studies of the rooiberg group bushveld complex south africa no evidence for an impact origin abstract it has been suggested that the bushveld complex of south africa could be the result of multiple large meteorite or comet impacts according to this hypothesis part of the lower rooiberg group which forms the roof of the complex represents a sheet of impact melt breccia and other impact breccias the present study is an attempt to test the viability of the impact hypothesis for the bushveld complex by interpreting newly acquired field geochemical petrographic and textural data for rooiberg group and associated rocks extensive field work throughout the rooiberg group and particularly at the contact between this unit and the underlying pretoria group metasediments has failed to identify any material that could be interpreted as impact related the rooiberg group is predominantly composed of individual volcanic flows and pyroclastic units representing several geochemically distinct magma types these volcanic units are interbedded with thin laterally extensive sedimentary units a few of which are sedimentary breccias the presence of needles of quartz that may represent paramorphs after tridymite in some rooiberg group units has been used as evidence to support the contention that these rocks represent superheated impact melt however quartz paramorphs after tridymite have been recognized in terrestrial volcanic provinces e g the north shore volcanic group in northeastern minnesota structural data including dips of rooiberg group strata suggest that the lobate shape of the complex which resembles several closely spaced ring features is the result of post rooiberg group deformation microdeformation features in quartz from bushveld related rocks do not satisfy the criteria of shock metamorphic planar deformation features pdfs which would be characteristic of impact induced shock pressures between 10 and 30 gpa these data especially the absence of macroscopic and microscopic evidence of shock deformation in pre bushveld rocks are inconsistent with formation of the bushveld complex by impact

geodynamics of rapid voluminous felsic magmatism through time abstract two end member geodynamic settings produce the observed examples of rapid voluminous felsic rhyolitic magmatism through time the first is driven by mantle plume head arrival underneath a continent and has operated in an identifiable and regular manner since at least 2 45 ga this style produces high temperature 1100 c low aspect ratio rheoignimbrites and lavas that exhibit high sio 2 al 2 o 3 ratios high k 2 o na 2 o ratios and where available data exists high ga al 2 o 3 ratios 1 5 with high f in thousands of parts per million and low water content f concentration is significant as it depolymerizes the silicate melt influencing the magmas physical behavior during development and emplacement these rhyolites are erupted as part of rapidly emplaced 10 15 myr mafic lips and are formed primarily by efficient assimilation fractional crystallization processes from a mafic mantle parent the second is driven by lithospheric extension during continental rifting or back arc evolution and is exclusive to the phanerozoic slips silicic large igneous provinces develop over periods 2 al 2 o 3 ratios and lower k 2 o na 2 o ratios rapid voluminous felsic magmatism represents both extreme examples of continental growth since the archean and also dramatic periods of crustal recycling and maturation during the phanerozoic

