Search

Zircon is not a common accessory mineral of banded iron formation (BIF), but it occurs in all BIF samples from the Kostomuksha Greenstone Belt (KGB) in the Karelian Craton of the Fennoscandian Shield. The Kostomuksha BIF is of Algoma type and is part of three variably old rock associations: BIF-1 is part of a sedimentary sequence in Mesoarchean (2.87–2.84 Ga) basaltic komatiites with dacites, BIF-2 is located in Meso-Neoarchean (2.8–2.79 Ga) felsic volcanics, and BIF-3 is part of a Neoarchean (2.76–2.74 Ga) greywacke unit with felsic volcanics. Analysis of zircon has revealed grains with cores and several generations of rims. The U-Th-Pb systems of zircon indicate that its isotopic age varies considerably from 2.98 to 1.89 Ga, forming up to 4 age clusters in each sample. Most samples contain single inherited grains. The bulk of zircon in BIF is younger than the host rocks and is interpreted as metamorphic. Most of metamorphic zircons display an oscillatory zoning, and their Th/U ratio varies from 0.01 to 4.7, i.e. their origin is hard to interpret reliably, based on the above characteristics. Paleoproterozoic (1.89–1.85 Ga) zircons from BIF-1 and 3, occurring as rims and individual grains, are related to local tectono-thermal processes simultaneous with the formation of the Svecofennian orogen and were described for the central Karelian Craton for the first time. Thus, both inherited and metamorphic zircons have been revealed in Meso-Neoarchean BIF metamorphosed repeatedly in the Neoarchean-Paleoproterozoic under up to amphibolite-facies conditions. All metamorphic generations of zircons from BIF are correlated with the tectono-thermal events in the Karelian Craton, including the effect of the Svecofennian orogeny. [ABSTRACT FROM AUTHOR]

The results of studies of carbonaceous matter (CM) and the S isotopic composition of associated sulfides in metapelites of the Neoarchean banded iron formation of the Kostomuksha greenstone belt of Karelia (Karelian Craton of the Fennoscandinavian Shield) are presented. The petrographic observations show that the CM occurs inside and between silicates, in sulfides, or at the grain boundaries between the sulfide crystals and biotite or amphibole. The results of scanning electron and atomic force microscopy showed the presence of several CM types distinct in structure and C content. The analysis of Raman spectra of CM revealed the presence both of well-ordered graphite and weakly structured kerogene in samples. The isotopic composition of the total organic carbon is typical of biogenic processes. The δ13Corg values from –27.9 to –30.6‰ are consistent with the fixation of carbon by photo- or chemoautotrophs. The S isotopic composition of associated sulfides is characterized by a positive Δ33S anomaly (up to +0.94‰) and negative δ34S values (from –2.06 to –4.1‰). The positive Δ33S values indicate a genetic link with photochemical elemental sulfur (S8) from the atmosphere, whereas the negative δ34S values reflect the fractionation of isotopes during bacterial-related processes. Based on these observations, we suggest that the primary CM mostly has a biogenic origin. [ABSTRACT FROM AUTHOR]

We report abundant small calcareous mounds associated with fossilized kerogenous microbial mats in tidal-facies sandstones of the predominantly siliciclastic Moodies Group (ca. 3.22 Ga) of the Barberton Greenstone Belt (BGB), South Africa and Eswatini. Most of the bulbous, internally microlaminated mounds are several centimeters in diameter and formed at the sediment-water interface contemporaneously with sedimentation. They originally consisted of Fe-Mg-Mn carbonate, which is now largely silicified; subtle internal compositional laminations are composed of organic matter and sericite. Their presence for >6 km along strike, their restriction to the inferred photic zone, and the internal structure suggest that mineral precipitation was induced by photosynthetic microorganisms. Similar calcareous mounds in this unit also occur within and on top of fluid-escape conduits, suggesting that carbonate precipitation may either have occurred abiogenically or involved chemotrophic metabolism(s) utilizing the oxidation of organic matter, methane, or hydrogen, the latter possibly generated by serpentinization of underlying ultramafic rocks. Alternatively or additionally, carbonate may have precipitated abiotically where heated subsurface fluids, sourced by the intrusion of a major Moodies-age sill, reached the tidal flats. In summary, precipitation mechanisms may have been variable; the calcareous mounds may represent "hybrid carbonates" that may have originated from the small-scale overlap of bioinduced and abiotic processes in space and time. Significantly, the widespread occurrence of these stromatolite-like structures in a fully siliciclastic, high-energy tidal setting broadens search criteria in the search for life on Mars while their possible hybrid origin challenges our ability to unambiguously identify a biogenic component. [ABSTRACT FROM AUTHOR]

This study investigates the structural control of the numerous gold occurrences in the southern part of the Barberton Greenstone Belt in the Malolotja and Steynsdorp areas. The gold-bearing event distribution is studied using field structural geology associated with a petrological and microstructural analysis. Three major tectonic events have been identified in quartz veins and direct country rocks. The first event (De) created a regional schistosity (Se), probably associated with an early thrusting event. The second event (Df) is related to a large-scale folding, which formed the anticline at Steynsdorp and synform at Malolotja. It resulted from a main E–W direction of shortening and is responsible for the folding of the Se schistosity. It formed a N–S-striking axial planar cleavage (Sf), observed and associated with the emplacement of the main gold-bearing veins. The mineralised system exhibits a complex vein network, in which shallow dipping veins have developed coevally with steep west- and east-dipping veins. A third deformation event (Dl) produced by NE–SW shortening is related to the late barren hydrothermal quartz vein formation. Mineralogically, the veins are filled with abundant quartz, and scarce tourmaline and feldspars. Alteration halos composed of neoformed tourmaline and plagioclase within a talc and white mica matrix developed specifically during the late Dl event. A model of regional deformation giving rise to large-scale folds and quartz vein formation formed in response to E–W and NE–SW directions of shortening globally is proposed and discussed in this paper. [ABSTRACT FROM AUTHOR]

The first data on the multi-isotopic S composition of sulfides of the Neoarchean banded iron formations (BIFs) of the Kostomuksha greenstone belt of Karelia (Karelian Craton of the Fennoscandian Shield) formed 2760–2740 Ma ago are presented in this paper. Pyrite associated with fine-grained magnetite has negative δ34S (to –7.8‰) and positive Δ33S (to +0.7‰) values. Pyrrhotite from stringer-disseminated ores associated with schists exhibits negative values of both δ34S (to –6‰) and Δ33S (to –0.46‰). Our data provide grounds to suggest that the BIF sulfides sourced photolitic elementary sulfur from the atmosphere, sulfate from seawater, and sulfur from magma. The S isotope ratios of sulfides reflect the interaction between abiotic (atmospheric, hydrothermal) and biotic (microbial dissimilation reduction) processes during the formation of iron deposits of the Kostomuksha greenstone belt of Karelia. [ABSTRACT FROM AUTHOR]

Our ability to understand the geological evolution of Archean cratons is fundamentally dependent on stratigraphic reconstructions across greenstone belts. Here, we integrate published stratigraphic, geochemical and geochronological data to produce a revised stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Yilgarn Craton. The Agnew-Wiluna Greenstone Belt records ca 130 m.y. of magmatism that occurred between ca 2820 and 2690 Ma and can be divided into three cycles. Cycle 1 is marked by ca 2820–2740 Ma mafic–ultramafic volcanic rock successions. Cycle 2 is associated with ca 2740–2720 Ma felsic volcanic rocks, associated fragmental deposits and contemporaneous mafic–ultramafic intrusions. Cycle 3 consists of ca 2720–2690 Ma mafic–ultramafic volcanic rock successions and co-magmatic intrusions. Exhumation and reworking of these continental rift basin successions after ca 2690 Ma resulted in two younger, additional, depositional cycles in the Agnew-Wiluna Greenstone Belt. Cycle 4 is associated with intermediate to felsic volcanoclastic rocks and epiclastic rocks erupted/deposited between ca 2690 and 2665 Ma. Cycle 5 is associated with the deposition of polymict conglomerates and sandstones in localised depocentres between ca 2665 and 2655 Ma. The stratigraphic scheme identified at the scale of the Agnew-Wiluna Greenstone Belt is the local expression of craton-wide tectono-thermal events. Our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt and proposed correlations at the craton scale support a para-autochthonous model whereby crust from a proto-Yilgarn continental mass was extended/thinned, favouring the activation of translithospheric pathways associated with the ca 2720–2690 Ma Kalgoorlie Large Igneous Province. Inversion of this intra-cratonic rift basin after ca 2680 Ma led to the protracted activation of shear zones and multiple shifts in sedimentary depocentres. The widespread occurrence of previously unidentified fragments of ca 2820–2720 Ma substrate within the Kalgoorlie-Kurnalpi Rift has significant implications for craton-scale stratigraphic reconstructions and challenges existing terrane and domain boundaries. We propose a revised, coherent and testable stratigraphic model for the Agnew-Wiluna Greenstone Belt of the Kalgoorlie Terrane. We compare our revised stratigraphic model for the Agnew-Wiluna Greenstone Belt with that of the Murchison and Southern Cross domains of the Youanmi Terrane in order to build a new space–time–integrated stratigraphic cross-section across the western half of the Yilgarn Craton. We demonstrate for the first time the existence of a magmatic continuum (i.e. no significant magmatic hiatus) between ca 2820 and 2690 Ma, providing the missing link between the geological record of the Youanmi Terrane and that of the Eastern Goldfields Superterrane. [ABSTRACT FROM AUTHOR]

The paper summarizes major and trace-element compositions and Sm–Nd isotope data on metabasites (amphibolites) and gabbroids of the Onot granite–greenstone block in the Sharyzhalgai basement uplift, southwestern Siberian craton. The Onot block consists of tectonically combined nappes of the Paleoarchean tonalite–trondhjemite–granodiorite (TTG) complex and the metasedimentary-volcanic complex of the greenstone belt (GB). The Mezoarchean (∼2.88 Ga) metabasalts of the greenstone belt and Paleoproterozoic (∼1.86 Ga) gabbronorites and vein gabbros were formed at rifting and postcollisional extension, respectively. The Archean metabasites of the greenstone belt and enclaves in the TTG complex compositionally correspond to low-Ti tholeiitic basalts and basaltic andesites. The basaltic rocks are characterized by flat REE patterns [(La/Sm)n = 0.9–1.9], depletion in Nb relative to Th and La (Nb/Nb* = 0.4–1.1), and a wide range of mostly positive εNd(T) values (from +5.2 to –1.0). The enrichment of the basaltic andesite in incompatible elements, its Eu minimum, and negative εNd(T) values resulted from contamination by Paleoarchean TTG gneisses, that form the basement of GB. The Paleoproterozoic gabbronorites have high Mg# and extremely low concentrations of Ti and incompatible elements. The rocks are characterized by low (Nb/Y)PМ (0.8–1.0), negative εNd(T) values (from 0 to –1.4), and weak enrichment in Th and LREE relative to Nb. The vein gabbros have low (La/Sm)n, positive εNd(T) values of +2.8 and +0.3, and a negative Nb anomaly (Nb/Nb* = 0.3–0.4). The trace element-composition of the amphibolites, gabbronorites, and gabbros and the results of geochemical modeling indicate that the parental melts were derived mainly from weakly depleted mantle sources. The Nd isotope composition of the Paleoproterozoic gabbroids resulted from the evolution of the heterogeneous Archean lithospheric mantle. Variations in the isotope and trace-element composition of the amphibolites reflect the initially depleted nature of the Mezoarchean mantle and its metasomatic alteration by fluids/melts, which occurred before its melting at ∼2.88 Ga. The geochemical and Nd isotopic characteristics of gabbronorites and gabbros indicate that the lithospheric mantle had become progressively more heterogeneous by the Paleoproterozoic due to preceding Archean processes. The variable depletion of both the Archean and the Paleoproterozoic mafic rocks in Nb relative to Th and La may be explained by mantle metasomatism and does not reflect the geodynamic settings of the mafic magmatism. [ABSTRACT FROM AUTHOR]

