an early ecosystem of archean tidal microbial mats moodies group south africa ca 3 2 ga abundant microbial mats from the mesoarchean moodies group barberton greenstone belt south africa ca 3 2 ga are densely interbedded with coarse grained and gravelly sandstones in a nearly mud free setting they apparently grew in marginal marine and possibly subaerial coastal and tidal environments characteristic sedimentary structures include anastomosing bedding gas or fluid escape structures sand volcanoes biomat doming patchy silicified microstromatolites and microbial sand chip conglomerates they indicate rapid growth of mechanically tough microbial mats possibly aided by early seafloor silicification in a high energy high sedimentation rate environment the observations expand the knowledge of the habitat of archean microbial mats and life on early earth if tidal environments were more widespread in the archean than today biomats in extensive intertidal settings may have contributed to immobilize large volumes of sand

evidence for cavity dwelling microbial life in 3 22 ga tidal deposits cavities are considered plausible and favorable habitats for life on early earth in such microenvironments organisms may have found an adequate protection against the intense ultraviolet radiation that characterized the archean ozone free atmosphere however while there is clear evidence that benthic life existed in the paleoarchean the oldest traces of cavity dwelling microbes coelobionts have been found in neoarchean rocks here we present the results of a detailed investigation of early silicified cavities occurring in the oldest well preserved siliciclastic tidal deposits the 3 22 ga moodies group of the barberton greenstone belt south africa downward growing microstromatolitic columns composed of kerogenous laminae are commonly present in planar bedding parallel now silica filled cavities that formed in sediments of the peritidal zone in situ 13cpdb pdb peedee belemnite measurements of the kerogen range from 32 3 to 21 3 and are consistent with a biogenic origin scanning electron microscopy analysis of the silicified cavities shows well preserved chains of cell sized molds that are interpreted as fossil filamentous microorganisms the geological context the morphology of the microstromatolites the 13c composition of the kerogen and the presence of microfossils all suggest that a microbial community inhabited the cavities these results extend the geological record of coelobionts by 500 m y supporting the view that cavities were among the first ecological niches to have been occupied by early microorganisms

earth s early o2 cycle suppressed by primitive continents a decrease in mafic continental crust coincides with the rise of o2 in the earth s surface environments about 3 billion years ago according to an analysis of sediment chemistry reduced rates of serpentinization of mafic material which produces chemicals that react with o2 could explain the link

tectono sedimentary characteristics of late miocene quaternary extensional basins of the northern apennines italy abstract extensional and compressional regimes exist at the same time in adjacent parts of the northern apennines italy and the former regime succeeded the latter at the same place as the thrust mountain front moved eastward since miocene times numerous extensional basins have developed west of the present mountain divide the westernmost ones formed over attenuated continental crust since late miocene and have been subjected to several subsidence and uplift events with rates of subsidence generally faster than rates of sedimentation they contain continental deposits overlain by gypsum bearing upper miocene marine sequences and siliciclastic marine pliocene deposits these basins were affected by shallow magmatism and are still experiencing high geothermal gradient and late to post magmatic activities such as in the renowned thermal region of larderello to the east closer to the present mountain divide the extensional basins formed later from pliocene to quaternary contain continental fluvial and lacustrine deposits and have not experienced near surface magmatic activities the sedimentary fill architecture of the apennine extensional basins is similar to that of other european american and african extensional or transtensional systems for instance a common characteristic is the prevalence of large alluvial fan deposits at the hanging wall shoulder of half grabens coarser smaller alluvial fans at the footwall shoulder and some fluvial deposits introduced longitudinally unlike the african rifts though the sedimentary facies distribution of extensional basins which developed in active orogenic zones such as in the apennines may be strongly affected by uplifting steep mountain slopes and can have large thick coarse alluvial fans also on the footwall side of the half grabens

ion microprobe ages of zircons from early archaean granite pebbles and greywacke barberton greenstone belt southern africa abstract we report single grain and grain domain u pb zircon ages for three granitoid clasts from the moodies conglomerate and for detrital zircons from a fig tree greywacke from clastic sediments in the early archaean barberton greenstone belt of southern africa the crystallization ages obtained for the moodies clasts vary between 3570 6 ma and 3518 11 ma while the fig tree zircons show a surprisingly uniform source terrain age of 3453 9 ma that may date the stratigraphically underlying onverwacht group felsic metavolcanics the discordancy patterns are ascribed to a combination of variable pb loss at unspecified times in the past and new zircon growth and or recrystallization the shrimp data record details of isotopic variation even within single zircons that result in precise definition of upper concordia intercept ages our results suggest an age range between 3460 and 3570 ma for the source region of the barberton belt clastic metasediments presumably the ancient gneiss complex of swaziland and felsic volcanics of the overwacht group this source included geochemically evolved granitoids that predate the oldest greenstones by some 100 ma

continental growth and convergence related arc plutonism in the mesoarchaean evidence from the barberton granitoid greenstone terrain south africa u pb zircon ages from trondhjemitic gneisses of the badplaas domain located in the southwestern parts of the mesoarchaean barberton granitoid greenstone terrain of south africa document a hitherto largely unrecognized plutonic event the geochronological results indicate the addition of juvenile felsic crust over a period of 60 myr between ca 3290 and 3230ma and prior to the main collisional event in the granitoid greenstone terrain at 3230 ma the timing and duration of plutonism together with the structural and compositional heterogeneity of the badplaas domain suggest that the badplaas domain represents part of a convergence related magmatic arc on a regional scale the spatial and temporal relationships between plutonism metamorphism and deformation are interpreted to preserve an almost complete inventory of a mesoarchean arc trench system this includes 1 the 3290 3230ma convergence related magmatic arc 2 a largely coeval deposition of the back arc type volcano sedimentary 3260 3225ma fig tree group of the barberton greenstone belt 3 relics of the underplated crust preserved in high p low t rocks to the immediate east of the magmatic arc juxtaposed against 4 high t medium p rocks of the overlying plate along the major crustal structure of the inyoni shear zone these findings corroborate the notion that thermal and rheological conditions on the early earth supported at least locally crustal growth and the extraction of buoyant felsic crust along convergence related magmatic arcs

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

northward channel flow in northern tibet revealed from 3d magnetotelluric modelling abstract the kunlun fault defines one of the major northern tectonic boundaries of the tibetan plateau previous geophysical studies have detected a major change in rheology across this boundary but it is not clear how this is related to models that have invoked crustal flow the lithospheric resistivity structure of the kunlun fault has been investigated by both the indepth iii and iv magnetotelluric mt transects all the mt data were processed using modern statistically robust methods and have been analysed for directionality and dimensionality in order to improve understanding of the anisotropic distribution of melt previously revealed by our remodelling of the indepth iii mt data a variant approach on 3d inversion of 2d profiles was investigated to explore and improve lateral resolution in addition to the apparent surficial deformation associated with the sinistral strike slip kunlun fault the 3d modelling of the indepth mt data reveals that complex deformation processes are occurring at mid crustal depths in northern tibet the 3d mt inversion results supported by synthetic modelling particularly confirm and highlight the presence of separate north south intrusions of conductive material crossing the kunlun fault into the more resistive kunlun qaidam block these north south intrusions are interpreted to be associated with the horizontal channel flow of partially molten songpan ganzi crust into two or more separated channels moving northwards and crossing the surficial trace of the kunlun fault at mid to lower crustal depths

braided fluvial to marine transition the basal lower cambrian wood canyon formation southern marble mountains mojave desert california abstract the lower cambrian wood canyon formation recognized over a 39 000 km2 belt through the southern great basin and mojave desert was examined in the southern marble mountains eastern mojave desert southeastern california there it crops out as a well exposed 90 130 m thick feldspathic sandstone nonconformably overlying proterozoic basement previous investigators have suggested a tide dominated nearshore shallow marine origin for the entire unit in the eastern mojave based on broad scale stratigraphic trends and localized fossil occurrences in the upper member of the formation in contrast to a regional approach the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata the middle and upper members of the wood canyon formation are recognized in the southern marble mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed cross stratified sandstone facies a mediumpebble conglomerate facies b planar stratified sandstone facies c sandstone and thin mudstone facies d mudstone with intercalated sandstone facies e and horizontally laminated siltstone facies f facies a d contained within the middle member comprise the bulk of wood canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced fluvial dominated braid delta complex cross stratification in the braid plain sediments displays a strongly unimodal pal ocurrent pattern suggestive of downstream migrating sinuous crested megaripples braid delta sediments however show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage a change from fluvial dominated to marine dominated processes is recorded in upper member rocks facies e and f facies e contains many of the traits characteristic of a low to mid tidal flat sequence such as coarsely interlayered bedding flaser and lenticular bedding microripple marks and locally abundant rusophycus and planolites trace fossils facies f which dominantly consists of horizontally laminated siltstone is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars

paleoarchean detrital zircon ages from the earliest tectonic basin in the barberton greenstone belt kaapvaal craton south africa abstract detrital zircon grains in coarse grained sedimentary rocks of the noisy formation traditionally considered as the h6 unit of the uppermost hooggenoeg formation in the paleoarchean barberton greenstone belt bgb south africa are investigated by laser ablation la icp ms to constrain their u pb and 207pb 206pb ages for depositional age and provenance determination drill core and field observations indicate that these rocks are clastic polymictitic diamictites and sandstones a wide range in 207pb 206pb ages between ca 3600 and 3430 ma is reported the youngest detrital zircon grain identified has an age of 3432 10 ma constraining the maximum depositional age of the sedimentary sequence and the formation of a major underlying erosional unconformity the youngest grains correspond in age to the ca 3460 3430 ma tonalite trondjemite granodiorite ttg gneisses of the stoltzburg terrane and felsic intrusions of the uppermost hooggenoeg formation that constitute most of the songimvelo block of the bgb on the kaapvaal craton the 207pb 206pb ages of the oldest grains identified corresponds to plutonic sources identified in older proto continental blocks of the kaapvaal craton namely the ca 3509 ma steynsdorp block and ca 3667 3223 ma ancient gneiss complex agc this new data indicates provenancial linkage between the songimvelo block and older proto continental blocks exposed on the kaapvaal craton at ca 3432 ma it is argued here that the noisy formation is a remnant of the earliest tectonic basin in the bgb that developed during major tectonic accretion and uplift at ca 3432 ma that lead to formation of the erosional unconformity in conjunction with new drill core and field observations the geochronological constraints presented here support plate tectonic processes operating in the proto kaapvaal craton that resulted in its formation as a stable continental landmass as early as ca 3432 ma

zircon geochronology and hf isotopes of the dwalile supracrustal suite ancient gneiss complex swaziland insights into the diversity of palaeoarchaean source rocks depositional and metamorphic ages abstract we report the results of combined u pb and lu hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano sedimentary rocks of the dwalile supracrustal suite dss from the ca 3 66 to 3 20 ga ancient gneiss complex agc in swaziland the results indicate that the dss is made up by palaeo and mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the neoarchaean the oldest dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca 3 46 ga and are derived from the same magma source as the tsawela gneisses based on the combined u pb hf isotope data two groups of metasedimentary rocks can be distinguished an older dwalile group i that is similar in age to the hooggenoeg and kromberg formations 3 46 3 40 ga but with a different source area a younger dwalile group ii is coeval with the fig tree and possibly moodies groups the u pb hf isotope data and petrological observations demonstrate that the dss was affected by three stages of metamorphic overprint an event at 3 23 ga pervasive amphibolite facies metamorphism at 3 15 ga and low p granulite facies metamorphism at 2 99 ga metamorphic zircon in the amphibolite facies rocks resulted from new growth in aqueous fluids at 3 15 ga whereas in the granulite facies rocks new zircon was formed in the presence of melt the 3 23 ga event coincides with the last significant metamorphic overprint documented in the barberton greenstone belt bgb whereas as two younger events are restricted to the agc the data support the model suggesting amalgamation between the agc and bgb terranes at ca 3 23 ga in addition they require crustal thickening in the agc terrane at ca 3 15 ga causing barrovian type metamorphism in the dss and granulite facies metamorphism in the luboya kubuta terrane low p granulite facies metamorphism at ca 2 99 ga is most likely related to crustal extension during the onset of pongola basin rifting causing mantle upwelling mafic magmatism and heat transfer from the mantle into the thinned crust

pre 3000 ma thermal history of the archean kaap valley puton south africa the kaap valley pluton is one of several early archean 3200 3500 ma tonalitetrondhjemite plutons that surround the barberton greenstone belt southern africa precise dating using single grain 207 pb 206 pb evaporation of zircon and 40 ar 39 ar laser step heating of hornblende and biotite indicates that in its interior the kaap valley pluton preserves a memory of its initial intrusion and cooling which spanned a time from 3225 to 3142 ma the pluton also records the effect of a low temperature thermal event at its margin as seen by a 40 ar 39 ar biotite age of 3035 ma which is perhaps related to hydrothermal activity and gold mineralization in the adjacent barberton greenstone belt these pre 3000 ma ages are not in agreement with results of dating studies from sedimentary rocks in the barberton greenstone belt and plutons south of the belt which show evidence of having been overprinted by late archean events 2650 2700 ma and the bushveld complex intrusion 2050 ma these events have been interpreted as affecting most of the kaapvaal craton that the kaap valley pluton has escaped these and all other events since 3035 ma with temperatures never reaching 250 c implies that these large scale events did not affect the entire craton and the overprinting seen elsewhere is of a more local nature thus it is possible to determine the intrusive and cooling history of the pluton these data can be used in developing models of heat flow paleomagnetic remanence acquisition and deformation events

influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the archean beaulieu rapids formation northwest territories canada abstract the 0 2 1 km thick archean beaulieu rapids formation is a late orogenic tectonically controlled sedimentary sequence overlying the beaulieu river volcanic belt unconformably on the west and bordering the major n trending beniah lake fault on the east local porphyry stocks characteristic of late orogenic successions were emplaced along strike of the fault four lithofacies comprise the beaulieu rapids formation 1 the 20 320 m thick conglomerate i 2 the 10 305 m thick siltstone sandstone 3 the 20 120 m thick conglomerate ii and 4 the 40 320 m thick quartz rich sandstone lithofacies stacking displays two distinct large scale fining upward sequences represented by the 300 375 m thick conglomerate i and sandstone siltstone sequence a and the 300 400 m thick conglomerate ii and quartz rich sandstone sequence b fining upward sequences a bounding unconformity and the beniah lake fault are compelling indicators of tectonic influence on sedimentation conglomerate i was deposited by debris flows traction currents and sheetfloods on the proximal medial and distal portions of alluvial fans or fan deltas respectively the overlying siltstone sandstone contains two distinct lithological units a sandstone dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds whereas sedimentary structures in the siltstone dominated unit are consistent with lacustrine deposition a second phase of tectonic uplift produced sequence b conglomerate ii was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars the quartz rich sandstone developed from dune migration on a sandy braidplain mineralogy and textures reflect high sediment influx limited transport distance local sources and climatic influence clast populations in conglomerate indicate a prevalent mafic volcanic source whereas sandstones contain prominent quartz av 76 with subordinate feldspar 13 and lithic fragments 11 the disparity between clast and sandstone composition is a function of chemical weathering typical of a humid climate source areas are restricted to the mafic dominated beaulieu river volcanic belt the 2 8 ga gneisso plutonic sleepy dragon complex and fault related porphyry stocks the humid weathering aggressive archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase accounting for the high quartz component in the sandstones the beaulieu rapids formation is similar to modern strike slip basins that form along major fault zones in terms of basin geometry faulted and unconformable contacts lithological units and sedimentary facies stacking and organization fault controlled basins of this nature commonly develop during the last stage of terrane accretion on archean cratons

on the reactivation of extensional fault systems in terranes that have undergone substantial extension three sets of faults dominate a shallow to steep dipping commonly rotational normal faults b a really extensive shallow dipping normal detachment faults and c steep dipping transfer faults that strike at high angles to the normal faults these fault systems may extend through a large fraction of the crust reactivation of these fault systems will depend primarily on the relative strengths of the faults shear zones and their host rock and their orientation in the prevailing stress field it is concluded that reactivation is generally mechanically favoured but that it will probably only take place when the fault shear zones are in near ideal orientations consideration of the tectonic setting of extended terranes and of the limited number of well described examples suggests that reverse thrust reactivation of the normal and detachment faults and wrench reactivation of transfer faults are the most likely styles examples of these styles are described from the bass strait basins of southeastern australia because extended terranes commonly underlie sedimentary basins for example on passive continental margins reactivation of extensional faults may be a key control on the tectonic evolution of such basins i e basin inversion

siliciclastic associated banded iron formation from the 3 2 ga moodies group barberton greenstone belt south africa most models proposed for banded iron formation bif deposition are based on observations of well preserved late archean and paleoproterozoic bif efforts to push the understanding gained from younger successions deeper in time have been hampered by the high metamorphic grades that characterize early archean bif this study focuses on a unique occurrence of well preserved and contextualized bif from the early archean 3 2 ga moodies group in the barberton greenstone belt south africa the moodies bif occurs thinly interbedded with fine grained and cross stratified sandstones indicating deposition during times of decreased clastic sediment supply in the moodies bif chert is present as concretions and is never observed in direct contact with the siliciclastic material but is always associated with iron minerals this observation suggests that the processes leading to the formation of both chert and iron minerals were coupled the dominant iron rich minerals within unweathered moodies bif are hematite and magnetite with less common occurrences of fe carbonate phases mainly ankerite petrographic textures reveal that hematite constitutes an early mineral phase while magnetite and ankerite display textures indicative of a late diagenetic or metamorphic origin carbonaceous particles are present in close association with the magnetite crystals these c bearing phases may be the preserved organic matter of microbes involved in the production of the ferric iron precursor phases though it is difficult to rule out an origin from abiotic processes involving thermal decomposition of siderite to magnetite and organic carbon compounds nonetheless the range of textures mineralogies and valence states supports the view that diagenetically stabilized bif mineralogies reflect the interaction of ferric iron phases with reducing fluids during diagenesis these patterns are commonly observed in younger archean and paleoproterozoic iron formations and imply a continuity of processes operating in the iron and silica cycles across both a range of paleoenvironments and long intervals of archean time