a long lived hydrothermal system in bushveld granites at the zaaiplaats tin mine lead isotope evidence tin mineralization at zaaiplaats is hosted in alkali feldspar granites of the lebowa granite suite which intrude the rashoop granophyre suite from the upper contact downward the alkali feldspar granites include the marginal pegmatite fine grained lease granite and the coarse grained bobbejaankop granite the major mineralization styles consist of subhorizontal tabular low grade orebodies in the lease granite a lower zone of disseminated cassiterite mineralization in the bobbejaankop granite and shallowly plunging and branching pipe orebodies in both granites fluid inclusion data show a continuous evolution of the magmatic hydrothermal system from 600 degrees to 200 degrees c with cassiterite and scheelite precipitation above 400 degrees c mineral compositions and oxygen isotope fractionations indicate reequilibration of minerals within the granites during pervasive alteration at temperatures generally between 200 degrees and 300 degrees c the emplacement age of these and other bushveld granites is ca 2050 ma yet pb pb whole rock isochron ages for the different variably mineralized granite bodies at zaaiplaats show a relatively restricted range from 961 or 129 ma for the rashoop granophyre to 1187 or 51 ma for the lease granite thus the analyzed sample suites consistently record an event approximately 1 b y after the emplacement of the granites modeling their pb isotope evolution indicates that each suite evolved independently between 2050 and ca 1100 ma suggesting that hydrothermal fluid circulation was essentially confined within each granite body and or fluid rock ratios were low and that the overall 238 u 204 pb within each suite from 2050 to ca 1100 ma was not unusually high or low for fractionated granites suggesting largely closed system behavior of u and pb within each suite during this interval the ca 1100 ma event which caused closure of the u pb systems at zaaiplaats is interpreted to reflect a blocking temperature age related to uplift and cooling of a long lived hydrothermal system and precipitation of the latest stage minerals e g galena at 200 degrees to 300 degrees c the high steady state temperature required to maintain the hydrothermal system at zaaiplaats at temperatures above 200 degrees c for ca 1 b y was provided by radioactive decay current heat production characteristics calculated from present day u th and k 2 o concentrations range from 4 2 mu w m 3 for the rashoop granophyre to 11 5 mu w m 3 for the lease granite preweathering heat production of 30 3 mu w m 3 is inferred for the mineralized bobbejaankop granite after accounting for dramatic recent losses of u and th the low level of hydrothermal fracturing combined with stable and pb isotope evidence effectively rules out convective cooling by external fluids as a significant heat loss mechanism at zaaiplaats fluid pathways within the granites were controlled dominantly by grain boundaries microfractures and vugs which allowed internal circulation and elevated subsolidus temperatures to be maintained indefinitely under a thermal blanket of an estimated 4 km of postemplacement cover uplift and erosion of the terrain caused cooling through the pb exchange blocking temperatures at ca 1100 ma

the conformable emplacement of the bushveld mafic rocks along a regional unconformity in the transvaal succession of south africa abstract a previously unrecognized regional unconformity occurs in the proterozoic transvaal succession of the kaapvaal province of southern africa the predominantly marine and siliciclastic rocks of the lower two thirds of the pretoria group are unconformably overlain by a terrestrial and largely volcanic sequence the most voluminous portion of which is the rooiberg felsite if the bushveld complex is mentally subtracted from map patterns the rooiberg felsite also locally overlies pre pretoria sequences and the archean crystalline basement the bushveld complex is not lopolithic as commonly supposed instead regionally condordant emplacement of the complex along the unconformity explains both the seemingly discordant footwall features and the lack of pretoria and older strata other than a few xenoliths in the hanging wall the lack of any geochemical evidence for major crustal contamination of bushveld magmas precludes disposing of any such hanging wall strata by wholesale assimilation the bushveld complex the unconformity and the enclosing strata are synformally folded making the bushveld appear to be lopolithic due to this folding and multiple periods of erosion the original extents of the bushveld complex the unconformity and the enclosing strata are now much reduced other smaller mafic intrusions along or immediately below the same regional unconformity probably are bushveld related

cumulatic diorites related to post collisional brasiliano pan african mafic magmatism in the vila nova belt southern brazil abstract the capivaras diorite in the vila nova region nw of the sul rio grandense shield is composed of six ne to nne oriented rock bodies of late tectonic emplacement relative to the d3 deformation phase which forms subvertical high strain zones in basement gneiss sequences within these intrusive bodies a shape foliation is present generally parallel to contacts and displaying a local solid state deformational component the internal structure of the capivaras diorite main intrusion is marked by a zone of intense flow and mingling characterized by strong shape foliation and layers of variable texture and composition which result from cumulative processes heterogeneous flow and interaction of coeval compositionally contrasting magmas the central part of this intrusion is texturally homogeneous and slightly foliated even though cumulative processes have remained important during its formation along the contact with basement gneisses fine grained diorites are found which are considered to be compositionally close to the parental magma of the capivaras diorite this magma has a mildly alkaline affinity and shows moderate to high contents of zr ti and p highly fractionated ree patterns low nb contents as well as high contents of k sr ba and rb are suggestive of its provenance from mantle sources which have been previously affected by subduction processes such as those of mature magmatic arcs or post collisional settings magmatic evolution was controlled by cumulative processes and gave origin to pyroxene orthocumulates plagioclase pyroxene orthocumulates pyroxene adcumulates and more rarely plagioclase adcumulates the cumulative origin of these rocks is indicated by field textural and geochemical features which are distinct from those of crystallized liquids the compositional diversity of cumulates has led to the generation of compositionally different melts the early formed pyroxene cumulates have caused ca al na ba and sr enrichment in the magmatic liquid leading to plagioclase crystallization and accumulation coarse grained mafelsic cumulates were formed during the late stages of magmatic crystallization due to volatile enrichment of the intercumulus liquid considering geological relations as well as tectonic and compositional features of the capivaras diorite it is interpreted as part of neoproterozoic magmatism related to the post collisional stage of brasiliano pan african orogenic cycle in southern brazil