• Hadean mantle was involved into formation-reworking process of NCC from 3.8Ga until 2.45Ga. • Early basalts can derive from a common 4.4Ga mantle formed shortly after lunar impact. • Eoarchean 142Nd heterogeneity seems analytical, rather than truly heterogeneous. • Hadean depleted domains can remain convectively isolated for 1.7 billion years. The 146Sm-142Nd systematics of Archean rocks have been extensively studied to decipher chronological and compositional constraints on Hadean differentiation processes. However, the paucity of 142Nd data on Meso- and Neoarchean mantle-derived rocks relative to earlier periods contributes to uncertainty about the ultimate fate of primordial silicate reservoirs in the convective system of the early Earth. Here we present 146,147Sm-142,143Nd and 176Lu-176Hf zircon data for Mesoarchean (3.0–3.1 Ga) to Paleoproterozoic (2.45 Ga) granitoids, amphibolites and sedimentary rocks representing the main lithological units of the eastern North China Craton (NCC). The analyzed samples have well-resolved positive 142Nd anomalies, suggesting that the Hadean materials have been involved into the generation and evolution of the Eoarchean-Neoarchean continental crust of the eastern NCC. The model age of differentiation (4. 44 − 0.07 + 0.05 Ga) and the 147Sm/144Nd source ratio (0.211 ± 0.002) estimated from the 3.1 Ga Caozhuang amphibolites are identical to those of the 3.8–3.6 Ga rocks from the North Atlantic Craton and the 2.7 Ga pyroxenites from the Abitibi Greenstone Belt, suggesting that these mantle-derived rocks originated from a common, moderately depleted reservoir that differentiated in the wake of the Moon-forming impact and evolved in closed-system until at least 2.7 Ga. These observations contradict the commonly accepted notion of progressive homogenization of 142Nd anomalies in the Archean due to crustal recycling and convective mixing, and instead suggest that primordial differentiation produced a long-lived, depleted reservoir that remained largely isolated from convective mixing on a timescale of several billion years. [ABSTRACT FROM AUTHOR]

On the basis of the available geochronological data, the main stages of formation of Archean rocks belonging to the tonalite-trondhjemite-granodiorite (TTG) association, with ages of about 3240, 3150, 2900, 2850, 2800, and 2750 Ma, are identified and characterized within the Karelian Province of the Fennoscandian Shield. The distinctive features of geological positions and chemical compositions of TTG rocks of different stages are shown. For example, early TTG melts could have been melted not from basalts of the greenstone belts of the Karelian Province, but from lower crustal basites or from early amphibolites of the Vodlozero domain. Features of the geological position, chemical composition, and εNd(t) isotopic evolution do not support the assumption that most of the TTGs formed from tholeiites of the greenstone belts, thus excluding their genetic connection with subduction of the ancient oceanic crust. The most acceptable model seems to be the formation of TTGs in the Karelian Province as a result of plume-affected melting of lower crustal mafites. Archean TTGs differ markedly from Phanerozoic plagiogranitoids not only in the shape and size of the massifs but also in a number of geochemical features, in particular, lower HREE, Sc, Y, and Ti contents. [ABSTRACT FROM AUTHOR]

Past orbital parameters of the Moon are difficult to reconstruct from geological records because relevant data sets of tidal strata are scarce or incomplete. The sole Archean data point is from the Moodies Group (ca 3.22 Ga) of the Barberton Greenstone Belt, South Africa. From the time‐series analysis of tidal bundles from a well‐exposed subaqueous sand wave of this unit, Eriksson and Simpson (Geology, 28, 831) suggested that the Moon's anomalistic month at 3.2 Ga was closer to 20 days than the present 27.5 days. This is in apparent accordance with models of orbital mechanics which place the Archean Moon in a closer orbit with a shorter period, resulting in stronger tidal action. Although this study's detailed geological mapping and section measuring of the site confirmed that the sandstone bed in question is likely a migrating dune, the presence of angular mud clasts, channel‐margin slumps, laterally aggrading channel fills and bidirectional paleocurrents in overlying and underlying beds suggests that this bedform was likely located in a nearshore channel near lower‐intertidal flats and subtidal estuarine bars; it thus carries risk of incomplete preservation. Repeated measurements of foreset thicknesses along the published traverse, measured perpendicular to bedding, failed to show consistent spectral peaks. Larger data sets acquired along traverses measured parallel to bedding along the 20.5 m wide exposure are affected by minor faulting, uneven outcrop weathering, changing illumination, weather, observer bias and show a low reproducibility. The most robust measurements herein confirm the periodicity peak of approximately 14 in the original data of Eriksson and Simpson (Geology, 28, 831). Because laminae may have been eroded, the measurements may represent a lower bound of about 28 lunar days per synodic month. This estimate agrees well with Earth–Moon dynamic models which consider the conservation of angular momentum and place the Archaean Moon in a lower orbit around a faster‐spinning Earth. [ABSTRACT FROM AUTHOR]

It is unresolved when eolian processes began to significantly affect global mean sand composition and texture through abrasion and enhanced sorting. Reports of Proterozoic eolian textures and sedimentary structures are common, in particular from 1.8 Ga on, but the Archean (4.0–2.5 Ga) geologic record possesses only seven reported eolian occurrences which generally do not manifest more than a few diagnostic features of eolian transport. We investigate in detail sandstone units within the Moodies Group (ca. 3.22 Ga) of the Barberton Greenstone Belt, South Africa, one of the oldest well-preserved terrestrial–marine sandy successions. Eight examined locations show terrestrial facies associations, high degrees of compositional and textural maturity, and large-scale foresets; however, none show convincing evidence of eolian transport but several can be excluded with confidence. In comparison to recent eolian strata, grains are generally slightly too coarse-grained, too angular, and mostly too poorly sorted. The common presence of shale laminations, granules, rare pebbles, and shale rip-up clasts demonstrates that the investigated strata were either of aqueous origin or had been formed by eolian processes but were subsequently aqueously reworked. This is vexing because the sandy-gravelly fluvial, coastal floodplain, and estuarine depositional environments of Moodies Group strata may have been hospitable to potentially powerful and long-lasting Archean eolian processes. Early Archean eolian strata may have had a limited depositional area, may have been extensively reworked, or formed under non-actualistic conditions. Thus, the significance of eolian processes on early Earth and their potential contribution to the high degree of textural and compositional maturity of Archean quartzarenites remains poorly constrained. [ABSTRACT FROM AUTHOR]

The paper summarizes major, trace, and Sm-Nd isotopic data on metabasites of the Urik greenstone belt (GB) (Sharyzhalgai uplift, southwestern Siberian craton). The Archean metabasalts (amphibolites and garnet amphibolites) of the supracrustal complex and sheeted bodies represent four geochemical types. The first type includes low-Ti amphibolites of the Urik area. Their characteristic features are a clear LREE depletion ((La/Sm)n = 0.8–1.1), the absent negative Nb anomaly, and positive εNd(t) values (up to +4.1). The second type corresponds to moderate-Ti amphibolites of the Urik area. They differ from the rocks of the first type in the higher (La/Sm)n ratio (1.0–1.5), Th, Zr, Nb, Y contents, and lower εNd(t) values (+0.4...–1.6), but, like the low-Ti amphibolites, they do not show Nb depletion. The third type includes amphibolites of the Tagna area, which show an increase in the concentrations of TiO2, LREE, Th, Zr, and Nb with a decrease in Mg# and have εNd(t) values from +1.3 to –0.5. They are characterized by the elevated (La/Sm)n ratio (1.1–2.3) and Zr and Nb contents and the presence of a clear Nb minimum. The fourth type includes all high-Ti amphibolites of sheeted bodies, which are enriched in LREE ((La/Sm)n = 1.9–3.4), Th, Zr, and Nb, have an increased (Gd/Yb)n (1.4–2.0), a sharp Nb depletion, and negative εNd(t) values. Geochemical modeling coupled with trace element and isotopic composition indicates the formation of the low- and moderate-Ti metabasalts of the Urik area from weakly depleted and nearly primitive mantle sources, respectively. Their protoliths were oceanic basalts. The metabasalts of the Tagna area were formed from a weakly depleted and hydrated mantle source contaminated with crustal material. The enrichment of the source with incompatible elements could be caused by sediment subduction shortly before basalt formation. The metabasalts of the fourth type originated from a long-lived enriched source such as subcontinental lithospheric mantle. They resemble the present-day intraplate continental basalts. None of the three dominant types of metabasalts of the Urik GB shows signatures of sharply depleted or sharply enriched mantle sources, which indicates a weak differentiation of the mantle by the Meso- Neoarchean boundary (~2.8 Ga). The formation of a subduction-modifed lithospheric mantle (hydrated mantle) has only occurred since ~ 2.8 Ga. Different compositional and isotopic characteristics of the metabasalts of the Urik GB suggest that the Bulun terrane was formed by accretion and tectonic juxtaposition of fragments of the oceanic crust, an oceanic island arc, and an older TTG continental crust at ~ 1.86 –1.82 Ga. [ABSTRACT FROM AUTHOR]

The article analyzes the position of individual and average data points of the Archean and Early Proterozoic metaaleuropelites (Isua and Akilia associations, West Greenland; Beit Bridge Complex, Limpopo Province, South Africa; Moodies, Mozaan, and Pretoria groups, Barberton Mountain Land, South Africa; Kola Group, Baltic Shield, Russia; Nurmes paragneisses, Eastern Finland; Onot greenstone belt, southeastern Sayan region, Russia; Rampur Group, Lesser Himalayas, India; Kan metamorphic complex, eastern Sayan region, Russia; Ladoga Group, northern Ladoga region, Russia; Yenisei metamorphic complex, Russia; and others) in the (La/Yb)N–Eu/Eu* and (La/Yb)N–Th diagrams with classification fields constructed from data on the content of REE and Th in the fine grained clastic/aleuropelitic sediments near the estuary of different-category modern rivers, according to (Bayon et al., 2015). It has been shown that the vast majority of data points of metaaleuropelites are localized in fields 1 (particulates of major rivers in the world), 2 (particulates of rivers draining sedimentary substrates) and 4 (particulates of rivers flowing through volcanic rock terrains), as well as the zone overlapping fields 1–3 (particulates of rivers feeding on erosion products of the magmatic/metamorphic terrains). Data points of all examined objects with an age of more than 2.8 Ga in the (La/Yb)N–Eu/Eu* diagram are concentrated mainly in field 4. If all our assumptions are correct, these observations suggest the following conclusion: until the above-indicated time, large rivers and, most likely, rivers draining mainly sedimentary rocks did not exist, and processes of the recycling of fine-grained aluminosiliciclastics were reduced. [ABSTRACT FROM AUTHOR]