petrogenetic evolution of metabasalts and metakomatiites of the lower onverwacht group barberton greenstone belt south africa abstract a well preserved sequence by archean standards of mantle derived metabasalts and metakomatiites forms large parts of the lower onverwacht group of the barberton greenstone belt south africa to elucidate the origin of mafic and ultramafic rocks from this 3 55 to 3 45 ga sequence we present a comprehensive geochemical dataset including major and trace elements as well as lu hf and sm nd isotope compositions for a variety of metavolcanic rocks these include metabasalts of the amphibolite facies sandspruit and theespruit formations as well as metabasalts and metakomatiites of the lower greenschist facies komati formation based on their incompatible trace element patterns the basalts of the sandspruit and theespruit formations can be subdivided into a light rare earth element lree depleted group a lree undepleted group and a lree enriched group positive ehf t and end t values of ca 3 to 4 and 0 to 2 respectively together with depletions in th and lacn ybcn indicate derivation of the lree depleted basalts from a depleted mantle source however chondritic ehf t and end t values combined with positive th and lacn ybcn of the lree enriched samples indicate a contribution from older granitoid crust in the petrogenesis of these samples trace element patterns of komatiites and basalts of the komati formation are generally flat relative to primitive mantle with slight depletions in heavy rare earth elements and th and overall positive ehf t of 2 5 3 5 2 s d and end t of 0 5 2 2 2 s d the coherence in trace element characteristics suggests a common magmatic origin for basalts and komatiites this study reveals that the two lavas were derived from the same mantle plume i e komatiites were formed by high degrees of melting of a depleted mantle source containing residual garnet and the basalts were formed by moderate degrees of partial melting in shallower regions of the mantle based on the current dataset combined with published data we propose a geodynamic model for the oldest units of the barberton greenstone belt that describes the development from a submerged continental setting for the sandspruit and theespruit formations to a submarine plateau setting for the komati formation as a consequence of continental rifting

weathering and sediment generation in the archean an integrated study of the evolution of siliciclastic sedimentary rocks of the 3 2ga moodies group barberton greenstone belt south africa abstract alluvial and braided fluvial deposits of the 3 2 ga moodies group in the barberton greenstone belt south africa are analyzed petrographically and geochemically to identify their source rocks provenance and assess the degree and type s of weathering required to produce the observed moodies sediment compositions conglomerate sandstone and shale data give somewhat differing pictures of provenance and weathering due to derivation from different components within the source terrane and size and compositional fractionation during transport the results suggest that shale geochemistry provides the most accurate estimate provenance although both rare earth element and trace element th sc zr cr and ti data must be used in combination to give the best results the source area for moodies group sediments was dominated by tonalite felsic volcanic rock komatiite basalt and granite based on mineralogical and major element divergence from estimated source area composition the moodies group sediments are remnants of an aggressive weathering environment labile materials such as komatiite basalt and coarse plagioclase grains decomposed almost entirely to clays and solutes and the chemical index of alteration for moodies shale is well above the global average an aggressive weathering environment in the archean may have been achieved by increased rainfall higher temperatures and or higher atmospheric p c o 2 more likely a combination of these conditions worked to offset the inhibitory weathering effects of a plant free environment

early permian giant cross beds near nqutu south africa interpreted as part of a shoreface ridge abstract sandstones and siltstones of a giant cross bed sedimentary association at the base of the coal bearing vryheid formation cannot be satisfactorily explained in terms of the currently accepted fluviodeltaic model for these strata in view of the sedimentary structures and associational characteristics of the facies a shoreface ridge interpretation is adopted instead the sandridge probably detached from the shoreface was situated on the slope towards the basin plain and associated with some elements of a minor submarine fan complex rapid shoreface by pass in the course of a marine transgression seems to have provided favourable conditions for sandridge formation the major bedform appears to have been in water of about 45 m depth possibly some kilometres from the contemporary shoreline there is evidence that it was mainly constructed by the action of strong easterly flowing currents probably of relaxant or geostrophic origin these were aided by ebb dominant tidal currents possible actualistic models include north sea and bering sea sandridges currents of the strength needed to build such large bedforms could only have existed in an open ended seaway and it is suggested that the ecca sea must have communicated at least intermittently with open ocean to the northeast and possibly the southeast as well as towards the west

organic walled microfossils in 3 2 billion year old shallow marine siliciclastic deposits the discovery of relatively large cell like structures in early archaean shale and siltstone deposits adds further evidence that diversified living organisms thrived on earth at a very early date claims for fossils of this age are often controversial as non biological processes can produce life like microstructures and chemical signatures resembling those of the remains of living organisms extensive efforts were therefore made to rule out a non biological origin for these microstructures they survive this examination and appear to be organic walled microfossils derived from large microorganisms that cohabited with previously reported microbial mats living in the sunlit shallows of earth s early oceans around 3 2 billion years ago claims that life existed on earth in the early archaean eon 3 2 billion years ago are often controversial as non biological processes can produce life like microstructures and chemical signatures that mimic those of the remains of living organisms now however the discovery of relatively large carbonaceous spheroidal microstructures interpreted as organic walled microfossils in early archaean deposits adds further evidence that life existed thrived and survived on earth at a very early date although the notion of an early origin and diversification of life on earth during the archaean eon has received increasing support in geochemical sedimentological and palaeontological evidence ambiguities and controversies persist regarding the biogenicity and syngeneity of the record older than late archaean1 2 3 non biological processes are known to produce morphologies similar to some microfossils4 5 and hydrothermal fluids have the potential to produce abiotic organic compounds with depleted carbon isotope values6 making it difficult to establish unambiguous traces of life here we report the discovery of a population of large up to about 300 m in diameter carbonaceous spheroidal microstructures in mesoarchaean shales and siltstones of the moodies group south africa the earth s oldest siliciclastic alluvial to tidal estuarine deposits7 these microstructures are interpreted as organic walled microfossils on the basis of petrographic and geochemical evidence for their endogenicity and syngeneity their carbonaceous composition cellular morphology and ultrastructure occurrence in populations taphonomic features of soft wall deformation and the geological context plausible for life as well as a lack of abiotic explanation falsifying a biological origin these are the oldest and largest archaean organic walled spheroidal microfossils reported so far our observations suggest that relatively large microorganisms cohabited with earlier reported benthic microbial mats8 in the photic zone of marginal marine siliciclastic environments 3 2 billion years ago

successive midcrustal high grade metamorphic events provide insight into mid archean mountain building along the se margin of the proto kaapvaal craton in this study we investigate the evolution of continental crust at the paleoarchean to mesoarchean boundary by documenting the deposition deep burial and partial melting of metasedimentary gneisses along the se margin of the proto kaapvaal craton successive high grade metamorphic events recorded by the gneisses between ca 3 23 and 3 07 ga not only coincided with the timing of subduction accretion in the adjacent barberton greenstone belt at ca 3 23 3 22 ga but also with discrete pulses of potassic granitic magmatism during differentiation and consolidation of the newly assembled lithosphere mineral equilibria modeling demonstrates that the granulites evolved along pressure temperature p t paths similar to those documented for metamorphism in modern collisional orogens high temperature deformation at ca 3 11 3 07 ga during uplift of the terrane was coaxial with the main ne sw trending structural grain of the barberton greenstone belt and reflects the regional nw se shortening and ne sw orogen parallel extension exhibited by the younger potassic granites we reconcile these features with mesoarchean terrane assembly in barberton via two nw dipping subduction zones the trace of the first is represented by the main terrane boundary within the barberton greenstone belt the second resulted in accretion burial and high temperature metamorphism along the se margin of the kaapvaal craton with the granulites providing valuable insight into the mid to lower crustal response to what appears to have been a protracted accretionary orogenic event