space and time in granite plutonism granitic suites formed in the last 2500 ma are either calc alkaline or less commonly alkalic the space time trends of granitoid rocks from modern compressional arcs include variations towards more silicic and potassic products but even the most mature magmas of these arcs have fe mg ratios near to 1 in contrast with the strongly iron enriched alkalic suites that characterize zones of crustal extension both calc alkaline and alkalic suites are mainly mantle derived they evolved respectively from tholeiitic and alkali basalt parental magmas which have been subject in most examples to a two stage fractionation history other factors that may influence the evolution of these parental magmas include first the addition of variable but minor amounts of crustal melt and secondly the persistence of refractory mineral phases in the zone of melting contributions from subducted continental detritus are thought to be of minor significance a long term trend over the entire history of the crust from more sodic to more potassic calc alkaline magmatism may reflect the declining importance of subducted ocean lithosphere in magma generation and the increasing role of the overlying volatile fluxed mantle wedge this trend and in particular the widespread development of alkalic granite suites in mid proterozoic times may be linked to the declining vigour of tectonic and associated magmatic processes during the earth s history these alkalic suites may indicate a period when the continental lithosphere had become stable enough to resist fragmentation leading to intracontinental rift and swell zones of magmatism in some cases new plate cycles with more normal calcalkaline igneous rocks seem to have occurred in the same zones although much of the evidence favours the operation of plate tectonic and subduction processes in magma generation during archaean and early proterozoic times this link cannot be taken as proved except for the last ca 1000 ma

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

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

stability of the nitrogen cycle during development of sulfidic water in the redox stratified late paleoproterozoic ocean nitrogen cycling has been evaluated across a depth transect in the late paleoproterozoic animikie basin north america spanning the end of earth s final period of global iron precipitation and a major transition to euxinic conditions in areas of high productivity sediments from near shore where productivity was highest have 15n compositions up to 3 higher than at more distal sites this suggests that as nh4 mixed vertically upward into the oxic photic zone from the anoxic ocean interior it was either assimilated by organisms or oxidized subsequent enhanced production of n2 by denitrification or anammox anaerobic ammonium oxidation led to the observed increase in 15n close to shore any deficit in biologically available n was overcome by n2 fixing organisms but the input of n with low 15n from this process did not overwhelm the increase in 15n from denitrification because there is no evidence for conditions of severe n stress arising from trace metal limitation particularly mo of n fixation during the transition to euxinic conditions losses of n were either very small potentially because low o2 levels limited nh4 oxidation or alternative pathways that retained n were important the fact that mo appears to have remained bioavailable for n fixation either suggests that the extent or severity of sulfidic water column conditions was not sufficient to quantitatively sequester mo on a global scale or that rivers directly delivered mo to surface waters on the inner shelf the effects of n2 fixation on 15n increased to more distal parts of the shelf consistent with models invoked for modern upwelling zones over broad continental margins

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

primary geochemical characteristics of mineral deposits implications for exploration extract the old adage mineral exploration is like finding a needle in a haystack recaps the difficulty of finding economic mineral deposits near or beneath the surface in general two main approaches can be used to detect the presence of mineral deposits near or beneath the ground surface one employs a mineral deposit model and the other aims at the identification of a primary halo about a mineral deposit a mineral deposit model provides systematic empirical descriptive and or theoretical genetic descriptions of the essential geological geochemical and geophysical features of a type of mineral deposits cf cox and singer 1986 and roberts et al 1988 the primary halo of a mineral deposit as defined originally by safronov 1936 is an area including rock surrounding mineral deposit ore bodies and enriched elements that make up that deposit thus descriptions of primary haloes may form part of a mineral deposit model but both deposit model and primary halo approaches to mineral exploration are based on studies of primary geochemical characteristics of mineral deposits because chemical processes during mineralization are the ones that ultimately bring about metal precipitation or mineral formation however a deposit model approach to mineral exploration is apt for identifying which haystacks contain a needle whereas a primary halo approach to mineral exploration is proper for deducing where in a needle bearing haystack is the needle located