The paper presents results of the first geochronologic study (LA-ICP-MS zircon method) of metasedimentary rocks that host the banded iron formation (BIF) and the metarhyolites (halleflinta) of the Gimoly Group in the Kostimuksha greenstone belt,Karelian Craton. Felsic magmatism that produced the rhyolite sills and dikes was dated at 2759 ± 19–2743 ± 15 Ma. The clearly predominant group of zircons in the schists (metagraywacke) has an isotope age of 2753 ±19 Ma, and scarce zircon grains of these rocks vary in age from 3.1 to 2.8 Ga. Geochemically, zircons from metarhyolites fall into three types (1) Eu-depleted and Ce-enriched, (2) Eu- and Ce-enriched, and (3) LREE- and MREE-enriched. The zircons of geochemical types (1) and (2) are also predominant in the sediments. The morphology and structure of zircon crystals in the rhyolites and sediments are also identical. The rhyolites were thus a major source of zircons for the metagraywacke, which alternates with BIF, while the Mesoarchean rocks played a subordinate role. The BIF of the Gimoly Group in the Kostomuksha Greenstone Belt was formed simultaneously with felsic volcanism at 2760–2740 Ma. [ABSTRACT FROM AUTHOR]

• Zircon petrochronological (U/Pb-O-Hf) constraints for the far-east Yilgarn Craton. • Relatively old (2840 – 2775 Ma) crustal block at the eastern edge of the craton. • Identification of the newly defined Dorothy Hills Block and Yamarna Rift. • Re-assessment of the nomenclature in the Eastern Goldfields Terrane (EGT). • The EGT formed via sustained, autochthonous crustal growth. The Archean Yilgarn Craton in Western Australia represents one of the key areas for investigating the mechanisms of crustal growth during Earth's infancy. The far-eastern margin of the Yilgarn Craton has, however, received limited scientific scrutiny. In this study, in-situ zircon petrochronology (U-Pb, O, and Lu-Hf isotopes) is used to constrain the formation and evolution of the easternmost Yamarna and Burtville regions as well as the encompassing Eastern Goldfields (Super)Terrane (EGT) of the Yilgarn Craton. The new data highlight the unequivocal existence of relatively old (ca. 2840 – 2775 Ma), compared to the EGT, and isotopically juvenile (εHf: +2.1 to +3.5) and mantle-like (δ18O: 5.2 to 5.9‰) crust at the eastern edge of the exposed craton. We refer to this crust as the 'Dorothy Hills Block'. The Dorothy Hills Block shows geochronological and isotopic similarities with the adjacent 'Burtville Block' and the Youanmi Terrane (>400 km further west), which suggests a shared history up to ca. 2720 Ma. In addition, linear greenstone belts characterised by younger magmatism (ca. 2720 – 2660 Ma) with a constant, mildly supra-chondritic Hf isotope composition (εHf: 0.0 to +3.5) are identified between the Burtville and Dorothy Hills Blocks. We refer to this region as the 'Yamarna Rift', by analogy to the better-understood 'Kalgoorlie-Kurnalpi Rift'. It is inferred that the Burtville and Dorothy Hills Blocks formed via peri-cratonic magmatism (ca. <2970 Ma), associated with mantle upwelling focused along the eastern edge of the ca. >3300 Ma Yilgarn proto-craton. The Yamarna and Kalgoorlie-Kurnalpi Rifts (ca. 2720 – 2660 Ma) overlie this extended older substrate and formed via intra-cratonic rifting broadly coeval with a major mantle upwelling event. The revised Meso- to Neoarchean crustal evolution model for the EGT, including new nomenclature, suggests a sustained (>200 Myr) and autochthonous geological development of the eastern Yilgarn Craton resulting from mantle upwelling events, potentially within a back-arc tectonic setting. Comparable crustal evolutionary trends are recorded in other Archean cratons (e.g., Superior Craton), which suggests that similar geodynamic regimes operated on a global scale in the Archean. [ABSTRACT FROM AUTHOR]

Archean komatiites form an important host rock for Ni-Cu sulfide deposits in the world. In this study, we examined Archean komatiites and komatiitic basalts from four areas in the Vedlozero-Segozero greenstone belt and two areas in the Tikshozero greenstone belt, Russian Karelia, to constrain their Ni-Cu sulfide mineralization potential using chalcophile element geochemistry. The platinum-group element (PGE) concentrations in these Munro-type komatiitic rocks are at a level of other similar S-undersaturated komatiites and komatiitic basalts globally, with Pt and Pd concentrations in the range of 5–20 ppb and Pd/Ir varying from < 10 (komatiites) to > 15 (komatiitic basalts and basalts). Generally, the metals of the iridium-group elements (IPGEs; Ir, Ru, Os) show a compatible behavior, decreasing in abundance with decreasing MgO, whereas the metals of the palladium group (PPGEs; Pt, Pd, Rh) exhibit an incompatible behavior. The poor correlation between Ir and MgO suggests that olivine fractional crystallization is not the main control on the behavior of IPGE. In contrast, Ir, Ru, and Os show positive correlations with Cr, which is consistent with the compatible behavior of IPGEs in chromite or during the co-precipitation of chromite and platinum-group minerals (PGMs). Palladium, Cu, and Au have been variably mobile during alteration and metamorphism whereas Pt appears to have been less mobile. Some samples from the Khizovaara area show low (Pt/Ti)N ratios, low Ni, and high La/Sm and La/Nb, suggesting localized sulfide saturation in response to crustal contamination. However, the potential of the Russian Karelian greenstone belts for Ni-Cu sulfide mineralization is considered relatively low because most samples show neither enrichment nor depletion of chalcophile elements, and the paucity of dynamic lava channel environments, as indicated by the scarcity of olivine-rich adcumulates. In addition, there appears to be a lack of exposed sulfidic sedimentary rocks in the region. [ABSTRACT FROM AUTHOR]

On the basis of phylogenetic studies and laboratory cultures, it has been proposed that the ability of microbes to metabolize iron has emerged prior to the Archaea/Bacteria split. However, no unambiguous geochemical data supporting this claim have been put forward in rocks older than 2.7–2.5 giga years (Gyr). In the present work, we report in situ Fe and S isotope composition of pyrite from 3.28‐ to 3.26‐Gyr‐old cherts from the upper Mendon Formation, South Africa. We identified three populations of microscopic pyrites showing a wide range of Fe isotope compositions, which cluster around two δ56Fe values of −1.8‰ and +1‰. These three pyrite groups can also be distinguished based on the pyrite crystallinity and the S isotope mass‐independent signatures. One pyrite group displays poorly crystallized pyrite minerals with positive Δ33S values > +3‰, while the other groups display more variable and closer to 0‰ Δ33S values with recrystallized pyrite rims. It is worth to note that all the pyrite groups display positive Δ33S values in the pyrite core and similar trace element compositions. We therefore suggest that two of the pyrite groups have experienced late fluid circulations that have led to partial recrystallization and dilution of S isotope mass‐independent signature but not modification of the Fe isotope record. Considering the mineralogy and geochemistry of the pyrites and associated organic material, we conclude that this iron isotope systematic derives from microbial respiration of iron oxides during early diagenesis. Our data extend the geological record of dissimilatory iron reduction (DIR) back more than 560 million years (Myr) and confirm that micro‐organisms closely related to the last common ancestor had the ability to reduce Fe(III). [ABSTRACT FROM AUTHOR]

This study presents extensive new field, petrographic, and whole-rock major and trace element and Sm–Nd and Pb isotope data for the Neoarchean megacrystic anorthosite-bearing Bird River Sill, spatially associated Neoarchean supracrustal rocks of the Bird River greenstone belt, and the Mesoarchean Maskwa Lake TTG Batholith I, Manitoba, Canada. Field observations indicate that the 2743 Ma Bird River Sill was emplaced into the Northern Lamprey Falls Formation in an oceanic setting and subsequently intruded by the ca. 2725 Ma Maskwa Lake Batholith II. The Northern Lamprey Falls Formation is in fault contact with the overlying Peterson Creek Formation, which in turn is unconformably overlain by the Bird River Formation. The peridotites of the Bird River Sill were subjected to intense alteration and ductile shearing, resulting in the redistribution of many major and trace elements and resetting of their Sm–Nd and U–Th–Pb isotopic systems. The gabbro, leucogabbro, and anorthosite units of the sill largely retain their primary igneous textures and minerals, REE and HFSE compositions, and Sm–Nd and Pb isotope signatures. These geochemical and petrographic data indicate that these units of the Bird River Sill crystallised from a hydrous, Ca- and Al-rich tholeiitic magma that fractionated from a hydrous primitive arc tholeiitic parental magma. The major and trace element geochemistry and initial εNd values of the Bird River Sill (+ 0.53 to + 4.72) and the Northern Lamprey Falls Formation (+ 1.40 to + 2.66) of the Bird River greenstone belt crystallised from parental magma batches that were derived by partial melting of variably-depleted mantle sources. High-magnesian andesites from the Bird River Formation formed by partial melting of subducting oceanic crust and/or slab sediments and subsequent interaction with sub-arc mantle peridotite, whereas the Peterson Creek Formation dacites and Maskwa Lake TTG Batholith I tonalites formed by partial melting of juvenile lower arc crust. The major and trace element geochemistry of the Bird River Sill and greenstone belt and the Maskwa Lake TTG Batholith I, and geological relationships and lithological associations point to a transition from continental arc to continental back-arc magmatism along the southern active margin of the North Caribou terrane at ca. 2743 Ma, resulting in the formation of the Maskwa Lake microcontinent that occurred prior to subsequent continental arc magmatism along the southern margin of this microcontinent. Combined field observations and geochemical data indicate that the Bird River Sill and Bird River greenstone belt are a dismembered Archean subduction-related ophiolite that marks a suture zone between the Winnipeg River subprovince and the Maskwa Lake microcontinent. [ABSTRACT FROM AUTHOR]

Most Archean orogenic gold deposits are associated with major faults such as the Larder Lake-Cadillac fault zone (LLCFZ) in the Superior craton and are hosted in or adjacent to second-order structures as quartz vein systems. Microstructures induced by quartz deformation in these faults or shear zones, such as typical textures of recovery and dynamic recrystallization, give indication of the P-T conditions during deformation. Electron backscatter diffraction (EBSD) analysis of grain lattice orientation and grain size determination, for flow stress calculation, are commonly performed on mylonite or quartzite samples from shear zones. This study tests the validity of such methods on hydrothermal auriferous quartz veins to better constrain the conditions of quartz vein deformation in the vicinity of the LLCFZ and evaluate the effect of depth and local variations on the microstructures and flow stress. Samples were collected in 24 gold deposits and showings along the LLCFZ. The microstructural observations and EBSD analysis allowed the distinction of six microstructural types within the studied samples. Samples analyzed with EBSD display a log-normal distribution of recrystallized grain sizes that is consistent with data for natural samples. The calculated flow stress is highly variable between samples and ranges from 38.2 + 5.1/-6.1 to 74.5 + 2.2/-3.0 MPa in surface samples only. Quartz recrystallization overprints primary textures in all but one samples, which indicates that recrystallization and its associated deformation occurred after the complete crystallization of the veins. Samples with bimodal distribution of recrystallized quartz grain sizes highlight the potential of quartz veins to record several deformation events. Samples are recrystallized at higher temperature and lower stress in the east than in the west of the LLCFZ, which could indicate the exposure of a deeper crust in the east. These data underline the potential of quartz vein microstructures to contribute to the evaluation and evolution of deformation conditions of auriferous quartz vein in an orogenic context. • Archean orogenic gold quartz veins from the southern Abitibi greenstone belt are overprinted by dynamic recrystallization. • Flow stress calculated from recrystallized grain size ranges from 21.2 + 4.9/-5.4 MPa to 103 + 3/-2 MPa. • Microstructural and EBSD analyses allowed to decipher at least two deformation events for some hydrothermal gold quartz veins. • DRX microstructures indicate a deeper crustal level in the eastern part of the Larder Lake-Cadillac fault zone. [ABSTRACT FROM AUTHOR]

