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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]

Vast clay deposits of Chitradurga and Sandur greenstone belts of the Dharwar Craton, southern peninsular India, hosting pockets of high‐grade manganese (Mn) ore, provide remarkable clues related to the ore‐forming processes and palaeo‐environmental conditions. In this study these Fe–Mn‐rich clays were investigated through mineralogical and geochemical characteristics to comprehend their palaeo‐weathering and genetic constraints and their role in the supergene accumulation of Mn ore. Their mineralogy is characterized by predominant kaolinite, muscovite, birnessite, haematite, goethite, halloysite along with quartz and traces of ilmenite. Chemical indices of alteration and weathering indicate intense weathering conditions under high palaeo‐precipitation rates in a humid, tropical climate under shallow burial conditions and K‐metasomatism. The presence of authigenic kaolinite and absence of illite reflect on diagenesis up to mesodiagenetic stage. Positive to negative Ce anomalies (Ce/Ce* = 0.67–5.93) of these clays suggest varying oxic‐anoxic conditions during their genesis, whereas conspicuous negative Eu anomalies (Eu/Eu* = 0.61–0.99) are attributed to precursor sediment signatures and hydrothermal imprints of the Mn ore. These kaolinite‐rich clays are suggested to have been formed by in situ chemical alteration, whereas kaolinitization of the Fe–Mn‐rich arenaceous and argillaceous sediments was derived from predominant tonalite and felsic provenance in a shallow marine passive margin setting. Based on the morphologies of birnessite, haematite and goethite in the investigated clays, we propose that these clays have played a key role in the oxidation and deposition of Mn (II) from the migrating aqueous solutions and acted as organo‐polymerization templates for the proliferation of ancient microbial life on the planet Earth. [ABSTRACT FROM AUTHOR]

Singhbhum Craton (SC), one of the oldest continental crusts, has a nucleus of Singhbhum Granite Complex with volcano-sedimentary belts in peripheral parts. Deciphering the inter-relationship between volcano-sedimentary packages and surrounding granitoids offers considerable challenges. The present study area lies in the northwestern parts of SC. Lithounits of volcano-sedimentary packages drapes around the Paleoarchean Bonai Granite Complex (BGC). An attempt has been made to understand the basin development over BGC based on the mutual relationship between lithopackages of Iron Ore Group (IOG) and their structural disposition. It is proposed that two distinct basins, Koira and Gurundia, developed over BGC during Mesoarchean. While the Eastern Koira basin witnessed continuous development accompanied by subsidence wherein lithopackages of Bonai–Kendujhar Iron-ore Belt were deposited, Western Gurundia Basin is marked by a hiatus after the deposition of Gurundia quartzite and mafic volcanics in a shoreline to shallow marine environment. Darjing Group of rocks deposited as platformal package over Gurundia, Koira groups and BGC. Litho packages of Koira, Gurundia and Darjing groups exhibit structural unity as they co-deformed during Iron ore orogeny. The first deformation phase has led to a series of NE–SW to ENE–WSW trending low plunging overturned folds with southeasterly vergence. The second deformation, near coaxial with the first, has led to low northerly plunging open folds with steep northwesterly dipping axial planes. BGC occupies the core of a major second-generation fold with volcano-sedimentary sequence draping around it. It is proposed to put them under Iron Ore Super Group (IOSG), where IOGs (Koira and Gurundia groups) represent the lower part and Darjing Group as the upper part. Research Highlights: IOSG consists of IOGs (Koira and Gurundia groups) and Darjing Group as per present observation. Volcano sedimentary litho packages of IOG were deposited in two distinct basins (Koira and Gurundia basins) developed over BGC in a continental shelf-slope facies. Darjing Group deposited all over Gurundia and Koira groups and BGC after a hiatus which is marked by polymictic basal conglomerate. All the litho members of IOSG were co-deformed during Iron-Ore Orogeny (during the formation of Ur) and exhibit two distinct phases of folding as observed from field and this is reported for the first time with an attempt to bring Darjing group within Iron Ore Supergroup (IOSG). [ABSTRACT FROM AUTHOR]

Greenstone belt dynamics are a key to understanding the formation and evolution of continental crust but the scarcity, commonly poor exposure, strong deformation, and high metamorphic grades of many Archaean supracrustal rocks preserved in greenstone belts often hinder geological insights. Exceptionally, the largely siliciclastic Palaeoarchaean (ca. 3.22 Ga) Moodies Group, uppermost unit of the Barberton Greenstone Belt (BGB) in South Africa and Eswatini, allows a detailed understanding of regional depositional processes and stratigraphic architecture due to its low degree of deformation, considerable thickness, generally good exposure, and diverse sedimentary facies. We here investigate the Powerline Road Syncline (PRS) and the adjacent Maid-of-the-Mists Syncline (MMS), two east-west-trending, tightly to isoclinally folded, locally northwardly overturned synclines in Moodies Group strata in the central BGB. The northern and southeastern margins of the PRS and the southern to southwestern margin of MMS are bounded by the bedding-parallel 24-Hour-Camp Fault. Along this fault, Moodies Group strata and stratigraphically underlying units of the Auber Villiers Formation of the upper Fig Tree Group structurally overlie lower Fig Tree strata above Onverwacht Group strata. Stratigraphic architecture and palaeocurrent analysis of Moodies Group strata in both synclines document a northeastward (depositionally downdip) facies transition from proximal fan delta conglomerates to fluvial- to coastal-plain and estuarine sandstones in which thick foresets likely represent subtidal channel fills. The overall deepening-and fining-upward trend in this sequence, corresponding to an increasing mineralogical maturity of sandstones, is partially obscured by local hydrothermal alteration which was contemporaneous with sedimentation, minor volcanism, and sill intrusion. Moodies Group conglomerates and sandstones were largely shed from the rising Onverwacht Anticline to the southwest. Lithologies and stratigraphy in the PRS-MMS region closely resemble strata of the lower Moodies Group in the Sadddleback Syncline, located nearby to the north but across the Inyoka Fault. If the 24-Hour-Camp Fault was part of a group of extensional faults accompanying the rise of the Onverwacht Anticline and of tonalitic-trondhjemitic-granodioritic plutons around the BGB margins at ca. 3 224 Ma, it would have acted as one of numerous basin-bounding normal faults during early Moodies time. This would place the deposition of Moodies Group strata in the PRS-MMS region in a supra-detachment fault environment which was subsequently tightly folded. [ABSTRACT FROM AUTHOR]

The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ15N values found in well‐preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much‐needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ15N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non‐quantitative oxidation of ammonium via the Feammox pathway, a metabolic co‐cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ15N signatures in Archean sediments. [ABSTRACT FROM AUTHOR]

New geochemical data demonstrate the similarity between Neoarchaean TTGs found in the middle crust of the Zimbabwe Craton and the lower crust of the Northern Marginal Zone of the Limpopo Belt. Hence the Neoarchaean segment of the Zimbabwe Craton provides a complete crustal section—from lower crustal granulites to the supracrustal rocks of the greenstone belts. Mantle activity recorded in the basalts of the greenstone belts is interpreted as plume volcanism, setting a context for the process of crustal generation. However, published isotopic data and new trace element data reported here show that magma genesis in Zimbabwe Craton in the Neoarchaean incorporated up to 20% of older crust, indicating that not all the granitoids are juvenile. This paper seeks to identify the trace element composition of the parental melts to the TTGs of the Zimbabwe Craton, define the petrological processes by which they were generated and set limits on possible petrological models for the process of crust generation. The least contaminated TTGs have low SiO2 and moderately fractionated trace element patterns. Calculations indicate that they were derived by the partial melting of a garnet amphibolite source at ca 10 kb depth. Rare intrusive diorites represent a low fraction melt of an enriched mantle source derived from refertilised depleted mantle. These observations lead to a model in which the source material of the TTGs was basaltic and the thermal energy to melt the basalts came from continued basaltic and komatiitic magmatism. Partial melts of old felsic crust were incorporated into the TTG magmas. The resulting 'enriched' TTGs provided a fertile lower crust from which an extensive suite of late tectonic granites could be generated. These data do not support a modern-style plate tectonic model for the growth of the Zimbabwe Craton in the Neoarchaean. [ABSTRACT FROM AUTHOR]