the age of the gunflint formation ontario canada single zircon u pb age determinations from reworked volcanic ash the gunflint formation a paleoproterozoic chemical clastic sedimentary assemblage outcropping to the immediate northwest of lake superior became famous in 1954 as containing the oldest fossil assemblage known at that time older microfossils have since been discovered but the gunflint procaryotes remain one of the most diverse precambrian fossil communities the finding of possible multicellular organisms in correlative lithic units in michigan has recently added to the need for an exact age of the formation zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the gunflint formation a euhedral zircon population has yielded a 1878 3 1 3 million years bp u pb age believed to be nearly synchronous with the depositional age this not only provides a precise age for the community of organisms but also strongly supports a back arc extensional setting for the animikie basin rather than a foreland trough resume la formation de gunflint un assemblage sedimentaire chimique et clastique datant du paleoproterozoique affleurant immediatement au nord ouest du lac superieur est devenue celebre en 1954 car elle contenait les plus an ciens assemblages fossiliferes connus a cette epoque de plus anciens microfossiles ont ete decouverts depuis mais les procaryotes de gunflint demeurent l une des communautes fossiles precambriennes les plus diversifiees la decouverte

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

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

an integrated approach to the late stages of neoproterozoic post collisional magmatism from southern brazil structural geology geochemistry and geochronology of the corre mar granite abstract the neoproterozoic post collisional stage in south brazil is marked by intense granitic magmatism controlled by transcurrent shear zones related to the southern brazilian shear belt sbsb the sbsb has controlled the ascent and emplacement of magmas from crust and mantle in such scenario the corre mar granite cmg represents a small intrusion emplaced in a low strain zone located between two important segments of the sbsb the major gercino and itajai perimbo shear zones the cmg has a diagnostic subvertical s c foliation pair present in all intrusions regardless of their size it was emplaced within a conjugate system where sinistral nne shearing and nw se extension were both active generating space along the ne direction solid state deformation associated to the nne shearing is attested to by microstructures as feldspar recrystallization and asymmetric recrystallization tails the opening is attributed to the regional dextral transcurrent dynamics of the major gercino and itajai perimbo shear zones and magma emplacement was essentially conditioned by the nw extension component zircon u pb la mc icp ms crystallization age of cmg at 615 4 ma very close to other regional granites as the 611 ma serra dos macacos smg and 620 ma rio pequeno granite rpg points these three granitic bodies as synchronous the strong deformation features present in the cmg as opposed to the other neoproterozoic granites demonstrate that space rather than time must be called upon to explain the difference in structural patterns identified in the cmg geochemical and sr nd isotopic signatures as the slightly peraluminous character high k contents high lree hree ratios moderate rb nb zr and ree contents to regular sio 2 together with low 87 sr 86 sr i and the strongly negative nd t values indicate that the cmg is derived from old crustal sources possibly related to the paleoproterozoic camboriu complex quartz feldspathic orthogneisses the coincidence of cmg archean to paleoproterozoic inheritance ages with the age of a migmatization event identified in the camboriu complex and moreover the coincidence of the crystallization age intervals of the granites 615 611 ma with the last migmatization event at 640 610 ma reinforces the genetic link between them the paleoproterozoic tdm ages the geochemical and isotopic signatures the zircon u pb crystallization and inheritance age and the resemblance of the cmg and the smg suggest that they represent comagmatic and contemporaneous granitic pulses with a possible genetic link with the neoproterozoic migmatization event in the camboriu complex