geology of the pitangui greenstone belt minas gerais brazil stratigraphy geochronology and bif geochemistry abstract based on drillcore description of rocks at the sao sebastiao gold deposit and surroundings a new geological map and a detailed stratigraphic column are presented for the pitangui greenstone belt which is located northwest of the quadrilatero ferrifero southern sao francisco craton brazil this belt comprises three units a lower unit predominantly of metavolcanic ultramafic and mafic rocks interbedded with thinner metasedimentary intervals of banded iron formations bifs turbiditic metasandstones and metapelites a middle unit of turbiditic metasediments interbedded with metavolcanic intermediary rocks minor bifs and quartzites and an upper unit composed of quartzites and polymictic metaconglomerates rocks of all three units are metamorphosed under upper greenschist to lower amphibolite facies conditions and are bounded by domes ttgs and late to post tectonic alkaline granitoids the pitangui greenstone belt records the geologic evolution of an ancient oceanic basin that evolved from a thick succession of metavolcanic ultramafic and mafic rocks interbedded with metasedimentary intervals of bifs and turbidites the latter representing moments of quiescence in volcanic activity this basin experienced a transient period dominated by andesitic volcanism turbidite deposition and minor bif and a final stage of clastic only sedimentation at the same time when volcanic activity ceased combined u pb and lu hf isotope data of detrital zircon from lower unit metasandstone revealed a maximum depositional age of 2859 11 ma this age is similar to those found in the basal unit of the nova lima group 2 90 2 78 ga in the rio das velhas greenstone belt located approximately 100 km southeast both belts can be correlated on the basis of similarities in stratigraphy and isotopic ages indicating that they possibly represent parts of an ancient larger domain geochemical data of bif coupled with stratigraphic analysis place constraints on paleoenvironmental conditions during the development of the mesoarchean pitangui greenstone belt shale normalized rare earth elements ree y in some bif samples display negative cerium ce anomalies which may be interpreted as evidence for the oldest oxygenation pulse documented in brazil to date 2 86 2 78 ga such anomalies endorse the presence of episodic periods of seawater oxygenation before the great oxidation event

gliding and overthrust nappe tectonics in the barberton greenstone belt abstract regional recumbent folds inverted stratigraphy nappes and olistostromes are described from the southern part of the 3 3 3 5 ga barberton greenstone belt overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km2 and up to 500 m in thickness occurred over minimum distances of 86 km more ductile and coherent units were overfolded up to at least 2 km during their emplacement the glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns driven by heat sources from igneous intrusions the upwelling areas of the geothermal convection cells were sites of mud pools and hydrothermal vents which may mark the trailing edges pull aparts of the overthrust units progressive silica and carbonate precipitation due to decreasing temperatures within the zones of fluid migration distant from the areas of high heat flow probably acted as built in braking systems below the travelling slabs active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt the recognition of nappe and overthrust tectonics in the barberton belt processes which may have been commonplace in archaean terrains necessitates a re evaluation of the stratigraphy of this belt

late syndepositional deformation and detachment tectonics in the barberton greenstone belt south africa the thick bedded competent quartzose sandstones conglomerates and siltstones of the moodies group 3 225 ma influence structural style in the 3 1 3 5 ga barberton greenstone belt moodies group rocks are folded into a train of tight subparallel doubly plunging overturned synclines separated by narrow strike parallel fault zones structural analysis along a transect across the central part of the greenstone belt suggests that much of the volcanic sedimentary greenstone sequence was buckle folded above a detachment within altered ultramafic rocks located at a depth of 3 6 km early development of brittle structures concentrated shortening and displacement near the fold hinges and in the anticlinal fault zones penetrative strain during folding was low the sense of displacement along faults the original fold vergence and the provenance of moodies group sandstone and conglomerate suggest that tectonic transport was directed toward the interior of the belt from its northwestern and southeastern margins orogenic contraction due to folding alone was at least 56 and subsequent ductile shortening strain accounted for an additional contraction of approximately 10 buoyant rise of the kaap valley tonalite to the north at approximately 3 22 ga and lateral intrusions of sheetlike granitic plutons along both sides of the preserved greenstone belt at 3 1 and 3 0 ga respectively imposed significant ductile strain on the greenstone belt margins and obscured or displaced the orogenic hinterlands of the facing fold and thrust belts the syn and post moodies tectonic history of the central barberton greenstone belt involves the formation and deformation of several shortlived alluvial to shallow marine sedimentary basins on unstable protocontinental crust the probable extensional origin of at least some of these basins and their subsequent shortening buckle folding and displacement above a subhorizontal detachment emphasizes the dominance of horizontal tectonics in the late stage deformation of this archean greenstone belt

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

morphological adaptations of 3 22 ga old tufted microbial mats to archean coastal habitats moodies group barberton greenstone belt south africa microbial life was well established and widespread by the paleoarchean however the degree of evolutionary advancement such as microbial motility intra and inter species interactions phototropism or oxygenic photosynthesis by that time remains highly debated the 3 22 ga moodies group in the barberton greenstone belt bgb south africa are earth s oldest well preserved siliciclastic tidal deposits they exhibit a unique assemblage of microbial mats providing an excellent opportunity to decipher the morphological adaptations of microbial communities to different paleoenvironmental settings the fossil mats are preserved as kerogenous laminations 0 5 1 mm thick that can be traced laterally for 15 km in a 1000 m thick succession of fine to coarse grained tidal sandstones and conglomerates we here present a detailed stratigraphic and depositional facies analysis documenting the association of the three principal mat morphotypes with specific environmental settings 1 planar type in coastal floodplain 2 wavy type in intertidal and 3 tufted type in upper inter to supratidal facies all mat types indicate a flourishing phototrophic biota moreover the tufted morphology suggests an intricate level of coordinated growth commonly known from cyanobacterial mats in modern environments

braided fluvial sedimentation in the lower paleozoic cape basin south africa abstract lower paleozoic braided stream deposits from the piekenier formation in the cape province south africa provide information on lateral and vertical facies variability in an alluvial plain complex influenced by a moderate to high runoff four braided stream facies are recognized on the basis of distinct lithologies and assemblages of sedimentary structures a lower facies dominated by upward fining conglomerate to sandstone and mudstone channel fill sequences is interpreted as a middle to lower alluvial plain deposit with significant suspended load sedimentation in areas of moderate to low gradients these deposits are succeeded by longitudinal conglomerate bars which are attributed to middle to upper alluvial plain sedimentation with steeper gradients this facies is in turn overlain by braid bar complexes of large scale transverse to linguoid dunes consisting of coarse grained pebbly sandstones with conglomerate lenses these bar complexes are compared with environments of the recent platte river they represent a middle to lower alluvial plain facies with moderate gradients and no significant suspended load sedimentation or vegetation to stabilize channels these bar complexes interfinger basinward with plane bedded medium to coarse grained sandstones interpreted as sheet flood deposits over the distal portions of an alluvial plain with low gradients and lacking fine grained detritus or vegetation

a comment on weathering of quartz as an archean climatic indicator by n h sleep and a m hessler earth planet sci lett 241 2006 594 602

the waning stage of a greenstone belt the mesoarchaean mosquito creek basin of the east pilbara western australia abstract the mosquito creek basin stands apart in the archaean granite greenstone terrane of the east pilbara craton because of the rectangular outcrop but crescent shaped subcrop and its composition of low grade metamorphic mainly turbiditic clastic sedimentary rocks these sediments belong to the c 2 9 ga mosquito creek formation tectonic interpretations of the basin have varied from a simple synclinorium to an accretionary prism an extensional passive margin and more recently into a rift basin separating the kurrana terrane from the east pilbara terrane ept combined sedimentological and structural geological investigations show coarse clastic fan delta marginal facies with stacked unconformities rimming an e w basin centre with predominantly finer grained mass flow dominated sediments a n s traverse therefore contains a complete cross section of the mosquito creek basin shortened to about 50 of its original width by tight folding and thrusting the basin fill shows one major fu sequence from fan delta sandstone to distal turbidites unconformably overlying an older partially preserved clastic sequence of comparable facies a large slump unit occupies the basin axis the unconformities along the margins interfere with folding of the sediments which also influenced basin centre turbiditic palaeocurrents the basin is interpreted as a near symmetrical underfilled compressional intramontane clastic sedimentary basin unconformably superposed on a volcanic greenstone belt rock assemblage with enclosed low angle shear zones compression started during sedimentation under conditions of net subsidence within a rim syncline between adjacent domes this occurred after phases of low angle extensional and compressional deformation of the substrate not related to the present day geometry of the granitoid domes the geological setting of the mosquito creek basin does not differ fundamentally from that of other adjacent e woriented greenstone belts of the ept the basin is inferred to represent the last stratigraphically uppermost and because of subsequent erosion rarely preserved stage of greenstone belt evolution

origin of negative ce anomalies in barberton sedimentary rocks deduced from la ce and sm nd isotope systematics abstract the origin of sedimentary rocks with negative ce anomalies among several samples derived from the barberton greenstone belt 3250 ma south africa was investigated using la ce and sm nd isotopic systematics together with light rare earth element lree abundances negative ce anomalies were recognized in two sedimentary rocks a banded ferruginous chert and a shale their ce ce values are 0 40 and 0 81 respectively initial ce isotopic ratio calculated for the ferruginous chert 138 ce 142 ce 3250 ma 0 0225059 is far lower than chur value at 3250 ma and even the value at earth formation 138 ce 142 ce chur 4560 ma 0 0225409 the calculated initial ce isotopic ratio for the shale with a ce anomaly is also lower than that of other rocks without ce anomalies these results indicate that the negative ce anomalies were not inherited from an archean redox change but occurred following later episodes of alteration two possible formation models were examined to estimate a plausible occurrence age for the negative ce anomalies one is characterized by ce extraction and the other is by an addition of la nd and sm both models indicate that negative ce anomalies arose before 1100 ma this study suggests that la ce isotopic systematics provides a unique tool to constrain the occurrence age of negative ce anomalies observed in archean sedimentary rocks and gives some hints about the redox condition in the archean ocean and the evolution of the earth

