oxygen and hydrogen isotope stratigraphy of the rustenburg layered suite bushveld complex constraints on crustal contamination new do values for plagioclase pyroxene and olivine and limited whole rock dd values are presented for samples from the rustenburg layered suite of the bushveld complex south africa in combination with existing data these provide a much more complete composite o isotope stratigraphy for the intrusion throughout the layered suite mineral do values indicate that the magmas from which they crystallized had do values that were about 7 1 that is 1 4 higher than expected for mantle derived magmas suggesting extensive crustal contamination more limited h isotope data suggest that the oh present within whole rocks regardless of the degree of alteration is of magmatic origin and not an alteration phenomenon there appears to be no systematic change in do value with stratigraphic height and this requires the contamination to have taken place in a staging chamber before emplacement of the magma s into the present chamber large amounts 30 40 of contamination by the lower to middle crust are needed to explain these do values which is in general agreement with previous estimates based on srand nd isotope data alternatively smaller amounts of contamination 20 by sedimentary rocks or their partial melts represented by the country rock can explain the data but it is not apparent how such material could have been present at the depth of the staging chamber in the lower to middle crust

hf isotopic evidence for a cogenetic magma source for the bushveld complex and associated felsic magmas abstract here we test the hypothesis that the rhyolitic lavas of the rooiberg group and granophyres associated with the roof of the bushveld complex are differentiation products of bushveld age mafic liquids we present lu hf isotopic compositions in zircons from roof rocks that have been interpreted to represent thermally metamorphosed and remelted rooiberg group lavas and from granophyres interpreted to be differentiation products of the cumulate rocks that make up the bushveld complex all of these rocks were found to possess e hf 2 06 ga statistically indistinguishable from the intrusion wide average e hf 2 06 ga value of 8 6 1 2 of the bushveld complex our results combined with chronologic and field relations suggest that the felsic rocks were generated by fractional crystallization of bushveld mafic liquids including those that gave rise to the cumulate rocks of the bushveld complex

chemical characteristics of migmatites accessory phase distribution and evidence for fast melt segregation rates equilibration between melt and solid is inhibited by rapid melt extraction and by restricted equilibration armouring slow dissolution when segregation occurs by channelised migration along high porosity pathways melt migration is more rapid than trace element diffusion rates in silicates and faster than accessory phase dissolution rates evidence for channelised flow and deformation enhanced melt segregation into boudin necks fractures and micro shears at low melt fractions is present in the moine kirtomy migmatitie suite kms in sutherland scotland melt migration distances are on a metre to tens of metres scale concordant leucosomes in stromatic migmatities in the kms have low zr contents low lree light rare earth element and h heavy ree contents and positive eu anomalies ree patterns of this type can be produced by removal of leucosome before complete equilibration with source due to the inhibited dissolution of lree and hree bearing accessory phases in water undersaturated melts melting in the kms however occurred at or near the wet granite solidus leaving biotite as a residual phase detailed back scattered electron imaging shows that ree bearing accessory phases remained as residual phases and were concentrated in the melanosome and at the melanosome leucosome boundary irregularly shaped patches of diatexite contain a small proportion of excess zr consistent with entrainment of melanosome schlieren enriched in zircon these data indicate that deformation enhanced melt extraction led to the rapid migration of small melt fractions from the melting site on a time scale less than that required to saturate the melt in zr leucosomes were thus prevented from equilibrating with accessory phases before extraction

u pb and hf isotope record of detrital zircons from gold bearing sediments of the pietersburg greenstone belt south africa is there a common provenance with the witwatersrand basin abstract results of combined u pb and hf isotope analyses on detrital zircons from coarse grained clastic sediments of the pietersburg greenstone belt provide evidence for four major magmatic activities in the hinterland at ca 3 29 3 26 3 21 and 2 96 ga during which new crust was formed from depleted mantle sources the latter interpretation is supported by mostly superchondritic hf t of 4 3 to 4 2 at 2 96 ga 0 1 to 5 3 at 3 21 ga 0 2 to 3 2 at 3 26 ga and 0 0 to 6 2 at 3 29 ga the observed hf t variations and the finding of abundant magmatic but also metamorphic zircon overgrowths with ages at ca 2 96 ga furthermore suggests that the youngest magmatic event took place in a continental arc setting whereby juvenile magmas were mixed with older reworked crust a similar setting is also suggested for the three older magmatic events based on our datasets deposition and metamorphism of the pietersburg sediments is bracketed between 2 88 and 2 69 ga in agreement with the deposition age of the gold bearing central rand group of the witwatersrand basin the occurrence of 2 98 2 88 ga zircon populations with mostly superchondritic but variable hf t in the pietersburg and central rand group sediments indicate that a vast amount of detritus for both areas was supplied from proximal relative juvenile magmatic arc sources however the general absence of 3 15 3 00 ga zircons in the pietersburg sediments but their abundant occurrence in the witwatersrand basin indicates that detritus supply for both basins came either from completely different sources or that 3 15 3 00 ga basement was additionally sampled during southward transport

alkenone and boron based pliocene pco2 records abstract the pliocene period is the most recent time when the earth was globally significantly 3 c warmer than today however the existing pco2 data for the pliocene are sparse and there is little agreement between the various techniques used to reconstruct palaeo pco2 this disagreement coupled with the general low temporal resolution of the published records does not allow a robust assessment of the role of declining pco2 in the intensification of the northern hemisphere glaciation inhg and a direct comparison to other proxy records are lacking for the first time we use a combination of foraminiferal 11b and organic biomarker alkenone derived carbon isotopes proxies to determine the concentration of atmospheric co2 over the past 5 ma both proxy records show that during the warm pliocene pco2 was between 330 and 400 ppm i e similar to today the decrease to values similar to pre industrial times 275 285 ppm occurred between 3 2 ma and 2 8 ma coincident with the inhg and affirming the link between global climate the cryosphere and pco2

the bushveld metamorphic aureole in the potgietersrus area evidence for a two stage metamorphic event the metamorphic aureole of the bushveld complex in the potgietersrus area was created in a two stage metamorphic event in the first stage which was related to the intrusion of the lower zone magma maximum temperatures of 750 degrees c were attained at 1 5 kb pressure pressure estimates are based on the rare cordierite olivine orthopyroxene or spinel or quartz assemblage at lower temperatures the cordierite chloritoid assemblage also appears to have been stable equilibrium temperatures and pressures during the second stage which was brought about by the intrusion of more voluminous gabbroic liquids that gave rise to the upper critical main and upper zones were considerably higher at 900 degrees c and 4 to 5 kb pressure geothermobarometry is based on the garnet cordierite orthopyroxene biotite clinopyroxene plagioclase quartz and cordierite spinel or sillimanite or corundum or orthopyroxene or olivine assemblages the temperature dependence of the fe mg exchange reaction between cordieritc and spinel was used as the basis of a geothermometer calibrated against published data for natural mineral assemblages high alumina 44 wt low silica 36 wt and low alkali pelites from the timeball hill formation also originated during this stage through the removal of about 65 percent granitic melt from normal pelites the maximum pressures calculated for the second stage of the metamorphic event are about 2 kb higher than the lithostatic pressure that could have been exercised by the presently exposed succession of layered and roof rocks this difference can be accounted for by a directed stress contribution for which there is evidence in the form of a large fold in the floor rocks and by assuming that the lithostatic pressure during the second stage of the metamorphic event was about 1 0 kb higher than the value suggested by the current field relations brought about by superincumbent load of layered rocks