the role of xenoliths and flow segregation in the genesis and evolution of the paleoproterozoic itapema granite a crustally derived magma of shoshonitic affinity from southern brazil abstract the paleoproterozoic itapema granite ig is a large granitic body that intrudes orthogneisses and amphibolites altogether representing part of the basement for neoproterozoic granitic magmatism in southern brazil it is composed of fine to medium grained hornblende biotite monzogranites to granodiorites that host abundant country rock xenoliths and irregularly distributed mafic aggregates interaction between magmatic flow and low flow velocity zones related to xenolith rich portions resulted in flow segregation and mineral fractionation as the main mechanisms that led to compositional and textural diversity expressed also in geochemical trends the geometry and distribution of flow foliation suggest that the itapema granite is a subhorizontal sheet like intrusion and that its emplacement was controlled by thrusting under upper amphibolite facies conditions many geochemical characteristics of the itapema granite are similar to those of shoshonitic granitoids however lower alkali contents lack of basic or intermediate associated rocks and intimate temporal association with high grade gneisses and migmatites suggest that it was crustally derived its origin is compatible with melting of amphibolitic orthogneisses derived from continental tholeiitic or shoshonitic sequences under high geothermal gradient conditions where temperatures reached at least 850 c

gushan magnetite apatite deposit in the ningwu basin lower yangtze river valley se china hydrothermal or kiruna type abstract the gushan deposit is one of the typical magnetite apatite deposits associated with dioritic porphyries in the lower yangtze river valley belt of the eastern yangtze craton the origin of this deposit is still uncertain and remains a controversial issue divergent opinions are centered on whether the iron deposits are magmatic or hydrothermal in origin however our field observations and mineralogical studies combined with previous published petrological and geochemical features strongly suggest that the main ore bodies in the gushan magnetite apatite deposit are magmatic specific evidence includes the existence of gas bubbles tubes and miarolitic and amygdaloidal structures melt flow banding structure and the presence of ore breccia new electron microprobe analyses of the pyroxene phenocrysts of the dioritic porphyry genetically associated with the gushan magnetite apatite deposit show that the fe contents in the evolving magma dramatically decrease and then gradually increase because there is no evidence of mafic magma recharge this scenario decreasing fe could be plausibly interpreted by fe rich melts separated from fe poor silicate melts i e liquid immiscibility was triggered by minor addition of phosphorus by crustal contamination the occurrence of massive iron ore bodies can be satisfactorily explained by the immiscible fe rich melt with enormous volatile contents was driven to the top of the magma chamber due to the low density the hot and volatile rich iron ore magma was injected along fractures and spaces between the dioritic intrusions and wall rocks and led to an explosion near the surface resulting in the immediate fragmentation of the roof of the intrusion and wall rocks forming brecciated ores moreover other types of ores can be considered as a result of post magmatic hydrothermal activities our proposed metallogenic model involving the kiruna type mineralization is consistent with the observed phenomenon in the gushan deposit

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

petrogenesis of syntectonic granites emplaced at the transition from thrusting to transcurrent tectonics in post collisional setting whole rock and sr nd pb isotope geochemistry in the neoproterozoic quatro ilhas and mariscal granites southern brazil abstract the neoproterozoic post collisional period in southern brazil 650 580 ma is characterized by substantial volumes of magma emplaced along the active shear zones that compose the southern brazilian shear belt the early phase syntectonic magmatism 630 610 ma is represented by the porphyritic high k metaluminous to peraluminous quatro ilhas granitoids and the younger heterogranular slightly peraluminous mariscal granite quatro ilhas granitoids include three main petrographic varieties muscovite biotite granodiorite mbg biotite monzogranite bmz and leucogranite lcg that although sharing some significant geochemical characteristics are not strictly comagmatic as shown by chemical and sr nd pb isotope data the most primitive muscovite biotite granodiorite was produced by contamination of more mafic melts possibly with some mantle component with peraluminous crustal melts the biotite monzogranite although more felsic has higher ca mgo tio 2 and ba and lower k 2 o feo t sr and rb contents possibly reflecting some mixing with coeval mafic magmas of tholeiitic affinity the leucogranite may be derived from pure crustal melts the mariscal granite is formed by two main granite types which occur intimately associated in the same pluton one with higher k 5 6 5 wt k 2 o high rb and lower cao na 2 o ba and zr as compared to the other 3 5 wt of k 2 o the two mariscal granite varieties have compositional correspondence with fine grained granites fgg that occur as tabular bodies which intruded the quatro ilhas granoitoids before they were fully crystallized and are inferred to correspond to the mariscal granite feeders an interpretation that is reinforced by similar u pb zircon crystallization ages the initial evolution of the post collisional magmatism marked by the emplacement of the quatro ilhas granitoids varieties activated sources that produced mantle and crustal magmas whose emplacement was controlled both by flat lying and transcurrent structures the transition from thrust to transcurrent related tectonics coincides with the increase in the proportion of crustal derived melts the transcurrent tectonics seems to have played an essential role in the generation of mantle derived magmas and may have facilitated their interaction with crustal melts which seem to be to a large extent the products of reworking of orthogneiss protoliths