comparative sedimentology of the principal volcanic sequences of archean greenstone belts in south africa western australia and canada implications for crustal evolution abstract significantly different assemblages of sedimentary rocks are present in the volcanic sequences of archean greenstone belts in the barberton mountain land pilbara block and canadian shield thin sedimentary layers in the 3 55 3 3 ga old onverwacht and warrawoona groups in the barberton mountain land and eastern pilbara respectively are made up of silicified volcaniclastic sediments banded cherts carbonaceous cherts silicified evaporites and locally stromatolites deposited under shallow water anorogenic conditions the volcanic sequences in these belts essentially lack terrigenous debris derived by weathering and erosion algoma type iron formation volcanogenic massive sulfide deposits hyaloclastic breccias and evidence for the existence during their accumulation of older sialic basement sedimentary rocks in the volcanic sequences of the 2 8 2 6 ga old superior and slave provinces canada and perhaps in other late archean belts in the rhodesian craton and yilgarn block include generally unsilicified volcaniclastic units mafic hyaloclastic breccias terrigenous clastic layers abundant algoma type iron formation and massive sulfide deposits sedimentation took place under relatively deep water conditions local tectonic activity was widespread and older sialic basement was present these younger volcanic sequences contain few shallow water deposits such as carbonaceous cherts evaporites and stromatolites the volcanic sequences of both generations of greenstone belts seem to have formed platforms that stood topographically above the surrounding sea floor differences between older and younger greenstone belt volcanic sequences appear to reflect the increasing abundance and influence of sialic crust from early to late archean time and an accompanying increase in ocean water depth over the volcanic platforms evolution of the sialic crust can be traced through at least five archean and early proterozoic stages 1 1 an early period of ensimatic shallow water greenstone belt formation 3 55 3 3 ga ago 2 2 an early episode of intrusive and metamorphic activity from ca 3 3 2 9 ga ago during which some of the older greenstone belts were cratonized 3 3 a late period of greenstone belt formation 2 8 2 6 ga ago during which volcanism occurred under relatively deep water conditions upon and or adjacent to areas of older sialic basement 4 4 a late archean interval of continental crust formation during which the younger greenstone belts were cratonized 2 7 2 5 ga ago 5 5 a latest archean and early proterozoic period dominated by stable cratons and relatively minor greenstone belt activity

determining the channel sinuosity of ancient fluvial systems from paleocurrent data abstract the classification of modern river channels is based primarily on their sinuosity values which can be determined directly from aerial photographs ancient channel systems are seldom sufficiently exposed to allow direct measurement of thalweg and valley lengths but paleocurrent directions can be used to calculate fairly accurate sinuosity values aerial photographs of four modern meander belts were studied to determine the relationship between local current directions within the meander belts and the actual sinuosity of the active river channels a value of 3 2 times the standard deviation gave the best approximation of the operational channel range resulting in a close correspondence between calculated and actual sinuosity values this method can therefore be used to estimate the s nuosity of ancient fluvial systems

upb zircon ages bearing on the nature of early archaean greenstone belt evolution barberton mountainland southern africa abstract u pb zircon systematics for granitic and gneissic clasts from the moodies conglomerate of the barberton greenstone belt south africa exhibit discordancy patterns that suggest variable and multi episodic lead loss and indicate crystallisation ages for the parent granitoids between 3 3 and 3 47 ga metamict zircons in some of these clasts indicate complete resetting of the older material or new growth at about 2 8 2 9 ga ago that is ascribed to a regional thermal event as also shown by published rb sr data for a nearby granitoid pluton since the moodies clasts consist largely of potassic granites our ages suggest the presence of highly evolved continental crust in the source region of the greenstone belt sediments a feature that contradicts the commonly accepted model for the primitive nature of the granitoid crust in early archaean granite greenstone terrains the maximum age of 3 3 ga for the moodies group also shows that greenstone belt evolution in the barberton region took place over a period of at least 150 ma from about 3450 to 3300 ma ago the kaap valley and stentor plutons intrude the lower part of the greenstone succession and their diapiric emplacement caused large scale deformation of earlier structures in the entire greenstone belt sequence at about 3 23 3 25 ga ago detrital zircons in the sediments of the fig tree group date the mean age of their source terrain at c 3520 ma which is also the maximum age of deposition for these strata this age is indistinguishable from the oldest age reported from the barberton greenstones and suggests the possibility that pre greenstone granitoid crust may exist in the eastern kaapvaal craton

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

steep rock lake sedimentology and geochemistry of an archean carbonate platform abstract the 2800 ma mosher carbonate formation at steep rock lake in south central canada is one of earth s oldest limestone deposits it is both thick up to 500 m and relatively well preserved and was one of the first precambrian successions to be noted for evidence of early life it continues to provide information on the antiquity of such processes as photosynthesis the carbonate contains a variety of fabrics and facies from apparently fine grained bedded limestones with domal and columnar stromatolites atikokania radial fans and sheet cracks to closely packed metric scale hybrid domes of originally aragonite seafloor crust interlayered with fenestral fabric here we review previous work provide new observations concerning these sediments and their geochemistry and discuss the inception overall development and ultimate demise of the steep rock platform we divide the mosher carbonate into the lower hogarth member and upper elbow point member these units constitute a shallow water carbonate platform succession the hogarth member overlies eroded 3000 ma tonalite and is at least 120 m thick thin horizons of domal pseudocolumnar columnar and fenestral stratiform stromatolites and occasional layers of atikokania radial crystal fans occur within relatively fine grained thin bedded limestone with iron carbonate horizons in the upper part meter scale domes occur locally with sheet cracks and stromatactis the irregular laminae that build the small domal and columnar stromatolites are interpreted to be lithified mats and biofilms presumably cyanobacteria dominated fine grained well bedded facies of the lower hogarth member and its varied association of small stromatolites is interpreted to have accumulated in a relatively protected inner platform environment numerous very thin horizons of iron carbonate in the lower and upper parts of the hogarth member suggest temporally limited influence of iron rich offshore waters vertical trends in fe mn ba sr ce and 13 c probably reflect progressive restriction of circulation with at least one abrupt but short lived change back to more open seawater conditions the upper hogarth member with large banded domes and fenestral crusts and stromatolites is interpreted as back margin facies the overlying elbow point member is at least 70 m thick and appears to be entirely composed of innumerable juxtaposed elongate smooth and thickly layered domes lacking intervening sediment each dome is typically several meters across and composed of centimetric alternations of cuspate fenestral fabric possibly microbial and crystal fan fabric that is probably abiotic seafloor aragonite precipitate adjacent cuspate fenestral and crystal fan fabric show significant differences in concentration of 13 c sr ba fe and mn this giant dome facies is interpreted as a platform margin deposit in which alternating calcite aragonite mineralogies within each dome reflect a laterally fluctuating offshore redox boundary negative ce anomalies positive gd anomalies and reduced positive eu anomalies relative to laterally correlative deep water iron formation occur in the majority of crystal fan samples and in some cuspate fenestral fabric samples ca carbonate precipitation at the margin of an anoxic iron rich sea would have been promoted by oxygenic removal of iron from seawater otherwise fe carbonate would have been favored over ca carbonate precipitation the mosher carbonate formation is interpreted as an early marine oxygen oasis a shallow platform that favored cyanobacterial productivity was sufficiently isolated from open marine circulation for the seawater to become relatively oxygenated and where carbonate sediment aggradation was a positive feedback that sustained these shallow water conditions limestone accumulation terminated as transgressive suboxic and anoxic seawater deposited first manganese oxide rich and then manganese oxide poor iron formation sediments

sm nd rb sr pb pb dating of carbonates from the early archaean barberton greenstone belt south africa evidence for post depositional isotopic resetting at low temperature abstract two silicic carbonate sequences occurring in the onverwacht group og and the overlying fig tree group ftg of the early archaean barberton greenstone belt south africa contain silicified dolomites with primary structures and textures they occur with volcanic rocks dated by single zircons the associated onverwacht felsic volcanics have an age of 3440 3416 ma whereas the fig tree volcanics are 3225 ma old therefore the depositional age of both sequences is between 3440 and 3225 ma pb isotopic data for different parts of a single og whole rock sample form a linear array corresponding to a pb pb age of 2632 99 ma sm nd and rb sr isotopes were analysed in four whole rock samples of the ftg silicified carbonates and form an sm nd array corresponding to an age of 2612 84 ma with initial end 8 1 0 5 and an rb sr array of 2743 36 ma with initial 87 sr 86 sr 0 7013 0 0003 these whole rock samples have rb sr ratios of 1 and unusually high rb concentrations for carbonates 100 ppm which derive from processes accompanying silicification all three isotopic systems of whole rocks indicate ages of 2 7 ga much younger than the depositional age of the successions the agreement provides strong evidence that an alteration event reset all of the isotopic systems sm nd as well as rb sr and u pb we suggest that the ages represent the timing of silicification and sericitization of the carbonates by large scale fluid movement at an apparent temperature of 200 c

