zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

polydiapirism of the archean mount edgar batholith pilbara block western australia structural analysis of the mount edgar batholith and the enveloping greenstone belt indicates that the batholith and rocks around its margin were subjected to similar deformation events that involved progressive and stepwise diapiric uplift of the batholith these events may be correlated on the basis of fold style orientation and fabric development early recumbent to reclined structures d1 including thrusts were buckled d2 by rounded upright folds prior to shearing d4 along normal faults parallel with the batholith margin d4 deformation was responsible for uplifting the batholith to its present structural position a deep seated linear deformation zone d3 within the batholith did not affect the supracrustal belt syntectonic granite intrusion amphibolite facies metamorphism and migmatisation were associated with d1 in the batholith whereas greenschist facies metamorphism was associated with d1 in the lowermost unit of the greenstone belt the talga talga subgroup amphibolite facies conditions were maintained within the batholith throughout the structural history but this grade was reached in the supracrustal belt only near the batholith margin progressive deformation d1 d2 and metamorphism occurred in response to early massive granite generation and emplacement producing a first order plutonic dome that reached full maturity during d2 solid state diapirism d4 followed producing a gneiss dome of similar dimension restriction of early recumbent folds and thrusts in the supracrustal belt to areas within or adjacent to the batholith is consistent with the model elsewhere deformation is minor the late stage diapirism brought a gneiss complex within the batholith into juxtaposition with lowermost greenschist facies rocks of the pilbara supergroup this complex represents in part sialic material that existed below the pilbara supergroup before diapirism began the complex closely resembles other archean high grade gneiss terranes which suggests that some of these terranes are deeper crustal exposures of granite greenstone terranes

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

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

geological evidence for a strewn field of impact spherules in the early precambrian hamersley basin of western australia predominantly sand sized spherules of k feldspar with spherulitic vesicular and other crystalline textures help to define a unique marker bed in the 2 6 ga wittenoom formation which persists for more than 300 km across the hamersley basin of western australia these spherules usually occur as a minor constituent of a single turbidite but locally they are restricted to centimeter thick lenses of pure spherules along a single horizon in the argillite close beneath it in the shallower northeastern part of the basin similar spherules as well as larger more heterogeneous particles likewise occur at only one horizon but here they are a minor constituent of a thick dolomitic debris flow deposit as the wittenoom formation and the carawine dolomite are thought to be stratigraphically equivalent this debris flow deposit is believed to be a proximal equivalent of the spherule bearing turbidite on the basis of their similarity to microtektites and microkrystites in shape size and mode of occurrence the spherules are interpreted as droplets of silicate melt that were generated and dispersed across the hamersley basin by a major bolide impact moreover the mass of the spherules preserved in the hamersley basin is of the same order of magnitude as the estimated masses of microtektite glass in major cenozoic strewn fields despite the fact that the spherules cover an area which is 2 to 3 orders of magnitude smaller the layers which host the spherules are interpreted as the deposits of a major sediment gravity flow which exhumed and redeposited most of the spherules after shallow burial the flow however is not believed to have been a direct result of the proposed impact if the spherule bearing layers constitute a chronostratigraphic marker as suggested here there are important implications for the deposition of carbonates and banded iron formations bif within the hamersley basin similar spherules also occur at a single horizon 250 m higher stratigraphically in the brockman iron formation suggesting that two impacts occurred near the hamersley basin in relatively rapid succession correlative strata may be present in the transvaal basin of south africa

iridium emissions from kilauea volcano during may 1983 gas and particulate samples were collected at the cooling vents of kilauea volcano in hawaii three vents on the southeast rift zone were sampled using base treated filter packs to absorb acidic gases and teflon filters for particles the samples were analyzed for 40 elements by nondestructive neutron activation analysis as with other volcanoes the chalcophilic and volatile elements were enriched by up to seven orders of magnitude relative to the erupted basalt unlike the case of other volcanoes iridium was observed to be highly enriched 105 fold at the two higher temperature vents and the degree of enrichment appears to be related to both high temperatures and high fluorine content of the gases estimates of the emission rates of ir during eruptions of kilauea based on the measurements reported here and those of the hawaiian volcano observatory yield an emission rate of about 3 g ir per 106 m3 of magma this rate amounts to only about 0 3 of the ir present in the magma that is being released by the volcano this previously unidentified ir source to the atmosphere and ocean may have a significant influence on the geochemical cycle of ir on the earth the uniqueness of kilauea in its emissions of ir is probably related to the deep source of magma which contains significant levels of ir 0 32 ppb and high levels of the halogens f and cl

asteroids and early precambrian crustal evolution abstract major enigmas arising from the archaean crustal record are capable of resolution in terms of the combined role of episodic mega impacts and of long term mantle processes the terrestrial impact record suggests that for very large impact structures the degree of preservation of proximal deformation effects decreases with the increase in the magnitude of the associated thermal effects a consequence of extensive recrystallization and melting this focuses attention on thermal igneous and metamorphic peaks as possible clues of impact events isotopic age studies define principal thermal events about 3 5 3 0 and 2 7 0 1 gyr and less well defined episodes succeeded by protracted thermal fluctuations in infracrustal high grade metamorphic zones the bulk of the igneous activity associated with these episodes took place over relatively short time spans of c 20 30 106 years interpretations of these episodes in terms of internal dynamics of the earth are inconsistent with the thermal behavior of silicate rheologies in a continuously convecting mantle regime a triggering of these episodes by the mantle rebound response to intermittent extra terrestrial asteroid impacts is considered on the basis of 1 identification of major impacts from distal ejecta horizons marked by iridium anomalies and impact earthquake triggered diamictite breccia and olistostromes 2 the relationships between microtektite horizons and volcanic activity 3 geochemical and experimental evidence for mantle upwelling from the transition zone and catastrophic adiabatic melting required to generate peridotitic komatiites episodic differentiation accretion growth of sial consequent on these events is capable of resolving the volume problem which arises from comparisons between modern continental crust and sial produced by continuous two stage mantle melting processes taking the temporal decline in heat flow into account impact shock effects on the archaean crust have been largely obscured by 1 outpouring of voluminous basic ultrabasic lavas inundating shock deformed crust and extending beyond the perimeters of impact excavated basins 2 gravity subsidence and downfaulting of terrestrial mare accounting for the anatexis of sub greenstones basement 3 extensive shearing and recrystallization of impact structures breccias and mineral deformation features beneath impact excavated basins relics of which may be retained in structural windows in high grade metamorphic terrains isostatic subsidence and anatexis of thick mare type piles and underlying impacted crust resulted in intracrustal comagmatic plutonic and volcanic suites within periods in the order of15 30 106 years as controlled by post impact mantle convection cooling subsequent isostatic updoming of remobilized basement gneiss terrains and the rise of their anatectic products along major normal shears bearing an analogy to metamorphic core complexes resulted in the multiple gneiss granite complexes amidst greenstone dominated terrains recurrent post tectonic thermal fluctuations and magmatic activity reflect a long term existence of anomalous mantle regions beneath original mega impact sites and thermal perturbations related to distal impacts a broad age zonation in some archaean shields suggests lateral migration and terrane accretion in a convection driven plate tectonic regime