discovery of distal ejecta from the 1850 ma sudbury impact event a 25 70 cm thick laterally correlative layer near the contact between the paleoproterozoic sedimentary gunflint iron formation and overlying rove formation and between the biwabik iron formation and overlying virginia formation western lake superior region contains shocked quartz and feldspar grains found within accretionary lapilli accreted grain clusters and spherule masses demonstrating that the layer contains hypervelocity impact ejecta zircon geochronologic data from tuffaceous horizons bracketing the layer reveal that it formed between ca 1878 ma and 1836 ma the sudbury impact event which occurred 650 875 km to the east at 1850 1 ma is therefore the likely ejecta source making these the oldest ejecta linked to a specific impact shock features particularly planar deformation features are remarkably well preserved in localized zones within the ejecta whereas in other zones mineral replacement primarily carbonate has significantly altered or destroyed ejecta features

pb o and c isotopes in silicified mooidraai dolomite transvaal supergroup south africa implications for the composition of paleoproterozoic seawater and dating the increase of oxygen in the precambrian atmosphere abstract the mooidraai dolomite formation is a unit of marine sedimentary carbonates in the upper transvaal supergroup which recorded significant changes in the composition of the earth s atmosphere and ocean previously available data had suggested that this dolomite was about 2 2 ga old and showed 13ccarb values around 0 8 pdb and hence was an exception to the positive excursion of 13ccarb values observed worldwide in marine carbonates deposited between 2 25 and 2 05 ga ago we studied the pb o c isotope systematics of drill core samples from a highly silicified and aluminosilicate free sub unit of the mooidraai dolomite formation that yields well preserved micritic dolomite grains selective leaching of the silicified dolomite revealed a significant pb isotopic contrast between the carbonate and the quartz fraction the former shows only a narrow range of 208pb 204pb 35 35 35 58 which does not correlate with 206pb 204pb indicating the absence of th derived detrital pb the carbonate bound pb defines an isochron that corresponds to an age of 2394 26 ma 2 mswd 1 8 n 22 which we interpret as the diagenetic age of the mooidraai dolomite the 13ccarb values range from 0 51 to 0 64 pdb and confirm the previous data the 18ocarb values range from 2 08 to 0 18 pdb and are amongst the heaviest reported yet from early precambrian marine sedimentary carbonates the quartz fraction is considerably lighter than the carbonate fraction and shows 18oqtz values between 9 22 and 6 73 pdb 21 40 and 23 97 smow together with evidence from fluid inclusion microthermometry suggesting that the quartz formed at minimum temperatures between 130 and 155 c this indicates that the intense silicification was post depositional comparison of 18ocarb values of silicified and non silicified mooidraai samples suggests that the post depositional silicification had sealed the system and thereby protected the primary isotopic compositions of the micritic dolomite the heavy 18ocarb values suggest that 2 4 ga ago the oxygen isotopic composition of seawater differed only little from that of today the new pb pb carbonate age for the mooidraai dolomite indicates that the upper transvaal supergroup is about 200 my older than previously thought this explains the normal carbon isotopic composition of the mooidraai dolomite and suggests that the pronounced increase of the oxygen content in the precambrian earth s atmosphere that is seen in strata underneath the mooidraai dolomite occurred before 2 4 ga ago on this background it appears unlikely that the positive excursion of the carbon isotopic ratios of 2 25 2 05 ga old marine sedimentary carbonates is related to this increase in atmospheric oxygen

