, interpreted to be the age of intrusion, with evidence of lead loss events at 3086 ± 17 Ma and in the Phanerozoic. The ~3.25 Ga age is consistent with late Palaeoarchaean ages obtained from similar granitoid units throughout the southeastern Kaapvaal Craton. The intrusive relationship with the adjacent Ilangwe Greenstone Belt provides a minimum age for the, as yet undated, greenstone succession. Initial hafnium isotopic compositions range from εHf = -2 to +7, indicating a role for both mantle input and crustal reworking. [ABSTRACT FROM AUTHOR]

This study focuses on the eastern flank of the Santa Rita syncline (Dorr 1969), with specific emphasis on the region known as Fábrica Nova. Important iron ore deposits are located on the flanks of this structure, such as Timbopeba, Alegria, São Luiz, Tamanduá, Almas and Fábrica Nova. The Santa Rita syncline is a fold with N-S axial direction and of subregional scale, with roots in the adjacent basement of the Santa Bárbara Complex and sectioned by the Água Quente thrust fault. The hypothesis of this study is that the structural framework of the region resulted from the superposition of at least three deformation phases on the Ouro Preto nappe. The Fábrica Nova mine, located in the central portion of the study area, is embedded in a synformal structure with a 100/20 trending axis named Fábrica Nova synform. The proposed model to explain the particular structural geometry of this region is based on the flanking folding mechanism (Passchier 2001). This mechanism may have been developed by E-W crustal shortening during the F4 tectonic deformation phase. [ABSTRACT FROM AUTHOR]

The metamorphic history of the Southern Marginal Zone (SMZ) of the Limpopo Belt, South Africa, possibly provides insight into one of the oldest preserved continental collision zones. The SMZ consists of granitoid gneisses (the Baviaanskloof Gneiss) and subordinate, infolded metasedimentary, metamafic and meta-ultramafic lithologies (the Bandelierkop Formation) and is regarded as the c. 2700 Ma granulite facies reworked equivalent of the Kaapvaal craton basement. The granulite facies metamorphism is proposed to have occurred in response to collision between the Kaapvaal and Zimbabwe cratons. Previous studies have proposed a wide variety of P-T loops for the granulites, with considerable discrepancy in both the shapes of the retrograde paths and the magnitude of the peak P-T conditions. To date, the form of the prograde path and the timing of the onset of metamorphism remain unknown. This study has used a range of different metasedimentary rocks from a large migmatitic quarry outcrop to better constrain the metamorphic history and the timing of metamorphism in the SMZ. Detrital zircon ages reveal that the protoliths to the metasedimentary rocks were deposited subsequent to 2733 ± 13 Ma. Peak metamorphic conditions of 852.5 ± 7.5 °C and 11.1 ± 1.3 kbar were attained at 2713 ± 8 Ma. The clockwise P-T path is characterized by heating in the sillimanite field along a P-T trajectory which approximately parallels the kyanite to sillimanite transition, followed by near-isothermal decompression at peak temperature and near-isobaric cooling at ~6.0 kbar. These results support several important conclusions. First, the sedimentary rocks from the Bandelierkop Formation are not the equivalent of any of the greenstone belt sedimentary successions on the Kaapvaal craton, as has been previously proposed. Rather, they post-date the formation of the Dominion and Witwatersrand successions on the Kaapvaal craton. From the age distribution of detrital zircon, they appear to have received significant input from various origins. Consequently, at c. 2730 Ma, the Baviaanskloof Gneiss most likely acted as basement onto which the sedimentary succession represented by the Bandelierkop Formation metapelites was deposited. Second, the rocks of the SMZ underwent rapid evolution from sediment to granulite facies anatexis, with a burial rate of ~0.17 cm yr−1. Peak metamorphism was followed by an isothermal decompression to 787.5 ± 32.5 °C and 6.7 ± 0.5 kbar and isobaric cooling to amphibolite facies conditions, below 640 °C prior to 2680 ± 6 Ma. This age for the end of the high-grade metamorphic event is marked by the intrusion of crosscutting, undeformed pegmatites that are within error the same age as the crosscutting Matok intrusion (2686 ± 7 Ma). Collectively, the burial rate of the sedimentary rocks, the shape of the P-T path, the burial of the rocks to in excess of 30 km depth and the post-peak metamorphic rapid decompression argue strongly that the SMZ contains sediments deposited along an active margin during lateral convergence, and that the SMZ was metamorphosed as a consequence of continental collision along the northern margin of the Kaapvaal craton at c. 2700 Ma. [ABSTRACT FROM AUTHOR]