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

apparent episodic crustal growth arising from a smoothly evolving mantle crustal growth curves are the same as crustal age distributions only in the absence of crustal recycling into the mantle and are likely to be very different for earth phanerozoic crust is more elevated on average compared to the ancient cratons and thus may be preferentially eroded and deposited onto the mobile ocean floor ultimately this may lead to the preferential recycling of young crust into the mantle a simplified crustal growth model shows that preferential recycling of young crust can lead to an apparent peak in crustal growth at 2 3 ga furthermore the time dependence and uneven positioning of ocean continent convergence sites of contemporaneous crustal growth and removal with respect to the continents are likely to lead to rather irregular age distributions for each individual continent with an apparent episodicity on a scale of hundreds of millions of years finally considerations of the physics of convection reveals that rapid changes in mantle temperature are strongly inhibited because of the temperature dependence of silicate rheology episodic global magmatism and hence episodic crustal growth are not expected from our understanding of mantle convection

gneiss domes and extensional deformation in the highly mineralised archaean eastern goldfields province western australia abstract the recognition of large areas of gneiss within granitic complexes of the northeastern part of the eastern goldfields province of western australia based on analysis of regional aeromagnetic data has resulted in a re evaluation of the structural data for that region five magnetic domains have been established in the granitic complexes which are marginal to the main greenstone belts at least two of these domains have a distinctive domal form and also have internal domeshaped regions the ballard and laverton domains have extensive areas of layered gneissic and migmatitic rocks which flank the internal domes boundary relationships at the laverton and ballard domains show that the domes are concordant sheets and the metamorphic conditions of the boundary rocks imply that the sheets were intruded at a middle crustal level strain conditions in the boundary zones indicate that emplacement of the gneiss domains to the crustal level of the surrounding lower grade greenstones was initially along relatively gently dipping shear zones which were active during uplift structural relationships around both the domal granitic complexes and the later granite batholiths however show that regional shortening deformation events were not caused directly by the emplacement of any of the granitic rocks the tectonic hypothesis presented for the eastern goldfields is similar to that proposed for the formation of cordilleran metamorphic core complexes the uplift event responsible for the crustal extension required to allow the emplacement of the gneiss may have been driven by the same thermal input responsible for the generation of the voluminous granites intrusive into both the gneiss complexes and the surrounding greenstone belts

growth and differentiation of the continental crust declining radiogenic heat production since the archaean has resulted in a secular evolution from a regime of numerous fast moving small thin torsionally weak plates to the present regime of larger thicker torsionally stronger plates moving at an average rate of less than one sixth of the archaean rate this has been accompanied by episodic changes in geological effects by 2500 ma b p about 85 of the present crustal mass had grown by the addition and amalgamation of mafic and calc alkaline rocks in oceanic arcs at an average rate of 11 17 pg a during the early proterozoic the first large cratons were stabilized and locally thickened and differentiated the proterozoic was an era of little continental growth falling average sea level and intracontinental deformation by 700 ma b p cratons had become much more stable marginal accretionary terrians had begun to develop with an average phanerozoic growth rate of 1 64 pg a and blueschists and ophiolites sensu stricto witness the advent of the extant plate tectonic regime

the development of continental crust through geological time the south african case nd isotopic compositions and147sm144nd ratios were measured in fifty eight south african shales and greywackes with depositional ages ranging from 0 2 to 3 3 b y elements such as the rare earths which are poorly soluble in water and not fractionated during exogeneous processes preserve the signature of the original crustal source the147sm144nd ratios appear to be approximately constant throughout the time interval sampled we calculated nd model ages of crustal differentiation knowing that the shales represent a true blend of different continental areas we consider these model ages representative of the mean ages of their primitive continental sources then using the inverse technique developed by alle gre and rousseau in 1984 we computed a growth curve for the continental crust in south africa two periods of important crustal genesis archaean and around 1 5 b y can be compared with the observed geology and with other continental crust growth curves obtained in previous studies in southern africa and in australia the observation of large variations in the mgo content and ni cr u and th concentrations between archaean south african shales and post archaean samples compared to the constancy of the147sm144nd ratios leads us to propose that the archaean crust was composed of both granite 70 5 and a mafic component 29 5 which could have been komatiite the small dispersion of 147sm 144nd ratios suggests that erosion and sedimentation processes yielded homogeneous archaean shales the present day continental crust is much more heterogeneous because it has undergone several episodes of recycling thus recent shales are characterized by more variable 147sm 144nd ratios

