geodynamo solar wind and magnetopause 3 4 to 3 45 billion years ago stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss although the early earth retained its water and atmosphere and thus evolved as a habitable planet little is known about earth s magnetic field strength during that time we report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3 4 to 3 45 billion years ago the measured field strength is 50 to 70 that of the present day field when combined with a greater paleoarchean solar wind pressure the paleofield strength data suggest steady state magnetopause standoff distances of 5 earth radii similar to values observed during recent coronal mass ejection events the data also suggest lower latitude aurora and increases in polar cap area as well as heating expansion and volatile loss from the exosphere that would have affected long term atmospheric composition

zircon shrimp dating confirms a palaeoarchaean supracrustal terrain in the southeastern kaapvaal craton southern africa abstract we report shrimp zircon ages for felsic volcanic rocks of the early archaean nondweni greenstone belt ngb located in northern kwazulu natal province south africa the ngb is part of a chain of greenstone belt remnants that occurs south of the well known barberton greenstone belt bgb two samples of felsic volcanic rocks of the basal toggekry formation yielded zircon ages of 3 53 ga whereas significantly younger ages of 3 41 ga were obtained for two samples of felsic rocks from near the top of the witkop formation the latter date now constrains the age of stromatolite like structures in cherts associated with the felsic rocks which probably represent some of the oldest preserved evidence of life on earth the zircon ages also indicate that the ngb correlates well chronologically with the bgb and is one of the oldest volcano sedimentary successions in the southeastern kaapvaal craton geochemical data for the felsic ngb rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust possibly in a back arc environment initial e nd values for ngb rocks range from 1 2 to 0 3 and suggest involvement of pre greenstone continental crust that was possibly extensive in the southeastern part of the kaapvaal craton during palaeoarchaean times

sedimentary geology of the palaeoarchaean buck ridge south africa and kittys gap western australia volcano sedimentary complexes abstract the two palaeoarchaean volcano sedimentary complexes of the buck ridge barberton greenstone belt south africa and kittys gap coppin gap greenstone belt east pilbara australia have a similar geological setting and age 3 45 ga the predominantly volcaniclastic sediments are concentrated at the top of these complexes and experienced thorough very early diagenetic silicification in many places the silicification process has led to excellent preservation of the primary sedimentary structures elsewhere it has resulted in their obliteration or replacement by diagenetic structures the buck ridge chert forms a regressive transgressive succession deposited around base level with lacustrine and littoral marine facies deposition of the kittys gap chert was also close to base level almost exclusively subaqueous with tidal influence and a regressive sequential trend in both volcano sedimentary complexes these low energy sediments are juxtaposed with high energy breccia pods and layers with often a high fe oxide content the breccias are interpreted as being the result of explosive hydrothermal activity sedimentation was strongly controlled by normal faulting

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

granite greenstone terranes in the pilbara block australia as coeval volcano plutonic complexes evidence from u pb zircon dating of the mount edgar batholith the mount edgar batholith in the pilbara block western australia consists in part of a gneiss complex that resembles other archean high grade gneiss terranes the complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession the warrawoona group the contact between the greenstone belt and the gneiss complex is tectonic field evidence indicates that the banded gneisses are the oldest part of the batholith zircon u pb ages for three samples are 3429 13ma 3443 10ma and3448 8ma all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession pidgeon 1978 1 two granitoids representing pre and post tectonic phases respectively give zircon ages of 3304 10ma and3314 13ma thereby constraining the major structural events to be contemporary with granite magmatism consistent with a diapiric model for the tectonic evolution of the batholith collins 1989 2 these ages are similar to the zircon age for terminal volcanism in the warrawoona group pidgeon 1984 3 but are 100ma older than the precisely defined rb sr whole rock ages obtained throughout the batholith collins and gray 1989 4 batholith wide resetting or late closure of rb sr isotopic systems is implied possibly associated with late fluid circulation the temporal coincidence between plutonism and volcanism in the mount edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the 3300ma granites and the 3440ma gneisses in the batholith the isotopic evidence combined with the structural data indicates that the granites cannot be derived from the surrounding greenstone belt further there is no compelling evidence for a sialic basement to the greenstone succession

geochemistry of a silicified felsic volcaniclastic suite from the early archaean panorama formation pilbara block western australia an evaluation of depositional and post depositional processes with special emphasis on the rare earth elements abstract many elements in a suite of pervasively silicified dacitic volcaniclastic and volcanic rocks from the early archaean panorama formation warrawoona group western australia were strongly depleted or enriched relative to those in equivalent unaltered units during widespread post depositional metasomatism the present compositions of the altered rocks are dominated by al 2 o 3 k 2 o and sio 2 and lie on or close to a quartz sericite mixing line the al contents were probably inherited from the original dacitic rocks while si some k and possibly rb may have been introduced during metasomatism the elements mg ca na and to a lesser extent fe mn and sr have been depleted relative to al during alteration whereas silica has been greatly enriched the high and variable ba concentrations suggest that ba like si was introduced during silicification the ree and th were relatively immobile during alteration and were diluted during silicification in the same proportion as al most of the rocks represent detrital sand and silt sized felsic tephra debris eroded from felsic volcanic rocks or ash fall deposits that underwent a short interval of transport and deposition dominated by physical rather than chemical processes a few siltstones contain clay minerals that probably formed by the chemical weathering of warrawoona group basalts the siltstones have lower th and la concentrations lower la lu ratios and higher sc and cr concentrations than felsic rocks in the suite regional alteration pervasively silicified the original sediments shortly after deposition the replacement mineral assemblage indicates a maximum temperature for hydrothermal alteration of about 350 c because of the lack of chemical weathering and the low temperature of post depositional alteration ree abundances in volcaniclastic rocks of the panorama formation reflect their felsic volcanic provenance and are interpreted to have preserved a relict magmatic signature the lack of a negative eu anomaly and high la lu ratio chondrite normalized ratio is 7 to 39 suggest that magmas of the panorama formation formed by direct melting of an eclogite residue without subsequent crystallization of feldspar during the commencement of cratonization of the pilbara block this origin differs from the origin of some other silicic volcanic rocks of the duffer formation representing earlier felsic volcanic activity of the warrawoona group that were produced by fractional crystallization from basaltic parents this suggests a polygenetic origin for felsic magmas of the warrawoona group

crustal fracturing and chert dike formation triggered by large meteorite impacts ca 3 260 ga barberton greenstone belt south africa the ca 3260 ma contact between the largely volcanic onverwacht group and overlying largely sedimentary fig tree group in the barberton greenstone belt south africa is widely marked by chert dikes that extend downward for up to 100 m into underlying sedimentary and volcanic rocks of the mendon formation onverwacht group in the barite valley area these dikes formed as open fractures that were filled by both precipitative fill and the downward flowage of liquefied carbonaceous sediments and ash at the top of the mendon formation spherules that formed during a large meteorite or asteroid impact event occur in a wave and or current deposited unit spherule bed s2 which widely marks the onverwacht fig tree contact and as loose grains and masses within some chert dikes up to 50 m below the contact four main types of chert dikes and veins are recognized type 1 irregular dikes up to 8 m wide that extend downward across as much as 100 m of stratigraphy type 2 small vertical dikes most 1 m wide which are restricted to the lower half of the mendon chert section type 3 small crosscutting veins most 50 cm across filled with precipitative silica and type 4 small irregular to bedding parallel to irregular veins mostly 10 cm wide filled with translucent precipitative silica type 2 dikes formed first and reflect a short lived seismic event that locally decoupled the sedimentary section at the top of the mendon formation from underlying volcanic rocks and opened narrow vertical tension fractures in the lower lithified part of the sedimentary section later seismic events triggered formation of the larger type 1 fractures throughout the sedimentary and upper volcanic section widespread liquefaction of soft uppermost mendon sediments and flowage of the liquefied sediments and loose impact generated spherules into the open fractures late stage tsunamis everywhere eroded and reworked the spherule layer the coincidence of crustal disruption dike formation spherule deposition and tsunami activity suggests that all were related to the s2 impact or impact cluster crustal disruption at this time also formed local relief that provided clastic sediment to the postimpact fig tree group including a small conglomeratic fan delta in the barite valley area remobilization and further movement of debris in the subsurface continued for some time locally the deposition of dense baritic sediments over soft dike materials induced remobilization of material in the dike causing foundering of s2 and 1 2 m of overlying baritic sediments into the dike spherule beds occur at the base of the fig tree group over wide areas of the barberton belt marking the abrupt change from 300 m y of predominantly anorogenic mafic and komatiitic volcanism of the onverwacht group to orogenic clastic sedimentation and associated felsic volcanism of the fig tree group this area never again returned to onverwacht style mafic and ultramafic volcanism but evolved 100 m y later into the kaapvaal craton these results indicate that this major transition in crustal evolution coincided with and was perhaps triggered by major impact events ca 3260 3240 ma