• Archean barite in the Barberton Greenstone Belt was deposited along unconformities. • Barium was sourced from cold springs venting to the surface through fractures. • Sulfate was sourced from photolysis reactions in the atmosphere. • Deposition involved Ba-bearing spring water contacting sulfate-bearing meteoric water. • Barite geochemistry carries little information on composition of Archean sea water. Small sedimentary barite deposits are widespread in the 3.258–3.225 Ga Fig Tree Group, Barberton Greenstone Belt (BGB), South Africa. This report explores the stratigraphy of these units, the types of barite present, their depositional settings, and the controls on sedimentation in the Barite Valley area. Five morphological types of barite are recognized: masses of irregularly layered bladed barite termed barite plugs, mounded barite, barite splays, flat-layered bladed barite, and barite sand. In all areas studied, barite deposition was closely associated with active faults and fractures in the underlying rock sequences that are inferred to have been conduits by which barium-bearing fluids made their way to the surface. The barite deposits are also associated with unconformities below, within, and/or above the barite layers. Associated sedimentary rocks show evidence of deposition within intertidal, fan delta, and shallow water to subaerial settings. Previous sulfur isotopic studies have shown that the sulfate originated by photolysis in the atmosphere. We infer that the barite was deposited where barium-bearing, shallow subsurface waters vented to the land surface via mostly small, cool springs and interacted directly with sulfate-bearing meteoric waters, probably often rainwater, triggering barite precipitation. Plug barite marks spring vents, mounds developed in pools, and the outflow systems were characterized by layered bladed barite with increasing barite sand toward more distal settings. Associated thin jasper beds are thought to have formed by oxidation of sedimentary pyrite and siderite by meteoric waters carrying photolysis-produced oxidants, probably H 2 O 2 and perhaps O 2. The Barite Valley barite deposits do not appear to have developed in open marine or evaporitic settings and their geochemistry may carry little information on the composition of Archean sea water or nature of the underlying crust. [ABSTRACT FROM AUTHOR]

• 3 m-thick Kuksha paleoweathering profile developed at 2.5–2.44 Ga is described on mafic volcanics in Imandra-Varzuga Greenstone Belt, northwestern Russia. • Paleoweathering indices and the formation of Al-rich phyllosilicate phase preserved as dickite suggest an intensive weathering. • Siderite in the uppermost part of the weathered section indicates CO 2 partial pressures possibly exceeding 0.1 atm. • Paleo-pH proxies estimate pH values around 5.0–5.5 in the upper part of the Kuksha paleoweathering crust. • Redox-sensitive element proxies suggest weathering under low-oxygen conditions. Chemical weathering of continental crust plays an important role in Earth's surface evolution and also controls the nutrient flow to the ocean. Major changes in the development of Earth's geochemical system were taking place at a critical time coinciding with the Archean-Proterozoic transition when Earth's atmosphere became oxygenated, thus triggering a whole new set of chemical weathering reactions. A paleoweathering crust on basalts of the Kuksha Volcanic Formation in the Paleoproterozoic Imandra-Varzuga Greenstone Belt, northwestern Russia developed between 2.5 and 2.44 Ga, just before the Great Oxidation Event and can potentially shed light on the paleoatmosphere composition and weathering modes of this time. The 3 m-thick Kuksha paleoweathering profile is characterized by Al-phyllosilicate clay minerals that are typical products of strong chemical weathering of igneous rocks. The mineralogy of the uppermost part of Kuksha weathering crust confirms formation under high CO 2 partial pressures, consistent with previous estimates of p CO 2 exceeding 0.1 atm during that time interval. That is in agreement with the geochemical and mineralogical proxies thereby also suggesting an intense acidic weathering under low-oxygen conditions. [ABSTRACT FROM AUTHOR]

Abstract A lens of black schist within 3.7 Ga quartzites of Greenland may be Earth's oldest known alluvial paleosol. The suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite, but it has a truncated top above dark gray grading down to gray color, ptygmatically folded surface cracks filled with silt grains, and large sand crystals, unusual for sedimentary or metamorphic rocks. The paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends, and by chemical analysis showing molar weathering ratios and REE distribution like those of soils. The schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the Isukasia area. The protolith can be reconstructed as a saponite clay with a salt-rich horizon of kieserite, like other acid-sulfate paleosols of the early Earth. Models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 ± 510 ppm O 2 and 820 ± 201 ppm CO 2 , and humid, cool temperate paleoclimate. The profile has organic δ13C PDB consistently of −24.2 to −27.4‰, and modest Raleigh distillation near the top. Similar consistent values and trends are produced by decay of organic matter in living soils, but biotic carbon isotopic composition of sediments is erratic from bed to bed, and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle. Thus life in this very ancient soil is not precluded by our analyses, but ultrastructural and geochemical testing of carbon particles would further test this hypothesis. Graphical abstract Unlabelled Image Highlights • 3.7 Ga schist has crack fills, desert roses, and profile changes like a paleosol. • Chemical model of atmosphere is 36 ± 510 ppm O 2 and 820 ± 201 ppm CO 2. • Degree of weathering is like modern soils of humid, cool temperate climate. • Organic δ13C PDB of −24.2 to −27.4‰ is compatible with life in the paleosol. • This Greenland profile may be the oldest known paleosol on Earth. [ABSTRACT FROM AUTHOR]

Highlights • The migmatitic core of the Yalgoo Dome records map-scale vertical sheath folds. • Sheath folds reflect stretching during diapirism rather than fold interference. • Sheath folds are overprinted by structures indicative of horizontal shortening. • Structural evolution indicates a shift from vertical to horizontal tectonics. Abstract In Archean granite-greenstone terrains, both vertical and horizontal tectonic processes can generate the typical dome-and-keel structural geometries. One of the classic Archean domes interpreted to result from a horizontal tectonic regime is the Yalgoo Dome in the Yilgarn Craton of Western Australia. In this contribution, we reassess the structural evolution of the Yalgoo Dome by detailing the structures exposed in the migmatitic core. The core contains multi-scale domes-and-basins. These structures were previously interpreted as the result of superposed folding. Our structural mapping shows that the dome-and-basin patterns result from vertical sheath folds developed in a single, progressive deformation event (D 1). This first event most likely resulted from diapiric ascent of a buoyant partially molten tonalitic crust and was overprinted by two contractional episodes (D 2 –D 3) both reflecting bulk EW shortening. The structural evolution proposed here points towards a shift from an early history of vertical tectonics dominated by diapirism to a regional regime dominated by horizontal tectonics. [ABSTRACT FROM AUTHOR]

Abstract Geodynamic regimes responsible for the formation of Eoarchean-to-Mesoarchean continental crust may be investigated using numerical modeling in which deeper mantle processes are coupled with shallower processes responsible for the formation and modification of the crust. In an earlier study using a 2D coupled petrological–thermomechanical tectono-magmatic numerical model constrained by information from the geological record, experiments with initial conditions appropriate to the Eoarchean–Mesoarchean revealed multiple tectonic processes by which mostly felsic continental crust could have been formed from an initial primary mafic crust in a non-uniformitarian geodynamic regime. We use the model pressure–temperature–time (P – T – t) paths for material particles from our previous study to test the hypothesis that the range of metamorphic P – T conditions recorded in Archean granite–greenstone domains could have been generated without subduction and plate tectonics. We compare the experimental results with P – T and age data recovered from two early Archean domains, the East Pilbara Terrane (EPT), part of the Pilbara craton in Western Australia, and the Barberton greenstone belt (BGB), part of the Kapvaal craton in South Africa. The dome-and-keel structures that develop in the crust in the numerical experiment and the associated P – T – t paths at various locations within these structures are similar to the structures and P – T and age data recovered from the early Archean crust in the EPT and BGB. This study shows that metamorphism in granite–greenstone-like crust could have been the result of both vertical and horizontal tectonic processes (coupled with crustal convective overturns) that occurred over linked sites of crustal delamination and mantle upwelling. The range of thermal gradients derived from peak P – T values for markers in the experiment matches the range of apparent thermal gradients retrieved from the EPT and BGB, demonstrating that even the lowest apparent thermal gradients associated with dome-and-keel structures in greenstone belts can be achieved in the absence of subduction. Graphical abstract Image 1 Highlights • We use a numerical model with appropriate thermal parameters to investigate Archean metamorphism. • For a reference experiment, we analyze the P – T – t path of markers located in the crust. • We demonstrate similarity between the numerical results and natural data from granite–greenstone belts. • In the experiment, the granite–greenstone-like crust formed over delaminating–upwelling mantle. • We show that low thermal gradients are possible in Archean crust without subduction. [ABSTRACT FROM AUTHOR]

This review paper reports on the discovery of komatiites and the impact this has had on the geosciences internationally. Komatiites were first discovered by Richard and Morris Viljoen in the Barberton greenstone belt of South Africa and proposed as a new class of ultramafic volcanic rock with >18 wt% MgO. Komatiites are in places associated with spinifex textures (elongated bladed and plumose olivine crystals). The discovery and petrological study of komatiites and understanding of the field controls have given rise to intense debate about their mantle origin, water content and geodynamic setting of eruption. The observation that komatiites are found predominantly in the Archean and to some extent in the Palaeoproterozoic, has led to the proposition that the early Earth was much hotter (by c. 400 °C) than the present day. However, some workers have challenged this model involving secular cooling of the Earth since Archean times. These workers argue that komatiites formed in a cooler Archean subduction zone, rather than in a mantle plume. Komatiites of the mid-to late-Archean have also proven to be important economically (e.g. Yilgarn Craton, Australia; Superior Craton, Canada) as they host important nickel-sulphide deposits. Today, novel petrological techniques (e.g. secondary ion microprobe analyses of olivine melt inclusions) are being applied to komatiites to address important outstanding questions in the geosciences, including the question of whether or not plate tectonics was operating in the early Archean. • Komatiites were discovered in 1969 in the Barberton greenstone belt, South Africa. • Significance of komatiites (ultramafic volcanics) to the geosciences is explored. • Origin of komatiite spinifex textures is explored. • Komatiites are found largely in the Archean suggesting the early mantle was hotter. • Debated aspects: their mantle origin, water content, and geodynamic setting. [ABSTRACT FROM AUTHOR]