Mass-independent fractionation of sulfur isotopes (reported as Δ33S) recorded in Archean sedimentary rocks helps to constrain the composition of Earth's early atmosphere and the timing of the rise of oxygen ~2.4 billion years ago. Although current hypotheses predict uniformly negative Δ33S for Archean seawater sulfate, this remains untested through the vast majority of Archean time. We applied x-ray absorption spectroscopy to investigate the low sulfate content of particularly well-preserved Neoarchean carbonates and mass spectrometry to measure their Δ33S signatures. We report unexpected, large, widespread positive Δ33S values from stratigraphic sections capturing over 70 million years and diverse depositional environments. Combined with the pyrite record, these results show that sulfate does not carry the expected negative Δ33S from sulfur mass-independent fractionation in the Neoarchean atmosphere. [ABSTRACT FROM AUTHOR]

It is widely believed that atmospheric oxygen saturation rose from <10-5 present atmospheric level (PAL) in the Archean to >10-2 PAL at the Great Oxidation Event (GOE) at ca. 2.4 Ga, but it is unclear if any earlier oxygenation events occurred. Here we report U-Pb zircon data indicating that a pyrophyllite-bearing paleosol, from Keonjhar in the Precambrian Singhbhum Craton of eastern India, formed between 3.29 and 3.02 Ga, making it one of very few known Archean paleosols globally. Field and geochemical evidence suggests that the upper part of the paleosol was eroded prior to unconformable deposition of an overlying sequence of shallow-marine siliciclastic sediments. A negative cerium anomaly within the currently preserved level of the paleosol indicates that ancient oxidative weathering occurred in the original upper soil profile. The presence of redox-sensitive detrital uraninite and pyrite together with a complete absence of pyrophyllite in the overlying sediments indicate that the mineralogical and geochemical features of the paleosol were established prior to the unconformable deposition of the sediments and are not related to subsequent diagenetic or hydrothermal effects. We suggest that a transient atmospheric oxygenation event occurred at least 600 m.y. prior to the GOE and ~60 m.y. prior to a previously documented Archean oxygenation event. We propose that several pulsed and short-lived oxygenation events are likely to have occurred prior to the GOE, and that these changes to atmospheric composition arose due to the presence of organisms capable of oxygenic photosynthesis. [ABSTRACT FROM AUTHOR]

We present new isotopic data (U-Pb and Sm-Nd) from the Neoarchean granitoids of the Lentua complex, which is a part of the Western Karelia subprovince of the Fennoscandian Shield. Compositionally, the samples are granitoids belonging to the sanukitoid suite and K-rich granitoids. Certain samples interpreted as partial melts of pre-existing crust in this study display more mafic compositions than previously described from the surrounding areas. This indicates, at least locally, a source poorer in SiO2 than the TTG (Tonalite-Trondhjemite-Granodiorite) suite granitoids, which are the likely sources for the majority of the anatectic granitoids within the Lentua complex. Based on new geochronological data (LA-MC-ICPMS U-Pb on zircon), the sanukitoids and anatectic granitoids are at least partly coeval (2.69 Ga) in the area, but interaction between the two felsic magmas was limited. The dated sanukitoid sample is distinctly younger (2687±8 Ma) than the average age of the sanukitoids of the Western Karelia subprovince (2718±3 Ma) indicating, together with one previously published sanukitoid age, the existence of a younger sanukitoid phase. This study gives new important constraints to understanding the formation of the Western Karelia subprovince by expanding the observed compositional heterogeneity and the temporal overlap of the different Neoarchean granitoid suites. [ABSTRACT FROM AUTHOR]