mantle convection and early crustal evolution abstract the existence of peridotitic komatiites in the archaean suggests that the archaean mantle was significantly hotter than the modern mantle this evidence is contradicted by estimates of archaean continental geothermal gradients based on the pressure and temperature recorded in metamorphic rocks which suggest that there is no marked difference between archaean and modern continental geothermal gradients numerical modelling shows that small changes in the mantle temperature can have an important influence on convection if the average temperature of the upper mantle is increased by 200 c convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe the crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction later when the mantle temperature falls to 100 c above its present level the upper mantle partial melt zone contracts away from subduction areas it is suggested that the first primitive felsic magmas were generated at subduction zones the appearance of these magmas at 3 8 ga permitted the formation of buoyant continents and eventually led to crustal thickening as a consequence of this thickening the proto continents consisting of a bimodal suite of basalts and sodic granodiorites contained two types of latent energy 1 radioactive energy held in elements such as th k and u and 2 potential energy resulting from the elevation of the continents above sea level the potential energy of the continents led to sedimentation the increase in the rate of sedimentation during the archaean resulted from increased crustal buoyancy at the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust this caused a major metamorphic metasomatic and crustal melting event which produced the potassic granites of the late archaean once the radioactive elements had been removed from the lower crust that region of the continent become tectonically stable the proterozoic shelf sediments were deposited at the margins of these stable cratons convection models of the archaean mantle show hot diapirs rising from the boundary layer above the core mantle interface we suggest that these diapirs began to melt at a depth of 450 km giving rise to komatiitic magmas this model requires the average temperature of the archaean upper mantle to be 100 c above that of the modern mantle the similarity between archaean and modern continental geothermal gradients can be explained if archaean continents formed above subduction zones raising the temperature of the archaean mantle by 100 c 1 halves the thickness of the oceanic lithosphere 2 increases the oceanic geothermal gradient at the mid point of a convection cell 3 decreases the viscosity of the mantle by at least an order of magnitude the combination of these effects produces a marked decrease in the strength of the archaean lithosphere and mantle thus the form of archaean tectonics can be expected to have been very different from modern tectonics

iridium enrichment in volcanic dust from blue ice fields antarctica and possible relevance to the k t boundary event abstract blue ice fields in antarctica well known as stranding zones of antarctic meteorites contain ice that is several tens to hundreds of thousand years old which records numerous events that have taken place since its formation among the most important structures on the ice fields are dust bands which are mostly of volcanic origin and consist of small volcanic glass shards and lithic fragments petrographic and chemical studies of a number of dust bands during earlier investigations have shown that significant ash layers are deposited at distances of up to several thousand kilometers from the source volcanoes samples of dust layers taken in the lewis cliff beardmore glacier area in antarctica analyzed in the course of this work for major and trace elements were identified as consisting of volcanic ash probably originating from the pleiades melbourne volcanic province northern victoria land some of these dust samples were found to contain iridium in concentrations of up to 7 5 ppb a positive correlation between ir and se as well as enrichments in as sb and other volcanogenic elements has been observed the dust samples investigated here have very small grain size consistent with the distance from the source volcanoes the small grain size of the dust suggests that the ir enrichment is a surface effect and was probably caused by condensation from a vapor phase this is the first time that significant ir enrichments have been found in volcanic dust which demonstrates that ir and possibly also other platinum group metals may be present in some volcanic ash deposits although this has been doubted by the proponents of the pure impact theory for the k t boundary event the results of this study suggest that the ir in the k t boundary clays is not necessarily of cosmic origin but alternatively may have originated from mantle reservoirs that were tapped during extensive volcanic eruptions which may been triggered by impact events

rapid growth of some major segments of continental crust some major segments of continental crust display a narrow range of sm nd crustal formation ages the sizes of the canadian shield the svecokarelian province of northern europe the west central united states and the arabian nubian shield suggest rapid crustal growth island arc accretion models rank among the most favored tectonic models for the formation of these areas a quantitative comparison of the growth rates of these crustal segments to mesozoic cenozoic arc addition rates shows however that island arc accretion alone seems insufficient to account for the amount of crust that was produced in each of these terrains other additional mechanisms such as hot spot volcanism and underplating may have been active in addition to arc accretion alternatively large amounts of preexisting basement have gone so far undetected

absence of shock metamorphic effects in the bushveld complex south africa results of an intensive search abstract an extensive search was made for distinctive shock metamorphic effects in order to test the theory that the bushveld complex bc was formed by the impact of large extraterrestrial objects field and petrographie studies concentrated on two geological environments in which current impact cratering theories predict the formation and preservation of such shock metamorphic effects as shatter cones unusual fragmental or melt breccias and shock lamellae in quartz these are 1 pre bc quartzite xenoliths in the rooiberg group a thick sequence of acid lavas rhyolites and rhyodacites and related rocks proposed by earlier workers as a possible crater filling impact melt and 2 the locally observable contact between the rooiberg group and the underlying pre bc basement pretoria group a contact possibly corresponding to the original crater floor in established impact structures more than 320 samples of pre bc quartzite were collected and examined petrographically for shock metamorphic effects no megascopic shock metamorphic effects shatter cones or unusual breccias were observed in any outcrop in thin section no petrographic shock effects e g shock lamellae hi quartz and kink bands in biotite were observed in any sample the quartzites from both the basement 80 samples and the rooiberg xenoliths 245 samples are virtually undeformed cataclastic textures are rare and individual quartz grains show only normal metamorphic deformation lamellae bohm lamellae and single or multiple sets of fluid inclusions arranged along irregular planes which appear to be healed fractures although some of these parallel sets superficially resemble so called decorated shock lamellae universal stage measurements of their orientation demonstrate that they are totally different the absence of both megascopic and microscopic shock metamorphic effects in a large suite of carefully selected samples strongly indicates that impact models for the bc are not correct and that the bc is not an impact structure this conclusion also agrees with 1 the complex stratigraphy now recognized in the lower rooiberg group which is not consistent with an impact melt origin 2 the absence in the rooiberg group of any of the distinctive melt rich fragmentai breccias which dominate the crater filling deposits in established large impact structures and 3 the undeformed character of the basement rocks below the rooiberg group and the apparently concordant contact between them