tectonic controls on atmospheric climatic and biological evolution 3 5 2 4ga abstract this paper presents a speculative scenario for the evolution of the earth s surface environment and biological community 3 5 2 4 ga based on the geologic record and its interpretation available geologic evidence suggests that the earth s climate before 3 2 ga was hot probably 60 73 c as a result of a co 2 and ch 4 greenhouse the early biological community was probably dominated by anoxygenic photosynthetic thermophiles cyanobacteria if they evolved before 3 1 ga would have struggled to survive at such high temperatures as a result the atmosphere contained extremely low levels of o 2 and 33 s shows wide departures from 0 indicating mass independent fractionation of s isotopes in the upper atmosphere this hot early climate collapsed 3 1 3 0 ga due to growth of large new blocks of continental crust cratons the weathering of which resulted in the depletion of atmospheric co 2 and eventual drawdown of ch 4 due to formation and rainout of methane aerosols cooling may have culminated in glaciation about 3 0 2 9 ga this climatic catastrophe and attendant changes in atmospheric composition drove a major biological revolution 3 0 2 7 ga characterized by the emergence of new low temperature taxa including cyanobacteria and their spread throughout surface environments at the expense of extreme thermophiles including methanogens during a period of general tectonic stability 3 0 2 7 ga the earth s surface was characterized by overall clement conditions with temperatures 2 after 2 75 ga and by 2 7 ga surface temperatures had returned to 60 c cyanobacteria were again suppressed o 2 production waned and atmospheric mass independent fractionation is again indicated by extreme variations in 33 s these conditions persisted perhaps intermittently until about 2 5 2 4 ga when weathering and erosion of vast new blocks of continental crust formed about 2 65 2 6 ga caused the second and probably last collapse of a 60 c surface climate broad tectonic climatic and biological events 3 5 2 9 ga are remarkably parallel to those of the late archean and paleoproterozoic 2 75 2 2 ga and neoproterozoic 1 0 0 50 ga these 500 myr long cycles of greenstone volcanism and crustal generation climatic and atmospheric instability and biological innovation reflect the long term interaction of the tectonic atmospheric climatic and biological components of the earth s surface system they suggest that the evolution of the earth s interior expressed through its control on the formation of large blocks of continental crust has influenced atmospheric composition and climate which in turn have provided a fundamental control on the timing and directions of biological evolution throughout earth history

detrital zircon geochronology of sandstones of the 3 6 3 2 ga barberton greenstone belt no evidence for older continental crust

biogeochemical cycling of iron in the archean paleoproterozoic earth constraints from iron isotope variations in sedimentary rocks from the kaapvaal and pilbara cratons abstract iron isotope compositions of low metamorphic grade samples of archean paleoproterozoic sedimentary rocks obtained from fresh drill core from the kaapvaal craton in south africa and from the pilbara craton in australia vary by 3 in 56 fe 54 fe ratios reflecting a variety of weathering and diagenetic processes depositional ages for the 120 samples studied range from 3 3 to 2 2 ga and fe c and s contents define several compositional groups including samples rich in fe organic carbon carbonate and sulfide the 56 fe values for low c org low c carb and low s sedimentary rocks are close to 0 the average of igneous rocks this range is essentially the same as that of c org poor late cenozoic loess aerosol river loads and marine sediments and those of c org poor phanerozoic proterozoic shales that these 56 fe values are the same as those of igneous rocks suggests that fe has behaved conservatively in bulk sediments during sedimentary transport diagenesis and lithification since the archean these observations indicate that if atmospheric o 2 contents rose dramatically between 2 4 and 2 2 ga as proposed by many workers such a rise did not produce a significant change in the bulk fe budget of the terrestrial sedimentary system if the archean atmosphere was anoxic and fe was lost from bedrock during soil formation any isotopic fractionation between aqueous ferrous fe fe aq 2 and fe bearing minerals must have been negligible in contrast if the archean atmosphere was oxic fe would have been retained as fe 3 hydroxides during weathering as it is today which would produce minimal net isotopic fractionation in bulk detrital sediments siderite rich samples have 56 fe values of 0 5 0 5 and experimentally determined fe aq 2 siderite fractionation factors suggest that these rocks formed from fe aq 2 that had similar or slightly higher 56 fe values the 56 fe values calculated for fe aq 2 overlaps those of modern submarine hydrothermal fluids but it is also possible that fe aq 2 had 56 fe values higher than those of modern hydrothermal fluids depending upon the fe aq 2 fe carbonate fractionation factor that is used in contrast c org rich samples and magnetite rich samples have strongly negative 56 fe values generally between 2 3 and 1 0 and available fluid mineral fractionation factors suggest that the fe bearing minerals siderite and magnetite in these rocks formed in the presence of fe aq 2 that had very low 56 fe values between 3 and 1 reduction of fe 3 hydroxide by sulfide precipitation of sulfide minerals or incongruent dissolution of silicate minerals are considered unlikely means to produce significant quantities of low 56 fe fe aq 2 we interpret microbial dissimilatory fe 3 reduction dir as the best explanation for producing such low 56 fe values for fe aq 2 and our results suggest that dir was a significant form of respiration since at least 2 9 ga

mesoproterozoic delta systems of the acurua formation chapada diamantina brazil abstract facies analysis distinct depositional paleoflow directions and grain size ranges suggest that different fluvial systems fed coeval distinct independent river dominated and tide dominated braid delta systems in the mesoproterozoic acurua formation the river dominated delta system is characterized by a vertical stacking of parasequences up to 30 m which are characterized by coarsening and thickening upward trends b gradationally based vertical succession of prodelta delta front distributary channels and floodplain c grain size of sandstone from delta front and distributary channels varies respectively from very fine to medium and medium to coarse d lacking of wave or tide reworking suggests delta development in a relatively low energy marine basin e paleocurrent readings indicate delta front sandstone bodies shifting laterally towards ne to se directions the tide dominated delta system is characterized by a vertical stacking of parasequences up to 25 m which are characterized by coarsening and thickening upward trends b gradationally based vertical succession of prodelta delta front and tide influenced distributary channels c grain size of sandstone from delta front and tide influenced distributary channels varies respectively from very fine to coarse and fine to granular d abundance of tide influenced sedimentary structures suggest tidal dominated delta development e consistent bidirectional paleocurrents with northwards ebb tide direction and no lobe shifting the studied braid delta systems belong to a highstand system tract and as such they exhibit classic parasequence sets with progradational stacking pattern the facies association variability and overall characteristics of these mesoproteozoic delta systems are compatible with delta systems of all ages however contrary to typically phanerozoic deltas which are characterized by progradational clinoforms the coalescence of the studied braid delta facies associations promoted fairly sheet like geometry limited by marine flooding surface that more likely resulted from the response of deposition on shallow and wide epeiric sea

comment on provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the palaeogeographic evolution of southwestern gondwana by blanco et al precambrian res 171 2009 57 73 thus it is truly tectonics which governs stratigraphy and thetwobranchesofthegeologicalsciencesareinseparable astruc tural geologists who is not a stratigrapher is only a geometer not a geologist for he reasons about abstract surfaces and vol umes emptied of their history and a stratigrapher who neverconcerned himself with tectonics would produce only a deadstratigraphy gignoux 1950 p 3 the paper provenance of the arroyo del soldado group ediacaran to cambrian uruguay implications for the palaeogeo graphicevolutionofsouthwesterngondwana blancoetal 2009 presented new u pb shrimp sm nd and geochemical data ofsome units of arroyo del soldado group asg in order to betterconstraining the evolution of sedimentary protoliths and there fore to justify a rift tectonic setting for deposition of those units unfortunately the data presented are incapable of answering thetectonic evolution of neoproterozoic lower paleozoic brasilianounits located at the east of the uruguayan territory we point outthattheauthors interpretationisnotsubstantiatedonthedata thescarcity of geological and isotopic data obtained for dom felicianobelt dfb determinesawidespanofgeotectonicmodels blancoetal 2009 use force andadapttheavailableisotopicdatatosustaintheir tectonic model other plausible models can also explain the