Abstract: This study investigates the origin of the 2.69Ga-old Matok pluton, emplaced in the Pietersburg block, northern Kaapvaal Craton (South Africa), forthwith after a major tectono-metamorphic event ascribed to continent–continent collision. The Matok pluton consists of diorites, granodiorites and monzogranites. Petrography and whole-rock major- and trace element compositions of the Matok samples are similar to those of post-collisional Fe–K suites, which are very common in Proterozoic terranes. These granitoids are particularly rich in FeOt, TiO2, P2O5, span a wide range of SiO2 contents and display elevated concentrations in K2O, Ba, HFSE and REE, with moderately fractionated REE patterns. All samples of the Matok pluton have unradionegic Nd isotopic compositions (εNd(2.69 Ga) =−2.7 to −4.6), but only a few monzogranite samples derive from reworking of older crust. Crustal contamination of basaltic melt cannot explain either the observed compositions. Most of the suite rather fractionated from a common mafic parent, either by partial melting or crystallization. Geochemical modeling shows that this parent magma or source rock is basaltic in composition, intermediate between calc-alkaline and tholeiitic groups, and enriched in incompatible trace elements with respect to the primitive mantle. It ultimately derives from the involvement of two distinct mantle sources: (1) enriched, sub-continental lithospheric mantle, which was metasomatized by sedimentary material derived from local crust of the Pietersburg block, <0.3Ga before the pluton emplacement; and (2) asthenospheric mantle. This model accounts for the differences between Fe–K suites, such as the Matok pluton, and Mg–K suites, such as sanukitoids, the origin of which only requires metasomatized mantle. We propose that the most appropriate geodynamic setting for the interaction between enriched lithospheric and asthenospheric mantle sources is an episode of “lithospheric reworking” (through slab breakoff, retreat or sub-continental mantle delamination) in response to continental collision. Depending on the relative contribution of the two mantle sources in this environment, the resulting magmas can show a wide range of compositions from Fe–K to Mg–K end-members. From a regional point of view, this conclusion supports that the Kaapvaal Craton and the Central Zone of the Limpopo Belt were amalgamated between 2.80 and 2.75Ga owing to continent–continent collision. [Copyright &y& Elsevier]

Abstract: A scheme is proposed for the stratigraphic division and correlation of the Cambrian sediments of the Upper Proterozoic-Paleozoic Cis-Yenisei basin. It is based on data from the drilling of parametric wells (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-1, Vostok-3, Vostok-4, and others). Two structure-facies zones are recognized in the study area: Kas zone (Lemok-1, Averinskaya-150, Tyiskaya-1, Vostok-4, and Eloguiskaya-1 wells), in which the sedimentary complexes accumulated in a salt subbasin, and Ket’ zone (Vostok-1 and Vostok-3 wells) with open-sea-basin sedimentation. The boundary between the structure-facies zones passes along the reconstructed zone of a barrier reef stretching in the N-S direction. The Vostok-4 well is localized in the western Kas structure-facies zone, at the salt subbasin/barrier reef boundary. Local stratigraphic units (formations) are described and compared with the adjacent Turukhan-Irkut-Olekma facies region of the Siberian Platform. [Copyright &y& Elsevier]

Abstract: We discuss the current understanding of the Vendian Regional Stages in the northeast of the Nepa-Botuobiya Anteclise, establish a new correlation of the Vendian sections across different facies, and propose stratotypes for the introduced stratigraphic units. A new stratigraphic chart is constructed for the Vendian terrigenous strata in the northeast of the Nepa-Botuobiya Anteclise and the adjacent territories. [Copyright &y& Elsevier]

Abstract: 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 ~4.5Ga. We know little about Hadean crustal geology (>3.9Ga) 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 (<3.9Ga) crustal geology integrate well. 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 2.2Ga. 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 ~600Ma, with a transition regime beginning ~850Ma. [Copyright &y& Elsevier]

Abstract: Undisturbed mid Archean crust (stabilized by 3.0–2.9Ga) has several characteristics that distinguish it from post Archean crust. Undisturbed mid-Archean crust has a low proportion of internal seismic boundaries (as evidenced by converted phases in seismic receiver functions), lacks high seismic velocities in the lower crust and has a sharp, flat Moho. Most of the seismic data on mid-Archean crust comes from the undisturbed portions of the Kaapvaal and Zimbabwe (Tokwe segment) cratons. Around 67–74% of younger Archean crust (stabilized by 2.8–2.5Ga) has a sharp, flat Moho. Much of the crust with a sharp, flat Moho also lacks strong internal seismic boundaries, but there is not a one to one correspondence. In cases where its age is known, basaltic lower crust in Archean terranes is often but not always the result of post Archean underplating. Undisturbed mid-Archean cratons are also characterized by lower crustal thicknesses (Archean median range=32–39km vs. post-Archean average=41km) and lower crustal seismic velocities. These observations are shown to be distinct from those observed in any modern-day tectonic environment. The data presented here are most consistent with a model in which Archean crust undergoes delamination of dense lithologies at the garnet-in isograd resulting in a flat, sharp Moho reflector and a thinner and more felsic-intermediate crust. We discuss the implications of this model for several outstanding paradoxes of Archean geology. [Copyright &y& Elsevier]