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

on the relative and absolute ages of seven lunar front face basins i from viscosity arguments the seven basins orientale imbrium crisium nectaris humorum and an unnamed basin between werner and the altai ring show rims whose absolute and relative heights are correlated with the sharpness and crispness of the features on the assumption that the decline in average outer rim height not scarp height measures the age of the basin and also that the decline represents a hot creep of rocks of very high viscosity absolute ages were derived basins were found to increase in age in the sequence listed above with a range from about 3 82 to 4 30 109 years the average or effective viscosity of the surface layers down to whatever level was involved in the creep was calculated as increasing from 9 46 1024 poises at about 4 30 109 years to 1 86 1030 poises at present it should be clear at the outset what the assumptions and associated observations are and why they are necessary to a solution they will be listed in this abstract and expanded upon in the text 1 1 the original rim height of each basin was a function only of basin diameter 2 2 the original rim height was given by pike s 1983 relation for fresh craters extrapolated to basin diameters 3 3 the present rim height is that of the most prominent ring structure 4 4 the smaller rim height of all seven basins relative to the height predicted by 2 is due largely to creep in the lunar rocks down to some undetermined level other forces may contribute to the sinking of the rims but these are considered to be of lesser importance and are discussed in the text 5 5 the relative ages of the seven basins are as given in table i this sequence differs slightly from that of wilhelms 1984 for example but it is that found in baldwin 1974 1987 and is consistent with the results of this paper 6 6 the age of formation of the imbrium basin 3 85 109 years inferred from lunar sample studies particularly apollo 15 is correct 7 7 the age of formation of the serenitatis basin 3 87 04 109 years inferred from petrologic and geochemical studies of apollo 17 boulders is incorrect this is not an assumption but is a result of the analysis of this paper 8 8 the rheology of the moon may be described for the purposes of this paper by an effective viscosity valid throughout the layers involved in the creep 9 9 this effective viscosity is used as a tool to determine basin ages and is not important in itself it does appear to vary in the same range as terrestrial rocks but not the lithosphere 10 10 other factors such as isostasy shaking due to jar from later impacts modification due to rim relief by ejecta and erosion from small impacts are all close to exponential in nature declining toward the present and hence may be included in the determination of the effective viscosity 11 11 the rim height of the imbrium basin subsided by 25 m in the last 2 5 109 years this value was chosen arbitrarily it could have been 250 300 m and the basin ages would not have been affected except for orientale and that only minutely 12 12 the effective viscosity of the moon was observed to change continuously and monotonically with time 13 13 judging by table iii the probable error of an absolute age is in the range of 10 to 50 106 years it is difficult to determine exactly what this means it will be constrained by points 14 15 and 16 14 14 an error in the age determination should not be large enough to alter the relative ages of the basins judging by crater counts baldwin 1974 1987 15 15 if the viscosity of the moon declined in the post imbrium period of mare formation the only basin to be affected would be orientale and this by no great amount inasmuch as the basin is older than nearby maria 16 16 if the effective viscosity were less than about 1022 poises at the time of the oldest basin then presaturation surfaces would not show the numerous craters and portions of craters that are obvious in this time span 17 17 considerably prior to the time of saturation the outer layers of the moon had a low enough viscosity so that they could not retain the record of the then occuring cratering 18 18 the approximations of this paper were adopted because it does not appear possible to make an unambiguous selection from the more elegant mathematical treatments of creep and isostasy that would lead toward reasonable ages for the giant basins

the distribution of 3 0 ga and 2 7 ga volcanic episodes in the yilgarn craton of western australia abstract the results of a conventional zircon u pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the yilgarn craton of western australia indicate that volcanism took place during two distinct episodes at ca 3 0 and ca 2 7 ga each episode spans a time period of about 75 ma and the two ages of volcanism are spatially separated 3 0 ga volcanic rocks are found in greenstone belts in the northern part of the western gneiss terrain and in the murchison province whereas 2 7 ga volcanic rocks are present in the southern part of the western gneiss terrain and in the southern cross and eastern goldfields provinces there is no evidence of inheritance in the zircon u pb systems which would support an older crustal origin for these rocks present results together with previous studies of the ages of granites and gneisses suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism it is suggested that the evolution of the yilgarn craton took place in three main stages the first was the development of a stable 3 4 ga granitic nucleus which incorporated earlier generations of granitoid a 3 7 ga layered complex and possibly 4 2 ga primitive crust this was followed by the two separate granite greenstone episodes at ca 3 0 and ca 2 7 ga involving the addition of significant amounts of new crust each episode was characterised by the accretion of extensive granite greenstone terranes at the margins of the stable craton the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems

origin of komatiite at high pressures abstract experimental melting relations to 20 gpa demonstrate that the commonly accepted model of komatiite formation by non invariant melting olivine liquid is too restrictive komatiites could also have formed by pseudoinvariant melting olivine pyroxene garnet liquid modified spinel majorite liquid along the anhydrous solidus at high pressures the mgo content of komatiites cannot be used to infer the degree of partial melting in the mantle there would have been little change in mgo at a high pressure pseudoinvariant point even for degrees of partial melting that ranged from very small to over 80 instead the mgo content of komatiites may be a better indication of the depth of partial melting than it is the degree of partial melting late archean komatiites containing 20 28 mgo could have been generated at depths ranging from about 130 to 260 km early archean komatiites from the barberton mountain land could have formed at depths corresponding to the present day transition zone or at shallower levels from an unusually primitive mantle source komatiites which originated by partial melting in a lower mantle formed by majorite fractionation should have geochemical characteristics that have not yet been reported this indicates that either melting did not extend into the lower mantle or if it did the upper and lower mantles were isolated by a chemical and thermal boundary layer in archean times

geologic evolution of the late archean abitibi greenstone belt of canada the southern volcanic zone svz of the late archean abitibi belt of the superior province of canada is dominated by komatiitic to tholeiitic volcanic plateaus and large bimodal mafic felsic volcanic centers these volcanic rocks were erupted between about 2717 and 2700 ma in a series of rift basins that formed as a result of wrench fault tectonics they overlie and juxtapose a volcano plutonic assemblage characterized in the northern abitibi belt the age of the assemblage is about 2720 ma or older and it comprises basaltic to andesitic and dacitic subaqueous massive volcanics cored by comagmatic sills and layered anorthositic complexes and overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma a tectonic model is proposed in which the svz formed in a series of rift basins that dissected an earlier formed volcanic arc comparisons are made with rift environments that have been postulated for phanerozoic areas such as the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs

continental rifting and the archean proterozoic transition in the western australian shield the norseman wiluna belt dominated by lowermost sequences of deformed and metamorphosed basalts and komatiites and the fortescue group with successions of gently dipping little metamorphosed basalts and rare komatiites are typical examples of archean style greenstones and proterozoic style platform sequences respectively isotopic evidence however indicates that they are broadly coeval and both archean ca 2 7 0 05 ga and geologic evidence suggests that both relate to continental rifting their contrasting volcanism and subsequent deformation are inferred to result from their positions relative to rift axes and the state of the basement the norseman wiluna belt representing a failed rift in stretched relatively hot crust and the fortescue group representing a continental basalt succession on thick relatively cold crust the fortescue group immediately follows a major period of crustal reworking and stabilization whereas eruption of the greenstones of the norseman wiluna belt was closely followed by such an event thus the diachronous change in tectonic style from greenstones to platforms the archean proterozoic transition in this case appears to relate to a finite period of stabilization of continental crust rather than to a fundamental change in tectonic style or process with time