on the relationship between the bushveld complex and its felsic roof rocks part 2 the immediate roof emplacement of large volumes of mafic magma into the crust undoubtedly causes significant thermal perturbation to the overlying crust despite the clear importance of the country rock in modulating the thermal evolution the bushveld complex little is known about the nature and extent of its roof zone this manuscript details the lateral variability of the rocks that make up the immediate roof of the intrusion in the eastern limb in the northern segment of the eastern bushveld the roof is dominated by thermally metamorphosed metapelites in the central segment the roof is dominated by highly metamorphosed meta volcanic rocks and their partially molten equivalents and in the southern segment the roof is likely composed of modestly thermally metamorphosed felsic volcanic rocks the variability of roof lithology is also reflected in the variability of floor rocks to the intrusion a new model for the emplacement of the eastern bushveld complex is proposed in which the mafic magmas intrude at a deeper level in the north and become shallower to the south

archaeomagnetic studies in central mexico dating of mesoamerican lime plasters abstract for the first time results of an archaeomagnetic study of unburned lime plasters from teotihuacan and tenochtitlan in central mesoamerica are presented plasters made of lime lithic clasts and water appear during the formative period and were used for a variety of purposes in floors sculptures ceramics and supporting media for mural paintings in the oaxaca and maya area in central mexico grinded volcanic scoria rich in iron minerals is incorporated into the lime plasters mixture samples were selected from two archaeological excavation projects in the teopancazco residential compound of teotihuacan and the large multi stage structure of templo mayor in tenochtitlan where chronological information is available the intensity of remanent magnetization natural remanent magnetization nrm and low field susceptibility are weak reflecting low relative content of magnetic minerals nrm directions are well grouped and alternating field demagnetization shows single or two component magnetizations rockmagnetic experiments point to fine grained titanomagnetites with pseudo single domain behavior anisotropy of magnetic susceptibility ams measurements document a depositional fabric with normal to free surface minimum ams axes characteristic mean site directions were correlated to the paleosecular variation curve for mesoamerica data from templo mayor reflect recent tilting of the structures teopancazco mean site declinations show good correspondence with the reference curve in agreement with the radiocarbon dating dates for four stages of teotihuacan occupancy based on the study of lime plasters range from ad 350 to 550 a date for a possible mazapa occupation around ad 850 or 950 is also suggested based on the archaeomagnetic correlation the archaeomagnetic record of a plaster floor in teopancazco differed from the other nearby sites pointing to a thermoremanent magnetization comparison with the reference curve suggests dates around ad 1375 or 1415 the burning of the stucco floor likely occurred during a late re occupation of the site by the aztecs our results suggest that archaeomagnetic dating can be applied to lime plasters which are materials widely employed in mesoamerica

crustally contaminated komatiite primary source of the chromitites and marginal lower and critical zone magmas in a staging chamber beneath the bushveld complex in the lowermost rocks of the rustenburg layered suite bushveld complex levels of total cr 2 o 3 are up to ten times higher than its putative solubility 0 16 cr 2 o 3 in the magmas that many investigators have accepted as parental to the dunites harzburgites pyroxenites and norites of the lower lower critical and upper critical zones various existing models fail to explain this chromium paradox in so far as they may explain why chromite is precipitated but do not account for its abundance in many cumulates simple mixing calculations and quantitative adiabat 1ph runs support our model in which a heterogeneous liquid crystals system was evolved in a staging chamber beneath the complex by mixing of komatiitic melts with 20 to 30 of crustal melts at depths appropriate to pressures between 10 and 4 5 kbars we contend that crystal settling and the effects of elutriation during magma ascent had generated a high sio 2 cap with modal olivine orthopyroxene and chromite increasing with depth in the staging chamber the magma from the high sio 2 cap of the staging chamber was first injected into the largely sedimentary transvaal supergroup generating what is now termed the marginal zone slurries of crystals plus liquid from the spasmodically rejuvenated main body of the staging chamber were then injected into the shallower bushveld chamber to form the lower and critical zones chromite within the slurries introduced in this way could account for their high gross cr content frequent reversals of normal evolutionary trends evident in the cumulates are attributable to the episodic nature of the influxes from the staging chamber the lithologically diverse nature of the floors upon which the chromitite layers accumulated peridotites pyroxenites norites and anorthosites was therefore more a function of the timing of emplacement of the cr rich slurries generated in the deeper reservoir than of specific evolutionary processes within the bushveld chamber itself

zircon u pb hf isotope systematics of transvaal supergroup constraints for the geodynamic evolution of the kaapvaal craton and its hinterland between 2 65 and 2 06 ga abstract the transvaal basin in south africa hosts a 15 km thick pile of sedimentary successions deposited over a period of 600 ma during the neoarchean to paleoproterozoic presently little is known about the source of these sediments as well as about the tectono magmatic evolution in the hinterland of the transvaal basin preventing detailed geotectonic correlations of the kaapvaal craton kc with other cratons worldwide to solve this problem we present the first systematic study of combined u pb and lu hf isotope data of 2000 detrital zircons from fourteen formations of the transvaal supergroup these reveal that clastic sedimentary rocks were supplied from sources on and off the present day kc detrital zircons in conglomerates of the wolkberg and black reef formations maximum deposition ages at 2769 8 and 2618 11 ma respectively were mainly supplied from surrounding kc either from pietersburg block basement pbb and or from eroded sedimentary successions of the witwatersrand pongola and or ventersdorp supergroups in contrast clastic sedimentary rocks of the rooihoogte duitschland and timeball hill formations maximum deposition ages at 2353 18 ma 2342 18 and 2290 8 ma respectively were predominately supplied from a juvenile neoarchean terrane junat formed at 2570 2500 ma ehf2500 ma 2 to 9 and intensely reworked at 2400 ma and to a minor amount from a composite archean terrane cat emplaced by granitoids between 3540 and 2680 ma and affected by crust reworking at 2570 2430 ma ehf2 5ga 3 to 12 in a neoarchean to paleoproterozoic continental arc terrane npcat subsequent periodic reworking of junat at 2250 2220 ma and 2120 ma is recorded by detrital zircons in sandstones of the overlying boshoek dwaalheuwel daspoort magaliesberg and post magaliesberg formations having maximum deposition ages at 2243 7 2242 7 2240 7 2080 7 and 2068 7 ma respectively the archean zircons age 2650 ma in all these formations were mainly supplied from pbb the new data sets also suggest that the kc was connected to cat npcat and junat at

the halogen geochemistry of the bushveld complex republic of south africa implications for chalcophile element distribution in the lower and critical zones solidification of the interstitial liquid the stratigraphic distribution halogen bearing minerals especially apatite are minor but ubiof s cu and the pge in the critical zone cannot readily be quitous phases throughout the bushveld complex interstitial apatite explained either by precipitation of sulfide as a cotectic phase or as is near end member chlorapatite below the merensky reef lower a function of trapped liquid abundance evidence from potholes and and critical zones and has increasingly fluorian compositions with the pge rich driekop pipe of the bushveld complex imply that increasing structural height above the reef main and upper zones migrating cl rich fluids mobilized the base and precious metal cl f variations in biotite are more limited owing to crystal chemical sulfides we suggest that the distribution of sulfide minerals and controls on halogen substitution but are also consistent with a the chalcophile elements in the lower and critical zones reflects a decrease in the cl f ratio with structural height in the complex general process of vapor refining and chromatographic separation of a detailed section of the upper lower zone to the critical zone is these elements during the evolution and migration of a metalliferous characterized by an upward decrease in sulfide mode from 0 01 cl rich fluid phase 0 1 to trace 0 001 cu tends to correlate with other incompatible elements in most samples whereas the platinum group elements pge can behave independently particularly in the critical zone the decrease in the cl f ratio of apatite in the

