retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

the karoo igneous province a problem area for inferring tectonic setting from basalt geochemistry tholeiitic basalts and associated intrusives are the major component of the karoo igneous province they are of mesozoic age and constitute one of the world s classic continental flood basalt cfb provinces it has been argued that most karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the proterozoic the only exceptions to this are late stage morb like dolerites near the present day continental margins which are considered to be of asthenospheric origin when data for the southern karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting essentially all of them would be classified as calc alkali basalts cab s or low k tholeiites virtually none of them plot in the compositional fields designated as characteristic of within plate basalts there is little likelihood that the compositions of the karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a subduction component has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out it must be concluded that the mantle source areas for cab s and the southern karoo basalts have marked similarities in contrast the data for northern karoo basalts largely plot in the within plate field on geochemical discriminant diagrams available data suggest that the source composition and or the restite mineralogy and degree of partial melting are different for southern and northern karoo basalts there is no evidence for any difference in tectonic setting between the southern and northern karoo basalts at the time they were erupted this appears to be clear evidence that specific mantle source characteristics and or magmatic processes can vary within a single cfb province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks

the p t record of synchronous magmatism metamorphism and deformation at petrel cove southern adelaide fold belt porphyroblastic schists in the thermal aureole of the victor harbor granite at petrel cove in the southern adelaide fold belt south australia preserve a record of sequential cordierite andalusite staurolite fibrolite chlorite and muscovite growth along with biotite plagioclase quartz ilmenite during progressive deformation a p t pseudo section appropriate to biotite saturated assemblages in kfmash shows that the sequence of mineral reactions records increasing pressure of at least 1 kbar from c 3 to c 4 kbar during cooling from around 580 c heating at pressures below c 3 kbar is inferred for growth of early formed cordierite porphyroblasts and is attributed in part to the thermal effects of granite emplacement while the pressure increase is attributed to tectonic burial accruing from ongoing deformation the anticlockwise p t path is consistent with convergent deformation being focussed as a consequence of heating as to be expected for a lithospheric rheology that is strongly temperature dependent

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

precise u pb titanite age constraints on the emplacement of the bushveld complex south africa the c 2 06 ga bushveld complex intrudes the transvaal supergroup south africa calcsilicate xenoliths within the mafic phase of the bushveld complex rustenburg layered suite or rls preserve calcsilicate xenoliths with high temperature c 1200 c mineralogies that were later metasomatized by hydrous retrograde c 600 700 c fluids whose timing has been unconstrained new u pb isotope data from newly grown titanite within a completely retrogressed xenolith indicates that retrogression occurred at 2058 9 0 8 ma this suggests that retrogression was due to hydrothermal circulation associated with the cooling rls and not slightly later bushveld related granites the new data place a tight constraint on the minimum age of emplacement of the rls

the problem of inherited 40ar in dating impact glass by the 40ar 39ar method evidence from the tswaing impact crater south africa abstract the tswaing meteorite impact crater is a 1 13 km diameter structure located in the 2 05 ga nebo granite of the bushveld complex the impact age had previously been determined by fission track dating to 220 104 ka 40ar 39ar step heating and total fusion experiments performed on single and multi grain impact glass aliquots gave apparent ages ranging from 1 0 0 3 ma to 204 6 ma these ages indicate that the radiogenic ar derived from the target rocks has not been completely degassed as a result of the impact process despite fusion of the target material results of step heating experiments imply that the 40 ar inherited trapped within the glass is located in two distinct reservoirs thought to be the glass matrix and fluid vapor inclusions or un melted residual clasts calculations assuming an age of 0 2 0 1 ma for tswaing fission track data reveal that the amount of inherited 40ar 40 ar inherited relative to the pre impact concentration varies from 0 015 to 4 15 the spread defined by 40 ar inherited likely reflects the various quench rates experienced by the glass most certainly due to the pre impact position of the sample relative to the center of the crater we compare the influence of 40 ar inherited on the apparent 40ar 39ar age determination of five impact structures our calculations show that the main characteristic controlling the age offset for a given proportion of 40 ar inherited is the age difference between the impact and the target rocks i e the 40ar concentration in the target rock the buffer effect for a given crater structure can be predicted knowing the age of the basement and having a rough estimation of the age of the crater structure itself the occurrence of 40 ar inherited is likely influenced by 1 the degree of polymerization i e silicate structure complexity of the target rock and presumably related to the diffusivity of ar in the melt and glass 2 the ar partial pressure at the grain boundary 3 the quantity of energy involved in the impact and 4 the porosity of the target rocks for glass that inevitably suffers inherited and or excess 40ar the use of the inverse isochron technique can be appropriate but should be applied with careful statistical treatment

petrography and geochemistry of sandstones interbedded with the rooiberg felsite group transvaal sequence south africa implications for provenance and tectonic setting abstract the 2150 ma rooiberg felsite group transvaal sequence contains thin sandstone interbeds within its 3500 5000 m thick volcanic succession whereas uppermost feldspathic arenites and wackes are thought to represent reworked felsitic material quartz arenites and subordinate lithic sandstones present throughout most of the rooiberg sequence comprise mainly reworked sedimentary detritus probably belonging to the underlying pretoria group sandy braided river systems probably transported clastic material into the basin from relatively stable source areas subject to intense chemical weathering geochemistry and sandstone petrography indicate that mixing of pretoria group detritus with sediment derived from erosion of felsitic material occurred within the basin during late rooiberg ti es the inferred hiatuses in volcanism represented by the predominant siliceous sedimentary interbeds appear to have been of relatively short duration and occur throughout much of the rooiberg stratigraphy upper arkosic sandstones indicate longer breaks in volcanism as rooiberg eruptions came to an end the sandstones provide evidence compatible with an impact origin for the rooiberg felsite group and for its successor the bushveld complex