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

zircon rock partition coefficients of rees y th u nb and ta in granitic rocks uses for provenance and mineral exploration purposes abstract the trace element composition of magmatic zircon is highly variable even within the same grain and its study is complicated by the abundance of microinclusions metamict portions and strong compositional zoning among other factors to contribute to the understanding of the geochemistry of trace elements in zircon we determined the rare earth y nb ta th and u contents in zircon grains by laser ablation inductively coupled plasma mass spectrometry la icp ms with the use of cathodoluminescence and backscattered mev images zircon rock partition coefficients were determined in nine different types of granitoids and compared with experimental and natural data the obtained data are comparable with those reported by other authors by using the partition coefficient we estimated the composition of the host granitoids based on the zircon grain composition which was interpreted as evidence that the patterns of the trace elements in zircon grains are controlled by the liquid composition at the moment of crystallization a new set of partition coefficients for rees y nb ta th and u is suggested for provenance and exploratory studies la 0 8 ce 1 7 pr 2 1 nd 3 4 sm 9 8 eu 7 9 gd 24 tb 37 dy 69 ho 117 er 167 tm 281 yb 379 lu 437 nb 1 6 ta 2 9 th 27 u 177 and y 109 trace element diagrams eu eu vs yb nb vs th and y 200 lu 10 nb vs nb ce are proposed for discriminating zircon from different types of granitoids particularly for a type i type and shoshonitic rocks the trace element patterns of zircon are useful as a tool for mineral exploration studies because they often reproduce the patterns of mineralized granites

spatial variability in oceanic redox structure 1 8 billion years ago the deposition of iron formations ceased about 1 84 billion years ago reconstructions of ocean chemistry suggest that the advent of euxinic conditions along ocean margins preferentially removed dissolved iron from the water column in the form of the mineral pyrite inhibiting widespread iron oxide mineral deposition

early post collisional granitic and coeval mafic magmatism of medium to high k tholeiitic affinity within the neoproterozoic southern brazilian shear belt abstract the post collisional stage of the brasiliano pan african orogenic cycle in the garopaba region southern brazil began at 630 620 ma by the emplacement of large granitic massifs with associated mafic rocks the paulo lopes suite pls is composed of metaluminous to slightly peraluminous biotite hornblende paulo lopes and garopaba granitoids and by the silveira gabbro containing augite pigeonite and orthopyroxene mafic microgranular enclaves and mutual intrusive relations attest to the coeval character of mafic and felsic magmas the pls intruded during regional transcurrent shearing and microstructures indicative of high temperature solid state deformation are described in the mafic and granitic rocks mafic rocks are medium to high k tholeiitic and show mineralogical and geochemical features including isotopic ones that are compatible with magmas from sources modified by previous lithospheric subduction such as oib em1 the pls granitic magmas are interpreted as evolved from a mafic magma that was compositionally equivalent to the silveira gabbro contaminated by crustal melts or by more complex mechanisms hence they may be considered as granitic magmas of tholeiitic affinity transcurrent shearing seems to be essential for promoting the generation of mantle derived magmas and their interaction with crustal melts with consequent generation of abundant granitoids basic magmas have resulted from melting of heterogeneous mantle reservoirs and their highly evolved character is indicated by their low contents of mgo ni and cr the compositional similarity of pls granitoids particularly the garopaba granitoids with a type granitoids discriminates them from the medium to high k granitoids of mature magmatic arcs and this difference probably reflects the tholeiitic character of their primary magmas typical of the early magmatic stages of post collisional settings in southern brazil trace element and isotopic geochemical data indicate subduction modified oib mantle sources for the mafic magmas which is coherent with the post collisional character of this magmatism

geochemical evolution of the rooiberg silicic lavas in the loskop dam area southeastern bushveld the rooiberg felsite is the voluminous volcanic precursor to the plutonic episodes of bushveld magmatism in the area north of the loskop dam a 3 520 m thickness of felsite is dominated by flows of dacitic and rhyodacitic lava occasional silicic andesites are developed near the base whereas true rhyolites become overwhelmingly dominant toward the top of the sequence nine major stratigraphic units of lava are present these are usually separated by and sometimes include volumetrically minor pyroclastic and sedimentary deposits whole rock chemical data on 165 lava samples embracing the entire stratigraphy suggest that nonuniform alteration processes have been most effective in the upper units nevertheless field petrographic and chemical criteria enable identification of three felsite magma types a high mg type mgo 1 7 containing low zr and y contents is present only within the lowermost 1 400 m of section a low mg type mgo 2 asymp 1 p 2 o 5 asymp 0 35 type which only occurs in the lower part of the succession the high mg lavas exhibit a systematic eruptive progression from more to less evolved compositions apparently reflecting continuous tapping of successively deeper levels of a normally zoned magma chamber during the early volcanism the low mg lavas were erupted intermittently with the high mg types probably from a separate and much larger chamber pronounced differentiation of this magma is only evident in the youngest volcanic units where several element profiles display conspicuous steplike variations across the prominent ash flow horizons the steps reflect upward enrichments in nb zr and y and corresponding incremental depletions in ti p and sc such trends imply the existence of a discontinuous but reversed compositional zonation in the evolved low mg chamber the high fe ti p lavas were also erupted from the low mg chamber this may indicate the development of an early type of chemical stratification that was markedly different from the compositional layering in the evolved system

time frame of 753 680 ma juvenile accretion during the sao gabriel orogeny southern brazilian shield abstract the time frame of the three main geological events in the neoproterozoic cambai complex juvenile sao gabriel belt in the southern brazilian shield is established by integrating field mapping back scattered electron imaging and sensitive high resolution ion microprobe shrimp ii u pb dating of 96 zircon crystals from nine granitic and metasedimentary rock samples the three events are 1 voluminous flat lying paragneisses cambaizinho complex and orthogneisses vila nova gneisses between 735 and 718 ma 2 tonalite trondhjemite association lagoa da meia lua suite between 710 and 690 ma and 3 late granodiorite intrusions sanga do jobim suite at 680 ma an additional older volcanic event campestre formation was dated at 753 ma these results are most significant for the reconstruction of west gondwana

melt collecting structures and the formation of extraction dykes during syntectonic anatexis of the camboriu complex south brazil abstract the camboriu complex metamorphic sequence comprises orthogneisses and amphibolites with widespread melting features generated at upper amphibolite facies conditions regional to mesoscale structures are shown to control generation collection and extraction of melts as partial melting starts it is locally focused as layer parallel leucosomes in banded gneisses and isolated pockets of in situ leucosomes in massive amphibolites the further tightening of cm to m scale upright folds and conjugate shear planes oblique to axial planar position trigger the coalescence and migration of melts first along the main banding then along axial planes of folds and finally giving rise to a network where melt is collected the melt network eventually acquires enough mobility to form m wide extraction dykes roughly parallel to axial planes of folds capable of transferring the products of migmatization to upper crustal levels to feed granitic plutons the orientation and geometry of pure shear dominated structures in the camboriu complex are in accordance with deformation partitioning between this and adjancent shear zones which is expected for the transpressive character of the neoproterozoic southern brazilian shear belt

an isotopic study on the volcanics of the rooiberg group age implications and a potential exploration tool many geochronological studies on silicic magmatic rocks associated with the bushveld complex rhyolitic lavas of the rooiberg group and granites of the lebowa granite suite have shown evidence of open system behaviour of the rb sr and pb pb isotopic systems until 1600 1000 ma many hundreds of million years after crystallisation of these rocks this pervasive open system behaviour has been attributed to sustained hydrothermal circulation driven by the high heat productivity of the bushveld granites new sr and pb isotopic data are presented for basaltic to rhyolitic volcanics from the rooiberg group of the transvaal sequence in the dullstroom loskop dam area of the eastern transvaal these data show little evidence of open system behaviour after about 1950 ma and many sample suites retain ages which could reflect the formation of the rooiberg group i e older than 2070 ma it is argued that this preservation is due to the absence of fractionated fluid vapour rich bushveld granites in the immediate vicinity of the volcanic occurrences rooiberg group volcanics with extensively perturbed rb sr and particularly pb pb isotopic systems reflect the action of granite derived hydrothermal fluids as a consequence the isotope systematics in these volcanics could prove a useful exploration tool for sites of granite derived metal deposits

juvenile accretion at 750 700 ma in southern brazil a revista economic geology nao autoriza a publicacao de seus artigos em repositorios institucionais