significance of 87 sr 86 sr ratios in the merensky cyclic unit of the bushveld complex in a number of mafic layered intrusions layers with platinum group element pge mineralization coincide with breaks in cumulus mineral composition and isotopic variation trends through the layered sequence1 2 for example in the bushveld complex in south africa abrupt increases in initial 87sr 86sr ratios occur at the ug 2 and merensky pegmatoid or reef horizons both of which are exploited for pges we report here the results of a detailed sr isotope study across the merensky unit and its immediate foot and hangingwall rocks which when considered together with new ideas on processes operating in magma chambers3 5 suggest that magma mixing and post cumulus infiltration of liquids displaced from below were important during crystallization of the merensky pegmatoid

volcanism of the palaeoproterozoic bushveld large igneous province the rooiberg group kaapvaal craton south africa abstract the volcanic rocks of the rooiberg group represent the uppermost unit in the palaeoproterozoic transvaal supergroup and form one of the largest provinces of silicic volcanic rocks in the world although stratigraphically associated with the transvaal basin fill the rooiberg group is petrogenetically linked with the larger bushveld magmatic event for which emplacement was preceded by the extrusion of the vast rooiberg lava flows in the northern part of the kaapvaal craton like many silicic dominated large igneous provinces lips which are increasingly recognised in the rock record the rooiberg group volcanics are intracontinental subaerial and are dominated by voluminous lava flows originally the rooiberg group is inferred to have covered an area of more than 200 000 km2 of which after erosion an area of 50 000 67 000 km2 remains the lava flows form a stratigraphic succession up to 6 km thick and are divided into four formations in ascending order dullstroom damwal kwaggasnek and schrikkloof due to a scarcity of reliable geochronological data the temporal span of the rooiberg group is poorly understood the rooiberg group consists of basaltic to rhyolitic lava erupted from fissural volcanism with estimated eruption temperatures of the rhyolitic lavas exceeding 1000 c minor explosive eruptions are represented by pyroclastic rocks and subordinate sedimentary interbeds originated from sandy fluvial and lacustrine processes the rocks are essentially undeformed and have not been buried so that their original textures are well preserved the bushveld complex and the associated rooiberg group lava flows are proposed to have formed as a result of partial melting of subcontinental lithosphere and lower crust by a mantle plume this thorough review of the geochronology physical volcanology and geochemistry of the rooiberg group enables construction of a geodynamic model

in situ oxygen isotope major and trace element constraints on the metasomatic modification and crustal origin of a diamondiferous eclogite from roberts victor kaapvaal craton a subducted oceanic crustal origin for most eclogite xenoliths in kimberlites has long been a cornerstone of tectonic models for craton development however eclogite xenoliths often have protracted and complex histories involving multiple metasomatic events that could overprint some of the key geochemical indicators typically taken as evidence of a subducted origin e g garnet 18o values and mineral 87sr 86sr compositions to assess the potential for disturbance of oxygen isotopic compositions in mantle eclogites via diamond forming and other possible metasomatic fluids we have conducted a multi technique in situ study of a diamondiferous eclogite xenolith from the roberts victor kimberlite s africa using sims we provide the first texturally controlled in situ measurements of 18o values in eclogitic garnet in close proximity to diamond garnet and clinopyroxene modal proportions are heterogeneous in the xenolith and garnet compositions vary from mg 75 8 79 2 grossular proportions 8 05 10 14 mol and omphacitic pyroxene has jd13 24 and mg 86 6 90 0 rare earth element patterns of minerals across the xenolith including grains close to diamond are typical lree depleted garnets and markedly lree enriched pyroxenes these silicate minerals also record detectable intra and inter grain lree abundance variations clinopyroxenes of the studied xenoliths show hfse and sr abundance variations that are decoupled from lree contents and major element variations mineralogical constraints and bulk rock reconstructions indicate that the studied sample likely experienced selective incompatible element enrichment during small volume 0 03 wt infiltration of metasomatic fluid s potentially linked to ancient diamond evolution intra grain major element lree and hfse variations in clinopyroxene resulted from late stage metasomatism oxygen isotope compositions in garnet are decoupled from all major and trace element variations with garnet 18o values being uniform across the xenolith in a wide variety of textural settings garnet 18o values of 6 5 0 2 are higher than the mean 5 19 0 26 of the mantle garnet range 4 8 5 5 modelling of the buffering effect of mantle peridotite on co2 rich and h2o rich metasomatic fluids at temperatures within the diamond stability field indicates that the likelihood of a metasomatic fluid with exotic oxygen isotopic composition arriving at a mantle eclogite body with its isotopic composition unmodified after percolative flow through dominantly peridotitic mantle at great depth is very low as we find no evidence of metasomatically induced garnet oxygen isotope variations in the studied diamondiferous eclogite xenolith we conclude that the most likely origin for the elevated garnet 18o values is via inheritance from a crustal protolith altered at relatively low temperatures these results have broader relevance and support the hypothesis of a low pressure protolith for mantle eclogite xenoliths demonstrating the robust nature of garnet oxygen isotope compositions even in diamond bearing eclogites

trapped intercumulus liquid in the main zone of the eastern bushveld complex south africa lateral variations in the amount of trapped intercumulus liquid in the main zone of the eastern bushveld complex are constrained by new sr isotopic whole rock and mineralogical data from three profiles that are separated laterally by ca 100 km and represent thicknesses of 551 1 127 m of main zone gabbronorites below the pyroxenite marker an analysis of the an contents 100 ca ca na of plagioclase cores within the thornhill north roossenekal and stoffberg south profiles show similar systematic up section variations from an66 to an59 in contrast both the an contents of bulk plagioclase separates and the mg numbers 100 mg mg fet of orthopyroxenes show pronounced lateral variations from thornhill an67 61 mg 67 61 through roossenekal an64 58 mg 64 55 to stoffberg an59 55 mg 59 50 these mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0 at thornhill through 10 30 at roossenekal to 30 45 at stoffberg a two to eightfold southward increase in whole rock concentrations of p ti y zr and ba is consistent with the trapped liquid model however the 14 fold increase in rb from thornhill to stoffberg is too great to be accounted for by trapped liquid alone but can be explained by local assimilation of partial melts of the country rock constant initial 87sr 86sr isotopic ratios of main zone plagioclase separates 0 7081 0 7085 in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0 6924 0 7096 the southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at stoffberg

geochemistry of contrasting siliceous magmatic suites in the bushveld complex genetic aspects and implications for tectonic discrimination diagrams abstract the intracratonic bushveld igneous province that formed 2 1 1 9 ga ago contains three contrasting suites of siliceous rocks that are demonstrably of magmatic origin the oldest of these are the high mg hmf and low mg lmf felsites which form interstratified flows in the rooiberg group bushveld granites intrude the rooiberg group and constitute the youngest component of the province well defined interelement variation trends illustrate that the granites do not share the same petrogenetic history as the rooiberg magmas nd isotope measurements indicate that the two eruptive suites probably formed under similar differentiation conditions but from parental magmas that were derived from compositionally different sources on trace element discrimination diagrams the bushveld granites and lmf are usually correctly assigned to a within plate setting but conversely the hmf are generally misclassified as subduction related eruptives it is argued that the trace element signatures of the granites and felsites do not identify their tectonic setting per se but rather point to the melting and crystallization histories of the source regions from which their parent magmas were extracted as such tectonic discrimination diagrams may provide valuable pointers to processes that have affected igneous source materials in much earlier magmatic cycles

a linear hf isotope age array despite different granitoid sources and complex archean geodynamics example from the pietersburg block south africa abstract combined u pb and lu hf isotope data from zircon populations are widely used to constrain hadean archean crustal evolution linear hf isotope age arrays are interpreted to reflect the protracted internal reworking of crust derived from the depleted mantle during a short lived magmatic event and related 176lu 177hf ratios are used to constrain the composition of the reworked crustal reservoir results of this study however indicate that hf isotope age arrays can also result from complex geodynamic processes and crust mantle interactions as shown by u pb and lu hf isotope analyses of zircons from well characterized granitoids of the pietersburg block pb northern kaapvaal craton south africa apart from scarce remnants of paleoarchean crust most granitoids of the pb with ages between 2 94 and 2 05 ga n 32 define a straight hf isotope age array with low 176lu 177hf of 0 0022 although they show a wide compositional range were derived from various sources and emplaced successively in different geodynamic settings the crustal evolution occurred in five stages i predominately mafic crust formation in an intra oceanic environment 3 4 3 0 ga ii voluminous ttg crust formation in an early accretionary orogen 3 0 2 92 ga iii internal ttg crust reworking and subduction of ttg derived sediments in an andean type setting 2 89 2 75 ga iv post collisional high k magmatism from both mantle and crustal sources 2 71 2 67 ga and v alkaline magmatism in an intra cratonic environment 2 05 2 03 ga the inferred array results from voluminous ttg crust formation during stage ii and involvement of this crust during all subsequent stages by two different processes i internal crust reworking through both partial melting and assimilation at 2 89 2 75 ga leading to the formation of biotite granites coeval with minor ttgs and ii subduction of ttg derived sediments underneath the pb causing enrichment of the mantle that subsequently became source for high k granitoids and mafic rocks at 2 68 and 2 05 ga some scatter along the array might have resulted either from significant assimilation of ancient crust intracrustal lu hf fractionation or melting of heterogeneous mantle sources those results show that without any information about the nature and composition of zircon hosting granitoids hf isotope age data are of limited use to constrain hadean archean magmatogenesis and geodynamics

hafnium isotopic composition of the bushveld complex requires mantle melt upper crust mixing new evidence from zirconology of mafic felsic and metasedimentary rocks

an isotopic and geochemical study of the northern kaapvaal craton and the southern marginal zone of the limpopo belt are they juxtaposed terranes abstract to test existing lateral accretion models for the archean kaapvaal craton in south africa metapelites and leucocratic granitoids from two different provinces the granulite facies southern marginal zone of the limpopo belt and the northern part of the adjacent greenschist to amphibolite facies kaapvaal craton have been studied both provinces resemble each other in terms of isotope and trace element characteristics they show a general depletion of incompatible elements independent from the grade of metamorphism this observation conflicts with theories involving mobilisation during granulite facies metamorphism a further common fingerprint is the unusually high cr and ni concentrations in the metapelites in addition they define a very narrow range of nd model ages 2 9 3 05 ga constraining the average crustal age of their cratonic source region the granitoid gneisses show somewhat larger variations of nd model ages indicating some age heterogenity within the crust however the data from both provinces completely overlap the southern marginal zone and the kaapvaal craton also show uniform low 207 pb 204 pb relative to their 206 pb 204 pb ratios that differ clearly from other southern african provinces these results support the view that the southern marginal zone represents a part of the kaapvaal craton exposed at a deeper crustal level it rules out a hypothesis that both provinces are separate amalgamated terranes furthermore there is very little difference between granitoids and metapelites which indicates that the latter largely derived from the former the exposure of the granulite facies metapelites of the southern marginal zone at the same level as the lower grade metamorphic rocks of the kaapvaal craton indicates uplift along a sharp tectonic boundary

melt generation and fluid flow in the thermal aureole of the bushveld complex granite sheets emplaced into the migmatite zone of the eastern contact aureole of the bushveld complex resulted from fluid enhanced incongruent biotite melting of the underlying silverton formation shales during prograde metamorphism ba concentrations are extreme in both the sheets 1000 ppm and the hornfels 800 ppm into which they have been emplaced we conclude that a ba rich hydrothermal fluid induced melting in the aureole and that fluid transport of ba2 and to a lesser extent sr2 and eu2 persisted in the melt zones under subsolidus conditions sr isotope systematics from high ba localities define an errorchron of 2161 106 ma with an initial 87sr 86sr ratio of 0 705 0 001 metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids there is no evidence of regional homogenization oxygen isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact suggesting the infiltrating fluid equilibrated isotopically with the metasediments their elevated 18o values 11 3 12 1 are consistent with a fluid source from devolatilization of the sedimentary lithologies textural analysis at both outcrop and thin section scale of arkose and psammites in the aureole lakenvalei and magaliesberg formations shows that the extent of melting was highly heterogeneous even on the grain scale and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt which preserve textures inherited from the regionally metamorphosed protolith simple finite difference thermal modelling of the aureole suggests that the width of the melt zone 500 m is inconsistent with conductive heat transfer and hence that the thermal structure has been modified by fluid advection

accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf acceleration exceeds 27 km 3 per year and ice is thinning at rates of tens of meters per year we attribute this abrupt evolution of the glaciers to the removal of the buttressing ice shelf the magnitude of the glacier changes illustrates the importance of ice shelves on ice sheet mass balance and contribution to sea level change index terms 1827 hydrology glaciology 1863 1863 hydrology snow and ice 1827 3349 meteorology and atmospheric dynamics polar meteorology 6924 radio science interferometry 9310 information related to geographic region antarctica citation rignot e g casassa p gogineni w krabill a rivera and r thomas 2004 accelerated ice discharge from the antarctic peninsula following the collapse of larsen b ice shelf geophys res lett 31 l18401 doi 10 1029 2004gl020697

on the relationships between the bushveld complex and its felsic roof rocks part 1 petrogenesis of rooiberg and related felsites a major question concerning the bushveld complex is the relationship between the layered mafic rocks and the overlying rooiberg group felsites and related granophyres here we assemble bulk rock analyses to gain insight into this question and investigate the petrogenesis of the felsic rocks the data indicate that the rooiberg group consists of distinctive magnesian and ferroan lavas the former dominates the basalts to rhyolites of the basal dullstroom formation while nearly all the dacites to rhyolites of the overlying damwal kwaggasnek and schrikkloof formations are ferroan the ferroan rocks also include the stavoren granophyre which exists regionally as a several hundred meter thick concordant sheet between the bushveld complex and rooiberg lavas the compositions of the magnesian lavas are similar to calc alkaline granitoids found in convergent margins suggesting that the lavas could have originated in a mantle affected by previous archean subduction events that are recorded by xenoliths and inclusions in diamonds from most kaapvaal kimberlites in contrast the compositions of the ferroan lavas indicate formation by fractional crystallization of basaltic liquids and are essentially identical to ferroan rhyolites associated with mafic rocks from other settings the hypothesis that these rocks are fractional crystallization products of bushveld mafic liquids is consistent with published radiogenic and stable isotope data and known age relations based on compositional characteristics and geologic relations the stavoren granophyre is the most likely candidate for the residual liquid that escaped from the top of the bushveld complex whether the bulk of the bushveld province ferroan rhyolites formed in the chamber of the extant layered mafic sequence or in a deeper hidden crustal magma reservoir remains unclear

pge tenor and metal ratios within and below the merensky reef bushveld complex implications for its genesis the merensky reef is the consequence of several major influxes of magma into the bushveld magma chamber that gave rise to at least two cyclic units including the pre merensky and merensky cyclic units analysis of the s ni cu platinum group element pge and au contents of 1140 samples from 24 profiles through the rocks forming these units and two profiles extending downward from the reef to close to the ug 2 horizon of the western bushveld complex provide important petrogenetic constraints on the origin of the reef particular attention is paid to variations in the calculated pge tenors and cu pd ratios of sulfides both vertically and laterally all of the profiles show a decrease in pt tenor and concomitant increase in cu pd ratio upward across the pyroxenite of the merensky unit and laterally away from certain centres that are interpreted as feeder zones the data also document that more mafic horizons within the merensky to ug 2 interval of the southwestern bushveld are characterized by much lower cu pd 100 400 than the ambient ratio 4000 this is interpreted as demonstrating that these horizons are the consequence of incursions of magma whose composition has been modified in a staging chamber by reaction with sulfide resident there the favoured interpretation for the merensky reef is that the bushveld chamber s was occupied by a resident magma crystallizing orthopyroxene and plagioclase pre merensky and subsequently merensky magmas entered the chamber s intermittently from localized feeder structures mixing or mingling with the resident magma and giving rise to the more mafic cumulates at specific horizons including the merensky reef the mixing or mingling caused sulfide immiscibility in these cases as the new magma pulses flowed laterally away from the entry zones they deposited sulfide becoming depleted in pge on coming to rest the magma continued to segregate sulfide and crystallize pyroxene so that the sulfides became progressively depleted in pge and acquired a higher cu pd upward through the pyroxenite this model requires that the merensky magmas could dissolve up to two orders of magnitude more pge than current measurements on dry diopside anorthite melts have indicated it is suggested that very pge rich magmas can develop as a result of sulfide unsaturated magma reacting with early formed sulfide in a staging chamber dissolving fes and enriching the remaining sulfide in pge later magmas equilibrating with this enriched sulfide will themselves become highly enriched in pge

voluminous acid volcanism in the busheveld complex a review of the rooiberg felsite the 2 1 b y old rooiberg felsite roofs and is intruded by the mafic layered rocks and granites of the bushveld complex the felsite unit which locally exceeds 5 km in thickness and may represent an originally erupted volume of more than 300 000 km3 is dominated by rhyolitic to dacitic lavas with minor pyroclastic and sedimentary rock types volcanic rocks of more mafic composition occur towards the base of the sequence the rooiberg episode essentially terminated the volcanic activity in the transvaal basin and heralded the emplacement of the bushveld complex

incompatible trace element behavior in the bushveld complex borehole core from the uppermost 500 m of the rustenburg layered suite of the bushveld complex has been analyzed for rb sr zr y and nb plots of absolute trace element abundance and trace element ratios against stratigraphic height are presented these are used to test for multiple injection of magma and postcumulus migration of residual liquid these and previously published data are used to support the hypotheses that there was a new injection of magma above the merensky reef and that large scale infiltration metasomatism did not occur no evidence from incompatible element data exists for an addition of magma near the pyroxenite marker where a distinct reversal in cumulus mineral composition occurs however this may be the result of relatively small degrees of fractionation of the host magma prior to the addition of new magma the ability to recognize the entry of new magma may be restricted by the analytical precision the uppermost 500 m of the layered sequence appears to behave as a single broadly homogeneous magma in showing a systematic predictable upward increase in incompatible element abundances consistent with a model assuming 10 to 30 percent trapped liquid in the cumulate deviations from the predicted trends for zr y and nb are attributed to these elements being compatible in magnetite for zr and apatite for y and nb which are present in the uppermost rocks

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

geochemistry and mineralogy of sediments from the ventersdorp and transvaal supergroups south africa cratonic evolution during the early proterozoic approximately 100 pelite and 12 quartzite samples from the ventersdorp 2 7 ga and transvaal supergroups 2 6 2 1 ga have been analyzed to monitor the early proterozoic evolution of the kaapvaal craton southern africa from oldest to youngest pelites were sampled from the ventersdorp bothaville bot transvaal selati sel black reef br timeball hill th strubenkop str and silverton sil formations paleocurrent measurements in transvaal quartzites indicate sources lying predominantly to the north and east relative to the bot sel br pelites from the th str sil are enriched in heavy ree lile zr hf nb and ta depleted in k2o mgo ni and cr and have lower cr zr sc th k2o na2o and k rb ratios compared to sel br bot th str sil pelites have higher light ree contents and la yb ratios and lower eu eu ratios 0 61 0 66 relative to nasc north american shale composite thstr sil pelites are enriched in light ree th u ta nb sc cs have higher la yb ratios and are depleted in k2o and mgo bot sel br pelites are enriched in k2o mgo cr and ni have higher k2o na2o sc th and eu eu ratios and are depleted in th u heavy ree and high field strength elements hfse relative to nasc compositions of th str sil pelites suggest a provenance similar to average phanerozoic uppercontinental crust this source is more evolved than that of bot sel br pelites indicating a transformation from primitive mafic rich to evolved felsic rich upper crust at 2 2 ga this transition follows earlier primitive to evolved trends in moodies pongola 3 3 3 0 ga and witwatersrand 2 8 ga successions these data suggest that several cycles of changing upper continental crust occurred in the kaapvaal craton between 3 3 2 1 ga

the palynology of odp site 1165 prydz bay east antarctica a record of miocene glacial advance and retreat abstract odp site 1165 recovered around 1000 m of early to late miocene mixed hemi pelagic sediments from the wild drift offshore prydz bay samples from between the bottom of the hole and 110 m below sea floor yielded distinctive palynomorph assemblages ranging in age from 22 2 ma at the base to 8 ma at 110 mbsf many assemblages contain large numbers of reworked terrestrial pollen and spores varying in age from devonian to early eocene in situ palynomorph assemblages are dominated by the acritarch leiosphaeridia other components of the assemblages are dinoflagellate cysts prasinophyte algae and acanthomorph acritarchs comparison with modern arctic assemblages suggests that the presence of these forms implies significant fresh water input resulting in complex stratified water masses unlike the situation in prydz bay today based on the palynomorph assemblages which are alternately dominated by glacially derived reworked terrestrial miospores and in situ leiospheres typical of ice edges the interval between the bottom of the hole and 310 mbsf 22 2 15 ma can be divided into three intervals samples from the base of each interval are dominated by high percentages of reworked terrestrial miospores and are interpreted as representing a period of time dominated by increased glacial flow down the lambert glacier and an expanded amery ice shelf samples from the upper part of each interval are characterised by low percentages of reworked terrestrial miospores and generally high percentages of leiospheres this is thought to represent a period of time of low glacial flow and retreat of the amery ice shelf at the same time an increase in the minor components of the marine palynomorph assemblage is recorded possibly in response to an overall climatic amelioration onto which the three glacial cycles are superimposed overlaying these glacial cycles between 310 and 99 1 mbsf 15 8 ma the assemblages are completely dominated by reworked material including an eocene dinoflagellate assemblage previously recoded on the continental shelf in odp drill hole 1166 leiospheres or other in situ marine palynomorphs are either absent or only make up a very small percentage of the assemblage this is interpreted as representing an interval where the climatic amelioration recorded during the underlying three cycles has been sharply reversed and the amery ice shelf has expanded to the point where it is grounded across the continental shelf eroding the eocene material present there the age of this climatic reversal 15 ma coincides with the well known shift in the oxygen isotope curve thought to be associated with the development of permanent ice sheets on antarctica

the uitkomst intrusion and nkomati ni cu cr pge deposit south africa trace element geochemistry nd isotopes and high precision geochronology the uitkomst intrusion is a tubular mafic ultramafic layered body that hosts one of south africa s largest ni cu cr pge deposits nkomati the sulphide ore occurs in the form of massive lenses in the immediate quartzitic footwall and as disseminations within peridotite the chromite ore forms an up to 10 m thick layer in the lower portion of the intrusion uitkomst has generally been interpreted as a magma conduit possibly related to the bushveld event here we present a new high precision u pb zircon date of 2057 64 0 69 ma that overlaps with the age of the merensky reef of the bushveld complex and thus demonstrates a coeval relationship between the intrusions based on incompatible trace elements as well as o and nd isotope data end 4 5 to 6 2 we show that the uitkomst parent magmas were contaminated with up to 20 archean upper crust prior to emplacement and with up to 15 dolomitic country rock during emplacement ore formation at nkomati was critically aided by substantial devolatisation and removal of dolomitic floor rocks leading to hydrodynamic concentration of sulphide and chromite during slumping of crystal mushes into the trough like centre of the subsiding intrusion and its footwall

how likely was a green antarctic peninsula during warm pliocene interglacials a critical reassessment based on new palynofloras from james ross island the question of whether pliocene climate was warm enough to support a substantial vegetation cover on antarctica is of great significance to the ongoing and controversial debate on the stability or dynamism of antarctic ice sheets during warm periods with high atmospheric greenhouse gas concentrations here we present a systematic palynological comparison of pollen and dinoflagellate cyst assemblages from early pliocene diamictites and underlying late cretaceous sediments collected from james ross island northern antarctic peninsula the diamictites are dated using a combination of 40ar 39ar and 87sr 86sr isotope ages on interbedded lavas and pristine bivalves well preserved pectinid shells and cheilostome bryozoans suggest that the palynomorph bearing sediments were probably deposited during warmer pliocene interglacials and later amalgamated into a diamictite formed by a major ice advance during cold glacial cycle the palynological analyses presented herein do not identify any in situ pollen and spores which indicate the presence of a substantial vegetation cover our study suggests a local i e james ross island provenance for most of the diamictites whilst sediments from the western coast might have been delivered by ice sheets from the antarctic peninsula whilst the acritarch leiosphaeridia might imply the presence of sea ice and near modern climate conditions during the late neogene the presence of the dinoflagellate cyst bitectatodinium tepikiense at one location suggests that sea surface temperatures might have been substantially warmer during some interglacials the absence of in situ pollen and spores in the james ross island diamictites cannot be taken as proof of non existence of vegetation however the combined palynological and geological evidence presented in this paper makes the presence of a substantial pliocene vegetation cover on james ross island unlikely and supports previous reconstructions of a permanent ice sheet on the antarctic peninsula throughout the late neogene

the difference between oceanic and continental tholeiites a fluid dynamic explanation continental tholeiites have higher sio2 k2o and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts these differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface although there is little doubt that most continental tholeiites have assimilated crustal material the lcoation and mechanism of assimilation remain uncertain longhi 1981 has argued that magmas derived directly from the mantle should crystallize little orthopyroxene the abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust since orthopyroxene is an early crystallizing phase in layered intrusions this assimilation must occur early before the magma enters the chamber assimilation can occur at the margins of the dykes which feed magma chambers depending on the nature of the flow if the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls if the flow is laminar a conductive profile develops at the margin and the flowing magma chills against the walls protecting them from thermal erosion the nature of flow in a dyke can be predicted from the reynolds number the criteria for turbulence reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and if this is the case assimilation of low melting point components in the walls of the dyke is inevitable it is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust

trace elements in the merensky reef and adjacent norites bushveld complex south africa trace elements were analysed in rocks and minerals from three sections across the merensky reef in the rustenburg platinum mine in the bushveld complex of south africa whole rocks and separated minerals were analysed by inductively coupled plasma mass spectrometer icp ms and in situ analyses were carried out by ion microprobe and by laser source icp ms merensky reef pyroxenites contain extremely high concentrations of a wide range of trace elements these include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements for major elements and compatible trace elements the measured concentrations in cumulus phases and the bulk rock compositions are similar for highly incompatible elements however concentrations in bulk rocks are far higher than those measured in the cumulus phases in situ analyses of plagioclase have far lower concentrations of th zr and rare earth elements than icp ms analyses of bulk separates of plagioclase a difference that is attributed to the presence of trace element rich accessory phases in the bulk mineral separates we used these data to calculate the trace element composition of the magmas parental to the merensky unit and adjacent norites we argue that there is no reason to assume that the amount of trapped liquid in the merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites we propose that the bushveld complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks the magma that gave rise to the merensky unit was the more contaminated and unusually rich in incompatible trace elements and when it entered the main bushveld chamber it precipitated the unusual phases that characterize the merensky reef the hybrid magma segregated sulphides or platinum group element rich phases during the course of the contamination in the lower chamber these phases accumulated following irruption into the main bushveld chamber to form the merensky ore deposits

rb sr and sm nd isotopic compositions of the rooiberg group south africa early bushveld related volcanism the rooiberg group is a 6 km thick sequence of mostly volcanic rocks which represent the first phase of magmatic activity associated with the bushveld complex these strata include in ascending stratigraphic order the dullstroom damwal kwaggasnek and schrikkloof formations units of the lower dullstroom formation range from basalts to andesites and comprise two compositional suites high ti and low ti compositional data indicate that melts represented by the overlying more siliceous volcanic rocks which include dacites and rhyolites were derived from low ti melts by fractional crystallization and assimilation of crustal material afc processes rb sr isotopic data 28 samples for units of the dullstroom and damwal formations loosely constrain a crystallization age of 2071 94 65 ma these errors and those below 95 confidence limits which agrees with previously reported age data these data suggest an initial value for 87sr 86sr of 0 70655 0 00087 0 00051 for the rooiberg group in contrast rb sr isotopic compositions of six samples of the kwaggasnek formation indicate post crystallization alteration which was probably associated with the lebowa granite suite of the bushveld complex sm nd isotopic data 29 samples for volcanic units of the rooiberg group provide a poorly constrained age of 1837 360 320 ma with an initial value for 143nd 144nd of 0 50976 0 00026 0 00035 these rb sr and sm nd isotopic compositions are similar to those indicated for melts that crystallized to form the rustenburg layered suite rls of the bushveld complex extruded and intruded melts probably were derived from the same or similar sources and may have resided in the same magma chambers before emplacement

the mendel formation evidence for late miocene climatic cyclicity at the northern tip of the antarctic peninsula abstract a detailed description of the newly defined mendel formation is presented this late miocene 5 9 5 4 ma sedimentary sequence with an overall thickness of more than 80 m comprises cyclic deposition in terrestrial glacigenic glaciomarine and marine environments subglacial till was deposited by a thick grounded antarctic peninsula ice stream advancing eastwards through the prince gustav channel crossing the northernmost part of present day james ross island with most of the material carried actively at the base of the warm based glacier the form of the prince gustav channel originated before the late miocene and its present glacial over deepening resulted in multiple grounded glacier advances during the neogene and quaternary the sea prograded from the east and the glacier margin became buoyant building a small ice shelf below and in front of which glaciomarine and marine sediments were deposited these sedimentary deposits are composed of material that was transported mostly by small glaciers and subsequently by the floating ice shelf and by calving icebergs towards the open sea these environmental and glaciological conditions differ strongly from the present cold based local glaciers of james ross island and the antarctic peninsula these changes were triggered by the global sea level fluctuation connected with climatic changes due to antarctic ice sheet build up and decay this reveals late miocene obliquity driven cyclicity with a 41 ka period the mendel formation sedimentary sequence is comprised of at least two glacial periods and one interglacial period sea level rise at the northern tip of the antarctic peninsula between glacial lowstand and interglacial highstand was more than 50 m during the late miocene and was followed by a subsequent sea level fall of at least the same magnitude the presence of well preserved articulated pectinid bivalves support interglacial open sea conditions the re deposition of palaeontological material was shown to be important for the mendel formation reworked pectinid shells revealed an open marine interglacial condition in this part of the antarctic peninsula during the early to mid miocene 20 5 17 5 ma unfortunately relevant sedimentary deposits have not been found on land

an overview of the geology of the transvaal sequence and bushveld complex south africa the late archaean early proterozoic transvaal sequence is preserved within the transvaal kanye and griqualand west basins with the 2050 ma bushveld complex intrusive into the upper portion of the succession within the transvaal basin both transvaal and bushveld rocks are extensively mineralized the former containing large deposits of iron manganese asbestos andalusite gold fluorine lead zinc and tin ores and the latter some of the world s major occurrences of pge chromium and vanadium ores transvaal sedimentation began with thin predominantly clastic sedimentary rocks black reef vryburg formations which grade up into a thick package of carbonate rocks and bif chuniespoort ghaap taupone groups these lithologies reflect a carbonate bif platform sequence which covered much of the kaapvaal craton in reaction to thermal subsidence above ventersdorp aged rift related fault systems an erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the pretoria postmasburg segwagwa groups within the three basins under largely closed basin conditions an uppermost predominantly volcanic succession rooiberg group loskop formation is restricted to the transvaal basin a common continental rift setting is thought to have controlled pretoria group sedimentation rooiberg volcanism and the intrusion of the mafic rocks of the rustenburg layered suite of the bushveld complex the dipping sheets of the rustenburg magmas cut across the upper pretoria group stratigraphy and lifted up the rooiberg lithologies to form the roof to the complex subsequent granitic rocks of the lebowa and rashoop suites of the bushveld complex intruded both upper rustenburg rocks and the rooiberg felsites

are bushveld u type parent magmas boninites or contaminated komatiites three distinct categories of magmas bushveld u type parent magmas boninites and siliceous high magnesium basalts from archaean greenstone belts share the distictive geochemical characteristics of high mgo 9 19 low tio2 less than 1 and high sio2 greater than 52 boninites are generally thought to form by hydrous melting of metasomatized previously depleted upper mantle while siliceous high magnesium basalts shmb in greenstone belts have recently been recognized as the products of combined fractionation and crustal contamination of komatiites both these mechanisms can apparently give rise to similar end products and both mechanisms have been proposed for the petrogenesis of bushveld u type magmas

low temperature zr mobility an in situ synchrotron radiation xrf study of the effect of radiation damage in zircon on the element release in h2o hcl sio2 fluids the release of zr u and pb from nearly metamict zircon and its recrystallized analog and of zr from fully crystalline and slightly radiation damaged zircon in h 2 o hcl sio 2 fluids was investigated in situ at temperatures between 200 and 500 c using a hydrothermal diamond anvil cell and time resolved synchrotron radiation xrf analyses dissolution of nearly metamict zircon proceeded much faster than that of zircon with little or no radiation damage and resulted in a 1 5 to 2 log units higher zr molality in 6 to 7 m hcl fluids extensive recrystallization of the almost fully amorphous material started at 260 to 300 c in h 2 o hcl and at about 360 c if quartz was added and was coupled with a decrease of the zr concentration in the fluid by more than an order of magnitude re crystallization in 7 m hcl had little effect on the aqueous u and pb concentrations whereas addition of quartz caused a more sluggish decrease of the zr concentration in the fluid upon recrystallization and lowered the release of u the data presented here support the interpretation that enhanced zr mobility in low grade metamorphic rocks may be related to dissolution of metamict zircon by aqueous fluids and illustrate the significance of the silica activity on the kinetics of dissolution and recrystallization during zircon fluid interaction

crustal contamination and fluid rock interaction during the formation of the platreef northern limb of the bushveld complex south africa the platreef is the main platinum group element pge bearing and contains some of the most important magmatic ore deposits yet discovered the intrusion covers an area of facies of the northern limb of the bushveld complex but unlike the merensky reef of the eastern and western limbs it is in direct roughly 65 000 km e g tankard et al 1982 and lies almost entirely within the bounds of the transvaal basin contact with the country rock mineral separate o values for samples from the upper zone and main zone of the northern limb fig 1 the layered igneous rocks the rustenburg layered suite sacs 1980 can be divided into eastern indicate crystallization from a well mixed already contaminated magma having a o value of 7 5 pyroxenes from the pyroxenites and western limbs of approximately the same size and a smaller northern limb the northern limb differs in facies of the platreef at sandsloot mine have o values that are up to 2 4 higher than pyroxenes from the upper and main several important aspects from the more well known eastern and western limbs e g van der merwe 1976 zones these differences can be explained by additional assimilation of up to 18 dolomite which is in contact with the intrusion at buchanan et al 1981 cawthorn et al 1985 eales cawthorn 1996 first the layered rocks show a this locality samples from the platreef that have plagioclase and pyroxene not in oxygen isotope equilibrium appear to have interacted pronounced transgression across the floor rocks from a with fluids during slow cooling quartz veins with granophyric level above the magaliesberg quartzite formation south margins have o values between 10 1 and 12 2 which of the town of potgietersrus across the whole pretoria suggest that the fluid that passed through the cooling platreef had group and the banded iron formations and dolomites a o value of 7 9 these data together with hydrogen isotope of the older chuniespoort group onto archaean granitic data from minerals and whole rock samples d of biotite 60 rocks fig 1 second the igneous layering shows an to 88 of the platreef suggest interaction with magmatic fluid apparent transgressive relationship with respect to the at low water rock ratios interaction of this fluid with the calcsilicate contact such that ultramafic rocks of the lower zone footwall rocks lowered their o values abut against the contact south of potgietersrus whereas to the north rocks forming the base of the intrusion are progressively higher in the stratigraphic succession of the

trace element and platinum group element distributions and the genesis of the merensky reef western bushveld complex south africa the merensky reef of the bushveld complex is one of the world s largest resources of platinum group elements pge however mechanisms for its formation remain poorly understood and many contradictory theories have been proposed we present precise compositional data major elements trace elements and platinum group elements pge for 370 samples from four borehole core sections of the merensky reef in one area of the western bushveld complex trace element patterns incompatible elements and rare earth elements exhibit systematic variations including small scale cyclic changes indicative of the presence of cumulus crystals and intercumulus liquid derived from different magmas ratios of highly incompatible elements for the different sections are intermediate to those of the proposed parental magmas critical zone and main zone types that gave rise to the bushveld complex mingling but not complete mixing of different magmas is suggested to have occurred during the formation of the merensky reef the trace element patterns are indicative of transient associations between distinct magma layers the porosity of the cumulates is shown to affect significantly the distribution of sulphides and pge a genetic link is made between the thickness of the merensky pyroxenite the total pge and sulphide content petrological and textural features and the trace element signatures in the sections studied the rare earth elements reveal the important role of plagioclase in the formation of the merensky pyroxenite and the distribution of sulphide

u th pb and lu hf systematics of zircon from ttg s leucosomes meta anorthosites and quartzites of the limpopo belt south africa constraints for the formation recycling and metamorphism of palaeoarchaean crust abstract results of more than 400 in situ u th pb and lu hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the limpopo belt musina area formed during the palaeoarchaean between 3 40 and 3 27 ga experienced a first anatexis at 3 24 3 07 ga m1 and were overlain by sediments mainly between 3 24 and 3 0 ga subsequently the infra and supracrustal rocks together underwent two high grade metamorphic overprints a first during the neoarchaean at 2 65 ga m2 and a second at 2 025 ga m3 hafnium isotope data additionally reveal that the palaeoarchaean magmatic rocks result from substantial recycling of much older crust this is well reflected by nearly chondritic hf int obtained from primary magmatic zircon domains of four sand river ttg gneiss samples hf 3 27 3 28 ga 1 3 to 1 8 from leucosomes within the sand river gneisses hf 3 40 ga 2 0 hf 3 28 ga 1 5 and from two meta anorthosite samples of the messina layered intrusion hf 3 35 ga 0 1 1 4 crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples these data reflect a general increase of the hf int between 3 65 and 3 2 ga from ca 4 0 to 0 0 indicating that an eo to palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the limpopo belt the present study also demonstrates that hf isotope data provide a powerful tool which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events from such affected by a pseudomorphic alteration even if the investigated zircon grain underwent multiple alteration processes

geochemical and geochronological constraints on the nature of the immediate basement next to the mesoarchaean auriferous witwatersrand basin south africa acombined petrological geochemical and geochronological rb srand sm nd whole rock u pb and lu hf zircon and ar arhornblende study on a section of pre witwatersrand basementdrilled at the northwestern margin of thewitwatersrand basin hasrevealed new insights into the nature and tectonic setting of thelikely source area for some of the mesoarchaean auriferouswitwatersrand sediments the protolithsof intersected alteredgran ite and hornblende metagabbro are of indistinguishable age 3062 5 ma and have very similar geochemical signatures trace element characteristics typical of calc alkaline magmatismand evidence of variable contamination with older crust subchon dritic e

genetic aspects of the granophyric rocks of the bushveld complex this study of the granophyric rocks of the bushveld complex was undertaken in order to distinguish the different types of granophyre and to determine their probable origin aspects of the granophyre and associated rock types that were studied include their field relations geochemistry and rb sr isotope chemistry thermal modeling of causal mechanisms was also carried out the results obtained demonstrate that a number of genetically different types of granophyre exist in the bushveld complex two types the diepkloof granophyre and the zwartbank pseudogranophyre were formed by metamorphic processes related to the intrusion of the bushveld basic rocks a third type the stavoren granophyre was formed by magmatic processes it is also concluded that the stavoren granophyre is genetically related to the volcanic pre bushveld rooiberg group and forms the shallow intrusive equivalent thereof

emplacement and crystallization time for the bushveld complex the bushveld complex formed by the crystallization of successive basaltic magmatism may be of the order of several million years for example the columbia river basalts hooper injections of magma which were sufficiently closely spaced in time that each previous magma had not cooled and differentiated 1988 were erupted in the period 17 12 ma with minor eruptions for a further 5 my although most outpouring significantly before the addition of the next one to constrain the emplacement and crystallization times a thermal model is presented occurred within the first 2 my it is now recognized that large intrusions were not emplaced in a single pulse but which permits the investigation of the rate of cooling of magma in an intrusion repeatedly subjected to magma addition and subresult from multiple magma injection the question is how rapidly were magma chambers such as the bc traction such modelling indicates that magmas injected into the bushveld complex were emplaced within 75 000 years at that filled and how much magma was involved answers are relevant to the dynamics of melt production storage and time injection into the complex ceased the volume of rock in the eastern and western limbs is 370 000 600 000 km however transport in the mantle and crust this paper describes a thermal modelling technique not previously applied a quantitative evaluation of the cr budget in the formation of chromitite layers indicates that large volumes of magma cannot be to magma chambers which can be used to analyse this process and to obtain an estimate of the emplacement accounted for in the preserved rock sequence similarly an evaluation of the incompatible trace element abundances such as those for zr time to present this model it is necessary to discuss the stratigraphy size and connectivity of the different limbs and k suggests that the chamber was open and that large volumes of differentiated magma escaped the volume of magma therefore of the bushveld complex and to consider the extent of tapping of the magma chamber as well as its filling greatly exceeded the preserved volume of cumulate rocks giving an estimated magma volume of over 1 10 km an average the term bushveld complex has been given several meanings in the literature and according to the south emplacement rate of 13 km year is indicated by these calculations african commission on stratigraphy 1980 includes not only the ultramafic mafic layered rocks but also the sills beneath the intrusion volcanic rocks which pre date the

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

progressive crustal contamination of the bushveld complex evidence from nd isotopic analyses of the cumulate rocks we report the first nd isotopic data on the cumulate rocks of the bushveld complex south africa we analysed 17 whole rock samples covering 4700 m of stratigraphy through the lower critical and main zones of the intrusion at union section north western bushveld complex the basal ultramafic portions of the complex have markedly higher nd t 5 3 to 6 0 than the gabbronoritic main zone nd t 6 4 to 7 9 the rocks of the upper critical zone have intermediate values these results are in agreement with new nd isotope data on marginal rocks and sills in the floor of the complex that are generally interpreted as representing chilled parental magmas and with published sr isotopic data all of which show a larger crustal component in the upper part of the intrusion in contrast the concentrations of many highly incompatible trace elements are decoupled from the isotopic signatures the basal portions of the complex have higher ratios of incompatible to compatible trace elements than the upper portions the variations of isotopic and trace element compositions are interpreted in terms of a change in the nature of the crustal material that contaminated bushveld magmas those magmas that fed into the lower part of the complex had assimilated a relatively small amount of incompatible trace element rich partial melt of upper crust whereas magmas parental to the upper part of the complex had assimilated a higher proportion of the incompatible trace element poor residue of partial melting

changing oligocene climate recorded by palynomorphs from two glacio eustatic sedimentary cycles cape roberts project victoria land basin antarctica abstract two marine palynomorph assemblages recovered from two glacio eustatic sedimentary cycles from the cape roberts project crp which recovered over 1500 m of oligocene and miocene strata from the western margin of the victoria land basin are investigated we present results from one cycle dated as early oligocene and the other dated as late oligocene modest numbers of marine palynomorphs were recovered with an average abundance of 56 specimens per gram the assemblages recovered from both cycles are broadly similar and dominated by acritarchs and the phycoma of prasinophyte algae these include leiosphaeridia sp 2 leiosphaeridia sp 3 and the prasinophyte alga cymatiosphaera sp 2 in detail however the lower oligocene assemblage is different from that of the upper oligocene the lower oligocene cycle includes impagidinium cf dispertium and impagidinium cf elegans cymatiosphaera sp 6 and pyxidinopsis sp are abundant the upper oligocene cycle is characterized by an absence of cymatiosphaera invaganata cymatiosphaera sp 8 and pterospermella sp a it has very low numbers of acritarch sp b impagidinium and pyxidinopsis it contains tasmanites sp and brigantedinium pynei and it has higher abundances of lejeunecysta sp 1 and sigmopollis than the lower oligocene cycle the environmental interpretations of the crp core from other workers using sedimentological and paleontological indicators are used to infer environmental conditions that are likely to have influenced the marine palynomorph assemblages paleontological and sedimentological analysis suggests that both cycles were deposited in cold conditions with sea ice and melt water influence although the upper oligocene assemblage is inferred to have existed in colder marine conditions with more prolonged sea ice cover and less associated fresh water input than the assemblage of early oligocene times the application of modern analogues for some of the marine palynomorph taxa identified suggest the presence of sea ice and occasionally lowered salinity in both cycles this is broadly consistent with the findings of the other paleoenvironmental studies on the crp cores

a felsic end to bushveld differentiation the similarities in age c 2 06 ga and isotopic composition of the bushveld igneous complex and the overlying rooiberg group felsic volcanic rocks suggest the possibility that the two are cogenetic to investigate this possibility we derive a robust new estimate of the major and trace element bulk composition of the bushveld upper zone and upper main zone uumz using melts thermodynamic modeling we show that this composition fails to reproduce fundamental features of the cumulate sequence such as the presence of primary orthopyroxene at the base of the magma column we investigate the possibility that some amount of evolved magma escaped from the magma chamber and derive a new estimate of the uumz parent magma composition that reproduces generally the observed cumulate sequence our results constrain the amount and composition of the escaped magma and provide evidence of a genetic link between the uumz of the bushveld layered series and the felsic volcanic rocks immediately overlying it we conclude that between 15 and 25 of the original magma volume was expelled and is manifest either as part of the upper rooiberg group lava sequence or the rashoop granophyre distinction is hindered by their almost identical major and trace element compositions a parent magma in which 15 25 rooiberg or rashoop material is added back to the extant cumulate sequence is initially saturated in orthopyroxene and is in major and trace element equilibrium with the phases at the base of the uumz