single zircon dating constraining the maximum age of the barberton greenstone belt southern africa one of the central issues in early precambrian crustal evolution is the age and genetic relationship between greenstone belt supracrustal assemblages and nearby high grade gneiss terranes the crucial question in most cases is which of the two is older we report zircon ages using the new single grain evaporation technique from a metaquartzite and a felsic volcanic flow of the onverwacht group in the barberton greenstone belt bgb south africa and swaziland that constrain the age of this basal part of the greenstone sequence between 3451 15 and 3438 6 ma this is almost 100 ma younger than previous age estimates assigned to the onverwacht and agrees well with recent sm nd pb pb and zircon ion microprobe dating tonalitic orthogneisses of the ancient gneiss complex agc adjacent to the bgb have ages up to 3644 4 ma and are thus demonstrably older than the greenstones also they cannot be derived from them by partial melting as has been proposed previously this age relationship as well as the presence of metaquartzites and thrust slices of pregreenstone tonalite in the onverwacht tectonic sequence rule out an entirely oceanic evolution for the early history of the bgb and suggest that continental crust was either below or nearby

spinel from archean impact spherules abstract large meteorite impacts are recorded in distal fall deposits preserved in two early archean greenstone belt sequences 3 2 3 5 ga mapping stratigraphie and geochemistry studies have been used to identify at least four impact layers in the barberton greenstone belt of southern africa and one in the pilbara greenstone belt of western australia spherules that represent impact melt droplets distinguish these impact layers from associated tuffs spinel with unusual compositional traits occurs within spherules in two of these impact layers spinel is the only preserved primary phase and is found within up to 10 of the spherules in the best preserved layers some spherules contain up to 40 spinel which is often the only primary phase that crystallized within the spherules spinel morphologies include dendrites and skeletal octahedra typical of those found in high temperature melts spinel compositions are similar to those of cenozoic impacts including very high ni and ferric iron contents but the archean spinel is distinctive in being composed dominantly of the chalcophile elements fe ni cr and v this spinel is not metamorphic or metasomatic in origin closely associated komatiites in the underlying onverwacht group both fresh and highly altered have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel characteristics of spinel preserved in these archean impact spherules suggest a rather complex series of steps following a large bolide impact initial high temperature reduction produced melt droplets some of which were immiscible silicate metal sulfide density differences allowed separation of some of these immiscible melts which subsequently evolved independently and formed spherules with highly variable chalcophile element contents oxidation during atmospheric reentry increased ni fe and fe 3 fe 2 in the chalcophile siderophile melts archean impact spinel has higher ni fe than cenozoic impact spinel though distinctly lower fe 3 fe 2 which probably reflects a substantially less oxidizing atmosphere during the archean the archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric h 2 o h 2 bolide size is estimated to be 24 km in diameter based on the average diameter of 0 85 mm for the spinel bearing spherules this estimate is consistent with the observed ir fluence in spinel bearing impact layers of approximately 5 3 mg cm2 which would be produced by a 30 km diameter chondritic bolide

microfossils and possible microfossils from the early archean onverwacht group barberton mountain land south africa there is widespread textural evidence for microbial activity in the cherts of the early archean onverwacht group layers with fine carbonaceous laminations resembling fossil microbial mats are abundant in the cherty metasediments of the predominantly basaltic hooggenoeg and kromberg formations in rare cases filamentous microfossils are associated with the laminae the morphologies of the fossils as well as the texture of the encompassing laminae suggest an affinity to modern mat dwelling cyanobacteria or bacteria a variety of spheroidal and ellipsoidal structures present in cherts of the hooggenoeg and kromberg formations resemble modern coccoidal bacteria and bacterial structures including spores the development of spores may have enabled early microorganisms to survive the relatively harsh surficial conditions including the effects of very large meteorite impacts on the young earth

the origin of carbonaceous matter in pre 3 0 ga greenstone terrains a review and new evidence from the 3 42 ga buck reef chert abstract the geological record of carbonaceous matter from at least 3 5 ga to the end of the precambrian is fundamentally continuous in terms of carbonaceous matter structure composition environments of deposition preservation and abundance in host rocks no abiotic processes are currently known to be capable of producing continuity in all four of these properties although this broad view of the geological record does not prove that life had arisen by 3 5 ga the end of the early archean it suggests a working hypothesis most if not all carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms this hypothesis must be tested by studies of specific early geological units designed to explore the form distribution and origin of enclosed carbonaceous matter the carbonaceous environmentally diverse 3416 ma buck reef chert brc of the barberton greenstone belt south africa provides an opportunity for such a study upward facies progressions in the brc reflect deposition in environments ranging from shallow marine evaporitic brine ponds to a storm and wave active shelf to a deep low energy basinal setting below storm wave base abundances and ratios of al2o3 zr tio2 and cr track inputs of various types of volcaniclastic and terrigenous clastic materials in particular zr al2o3 and zr serve as proxies for concentration of windblown dust and indirectly as proxies for sedimentation rate cu zn ni and feo were concentrated in the most slowly deposited transitional and basinal sediments inconsistent with a hydrothermal setting but consistent with a normal marine setting the distribution of microfacies defined by associations and layering of clastic ferruginous and carbonaceous grains correlates with facies transitions fine carbonaceous laminations which occur only in shallow platform settings represent photosynthetic microbial mats these were ripped up and the debris widely redistributed in shallow and deep water by waves and storms the isotopic composition of carbonaceous matter ranges from 35 to 30 in shallow water settings and to 20 in deep water units the heavier 13c in deep water carbonaceous matter is thought to reflect microbial processing possibly by fermentation and methanogenesis of organic matter originally produced in shallow water hydrothermal origins for brc carbonaceous matter are clearly excluded by the inferred depositional setting of the rocks as a whole an inference supported by field petrographic and geochemical analysis we suggest that the biological model proposed here for brc carbonaceous matter is the best currently available the hypothesis that at least some carbonaceous matter present in rocks older than 3 0 ga was produced by living organisms should be regarded as likely until extraordinary contradictory evidence is presented

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

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

growth of early archaean crust in the ancient gneiss complex of swaziland as revealed by single zircon dating abstract we present new single grain zircon ages derived from ion microprobe and evaporation analysis for rocks of the ancient gneiss complex agc of swaziland southern africa that enable us to document a more extended history of early archaean crustal growth in the eastern kaapvaal craton than previously documented the oldest rocks appear to be strongly flattened tonalitic trondhjemitic orthogneisses dated as 3644 4 ma compston and kroner 1988 which are in faulted contact with rocks of the barberton greenstone belt bgb in an agc enclave in northwest swaziland and which also record subsequent thermal metamorphic plutonic events at 3580 3500 3430 3200 and 3000 ma these rocks are tectonically interbanded with 3200 ma old granodiorites and k rich leucogranites thereby providing evidence for a severe and important post 3 2 ga deformation event that is also inferred for 3500 to 3200 ma old tonalitic to trondhjemitic gneisses in the agc of northeast swaziland in central swaziland zircons from the oldest tonalitic gneiss of the agc have an age of 3560 ma whereas younger additions of granitoid which also intrude the dwalile greenstone enclave in the agc record ages of 3450 3460 ma intense ductile deformation has repeatedly obliterated original intrusive contacts so that preserved field relationships often cannot resolve the above chronology chemical and whole rock isotopic data are consistent with derivation of some agc gneisses from mantle sources and thus imply very short crustal residence times for their granitoid precursors but this is not generally true for the entire agc we suggest that at least some gneisses resulted from intracrustal melting perhaps during crustal thickening events as recorded in granulite parageneses within the agc documentation of 3 5 ga or older granitoid crust in all major regions of the agc now exposed suggests the presence of continental terrains of up to at least 100 km in diameter before the barberton greenstone succession was deposited the combined geochronologic and structural metamorphic data for the agc and bgb reveal a complex history of growth and deformation for the eastern kaapvaal craton that lasted for at least 600 ma from 3 64 to about 3 ga ago the magmatic and tectonic evolution of the agc was broadly similar to that of the bgb and suggests that early archaean greenstone and gneiss terrains were coeval but formed and evolved in different settings and at different crustal levels our age data and the structural metamorphic evolution of the eastern kaapvaal craton are compatible with models suggesting crustal growth through vertical magmatic underplating and early crustal thickening uplift and erosion as a result of tectonic interleaving due to repeated horizontal shortening