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

seismic precursors of the february 10 1990 eruption of kelut volcano java abstract kelut volcano eastern java indonesia erupted on february 10 12 1990 seismic activity and an increase of temperature and acoustic noise in the crater lake appeared three months before the eruption and were the main precursory signs of this eruption during the pre eruptive stage the number of events was eight per day on average and the cumulative seismic energy released was 5 6 10 15 erg these low values are uncommon for an andesitic volcano seismic activity mainly consisted of swarms of very shallow volcano tectonic earthquakes many of which were part of seismic multiplets in the last three weeks before the eruption seismic activity remained at a very low level this quiescent period was followed 24 hours prior to the eruption initiation by an intense swarm of low energy events which progressively became continuous spasmodic tremor most of the data obtained during the pre eruptive stage are consistent with a process of fracturation of a shallow and low strength plug blocking the eruptive conduit before the eruption several observations suggest that water is an important element in the preeruptive and eruptive phenomena at kelut volcano in particular the monochromatic events observed at kelut are characterized by one narrow peak in their spectra the estimations of q value and impedance contrast for these events suggest that the fluid which fills the source resonator could be water

chemostratigraphy across the cretaceous tertiary boundary and a critical assessment of the iridium anomaly the elevated concentration of iridium one of the platinum group elements pge at the cretaceous tertiary boundary is still the most generally accepted evidence that a large bolide struck the earth at the time of the end cretaceous mass extinctions new chemostratigraphic data for cross boundary sections from both hemispheres are not easily explained in terms of such an impact event for example the observation that the pge patterns show marked differences between the hemispheres the new constraints indicate that models of mantle derived pge should be seriously considered and that pge anomalies might not be as useful as previously thought as unambiguous identifiers of large impact events in the earth s history

early archean silicate spherules of probable impact origin south africa and western australia layers of sand sized silicate spherules in 3 2 to 3 5 ga archean greenstone belts in south africa and western australia formed by the accumulation of quenched liquid silicate droplets having primary compositions resembling those of immediately underlying rock sequences the layers and particles appear to be unrelated to volcanic activity and we suggest that both occurrences may represent melt droplets formed during meteorite or comet impacts if so these spherules are the oldest known terrestrial impact products if these objects are representative of a larger population of early archean impact features it may be possible to directly estimate the meteorite flux on the early archean earth

dynamic deformation of volcanic ejecta from the toba caldera possible relevance to cretaceous tertiary boundary phenomena plagioclase and biotite phenocrysts in ignimbrites erupted from the toba caldera sumatra show microstructures and textures indicative of shock stress levels higher than 10 gpa strong dynamic deformation has resulted in intense kinking in biotite and with increasing shock intensity the development of plagioclase of planar features shock mosaicism incipient recrystallization and possible partial melting microstructures in quartz indicative of strong shock deformation are rare however and many shock lamellae if formed may have healed during post shock residence in the hot ignimbrite they might be preserved in ash falls peak shock stresses from explosive silicic volcanism and other endogenous processes may be high and if so would obviate the need for extraterrestrial impacts to produce all dynamically deformed structures possibly including shock features observed near the cretaceous tertiary boundary 38 references 3 figures

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

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

impacts tillites and the breakup of gondwanaland mathematical analysis demonstrates that substantial impact crater deposits should have been produced during the last 2 gy of earth s history textures of impact deposits are shown to resemble textures of tillites and diamictites of precambrian and younger ages the calculated thickness distribution for impact crater deposits produced during 2 gy is similar to that of tillites and diamictites leq 2 ga we suggest therefore that some tillites diamictites could be of impact origin extensive tillite diamictite deposits predated continental flood basalts on the interior of gondwanaland significantly other investigators have already associated impact cratering with flood basalt volcanism and continental rifting thus it is proposed that the breakup of gondwanaland could have been initiated by crustal fracturing from impacts

a two stage model for the formation of the granite greenstone terrains of the kalgoorlie norseman area western australia abstract accurate dating of granites and greenstones from the kalgoorlie norseman area shows that there is considerable overlap in time between the formation of granites and greenstones however within any given area the oldest granites are always younger than the oldest greenstones it is suggested that the long lived 100 ma thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting leading to the production of granites the time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma where this happens the basaltic magma ponds at the base of the crust supplying additional heat and accelerating the melting process eventually the granitic melt layer becomes unstable and rises within the crust areas intruded by granite become topographic highs with topographic lows forming above areas where light melt has been withdrawn from the lower crust when the topographic highs protrude above sea level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones thus the change from a basalt dominated lower greenstone succession to a sediment and felsic volcanic dominated upper whitestone sequence seen in many granite greenstone terrains results from the intrusion of granitic magma into the upper crust the crust of the kalgoorlie norseman area formed in two distinct stages separated by a time break of up to 700 ma the first stage involved the formation of primitive crust probably above subduction zones this primitive crust was characterized by low rb sr and high sm nd ratios the second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents this led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas melting reset the rb sr sm nd and u pb systems giving the impression that the second event which occurred at 2 7 ga was a major crust forming event it was in fact a crustal reworking process although some basalt was added to the crust at this time although the two stage history of the kalgoorlie norseman area is clearly seen in the lead isotopes and in zircons it is not readily discernible from the rb sr and sm nd systematics due to mantle like ratios of those elements in the primitive crust

