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

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

archaean granulite facies paragneisses from central swaziland inferences on palaeoarchaean crustal reworking and a complex metamorphic history we present a petrographic petrological geochemical and geochronological study u pb lu hf on granulite facies paragneisses of the mahamba gneiss complex in central swaziland eastern kaapvaal craton our data suggest that prograde metamorphism occurred at c 3 07 ga dating of detrital zircons of a metapelite in combination with geochronological and ambiguous structural relationships with granitoid gneisses suggests two possible scenarios 1 the time of deposition of the sedimentary protoliths is prior to c 3 58 ga 2 c 3 58 ga granitoid crust was the basement for the sedimentary protoliths furthermore enrichment in ni and cr in the mahamba metasediments may originate from erosion of a greenstone terrane similar in composition to the dominantly mafic and ultramafic lithologies of the barberton greenstone belt or dwalile supracrustal belt evidence for an older crustal basement is provided by the oldest detrital zircons which yield ages up to c 3 7 ga because the metamorphic conditions of metapelitic rocks have no similarity to p t paths of modern subduction collision tectonic settings we conclude that the metamorphic history of the mahamba gneiss complex cannot be explained by this model we propose that deformation and metamorphism occurred through intracratonic contraction and a thermal event possibly related to mpuluzi batholith emplacement supplementary material sample localities mineral chemistry analytical methods and mineral equilibria modelling method are available at www geolsoc org uk sup18786

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

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

determining accurate temperature time paths from u pb thermochronology an example from the kaapvaal craton southern africa thermochronology has revolutionized our understanding of the establishment and evolution of lithospheric thermal structure however many potential benefits provided by the application of diffusion theory to thermochronology have yet to be fully exploited this study uses apatite tc 450 550 c and titanite tc 550 650 c u pb id tims thermochronology at the single to sub grain scale to separate the variable effects of volume diffusion of pb from metamorphic over growth above and below the tc of a mineral data are presented from two ca 3227 ma tonalite samples from north and south of the barberton greenstone belt bgb southern africa two distinct populations of apatite from a sample north of the bgb record fast cooling followed by metamorphic growth 10 myr later both apatite and titanite dates from south of the bgb show a strong correlation with the grain size and record 100 myr of post emplacement cooling complex core rim zoning observed in cathodoluminescence images of apatite is interpreted to reflect metamorphic overgrowth above the tc the age and topology of grain size versus date curves from titanite and apatite are used in combination with a finite difference numerical model to show that slow non linear cooling and not thermal resetting is responsible for the observed distribution the thermal histories from either side of the bgb are very different and provide unique insight into the bgb s tectonic evolution a 70 myr period of apparent stability after ca 3 2 ga terrane assembly was followed by fast exhumation south of the bgb that led to lower crustal melting and intrusion of granitic batholiths ca 3 14 3 10 ga

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

tide mediated warming of arctic halocline by atlantic heat fluxes over rough topography atlantic water brings heat to the subsurface arctic ocean pan arctic microstructure measurements of energy dissipation suggest that vertical mixing is substantial over the continental slopes tidally induced and insensitive to sea ice cover

sm nd studies of archaean metasediments and metavolcanics from west greenland and their implications for the earth s early history metasedimentary and metavolcanic rocks from the archaean of west greenland have been examined for evidence of crustal components greater than 3 8 ga in age and for their compatibility with the presently adopted bulk earth sm nd parameters sm nd isotopic data have been obtained for the garbenschiefer metagabbro unit metasediments from the isua supracrustal belt gneisses interior to the isua belt and metasediments from the malene supracrustal belt using estimates of emplacement age t of between 3 77 and 3 67 ga for the parental volcanics to the garbenschiefer unit initial143nd 144nd ratios yield positiveendt values between 1 0 and 3 1 relative to the chur parameters for seven out of eight samples model sm nd ages for the isua gneisses and metasediments are only compatible with their estimated stratigraphic ages if their sources were ca 2end relative to chur at those times similarly model sm nd ages for the malene samples are only compatible with stratigraphic age constraints when based on a source evolution with positiveendt implications of these results for the early development of the earth s mantle are discussed