Applying the Th/Yb–Nb/Yb plot of Pearce (2008) to the well-studied Archean greenstone sequences of Western Australia shows that individual volcanic sequences evolved through one of two distinct processes reflecting different modes of crust–mantle interaction. In the Yilgarn Craton, the volcanic stratigraphy of the 2.99–2.71 Ga Youanmi Terrane mainly evolved through processes leading to Th/Yb–Nb/Yb trends with a narrow range of Th/Nb (‘constant-Th/Nb’ greenstones). In contrast, the 2.71–2.66 Ga volcanic stratigraphy of the Eastern Goldfields Superterrane evolved through processes leading to Th/Yb–Nb/Yb trends showing a continuous range in Th/Nb (‘variable-Th/Nb’ greenstones). Greenstone sequences of the Pilbara Craton show a similar evolution, with constant-Th/Nb greenstone evolution between 3.13 and 2.95 Ga and variable-Th/Nb greenstone evolution between 3.49 and 3.23 Ga and between 2.77 and 2.68 Ga. The variable-Th/Nb trends dominate greenstone sequences in Australia and worldwide, and are temporally associated with peaks in granite magmatism, which promoted crustal preservation. The increasing Th/Nb in basalts correlates with decreasing ε Nd , reflecting variable amounts of crustal assimilation during emplacement of mantle-derived magmas. These greenstones are typically accompanied in the early stages by komatiite, and can probably be linked to mantle plume activity. Thus, regions such as the Eastern Goldfields Superterrane simply developed as plume-related rifts over existing granite–greenstone crust – in this case the Youanmi Terrane. Their Th/Nb trends are difficult to reconcile with modern-style subduction processes. The constant-Th/Nb trends may reflect derivation from a mantle source already with a high and constant Th/Nb ratio. This, and a lithological association including boninite-like lavas, basalts, and calc-alkaline andesites, all within a narrow Th/Nb range, resembles compositions typical of modern-style subduction settings. These greenstones are very rare, and were probably only preserved when fortuitously stabilised by granitic magmatism related to the evolution of later variable-Th/Nb greenstones. The rarity of constant-Th/Nb trends suggests that either processes forming them never dominated Archean greenstone evolution, or that such greenstones simply were rarely preserved. Metamorphic mobility of Th renders the Th/Yb–Nb/Yb plot inappropriate for interpreting Eoarchean greenstone units worldwide. Nevertheless, such sequences appear dominated by volcanic rocks that, in modern settings, reflect only the embryonic or initiation stages of subduction. They probably record subduction failure rather than anything resembling modern-style subduction. [ABSTRACT FROM AUTHOR]

The rare earth elements and Y (REY) geochemistry is considered to be an important tool to interpret the origin of banded iron formations (BIF) and paleo-ocean chemistry. The interpretations are based on the assumption that there was minimal fractionation of REY during precipitation of iron oxyhydroxide phases in the Precambrian oceans. However, the study of iron precipitation in laboratory and modern marine setting shows that precipitating iron oxyhydroxide phases significantly fractionate REY from solution. The purpose of this work is to inspect the validity of using BIF as a direct proxy for ancient seawater and establish the potentials of REY fractionation into BIF. The BIF magnetite of the 3.3–3.1 Ga Badampahar greenstone belt (BGB) is studied by LA-ICP-MS for trace and REY compositions. Considering the REY fractionation in BIF, the corresponding seawater (3.3–3.1 Ga) REY compositions are reconstructed from the BIF magnetite data using experimental REY partitioning coefficients and compared with modern seawater compositions. The calculated seawater REY patterns (shale normalized) show more pronounced W-type lanthanide tetrad effect relative to modern seawater implying high rate of iron precipitation. The calculated seawater data also show true negative Ce anomalies which indicate that iron was oxidized in presence of free oxygen derived from microbial photosynthesis. [ABSTRACT FROM AUTHOR]

• Zircon U–Pb and Hf data from a crustal cross-section of the Superior Province. • TTG magmatism from three isotopically distinct sources. • Upper and middle crust dominated by the youngest, most radiogenic magmatic source. • Only deep crust preserves evidence of all three source components. The formation of granitoids, predominantly tonalite-trondhjemite-granodiorite (TTG) suites was a key process in the differentiation of Earth's earliest crust. However, the relative importance of intracrustal reworking versus input from external sources remains unclear. U–Pb and Hf isotopic data collected from zircon in Archean granitoids frequently support reworking and partial melting of a common crustal reservoir over time to produce subsequent generations of melt. A less frequently observed isotopic signature, in a single region, implies the production of different generations of TTGs from unrelated sources. These end-member scenarios of granitoid generation have quite different implications for Archean crustal evolution. Here, we investigate TTGs from across a crustal cross-section known as the Kapuskasing Uplift in the Neoarchean Wawa terrane of the southern Superior Province. We report U–Pb and Hf isotopic data from zircon in TTGs from the lower, middle, and upper crust to evaluate their sources. Results indicate three groups of TTGs: a ca. 2885 Ma upper-crustal TTG with chondritic εHf, ca. 2835–2820 Ma lower-crustal TTGs with subchondritic εHf, and ca. 2750–2685 Ma TTGs with suprachondritic εHf emplaced at middle- to upper-crustal levels. Each group was derived from an isotopically distinct source. We propose that the Mesoarchean rocks exposed in the Kapuskasing Uplift represent a crustal fragment rifted from the margin of existing Mesoarchean terranes in the Superior Province. Subsequent Neoarchean juvenile magmatism dominates the upper and middle crust resulting in isotopic stratification of the Kapuskasing Uplift, as only the lower crust contains evidence of all three isotopic sources. Inferences about Archean crustal evolution in general rely on upper- to middle-crustal rocks (i.e., greenstone belts and grey gneiss), as these represent the majority of exposed Archean crust worldwide. The potential for the lower crust to be decoupled from overlying rocks implies that some parts of the Archean geological record may be preserved only at the deepest crustal levels—particularly rocks related to the most ancient sources. [ABSTRACT FROM AUTHOR]

The occurrence of Early Archean barite deposits is intriguing since this type of sediment requires high availability of dissolved sulfate (SO 4 2− ), the oxidized form of sulfur, although most authors argued that the Archean eon was dominated by reducing conditions, with low oceanic sulfate concentration (<10 μM) relative to present day levels of 28,000 μM. In order to better assess the redox state of the paleo-atmosphere and -oceans, we examined Fe and S isotope compositions in a sedimentary sequence from the 3.2 Ga-old Mendon and Mapepe formations (Kaapvaal craton, South Africa), recovered from the drill-core BBDP2 of the Barberton Barite Drilling Project. Major elements were also analyzed to constrain the respective imprints of detrital vs metasomatic processes, in particular using Al, Ti and K interrelations. Bulk rock Fe isotope compositions are linked to mineralogy, with δ 56 Fe values varying between −2.04‰ in Fe sulfide-dominated barite beds, to +2.14‰ in Fe oxide-bearing cherts. δ 34 S values of sulfides vary between −10.84 and +3.56‰, with Δ 33 S in a range comprised between −0.35 and +2.55‰, thus supporting an O 2 -depleted atmosphere (<10 −5 PAL). Iron isotope variations together with major element correlations show that, although the sediments experienced a pervasive stage of hydrothermal alteration, the rocks preserved a primary/authigenic signature predating subsequent hydrothermal stage. Highly positive δ 56 Fe values recorded in primary Fe-oxides from ferruginous cherts support partial Fe oxidation in a reducing oceanic environment (O 2 < 10 −4 μM), but are incompatible with a model of complete oxidation at the redox boundary of a stratified water column. Iron oxide precipitation under low O 2 levels was likely mediated by anoxygenic photosynthesis, and/or abiotic photo-oxidation processes. Our results are consistent with global anoxic conditions in the 3.2 Ga-old sediments, implying that the barite deposits were most likely sourced by atmospheric photolysis of S gases produced by large subaerial volcanic events, and possibly SO 4 2− produced by magmatic SO 2 disproportionation in hydrothermal systems. [ABSTRACT FROM AUTHOR]

The metamorphic map of Finland was constructed mainly on the basis of the observed metamorphic features in peraluminous metasedimentary rocks, especially mineral assemblages, and the onset and degree of partial melting that characterize a particular zone. The PT fields of these assemblages are shown in the PT pseudosections with a colour corresponding to the same colour in the map. The GIS map contains several layers. In the first one, the bedrock is classified into low, medium and high metamorphic grade domains. The second layer is based on the metamorphic facies. The third layer shows metamorphic zones based on the stable mineral assemblages in the PT pseudosections, wherein the assemblages are classified into low-pressure (LP), medium-pressure (MP) and high-pressure (HP) series. The pseudosection map has additional layers showing pre-peak and overprinting metamorphic events, wherever such classification is possible. Most of the Archaean bedrock was metamorphosed during 2.70-2.60 Ga in high amphibolite facies at low to medium pressures around 6-7 kbar. There are only a few low- to medium-pressure granulite facies areas. The Archaean greenstone belts were metamorphosed in mid-amphibolite facies at medium pressures, in Ilomantsi at around 5-6 kbar, but in the Kuhmo and Oijärvi greenstone belts, pressures close to 10 kbar or even higher are possible. The Archaean areas were overprinted by Proterozoic metamorphism, which can be seen as local rehydration (epidotization, chloritization), especially close to shear zones. In the Rautavaara and Tuntsa areas, the Proterozoic overprint was pervasive, extending from several kilometres to tens of kilometres in width. This retrogression was obviously related to metamorphism of the Proterozoic cover sequences of the Karelia Province, which were metamorphosed in greenschist to mid-amphibolite facies conditions. The cover sequences show low-pressure metamorphism (Peräpohja, Kiiminki), with andalusite and cordierite assemblages, to medium-pressure (North Karelia schist belt, Central Lapland) preserving kyanite. Most of the Svecofennia Province was metamorphosed during 1.88-1.79 Ga in upper amphibolite and granulite facies at low pressures of around 4-6 kbar. Within the abundant metatexitic and diatexitic migmatites, there are lesser low- to mid-amphibolite facies zones with low-pressure andalusite, sillimanite and cordierite assemblages in peraluminous schists. These commonly have tectonic boundaries with the surrounding higher-grade metamorphic rocks. In the Southern Finland Subprovince, the majority of the lower grade rocks are situated at the northern boundary of the subprovince. In the Western Finland Subprovince, there is a prograde increase in metamorphic grade from andalusite-bearing schists to diatexitic migmatites towards the contacts of the Vaasa complex. [ABSTRACT FROM AUTHOR]

The preserved Archean continental crust is dominantly comprised of tonalite-trondhjemite-granodiorite (TTG) suites associated with less abundant low-grade greenstone belts. The exact processes that form TTGs, as well as the source rock they are derived from, are difficult to constrain from the sparse Archean geological record. However, studies show that the water-present partial melting of metamorphosed basalt at temperatures of 750–950 °C is required to produce large volumes of partial melt with TTG compositions. In this contribution, we investigate if hydrated komatiites — a constituent of greenstone belts — played a vital role in TTG genesis. Using petrology, mineral chemistry and phase equilibria modelling of representative komatiite samples, combined with analysis of a global geochemical dataset of komatiites and basaltic komatiites, we show that during metamorphism hydrated komatiites can release at least 6 weight % mineral-bound water. Up to 5 weight % of this water is released by breakdown of chlorite and tremolite at temperatures between 680 and 800 °C, regardless of the P–T path (i.e., tectonic scenario) experienced by the komatiitic rocks. As the temperatures of komatiite dehydration are above the water-saturated basalt solidus, the released water can trigger voluminous partial melting of basalt to ultimately create TTG batholiths. This considerable hydration potential of komatiites is due to their high X Mg (X Mg = molar Mg/[Mg+Fe]), which stabilises water-rich minerals during oceanic alteration on the seafloor, but also extends the stability of Mg-rich chlorite to high temperatures. During prograde metamorphism, the X Mg , CaO and Al 2 O 3 content of the reactive rock composition determines the proportion of chlorite vs amphibole, and therefore the volume of water which can be transported to temperatures of >750 °C. Despite the low abundance of komatiites in greenstone belts, they potentially played a vital role in crustal formation and the Earth's early water cycle. • All komatiites from the Archean are altered and hydrated, likely on the ocean floor. • When metamorphosed, komatiites release ∼6 wt.% H 2 O at wet basalt melting conditions. • Komatiites are a source of H 2 O for basalt melting and TTG formation in the Archean. [ABSTRACT FROM AUTHOR]

Komatiites from Alexo and Pyke Hill in the Archean Abitibi greenstone belt provide petrological evidence for an early Earth carbon and water cycle, ingassing in the cool Hadean and outgassing in the hot Archean. The komatiites have SiO2 contents that are lower than those expected of advanced volatile-free melting of mantle peridotite. The SiO2 misfit cannot be plausibly accounted for by variations in model Bulk-Earth peridotite composition, perovskite fractionation in a magma ocean, addition of chondrites, a source that had recycled crust added to it, or by chemical alteration during serpentinization. One possible resolution to the silica misfit problem is obtained if the komatiites from Alexo and Pyke Hill were partial melts of carbonated peridotite, a conclusion based on reasonable agreement between the major element compositions of komatiites (i.e. SiO2, Al2O3, FeO, MgO, and CaO) and experimental melt compositions of carbonated peridotite. High-degree melts with olivine as the sole residual phase can have low SiO2 contents owing to carbonate addition. Furthermore, a role for significant H2O is indicated from recent olivine-hosted melt inclusion studies. More work is needed to constrain how much CO2 and H2O is required to resolve the SiO2 misfit, and the T-P conditions of melting. Failure to do so imposes significant uncertainty in Archean mantle potential temperature and geodynamic interpretations. These uncertainties notwithstanding, the komatiites appear to be recording important degassing events in the Archean. Depending on the extent of volatile degassing, hydrous and CO2-rich komatiites could have formed either in mantle plumes or in ambient mantle. Ingassing may have occurred in the Hadean when atmospheric CO2 and H2O were sequestered by reaction with impact ejecta, oceanic crust and mantle peridotite to produce carbonates and hydrous minerals. Parts of the Earth may have been sufficiently cool at some point in the Hadean to ingas the deep mantle, consistent with a variety of constraints from zircon and isotopic studies. Hydrous and CO2-rich komatiites formed from 'carbonated wetspots' in mantle plumes or ambient mantle later in the Archean. The drop in komatiite production at the end of the Archean may be a record of significant purging of the mantle in volatiles, affecting biogenic methane production and the evolution of oxygen in the atmosphere. [ABSTRACT FROM AUTHOR]

The fact that mineral deposit attributes such as the size frequency of orogenic gold deposits in specific provinces exhibit power law distributions similar to forest fires, earthquakes, and fault size populations, is a compelling motivation to examine their genesis from a systems context. Based on well-studied Earth systems such as climate, the systems related to mineral deposits are likely to be complex and potentially include sensitive dependent components that vary simultaneously and in subtly interconnected ways. Although a “systems approach” was enunciated for mineral exploration by Fyfe and Kerrich as early as 1976, it is yet to be fully embraced by the geosciences community that commonly retain models dependent primarily on deposit-scale characteristics. Orogenic gold deposits are well studied and widely considered to represent a single class of deposit that has formed over much of Earth history in settings ranging from Archean granite-greenstone belts to Phanerozoic turbidite sequences. Accordingly, the deposit type is well suited for assessment within a systems context. If orogenic gold deposits do in fact represent a single class of deposits, then the simplest application of a systems approach highlights the fact that the nature of the host upper crustal succession cannot be a fundamental control, with specific granite suites and pyritic sediments not universal, or at least not essential, components of the system. Furthermore the scale of orogenic gold systems implicates processes capable of tapping sub-crustal source regions. Increasingly, advances in orogenic gold systems, and mineral systems in general, are linked to application of systems science that emphasize importance of system-driven criticality. Orogenic gold systems and other mineral systems are typically short in duration and linked in time and space to tectonic triggers. The latter promote a rapid release of energy (‘avalanches’) that overcome system thresholds and are strong indicators of complex systems that may show power-law behavior. Only a rigorous application of a systems approach can cut through the confusion that arises from conflicting models based on local deposit studies. Only a systems approach can evaluate the significance of rare or anomalous features in a small number of deposits. Truly predictive models for mineral exploration will ultimately be developed by workers who adhere to the systems approach. [ABSTRACT FROM AUTHOR]

Measurements of 142Nd isotope signatures in Archean rocks are a powerful tool to investigate the earliest silicate differentiation events on Earth. Here, we introduce a new analytical protocol that allows high precision radiogenic and mass-independent Nd isotope measurements by MC-ICP-MS. To validate our method, we have measured well-characterized ∼3.72 to ∼3.8 Ga samples from the Eoarchean Itsaq Gneiss Complex and associated supracrustal belts, as well as Mesoarchean greenstones and a Proterozoic dike in SW Greenland, including lithostratigraphic units that were previously analyzed for 142-143Nd isotope systematics, by both TIMS and MC-ICP-MS. Our μ142Nd values for ∼3.72 to ∼3.8 Ga rocks from the Isua region range from +9.2 ± 2.6 to +13.2 ± 1.1 ppm and are in good agreement with previous studies. Using coupled 142,143Nd/144Nd isotope systematics from our data for ∼3.8 Ga mafic-ultramafic successions from the Isua region, we can confirm previous age constraints on the earliest silicate differentiation events with differentiation age of 4.390 −0.060 +0.045 Ga. Moreover, we can resolve a statistically significant decrease of 142Nd/144Nd isotope compositions in the ambient mantle of SW Greenland that already started to commence by Eoarchean time, between ∼3.8 Ga (μ142Nd = +13.0 ± 1.1) and ∼ 3.72 Ga (μ142Nd = +9.8 ± 1.0). Even lower but homogeneous μ142Nd values of +3.8 ± 1.1 are found in ∼3.4 Ga mantle-derived rocks from the Ameralik dike swarms. Our study reveals that ε143Nd (i) and εHf (i) values of Isua rocks scatter more than it would be expected from a single stage differentiation event as implied from nearly uniform μ142Nd values, suggesting that the previously described decoupling of Hf and Nd isotopes is not a primordial magma ocean signature. Instead, we conclude that some of second stage processes like younger mantle depletion events or recycling of subducted material affected the 147Sm 143Nd isotope systematics. The preservation of pristine whole-rock isochrons largely rules out a significant disturbance by younger alteration events. Based on isotope and trace element modelling, we argue that the temporal evolution of coupled 142,143Nd/144Nd isotope compositions in the ambient mantle beneath the Isua rocks is best explained by the progressive admixture of material to the Isua mantle source that must have had present-day-like μ142Nd compositions. In contrast, Mesoarchean mafic rocks from the ∼3.08 Ga Ivisaartoq greenstone belt and the 2.97 Ga inner Ameralik Fjord region as well as a 2.0 Ga Proterozoic dike within that region all have higher μ142Nd values as would be expected from our simple replenishment model. This argues for reworking of older Isua crustal material that carried elevated μ142Nd compositions. [ABSTRACT FROM AUTHOR]

Deformation adjacent to faults and shear zones is traditionally thought to correlate with slip. Inherited structures may control damage geometry, localizing fluid flow and deformation in a damage aureole around structures, even after displacement has ceased. In this paper we document a post-shearing anastomosing foliation and fracture network that developed to one side of the Mesoarchean Marmion Shear Zone. This fracture network hosts the low-grade, disseminated Hammond Reef gold deposit. The shear zone juxtaposed a greenstone belt against tonalite gneiss and was locked by an intrusion that was emplaced during the final stages of suturing. After cessation of activity, fluids channeled along fault- and intrusion-related fractures led to the pervasive sericitization of feldspars. Foliated zones resulted from flattening in the weaker sericite-rich tonalite during progressive alteration without any change in the regional NW-SE shortening direction. The anastomosing pattern may have been inherited from an earlier ductile fabric, but sericite alteration and flattening fabrics all formed post-shearing. Thus, the apparent foliated fracture network adjacent to the Marmion Shear Zone is a second-order effect of shear-related damage, distinct in time from shear activity, adjacent to an effectively dormant shear zone. This phenomenon has implications for understanding the relative timing of fault zone activity, alteration and (in this case) gold mineralization related to long-term fault zone permeability. [ABSTRACT FROM AUTHOR]

The Mendon Formation is the uppermost unit of the 3.5–3.26 Ga Onverwacht Group in the Barberton Greenstone Belt, South Africa. It consists of a cyclic stack of komatiitic volcanic units separated by thin cherty sedimentary layers. In most areas, the uppermost Mendon Formation is a sedimentary interval characterized by black chert, banded black-and-white chert, and banded ferruginous chert, although the detailed patterns of lithofacies in different sections are more complex. Previously reported zircon U/Pb ages suggest that Mendon deposition could represent more than 70 Myr of time between ∼3334 Ma and ∼3260 Ma. This study presents sedimentological and petrographic observations of the upper Mendon Formation from across the central part of the Barberton Greenstone Belt in order to investigate sediment sources, depositional processes, and environments of sedimentation. The dominant mode of sedimentation was quiet settling of carbonaceous grains and, in the deepest sections below storm wave base, fine ferruginous material, resulting in finely laminated black and grey chert. In situ carbonaceous laminations are rare, suggesting that benthic microbial mat growth had little direct influence on deposition. The hemipelagic background deposition was punctuated by occasional inputs of fine pyroclastic debris, formation and deposition of silica granules, and reworking by infrequent storm events. Storm deposits are represented by coarse-grained, poorly-sorted intraclast breccias, some of which include distinctive intraclasts sampling lithofacies that are not observed in situ . Despite considerable lateral variability, correlative temporal trends are resolvable in many Mendon sections: there is an upward-deepening of the overall depositional setting recorded in the oldest upper Mendon sections, consistent with the previous interpretation that Mendon time was characterized by rifting (Lowe, 1994a, 1999a). Younger Mendon cycles include thick, relatively ferruginous basal sections, interpreted to reflect the deepest water deposition. These sections are capped by black chert and silicified ashes with more evidence of disturbance and reworking by storms, reflecting gradual shoaling. This sedimentological analysis is broadly consistent with previous geochemical and tectonic analyses and provides a better picture of depositional patterns during uppermost Onverwacht time, before the distinct change in tectonic regime marked by impact spherule layer S2 and the onset of Fig Tree Group orogenesis and related siliciclastic deposition. [ABSTRACT FROM AUTHOR]

This paper shows the results of Short-Wavelength Infrared (SWIR) spectroscopy investigations of volcanic rocks sampled from low-grade metamorphic greenstone belts of the Archean Pilbara Craton in Western Australia. From the reflectance spectra a range of spectrally active minerals were identified, including chlorites, hornblende, actinolite, epidote and white micas. The rock samples were grouped into mineral assemblages based on their spectrally identified minerals and stratigraphic positions. The metamorphic amphibolite and greenschist facies could be identified from the SWIR spectroscopic data as well as three sub zones of the greenschist facies: 1) a zone containing Fe-chlorite; 2) a zone containing intermediate chlorite and epidote; and 3) a zone containing intermediate chlorite, actinolite and hornblende. Spectral parameters were calculated from the reflectance spectra to assess the metamorphic grade and zones. Plots of the depth parameters of the Fe-OH feature near 2250 nm versus the Mg-OH feature near 2390 nm differentiate the metamorphic amphibolite and greenschist facies and a transition zone between the two. The wavelength position parameter of the Mg-OH absorption feature near 2340 nm also serves to discriminate between the various metamorphic sub zones. The identification of the metamorphic grades of the volcanic sequences in greenstone belts with SWIR spectroscopy is useful for regional geological field studies, exploration for metamorphic mineral deposits hosted in the greenstone belts and the interpretation of hyperspectral remote sensing data sets covering similar types of terranes. [ABSTRACT FROM AUTHOR]

Lithic sandstones of braided-fluvial to supratidal facies in the Paleoarchean Moodies Group (∼3.22 Ga, Barberton Greenstone Belt, South Africa) include several regionally traceable units with common to abundant, in places rock-forming, nodular concretions of megaquartz pseudomorphs after gypsum, barite and calcite. Concretionary accumulations are stratiform and commonly associated with aqueously reworked, fine-grained, tuffaceous sediment of originally rhyodacitic composition and can grow to fist-sized agglomerates in crusts tens of m in lateral extent. Weathering of tuffaceous material and feldspar delivered alkali cations such as Ca, Ba, and K, while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a CO 2 -rich atmosphere. Sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic S or SO 2 . Concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone, dominated by seasonal fluctuations of the groundwater level under evaporitic conditions. The concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols. Their formation and composition constrain the local occurrence of sulfate in the Archean atmo- and hydrosphere, their interaction with the emerging biosphere, Archean weathering regime, local climate, and vadose-zone hydrodynamics. [ABSTRACT FROM AUTHOR]

The Hietaharju Ni–Cu–PGE deposit occurs in the Archean Suomussalmi greenstone belt of eastern Finland and is associated with high-Mg basalts and low-Mg komatiitic rocks. It represents a PGE-enriched type among komatiite-related Ni–Cu deposits. The Hietaharju deposit consists of several mineralized lenses in highly altered olivine–pyroxene cumulates (i.e., talc–carbonate–chlorite rocks) and contains disseminated, massive to semi-massive and brecciated sulfides. Post-magmatic processes have not significantly changed the primary magmatic sulfide assemblage (i.e., pyrrhotite–pentlandite–chalcopyrite), but has modified sulfide textures and possibly remobilized and reconcentrated the sulfides. Though the deposit is currently subeconomic, it has high PGE contents in the sulfide fraction, averaging 5.7 ppm Pd and 2 ppm Pt (average of all ore types), being highest in disseminated sulfides. The parental magma represents an intermediate type between Al-undepleted and Al-depleted komatiites and differs from most other komatiitic rocks in the Tipasjärvi–Kuhmo–Suomussalmi greenstone belt complex, which are normally of the Al-undepleted type. The relatively fractionated nature of the komatiitic basalt parental magma of the Hietaharju deposit is consistent with the low Ni/Cu (< 4) and Ni/Co (< 18) and high Pd/Ir (< 25) ratios of the sulfide fraction, similar to the values in the Paleoproterozoic Raglan belt deposits, Canada. Use of REE for constraining crustal contamination is hampered by the highly altered nature of komatiitic rocks in the Hietaharju area. Sulfides in the country rock metasediments that host the Hietaharju Ni–Cu–PGE deposit show clear mass-independent sulfur isotope fractionation (MIF-S) consistent with their age and published S isotope data from the Suomussalmi belt. In contrast, sulfides of the Hietaharju Ni–Cu–PGE deposit only possess a small degree of MIF-S, providing limited direct evidence for the role of local metasedimentary sulfur in ore formation. Archean base metal sulfide mineralization is among the potential sources of sulfur, however, and, as shown by the epithermal Taivaljärvi Au–Ag–Zn deposit, can have mass-independent sulfur isotope fractionation that is similar in magnitude and sign to that of the Hietaharju Ni–Cu–PGE deposit. [ABSTRACT FROM AUTHOR]

Insights into the source and partial-melting conditions of tonalite-trondhjemite-granodiorite (TTG) and other tonalite-dominated intrusive suites provide essential constraints on the tectonic regimes prevailing before 2.5 Ga. Most petrogenetic interpretations rely on whole-rock chemical data and neglect the role of differentiation on primitive melt composition. Indeed, the processes of mineral separation from residual melt (fractionation) and of mineral sorting in a low-melt-fraction mush are rarely evaluated for tonalite-dominated magmatism. Here, numerical modeling provides quantitative constraints on fractional crystallization and mineral sorting for tonalite-dominated suites of the Abitibi greenstone belt, Superior craton, Canada. It is concluded that differentiation can be responsible for most within-suite chemical heterogeneity. The modeling shows that amphibole fractionation alone is sufficient to explain the range of HREE observed in the studied intrusive suites, indicating that the relationship between HREE content and partial-melting depth is not always straightforward. The modeling also shows that mineral sorting can explain chemical heterogeneities as well as primitive melts produced at distinct pressures. Also, the fractional crystallization of amphibole under various temperature conditions explains the contrasting Nb and Ti contents of the studied intrusive phases. The chemical heterogeneity of TTG suites can be explained using different modeling assumptions (e.g., fractional crystallization, mineral sorting, melts produced at distinct pressures) and Neoarchean tonalite-dominated magmatism likely results from a diversity of petrogenetic processes. The petrogenesis of individual intrusions should thus be interpreted with caution before drawing conclusions on source processes and, by extension, the geodynamic settings. • The chemistry of TTG suites provide insights into ancient geodynamic processes. • Differentiation process could explain observed chemical heterogeneities in tonalite. • Numerical modeling provides important insights into differentiation processes. • Nb in TTG suites can help constrain the temperature of the Archean crust. [ABSTRACT FROM AUTHOR]

Nyankanga is the largest gold deposit in the Geita Greenstone Belt of the northern Tanzania Craton. The deposit is hosted within an Archean volcano-sedimentary package dominated by ironstones and intruded by a large diorite complex, the Nyankanga Intrusive Complex. The supracrustal package is now included within the intrusive complex as roof pendants. The ironstone fragments contain evidence of multiple folding events that occurred prior to intrusion. The supracrustal package and Nyankanga Intrusive Complex are cut by a series of NE–SW trending, moderately NW dipping fault zones with a dominant reverse component of movement but showing multiple reactivation events with both oblique and normal movement components. The deposit is cut by a series of NW trending strike slip faults and ~ E–W trending late normal faults. The Nyankanga Fault Zone is a major NW dipping deformation zone developed mainly along the ironstone–diorite contacts that is mineralised over its entire length. The gold mineralization is hosted within the damage zone associated with Nyankanga Fault Zone by both diorite and ironstone with higher grades typically occurring in ironstone. Ore shoots dip more steeply than the Nyankanga Fault Zone. The mineralization is associated with sulfidation fronts and replacement textures in ironstones and is mostly contained as disseminated sulphides in diorite. The close spatial relationship between gold mineralization and the ironstone/diorite contact suggests that the reaction between the mineralising fluid and iron rich lithotypes played an important role in precipitating gold. Intense brecciation and veining, mainly in the footwall of Nyankanga Fault Zone, indicates that the fault zone increased permeability and allowed the access of mineralising fluids. The steeper dip of the ore shoots is consistent with mineralization during normal reactivation of the Nyankanga Fault Zone. [ABSTRACT FROM AUTHOR]

Microbial life was well established and widespread by the Paleoarchean; however, the degree of evolutionary advancement such as microbial motility, intra- and inter-species interactions, phototropism, or oxygenic photosynthesis by that time remains highly debated. The 3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB, South Africa) are Earth's oldest well-preserved siliciclastic tidal deposits. They exhibit a unique assemblage of microbial mats, providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings. The fossil mats are preserved as kerogenous laminations (0.5–1 mm thick) that can be traced laterally for ∼15 km in a ∼1000 m-thick succession of fine- to coarse-grained tidal sandstones and conglomerates. We here present a detailed stratigraphic and depositional facies analysis, documenting the association of the three principal mat morphotypes with specific environmental settings: (1) planar-type in coastal floodplain, (2) wavy-type in intertidal, and (3) tufted-type in upper inter- to supratidal facies. All mat types indicate a flourishing phototrophic biota; moreover, the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments. [ABSTRACT FROM AUTHOR]

The geochemistry and zircon geochronology (U-Pb, SHRIMP-II) of Late Archean intermediate-felsic dikes and plagiogranites of the Shilossky massif of the South Vygozersky and Kamennozersky greenstone belts of Central Karelia were studied. Subvolcanic rocks of the dike complex vary in composition from andesitobasalts to rhyolites, in structural-textural peculiarities, and in the formation age, from 2862 ± 8 to 2785 ± 15 Ma. Compositionally and geochronologically (2853 ± 11 Ma), plagiogranites of the Shilossky massif of the South Vygozersky greenstone belts are close to the most ancient dacite and granodiorite porphyry dikes. Dikes intruded synchronously with intrusion of plagiogranites over a period of at least 70 m.y. Geochronologically, subvolcanic rocks of the dike complex and plagiogranites of the Shilossky massif are similar to granitoids of the TTG assemblages of I- and M-type granites. The Sm-Nd model age of some dikes (2970-2880 Ma) is close to the age of rock crystallization, which is evidence in favor of juvenile origin of magma. Dikes with more ancient model age (3050 Ma) are presumed to contain crustal material. Variations in age and ɛ (from −2.7 to +2.9) indicate the absence of a unified magmatic source. [ABSTRACT FROM AUTHOR]

The 2.97 Ga Prince Albert greenstone belt (PAGB) of Melville Peninsula (Nunavut, Canada) contains multiple komatiitic flows and laterally extensive units of komatiitic tuff in the Selkirk Bay area. These units are serpentinized, chloritized, amphibolitized, and less commonly, carbonate altered. The tuffs retain element abundances typical of Al-undepleted komatiites, identical to the associated komatiitic flows, indicating that the tuffs and flows are comagmatic. The flows exhibit excellent textural preservation with well-developed upper spinifex and lower cumulate zones typical of differentiated komatiite flows. However, the tuffs do not preserve primary microscale textures and are fine-grained and largely massive to planar-laminated. This, coupled with their extensive nature (up to 10 km laterally; up to 10 m thick vertically), lack of gradational contacts with komatiitic flows, and their komatiitic mineralogical and chemical composition support a pyroclastic origin, deposition via eruption-fed fallout and mass flow, and subaqueous deposition during intervals characterized by lower rates of effusive volcanism. The low volatile content of the komatiitic magma, indicated by lack of vesicles in komatiitic flows and the lack of scoriaceous lapilli in the tuff, suggests an origin through phreatomagmatic pyroclastic eruptions. The Selkirk Bay komatiitic lavas and tuffs have liquid compositions with up to 30 wt% MgO, consistent with their derivation from a high magnesium komatiitic parental magma. There is evidence for up to 15% crustal contamination, which is also supported by slight enrichments in the LREE and Th relative to Nb. Major and minor elemental trends are indicative of olivine being the dominant liquidus mineral during early crystallization. Komatiitic flows of the PAGB were erupted onto massive to pillowed basalt where they formed thick, channelized, massive, undifferentiated flows flanked by spinifex-textured sheet/lobate flows. Synvolcanic structural basins and down-faulting are considered responsible for flow channelization and also controlled the deposition of local, framework-supported, ultramafic mass flow deposits and localized deposits of felsic volcaniclastic rocks. The komatiitic tuffs occur at two stratigraphic intervals indicating that they are a product of at least two pyroclastic eruptions. The uppermost strata document a change to a more mafic-dominated lava sequence interlayered with both differentiated and undifferentiated komatiitic flows, which are overlain by thick deposits of felsic volcaniclastic rock. [ABSTRACT FROM AUTHOR]

The Hadean and Archean geologic history of the Earth is discussed in the context of available knowledge from different sources: space physics and comparative planetology; isotope geochronology; geology and petrology of Archean greenstone belts (GB) and tonalite-trondhjemite-granodiorite (TTG) complexes; and geodynamic modeling review to analyse plate-tectonic, plume activity, and impact processes. Correlation between the age peaks of terrestrial Hadean-Early Archean zircons and late heavy bombardment events on the Moon, as well as the Hf isotope composition of zircons indicating their mostly mafic sources, hint to an important role of impact processes in the Earth’s history between 4.4 and 3.8 Ga. The earliest continental crust (TTG complexes) formed at 4.2 Ga (Acasta gneisses), while its large-scale recycling left imprint in Hf isotope signatures after 3.75 Ga. The associations and geochemistry of rocks suggest that Archean greenstone belts formed in settings of rifting, ocean floor spreading, subduction, and plume magmatism generally similar to the present respective processes. The Archean history differed in the greater extent of rocks derived from mantle plumes (komatiites and basalts), boninites, and adakites as well as in shorter subduction cycles recorded in alternation of typical calc-alkaline andesite-dacite-rhyolite and adakite series that were generated in a hotter mantle with more turbulent convection and unsteady subduction. The Archean is interpreted as a transient period of small plate tectonics. [ABSTRACT FROM AUTHOR]

The Mendon Formation in the Barberton Greenstone Belt of South Africa marks the boundary between the Onverwacht and Fig Tree Groups. These groups are characterized by mafic to ultramafic volcanism and felsic volcanism with related epiclastic sedimentation, respectively. This transition marks the end of komatiitic volcanism in the Barberton Greenstone Belt and is accompanied by numerous impact-related spherule layers. This study characterizes the upper Mendon Formation texturally and geochemically over a wide areal extent and across structure and facies changes in an attempt to better understand the evolution of tectonic processes at this boundary. A suite of whole rock and handheld X-ray fluorescence analyses are presented in conjunction with textural information, stratigraphic relationships, and U/Pb ages to create a temporal and chemostratigraphic framework for the Mendon Formation. Local and regional stratigraphic variations, including absence of distinctive layers and variation in layer thickness, seen across the Mendon preclude ascription of a single stratigraphy that accurately describes the >1.2 km of section present in this formation. These variations indicate diachronous deposition of the Mendon Formation over a wide areal extent and into multiple basins or sub-basins by more than one magmatic source. 204 Pb-corrected 206 Pb/ 238 U and 207 Pb/ 235 U concordia model ages of 3279 ± 9.1 Ma and 3287.3 ± 2.9 Ma for two samples from upper portions of the Mendon Formation provide temporal context for deposition. Two samples from the basal 10 m of the Fig Tree Group, above the S2 spherule bed that marks the boundary between the Onverwacht and Fig Tree Groups, give model ages of 3267.8 ± 6.9 Ma and 3261 ± 18 Ma. These ages provide added constraints for the Onverwacht-Fig Tree boundary and confirm that the Weltevreden Formation is roughly age-correlative with the uppermost Mendon Formation. While the Mendon and Weltevreden Formations are in part age-correlative and have similar lithologies, they do not appear to be genetically related. The dominance of ultramafic volcanic rocks and the paucity of felsic volcanic and terrigenous sedimentary rocks within the Mendon and Weltevreden Formations indicate that the primary mode of crustal formation was likely plume-related magmatic accretion and not subduction. The relatively sharp transition within the Barberton Greenstone Belt from ultramafic volcanic sequences to more felsic volcanic and epiclastic sedimentary sequences is everywhere marked by impact-related spherule layers, which suggest that major impacts may have played a role in the evolution of early tectonics to more modern, subduction-related styles. [ABSTRACT FROM AUTHOR]

Structural analysis of a proposed Mesoarchean suture located immediately to the southwest of the Barberton Greenstone Belt, South Africa – known as the Inyoni Shear Zone (ISZ) – reveals that the main, steeply dipping, NNE-striking fabric is distributed across only 1 km width, is late (D3), and clearly overprints two earlier sets of fabric elements (D1, D2) that were originally oriented at right angles to the direction of proposed collision. Dating of a S2 foliated meta-trondhjemite is interpreted to indicate that D2 deformation was at, or younger than, 3238.2 ± 0.9 Ma. The D3 high strain fabric of the ISZ is localised around tightly folded, kilometre-scale supracrustal rafts, but dissipates in all directions away from the rafts, along and across strike. The kinematic asymmetry of S3 shear foliations changes across opposing limbs of tight, upright F3 folds of supracrustal rocks in the ISZ, indicative of simple shear related to buckle folding of more competent supracrustal rocks in a matrix of less competent granitic gneisses. F3 buckle folding was highly non-cylindrical on steep to subvertical axes that plunge to the south in the northern part of the zone and to the north in the southern part of the zone, with nests of vertically plunging sheath folds along amphibolite/metagranite contacts in the central area. High-pressure (HP), garnet-bearing amphibolites are restricted to close association with the vertical D3 sheath folds and do not extend along strike. Combined, the data indicate that the ISZ is not the product of west-directed subduction, as previously suggested. Rather, the strain and kinematic data indicate that the ISZ is just another drip of relatively cool, relatively rigid greenstones into hot, ductile granitic gneisses, formed during partial convective overturn of the upper (greenstone) and middle (granitic) crust. Uplift of the HP rocks was along a late (D4), brittle–ductile mylonite zone associated with emplacement of the c. 3.1 Ga Kees Zyn Doorns syenite. [ABSTRACT FROM AUTHOR]

We present data on the composition of metasedimentary rocks from the greenstone belt of the Onot terrane (Sharyzhalgay uplift) and results of U–Pb dating (SHRIMP II) and Lu–Hf isotope study of detrital zircon from garnet–staurolite schists. The metasedimentary rocks of the Onot greenstone belt are dominated by garnet- and staurolite-bearing schists alternating with amphibolites (metabasalts) in the upper part of the section. Compositionally the protoliths of garnet–staurolite schists correspond to sedimentary rocks, ranging from siltstone to pelitic mudstone. The trace-element characteristics of the garnet–staurolite schists indicate that the terrigenous material was derived from three different rock types, such as tonalite–trondhjemite plagiogneisses (elevated Gd/Yb ratios), mafic rocks (elevated Cr/Th ratios and reduced Th/Sc ratios), and felsic igneous rocks formed by crustal melting (the presence of a Eu minimum), which agrees with the set of potential source rocks from the Onot terrane. The age of predominant detrital zircon reflects the erosion of mainly Neoarchean igneous rocks; this fact, combined with the poor rounding of zircon and tectonically active sedimentation conditions accompanied by mafic volcanism, suggests that the probably depositional age is ca. 2.7 Ga. Older source rocks (2.80–3.35 Ga) contributed to the sediment deposition along with the Neoarchean ones. According to the Hf isotope composition of detrital zircon from the garnet–staurolite schists, the source provenances had different crustal prehistories. The source provenances include Paleoarchean and juvenile Neoarchean crust and rocks formed by the mixing of melts from ancient and juvenile crustal sources. [ABSTRACT FROM AUTHOR]

The finding of Fe-rich picritic basalts on Mars has stimulated interest in Fe-rich magmatism on the early Earth. Rare vestiges of Fe-rich magmatism have been identified in remnants of old crust, but many other lithologies reported as ‘enriched’ or ‘Al-depleted’ komatiites in Archean terranes may also be derived from Fe-rich primitive magmas. Although absent today and rare in the Phanerozoic, Fe-rich magmatism may thus have been widespread on the early Earth. Knowledge of the mineralogy and geological setting of Archean Fe-rich igneous rocks may reveal the nature of volcanic activity on the early Earth and possibly Mars. We conducted a detailed petrological study of some of the best exposed of these terrestrial Fe-rich igneous rocks, which occur in the Pulpwood-Playter Harbours sequence of the Heron Bay volcanics (∼2.6–2.7 Ga) in the Wawa oceanic plateau association of the Superior Province (Canada). Rocks from these Fe-rich igneous bodies range from serpentinized dunite and clinopyroxenite to differentiated gabbro. They followed the crystallization sequence: olivine ± chromite, clinopyroxene, Fe–Ti oxide and plagioclase. Whole-rock major and trace element analyses show that these Fe-rich bodies are mildly alkaline, and are enriched in incompatible trace elements. The parental magma that we calculated for the Fe-rich bodies is a strongly Fe-rich (∼19 wt.% FeO tot ) alkaline magma, with at least ∼9 wt.% MgO. It represents the most Fe- and trace element-enriched end-member of the Wawa greenstone belts and may be the Archean equivalent of modern alkaline basalt. Magmatic clinopyroxene compositions indicate that crystal fractionation occurred in a small closed-system under low-pressure conditions without significant recharge of magma. Our crystal accumulation models confirm that a closed body of this ferropicritic alkaline magma can undergo progressive crystal accumulation and differentiate to produce the Fe-rich rocks of Pulpwood-Playter Harbours. The observed range of trace elements is mainly controlled by increasing proportions of inter-cumulus liquid retained in the rocks during their crystallization and the gabbros represent residual liquid portions of thick lava flows or sub-volcanic sills undergoing extensive closed-system differentiation (∼80% crystal fractionation). Detailed mapping of the Fe-rich bodies revealed that they are underlain by tholeiitic lava and overlain by some volumetrically minor Fe-rich lava. These upper Fe-rich lavas are more enriched in Fe and incompatible trace elements compared to the lower tholeiites and correspond to mixtures between a tholeiitic magma and the Fe-rich alkaline magma parent to the cumulate rocks. The close spatial association between the tholeiites and alkaline ferropicritic bodies at Pulpwood-Playter Harbours suggests a mantle source in which local regions enriched in Fe- and trace elements melted to produce volumetrically minor, mildly alkaline ferropicritic magmas. [ABSTRACT FROM AUTHOR]