shocked quartz in the cretaceous tertiary boundary clays evidence for a global distribution shocked quartz grains displaying planar features were isolated from cretaceous tertiary boundary clays at five sites in europe a core from the north central pacific ocean and a site in new zealand at all of these sites the planar features in the shocked quartz can be indexed to rational crystallographic planes of the quartz lattice the grains display streaking indicative of shock in x ray diffraction photographs and also show reduced refractive indices these characteristic features of shocked quartz at several sites worldwide confirm that an impact event at the cretaceous tertiary boundary distributed ejecta products in an earth girdling dust cloud as postulated by the alvarez impact hypothesis

distribution of noble metals across the cretaceous tertiary boundary at gubbio italy iridium variation as a constraint on the duration and nature of cretaceous tertiary boundary events iridium pd pt and au were determined in sections from the bottaccione gorge and contessa valley gubbio italy by radiochemical neutron activation shales and limestones were sampled from 2 85 m above to 219 m below the cretaceous tertiary k t boundary metal enrichment was evaluated by comparing the boundary shale region with the lower part of the section background iridium is concentrated by 63 times in the boundary shales in comparison with the background whereas other metals are enriched by no more than 2 2 times the enrichment of ir is not confined to the boundary shales but extends approximately 2 m above and below this horizon within this ir rich region there are four distinct ir maxima in addition to the major ir enrichment in the k t boundary shales iridium maxima are stratigraphically coincident with maxima in abundances of shocked minerals characteristic of explosive volcanism limestones are much lower in noble metals than shales and their ir contents in the k t boundary region are largely accounted for by their minor clay mineral contents the time represented by the 4 m of ir rich section is at least 3 x 10 5 yr if published sedimentation rates are used to sustain an increased ir flux over this period and to account for the ir distribution near the k t boundary intense volcanic activity is a preferred alternative to impact of extraterrestrial material

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

chemical composition of archaean komatiites implications for early history of the earth and mantle evolution abstract estimates of the chemical composition of the archaean mantle derived from elemental abundance ratios in komatiites combined with ultramafic xenolith data support a model of a multistage heterogeneous accretion history of the earth and synchronous core formation 4 6 ga ago most refractory lithophile element abundance ratios in komatiites and xenoliths are close to chondritic except for v ti and ca al depletion of vanadium is likely due to its partial incorporation into the iron core whereas fractionation of ca al observed in archaean al undepleted komatiites 1 20 times chondrites and in some modern fertile spinel lherzolite xenoliths 1 15 times chondrites could be due to small amounts of garnet rich in al but poor in ca segregation into the lower mantle during partial or complete melting of the upper mantle in the very early history of the earth however this process may have had only a small effect on the overall chemical composition of the upper mantle simultaneous occurrence of early archaean al undepleted al ti chondrites and al depleted al ti 0 5 chondrites and depletion of sc and heavy ree peridotitic komatiites in the barberton area s africa and late archaean newton township canada argue against derivation of peridotitic komatiites from a circum global magma ocean garnet separation from a mantle diapir which intersects the solidus at great depth 200 km in a hotter early archaean mantle could explain the chemical characteristics of al depleted komatiites alternatively these two types of komatiites could have been derived from different layers in a fractionated mantle a limited amount of hf isotope data for archaean komatiites seems to suggest that both mechanisms are important this chemically and minerallogically layered mantle if it existed was homogenized by mantle convection after early archaean times constant p 2 o 5 tio 2 ni mg co mg fe mg ratios siderophile lithophile and pge abundances estimated for the mantle sources of komatiites from archaean to modern times strongly argue against continuous growth of the earth s core since the early archaean extensive crustal contamination might have been involved in the generation of archaean early proterozoic siliceous high magnesian basalts with boninite affinity however involvement of chemically modified ancient continental lithosphere may also be important in the generation of these basalts

geochronology of the lumby lake greenstone belt a 3 ga complex within the wabigoon subprovince northwest ontario the lumby lake greenstone belt lies within the central predominantly metaplutonic terrane of the western wabigoon subprovince it is conformable with gneiss of the marmion lake tonalite batholith u pb analyses of zircons from two felsic tuffs and a quartz porphyry within the greenstone belt give a scattered lead loss pattern indicative of early disturbance the ages of the oldest concordant fractions agree however and define an age of 2999 1 m y for emplacement of these felsic units zircons from a tonalite gneiss within the marmion lake batholith are also somewhat disturbed the oldest abraded fraction is concordant and has an age of 3003 5 m y indicating that the gneiss is probably close in age to the supracrustal rocks u pb analyses of rutile from one of the felsic units give an age of 2690 5 m y and record the effect of late archean metamorphism three titanite fractions from the tonalite gneiss agree within error and have an age of 2809 1 m y the age and consistency of the titanite fractions indicate that titanite in this rock was formed as a result of a metamorphic event and was never subsequently disturbed this age is therefore a minimum estimate for high grade metamorphism of the gneiss the presence of a pre 2809 ma fabric in a gneiss of regional extent suggests that an early metamorphic deformational event affected this area these data constitute the first firm evidence for extensive 3 0 ga crust in the southern superior province and give the first evidence of a regional deformational event which predates cratonization of the superior province

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

petrography chemistry and isotopic ages of peninsular gneiss dharwar acid volcanic rocks and the chitradurga granite with special reference to the late archean evolution of the karnataka craton southern india abstract new petrographic major and trace element and isotopic age data are presented for four suites of rocks from the low to medium grade part of the south indian archaean craton in central karnataka two suites are from the peninsular gneiss basement to the late archaean dharwar supergroup the third is a suite of dharwar potassic rhyolites and the fourth is from the chitradurga granite which has intrusive contacts with the dharwar supergroup the chikmagalur granite granodiorite which with its host gneisses forms part of the basement peninsular gneiss to the dharwar supergroup in the bababudan basin yields rb sr and pb pb whole rock isochron dates of 3080 110 ma and 3175 45 ma respectively an initial 87 sr 86 sr ratio of 0 7013 0 0009 and a 1 value 238 u 204 pb of 7 98 host tonalitic gneisses to the chikmagalur granite give an rb sr whole rock isochron date of 3060 160 ma with an initial 87 sr 86 sr ratio of 0 7015 0 0004 and a pb pb whole rock isochron date of 3190 100 ma with a model 1 value of 8 00 a suite of granites and granodioritic tonalitic gneisses chitradurga granitic rocks and gneisses forming part of the peninsular gneiss basement to the dharwar supergroup west of the chitradurga belt yields rb sr and pb pb whole rock isochron dates of 2970 100 ma and 3028 28 ma respectively with an initial 87 sr 86 sr ratio of 0 7035 0 0013 and a model 1 value of 7 62 the suite of dharwar acid volcanic rocks from north of honnali defines a poorly fitted 207 pb 206 pb linear array mswd 15 8 which gives an apparent age of 2565 28 ma and a model 1 value of 7 46 the poor fit of this isochron shows that a closed system was not maintained a conclusion supported by high k abundances which are attributed to a syn or post magmatic ion exchange process the chitradurga granite which intrudes the older rocks of the dharwar supergroup in the chitradurga belt yields a well fitted pb pb whole rock isochron date of 2605 18 ma with a model 1 value of 7 68 the u pb relations in these suites from karnataka are in marked contrast with the commonly severe u depletion and consequently unradiogenic pb isotopic compositions observed in deeply eroded high grade archaean gneiss terranes such as those bordering the north atlantic this contrast indicates that the cratonic rocks in central karnataka represent a relatively high level in the original archaean continental crust

majorite fractionation and genesis of komatiites in the deep mantle abstract melting experiments of the primitive mantle indicate that majorite fractionation is the key process to determine the geochemical nature of magmas generated in the deep upper mantle and the transition zone a model of petrogenesis of komatiite magma is presented by using the melting relations of the primitive mantle and the partitioning data of lithophile elements between majorite and the liquid alumina depleted komatiite generally observed in the early archean can be generated by any degree of partial melting at depths from 450 to 650 km this type of komatiite is also generated by a relatively low degree of partial melting at depths from 180 to 450 km alumina enriched komatiite can be generated by remelting of the residual mantle after separation of alumina depleted komatiite alumina undepleted komatiite generally observed in the late archean can not be formed by a low degree of partial melting of the primitive mantle at any depths this type of komatiite can be generated only by a high degree of partial melting of the mantle at the depths shallower than 450 km as an alternative model the alumina depleted komatiite can be formed by a high degree of partial melting of the archean upper mantle depleted in majorite such heterogeneity in the archean upper mantle was generated by a global melting event succeeding the accretion of the earth and erased by convection by the later archean hf isotope data seem to favour the former model of the petrogenesis of komatiite i e single stage melting of the primitive mantle at various depths

age and origin of granitic rocks in the kalgoorlie norseman region of western australia implications for the origin of archaean crust three granites from the norseman area at stennet rocks buldania rocks and east of lake johnston have u pb zircon ages of 2691 8 95 c l 2689 22 and 2661 8 ma respectively one stennet rocks contains 3100 3200 ma cores providing support for oversby s 1975 argument that all three plutons were derived by anatexis of much older basement common leads within the zircons show evidence of having been derived from within the rock and represent mixtures of initial rock leads and radiogenic leads the unusually 208pb rich compositions of the common leads suggest the breakdown of a th rich phase perhaps uranothorite as a major contributor to the radiogenic component available zircon geochemical and isotopic data are used to develop a two stage model for archaean crustal development in this region with formation of primitive crust of broadly andesitic composition from depleted mantle or basalt beginning at 3400 ma followed by extensive thermal reworking and possibly continued growth over the period 3000 3300 ma in the second stage at 2700 ma anatexis resulted in large scale intra crustal differentiation giving a thick upper granitic layer and a lower crust residual from anatexis it is this second major thermal event that is recorded by most geochronometers implicit in the andesitic crustal growth model is that because the sm nd and rb sr ratios of newly formed archaean crust are close to mantle values this process is transparent to the sm nd and rb sr isotope systems until recycling of incompatible element rich sediment becomes important following the late archaean production of a wide spread differentiated upper crust

a chronostratigraphic framework for the north central kaapvaal craton the bushveld complex and the vredefort structure abstract sufficient geochronological data are available pertaining to the north central part of the kaapvaal craton to construct a chronostratigraphic framework for the rock units involved in the vredefort event from the basement rocks at 3170 ma to the waterberg soutpansberg groups deposited between 2025 and 1425 ma the resulting framework forms a convenient chronological guide to the geological evolution of this area and the timing of events such as that at vredefort the basement on which the supracrustal rocks were deposited stabilised from 3170 to 3060 ma but metamorphism and metasomatism of these rocks occurred up to 2800 m y ago in different parts of the craton the first unit of the wits triad the volcaniclastic dominion group was deposited at 3060 ma on the stabilising basement this was followed by the dominantly clastic witwatersrand supergroup and the overlying ventersdorp supergroup the dominantly volcanic ventersdorp rocks are 2700 m y old and form the last unit of the wits triad a considerable time gap of 200 300 ma separates the triad from the lowermost units of the overlying transvaal sequence the age of these volcanic and clastic protobasinal rocks is poorly constrained between 2500 and 2300 ma they may in fact represent eroded remnants of a more extensive depositional cycle the mainly clastic sedimentary part of the transvaal sequence pretoria group has been dated at 2224 ma the upper part of the transvaal sequence rooiberg group is only constrained by the age of the intrusive bushveld complex the bushveld basic layered rocks are well dated at 2060 ma as are the subsequent granites at 2050 ma the vredefort event which post dates the bushveld complex is reasonably well constrained by the age of the bronzite granophyre at 2000 ma deposition of the waterberg and soutpansberg groups which post date the bushveld complex is poorly constrained between the age of the bushveld granites and younger alkaline intrusives at 1425 ma but these sediments and volcanics were probably formed between 2025 and 1752 ma the waterberg group may therefore contain debris from the vredefort structure

u pb zircon ages and the evolution of the michipicoten plutonic volcanic terrane of the superior province ontario the archean michipicoten greenstone belt of the superior province in ontario is made up of supracrustal rocks divided into lower middle and upper metavolcanic rocks with associated metasedimentary rocks the belt has been intruded by granitic rocks and is also surrounded by granitic terranes based on u pb zircon geochronology it appears that volcanism in the area extended from at least 2749 to 2696 ma and plutonism and tectonic activity extended from at least 2888 to 2615 ma the various granitic and also one gabbroic plutons both internal and external to the greenstone belt were emplaced concomitantly with the three volcanic cycles as well as before and after the formation of the volcanic rocks zircon ages reported here together with previously published ages show that the area evolved in six major volcanic and plutonic events i 2888 ma plutonism ii 2743 ma volcanism and plutonism iii 2717 ma volcanism and plutonism iv 2696 ma volcanism and plutonism v 2668 ma plutonism and vi

four zircon ages from one rock the history of a 3930 ma old granulite from mount sones enderby land antarctica ion microprobe u th pb analyses of zircons from a granulite grade orthogneiss from mount sones enderby land antarctica record the ages of four principal events in the history of the gneiss three of which already have been recognized through previous isotopic dating of other samples the structure of the zircons indicates at least four different stages of growth the several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate u pb discordance patterns for each stage the stratigraphically oldest zircon rare discrete cores is indistinguishable in age from the most common euhedrally zoned zircon both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927 10 ma ago making the orthogneiss currently the oldest known terrestrial rock the outer parts of most grains and some whole grains recrystallized at 2948 31 17 ma during or immediately after possibly 100 ma of high granulite grade metamorphism the recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the napier complex at 1000 ma in the intervening time at 2479 23 ma the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic pb which left most of the crystals with radiogenic pb deficiencies but produced local radiogenic pb excesses in others a new generation of zircon characterized by very high th u and low u grew at that time that event deformation and possibly a minor rise in temperature produced widespread perturbations of other isotopic systems throughout the napier complex

role for microorganisms in the formation of iridium anomalies we propose a microbial role in the enhancement erasure dispersal and creation of certain sedimentary iridium anomalies such as those of the cretaceous tertiary boundary the proposal is based on laboratory experiments that have demonstrated the ability of some bacteria and fungi to concentrate iridium from solution and to dissolve iridium from source igneous and meteoritic rocks

archaean chronology in the eastern part of the baltic shield a synthesis abstract twenty seven gneisses and granodiorites of the archaean basement of eastern finland have been chosen for rb sr geochronological studies the results were used together with other radiometric measurements to discuss the time space relationships between the various rock units this review can be summarized as follows from 3 0 to 2 8 ga a first generation of grey orthogneisses kivijarvi type was emplaced subsequently to an important tectono metamorphic event a second generation of grey orthogneisses naavala type intruded the kivijarvi gneisses 2 69 2 65 ga the two generations of grey gneisses form the basement on which were deposited the mafic and ultramafic magmas of the lower volcanic sequence of the kuhmo suomussalmi greenstone belt at 2 65 ga both greenstone and gneiss basement were intruded by calc alkaline intermediate to felsic magmas they were emplaced as volcanites in the upper part of the greenstone belt 2 5 ga or as granodioritic augen gneisses into the basement 2 50 2 55 ga the end of the archaean magmatic activity is marked in this part of the baltic shield by the intrusion of pink leucogranites 2 41 ga these age data are consistent with the field relative chronology and show that 1 the finnish archaean terrains developed over a period of at least 0 45 ga followed by a 0 40 ga period of geologic quiescence 2 there is a gneissic basement to the kuhmo suomussalmi greenstone belt the kivijarvi grey gneisses were emplaced at least 0 25 ga prior to the greenstones 3 the zircons extracted from recycled magmatic rocks give c 2 7 ga ages and are interpreted as retaining a memory of their ancient protolith and consequently do not indicate the emplacement age of their present host rock this interpretation is subject to great controversy and discussion among geologists 4 the rb sr whole rock system is not modified by the svecokarelian metamorphism but the strongly heterogeneous rocks as well as the minerals were affected and recorded the thermal influence of that orogeny

geochronology and geochemistry of precambrian granitic rocks of goa sw india abstract the precambrian rock succession in goa representing the nw extremity of the south indian precambrian craton includes 1 a low grade supracrustal sequence known as the goa group and 2 several acid granitic and gneissic bodies occurring in close spatial association with the supracrustals the anmod ghat trondhjemitic gneisses of ne goa which form the basement to the goa group yield a rb sr whole rock isochron age of 3400 140 ma the pervasive late archaean acid plutonism post dates evolution of the goa group supracrustal sequence and spans syn to post tectonic regimes with respect to the last major penetrative folding in the segment the rb sr whole rock ages of the granitic plutons and migmatites chandranath granodiorite granite 2650 100 ma dudhsagar granite 2565 95 ma londa migmatites 2650 130 ma canacona graniteporphyry 2395 390 ma suggest a minimum age of the goa group of c 2 6 ga and confirm the correlation of the goa group with the chitradurga group which occurs to the south of the craton geochemical data presented bring out essential differences between the c 3 4 ga old basement trondhjemitic gneisses and the younger group of granites c 2 6 2 4 ga the early archaean trondhjemitic gneisses show low rb sr 0 20 th u 1 45 1 93 highsr ba 0 5 low ree 24 14 52 81 with a slightly fractionated ree pattern moderately depleted in hree cen ybn 2 1 6 5 compared with the early archaean gneisses the younger granites are distinctly potassic with higher rb sr 0 50 0 87 th u 4 05 14 91 lower sr ba 0 22 0 43 a lree enriched pattern cen ybn 15 1 26 6 and much higher ree 132 06 180 84

age estimation of the deccan traps from the north american apparent polar wander path it has recently been proposed that flood basalt events such as the eruption of the deccan traps have been responsible for mass extinctions to test this hypothesis accurate estimations of the ages and duration of these events are needed in the case of the deccan traps however neither age nor duration of emplacement is well constrained measured ages range from 40 to more than 80 ma and estimates of duration range from 1 to 67 m y to make an independent age determination the authors use paleomagnetic and sea floor spreading data and estimate the associated errors the authors compare the deccan paleomagnetic pole with the reference apparent polar wander path of north america by rotating the positions of the paleomagnetic pole for the deccan traps to the reference path for a range of assumed ages uncertainties in the apparent polar wander path deccan paleopole position and errors resulting from the plate reconstruction are estimated the authors suggest 83 70 ma as the most likely time of extrusion of these volcanic rocks

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