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

sedimentology and geochemistry of archean silica granules the production of biogenic silica has dominated the marine silica cycle since early paleozoic time drawing down the concentration of dissolved silica in modern seawater to a few parts per million ppm prior to the biological innovation of the first silica biomineralizing organisms in late proterozoic time inputs of silica into precambrian seawater were balanced by strictly chemical silica and silicate precipitation processes although the mechanics of this abiotic marine silica cycle remain poorly understood cherty sedimentary rocks are abundant in archean sequences and many previous authors have suggested that primary precipitation of amorphous silica could have occurred in archean seawater the recent discovery that many pure chert layers in early archean rocks formed as sedimentary beds of sand sized subspherical silica granules has provided direct evidence for primary silica deposition here we provide further sedimentological and geochemical analyses of early archean silica granules in order to gain a better understanding of the mechanisms of granule formation silica granules are common components of sedimentary cherts from a variety of depositional settings and water depths the abundance and widespread distribution of silica granules in archean rocks suggest that they represented a significant primary silica depositional mode and that most formed by precipitation in the upper part of the water column the regular occurrence of silica granules as centimeter scale layers within banded chert alternating with layers of black or ferruginous chert containing few granules indicates episodic granule sedimentation contrasting silicon isotopic compositions of granules from different depositional environments indicate that isotopic signatures were modified during early diagenesis looking to modern siliceous sinters for insight into silica precipitation we suggest that silica granules may have formed via multiple stages of aggregation of silica nanospheres and microspheres consistent with this hypothesis archean ocean chemistry would have favored particle aggregation over gelling granule formation would have been most favorable under conditions promoting rapid silica polymerization including high salinity and or high concentrations of dissolved silica our observations suggest that granule sedimentation was often episodic suggesting that granule formation may have also been episodic perhaps linked to variations in these key parameters

a high continental weathering flux into paleoarchean seawater revealed by strontium isotope analysis of 3 26 ga barite controls on archean seawater chemistry remain controversial many studies have suggested that it was largely controlled by oceanic hydrothermal fluid circulation recent work however from clastic sequences hf o isotope data from detrital zircons and models for the rb sr evolution of the continental crust suggest that intense continental weathering and low temperature surface alteration were more important than previously thought during the early archean this is consistent with biogeochemical studies that suggest the archean had a diverse microbial ecology which would in part need to be sustained by nutrients e g phosphorus that were derived from continental weathering to further quantify continental weathering during the early archean we analyzed 3 26 ga barite from the fig tree group south africa for strontium oxygen and sulfur isotope compositions we propose that the seawater component of the barite is characterized by 87sr 86sr ratios 0 701 which is significantly more radiogenic than contemporaneous mantle 0 7007 0 7008 the radiogenic nature of seawater at this time suggests that the continental weathering flux at 3 26 ga had a large impact on ocean chemistry 400 million years earlier than previously suggested

a metamorphosed stratabound epigenetic origin for a gruneritic archean gold deposit barberton south africa the lily mine located some 24 km northeast of barberton in south africa occurs stratabound within carbonaceous fe and mn rich metapelites containing abundant grunerite it is interpreted to be a metamorphosed epigenetic archean gold deposit the host rocks form part of a graywacke shale sequence close to a major regional structure known as the lily fault they are intensely deformed close to the fault with strain partitioned into finely laminated carbonaceous phyllosilicate rich units the bulk geochemistry of the host rocks is dominated by pelitic ferruginous and quartz rich fractions mixed in varying proportions mineralization is concentrated in rocks composed largely of the pelitic and ferruginous fractions that have prograde metamorphic almandine biotite grunerite magnetite and fe carbonates magnetite rich rocks were produced by progressive metamorphism of greenalite and or siderite and do not reflect oxide facies iron formation much of the quartz rich fraction occurs in deformed and metamorphosed veins associated with the mineralization gold is concentrated in the main reef zone immediately adjacent to the lily fault native gold occurs with arsenopyrite pyrrhotite minor chalcopyrite sphalerite galena and stibnite in association with abundant deformed and metamorphosed veins composed of quartz albite grunerite ankerite sulfides in graphite depleted biotite grunerite tourmaline schists and massive grunerite ankerite rocks ultramafic schists in the footwall of the lily fault exhibit intense k co2 alteration phlogopite dolomite calcite and quartz veining but are essentially barren of gold primary gold concentration probably occurred by destabilization of bisulfide complexes through reactions with reduced iron silicates and carbonaceous matter during channelized fluid influx along the lily fault subsequent metamorphism occurred at t 500 550 c and p 3 5 kbar transitional greenschist amphibolite possibly in association with the emplacement of the nelspruit batholith at 3105 ma

an archean geomagnetic reversal in the kaap valley pluton south africa the kaap valley pluton in south africa is a tonalite intrusion associated with the archean barberton greenstone belt antipodal paleomagnetic directions determined from the central and marginal parts of the pluton record a geomagnetic reversal that occurred as the pluton cooled the age of the reversal is constrained by an 40 ar 39 ar plateau age from hornblende at 3214 4 million years making it the oldest known reversal the data presented here suggest that earth has had a reversing perhaps dipolar magnetic field since at least 3 2 billion years ago

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

geological and geochemical record of 3400 million year old terrestrial meteorite impacts beds of sand sized spherules in the 3400 million year old fig tree group barberton greenstone belt south africa formed by the fall of quenched liquid silicate droplets into a range of shallow to deep water depositional environments the regional extent of the layers their compositional complexity and lack of included volcanic debris suggest that they are not products of volcanic activity the layers are greatly enriched in iridium and other platinum group elements in roughly chondritic proportions geochemical modeling based on immobile element abundances suggests that the original average spherule composition can be approximated by a mixture of fractionated tholeiitic basalt komatiite and ci carbonaceous chondrite the spherules are thought to be the products of large meteorite impacts on the archean earth

the contrasted response of ash to wetting abstract after a wildfire the soil is covered by ash ash properties depend on vegetation type amount of fuel and fire intensity the ash layer controls the post fire soil hydrologic response but little is known about the effect of ash thickness and ash type on infiltration which is relevant for post fire runoff and soil losses and for ecosystems rehabilitation and restoration this paper analyses the role of i ash type black or white ii thickness 5 15 and 30 mm thick and iii temporal variation 0 15 and 40 days under three simulated rain events 55 mm for 1 h on soil surface hydrology the rainfall was simulated on 0 25 m2 plots and time to ponding runoff and runoff discharge were measured the infiltration rates the initial infiltration rate f0 the steady state infiltration rate fc and the infiltration decay factor k were calculated and the horton infiltration equation applied the results show that soils covered with white ash doubled the runoff rates of soils covered with black ash in general runoff decreases as the ash thickness increases and the runoff decreases with the number of rainfall events after the fire in plots covered with white ash ponding time and k are positively correlated by the ash thickness and f0 and fc are correlated by the rainfall events in three runs ash type and ash depth are key factors on the soil hydrology after a wildfire