Abstract: According to some previous studies, Archean continental crust is, on global average, apparently thinner than Proterozoic crust. Subsequently, the validity of this statement has been questioned. To provide an additional perspective on this issue, we present analyses of Moho signatures derived from recent seismic data along swaths 2000km in length across southern Africa and its flanking ocean. The imaged crust has a near continuous age range between ca. 0.1 and 3.7billion years, and the seismic data allow direct comparison of Moho depths between adjacent Archean, Proterozoic and Phanerozoic crust. We find no simple secular change in depth to Moho over this time period. In contrast, there is significant variation in depth to Moho beneath both Archean and Proterozoic crust; Archean crust of southern Africa displays as much crustal diversity in thickness as the adjacent Proterozoic crust. The Moho beneath all crustal provinces that we have analysed has been severely altered by tectono-metamorphic and igneous processes, in many cases more than once, and cannot provide unequivocal data for geodynamic models dealing with secular changes in continental crust formation. These results and conclusions are similar to those documented along ca. 2000km swaths across the Canadian Shield recorded by Lithoprobe. Tying the age and character of the Precambrian crust of southern Africa to their depth diversities is clearly related to manifold processes of tectono-thermal ‘surgery’ subsequent to their origin, the details of which are still to be resolved, as they are in most Precambrian terranes. Reconstructing pristine Moho of the early Earth therefore remains a formidable challenge. In South Africa, better knowledge of ‘fossilised’ Archean crustal sections ‘turned-on-edge’, such as at the Vredefort impact crater (for the continental crust), and from the Barberton greenstone belt (for oceanic crust) is needed to characterize potential pristine Archean Moho transitions. [Copyright &y& Elsevier]

Endogenic events in the form of intrusive activity and regional metamorphism developed asynchronously in various parts in the Svecofennian Orogen of Fennoscandia. The Early and Late Svecofennian stages of regional high-temperature metamorphism and related plutonism are distinguished from isotopic evidence. The composition, structural features, and asynchronous peaks of endogenic activity within the orogen indicate that at least two zones (inner and outer) should be distinguished in the Svecofennides. The lateral heterogeneity of the orogen in present-day coordinates is traced southward from the margin of the Archean craton. The conjugation zone of the Svecofennian Orogen and the Archean Karelian Craton is characterized by transition from negative to positive εNd (1.9 Ga) values as evidence for a decreased contribution of Archean crustal material to the source of Proterozoic granitoids from the north toward the Proterozoic domain in the south. With allowance for lateral compositional and isotopic heterogeneity of the Svecofennian Orogen and asynchronous culmination of endogenic events in different parts of this orogen, a new scheme of tectonic regionalization has been proposed. [ABSTRACT FROM AUTHOR]

The Archean-Paleoproterozoic terrane of central Brazil is an exotic and allochthonous part of the Tocantins Province, a large Brasiliano/Pan-African orogen of the South American Platform formed during the Brasiliano orogeny. The terrane amalgamated to the province during the late stages of the orogeny as a crustal segment consisting of six Archean orthogneiss complexes and five low-grade metamorphic, in part Paleoproterozoic (Rhyacian) greenstone belts. The Uvá complex is the southernmost orthogneiss association of the Archean-Paleoproterozoic terrane of central Brazil. New U–Pb LA-ICP-MS data from zircon crystals show that the complex formed at least during two magmatic stages. The older consists of polydeformed tonalite and granodiorite batholitic and diorite stock protoliths with igneous age of 3040 Ma to 2930 Ma. The youngest comprises tonalite, monzogranite and granodiorite tabular bodies formed between 2876 and 2846 Ma. As compared to the orthogneisses of the northern portion of the terrane, both the oldest and youngest granitogenesis stages of the Uvá complex are, in average, about 150 Ma older. This suggests that the northern and southern orthogneisses formed during different times as independent crustal segments, but when and why they amalgamated is still under investigation. [ABSTRACT FROM AUTHOR]

Highlights: [•] We present a depth profile of carbonation in the 2.6Ga seafloor basalts. [•] Carbon isotope ratio of calcite systematically changes with depth. [•] Degree of carbonation is lower than those in the Early and Middle Archean equivalents. [•] Seawater CO2 level likely decreased from the Middle Archean to the Late Archean. [ABSTRACT FROM AUTHOR]