geochemical characteristics of archaean and late proterozoic to palaeozoic fine grained sediments from southern africa and significance for the evolution of the continental crust chemical data for pelitic rocks shallow marine platform sediments of the 2 9 ga old pongola supergroup south africa and the 550 700 ma old nama group namibia revealed clearly different compositions in both groups a comparison of the pongola and nama data with literature values for sedimentary rocks shows a trend for cr tio2 and cr zn ratios as well as the absolute cr and ni concentrations to lower values with decreasing age whereas the cr ni ratios seem to remain broadly constant through time ree patterns of pongola and nama sediments show no significant difference thus giving the impression of a post archaean origin for the demonstrably archaean pongola sediments however a plot of the trace element data for these sediments normalized to the average shale of turekian wedepohl 1961 shows the pongola shales to have a clear archaean pattern whereas the nama pelites display a typical post archaean signature the results of this investigation confirm a model of rapid evolution of the earth s upper continental crust from a predominantly mafic composition in the early archaean to a more felsic character with increasing differentiation the geochemistry of post archaean sediments may reflect the onset of cannibalistic recycling of older crust as proposed byveizer 1973 and the end of major crustal growth

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

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

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

3820 ma zircons from a tonalitic armisoq gneiss in the godthab district of southern west greenland abstract zircons from a sample of the tonalitic amitsoq gneisses near godthhb west greenland exhibit several stages of crystal growth measurements of the u th pb isotopic composition of 30 m areas within individual grains using the ion microprobe shrimp reveal that components of magmatic zircon from the original igneous protolith are at least 3822 5 ma l old slightly but significantly older than the age of zircons from a fclsic volcanic unit in the isua supracrustal belt as measured at high precision by the same technique and older than all ages previously determined by mineral and whole rock isotopic techniques for amitsoq gneisses at godthab and at isukasia overgrowths of younger zircon formed at ca 3630 ma coinciding with an episode of major pb loss from the older grains these results indicate that previous whole rock isochron ages refer to later isotopic redistribution and or that other phases of the amitsoq gneisses intruded during this later period and dominate the isotopic systematics comparison of the ion probe results with previous conventional u th pb data for the same zircon sample demonstrates the strong influence of discordant high uranium parts of zircon on the mean isotopic composition of multi grain samples

the cr th ratio in precambrian pelites from the kaapvaal craton as an index of craton evolution the cr th ratio in precambrian pelites from the kaapvaal craton in southern africa correlates with pelite th sc and co th ratios and appears to monitor changes in source composition it is a more sensitive indicator of provenance than other commonly used ratios including eu eu la yb and th u pelite cr th ratios are controlled most strongly by the effects of local provenance and local tectonics secular decreases in this ratio the most prominent of which occurred at 2 4 2 3 ga in pelites of the kaapvaal craton record rapid uplift exposing granite decreases in the ratio reflect increases in the abundance of komatiite basalt in sediment sources in response to arc collisions rift hotspot volcanism or intracratonic uplift of older greenstones relatively constant cr th ratios appear to require widespread uplift or and efficient sediment mixing during tectonic stability changes in cr th ratio of kaapvaal pelites reflect a complex tectonic history of the craton from about 3 3 ga to 1 8 ga and do not favor single stage cratonization in the late archean

sm nd age and isotopic systematics of the bimodal suite ancient gneiss complex swaziland studies of the development and stabilization of the archaean crust often focus on the relative temporal relationships between the metamorphosed basaltic to ultramafic volcanic units greenstone belts and the sialic gneiss terrains that make up the oldest sections of the terrestrial crust at the heart of this interest are the questions of the processes responsible for crust formation in the archaean and whether or not the various units of an archaean crustal section represent new additions to the crust from the mantle or are products of the reprocessing of even older crustal materials one area where this controversy has been particularly pronounced is the archaean crustal section of south west africa1 6 the oldest rocks in the kaapvaal craton consist of the onverwacht group of mafic to ultramafic metavolcanics of the barberton greenstone belt and a grey gneiss complex termed the ancient gneiss complex agc of swaziland we report here the results of a whole rock sm nd isotopic study of the agc and the implications these data may have for crustal evolution in the kaapvaal craton