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

gneiss greenstone relationships in the ancient gneiss complex of southwestern swaziland southern africa and implications for early crustal evolution the ancient gneiss complex agc of swaziland is made up of a variety of archaean gneisses derived from granitoid intrusions of ttg composition emplaced between 3 6 and 3 2 ga ago and infolded amphibolite grade greenstone remnants locally referred to as dwalile supracrustal suite dss the greenstones were considered to postdate the oldest phases of the agc known as ngwane gneiss and this is now confirmed by conventional u pb zircon geochronology and shrimp single zircon ages on detrital grains from dwalile clastic metasediments suggesting that these rocks were derived from a granitoid source some 3 42 to 3 57 ga in age trondhjemitic ngwane gneiss structurally underlying the greenstones has a zircon age of 3521 23 ma and contains a 3683 10 ma old xenocryst this together with nd tdm model ages between 3 7 and 3 9 ga and a pronounced negative euanomaly for the metasediments strongly argues for the presence of very old crustal components in the agc nd isotopic systematics of dwalile metavolcanics suggest these rocks to be contaminated with older crustal material of ngwane type and resulting nd model ages 3 60 3 99 ga are likely to be too old the major and trace element chemistry of the dwalile metavolcanics is remarkably similar to that of onverwacht group rocks in the nearby barberton greenstone belt bgb and this together with a similar age for these two suites points to a genetic link and a similar mechanism of formation our data as well as field relationships do not favour an intra oceanic origin for the dss as has been inferred for the onverwacht group nor do they support a juvenile origin for the exposed ngane gneisses of the agc the ngwane precursors were apparently derived from older crustal protoliths whose subsequent modification particularly through remelting and deformation gave rise to the various compositional varieties now seen in the ngwane suite

emplacement features of archaean ttg plutons along the southern margin of the barberton greenstone belt south africa abstract the barberton greenstone belt in south africa and its surrounding granitoid terrane represents one of the type localities for archaean granite greenstone tectonics contact relationships between early archaean granitoids and greenstone sequences along the southern margin of the greenstone belt illustrate multiple simultaneously operating emplacement mechanisms for the plutons these emplacement mechanism which can be identified in individual plutons comprise ductile wall rock deformation stoping assimilation and the intrusion of ring dykes the interaction between these various mechanisms is interpreted to be responsible for the complex and highly variable discordant and concordant contact relationships observed in the granite greenstone terrane as well as the deformation observed within wall rocks and country rock xenoliths the absence of regionally consistent fabrics within the plutons suggests considerable competence contrasts between the tonalitic trondhjemitic plutons and in many cases the hydrated greenstone cover sequence during later subhorizontal crustal shortening which occurred parallel to the greenstone granitoid interface this competence contrast is interpreted to have resulted in the accentuation of cuspate lobate folds along the granitoid greenstone interface being partly responsible for the typical arcuate dome and keel geometry of the barberton greenstone belt

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

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

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

sedimentology of archean greenstone belts signatures of tectonic evolution abstract stratigraphic styles in archean greenstone belts are compared to those of modern and phanerozoic depositional basins in order to test the conformity of tectonic style through time six lithological associations in greenstone belts are recognized 1 mafic ultramafic volcanic 2 calc alkaline volcanic 3 bimodal volcanic 4 quartz arenite iron formation or carbonate 5 conglomerate wacke and 6 conglomerate arenite examples of the associations are described from the kaapvaal superior and zimbabwe provinces and the pilbara block each association differs only slightly between the four regions thereby emphasizing a common tectonostratigraphic theme sedimentary rocks are a minor component of the mafic ultramafic volcanic association they were deposited in two types of volcanic basin environments one similar to barred lagoons and bays around oceanic volcanic islands and the other similar to sediment starved platforms adjacent to coalesced volcanoes in inter arc intra arc and back are basins in contrast sedimentary rocks in the calc alkaline volcanic association comprise thick wedges of epiclastic and volcaniclastic detritus deposited in elongate sedimentary basins and thick wedges of pyroclastic and volcaniclastic detritus deposited adjacent to volcanic centers sedimentary facies were identical to those of forearc trench and marine volcanoplutonic arc settings chemogenic lithofacies in both volcanic associations were deposited in shallow to deep marine waters although shallow water profiles were selectively preserved in some terrains sedimentary rocks of the bimodal volcanic association are thick wedges of siliciclastic and volcaniclastic deposits interbedded with subaerial to submarine erupted flows of komatiite basalt and rhyolite depositional environments ranged from braid plain and braid delta to coastal marine and submarine fan starved basin deposits include banded iron formation and sulfidic black shale the association is identical to that of cratonic extensional basins in arc continent and intracontinental rifts sedimentary rocks of the quartz arenite iron formation or carbonate association onlap weathered basement or bimodal volcanic association rocks depositional environments were either transitional from fluvial to siliciclastic and carbonate marginal marine to offshore pelagic hemipelagic or from inshore intrabasinal clastic and volcaniclastic to offshore pelagic hemipelagic siliciclastic deposits are supermature two tectonic sites are represented those with siliciclastic facies represent continent adjacent syn to post rift stable shelves and those dominated by banded iron formation with volcaniclastic deposits represent arc adjacent post rift stable shelves sedimentary rocks of the conglomerate wacke association are represented by thick wedges of siliciclastic or volcaniclastic sandstone with minor volcanic components these were deposited in braid plain braid delta and submarine fan environments within elongate sedimentary basins coastal plain and pelagic basin facies are also present siliciclastic detritus was derived from syndepositional magmatic and metasedimentary thrust belts two stratigraphic styles are preserved one that records a flysch like stage of deep to marginal marine environments and the other that records a molasse like stage of marginal marine to fluvial environments the association is identical to that of compressional foreland basins of arc continent collisional and compressional arc tectonic affinities the conglomerate arenite association is preserved in elongate structural basins that transgress crustal components and all other supracrustal units it is dominated by thick wedges of siliciclastic sandstone although bimodal volcanic and volcaniclastic rocks can also be present depositional environments ranged from talus and alluvial fan and braid plain to either lacustrine or coastal marine local basin margins often show lateral offsets from source terrains and vertically skewed facies patterns longitudinal facies geometries are typical the association is similar to that of strike slip collisional graben in hinterland tectonic escape and terrane accretion orogens the examples show that despite all the ramifications of secular geophysical geothermal and geochemical global change the stratigraphic style of sedimentary rocks in archean greenstone belts can be matched with modern tectonic analogues thereby emphasizing the conformity of stratigraphic style through time the inseparable link between stratigraphic and tectonic styles implies that the tectonic style of greenstone belts was not temporally unique major differences exist with regards to the chemical composition of certain magmatic and sedimentary products but these are compensated for by their constant stratigraphic function in terms of depositional processes depositional environments and tectonic sites

fluid inclusion analysis of silicified palaeoarchaean oceanic crust a record of archaean seawater abstract in recent years the role of archaean seawater and hydrothermal fluid in the extensive silicification of palaeoarchaean volcano sedimentary successions has been a matter of considerable debate in an attempt to constrain the conditions of silica precipitation and the sources and chemical composition of the interacting fluids we used fluid inclusion microthermometry bulk crush leach and oxygen isotope analyses of chert and quartz veins in silicified komatiites and sediments from the barberton greenstone belt south africa chert vein margins consist of microcrystalline quartz and carbonaceous matter whereas the vein centres are often filled with macrocrystalline quartz that contains abundant fluid inclusions oxygen isotope ratios of vein chert and macrocrystalline quartz vary from 18 to 21 with the macrocrystalline quartz having slightly higher 18o values 0 7 0 3 the data are consistent with silica precipitation during low temperature 100 c hydrothermal processes on the archaean seafloor macrocrystalline quartz contains homogeneous 2 phase l v inclusions at room temperature with a relatively constant vapour fraction the inclusions have a salinity of 3 11 wt nacl equiv and homogenisation temperatures th of 150 200 c whereas some of the inclusions form intragranular fluid inclusion clusters that appear to be primary other inclusions form transgranular fluid inclusion trails and are clearly secondary both types of inclusions share the same microthermometric characteristics indicating that fluid entrapment occurred during a later metamorphic event and not near the seafloor the event likely coincided with regional deformation and metamorphism at 3 23 ga chlorite thermometry from vein host rocks reveals peak conditions of 257 31 c in conjunction with microthermometry the data correspond to a crustal depth of 3 6 km a secondary origin of fluid inclusions is also consistent with the cl br na cl and na k ratios of the macrocrystalline veins which are similar to those found in metamorphic quartz veins the fluid inclusions thus do not provide information on the conditions and temperatures during chert formation while the fluids potentially carry the geochemical signature of modified archaean seawater i e hydrothermal fluid characterised by low cl br and na k ratios and low mg contents the fluid composition was likely modified during regional metamorphism

the johannesburg dome south africa new single zircon u pb isotopic evidence for early archaean granite greenstone development within the central kaapvaal craton abstract the johannesburg dome located in the central part of the kaapvaal craton constitutes one of the key areas to better understand the archaean crustal evolution of this part of the craton the dome comprises a variety of archaean granitic rocks intruded into mafic ultramafic greenstone remnants this study presents new precise u pb single zircon dating for seven different granitoid samples and an amphibolite dyke collected from the johannesburg dome a trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of 3340 3 ma and represents the oldest granitoid phase recognized so far this result has important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstone remnants are older than c 3 34 ga this initial magmatic episode involving early greenstone development and the intrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome was followed by the emplacement of a 3201 5 ma hornblende biotite tonalite gneiss in the south following the trondhjemite tonalite gneiss emplacement a further period of magmatism took place on the dome which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites the age of these dykes has yet to be determined quantitatively but they fall within the time constraints imposed by the age of the trondhjemitic gneisses 3340 3200 ma and later crosscutting potassic granitoids these rocks consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome the southern and southeastern parts of the batholith consist mainly of medium grained homogeneous grey granodiorites dated at 3121 5 ma their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an age of 3114 2 3 ma an age of 3117 12 ma from zircons extracted from one of the mafic dykes possessing granitic microveins provided confirmation of the timing of this third magmatic event lastly pegmatites that crosscut all these earlier granitoid events are younger than 3114 ma and might be at least 3 0 billion years old these new data provide confirmation of the conclusion that the witwatersrand basin was deposited after c 3074 ma on an archaean basement as young as c 3120 ma the data combined with that from other parts of the kaapvaal craton further supports the view that the evolution of the craton was long lived and episodic and that it grew by accretionary processes becoming generally younger to the north and west of the c 3 5 ga barberton swaziland granite greenstone terrane situated in the southeastern part of the craton

tourmaline b isotopes as tracers of fluid sources in silicified palaeoarchaean oceanic crust of the mendon formation barberton greenstone belt south africa pervasive silicification is a common phenomenon in early archaean volcano sedimentary sequences reflecting the interaction between si rich fluids and the archaean oceanic crust in the palaeoarchaean barberton greenstone belt south africa the silicified zones locally contain abundant tourmaline in order to constrain the source of fluids responsible for the alteration six tourmaline bearing samples from the 3 3 ga mendon formation were investigated by in situ analyses of mineral chemistry and boron isotopes the samples comprise a silicified finely laminated sedimentary chert and five highly altered and silicified komatiites electron microprobe analysis empa indicates that tourmaline is mostly dravitic with one sample between dravite and mg foitite tourmaline has a weakly developed patchy colour and chemical zonation with variations in al fe and mg composition some samples show strong zoning in cr concentration with cr enrichment in the core and locally the high cr domains are at contacts with other cr rich phases such as magnesiochromite and chromian muscovite fuchsite overall the boron isotope composition 11b ranges from 20 7 to 10 2 two groups of tourmaline are distinguished based on the variation in 11b ratios within single samples 1 those with small 4 range in 11b values spinifex textured komatiites and silicified sediment and 2 those with a large range in 11b values of up to 18 brecciated and foliated samples positive boron isotope values 6 to 10 are found in one spinifex textured komatiite and are interpreted as being derived from seawater interacting with archaean oceanic crust in shallow hydrothermal systems the intermediate boron isotope values 5 to 8 found in all other samples are similar to morb and may represent boron derived from the archaean oceanic crust the rare and exceptionally light boron isotope values 10 to 21 were found in rims of group 2 tourmaline these values cannot be produced by fractionation alone and point to the presence of another fluid source the source of the light boron has yet to be identified but possible candidates are remobilized boron either derived from granitic crust or from marine evaporites that precipitated from seawater enriched in 10b in any case the isotopically light boron appears to have been present during seafloor hydrothermal alteration and deposition of the mendon formation at ca 3 3 ga

3230 3200 ma post orogenic extension and mid crustal magmatism along the southeastern margin of the barberton greenstone belt south africa abstract the barberton granitoid greenstone terrain south africa preserves a complex and protracted evolution involving several events of magmatism and terrain accretion along convergent tectonic boundaries recent studies propose that the main period of tectonic accretion and arc related magmatism is linked to a system of divergent subduction zones above which voluminous ttg magmas were emplaced between ca 3236 and 3227 ma our structural and la icp ms u pb geochronology study along the southeastern margin of the barberton greenstone belt bgb ties the waning stages of this ttg magmatism to a short ca

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

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

evolution of the archean continental crust some 20 years of plate tectonic theory combined with new insights into the fine structure of the lithosphere the application of multielement geo chemical and isotopic studies paleomagnetism and geophysical modeling of mantle processes have profoundly influenced present thinking on the origin and evolution of the earth s early continental crust previously our knowledge of the continental crust was based almost exclusively on field geological observations although there is now general agreement on how the earth worked for the last 200 m y because of observable evidence in the oceans and continents e g bird 1980 condie 1982 it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre mesozoic rock relationships in the continents dewey 1982 however preserved characteristic rock as semblages uniquely identifying modern type wilson cycle processes i e opening and closure of oceans underlain by oceanic crust have now been recognized in continental terranes as old as 900 m y and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late precambrian kroner 1977 1981a b goodwin 1981 profound disagreement on the older crustal history however prevails to the present day since typical features characterizing phanerozoic accretionary terranes such as obducted ophiolites blueschists and franciscan type melanges have not been found in more ancient regions thus two types of evolutionary models have been developed one type postulates uniformitarian development back to the earliest archean burke

evidence from zircon dating for existence of approximately 2 1 ga old crystalline basement in southern bohemia czech republic zircon ages are reported for three moldanubian amphibolite grade orthogneisses from the southern bohemian massif obtained by conventional u pb analyses for two of these orthogneisses conventional u pb data are supported by ion microprobe single zircon ages or single grain evaporation data the amphibolite grade orthogneisses occurring in three small tectonic lenses within the varied group close to the south bohemian main thrust are of tonalitic granodioritic or quartz dioritic composition

juvenile crust formation in the northeastern kaapvaal craton at 2 97 ga implications for archean terrane accretion and the source of the pietersburg gold abstract combined u pb and lu hf isotope data of detrital and magmatic zircon grains provide evidence that the murchison greenstone belt ne kaapvaal craton south africa hosts an important archean suture zone which primarily formed by a collision 2 97 ga ago between a back arc system to the north and an evolved terrane the proto kaapvaal craton to the south this model is supported by the observations that igneous rocks of the northern terrane comprising mafic rocks and tonalites of the rooiwater complex and felsic volcanics of the rubbervale formation were both emplaced at about 2 97 ga and show highly superchondritic hft of 4 4 to 5 1 in contrast u pb hf isotope data of detrital zircon grains from quartzitic schists of the murchison unit and the la france formation both part of the southern terrane provide evidence for a southern provenance which was affected by granitoid intrusions accompanied by crust re working at 3 53 3 42 ga hft 1 8 to 4 8 3 30 3 20 ga hft 1 8 to 6 3 and 3 13 3 05 ga hft 1 3 to 5 6 pointing to a connection with the barberton greenstone belt and swaziland a spatial separation of the two terranes prior to 2 97 ga is furthermore supported by the facts that the youngest detrital zircon grains of the southern terrane having ages between 2 99 and 2 97 ga show subchondritic hft between 6 5 and 1 5 in contrast to the highly superchondritic hft obtained from the contemporaneous magmatic rocks of the northern terrane the juvenile character of the rooiwater and rubbervale magmatic rocks along with the occurrence of vms deposits makes them a likely candidate for the source of the gold pyrite bearing conglomerates exposed in the adjacent pietersburg greenstone belt

u pb zircon and sm nd dating of moldanubian hp ht granulites from south ohemia czech republic conventional multigrain u pb systematics of zircon and single grain evaporation data of zircons in combination with nd model ages are presented for pyroxene free felsic garnetiferous granulites and orthopyroxene bearing granulites of felsic mafic and intermediate composition from the blansky les mountains granulite complex s bohemia zircons from mafic and intermediate granulites yield concordia intercept ages between 346 5 and 351 6 ma as well as a single grain evaporation age of 346 12 ma these ages define the time of variscan high grade metamorphism in s bohemia and are supported by an internal sm nd garnet zircon plagioclase whole rock isochron age of 343 21 ma euhedral zircons from felsic granulites of probable granitic derivation yield u pb ages of 366 5 ma and 373 12 ma interpreted as intrusion ages of the granitic precursor an inherited proterozoic crustal component is documented by poorly defined concordia upper intercept ages around 1700 ma and nd model ages between 1 3 and 2 1 ga

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

iib trioctahedral chlorite from the barberton greenstone belt crystal structure and rock composition constraints with implications to geothermometry iib trioctahedral chlorite in the barberton greenstone belt bgb metavolcanic rocks was formed during pervasive greenschist metamorphism the chem ical composition of the chlorite is highly variable with the fe fe mg ratio ranging from 0 12 to 0 8 among 53 samples the chemical variation of the chlorite results from the chemical diversity of the host rock especially the mgo content of the rock but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite all major cation abundances in the chlorite are strongly correlated with each other sil icon increases with mg and decreases with fe while aliv and alvi decrease with mg and increase with fe2 a complex exchange vector explains over 90 of the chlorite compositional variation mg4sife2 3alvi 1 aliv 1 which has 3 parts fe mg substitution coupled with one part tschermakite substitution this ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets the subtle change in al substitution in chlorite implies that alvi is preferentially ordered in the m 4 site and about 84 of the alvi present is in the m 4 sites when they are nearly filled with alvi based on 47 analyzed chlorite bearing rock samples chlorite chl composition is strongly correlated with the mgo content of the host rock calculated correlation coefficients are 0 91 for sio2chl mgorock 0 87 for al2o3chl mgorock 0 89 for mgochl mgorock and 0 85 for feochl mgorock only weak correlations have been found between chlorite oxides and other oxides of rock between same oxides in chlorite and rock sio2 0 67 al2o3 0 59 feo 0 41 however mgochl is saturated at about 36 wt in rocks that have mgo above 22 wt the mgochl is about 5 wt when the host rock approaches 0 wt of mgo this implies that mg substituting into the chlorite is approximately limited to 1 5 9 2 mg atoms per formula unit and 1 0 3 2 aliv chlorite geothermometers can not be applied to all bgb samples however the empirical chlorite geothermometer based on aliv of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study an estimated temperature of about 320 c for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers

temperature and salinity records of the tyrrhenian sea during the last 18 000 years abstract we have used the modern analogue technique to derive sea surface temperature sst records of the last 18 000 years from two well dated deep sea cores recovered in the tyrrhenian sea results show two major cooling events synchronous with the european and north atlantic oldest and younger dryas events a later cooling of lower amplitude is observed between about 8000 and 4500 yr bp tyrrhenian ssts were similar to those of the upper holocene only during the bolling allerod event and at the beginning of the holocene we compared the sst and the planktonic foraminiferal oxygen isotope records to estimate the surface salinity variations our reconstruction shows that salinity decreased in the tyrrhenian sea during the three cold events whereas high ssts correlate with high salinities during most of the last 18 000 years tyrrhenian sea salinity variations were similar to those of the north atlantic ocean near the entrance of the mediterranean sea the salinity gradient between the two basins was thus constant indicating that the western mediterranean circulation pattern and the freshwater budget were not significantly different from the modern one the case was however different during the middle holocene when the strong salinity decrease associated with the sapropel deposit in the eastern mediterranean basin had no equivalent in the north atlantic ocean this peculiar event can only be explained by a local climatic change and an increase of precipitation

stratigraphic and sedimentological evidence bearing on structural repetition in early archean rocks of the barberton greenstone belt south africa abstract the 3 5 3 3 ga old swaziland supergroup in the barberton greenstone belt south africa includes a lower predominantly volcanic unit the onverwacht group and an upper largely sedimentary section the fig tree and moodies groups although viljoen and viljoen concluded that the onverwacht group exceeds 15 km in stratigraphic thicknesses more recent workers have suggested that it represents a much thinner section of rocks repeated by folding and faulting our mapping indicates that the upper half of the group includes at least 8 km of unrepeated stratigraphic section inferences of structural repetition have arisen in part because a major shear zone is present in the upper half of the onverwacht group on the west limb of the onverwacht anticline shearing and the overturning of large blocks of the igneous sequence have combined with the later intrusion of felsic magma to produce a generally stratiform highly complex zone of deformation up to 2 km wide the displacement across the zone is unknown but the presence of unfaulted sections of this part of the stratigraphic sequence indicates that the gross stratigraphic arrangement of units within the group has not been significantly altered structures previously mapped as major fold hinges within the upper part of the onverwacht group can be shown to have formed by local deformation and reflect neither large scale folding nor structural repetition of the rocks several kilometers higher in the section rocks of the uppermost onverwacht and fig tree groups are stacked and repeated in a series of thrust nappes individual nappes extend for up to 15 km along strike and include as much as 900 m of allochthonous volcanic and sedimentary rocks they were apparently emplaced by faulting or gravity sliding at or near the earth s surface their formation involved the movement of sheets made largely up of sedimentary rocks along shear surfaces within underlying much weaker serpentinized ultramafic komatiites locally the sheets were deformed to form fold nappes the lower limbs of the fold nappes commonly show extreme attenuation and brittle deformation whereas the upper limbs are largely intact the thrust faults are the loci of small hypabyssal igneous bodies that appear to be co magmatic with a suite of upper fig tree volcanic rocks this style of faulting has not affected rocks of the overlying moodies group although estimates of the timing of deformation are as yet imprecise these features suggest that it occurred during a late fig tree phase of tectonism and volcanism

felsic igneous rocks within the 3 3 to 3 5 ga barberton greenstone belt high crustal level equivalents of the surrounding tonalite trondhjemite terrain emplaced during thrusting felsic igneous rocks within the circa 3 5 ga ophiolitic onverwacht group of the barberton greenstone belt have traditionally been mapped as recurring volcanic units within a continuous stratigraphic succession these felsic units have been interpreted as part of several mafic to felsic volcanic cycles making up this succession our field and geochemical data show that the felsic igneous rocks are not closely related to the mafic ultramafic rocks of the onverwacht group rather the data indicate that these felsic units are high level equivalents of the widespread gneissic to isotropic trondhjemite tonalite plutons which surround and intrude the greenstone belt the felsic igneous rocks are predominantly shallow level intrusions and subsurface felsic domes associated with minor volcanics and volcanoclastics and postdate the onverwacht group by 40 50 ma the felsic intrusions were preferentially emplaced along thrust zones cutting the ophiolite basement and its overlying sedimentary cover structural and chemical data imply that the hydrated ophiolitic rocks of the barberton greenstone belt are allochthonous and were thrust over similar simatic rocks from which the felsic igneous rocks were syntectonically derived by partial melting such structural imbrication of simatic crust can be best incorporated in a model advocating archean intraoceanic obduction associated with shallow angle subduction similar to that documented for the neotectonics of the western aleutians

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

age and tectonic setting of granitoid gneisses in the eastern desert of egypt and south west sinai strongly deformed and locally migmatized gneisses occur at several places in the southern eastern desert of egypt and in sinai and have variously been interpreted as a basement to pan african 900 to 600 ma supracrustal and intrusive assemblages a suite of grabbroic to granitic gneisses was investigated in the hafafit area which constitutes an i type calc alkaline intrusive assemblage whose chemistry suggests emplacement along an active continental margin and whose granitoid members can be correlated with the so called older granites of egypt 207pb 206pb single zircon evaporation from three samples of the hafafit gneisses yielded protolith emplacement ages between 677 9 and 700 12 ma and document granitoid activity over a period of about 23 ma a migmatitic granitic gneiss from wadi bitan south west of ras banas has a zircon age of 704 8 ma and its protolith was apparently generated during the same intrusive event as the granitoids at hafafit single zircons from a dioritic gneiss from wadi feiran in south west sinai suggest emplacement of the protolith at 796 6 ma and this is comparable with ages for granitoids in north east sinai and southern israel none of the above gneisses is derived from remelting of older continental crust but they are interpreted as reflecting subduction related calc alkaline magmatism during early pan african magmatic arc formation

early archaean spherule beds in the barberton mountain land south africa no evidence for impact origin abstract detailed petrographical and geochemical studies are reported for several well documented occurrences of spherule rich layers in the barberton mountain land south africa which were previously inferred by lowe and co workers to be the result of early archaean meteorite impacts in contrast we made the following observations the textures of most spherules are not necessarily the result of impact but could instead be the product of radial or intersertal growth of crystals during secondary mineral formation there is no difference in the content of siderophile elements between spherule layers and layers devoid of spherules the siderophile element abundances are high where sulphide minerals e g pyrite gersdorffite and chalcopyrite and or chromite are present independent of the presence or absence of spherules high contents of e g ir up to 2700 ppb ni 0 96 wt and cr 1 6 wt were found in various samples abundances of these elements in chondritic meteorites are approximately 600 ppb 1 4 wt and 0 35 wt respectively resulting in respective meteoritic components in these samples of 450 70 and 460 we do not accept these high concentrations as primary meteoritic signatures impact melt rocks have typically 1 of a meteoritic component furthermore spherules of any kind are very rare in known impact deposits including the k t boundary if present at all they are not usually associated with any significant ir or pge anomaly the high abundances of the siderophile elements in some barberton samples and their enrichment in secondary minerals indicate that these elements have been remobilized and reconcentrated the pge interelement ratios have changed during remobilization as well thus the pge abundance patterns and ratios are not primary and cannot be used as an argument in favour of an impact origin either nickel rich cr spinels which are found in some of the barberton spherule samples have low fe 3 fe total ratios these ratios are incompatible with an extraterrestrial or impact origin because such impact derived spinels including those found at the k t boundary are highly oxidized and also have different chemical compositions there is also no indication of any evidence of shock metamorphism associated with the barberton spherule layers which is the commonly accepted definitive criterion for recognition of an impact origin this is unusual because such evidence is preserved even in heavily altered samples from deeply eroded archaean impact structures we conclude that while these spherule layers are clearly unusual and deserve further attention there is no convincing evidence for an origin by impact we suggest that they were formed by volcanic processes followed by extensive hydrothermal alteration

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

chemical variations in clay minerals of the archaean barberton greenstone belt south africa abstract four rock profiles of shales carbonates and greywacke type sediments were sampled from the mafic compositional influenced sedimentary fig tree group and from overlying more felsic related sedimentary moodies group of the archaean barberton greenstone belt in south africa clay separates of forty sedimentary rocks were examined by xrd and sem to study the mineralogical chemical and morphological variations of the constitutive illite and chlorite the clay minerals showed a significant k enrichment in the illites and mg and fe depletions in the chlorites upwards the stratigraphic sequence three independent parameters influenced these variations 1 a change in the origin of the source rocks 2 the host rock lithologies and 3 a tectono thermal recrystallization process

pb loss patterns in zircons from a high grade metamorphic terrain as revealed by different dating methods upb and pbpb ages for igneous and metamorphic zircons from northern sri lanka abstract we report u pb and 207 pb 206 pb ages for multigrain fractions single grains and grain domains in zircons from highgrade igneous and sedimentary rocks of the vavuniya charnockite province vcp in northern sri lanka using the ionmicroprobe shrimp a new vapour digestion technique vdt and evaporation charnockitic enderbitic and mangeritic gneisses are derived from i type tonalitic syenitic granodioritic and granitic precursors for which the u pb systematics of zircons as determined by shrimp suggest crystallization ages between 1000 and 1100 ma many translucent round and multifacetted zircons in these rocks are not of metamorphic but of igneous origin and acquired their shape through partial zircon dissolution and recrystallization the large majority of zircons exhibit severe and variable pb loss some 540 580 ma ago which we ascribe to granulite grade metamorphism almost immediately followed by local retrogression most of the zircons lost pb from the entire grain with no domains remaining that reflect the original pb isotopic composition it was therefore impossible to obtain meaningful 207 pb 206 pb evaporation ages for such grains and this cautions against use of this method in zircons that experienced severe pb loss however zircons formed during high grade metamorphism apparently retain their primary pb isotopic composition and were dated successfully by the evaporation technique yielding ages between 553 26 and 560 30 ma conventional multigrain analysis and small fractions following vdt for two samples although much more precise than the shrimp data yielded only strongly discordant data points with patterns reflecting variable pb loss these data alone would not have made it possible to obtain reliable primary zircon crystallization ages 207 pb 206 pb evaporation ages for detrital zircons from two garnetiferous gneiss samples of sedimentary origin in the centre of the vcp revealed a maximum age of 1329 18 ma whereas a metaquartzite near the southern margin of the vcp contained zircons with 207 pb 206 pb ages between 1857 14 and 2543 14 ma we interpret the latter rock as belonging to the highland complex which occurs adjacent to the vcp our zircon data in combination with nd model ages milisenda et al 1994 suggest the vcp to be part of the wanni complex a distinct crustal domain in northern sri lanka characterized by relatively young ages in comparison with much older ages in the adjacent highland complex

deformed a type granites in northern malawi east central africa pre or syntectonic metamorphosed and deformed a type granites ranging in age from 1087 to 1119 ma occur in strike slip and reverse slip shear zones of the ubendian and irumide belts of northern malawi peak metamorphic conditions in the granites and their wallrocks reached about 12 13 kbar and 680 740 c microstructures reveal that the granites experienced a seemingly progressive deformation from magmatic to solid state magmatic deformation is manifested by the coexistence of aligned magmatic potassium feldspar plagioclase amphibole biotite and ilmenite with undeformed quartz and schlieren structures high temperature solid state deformation is characterized by widespread grain boundary migration dynamic recrystallization and myrmekitic potassium feldspar replacement the tectonic foliation is generally subparallel to the magmatic foliation in the granites and also subparallel to the borders of the plutons and to the foliation in the wallrock kinematic indicators in both granite and wallrock reveal sinistral movement in the ubendian belt shear zones and top to the ese thrusting in the irumide belt shear zones the overall parallelism of magmatic and tectonic foliations would suggest syntectonic emplacement of the granites with respect to a c 1100 ma irumide orogeny however similar deformation patterns of granite related dykes and some pan african pegmatites and geochronologic work suggests that the first deformation of the granites their related dykes and some of the pegmatites occurred during the pan african orogeny this study implies that a purely structural approach to distinguish syn and pretectonic granites is rendered difficult when post emplacement deformation occurred under high grade metamorphic conditions strongly heterogeneous deformation with a pronounced degree of coaxial flattening in the granites allows magmatic structures to be preserved in low strain zones and to be passively rotated into parallelism with penetrative deformation structures our work also implies that there was no irumide orogen in northern malawi

the non glacial and non cratonic origin of an early archean felsic volcaniclastic unit barberton greenstone belt south africa abstract recent studies have suggested that 3 445 ga felsic volcaniclastic rocks of member h6 of the hooggeneoeg formation in the barberton greenstone belt south africa show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older granitic continental crust these conclusions represent fundamental re interpretations of this sequence its crustal setting and processes of deposition we here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits storm influenced shallow marine units and turbidity current like storm driven submarine flows all of which lack any evidence of a glacial influence during sedimentation they accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface the debris consists entirely of volcanic derived materials no clasts of plutonic rock are found in the conglomerates low temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline previously thought to be characteristic of granitic plutonic rocks this potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the greenstone belt this succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center

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

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

pb pb age stable isotope and chemical composition of archaean magnesite barberton greenstone belt south africa abstract trace element carbon oxygen and lead isotope analyses were carried out to determine the formation mode and crystallization age of magnesite from the budd ultramafic complex of the archaean barberton greenstone belt south africa its significantly high ti contents probably relate to a soluble ti rich accessory mineral probably dissolved during magnesite precipitation primitive mantle normalized ree patterns of the magnesite show negative ce and eu anomalies induced by two events 1 the eu anomaly indicates reducing conditions probably induced by the emplacement of the ultramafic source rock 2 the ce anomaly implies oxidizing conditions probably during a hydrothermal event that favoured the precipitation of magnesite in veins of the rock this ce anomaly is among the oldest in earth history negative 13 c 2 9 to 3 3 pdb and high 18 o 30 5 31 2 smow suggest low temperature precipitation in a near surface kraubath type epithermal system the magnesite contains considerable amounts of radiogenic lead 206 pb 204 pb is 125 5 153 7 and 207 pb 204 pb is 41 3 47 7 the resulting pb pb age of 3043 59 ma is interpreted as dating a hydrothermal event related to the extensive plutonism episode that has been reported in the barberton area at about 3 1 ga previous models that proposed formation of the magnesite under recent climatic conditions can be discarded

provenance and tectonic implications of the 3 28 3 23 ga fig tree group central barberton greenstone belt south africa abstract sedimentary rocks provide a valuable record of non uniformitarian paleoarchean tectonic processes the rocks of the 3 28 3 23 ga mapepe formation mark the initiation of orogenesis in the barberton greenstone belt south africa however reconstruction of the mapepe basin s is challenging because the rocks occur in fault bounded structural belts that are characterized by lateral heterogeneity and whose correlation remains uncertain we have studied the petrography geochemistry and detrital zircon geochronology from outcrop and drill core samples from two adjacent structural belts the manzimnyama syncline and the eastern barite valley area to evaluate the timing of basin formation correlate between these two adjacent areas and provide constraints on the character of tectonic uplift in the eastern barite valley area sedimentation was initiated at about 3260 ma the section includes a lower deep water unit of mainly mudstone overlain by a shoaling upward sequence of fan delta sandstone and conglomerate the sandstone throughout the formation was derived by erosion of local uplifts of the underlying greenstone belt sequence except at the top of the section where sedimentation was dominated by dacitic volcaniclastic sediments dated at 3239 ma in contrast rocks of the mapepe formation in the manzimnyama syncline area 1 2 km south of the eastern barite valley area record the initiation of sedimentation at about 3277 ma with deposition of a rhyolitic tuffaceous unit followed by persistent deep water sedimentation of banded iron formation banded ferruginous chert and turbiditic sandstone siliciclastic sediments are immature and were derived by erosion of older supracrustal greenstone belt rocks no fig tree age volcanism younger than 3260 ma is recorded in this belt overall mapepe sedimentary rocks record the formation of numerous local uplifts of underlying supracrustal rocks with periodic volcanic activity and the dispersal of debris to form mainly small fan deltas flanked by deep water systems the contrasting ages lithologies depositional settings and provenance of the sedimentary rocks in the now adjacent eastern barite valley and manzimnyama syncline belts suggest that they were deposited in either different parts of a large complex fig tree basin or represent separate basins that have been juxtaposed during post fig tree tectonism and shortening

non subduction origin for 3 2 ga high pressure metamorphic rocks in the barberton granitoid greenstone terrane south africa the occurrence of high pressure hp garnet bearing metamorphic rocks in the southern barberton granitoid greenstone terrane bggt south africa has been proposed as a key indicator of the onset of modern plate tectonics at ca 3 2 ga here we report new zircon titanite u pb ages of garnet bearing hp metamorphic rocks and associated granitoids in the bggt that argue against such an interpretation the results show that hp metamorphism occurred synchronously with granitoid magmatism in the stolzburg domain the supposed subducted plate during two episodes at 3 4 and 3 2 ga and that these two episodes of magmatism occurred on both sides of the assumed suture zone a feature that cannot easily be explained by the subduction model in contrast the coupled magmatism and metamorphism probably resulted from partial convective overturn pco another viable mechanism for production and differentiation of continental crust during early archean

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

geophysics a plate boundary in flux the pacific and australian plates collide and interact in complex ways around new zealand electrical resistivity data reveal that subduction zone fluids exert an important influence on deformation in the region

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

trace element zoning of sulfides and quartz at sheba and fairview gold mines clues to mesoarchean mineralisation in the barberton greenstone belt south africa abstract the fairview and sheba mines are two of the major gold mines in the paleoarchean barberton greenstone belt of southern africa at these mines gold is associated with quartz carbonate rutile veins and occurs both as invisible gold finely dispersed in sulfides primarily pyrite and arsenopyrite and as visible electrum grains hosted in pyrite up to approximately 1000 ppm au are contained in pyrite and up to approximately 1700 ppm in arsenopyrite mapping of trace element distribution in sulfide minerals using electron microprobe and proton probe techniques revealed multiple events of ore formation and au mineralisation at fairview mine three stages of pyrite formation were identified the last of which is associated with arsenopyrite electrum and other sulfide minerals sphalerite chalcopyrite galena gersdorffite and sb sulfides at sheba mine pyrite was deposited in two stages and electrum is associated with the second stage at both mines the last stage of sulfide formation is the main stage of au deposition and is associated with mobilisation of au as sb cu zn and ni the host rock composition seems to have affected the composition of pyrite since higher ni and co concentrations up to 1 4 and 1 6 wt respectively have been measured in meta ultra mafic host rocks in comparison with chert and metagreywacke arsenopyrite is chemically zoned and has sb and s rich cores and as and ni rich rims this zoning indicates variations in fluid compositions decreasing sb and increasing ni and crystallisation conditions increasing as content for increasing temperature geothermometric estimates based on the as content of arsenopyrite as 32 at indicate temperatures up to 420 c for the crystal rims petrographic and cathodoluminescence observations of quartz associated with gold mineralisation show only local brittle deformation and no plastic deformation this supports the notion that the ore transporting veins were emplaced late in the deformation history variations of cathodoluminescence of quartz are correlated with changing al contents al 0 16 wt and can be related to fluctuations in the ph of the mineralising fluids

vaalbara earth s oldest assembled continent a combined structural geochronological and palaeomagnetic test the only remaining areas of pristine 3 6 2 7 ga crust on earth are parts of the kaapvaal and pilbara cratons general similarities of their rock records especially of the overlying late archean sequences suggest that they were once part of a larger vaalbara supercontinent here we show that the present geochronological structural and palaeomagnetic data support such a vaalbara model at least as far back as 3 1 ga and possibly further back to 3 6 ga vaalbara fragmented prior to 2 1 ga and possibly as early as 2 7 ga suggesting supercontinent stability of at least 400 myr consistent with neoproterozoic and phanerozoic analogues

oldest reliable 40ar 39ar ages for terrestrial rocks barberton mountain komatiites we report here the first 40ar 39ar ages for komatiites and komatiitic basalts from the barberton mountain greenstone belt on the transvaal swaziland border in southern africa1 both rock types display remarkable argon retentivity while some variation is found among the samples the best argon age estimate for the time of metamorphism is in the 3 450 3 490 myr range this age is only slightly less than that found in komatiites from the same area by sm nd dating 3 540 15 myr 2 within the uncertainties of the decay constants involved the two age estimates are not different the results show that the principal pervasive greenschist metamorphism1 in the area must have occurred within 100 myr of the eruption of the komatiite analytically precise ages were obtained despite potassium contents as low as 80 p p m in the presence of high calcium concentrations 3 5 ca to our knowledge the results represent by far the oldest reliable ages obtained for terrestrial rocks using the k ar system