Rhenium-Os isotope and highly siderophile element (HSE, including Re, Os, Ir, Ru, Pt, and Pd) abundance systematics of Archean komatiites can be used to estimate the stirring rates of the mantle for the HSE and the timing of homogenization of late accreted materials within the mantle. In this study, we report Re-Os isotope and HSE abundance data for ~2.9 Ga komatiites and basalts from the Sumozero-Kenozero greenstone belt in the SE Fennoscandian Shield. The lavas are characterized by excellent preservation of the primary textural, chemical, and Re-Os isotope characteristics. The Re-Os isotopic data for spinifex-textured and cumulate komatiite and massive basalt samples from the lowermost sequences define a precise 10-point isochron (MSWD = 2.6) with an age of 2904 ± 18 Ma and an initial 187Os/188Os = 0.10758 ± 18 (γ187Os(2904) = +0.45 ± 0.17). This is the first direct age determination for the Sumozero-Kenozero lower komatiite-basalt sequences. Our modeling indicates that the mantle source of the komatiites and basalts evolved with a time-integrated 187Re/188Os = 0.418 ± 6. This ratio is well within the uncertainty of the bulk chondritic average 187Re/188Os = 0.410 ± 51 (2SD), also consistent with the chondritic evolution of the majority of komatiite mantle sources observed globally. The mantle source of the Sumozero-Kenozero komatiites has been calculated to contain the total HSE abundances of 58 ± 7% of those in the estimates for modern Bulk Silicate Earth (BSE). This estimate is in the middle of the range for other late Archean and Proterozoic komatiite systems. Using the estimated HSE abundances in the sources of komatiite systems as a function of their ages and ISOPLOT regression analysis, we calculated the average time in the past by which late accreted materials have been completely homogenized within the mantle to be 2.48 ± 0.23 Ga. These data require that the residence times of the late accreted planetesimals within the mantle, before complete homogenization, were on average 1.92 ± 0.23 Ga. This estimate represents a constraint on the average mixing rates of the mantle in terms of the HSE abundances in the Hadean and the Archean. [ABSTRACT FROM AUTHOR]

This paper presents a complex interpretation of structural–petrological studies and new geochemical and isotope–geochronological data on the Paleoarchean metamorphic and metaintrusive rocks of Mts. Rymill and Bloomfield in the northern part of the granite-greenstone Ruker terrane (Prince Charles Mountains, East Antarctica). The detailed sequence of changing Archean tectonomagmatic processes in the northern block of the Ruker area has been revealed and the main stages of tectonic deformations and metamorphism of Precambrian geodynamic evolution are shown, which is a key for the understanding of the geological evolution of the Archean metamorphic complexes of East Antarctica and ancient Earth's regions. The northern part of the Ruker Terrane is composed of the Meso-Neoarchean granite–gneiss domes, which are tectonically framed by fragments of greenstone belt of the metavolcanosedimentary Meso-Neoarchean Menzies Series. The detached blocks or packages of sheets of metavolcanosedimentary sequences are a section of weakly metamorphosed intermediate–felsic sandstones, quartzites, and micaceous schists. An assemblage of rocks spans a significant period from 3.2 to 2.5 Ga. The composition of the Mesoarchean granite–gneiss domes of the Mawson Series (3.2–3.1 Ga) corresponds to those of ancient granitoids, is similar to tonalite–trondhjemite–granodiorite (TTG) complexes, and is comparable with similar associations of ancient cratons of Australia, Canada, and Fennoscandia. The rocks of the Mawson Series are polymetamorphic and a significant part of their recrystallization is related to the Mesoarchean stage of metamorphism, whose end is comparable with the formation of the Meso-Neoarchean blocks of the granite–gneiss association of the TTG dome of ~2.8 Ga identified by the author. The age of the striking tectonothermal event of ~2.7 Ga is determined by overgrowths of zircons from the TTG gneisses. The Mawson Orthogneiss and TTG gneisses are compared with intraplate A‑granitoids and are ascribed to low-Ti high-K Archean granitoids. The geochemistry of the rocks indicates that primary melts of the Meso-Archean Mawson Orthogneiss formed in crust at shallower depths (P < 8–10 kbar) in comparison with Meso-Archean TTG gneisses, whose formation is possibly caused by the increasing thickness of the crust followed by its extension and thinning at a boundary of ~2.5 Ga. [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]

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0–2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT. [ABSTRACT FROM AUTHOR]

It is well established that the major and minor element contents of chromites are subject to change during greenschist to amphibolite facies metamorphism. During upper amphibolite facies metamorphism, chromite can be completely converted to chrome magnetite. However, not all elements are affected to the same degree, the concentrations of +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable to modification. The degree to which trace elements, particularly the platinum-group elements (PGE), are affected has not been closely examined. The compositions and textures of chromites from komatiites of the Gorumahishani greenstone belt of the Singhbhum Craton (India) have experienced a range of metamorphic conditions from greenschist to amphibolite facies, providing the opportunity to study the changes of trace and platinum-group element composition with metamorphic grade. Five types of altered chromites are identified from the komatiitic suite of rocks in the ~120-km-long greenstone belt. The type-I chromites are non-porous and characterized by the least modified cores. These chromites are mostly present in the northern Maharajgunj-Tua Dungri section where rocks show metamorphism from greenschist to greenschist-amphibolite transition facies. The type-II and type-III chromites are porous and mostly found in the southern Kapili section of the greenstone belt where rocks show metamorphism up to the mid-amphibolite facies. Type-IV and type-V chromites are completely modified to ferritchromit and chrome magnetite, respectively, and are present in the komatiitic rocks from the entire greenstone belt. The central cores of the type-I and type-II grains have relatively higher concentrations of mobile trace elements (e.g. Zn, Co, and Mn) with higher Mg# [Mg/(Mg + Fe2+)], lower Cr# [Cr/(Cr + Al)], and lower Fe3+/R3+ (R3+ = Fe3+ + Cr3+ + Al3+) ratios than their respective rims. Significantly higher concentrations of the immobile trace elements (e.g. Ti and V) in the cores of the type-II grains relative to their chrome magnetite rims from the Kapili section and to the type-I varieties from other sections might be due to the metamorphism of the komatiitic rocks under higher-grade conditions (amphibolite facies). In situ LA-ICPMS analysis for PGE reveals a relatively higher concentration of Ru and Rh in the rims of the type-I chromites than in the cores which is due to the diffusion of these elements from the normal spinel structure of the cores towards the bivalent octahedral sites of the inverse spinel structure of the chrome magnetite rims during metamorphic processes. The lower concentrations of Os, Ir, Ru, and Rh in the cores of the type-II chromites from the Kapili section might be related to the metamorphism of the rocks under higher-grade conditions that facilitated the diffusion of these elements to associated sulphide or platinum-group mineral or alloy phases. The calculated partition coefficients of Sc, Ti, V, Mn, Ni, Ga, Os, Ir, Ru, and Rh from the least altered chromite cores assuming equilibrium with the parental komatiitic melt also suggest the variable effects of metamorphism when compared with global experimental and empirical values of the natural samples. [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]

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 Nellore Schist Belt (NSB) along the southeastern margin of Eastern Dharwar Craton (EDC) is similar to all other Late Archaean greenstone belts of EDC. The NSB of EDC is bounded by Proterozoic Cuddapah sediments including the deformed and metamorphosed Mesoproterozoic Nallamalai Fold Belt (NFB) in the west; and high-grade Palaeo- to Mesoproterozoic Eastern Ghat Mobile Belt (EGMB) towards further east. Two parallel Western and Eastern arms of different environmental facies of NSB found to be evolved in foreland and back-arc/continental-arc basinal setups, respectively. An initial Late Archaean subducting coupled with the east to west convergence mechanism with regards to evolution of the NSB is discussed in detail. Based on gravity and seismic studies, an elliptical gravity high over the Eastern arm and with its gradual decrease in intensity towards the Western arm indicates the presence of a high-density layer over the Eastern arm of NSB with its continuity and wedging out below the Western arm defines the subducting and convergence mechanism from east to west in concurrence with several established studies. Accordingly, the NSB is differentiated as one continuous single segment, i.e., Late Archaean in contrary to earlier views. The steep gravity gradient in the west defines the Cuddapah Eastern Marginal Thrust, which separates the NFB in the west and NSB in the east. A similar gravity gradient is differentiated as low-angle Malakondasatram Thrust corroborating with the geological features in the eastern part, which defines the eastern limit of EDC. In the NSB domain of Kanigiri–Pamuru area, an isolated structurally controlled Mesoproterozoic metasedimentary unit is established unconformably overlying both Western and Eastern arms of the NSB. This unit is transformed as high-grade kyanite schist in the Vinjamuru domain of extreme eastern part at the EGMB front. The occurrence of Mesoproterozoic high-grade schist over the low-grade schist of NSB is interpreted as a post-dated overprint with composition and layer specific metamorphism in response to the Proterozoic tectono-thermal events of Easternghat orogeny. [ABSTRACT FROM AUTHOR]

New data have been obtained on composition and age of the Archean supracrustal complexes of the Yalonvara–Pertinjarvi, Korpijarvi, and Pastajarvi structures in the marginal part of the Karelian Craton of the Fennoscandian Shield in the conjunction area with the Svecofennian Orogen. The structures were typically attributed to the single Yalonvara–Ilomantsi greenstone belt. On the basis of geochronological, petrographic, and geochemical data, only the Korpijarvi and Pastajarvi structures with an age of 2.73 Ga can be considered as a continuation of the Hattu greenstone belt in the Ilomantsi province (Finland). More ancient rocks of the Yalonvara–Pertinjarvi structure, estimated at 2.95–2.93 Ga, can be compared with the Mesoarchean basalt–andesite–dacite–rhyolite stratotectonic association of the Hautavaara greenstone structure. The younger Neoarchean rocks metamorphosed under the high-temperature amphibolite facies conditions are composed of gneisses and amphibolites. The ancient greenstone rocks metamorphosed under the greenschist and epidote–amphibolite facies conditions are of tuff nature. The rocks of both complexes are similar in terms of geochemistry to volcanic rocks of the suprasubduction settings, but they are different in proportions of the material of the mantle wedge and subduction plate. [ABSTRACT FROM AUTHOR]

The Banded iron formations (BIFs) and manganese formations of Chitradurga, Shimoga and Sandur greenstone belts of Dharwar craton, associated with the stromatolitic carbonates, carbonaceous phyllites and shales along with gold mineralization, are best geological entities to evaluate the Archean biogeochemical processes and transformation of a habitable Earth. The geochemical anomalies along with C, O and S isotopic signatures of stromatolitic carbonates, carbonaceous phyllites and sulphidic BIFs reflect on biogenic signatures, fluctuating Archean ocean temperatures from 25–75°C and anoxic to euxinic redox conditions. The U-Pb detrital zircon ages of these stromatolitic carbonates indicate 3.5 to 2.6 Ga whereas the carbonaceous shales indicate 3.2–2.2 Ga reflecting the transportation of organic matter to the ocean basin during the growth of stromatolitic carbonates. The gold content of carbonaceous phyllites and sulphidic BIFs indicates hydrothermal source. The O2 produced due to stromatolitic activity has deposited Mn and Fe of the Archean oceans as BIFs and Mn formations. The biogenic matter of the stromatolites along with other siliciclastic material, gold and sulphides derived from the volcanic activity mixed and formed as carbonaceous shales in the ocean basin under euxinic conditions. The comprehensive geological, geochemical including isotopic studies on these rock types collectively indicate the interaction of lithosphere-hydrosphere-atmosphere-biosphere in the Archean oceans which paved the way for the advanced forms of life. [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]

Data-driven methods have increasingly been applied to solve geoscientific problems. Incorporation of data-driven methods with hypothesis testing can be effective to address some long-standing debates and reduce interpretation uncertainty by leveraging larger volumes of data and more objective data analytics, which leads to increased reproducibility. In this study, lithogeochemical data from regionally persistent Archean shale units were aggregated from literature, with special reference to the Kaapvaal Craton of South Africa—namely, shales from the Barberton, Witwatersrand, Pongola, and Transvaal Supergroups—and the Belingwe and Buhwa Greenstone Belts of the Zimbabwe Craton. We examine the feasibility of using machine-learning algorithms to produce a geochemical classification and demonstrate that machine learning is capable of accurately correlating stratigraphy at the formation, group, and supergroup levels. We demonstrate the ability to extract highly useful scientific findings through a data-driven approach, such as geological implications for the uniqueness of the sediment compositions of the Central Rand and West Rand Groups. We further demonstrate that when lithogeochemistry and machine-learning algorithms are used, only about 50 samples per geological unit are necessary to reach accuracy levels of around 80%–90% for our shale samples. Consequently, for many traditional tasks, such as rock identification and mapping, some expensive analyses and manual labor can be replaced by an abundance of cheaper data and machine learning. This approach could transform large-scale geological surveys by enabling more detailed mapping than currently possible, by vastly increasing the coverage rate and total coverage. In addition, the aggregation of historical data facilitates data reuse and open science. These results justify the need to bridge data- and hypothesis-driven techniques for the stratigraphic correlation and prediction of rock units, which can improve the accuracy of the inferred stratigraphic correlation and basin setting. [ABSTRACT FROM AUTHOR]

A new lithostratigraphic framework based upon a review of historic data, field mapping and remote sensing, including aerial photography, high-resolution airborne aeromagnetic and radiometric data, is proposed for the Archaean geology along the southeastern margin of the Kaapvaal Craton, South Africa. A synthesis of new and existing data reveals that previously accepted lithostratigraphic schemes require complete revision, with reinterpretations identifying multiple major shear zones and previously unidentified granitoid successions along the margin of the craton. In this new lithostratigraphic framework, lithologies of the Southern Syncline previously correlated with the Nsuze Group of the Pongola Supergroup, are redefined as greenstone lithologies associated with the Ilangwe Greenstone Belt. The geology of the Nkandla region can be subdivided into five distinct geophysical domains including: (i) an extension of the Ilangwe Greenstone Belt, (Domain 1) which is subdivided into; a lower volcanic succession, the Thathe Formation, comprising pillow and amygdaloidal volcanics; the adjoining Sabiza Formation, comprising pillow volcanics exposed in the southeast of the study area; the volcano-sedimentary Mtshwili Formation, which overlies the Thathe and Sabiza formations, consisting of quartz (sericite) schist, phyllite, metavolcanics and iron formation; the Nomangci Formation, which occurs as a region of highly deformed quartz-kyanite-sericite schists, and the Simbagwezi Formation, which comprises maroon to green phyllites and schists in the north of the study area. (ii) granitoids of the Impisi Granitoid Suite (Domain 2) which border the greenstone succession to the north, intruding the Nomangci and Simbagwezi formations. (iii) a southern complex of sheared granitoids termed the Umgabhi Granitoid Suite (Domain 3), which intrudes the Thathe, Sabiza and Mtshwili formations. (iv) The two remaining domains, comprise the Mesoproterozoic Mfongosi and Ntingwe Groups (Domain 4) and Mesoarchaean volcano-sedimentary sequences of the Pongola Supergroup (Domain 5). [ABSTRACT FROM AUTHOR]

The chemical compositions of Phanerozoic basalts of all known geodynamic settings (mid-ocean ridges, oceanic plateaus, oceanic islands, island arcs, fore-arc and back-arc basins, continental rifts, large igneous provinces) are compared with Archean basalts, including basalts of greenstone belts and high-grade gneiss complexes. Linear discriminant analysis has shown that the Archean basalts differ significantly from Phanerozoic basalts in terms of the content of the least mobile major and trace elements. In this regard, the geochemical systematics of Phanerozoic basalts in most cases cannot be used to reconstruct the geodynamic settings of Archean basalts. The results obtained are illustrated by the example of the Mesoarchean basalts of the Olondo fragment of the Tokko–Khani greenstone belt. [ABSTRACT FROM AUTHOR]

Komatiitic magmatism is a characteristic feature of Archean cratons, diagnostic of the addition of juvenile crust, and a clue to the thermal evolution of early Earth lithosphere. The Slave craton in northwest Canada contains >20 greenstone belts but no identified komatiite. The reason for this dearth of komatiite, when compared to other Archean cratons, remains enigmatic. The Central Slave Cover Group (ca. 2.85 Ga) includes fuchsitic quartzite with relict detrital chromite grains in heavy-mineral laminations. Major and platinum group element systematics indicate that the chromites were derived from Al-undepleted komatiitic dunites. The chromites have low 187Os/188Os ratios relative to chondrite with a narrow range of rhenium depletion ages at 3.19 ± 0.12 Ga. While these ages overlap a documented crust formation event, they identify an unrecognized addition of juvenile crust that is not preserved in the bedrock exposures or the zircon isotopic data. The documentation of komatiitic magmatism via detrital chromites indicates a region of thin lithospheric mantle at ca. 3.2 Ga, either within or at the edge of the protocratonic nucleus. This study demonstrates the applicability of detrital chromites in provenance studies, augmenting the record supplied by detrital zircons. [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 sources of metals enriched in Archean orogenic gold deposits have long been debated. Metasedimentary rocks, which are generally accepted as the main metal source in Phanerozoic deposits, are less abundant in Archean greenstone belts and commonly discounted as a viable metal source for Archean deposits. We report ultralow-detection-limit gold and traceelement concentrations from a suite of metamorphosed sedimentary rocks from the Abitibi belt and Pontiac subprovince, Superior Province, Canada. Systematic decreases in the Au content with increasing metamorphic grade indicate that Au was mobilized during prograde metamorphism. Mass balance calculations show that over 10 t of Au, 30,000 t of As, and 600 t of Sb were mobilized from 1 km3 of Pontiac subprovince sedimentary rock metamorphosed to the sillimanite metamorphic zone. The total gold resource in orogenic gold deposits in the southern Abitibi belt (7500 t Au) is only 3% of the Au mobilized from the estimated total volume of high-metamorphic-grade Pontiac sedimentary rock in the region (25,000 km3), indicating that sedimentary rocks are a major contributor of metals to the orogenic gold deposits in the southern Abitibi belt. [ABSTRACT FROM AUTHOR]

The Neoarchean potassic granite from the Hosadurga region adjacent to the Chitradurga greenstone belt (CGB), part of the western Dharwar Craton (WDC), is studied for its petrogenesis. The combined field, petrographic, whole‐rock elemental, and Nd–Sr isotope data of granite and associated Tonalite–Trondhjemite–Granodiorite (TTG) gneiss from this region are presented here. The granites are alkali‐calcic to calc‐alkalic in terms of modified alkali lime index (MALI) and are weakly peraluminous (molar Al2O3/(CaO + Na2O + K2O) = 1.0–1.1). The geochemical data indicate that these are formed by the melting of TTG gneiss at depths above the garnet stability field. Isotopic data (εNd = −2.9 to −7.6 at T = 2,600 Ma, ISr = 0.715–0.729, TDM > 3.0 Ga) also show the involvement of Archean crustal sources. Trace element modelling was carried out using non‐modal equilibrium melting equation, which constraints partial melting of TTG source to a low degree (11%) for the formation of this granite. This model could also produce a major element composition consistent with this granite. The upper bounds on the magmatic temperatures (724–785°C) assessed from zircon saturation thermometry suggest that fluid influx is required. TTG samples show variable and very high positive epsilon Nd values (1.5–8.9), higher than the contemporaneous mantle, suggesting re‐melting of mafic crust. The observed high Nd isotope ratios in TTG indicate involvement of a mafic source that formed as early as ~3.8 Ga, probably as a result of global differentiation of the early Earth. [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 Kaapvaal and Zimbabwe cratons host some of the earliest evidence for life. When compared to the contemporaneous East Pilbara craton, cherts and other metasedimentary horizons in southern Africa preserve traces of life with far greater morphological and geochemical fidelity. In spite of this, most fossiliferous horizons of southern Africa have received relatively limited attention. This review summarises current knowledge regarding the nature of early life and its distribution with respect to environments and ecosystems in the Archaean (>2.5 Ga) of the region, correlating stratigraphic, sedimentological, geochemical and palaeontological understanding. There is abundant and compelling evidence for both anoxygenic photosynthetic and chemosynthetic biomes dominating Palaeoarchaean-Mesoarchaean strata dating back to around 3.5 Ga, and the prevalence of each is tied to palaeoenvironmental parameters deducible from the rock record. Well-developed, large stromatolites characteristic of younger Mesoarchaean-Neoarchaean sequences were probably constructed by oxygenic photosynthesisers. Isotopic evidence from the Belingwe greenstone belt and the Transvaal Supergroup indicates that both a full sulphur cycle and complex nitrogen cycling were in operation by the Mesoarchaean-Neoarchaean. The Archaean geological record of southern Africa is thus a rich repository of information regarding the co-evolving geosphere and biosphere in deep time. [ABSTRACT FROM AUTHOR]

A new 3D crustal-scale model has been created for the Kautokeino Greenstone Belt (KKGB) in northern Norway based on 3D density modelling and aeromagnetic data, integrated with qualitative interpretation of geophysical and geological data. Detailed geophysical analyses and their integration with an existing petrophysical database allowed us to identify and interconnect shallow and deeper structures throughout the study area. To have better control on the regional tectonic setting of the KKGB, the model area has been extended to include the northern part of Finland and Sweden. The new 3D density model reveals that the KKGB is broader than supposed previously. The 3D analysis indicates that the belt reaches depths of approximately 5-6 km and is emplaced as a highly deformed structure between the Archaean Jergul Gneiss Complex (JGC) to the east and the Rommaeno Gneiss Complex (RoGC) in Finland to the west. The Raiseatnu Gneiss Complex (RaGC) in the western part of the KKGB is characterised by a dominant NNW-SSE magnetic trend that is sub-parallel to the main trend of the KKGB. Moreover, the RaGC is similar to the KKGB in terms of the presence of a large number of sub-parallel magnetic anomalies. This gneiss complex shows mostly short-wavelength anomalies and some circular anomalies that differ from the more subdued and irregular anomaly pattern of the JGC and RoGC. In the new 3D model, the RaGC is assumed to constitute a migmatised part of the Kautokeino Greenstone Belt. At a large scale, this Proterozoic greenstone belt has some similarities to Archaean counterparts, where most of the deformation is caused by gravitational tectonics. This study shows that careful integration of geological and geophysical data can strongly improve the 3D understanding of the complex, poorly exposed, Precambrian terranes of the Finnmarksvidda region in Norway. [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]

Volcanic rocks in Archean and Paleoproterozoic greenstone belts are abundant and have been suggested as a potential Au source for orogenic Au deposits. The behavior of Au during metamorphism of these rocks is, however, poorly known. We present ultra-low-detection-limit Au analyses from a suite of variably metamorphosed rocks from the Archean La Grande subprovince, Canada, and the Paleoproterozoic Central Lapland greenstone belt, Finland. Both areas are well endowed in Au and have great potential for discovery of new orogenic Au deposits. The metavolcanic rocks in these belts are grouped into tholeiite and calc-alkaline magmatic series, for which the protolith Au contents are calculated using Au versus Zr/Y power-law regressions from greenschist facies samples. In the tholeiitic rocks, Au is compatible during magmatic processes and decreases with differentiation, whereas in the calc-alkaline rocks, Au is incompatible and increases with differentiation. Mass-variation calculations show that as much as 77% and 59% of the initial Au content is lost during progressive metamorphism to upper amphibolite facies conditions (>550 °C) in La Grande and Central Lapland respectively. This study highlights, first, that metavolcanic rocks release Au during metamorphism in Archean and Paleoproterozoic greenstone belts and are thus a good potential source rocks for orogenic Au deposits; second, that the Au fertility of the metavolcanic rocks is controlled by their mantle source and magmatic evolution; and third, that the metamorphic devolatilization model can be applied to Archean and Paleoproterozoic orogenic Au deposits. [ABSTRACT FROM AUTHOR]

The world-class Cuiabá gold deposit of the Archaean Rio das Velhas greenstone belt in Brazil is hosted in banded iron formation containing carbonaceous matter and carbonate, within the reclined, isoclinal Cuiabá fold. Mineralised quartz veins are hosted in andesite in the stratigraphic footwall of the banded iron ores and form some of the more recently discovered ore bodies. Fluid inclusion data of the quartz vein–associated "Veio de Quartzo" ore body are obtained from four quartz types (Qz1, Qz2, Qz3, Qz5) in gold-mineralised V1 shear vein and V2 extensional veins, barren V3 extensional vein array, and V4 breccia-style veins, all developed during the Archaean D1 event. Three fluid types are distinguished: (i) aqueous fluids of low salinity (1.8–3.8 wt% NaCl equiv), homogenisation (into liquid) at 220 to 230 °C; (ii) aqueous fluids of moderate salinity (5.3–12.7 wt% NaCl equiv), and homogenisation at 250 to 290 °C; and (iii) aqueous-carbonic fluids of moderate salinity (6.0–15.1 wt% NaCl equiv), with 30–91 mol% CO2, 8–41 mol% CH4 and up to 28 mol% N2 and decrepitation (into vapour) at 280 to 310 °C. Based on an independent pressure estimate, a pressure correction was applied to aqueous fluid inclusions, resulting in minimum trapping temperatures at 360 °C for V1 veins, 330 °C for V2 veins, 300 °C for V3 veins and 270 °C for the late-stage V4 veins. Ion chromatography analyses reveal a Br/Cl ratio of 0.7 × 10−3 in Qz1-V1, from 1.4 to 1.5 × 10−3 in Qz2-V2, 0.3 to 0.4 × 10−3 in Qz3-V3 and 0.7 to 0.9 × 10−3in Qz5-V4 veins. Zinc, Pb and Cu are relatively enriched with ~ 100 to 1000 ppm in aqueous and aqueous-carbonic fluid inclusion assemblages in all vein and quartz types, which is similar to other orogenic gold deposits hosted in the Rio das Velhas greenstone belt. The fluid inclusion data are consistent with a model invoking a metamorphic origin for the mineralising fluid. A two-step model of hydrothermal fluid flow and gold enrichment is suggested to have developed during the Archaean D1 event, with an early, aqueous-carbonic fluid pulse of relatively high temperature (from V1 up to V3) and an evolved, aqueous-carbonic fluid pulse of lower temperature (V4, breccia-style veins). The Rio das Velhas greenstone belt is dominated by regionally metamorphosed metasedimentary rocks, resulting in a complex hydrothermal fluid evolution and related gold mineralisation such as the shear zone–controlled Veio de Quartzo ore body. [ABSTRACT FROM AUTHOR]

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyzed a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold-bearing massive metabasalts (supracrustals) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), Western Dharwar Craton, southern India. The study involves the following steps: (1) analyzing the internal magnetic fabric, using anisotropy of magnetic susceptibility (AMS) studies, and determining strength of the host metabasalts, (2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R′), stress ratio (ϕ), and susceptibility to fracturing, and (3) deciphering the paleostress condition using fault-slip analysis. We interpret the NNW–SSE to NW–SE (mean 337/69 ∘ NE) oriented magnetic fabric in the rocks of the region as having developed during regional D1/D2 deformation on account of NE–SW shortening. However, D3 deformation manifested by NW–SE to E–W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (∼ 2.4 km). However, NNW–SSE orientations were prone to reactivate under both high- and low- Pf conditions, thereby attaining maximum vein thickness along these orientations. The deduced paleostress from fault-slip analysis along with the kinematics of the fractures and veins are in good agreement with previously estimated regional tectonics. Thus, integrating multiple domains of studies helps in the logical interpretation of fluid flow conditions and vein emplacement mechanisms in the study area that has not been ventured before. [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]

Data on the carbon and oxygen isotopic composition of carbonates from the rocks of the Giyani greenstone belt, Kaapvaal craton, South Africa are presented. This belt is immediately adjacent to the Hout River Shear Zone, along which the Southern Marginal Zone (SMZ) of the Limpopo granulite complex is overthrusted onto the Kaapvaal craton. The δ13C values are –2.1 and –2.2‰ for carbonates from two metapelite samples, –5.1‰ for a meta ultrabasite sample, and –7.1 and –7.7‰ for two metabasite samples, respectively. The δ18О values are 17.1 and 17.5‰ for carbonates from the metapelites, 14.3‰ for carbonates from the metaultrabasite, and 12.9 and 13.0‰ for carbonates from the metabasites. The comparison of the data obtained to the published data on the isotopic composition of carbonates from the rocks of the different greenstone belts indicates both hydrothermal (in the metabasites and the metaultrabasite) and sedimentary (in the metapelites) origins of the carbonate material in the rocks studied. Based on the comparison of these data to δ13C of magnesite that formed in ultrabasic granulites during the interaction of the Limpopo complex with the craton, the graphite and fluid inclusions in leucocratic garnet-bearing granitoids that transported fluids, and the carbon isotopic composition of graphite from metapelites of the SMZ, it is concluded that the carbon source was located outside the SMZ and represented a heterogeneous carbonate-bearing lithologies. It could be carbonate-bearing volcanogenic–sedimentary sequences of the greenstone belts of the Kaapvaal craton, which underwent prograde metamorphism during the interaction with the SMZ granulites. [ABSTRACT FROM AUTHOR]

Most of the upper crustal fluid flows are strongly influenced by the pre-existing fractures/foliations in the rocks under a certain state of tectonic stress and fluid pressure condition. In the present study, we analyze a wide range of crosscutting fractures that are filled with quartz veins of variable orientations and thicknesses, from the gold bearing massive metabasalts (supracrustal) of the Chitradurga Schist Belt adjacent to the Chitradurga Shear Zone (CSZ), western Dharwar craton, south India. The study involves the following steps: 1) analyzing the internal magnetic fabric using anisotropy of magnetic susceptibility (AMS) studies, and strength of the host metabasalts, 2) quantifying the fluid pressure condition through lower hemisphere equal area projection of pole to veins by determining the driving pressure ratio (R'), stress ratio (φ), and susceptibility to fracturing, and 3) deciphering the paleostress condition using fault slip analysis. We interpret that the NNW-SSE to NW-SE (mean 337°/69°& NE) oriented magnetic fabric in the rocks of the region developed during regional D1/D2 deformation on account of NE-SW shortening. However, D3 deformation manifested by NW-SE to E-W shortening led to the sinistral movement along CSZ. As a consequence of this sinistral shearing, fractures with prominent orientations formed riedel shear components, with CSZ as the shear boundary. Subsequently, all the pre-existing fabrics along with the riedel shear components were reactivated and vein emplacement took place through episodic fluid pressure fluctuation from high to low Pf at shallow depth (~2.4 km). However, NNW-SSE orientations were susceptible for reactivation under both high and low Pf conditions leading to a much greater thickness along the same. The deduced paleostress from fault-slip analysis, along with the kinematics of the fractures and veins are in good agreement with the previously revealed regional tectonics. Thus, integrating multiple domains of studies, help in the logical interpretation of fluid flow condition and vein emplacement mechanism in the study area that has not been ventured before. [ABSTRACT FROM AUTHOR]