early archean foredeep sedimentation related to crustal shortening a reinterpretation of the barberton sequence southern africa abstract the upper dominantly terrigenous sedimentary successions of the early archean barberton sequence kaapvaal province record an evolution similar to that of phanerozoic foredeeps and may represent the oldest known foredeep sequence rather than a passive continental margin sequence as previously proposed pre foredeep ocean crust the 3 5 ga old onverwacht group consists of bimodal volcanics with intercalated shallow water sediments gravity modeling and boundary shear zones indicate that deformed onverwacht crust was obducted northward onto sialic continental crust immediately before during or after burial by the synorogenic terrigenous clastic fill foredeep basin deepening due to crustal loading from the south is recorded by basinal iron formation chert and mudstone of the basal fig tree group which accumulated paraconformably on the onverwacht overlying sediments of the fig tree group consist of 2 km of submarine fan greywackes and mudstones shoaling of the basin is indicated by the transition to the overlying braided alluvial and shallow marine sediments of the ca 3 3 ga old moodies group 3 km thick changing ree patterns and relative abundances of ni cr sr th and ur in the sediments of the fig tree and moodies groups indicate a significant ultramafic mafic volcanic component diluted upward in the stratigraphic sequence by an increasing granitic contribution a similar trend is reflected in sandstone petrography in which the ratio of extrusive to intrusive rock fragments decreases upwards in the fig tree group whereas the moodies group sediments are enriched in quartz and potassium feldspar a persistent chert component in both units suggests that the tectonic setting of the basin is analogous to that of phanerozoic recycled orogens the distribution of facies in the fig tree and moodies groups and paleocurrent petrographic and geochemical data indicate progressive unroofing of a southerly source terrane consisting of the onverwacht volcanics and intercalated sediments and the 3 5 3 2 ga old ancient gneiss complex agc in addition older components of the fig tree group were recycled into overlying stratigraphie units the agc consists of mantle derived 3 5 ga old bimodal metavolcanics and metasediments a tonalite gneiss batholith metabasite dikes and meta anorthosites its structures record progressive northwestward uplift beginning around 3 4 ga ago intense homogeneous ductile strains were overprinted by northwest verging major folds rotated by large scale northwestward simple shear followed by imbricate faulting and pseudotachylite formation by 3 3 ga ago the fig tree and moodies groups had also shortened with similar kinematics to those of the agc by partly syndepositional folding and volume loss during the formation of transecting cleavage further folding and slide formation followed during subsolidus diapiric rise of the initially tabular kaap valley tonalite by 3 2 ga and emplacement of trondhjemite plutons from 3 3 to 3 0 ga ago

marine sand wave complex in the permian of central arizona abstract the permian rancho rojo member of the schnebly hill formation in central arizona displays distinctive large scale cross stratification in compound sets that are several meters thick and tens of meters long based on geometry stratigraphic sequences sedimentary structures and the processes which formed these features the rancho rojo is interpreted as a submarine or estuarine sand wave complex modern sand wave processes produce large scale intraset cross stratification and superimposed dunes which form complex internal sedimentary structures like those of the rancho rojo the rancho rojo is subdivided into five facies the planar tabular foreset facies is composed of giant westward dipping foresets that probably formed in large steep strongly asymmetrical sand waves the intraset cross stratified facies comprises complex compound cross stratification bounded by westward dipping planes of erosion it was deposited by large less asymmetrical sand waves with superimposed dunes on the surface the trough cross stratified facies consists of complex trough shaped intrasets the internal geometry of this facies probably resulted from deposition in complex nearly symmetrical sand waves that responded to oscillatory tidal currents the ripple laminated sand facies and homogeneous sand facies comprise ripple laminated and bioturbated sandstone that formed du ing waning sand wave deposition although the facies are complexly interbedded their overall stratigraphic succession from oldest to youngest typically is similar to the order just presented this suggests that through time large asymmetrical rather simple sand waves were succeeded by more complex lower less asymmetrical sand waves due to change of current regime from unidirectional to bidirectional this interpretation of a sand body that was once thought to be part of a continental redbed sequence requires revision of paleogeographic reconstruction for part of the permian of central arizona

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

petrography and geochemistry of granitoid and metamorphite pebbles from the early archaean moodies group barberton mountainland south africa pebbles of potassic granitoids and metamorphites constitute up to 5 of the basal conglomerate of the moodies group in a ratio of 2 1 the granitoid pebbles frequently show micrographic quartz feldspar intergrowth whereas the metamorphites of a modal composition similar to that of the granitoids are characterized by large quartz grains which could represent original quartz phenocrysts in felsic volcanic precursors the granitoids show high k2o sr k2o na2o and k rb small enrichment of light ree large negative eu anomalies and slightly depleted and fractionated heavy ree compared to the granitoids the metamorphites show higher fe2o3 tio2 and cr concentrations greater enrichment of light ree and also large negative eu anomalies there is little similarity between the moodies pebbles and the majority of the rocks of the ancient gneiss complex of swaziland agc there is only some similarity of the ree distribution patterns between the pebbles and the mkhondo metamorphic suite possibly an areally restricted phase of the agc the geochemical data and especially the large negative eu anomalies suggest that the moodies pebbles were derived from granites which represent residual magmas from which much plagioclase had been removed the granites crystallized at depths of 7 km from magmas with low h2o pressures in a rather thick sialic crust it appears possible that the pre moodies granitoids originated through partial melting of low al2o3 siliceous gneisses of the agc a chronologic connection of the formation of the granitoids with the late onverwacht group volcanicity is possible

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

3 2ga eolian deposits from the moodies group barberton greenstone belt south africa implications for the origin of first cycle quartz sandstones abstract the 3 2 ga moodies group in barberton greenstone belt south africa contains the oldest recognized evidence for eolian processes on earth these eolian facies indicate that significant wind regimes existed in the archean atmosphere eolian deposits are developed in the lower moodies group in the central part of the belt and consist of stacked inversely graded strata ranging in thickness from 2 to 10 mm individual strata consist of fine to medium grained quartz sandstone at the base that passes vertically into very coarse to coarse grained quartz sandstone strata make up cross bed sets up to 3 5 m thick sets of wind ripple strata are either tabular tangential or in the form of broad troughs this type of inversely graded stratification is the product of wind ripple migration the coarse grain size of the sediment combined with the low angle of inclination of the strata is best interpreted as the product of sand sheet or dune plinth preservation based on the geometry of eolian set boundaries dunes were probably simple barchan form the tens of meters thick eolian deposits are underlain by braided stream deposits composed of subfeldspathic pebbly sandstones the vertical change from braided stream subfeldspathic sandstones to eolian quartz arenites highlights the underappreciated capacity of eolian processes to generate first cycle quartz sandstones on landscapes pre dating the advent of land vegetation

depositional and tectonic setting of the archean moodies group barberton greenstone belt south africa the 3 22 3 10 ga old moodies group uppermost unit of the swaziland supergroup in the barberton greenstone belt bgb is the oldest exposed well preserved quartz rich sedimentary sequence on earth it is preserved in structurally separate blocks in a heavily deformed fold and thrust belt north of the inyoka fault moodies strata reach up to 3700 m in thickness detailed mapping correlation of measured sections and systematic analysis of paleocurrents show that the lower moodies group north of the inyoka fault forms a deepening and fining upward sequence from a basal alluvial conglomerate through braided fluvial tidal and deltaic sandstones to offshore sandy shelf deposits the basal conglomerate and overlying fluvial facies were derived from the north and include abundant detritus eroded from underlying fig tree group dacitic volcanic rocks shoreline parallel transport and extensive reworking dominate overlying deltaic tidal and marine facies the lithologies and arrangement of moodies group facies sandstone petrology the unconformable relationship between moodies strata and older deformed rocks presence of at least one syndepositional normal fault and presence of basaltic flow rocks and airfall fall tuffs interbedded with the terrestrial strata collectively suggest that the lower moodies group was deposited in one or more intramontane basins in an extensional setting thinner moodies sections south of the inyoka fault generally less than 1000 m thick may be correlative with the basal moodies group north of the inyoka fault and were probably deposited in separate basins a northerly derived southward thinning fan delta conglomerate in the upper part of the moodies group in the central bgb overlies lower strata with an angular unconformity this and associated upper moodies conglomerates mark the beginning of basin shortening by south to southeast directed thrust faulting along the northern margin of the bgb and suggest that the upper moodies group was deposited in a foreland basin timing orientation and style of shortening suggest that this deformation eventually incorporated most of the bgb into a major fold and thrust belt