ree geochemistry and isotopic data of archean silicic volcanics and granitoids from the pilbara block western australia implications for the early crustal evolution eighteen silicic volcanic rocks of the warrawoona group and ten associated plutonic rocks from the pilbara block western australia have been chosen for geochemical and isotopic studies silicic volcanics of the unsb upper member of north star basalt are dated at 3 56 3 57 by both the rb sr and the sm nd methods the respective 1 initial isotopic composition values are 0 7005 5 sr and 0 50810 39 nd this age is consistent with the stratigraphic interpretation that the talgatalga subgroup in which the north star basalt occupies the lowermost position is overlain by the duffer formation whose age was earlier established at 3 45 by the zircon u pb method the new rb sr data on six silicic lava samples from the duffer formation yield an isochron of 3 23 0 28 2v though imprecise this age agrees with the zircon age within error limits rb sr ages of 2 3 2 4 obtained for the panorama rocks and the wyman formation do not correspond to their initial eruption ages chemical arguments suggest that these ages represent the time of metasomatism associated with the widespread thermal event in this region about 2 3 2 4 ago geochemically most of these analyzed rocks volcanic and plutonic are of tonalite trondhjemitegranodiorite ttg composition a typical feature found in many other archean terrains they generally show fractionated ree patterns except the panorama formation rocks furthermore the wyman formation rhyolites and the post tectonic adamellites show significant negative eu anomalies suggesting a similar mode of magma generation and a probable genetic link theoretical considerations suggest that most of these ttg rocks could have been generated by partial melting of amphibolitic or basaltic sources followed by fractional crystallization although the archean granitic gneisses often possess mantle like isr values the trace element data indicate that they could not have been derived by direct melting of upper mantle materials the immediate tectonic implication is that in any archean terrain the formation of na rich continental crust of ttg suite must be preceded by the presence of basaltic crust the occurrence of this basaltic crust is a matter of controversy such crust might have been totally destroyed by repeated melting processes or its remnants are now represented by some of the mafic ultramafic enclaves within the tonalite trondhjemite batholiths

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

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

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

early archaean zircon ages from orthogneisses and anorthosites at mount narryer western australia abstract u th pb isotopic analyses have been made with an ion microprobe of 20 30 m areas within zircons from gneisses adjacent to the mount narryer quartzite western australia in which detrital zircons up to 4200 ma old have been found the cores of the zircons yield protolith crystallization ages of 3678 6 ma 95 confidence limits for the meeberrie banded monzogranite gneiss and 3381 22 ma for the dugel syenogranite leucogneiss in agreement with observed intrusive relationships zircons from inclusions of deformed leucogabbro and meta anorthosite within the dugel gneiss formerly part of the manfred layered igneous complex are 3730 6 ma old the remnants of the manfred complex are therefore the oldest rocks currently known on the australian continent analyses of discrete post magmatic grains and of overgrowths on older grains indicate an episode of subsolidus zircon growth in the region at 3296 4 ma which is interpreted as a time of local metamorphism of the gneisses subsequently the zircons have been rounded and embayed and have lost radiogenic pb in both ancient and recent times all the zircon ages determined in this study are within the age range for detrital zircons in the mount narryer quartzite it is therefore possible that these felsic gneisses and anorthositic rocks were a local source for some of the sedimentary material however the lateral extent of this early archaean terrain and the source rocks for the rare 4100 4200 ma old zircons in the metasediments are yet to be determined

applications of single grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites abstract 207 pb 206 pb ages determined by evaporation of single zircon grains from rocks of the new england appalachians in the usa permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age whose u pb upper intercept zircon ages obtained through isotope dilution methods show profound post crystallization discordance demonstrate that the evaporation method is successful in recovering protolith ages uncertainties are 10 15 my for rocks of paleozoic age or older preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics 207 pb 206 pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the new england appalachians reveal the distinct provenances of these rocks the missisquoi formation in southeastern vermont usa is comprised in part of detritus derived from the 1 0 1 2 ga grenvillian province of laurentia whereas this component is conspicuously absent from the plainfield formation in easternmost connecticut the detrital zircon age data suggest that the missisquoi formation accumulated peripheral to the laurentian craton during the early paleozoic whereas the plainfield formation detritus was supplied by non laurentian ossibly west african sources during the late proterozoic or early paleozoic in southern vermont the evaporation method successfully distinguishes between two groups of augen gneisses with ages of 960 ma and 1120 ma which were first recognized through isotope dilution methods collectively these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks

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

single zircon evaporation ages constraining the growth of the archaean aravalli craton northwestern indian shield single grain 207 pb 206 pb zircon ages were determined for granitoids and gneisses which constitute the archaean basement rocks of the aravalli craton of rajasthan northwestern indian shield the protolith ages for two gneisses collected from east of udaipur are 3230 ma and 2887 ma respectively the granitoids display an intrusive relationship with the gneisses and yielded ages ranging between 2666 ma and 2620 ma these ages provide the basis for a geochronological model of evolution of the oldest basement of the aravalli craton

evolution of continental crust in southern africa abstract nd isotopic data from the zimbabwe and kaapvaal cratons and the limpopo kalahari namaqualand and damara mobile belts imply that over 50 of present day continental crust in this region had separated from the mantle by the end of the archaean and that

gully head erosion processes on a semi arid valley floor in kenya a case study into temporal variation and sediment budgeting a three year monitoring programme of gully head retreat was established to assess the significance of sediment production in a drainage network that expanded rapidly by gully head erosion on the low angled alluvio lacustrine njemps flats in semi arid baringo district kenya this paper discusses the factors controlling the large observed spatial and temporal variation in gully head retreat rates ranging from 0 to 15 m a 1 the selected gullies differed in planform and in runoff contributing catchment area but soil material and land use were similar the data were analysed at event and annual timescales the results show that at annual timescale rainfall amount appears to be a good indicator of gully head retreat while at storm event timescale rainfall distribution has to be taken into account a model is proposed including only rainfall p and the number of dry days dd between storms which explains 56 per cent of the variation in retreat rate of the single headed gully of lam1 a detailed sediment budget has been established for lam1 and its runoff contributing area rca by measuring sediment input from the rca the sediment output by channelized flow and linear retreat of the gully head for nine storms it can be seen that erosion shifts between different components of the budget depending on the duration of the dry period dd between storms sediment input from the rca was usually the largest component for the smaller storms the erosion of the gully head occurred as a direct effect of runoff falling over the edge ghwaterfall and of the indirect destabilization of the adjacent walls by the waterfall erosion and by saturation ghmass storage the latter component ghmass storage was usually much larger that the former ghwaterfall the sediment output from the gully was strongly related to the runoff volume while the linear retreat because of its complex behaviour was not overall the results show that the annual retreat is the optimal timescale to predict retreat patterns more detailed knowledge about relevant processes and interactions is necessary if gully head erosion is to be included in event based soil erosion models copyright 2001 john wiley sons ltd

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

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

combined use of single zircon ages and sm nd isotopes in the analysis of early archaean crustal growth processes the ancient gneiss complex of swaziland southern africa

geochemical investigation of archaean bimodal and dwalile metamorphic suites ancient gneiss complex swaziland the bimodal suite bms comprises leucotonalitic and trondhjemitic gneisses interlayered with amphibolites based on geochemical parameters three main groups of siliceous gneiss are recognized i sio2 14 and fractionated light rare earth element ree and flat heavy ree patterns ii sio2 and al2o3 contents similar to i but with strongly fractionated ree patterns with steep heavy ree slopes iii sio2 73 al2o3 14 zr 500 ppm and high contents of total ree having fractionated light ree and flat heavy ree patterns with large negative eu anomalies the interlayered amphibolites have major element abundances similar to those of basaltic komatiites mg tholeiites and fe rich tholeiites the former have gently sloping ree patterns whereas the mg tholeiites have non uniform ree patterns ranging from flat 10 times chondrite to strongly light ree enriched the fe rich amphibolites have flat ree patterns at 20 30 times chondrite the dwalile metamorphic suite which is preserved in the keels of synforms within the bms includes peridotitic komatiites that have depleted light ree patterns similar to those of compositionally similar volcanics in the onverwacht group barberton basaltic komatiites and tholeiites the basaltic komatiites have ree patterns parallel to those of the bms basaltic komatiites but with lower total ree contents the dwalile tholeiites have flat ree patterns the basic and ultrabasic liquids were derived by partial melting of a mantle source which may have been heterogeneous or the heterogeneity may have resulted from sequential melting of the mantle source the fe rich amphibolites were derived either from liquids generated at shallow levels or from liquids generated at depth which subsequently underwent extensive fractionation

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

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