The subsurface geometry of five representative late Archaean 'Chilimanzi and Razi' suite plutons in the Zimbabwe craton (ZC) has been investigated by gravity modelling constrained in part by surface geology, density measurements and seismic information, to determine their 3D configuration and infer tectonic context of emplacement. The generally K-rich, massive, homogeneous monzogranites are characterised by large Bouguer gravity lows (up to − 30 mGal amplitude) whose gradients outline their spatial extent well. The southernmost plutons and their anomalies have general trends paralleling the North Marginal Zone (NMZ) of the Limpopo orogenic belt (LB). Predictive gravity models indicate that the density contrast of the Chivi batholith (CB) adjacent to the 'volcanic arc-like' Belingwe greenstone belt extends to a depth of about 13 km. The nearby Razi pluton (RP) which intrudes the ZC-LB boundary appears to have been emplaced at shallower depths/levels. The gravity model suggests a thickness of about 5–6 km, and a moderate to shallow dip to the southeast under the NMZ, compatible with syn-kinematic intrusion during overthrust of the LB over the ZC. The smallest Nalatale granite (Ng) is on average 2.5 km thick under the Fort Rixon greenstone belt but includes a root up to 4.5 km thick under the anomaly peak, and a steep contact with the tonalite/gneiss to the east. These granites follow the general power-law for pluton dimension and are similar in this respect to the classical wedge-shaped plutons, extending largely in one direction, with large aspect ratios (Length (L)/Thickness (T) > 7). However, the overall shape of the RP is typical of a diapir (Width (W) < T), although it may have been affected by the LB deformation. Gravity modelling along a NS traverse crossing the Chilimanzi batholith (ChB), the Masvingo greenstone belt (MGB) and the Zimbabwe granite (ZG) indicate a thickness of around 6 km for the dense greenstone belt with a thickness of about 8.5 km for the adjacent ZG. The 'complex' shaped ChB shows a 2 km thick tabular body with a root zone extending to ~ 4.5 km depth on the south end, adjacent to the greenstone belt; typical of the so-called flat-floored plutons with a gently dipping floor towards the root zone. These two plutons roughly follow the power-law for laccolith/batholith dimensions (W/T > 5; L/T > 15). Overall, the CB and the ZG are interpreted as massive, deep-rooted batholithic intrusions (L/T ≅ 10), contrary to some geological interpretations of these late, post-kinematic intrusions as sheet-like bodies emplaced at relatively shallow levels in the crust. On the other hand, the ChB appears to be a tabular intrusion, probably fed by dykes; it exhibits a lateral extent much greater than the vertical one, outlining a sheeted geometry (W/T > 7; L/T > 18). The geophysical evidence, together with geological and fabric data, support and/or confirm the two main granite configurations: sheets and batholith; and thus also confirm the two main modes of emplacement: dyke and diapirism or ballooning plutonism. This is consistent with other known batholiths on the ZC but considered unusual for plutons of the same age and spatially close when compared to other Archaean cratons. [ABSTRACT FROM AUTHOR]

The 920 km2 Archean Sandstone greenstone belt lies in the central-northern part of the Southern Cross Domain of the Youanmi Terrane, Western Australia. The belt, forming an arrow-head geometry, is bounded by the Youanmi and Edale shear zones on its eastern and western margins, respectively. The belt has a total gold endowment, including historic production, resources and reserves, of approximately 62 t (2.0 Moz). Most known deposits are situated within a central-southern ultramafic domain, although this is likely a function of exploration focus. The distribution of historical exploration efforts and domaining of deposit types, based on litho-structural controls on ore formation, highlight the potential for significant undiscovered deposits, hosted within arrays of shear zones parallel and adjacent to the lithosphere-scale Youanmi and Edale shear zones. [ABSTRACT FROM AUTHOR]

Archean spherule layers represent the only currently known remnants of the early impact record on Earth. Based on the lunar cratering record, the small number of spherule layers identified so far contrasts to the high impact flux that can be expected for the Earth at that time. The recent discovery of several Paleoarchean spherule layers in the BARB5 and CT3 drill cores from the Barberton area, South Africa, drastically increases the number of known Archean impact spherule layers and may provide a unique opportunity to improve our knowledge of the impact record on the early Earth. This study is focused on the spherule layers in the CT3 drill core from the northeastern Barberton Greenstone Belt. We present highly siderophile element (HSE: Re, Os, Ir, Pt, Ru, and Pd) concentrations and Re‐Os isotope signatures for spherule layer samples and their host rocks in order to unravel the potential presence of extraterrestrial fingerprints within them. Most spherule layer samples exhibit extreme enrichments in HSE concentrations of up to superchondritic abundances in conjunction with, in some cases, subchondritic present‐day 187Os/188Os isotope ratios. This indicates a significant meteoritic contribution to the spherule layers. In contrast to some of the data reported earlier for other Archean spherule layers from the Barberton area, the CT3 core is significantly overprinted by secondary events. However, HSE and Re‐Os isotope signatures presented in this study indicate chondritic admixtures of up to (and even above) 100% chondrite component in some of the analyzed spherule layers. There is no significant correlation between HSE abundances and respective spherule contents. Although strongly supporting the impact origin of these layers and the presence of significant meteoritic admixtures, peak HSE concentrations are difficult to explain without postdepositional enrichment processes. [ABSTRACT FROM AUTHOR]

Archean ultramafic–mafic magmatism associated with dynamic mantle melting processes represents an integral part of crustal evolution and continental growth. Archean mantle plumes and their high temperature derivatives in komatiitic magmas provide a cornerstone in understanding the thermal and chemical evolution of the early Earth and its mantle. In this study, we evaluate the geochemical characteristics of the temporally distinct Sargur Group and Dharwar Supergroup greenstone belts of Dharwar Craton in southern India. The komatiites associated with these greenstone belts are characterized by Al‐depleted and Al‐undepleted compositions. Their MgO, Ni, Co, and Cr concentrations reflect primitive, undifferentiated nature of precursor melts, whereas the HFSE and REE chemistry corroborates high‐degree (30–50%) partial melting of mantle with prominent role of majorite garnet and perovskite in the generation of komatiitic melts. The Al‐depleted komatiites were formed at a greater depth (~13 GPa) by equilibrium melting leaving a garnet‐bearing residue. The resultant komatiitic melt retained buoyancy under extreme pressure and high temperature conditions, segregated and accumulated in the ambient peridotite mantle source and subsequently escaped with progressive decrease in pressure in the ascending plume. The Al‐undepleted and relatively rare Al‐enriched komatiites were derived by fractional melting at intermediate to shallow depths after the discharge of a large proportion of melt ensued by the collapse and entrance of residual garnet into the melt phase. The Mesoarchean–Neoarchean Dharwar komatiites provide evidence for heterogeneous, hydrated Archean upper mantle trapped by ascending mantle plumes. The non‐plume, arc‐related signatures invoke contributions from (a) sub‐cratonic lithospheric mantle roots influenced by ancient subduction events and (b) hydrated Archean upper mantle. The Sargur Group and Dharwar Supergroup komatiites record a distinct temporal transition and geochemical heterogeneity in the Archean mantle beneath the Dharwar Craton and can be attributed to Archean upper mantle hydration by flat slab subduction of >3.3 Ga oceanic crust at shallow level with vestiges of primordial oceanic crust forming deep cratonic mantle roots beneath the Dharwar Craton. The temporal transition towards mantle hydration beneath Dharwar Craton during the Mesoarchean can be correlated with the onset of subduction‐driven plate tectonics and ocean–crust–mantle interactions. [ABSTRACT FROM AUTHOR]

The Neoarchaean era is characterized by rapid crustal growth corresponding to some fundamental global changes in geodynamic processes. However, the nature of crustal growth including the mechanism and tectonic setting of the Neoarchaean are controversial issues. The eastern Dharwar Craton (EDC) exposes widespread Neoarchaean granite‐greenstone belts, which provide an opportunity to evaluate the various models proposed for Neoarchaean crustal growth. In this study, we present field, petrographic, and geochemical data and discuss the petrogenesis and significance for crustal evolution for a suite of previously undescribed banded gneisses, TTG (tonalite–trondhjemite–granodiorite), biotite granites, alkali feldspar granite and gabbro. These rocks are associated with Neoarchaean metavolcanic and metapelites rocks of the Tsundupalle greenstone belt, in the eastern fringe of the EDC. Whole‐rock major and trace element geochemical data are consistent with diverse sources, including both crust and enriched mantle in an evolving subduction zone. A convergent orogenic setting is proposed for interpreting the association of various granitoids in the Tsundupalle area. Finally, intrusion of crustally derived, highly silicic, alkali‐rich granite, and mantle‐derived gabbro emplaced in a post‐subduction regime is proposed. Lithospheric delamination and attendant mantle melting are suggested as possible mechanisms for generation of these rocks. The understanding of generation of the different granitoid types along with gabbro provides significant insights into the mechanism of Neoarchaean crustal growth. [ABSTRACT FROM AUTHOR]

• Cherty stromatolitic carbonates of Dharwar Craton exhibit limited morphologies. • REE characteristics reflect marine water conditions with heterogenous hydrothermal input. • Negative to positive δ13C indicate oxic-anoxic conditions of Archean oceans. • Negative δ18O endorse local variations in the Archean sea water temperature. The well-preserved stromatolitic carbonates from Neoarchean Sandur, Chitradurga and Shimoga greenstone belts of Dharwar Craton were studied to constrain the depositional conditions, geochemical, isotopic characteristics and the Archean seawater temperatures. These rocks display limited diversity in their morphological features such as columnar (occasionally branched), pseudo-columnar, domal, crinkly laminated, and oval/elliptical shapes. The stromatolites are associated with Fe-Mn formations along with arenites, argillites, limestones and carbonaceous shales. These stromatolites are cherty dolomites having variable SiO 2 , MgO and CaO contents. The stromatolitic carbonates of the three greenstone belts possess low total Rare earth elements plus Ytterium (∑REY) along with positive La, Eu, Gd, Y anomalies and Y/Ho ratios reflecting their deposition in marine conditions with variable hydrothermal and terrigenous inputs. These stromatolites display predominantly negative δ13C values ranging from −1.61‰ to 0.01‰ VPDB of Sandur, −2.53‰ to 1.35‰ VPDB of Chitradurga and positive δ13C of Shimoga varying from −0.30‰ to 1.29‰ VPDB combined with the negative δ18O values (−20.95‰ to −7.72‰ VPDB). Our studies indicate fluctuating oxic to anoxic; acidic to alkaline environment responsible for the deposition of these stromatolites with locally varying Archean ocean temperatures from 25 to 75 °C. These conditions favoured the deposition of cherty dolomitic stromatolites in a shallow marine environment ranging from supra to subtidal zones with the minor influence of hydrothermal activity which is evident for the existence of Archean smaller plate tectonics. [ABSTRACT FROM AUTHOR]

The Mauranipur and Babina greenstone belts of the Bundelkhand Craton are formed of the Central Bundelkhand greenstone complex (CBGC). This complex represents tectonic collage which has not been previously studied in depth. The purpose of this study is to contribute to the understanding of the main features of the Archaean crustal evolution of the Bundelkhand Craton. The CBGC consists of two assemblages: (1) the early assemblage, which is composed of basic-ultramafic, rhyolitic–dacitic, and banded iron formation units, and (2) the late assemblage, which is a felsic volcanic unit. The units and assemblages are tectonically unified with epidote–quartz–plagioclase metasomatic rocks formed locally in these tectonic zones. The early assemblage of the Mauranipur greenstone belt is estimated at 2810 ± 13 Ma, from the U–Pb dating (SHRIMP, zircon) of the felsic volcanics. Also, there are inherited 3242 ± 65 Ma zircons in this rock. It is deduced that this assemblage is related to early felsic subduction volcanism during the Mesoarchaean that occurred in the Bundelkhand Craton. Zircons extracted from metasomatic rocks in the early assemblage's high-Mg basalts show a concordant age of 2687 ± 11 Ma. This age is interpreted as a time of metamorphism that occurred simultaneously with an early accretion stage in the evolution of the Mauranipur greenstone belt. The felsic volcanism, appearing as subvolcanic bodies in the late assemblage of the Mauranipur greenstone belt, is estimated to be 2557 ± 33 Ma from the U–Pb dating (SHRIMP, zircon) of the felsic volcanic rocks. This rock also contains inherited 2864 ± 46 Ma zircons. The late assemblage of the Mauranipur greenstone belt corresponds with a geodynamic setting of active subduction along the continental margin during Neoarchaean. The late assemblage Neoarchaean felsic volcanic rocks from the Mauranipur and Babina greenstone belts are comparable in age and geochemical characteristics. The Neoarchaean rocks are more enriched in Sr and Ba and are more depleted in Cr and Ni than the Mesoarchaean felsic volcanic rocks of the early assemblage. Through isotopic dating and the geochemical analysis of the volcanic and metasomatic rocks of the CBGC, this study has revealed two subduction–accretion events, the Meso–Neoarchaean (2.81–2.7 Ga) and Neoarchaean (2.56–2.53 Ga), during the crustal evolution of the Bundelkhand Craton (Indian Shield). [ABSTRACT FROM AUTHOR]

• Formation and evolution of a Neoarchean greenstone belt are demonstrated. • Three mafic-felsic magma-series are identified by bulk/zircon composition and Lu-Hf. • Geochemistry of some basalts and zircon is consistent with subduction zone setting. • Some volcanic rocks contain xenocrystic zircon indicating Mesoarchean inheritance. • Distinguishing rock types and zooming to greenstone belt subunits give new insights. Archean greenstone belts contain the oldest volcanic rocks on Earth and can provide critical insights into Earth's early evolution. This study chooses the Sturgeon Lake greenstone belt in the easternmost Western Wabigoon terrane (WWT) of the Superior craton to illustrate an option for better understanding the formation of Archean greenstone belts. Our dataset includes outcrop observations, bulk compositions of 77 mafic to felsic volcanic samples, and in-situ U-Pb, Lu-Hf, and trace elements of zircon from 14 intermediate to felsic volcanic samples. The results revealed that this greenstone belt had broadly continuous volcanism from ca. 2780 to 2725 Ma, followed by a tectonic quiescence prior to a ca. 2707–2703 Ma volcanism at the terrane boundary. The intermediate volcanic rocks are characterized by variable Th/Nb and highly depleted Hf (zircon ƐHf (i) = +3 to + 6), interpreted to be derived from mantle sources ranging from those of normal mid-ocean ridge basalts (N-MORB) to those of enriched MORB (E-MORB), which commonly assimilated various felsic crustal components. The mafic volcanic rocks are dominated by low La/Sm basalts with negligible Nb, Ta anomalies and limited range of Th/Nb, forming a mantle array on the Th/Yb-Nb/Yb plot and consistent with oceanic crust of N-MORB to enriched N-MORB affinities. Some basalts from the ca. 2738–2725 Ma assemblages, however, gave high La/Sm, negative Nb and Ta anomalies, and limited range of Th/Nb plotting above and parallel to the mantle array. These basalts might be associated with passive subcretion and rollback of oceanic crust of the stagnant-lids tectonics or subduction of oceanic lithosphere as in modern-style plate tectonics, both of which are supported by trace element compositions of zircon that indicate hydrous, oxidized, large-ion lithophile element-enriched, and medium- to high-pressure magmas. Identification of three magma-series would not be possible if the data were lumped together, which is commonly seen in terrane- or craton-scale studies that sometimes propose contrasting interpretations of rock origins and Archean geodynamic processes. This detailed approach of distinguishing rock types and zooming into the subunits of a greenstone belt offers a practical solution to more accurately characterize Archean volcanic rocks and to better understand the early Earth. [ABSTRACT FROM AUTHOR]

• Geochronological data suggested deposition maximum age for the Algodão Formation of 1958 ± 18 Ma. • The youngest zircon was dated as 894 ± 38 Ma in the Serra da Garapa Formation. • Zircon ages in the Santo Onofre Group were similar to glacial units of the Macaúbas Group. The evolutionary history of the Paramirim Aulacogen is characterized by the superposition of rifts that developed from 1770 to 675 Ma. In the Northern Serra do Espinhaço Fold Thrust Belt, two units of this aulacogen were analyzed in the present study: the Algodão Formation (Espinhaço Supergroup) and the Santo Onofre Group (São Francisco Supergroup). In the study area, the Algodão Formation consists of alternating of quartzose to feldspathic metasandstones, sericitic metasandstones, oligomictic metaconglomerates, metasandstones and quartzites, laminated metapelites, metarhythmites and metasiltites, and quartzites. In turn, the Santo Onofre Group, the Serra da Garapa Formation was subdivided into two lithofacies associations. The first is composed predominantly by quartzites and hematite-rich, graphite-rich and/or manganese-rich metapelites and phyllites. The second lithofacies association comprises staurolite-garnet-quartz-biotite aluminous schist and quartzites. In the Boqueirão Formation predominates metasubarkoses and lithic metasandstones and metasiltites, besides oligomictic metamicroconglomerates. U-Pb geochronological studies (LA-ICPMS) were performed on detrital zircons from these units. Ages ranging were 2870 ± 17–1958 ± 18 Ma and 3177 ± 42–894 ± 38 Ma, respectively. The main primary sources were related to rocks from the basement of the Western Bahia orogeny, of siderian-orosirian age, as well as volcanites and plutonites, both mafic and felsic, with ages between ca. 1.7 and 0.8 Ga, related to the evolution of the precursor basins of the Araçuaí-West Congo Orogen. As secondary sources, results suggest Metavolcanosedimentary sequences and Greenstone Belts of Archean and Paleoproterozoic ages, as well as rocks from the Espinhaço Supergroup and Macaúbas Group. The lithofacies of Serra da Garapa formation, as well as the ages found in detrital zircons, suggest that this formation is chronocorrelate with Rio Peixe Bravo Formation of the Macaúbas Group. Together, these formations, and Matão and Duas Barras Formation represent the pre-glacial unit of the Santo Onofre-Macaúbas basin initiated in Tonian. Some characteristics suggest that the Boqueirão Formation may be correlated to some of the glacial units of this basin. This work suggests that the formations Fazendinha and Serra da Vereda are excluded from the Santo Onofre Group. [ABSTRACT FROM AUTHOR]

Documenting evidence of fossil microbial life on early Earth is made difficult by the paucity of suitable Archean sedimentary rocks, their common metamorphic overprint, the small outcrop areas, and the small size of the objects of interest. Although a large number of putative microfossils dating back as far as 3700 Ma have been described, the syngeneity and biogenicity of many occurrences is debated, and some of the proposed fossils have been found to be either contaminants or abiotic artefacts. The ~3200 Ma Moodies Group of the Barberton Greenstone Belt (BGB), South Africa, contains locally abundant and remarkably well-preserved microbial mats which show indirect evidence of photosynthetic activity. They also contain microstructures which strongly resemble remains of microbial cells. Detailed morphological and geochemical analyses, however, show that these structures mostly represent fragments of volcanic tephra. Our study demonstrates that opaque microstructures within microbial mats can potentially be misidentified as microfossils even when a strict protocol is followed. It also posits the question to which degree volcanic air-borne fertilization contributed to the remarkable growth rate, high mechanical tenacity and wide extent of these oldest tidal microbial mats in siliciclastic environments. [ABSTRACT FROM AUTHOR]

Metarhyiolites are found along with metapelites and metabasites in sedimentary–volcanic sections of Archean greenstone belts of the Kursk block of the Eastern Sarmatia. The age of igneous protoliths of metarhyolites is about 3.12 Ga. A positive value of εNd(3122) = +0.9 for metarhyolite and a model age of TNd(DM) = 3.30 Ga, as well as the age of the inherited zircon grain (3250 Ma), testify to the contribution of a more ancient crustal component in the formation of rhyolite magmas. In terms of the geochemical characteristics, metarhyolites are quite similar to tonalites and trondhjemites of the TTG association of about 3 Ga in age within the framework greenstone belts of the Kursk block. These data confirm the hypothesis of the common geological history of the eastern part of the Ukrainian Shield and the Kursk block during the Mesoarchean. [ABSTRACT FROM AUTHOR]

Abstract The Johannesburg Dome – a tonalite-trondhjemite-granodiorite (TTG)-dominated terrane in the central Kaapvaal Craton – contains a suite of ultramafic-mafic complexes that are concentrated largely along its southern rim. These >3.3 Ga ultramafic-mafic complexes have recently been re-interpreted as fragments of an Archaean ophiolite (Anhaeusser, 2006a), challenging a longstanding hypothesis whereby the complexes represent the intruded remnants of an Archaean greenstone belt. As with similar interpretations of ultramafic-mafic units in other Archaean cratons, the ophiolite hypothesis is used as evidence in favour of Phanerozoic-style plate tectonic processes having operated in the Archaean, with this geodynamic regime the prevailing explanation for the rocks and structures displayed by the Kaapvaal Craton. Through detailed new geological mapping of the scarcely studied Modderfontein Complex, alongside petrography, bulk-rock geochemistry and mineral chemistry, we here assess the validity of both hypotheses. Moreover, having experienced amphibolite-facies metamorphism and substantial hydrothermal alteration, we assess the degree of element mobility experienced by the Modderfontein Complex and discuss the implications for subsequent geodynamic interpretations. The 1 km2 area mapped comprises separate northern and southern domains, with the former dominated by homogenous serpentinite that contains irregularly-shaped chromitite lenses, and the latter comprising coarsely-layered peridotite, pyroxenite, gabbro and amphibolite. The data indicate that the Modderfontein Complex has experienced significant mobility of Pd, the fluid-mobile lithophile elements (e.g., Ba, Rb and Cs) and potentially some elements generally considered immobile. Mobility of Pd is restricted to chromitite lenses, where Pd was originally hosted by sulphide mineral phases (e.g., pentlandite). This element was immobile in all other Modderfontein lithologies, where it is hosted by nano-scale PGM, demonstrating that PGE mobility is, in-part, controlled by the host phase(s). Moreover, based on a variety of petrographic and geochemical characteristics, including PGE mineralogy and spinel mineral chemistry, it is considered unlikely that the Modderfontein Complex represents an ophiolite fragment. Instead, the Complex is interpreted as the intrusive remnant of a greenstone belt that was subsequently intruded by TTG magmas. Graphical abstract Unlabelled Image Highlights • The Complex comprises lithologically distinctive domains separated by a shear zone • Chromitite lenses are geochemically distinct from Phanerozoic podiform chromitite • It is unlikely that the Modderfontein Complex represents an ophiolite fragment • The Complex likely represents the intrusive remnant of a greenstone belt [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 The 2722 ± 2 Ma Archaean megacrystic anorthosite-bearing Haines Gabbroic Complex in the Wawa subprovince of the western Superior Province formed contemporaneously with the spatially associated Greenwater Assemblage of the Shebandowan greenstone belt, Ontario, Canada. Both field and petrographic observations indicate that the Haines Gabbroic Complex and the spatially and temporally associated ultramafic to intermediate volcanic and intrusive rocks of the Greenwater and Burchell assemblages experienced extensive metamorphic recrystallization and alteration as a consequence of greenschist-facies metamorphism. These metamorphic processes resulted in pervasive replacement of igneous minerals by metamorphic counterparts. Metamorphic alteration also mobilised LILE and transition metals and slightly reset the U-Th-Pb and Sm-Nd isotope systems. Consequently, the Haines Gabbroic Complex and Greenwater Assemblage have respective Sm-Nd errorchron ages of 2765 +110/‐280 Ma and 3108 +120/‐380 Ma. Major and trace element compositions suggest that the least altered anorthosites, leucogabbros and gabbros of the Haines Gabbroic Complex are not cogenetic with the Greenwater and Burchell assemblages and have distinct parental magmas and depleted mantle sources. The calc-alkaline andesites of the Burchell Assemblage, the tholeiitic basalts and gabbros of the Greenwater and Burchell assemblages, the transitional gabbros of the Greenwater and Burchell assemblages and the picrites, serpentinites and peridotites of the Greenwater Assemblage are all petrogenetically distinct and have different parental magmas and depleted mantle sources. The pyroxenites are genetically distinct from the other Greenwater Assemblage lithologies, the Burchell Assemblage and the Haines Gabbroic Complex. The Haines Gabbroic Complex crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas that fractionated from more primitive, hydrous boninitic parental magmas. The initial ε Nd values of the Haines Gabbroic Complex (ε Nd = +3.0 to +4.1) and the Greenwater (ε Nd = +1.8 to +4.1) and Burchell (ε Nd = +2.0 to +3.7) assemblages are indicative of variably depleted mantle sources. The major and trace element characteristics of the Haines Gabbroic Complex and the Greenwater and Burchell assemblages and geological relationships indicate that they formed in a Japan-style mature intra-oceanic continental arc with a large volume of TTGs. Intra-oceanic arc rifting was most likely triggered by slab rollback, followed by mantle plume emplacement. On the basis of rock association and geochemical data, we suggest that the Haines Gabbroic Complex and the Greenwater and Burchell assemblages represent a dismembered subduction-related Archaean ophiolite that marks a Neoarchaean suture zone between the Wawa and Wabigoon subprovinces. Highlights • Haines Gabbroic Complex (HGC) was not cogenetic with the Shebandowan belt • HGC crystallised from hydrous, Ca- and Al-rich tholeiitic parental magmas • These parental magmas fractionated from more primitive, hydrous boninitic magmas • Parent magmas to HGC formed by high-degree partial melting of depleted mantle sources • HGC and adjacent assemblages formed in a Japan-style mature intra-oceanic arc setting [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]

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