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

earliest life on earth evidence from the barberton greenstone belt south africa abstract studies investigating the structure and diversity of earth s record of life older than 3 2 ga are restricted to two locations worldwide in which sedimentary rocks have escaped regional high grade metamorphism and penetrative deformation the pilbara craton of western australia and the barberton greenstone belt in the kaapvaal craton of south africa this paper provides a south african perspective on the evidence of paleoarchean life a record that is often overlooked in the literature it aims to summarize and critically review previously reported claims of early life in the bgb gives an overview of the latest findings and provides an outlook on potential future discoveries the 15 km thick volcanic sedimentary succession making up the barberton supergroup was deposited between 3 55 to ca 3 20 ga and can be subdivided in three stratigraphic units that provide a unique window into a diverse and widespread paleoarchean microbial ecosystem landscape putative biosignatures occur almost throughout the entire bgb stratigraphy and range from carbonaceous cherts containing filamentous spheroidal and lenticular microstructures traces of hydrothermal biofilms photosynthetic microbial mats remnants of pseudocolumnar stromatolites and large organic walled spheroidal microfossils of currently unknown affinity the bgb also contains one of the world s oldest known record of tufted microbial mats which extensively colonized tidally influenced siliciclastic shorelines and were most likely formed by filamentous photosynthesizers other mat associated biosignatures include silicified gas bubbles domes and lenses that likely formed due to metabolic activity or the decay of buried organic matter some of these subsurface voids beneath the cohesive mats were inhabited by the earliest known forms of cavity dwelling microbial communities that were probably dominated by chemotrophic or photosynthetic microbes recently discovered terrestrial microbial mats once thriving in a fluvial dominated setting represent the oldest macroscopically visible fossil traces of life on land which is also supported by the occurrence of nearby paleosols that carry signals of biogenic sulfur fractionation the wealth of preserved microbial biosignatures from marine fluvial hydrothermal and possibly planktonic settings combined with the high spatial and temporal resolution of the barberton greenstone belt deposits is truly exceptional consequently the bgb deserves an equal level of attention and protection for future generations like its australian counterpart

in situ iron isotope analyses of pyrites from 3 5 to 3 2ga sedimentary rocks of the barberton greenstone belt kaapvaal craton abstract the barberton greenstone belt bgb south africa consists of volcano sedimentary and sedimentary successions deposited between 3 5 and 3 2 ga which are subdivided into three groups the onverwacht fig tree and moodies after deposition the barberton greenstone belt underwent relatively low grade tectonothermal events enabling estimation of surface environmental events and biological evolution in the middle archean isotopic studies suggest the activity of methanogen sulfate reducing bacteria and photosynthetic bacteria at 3 4 ga microbial dissimilatory iron reduction dir is also considered one of the earliest expressions of metabolism on earth but the isotopic signal of microbial dir is still uncertain in the archean we performed in situ iron isotope analyses of individual pyrites in sedimentary rocks from the bgb using a femtosecond laser ablation multi collector icp ms technique fs la mc icp ms to find isotopic evidence for the microbial activity we obtained a large variation in 56 fe values ranging from 1 84 to 3 79 most of the 56 fe values of pyrites from the hooggenoeg complex show positive values whereas those from the noisy complex and subsequent sedimentary sequences show a wide variation from negative to positive 56 fe values one of the main differences between these complexes is their depositional depth the hooggenoeg complex was probably deposited in a deep ocean environment whereas the noisy complex was in shallow water the negative 56 fe values of pyrite grains in the noisy complex onverwacht group indicate the occurrence of partial reduction caused by microbial dir in a middle archean shallow sea

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

erosion of archean continents the sm nd and lu hf isotopic record of barberton sedimentary rocks abstract knowing the composition nature and amount of crust at the surface of the early earth is crucial to understanding the early geodynamics of our planet yet our knowledge of the hadean archean crust is far from complete limited by the poor preservation of archean terranes and the fact that less attention has been paid to the sedimentary record that tracks erosion of these ancient remnants to address this problem and get a more comprehensive view of what an archean continent may have looked like we investigated the trace element and sm nd lu hf isotopic records of archean metasedimentary rocks from south africa we focused our study on sandstone and mudstone from drill core in the fig tree group 3 23 3 26 ga of the barberton granite greenstone belt but also analyzed the 3 4 ga buck reef cherts and still older 3 5 3 6 ga meta igneous rocks from the ancient gneiss complex swaziland based on principal component analysis of major and trace element data the fig tree metasedimentary rocks can be classified into three groups crustal detritus rich sediments si rich sediments and ca fe rich sediments the detritus rich sediments have preserved the sm nd and lu hf isotopic signatures of their continental sources and hence can be used to constrain the composition of crust eroded in the barberton area in the paleoarchean period based on sm nd ratios we estimate that this crust was more mafic than today with an average sio 2 content of 60 5 2 wt this composition is further supported by isotopic mixing calculations suggesting that the sedimentary source area contained equal proportions of mafic ultramafic and felsic rocks this implies that the archean crust exposed to weathering was more mafic than today but does not exclude a more felsic composition at depth neodymium and hf crustal residence ages show that the eroded crust was on average 300 400 ma older than the deposition age of the sediments which highlights the importance of intracrustal reworking of older crust at 3 2 ga in the barberton area the si rich sediments have slightly positive e nd t 3 23ga but extremely radiogenic e hf t 3 23ga up to 11 based on analyses of 3 4 ga buck reef cherts we suggest that the radiogenic hf isotopic signature of the si rich sediments can be accounted for by the old chert clasts or detrital silicified rock fragments present in the rocks the latter have extremely high lu hf ratios such that their e hf values would increase dramatically by about 100 epsilon units every 100 ma in the ca fe rich sediments one sample contains carbonate that preserves the typical rare earth element features of seawater precipitates the initial nd isotopic composition of this sample e nd t 3 23ga 1 7 is within the range of previous estimates for archean anoxic seawater

climatic and geochemical implications of archean pedogenic gypsum in the moodies group 3 2 ga barberton greenstone belt south africa abstract lithic sandstones of braided fluvial to supratidal facies in the paleoarchean moodies group 3 22 ga barberton greenstone belt south africa include several regionally traceable units with common to abundant in places rock forming nodular concretions of megaquartz pseudomorphs after gypsum barite and calcite concretionary accumulations are stratiform and commonly associated with aqueously reworked fine grained tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent weathering of tuffaceous material and feldspar delivered alkali cations such as ca ba and k while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a co2 rich atmosphere sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic s or so2 concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone dominated by seasonal fluctuations of the groundwater level under evaporitic conditions the concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols their formation and composition constrain the local occurrence of sulfate in the archean atmo and hydrosphere their interaction with the emerging biosphere archean weathering regime local climate and vadose zone hydrodynamics

sm nd isotopic mapping of lithospheric growth and stabilization in the eastern kaapvaal craton whole rock sm nd isotope systematics of 79 archean granitoids from the eastern kaapvaal craton southern africa are used to delineate lithospheric boundaries and to constrain the timescale of crustal growth assembly and geochemical differentiation c 3 66 2 70 ga offsets in end values for 3 2 3 3 ga granitoids across the barberton greenstone belt bgb are consistent with existing models for c 3 23 ga accretion of newly formed lithosphere north of the bgb onto pre existing c 3 66 ga lithosphere south of the bgb along a doubly verging subduction margin the nd isotopic signature of c 3 3 3 2 ga magmatic rocks show that significant crustal growth occurred during subduction accretion after c 3 2 ga however the nd signature of intrusive rocks c 3 1 and 2 7 ga is dominated by intracrustal recycling rather than by new additions from the mantle signalling cratonic stability

multi stage alteration rheological switches and high grade gold mineralization at sheba mine barberton greenstone belt south africa abstract structural petrographic and geochemical data from sheba gold mine in the barberton greenstone belt of south africa indicate that gold mineralization formed at the end and as the result of a multi stage structural fluid flow and alteration history this multi stage evolution is a prerequisite for the formation of high grade 40 g t au ore bodies the sheba mine is situated in well bedded metaturbidites of the mesoarchaean fig tree group and structurally infolded ultramafic lavas in the first order refolded ulundi syncline in the northern parts of the belt original komatiitic lava flows are pervasively altered to talc carbonate greyschist this regionally widespread alteration relates to an early phase of deformation and associated fluid flow the reaction softening associated with this alteration accentuates rheological anisotropies within the heterogeneous wall rock sequence this results in the localization of strain along the contact between talc carbonate greyschist and overlying massive greywacke and chert units during later stage refolding and associated flexural slip pervasive quartz veining silicification k enrichment and the progressive replacement of the earlier talc carbonate assemblages by a high variance quartz fuchsite green schist alteration testify to the localized fluid flow along this contact the newly formed finely intergrown green schist assemblage leads in turn to a reaction hardening and embrittlement of the former greyschist chert contact high grade gold mineralization is hosted by brittle quartz carbonate vein networks and breccias developed along brittle ductile structures that cross cut the competent green schist at high angles the siting of gold and predominance of free gold suggest that gold precipitation is related to extreme fluid pressure cycling and phase separation of the mineralizing fluid during the formation of the brittle fracture networks the thrust kinematics of the late stage controlling structure studied here indicate that the mineralized structure accommodated regional nw se subhorizontal shortening when further tightening of the ulundi syncline passed the lock up stage could no longer be accommodated by flexural slip the results highlight that the formation of favourable conditions for high grade mineralization such as those at sheba mine may be the result of the interaction of temporally distinct fluid flow and alteration events and lithological heterogeneities during the progressive structural evolution of a region

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation