relationships between the vredefort structure and the witwatersrand basin within the tectonic framework of the kaapvaal craton as interpreted from regional gravity and aeromagnetic data abstract processed gravity and aeromagnetic data covering the witwatersrand basin and adjacent areas have been used to delineate the boundary of the basin and to investigate other features such as the vredefort structure the bethlehem gravity high and the colesberg magnetic anomaly trend a model is proposed in which the regional aeromagnetic and gravity signatures of these features can be related to the disposition of a semi continuous mid crustal magnetite rich layer which is exposed or has a shallow suboutcrop in three regions in the centre of the vredefort structure along the kaapvaal craton boundary and centrally through the craton northwards from colesberg the bethlehem high is thought to be due to elevation of lower levels of the crust along a nw trending geanticlinal arch the apex of this broad arch is termed the vredefort axis and plunges northwestwards beneath the witwatersrand basin the vredefort structure lies at the intersection of this axis and a major nne trending axis of crustal downwarp which also constitutes the long axis of the witwatersrand basin two contrasting scenarios for the development of these major axes are discussed with either scenario genesis of the vredefort structure should be integrated with the genesis and evolution of the vredefort axis and with the tectonic evolution of the kaapvaal craton

tracing groundwater components by chemical isotopic and physical parameters example schinznach switzerland dissolved ions stable and radioactive isotopes and physical data obtained from hydrologically related samples may be processed as anonym and as specific tracers answers are obtained to questions pertaining to the number and proportions of inter mixing water types the properties of each end member its recharge altitude depth of circulation nature of the rocks passed and mode of flow the methodology of the data processing is demonstrated on observations made over one year on a spring and a well at the spa of schinznach switzerland two end members are identified 1 warm 39 c saline up to 85 meq l 1 total dissolved ions circulating at a depth of over 640 m and passing through gypsum anhydrite and halite bearing rocks and 2 colder 21 2 c fresher 30 meq l 1 total dissolved ions circulating at a depth of 240 m passing through a sequence of gypsum anhydrite bearing carbonate rocks and flowing in a karstic mode both water types are recharged at an average altitude of 550 100 m a s l a value used in identifying the recharge area the existence of a separate hotter water body at greater depth is possible

the geology and gravity field in the central core of the vredefort structure the centre of the vredefort structure in south africa comprises an uplifted granitic basement with an associated 300 g u gravity high ringed by upturned sedimentary strata detailed geological mapping of the basement core has delineated a three fold concentric zoning of outer granite gneisses ogg steynskraal formation sf and inlandsee leucogranofels ilg an increase in metamorphic grade occurs toward the centre of the structure from greenschist relics on the basement perimeter to upper amphibolite and granulite facies in the sf and granulite facies in the ilg a marked lithological change occurs within the sf in which perimeter greenschists and inner amphibolites give way to paragneisses in the ilg paragneiss and mafic granulite relics predominate these divisions are consistent with the standing on edge model for the basement 1 three regional metamorphic episodes and two thermal events have been recognised m1 and m3 both attained granulite facies at low medium load pressures in the ilg m1 granulite facies assemblages extended into the ogg but the m3 facies was restricted to the ilg m2 was a k metasomatism event limited to the ogg an extensive thermal episode m4a is provisionally correlated with the collar metamorphism pyroxene hornfels facies t 800 c was attained in the centre as opposed to conditions of p 4 5 kbar and t 650 c for alkali granite stocks in the sedimentary collar 2 this event was thus pre or syn uplift but not post uplift a high level high temperature p 3 kbar and t 800 900 c event m4b occurred at a late stage in the overall sequence a suite of amphibole orthopyroxene rocks of basaltic komatiite type cao al2o3 1 are chemically related to younger high mg tholeiites these tholeiites are correlated with similar rocks in the sedimentary collar and were intruded syn to post m4a in several episodes of decreasing basicity shock metamorphic features granitic breccias and pseudotachylite are linked to the late stage history of the structure and a case is presented for a dynamic metamorphic front advancing outward and upward from the centre in response to unroofing of the basement the central gravity high3 is delineated by 1239 new gravity stations at 1 km and 0 5 km intervals a low frequency oval shaped anomaly with a long axis trending nw se has eccentrically positioned on it a plateau extending southwards as a spur a dyke is responsible for the spur but the source for the plateau is from a recent borehole thought to be largely due to sheared altered harzburgite modelling of the low frequency gravity field component was based on the mapped divisions of the ogg sf and ilg and a density contrast of 200 kg m3 was determined between the ogg and the sf and ilg lithologies a purely crustal solution for the excess mass distribution in the basement is conclusively demonstrated

window constrained inversion of gravity gradient tensor data using dike and contact models we have developed a constrained inversion technique for interpretation of gravity gradient tensor data for dike and contact models striking in the y direction the measured g xz and g zz com

moho depth and crustal composition in southern africa abstract we present new results on structure thickness and composition of the crust in southern africa based on 6300 seismic receiver functions at 85 stations application of hk stacking to the entire sase dataset and use of multi frequency bands improve resolution substantially we observe a highly heterogeneous crustal structure with short wavelength variations in thickness h vp vs ratio composition and moho sharpness which defines 20 blocks that do not everywhere coincide with surface tectonic features in the zimbabwe craton the tokwe block has h 35 38 km and vp vs 1 74 1 79 whereas the thicker crust in the tati block h 47 51 km may be related to deformation of the archean crust along the cratonic margin two distinct crustal blocks with similar crustal thickness 42 46 km but significantly different vp vs ratios are recognized in the limpopo belt extreme values of 1 90 1 94 at the dyke swarms in eastern limpopo and 1 84 at the olifants river dyke swarm and easternmost bushveld intrusion complex bic indicate voluminous magmatic intrusions in the whole crust we find no evidence for magmatic intrusions in the central inferred part of bic where the crust is thick 45 50 km and vp vs is low 1 68 1 70 this thick crustal root may have deflected rising magmas to form the two bic lobes most of central kaapvaal has thin 35 40 km crust and vp vs 1 74 these characteristics are similar to the tokwe block in zimbabwe craton and may indicate delamination of pre existing lower crust which is further supported by a very sharp moho transition the exposed cross section in the vredefort impact crater is non representative of cratonic crust due to shallow moho 34 km and high vp vs 1 80 attributed to shock metamorphism high vp vs 1 76 is typical of the witwatersrand basin and anomalously low vp vs 1 66 1 67 marks the kaapvaal kheis namaqua transition highly heterogeneous crust both in thickness and vp vs ratio is typical of the namaqua natal and cape fold belts

aspects of the dynamic and thermal metamorphic history of the vredefort cryptoexplosion structure implications for its origin abstract the vredefort structure is the oldest and the largest known cryptoexplosion structure on earth an approximately 36 km deep section through the archean sialic crust and the overlying precambrian strata of the kaapvaal craton is exposed in the core of the structure the geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo tectonic history which dates back to at least 3 5 ga the petrographie and geological observations in the basement rocks indicate that there is a complex interrelationship between the archean geology and the 2 0 ga dynamic and thermal metamorphic overprint some of which are postulated to be indicative of impact processes the dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures in particular dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the vredefort discontinuity a brittle ductile shear zone which separates upper crustal amphibolite facies rocks from lower crustal granulites in certain other lithological zones deformation phenomena are noticeably absent or diminished we suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2 0 ga thermal and dynamic metamorphic effects observed at vredefort in conclusion our overall impression of the vredefort structure is that it is a relic of an ancient meteorite impact crater but that there were thermo tectonic events which occurred both prior to and after the postulated impact event which complicates the interpretation of its origin

impact tectonics in the core of the vredefort dome south africa implications for central uplift formation in very large impact structures the 80 km wide vredefort dome presents a unique opportunity to investigate the deep levels of the central uplift of a very large impact structure exposure of progressively older strata in the collar of the dome and of progressively higher grade metamorphic rocks toward its center is consistent with differential uplift however the deepest levels exposed correspond to pre impact mid crust rather than lower crust as has been suggested previously pre impact archean gneissic fabrics in the core of the dome are differentially rotated with the angle of rotation increasing sharply at a distance of 1619 km from the center the present asymmetric dips of the collar strata with layering dipping outward at moderate angles in the southeastern sector but being overturned and dipping inward in the northwestern sector and the eccentric distribution of the pre impact metamorphic isograds around the core of the dome can be reconciled with symmetric rotation of an initially obliquely nw dipping target sequence during central uplift formation the rocks in the core of the dome lack distinctive megablocks or large slip magnitude faults such as have been described in other central uplifts we suggest that the large scale coherent response of these rocks to the central uplift formation could have been accommodated by small scale shear and or rotation along pervasive pseudotachylitic breccia vein fractures

mid crustal granulite facies metamorphism in the central kaapvaal craton the bushveld complex connection abstract the numerous greenstone remnants which occur as inclusions in the archaean gneissic granitoid basement exposed in the core of the vredefort dome display evidence of very high grades of granulite facies metamorphism and an unusually complex three stage metamorphic history 1 in metapelites migmatites and refractory restites were produced in a mid crustal 0 5 gpa m1 anatectic event at temperatures in excess of 900 c in the restites 2 in the migmatites the peak metamorphic assemblages are variably overprinted by a high grade retrograde reaction which involved crystallizing anatectic melts m2 3 both the peak metamorphic assemblages and the retrogression textures are cross cut by pseudotachylitic breccia veins which mark a shock metamorphic event associated with the formation of the vredefort dome and 4 the peak metamorphic assemblages and the m2 retrogression textures are overprinted by new generations of remarkably fine grained post shock high grade phases which crystallised at a pressure some 0 25 gpa lower than that of both the peak metamorphic conditions and the retrogression m3 these data indicate an anticlockwise p t evolution where mid crustal anatexis most probably resulted from the intraplating of bushveld complex related ultramafic magmas at 2 06 ga a period of approximately isobaric cooling and retrogression was followed by the vredefort catastrophe which resulted from the impact of a large meteorite into the terrane at 2 02 ga this shock event halted the natural mid crustal metamorphic progression prior to the terrane cooling below 650 c and resulted in the rapid exhumation of the terrane by some 9 km in the process it provided a window into the deep levels of the central kaapvaal craton not seen elsewhere in the region

ultramafic rocks at the center of the vredefort structure further evidence for the crust on edge model ultramafic rocks from the center of the vredefort structure previously have been interpreted either as early proterozoic igneous rocks genetically related to the bushveld complex or as archean upper mantle rocks re os isotopic data from these rocks display very low 187 os 188 os and 187 re 188 os suggesting that they are not magma chamber cumulates but rather the residues of melt extraction similar in re os systematics to subcontinental lithospheric mantle whole rock re depletion model ages for two samples are 3 3 and 3 5 ga indicating that the postulated melt extraction event must have occurred in the archean on the basis of these data we conclude that the ultramafic rocks are not related to the bushveld event but rather represent upper mantle that rebounded to crustal levels following an impact event this conclusion strengthens evidence for a model whereby the rocks exposed in the core of the vredefort structure represent the lower parts of a crust on edge profile that possibly contains a paleo moho very near the earth9s surface

nature of the archean midcrust in the core of the vredefort dome central kaapvaal craton south africa extreme uplift associated with the formation of the 2 02 ga vredefort dome has exposed a substantial cross section through the crystalline early archean basement complex rocks of the kaapvaal craton the rocks comprise polydeformed high grade tonalite trondhjemite granodiorite ttg gneisses migmatites and late tectonic intrusive granitoids that straddle the upper amphibolite to granulite facies transition field petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal scale layering as has been previously proposed no evidence has been found to support exposure of either a melt depleted refractory lower crust or an upper crustal batholithic granite layer however the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones granitic leucosomes in the upper amphibolite facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks rather than of in situ derivation leucosome compositions in the granulite facies migmatites are more variable ranging from granitic and charnockitic to enderbitic probably reflecting at least some local derivation leucosomes and small granitoid bodies show local scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration and fractionation of minerals such as k feldspar within the melts

radioelement concentrations in the deep profile through precambrian basement of the vredefort structure the vredefort basement granite in south africa forms the core of an undomed and overturned sequence of precambrian strata slawson 1976 proposed that the granite core has also been overturned exposing to view a thick section through the archean crust incrasing in depth toward the center of the dome profiles across the vredefort basement provide an opportunity for studying the distribution of u th and k with depth in the upper and middle crust detailed radioelement concentration profiles across the granitic basement showed that 1 the central part of the basement core is depleted in th and u relative to the perimeter and the concentration of th and u falls off regularly from the basement margin inward 2 the distribution of the radioelements and the distribution of heat production with depth over the outer 8 km are consistent with an exponential depth function 3 the felsic rocks of the central basement are characterized by high k u and th u ratios and the ratios k u k th and th u increase from the margin inward and 4 mafic granulites which occur in the central regions of the dome have normal k u k th and th u ratios high grade metamorphic environment resulted in the depletion of bothmore u and th relative to k in the greater proportion of the felsic rocks from the central regions of the vredefort basement depletion of u was far greater than that of th the normal th u k u and k th ratios in mafic rocks in close proximity to the felsic rocks suggest that mineral assemblages have a strong control on the behavior of the radioelements during regional metamorphism in the vredefort basement less

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

eigenvector analysis of gravity gradient tensor to locate geologic bodies we have developed a new method to locate geologic bodies using the gravity gradient tensor the eigenvectors of the symmetric gravity gradient tensor can be used to estimate the position of the source body as well as its strike direction for a given measurement point the eigenvector corresponding to the maximum eigenvalue points approximately toward the center of mass of the causative body for a collection of measurement points a robust least squares procedure is used to estimate the source point as the point that has the smallest sum of square distances to the lines defined by the eigenvectors and the measurement positions it s assumed that the maximum of the first vertical derivative of the vertical component of gravity vector gzz is approximately located above the center of mass observation points enclosed in a square window centered at the maximum of gzz are used to estimate the source location by increasing the size of the window the number of eigenvectors used in the robust least squares and

integrated geophysical modelling of a giant complex impact structure anatomy of the vredefort structure south africa abstract only three very large confirmed impact structures are known on earth the chicxulub crater mexico 65 ma ca 180 km wide the sudbury structure canada 1 85 ga 200 km in diameter and the vredefort structure in south africa 2 02 ga while extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on earth integrated modelling of gravity and magnetic data constrained by geological as well as refraction and reflection seismic data accomplished the reconstruction of the vredefort impact structure in south africa approximately 250 km wide the original vredefort impact structure covered the whole extent of the archaean witwatersrand basin distinguished by enormous gold resources as it is structurally preserved today in fact it is clear that the preservation of vast volumes of economically important witwatersrand strata is the direct result of the formation of the ring basin around the central uplift the vredefort dome of the impact structure this study is the first attempt to create an integrated and geophysically well constrained model of this very large complex impact structure

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

generation of fragment rich pseudotachylite bodies during central uplift formation in the vredefort impact structure south africa abstract target rocks underlying the central portions of large terrestrial impact structures are characterized by the pervasive presence of fragment rich pseudotachylite bodies debates regarding the formation of these bodies include the origin of pseudotachylitic melts i e friction versus shock induced melting melt mobility causes of target rock fragmentation and timing of fragmentation and melt emplacement with respect to stages of cratering comprehensive structural analysis of pre impact mineral fabrics and properties of fragment rich pseudotachylite in the vredefort dome suggests that melt is allochthonous and was emplaced at an advanced stage of cratering into tensional fracture zones within the crater floor both concentration of bending strains imparted on target rocks during central uplift formation and thermal stresses induced by the emplacement of allochthonous melt led to fragmentation of target rock tensional fracture zones formed in an overall dilational strain field towards the end of cratering likely during collapse of the central uplift and constitute low pressure zones into which melt was forcefully drawn melt may have been drained from the overlying impact melt sheet or from sites within the crater floor and transported into fragment rich dilation zones our field based analysis failed to identify the presence of bona fide shear faults that could potentially have generated in situ frictional melts and fragments rather target rock fragmentation and melt generation that resulted in fragment rich pseudotachylite bodies are processes separated in space and time during cratering

impact induced melting of archean granulites in the vredefort dome south africa i anatexis of metapelitic granulites mid crustal archean pelitic granulites in the vredefort dome experienced a static low p granulite facies overprint associated with the formation of the dome by meteorite impact at 2 02 ga heating and exhumation were virtually instantaneous with the main source of heat being provided by energy released from nonadiabatic decay of the impact shock wave maximum temperatures within a radius of a few kilometres of the centre of the structure exceeded 900 c and locally even exceeded 1350 c this led to comprehensive melting of the precursor archean granulite assemblages grt bt qtz pl ksp crd opx sil followed by peritectic crystallization of aluminous alkali feldspar crd spl crn sil parageneses and the segregation of small evolved biotite leucogranite bodies however at a distance of c 6 km from the centre pre impact rock features are largely preserved although partial replacement of garnet by symplectitic coronas of crd opx spl pl and biotite by orthopyroxene indicate that peak temperatures approached 775 50 c thin interstitial moats of k feldspar are closely associated with the orthopyroxene coronas they are interpreted as the remnants of low proportion partial melts generated by biotite breakdown both the textures and mineral compositional data support reduced equilibration volumes in these rocks which reflect rapid isobaric cooling following shock heating and exhumation the high temperatures and strong lateral thermal gradient are consistent with the modelled impact induced isotherm pattern for a 200 300 km diameter impact crater

insights into the morphology geometry and post impact erosion of the araguainha peak ring structure central brazil the 40 km wide araguainha structure is the largest and best exposed complex impact crater in south america it was excavated in flat lying sediments of the intracratonic parana basin where target rocks are well exposed and have remained undeformed since the impact event ca 245 ma despite the excellent state of preservation and exposure information available on the target rock stratigraphy post impact erosion and morphology of the structure is limited our combined field observations and remote sensing analysis demonstrate that araguainha preserves all features of a shallowly eroded peak ring crater the interior of the structure exposes a central peak surrounded by a 5 km wide annular basin and two main ring features 10 12 and 14 18 km from the center analysis of the pre impact stratigraphy present morphology and crater dimensions indicates that excavation related to the transient cavity formation was extensive in the annular basin but minimal to nonexistent beyond the inner ring feature the formation of the inner ring feature can be reconciled with compressional stresses during outward collapse of the central uplift and inward slumping of the crater walls regional stratigraphic data combined with field observations indicate two periods of post impact erosion associated with exhumation of the parana basin we estimate that 250 350 m of fallback deposits and target rocks have been removed by erosion erosion also accounted for removal of the proximal ejecta immediately outside the rim of the structure in contrast to previous suggestions that the impact took place in a shallow marine environment our observations are more consistent with an extremely shallow brackish to freshwater lacustrine environment with little effect on the developing impact crater

variable sulfur isotope composition of sulfides provide evidence for multiple sources of contamination in the rustenburg layered suite bushveld complex abstract the rustenburg layered suite rls of the bushveld complex bc is famous for its platinum group element pge ore which is hosted in sulfides the source of sulfur necessary to generate this type of mineralization is inferred to be the host rock of the intrusion the rls has a sulfur isotopic signature that indicates the presence of archean surface derived material 33 s 0 in the magma this signature with an average value of 33 s 0 112 0 024 deviates from the expected 33 s value of the mantle of 0 0 008 previous work suggested that this signature is uniform throughout the rls which contrasts with radiogenic isotopes which vary throughout the igneous stratigraphy of the rls in this study samples from key intervals within the igneous stratigraphy were analyzed showing that 33 s values vary in the same stratigraphic levels as sr and nd isotopes however the variation is not consistent in some levels there is a positive correlation and in others a negative correlation this observation suggests that in some cases distinct magma pulses contained assimilated sulfur from different sources textural analysis shows no evidence for late addition of sulfur these results also suggest that it is unlikely that large scale assimilation and or efficient mixing of host rock material in a single magma chamber occurred during emplacement the data do not uniquely identify the source of sulfur in the different layers of the rls but the variation in sulfur isotope composition and its relationship to radiogenic isotope data calls for a reevaluation of the models for the formation and evolution of the rls which has the potential to impact the knowledge of how pge deposits form

a 3d model of first order structural elements of the vredefort dome south africa importance for understanding central uplift formation of large impact structures abstract based on detailed structural mapping geophysical information and structural consideration we generated a 3d structural model of a segment of the vredefort dome this model provides insight into the mechanism of mass transfer during central uplift formation of large impact structures the model consists of geometrically and kinematically linked km scale radial and concentric faults that affected the overturned limb of the rim syncline to the vredefort dome the faults accomplished constrictional centripetal rock flow followed by radial spreading of uplifted and gravitationally unstable rocks in the crater centre more specifically concentric faults formed likely as normal faults during transient crater rim collapse steepened and were transformed to reverse faults during central rock uplift and finally were overturned during gravitational collapse of the central uplift radial faults by contrast formed at a later stage of convergent rock flow towards the crater centre and retained largely their orientation during central uplift formation displacement on individual faults is up to 2 km cumulative displacement notably on concentric faults can account for the large displacement magnitudes predicted by numerical models

the 2 65 ga a type granite in the northeastern yangtze craton petrogenesis and geological implications abstract the presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution due to the poor exposure of archean rocks the early evolution of the yangtze craton has not been well constrained yet in this study a combined study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes have been conducted for a granite pluton at huji town in the northern part of the yangtze craton u pb dating for magmatic zircons yielded a weighted mean age of 2656 6 ma revealing another archean outcrop in the yangtze craton besides the kongling terrane the granite samples show relatively evaluated k 2 o na 2 o cao tfeo mgo ratios and zr nb ce ga and y contents sharing the same features as the a type granite they have pronounced eu sr and ba negative anomalies on the primitive mantle normalized trace element spidergram implying the residual of plagioclase in the magma sources their high zr contents of 440 529 ppm correspond to initial magma temperatures of 930 962 c the zircons have negative hf t values of 2 7 to 0 1 and hf model ages of ca 3 2 ga respectively the 18 o values of the zircons range between 4 94 and 5 72 and give an average of 5 26 0 14 suggesting no contribution of supracrustal rock into the host magma therefore the hf model ages indicate the generation of the huji granite by reworking of ca 3 2 ga juvenile crustal rocks combined with previous studies the finding of the neoarchean huji granites with ca 3 2 ga hf model ages suggests the widespread of archean basement in the yangtze craton the occurrence of the ca 2 65 ga huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the yangtze block

geochemistry across an exposed section of archaean crust at vredefort south africa with implications for mid crustal discontinuities abstract the central region of the vredefort structure consists of a semi circular multi layered sequence of crystalline rocks which are nearly vertical in attitude and which increase in metamorphic grade towards the core of the structure together with the overlying precambrian strata this sequence provides a cross section through almost the entire crustal section of the kaapvaal craton 36 km the upper part of the vredefort crystalline crust consists of a 3 0 ga sequence of differentiated felsic rocks in amphibolite facies the lower crust consists of a complex and heterogeneous both chemically and isotopically high grade metamorphic terrain of charnockites granulites mafic and felsic and supracrustal rocks the upper crust is separated from the lower crust by the vredefort discontinuity a brittle ductile shear zone characterised by a high concentration of pseudotachylite and brecciated rock petrographic chemical and isotopic evidence suggest that the upper and lower crust have undergone very different styles of evolution this is indicative of different geological environments prior to their present juxtaposition we speculate that the upper and the lower parts of the vredefort crystalline crust were juxtaposed during intracratonic thrusting 2 8 ga ago and that the entire 36 km section of crust including the vredefort discontinuity were rotated and uplifted into their present vertical position during the 2 0 ga vredefort catastrophe

thermal metamorphic signature of an impact event in the vredefort dome south africa pseudotachylitic breccias and shock deformation features related to the 2 02 ga formation of the vredefort dome by meteorite impact were overprinted by a static metamorphic event the intensity of which decreased from granulite facies t 700 c in the center of the dome to greenschist facies t 400 c around its margins geobarometric estimates of 0 2 0 3 gpa for the metamorphic parageneses indicate some 8 11 km of erosion since the impact event the strong lateral thermal gradient implied by these p t results is attributed to the combined effects of differential uplift of mid crustal rocks heated along a pre impact geotherm and increased shock heating of the target crust toward the center of the impact structure we suggest that the exceptionally high grade of metamorphism in the center of the dome may in part reflect an elevated regional geothermal gradient of 25 c km in the target crust due to lingering thermal effects related to the 2 05 2 06 ga bushveld magmatic event

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

low pressure metamorphism and anatexis of carolina slate belt phyllites in the contact aureole of the lilesville pluton north carolina usa in the contact aureole of the lilesville granite and comagmatic pee dee gabbro n c greenschist facies phyllites of the carolina slate belt have been overprinted by a series of metamorphic reactions producing opx bearing hornfelses and migmatitic gneisses in the exterior aureole the slate belt assemblage chl ms ep ab qz gives way to the continuous reaction assemblages chl bt cd ms ab ep qz bt cd ms an8 29 qz bt cd kf ms pl qz bt cd als ms kf pl qz and bt cd ga kf pl qz from lowest to highest grade the interior aureole interpreted as part of the floor of the granite bears the continuous and discontinuous reaction assemblages bt cd als kf pl qz bt cd kf pl qz and near the gabbro bt cd ga opx kf pl qz the leucosomes of the migmatitic interior aureole are predominantly trondhjemites with the assemblage an35 45 qz bt cd kf restites in the migmatitic interior aureole contain the afm assemblages bt bt cd bt cd als and bt cd ga plus kf an40 50 and qz contact metamorphism was isobaric at 4 0 5 1 or 2 0 3 5 kb depending on choice of aluminosilicate triple point temperatures reached 650 c in the migmatitic interior aureole and approached 750 c near the gabbro text p text h text 2 text o was less than 0 8 in the migmatites and was lower in the interior aureole and in the high grade exterior aureole partial melting in the migmatitic interior aureole took place during dynamothermal metamorphism caused by the magmatic diapir incipient melting occurred by the reaction bt cd kf pl qz w liquid the melt was h2o undersaturated and coefficients of the reactants were weighted heavily toward the felsic minerals the proportion of felsic minerals in the leucosomes was controlled in part by modal abundance of kf pl and qz available for melting the incorporation of k into biotite by subsolidus reactions coupled with the high thermal stability and low solubility of biotite in a felsic melt are responsible for the trondhjemitic composition of the early anatectic liquids

characterization of magnetite particles in shocked quartz by means of electron and magnetic force microscopy vredefort south africa submicroscopic opaque particles from highly shocked granite gneisses close to the core of the vrede fort impact structure have been investigated by means of micro analytical techniques with high spatial resolution such as electron diaraction orientation contrast imagery and magnetic force microscopy the opaque particles have been identified as nano to micro sized magnetite that occur in several distinct modes in one sample magnetite occurs along relict planar deformation features pdfs in quartz generally accepted as typical shock lamellae the magnetite particles along shock lamellae in quartz grains virtually all show uniform crystallographic orientations in most instances the groups of magnetite within diaerent quartz grains are systematically misori entated such that they share a subparallelh101i direction the magnetite groups of all measured quartz grains thus appear to have a crystallographic preferred orientation in space in a second sample orientations of magnetite particles have been measured in microfractures non di agnostic of shock of quartz albite and in the alteration halos e g biotite grains breaking down to chlorite the crystallographic orientations of magnetite particles are diverse with only a minor portion having a preferred orientation scanning electron microscopy shows that magnetite along the relict pdfs is invariably asso ciated with other microcrystalline phases such as quartz k feldspar and biotite petrographic observations sug gest that these microcrystalline phases crystallized from locally formed micro melts that intruded zones of weakness such as microfractures and pdfs shortly after the shock event the extremely narrow widths of the pdfs suggest that heat may have dissipated rapidly re sulting in melts crystallizing relatively close to where they were generated magnetic force microscopy confirms the presence of magnetic particles along pdfs the smallest particles 5 lm with high aspect ratios 15 1 usually exhibit intense uniform magnetic signals characteristic of single domain magnetite consistent oasets between attractive and repulsive magnetic signals of individual single domain particles suggest consistent directions of magnetization for a large proportion of particles

metamorphism on the moon a terrestrial analogue in the vredefort dome south africa a new model is proposed to explain the origin of enigmatic fine grained granulite facies rocks sampled from the moon based on observations from the vredefort dome south africa the dome is the deeply eroded central uplift of the 300 km diameter vre defort impact structure in the dome fine grained granulites dis playing poikilitic or granoblastic microstructures were formed by relatively slow cooling of shock 6 friction melts derived at t 1350 8c slow cooling was achieved owing to the 7 km depth of burial of the rocks following the impact at least some of the lunar granulitic impactites may also have formed by shock heating and slow cooling at deep levels within the central uplifts of large impact structures without the need for additional heating by younger in trusive or impact melt bodies

formation of pseudotachylitic breccias in the central uplifts of very large impact structures scaling the melt formation abstract the processes leading to formation of sometimes massive occurrences of pseudotachylitic breccia ptb in impact structures have been strongly debated for decades variably an origin of these pseudotachylite friction melt like breccias by 1 shearing friction melting 2 so called shock compression melting with or without a shear component immediately after shock propagation through the target 3 decompression melting related to rapid uplift of crustal material due to central uplift formation 4 combinations of these processes or 5 intrusion of allochthonous impact melt from a coherent melt body has been advocated our investigations of these enigmatic breccias involve detailed multidisciplinary analysis of millimeter to meter sized occurrences from the type location the vredefort dome this complex archean to early proterozoic terrane constitutes the central uplift of the originally 250 km diameter vredefort impact structure in south africa previously results of microstructural and microchemical investigations have indicated that formation of very small veinlets involved local melting likely during the early shock compression phase however for larger veins and networks it was so far not possible to isolate a specific melt forming mechanism macroscopic to microscopic evidence for friction melting is very limited and so far chemical results have not directly supported ptb generation by intrusion of impact melt on the other hand evidence for filling of dilational sites with melt is abundant herein we present a new approach to the mysterium of ptb formation based on volumetric melt breccia calculations the foundation for this is the detailed analysis of a 1 5 3 0 04 m polished granite slab from a dimension stone quarry in the core of the vredefort dome this slab contains a 37 5 dm3 breccia zone the pure melt volume in 0 1 m3 ptb bearing granitic target rock outside of the several decimeter wide breccia zone in the granite slab was estimated at 5 2 dm3 this amount can be divided into 4 6 dm3 melt 88 for which we have evidenced a limited material transport at maximum 20 cm and 0 6 dm3 melt 12 with at most grain scale material transport which we consider in situ formed shock melt the breccia zone itself contains about 10 dm3 of matrix melt assuming melt exchange over 20 cm at the slab surface between breccia zone and surrounding melt bearing host rock volume the outer melt volume is calculated to contain the same amount of melt as contained by the massive breccia zone meso and microscopic observations indicate melt transport is more prominent from larger into smaller melt occurrences thus melt of the breccia zone could have provided the melt fill for all the small scale ptb veins in the surrounding target rock extrapolating this melt capacity calculation for 1 m3 ptb bearing host rock shows that a host rock volume of this dimension is able to take up some 52 dm3 melt scaling up 1000 fold to the outcrop scale reveals that exchange between a host rock volume of 2 m radius around a 37 m3 breccia zone could involve some 10 m3 melt these results demonstrate that large melt volumes i e large breccia zones can be derived in principle from local reservoirs however strong decompression would have to apply in order to exchange these considerable melt volumes which would only be realistic during the decompression phase of impact cratering upon central uplift formation or locally where compressive regimes acted during the subsequent down and outward collapse of the central uplift

archean crustal structure of the kaapvaal craton south africa evidence from the vredefort dome abstract crystalline archean basement rocks in the core of the vredefort dome present a profile through a substantial part of the middle and lower crust of the kaapvaal craton previously this profile has been subdivided into two terranes with allegedly distinct lithologies and tectonometamorphic histories that were juxtaposed along a crustal scale late archean brittle ductile thrust zone lithological and structural mapping across the dome indicates however that the basement lithologies share a common polyphase tectonic history culminating in high grade metamorphism and melting at 3 1 ga no evidence was found of the postulated tectonic terrane boundary but the alleged boundary does coincide with a 1 2 km wide transition zone between upper amphibolite facies migmatitic gneisses and more restitic granulite facies gneisses the implications of these results for archean regional tectonic models for the kaapvaal craton are discussed

archean age for the granulite facies metamorphism near the center of the vredefort structure south africa granulite facies metamorphic assemblages in rocks exposed near the center of the 2 02 ga vredefort impact structure previously have been interpreted either as early proterozoic genetically related to the 2060 ma bushveld complex or as archean and representative of lower crust that rebounded to upper crustal levels following an impact event zircon and monazite recovered from the granulite facies rocks record high grade metamorphism at 3107 9 ma and a primary age of 3425 ma for detrital zircon a shock deformed but otherwise pristine dolerite dike that intrudes the granulite terrane yields a u pb zircon age of 2560 ma providing a minimum age for the metamorphism these isotopic age data are difficult to reconcile with a regional high grade metamorphic event in the crust beneath vredefort at 2060 ma instead the preimpact high temperature metamorphic history observed in the vredefort lower crustal rocks indicates an enigmatic high temperature event during the stabilization of diamondiferous archean tectosphere

magnetic anomaly near the center of the vredefort structure implications for impact related magnetic signatures a strong magnetic anomaly near the center of the ancient and deeply eroded vredefort structure is attributed to remanent magnetization caused by a large meteorite impact at 2 0 ga the rocks underlying the anomaly are archean gneisses thought to represent mid crustal depths that were uplifted to the surface during the postulated impact event measurements of the remanent magnetization of the basement rocks yielded consistent vectors of declination 25 inclination 56 k 18 16 that correspond to the paleomagnetic pole position at time of impact petrologic studies indicate that during impact large volumes of rock were heated enough to cause thermal remagnetization in the ambient field thermal effects of all large impacts on cratons may induce a remanent magnetization of sufficient intensity to cause anomalies in the geomagnetic field that are detectable even by satellites

stabilization of the southern portion of the sao francisco craton se brazil through a long lived period of potassic magmatism abstract although the southern portion of the sao francisco craton in southeast brazil encompasses one of the largest segments of meso to neoarchaean crust in the south american plate there is little understanding of the processes leading to cratonization of this region our combined field and u pb age data show that the crust experienced massive intrusions of potassic k rich granitoids between 2750 and 2600 ma and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions thus deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the sao francisco crust attained buoyancy to survive recycling regional mapping indicates that the potassic granitoids are distributed over an area of 25 000 km 2 and are related to one of the most prolific periods of potassic magmatism in the craton la icp ms and shrimp u pb data from 16 samples of potassic granitoids together with published tims ages suggest a long lived period of magmatism from 2750 to 2700 ma and a small pulse of granite crystallisation at ca 2612 ma the u pb shrimp and la icp ms data also indicate that the archaean granitoids do not record evidence of u pb zircon resetting our interpretation is that partial melting and transport of granitoid melts rich in heat producing elements to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment in the craton area became sufficiently refractory and resistant to further partial melting

the formation of the sudbury breccia in the north range of the sudbury impact structure abstract in the north range of the ca 1 85 ga sudbury impact structure massive bodies of sudbury breccia cut across the archean levack gneiss complex and early proterozoic matachewan diabase dykes the complex underlies the sudbury igneous complex sic which represents an impact melt sheet that lined the floor of the impact crater the breccia occurs as 1 100 m wide irregular bodies that contain rounded clasts of diabase and tonalitic and dioritic levack gneiss within a fine grained to aphanitic black to dark grey matrix mineral chemical analyses and rietveld analyses based on x ray diffraction patterns show that the breccia matrix contains microclasts of magnesiohornblende oligoclase and quartz surrounded by metamorphic andesine laths actinolite kfeldspar albite and chlorite which crystallized during cooling of the sic on binary oxide sio2 diagrams breccia values plot on a compositional mixing line between those of the more felsic tonalitic gneiss and the more mafic dioritic gneiss and diabase the breccia has concentrations in rare earth elements rees intermediate between those of the dioritic and tonalitic gneisses and a ree pattern with a negative slope that is less pronounced than those of the gneisses this is due to the addition of a minor diabase component that has a slightly negative ree slope suggesting that the breccia is a mix of comminuted tonalitic gneiss dioritic gneiss and minor diabase comminution and cataclasis of these source rocks occurred along pre existing anisotropies and fractures that formed and were reactivated during the growth and collapse of the transient crater the comminuted materials mixed together as large bodies of breccia that were injected in dilational sites that opened during the upward expansion and collapse of the crater

emplacement history of granophyre dikes in the vredefort impact structure south africa inferred from geochemical evidence abstract the central vredefort impact structure is characterised by impact melt rocks known as the vredefort granophyre dikes the mode of emplacement of which is not well known whole rock and petrographic analyses of two dikes were conducted and compared to published geochemical data to elucidate the mode and timing of dike formation the dikes are characterised by compositional and textural heterogeneity between and within individual dikes specifically central dike portions are felsic and rich in wall rock fragments whereas marginal dike phases are more mafic and fragment poor collectively this suggests that melt was derived from compositionally different parental melts and emplaced in at least two pulses in addition the chemical heterogeneity between fragment rich and fragment poor dike zones can be explained by variable assimilation of a mafic component notably ventersdorp basalt at the base of the impact melt sheet from which melt of the granophyre dikes is derived this scenario accounts for the mafic and fragment poor character of melt emplaced first in the dikes and the more felsic and fragment rich nature of melts of the following emplacement pulse i e when the impact melt was less hot and thus less capable of digesting large quantities of mafic wall rock fragments differences in geometrical textural chemical and fragment characteristics between the granophyre dikes and pseudotachylite bodies can be explained by the same process i e impact melt drainage but operating at different times after impact

reply to comment by w u reimold r l gibson and h henkel on muundjua et al 2007 magnetic imaging of the vredefort impact crater south africa epsl 261 pp 456 468