ore shoots in folded and fractured rocks insights from 3d modelling of the fosterville gold deposit victoria australia abstract at the fosterville gold deposit high concentrations of visible and refractory gold are hosted in arrays of quartz and quartz carbonate veins they are distributed in elongate zones to form south plunging ore shoots within a folded and fractured turbidite sequence here we distinguish the influence of faults versus folds at fosterville by assessing the deposit scale structural framework and hydrothermal fluid flow using a combination of 3d implicit modelling structural analysis and 3d fault slip dilation tendency analyses gold assay and bedding trend data from a large drillhole dataset were spatially interpolated and used to analyse and measure the orientation of ore bodies and folded host rocks field measurements of fold axis orientations were compared to bedding measurements in oriented drillcore bingham axial analysis of bedding trends and orientation data from a 3d implicit model both field and drill core structural data show that ore shoots at fosterville do not parallel the first order gently 10 south plunging folds instead the orientation of the ore shoots is coincident with an intersection lineation between steeply west dipping bedding parallel faults and moderately dipping linkage faults that obliquely cut across the main structural grain resulting in ore shoot plunges larger than 20 slip and dilation tendencies suggest that linkage faults had a higher likelihood of activity in the inferred far field stress regimes at the time of mineralisation relative to other structures at fosterville these linkage faults are therefore interpreted to have been preferential fluid flux conduits in the system despite appearing to be subordinate structures at the scale of the deposit this new structural model for fosterville highlights that fluid flow and ore shoot formation are strongly controlled by obliquely oriented structures that are unlike the main structural grain of first order folds and faults

pulses of plagioclase laden magmas and stratigraphic evolution in the upper zone of the bushveld complex south africa cumulate rocks of the upper main zone and upper zone uumz of the bushveld complex south africa contain the world s major resources of fe ti v hosted in ti magnetite and apatite and are commonly considered as having crystallized from the last major injection of magma into the magma chamber in this study we present the petrography modal proportions whole rock major element chemistry 260 samples electron microprobe data 10 000 analyses for plagioclase olivine and pyroxene and compiled analyses of cr in magnetite 239 samples for the uumz sampled over 2 1 km of the bierkraal drill cores in the western limb of the complex the uumz section exhibits a broad normal fractionation trend upwards but a series of reversals to more primitive anorthite contents in plagioclase mg in pyroxenes and olivine cr in whole rocks and cr in magnetite separates are observed accompanied by the appearance or disappearance of various minerals anorthosite or leucogabbro layers are closely linked to these reversals the reversals in an of plagioclase are used as boundaries to divide the uumz into 18 cycles these cycles are interpreted as indications of magma chamber replenishment by plagioclase laden magmas up to 20 vol plagioclase and are also marked by spikes in cr content in addition abundant fe ti oxide bearing plagioclase rich rocks are identified in the lower half of the uumz these have crystallized from a hybrid melt produced by the mixing of a new plagioclase bearing magma batch and the resident magma further crystallization of this hybrid liquid may lead to the formation of magnetite layers in the lower part of the uumz the bushveld uumz therefore grew by multiple emplacements of crystal laden magmas coming from deep seated chambers slow cooling in a shallow chamber explains the systematic bottom up compositional evolution in the cumulate pile within individual cycles the residual melt reached silicate liquid immiscibility soon after the saturation of apatite thereafter segregation of conjugate fe rich and si rich melts and crystallization of the paired melts produces cumulates with a smooth upward decrease in fe ti oxides whereas plagioclase mode increases in each apatite bearing cycle a comparison of systematic geochemical analyses and a detailed lithological stratigraphy between the bushveld limbs demonstrates the possible connectivity between the western and eastern upper zone but indicates notable differences from the bellevue section of the northern limb

chemical alteration within the volcanic roof rocks of the bushveld complex field relations petrography and geochemistry identify mobile and immobile elements within the rooiberg volcanic rocks of the floor and roof of the mafic rustenburg layered suite of the 2 06 ga bushveld complex the floor rocks show thermal metamorphism accompanied by dehydration two major alteration processes affected the roof rocks the first followed extrusion of the felsic damwal formation and is marked by veins of pyrite and arsenopyrite the second is related to the intrusion of the rustenburg layered suite which produced an aureole 1 4 km thick in the overlying felsic rocks pb zn and mn attain maximum concentrations of 335 ppm 929 ppm and 0 45 wt percent in the metasomatized roof rocks the economic significance of elevated pb and zn concentrations at the top of the aureole deserves future attention primary concentrations of si mn ca na k fe mg st th ba u hf ni cu zn pb nb zr and y were modified to varying degrees depending on locality and distance from the bushveld intrusions ti al p ga so and heavy ree were immobile concentrations of these elements in the rooiberg volcanic rocks indicate that at least one geochemically distinct magma type is present in both the floor and the roof sequences this confirms that the volcanic floor and root successions originally formed as a continuous sequence the color of the roof rocks is controlled by the contact metamorphic processes and cannot be used for stratigraphic subdivision nor should the red color of these rocks be used for inferences about the evolution of the proterozoic atmosphere hydrothermal alteration was also responsible for the aberrant radiometric dates in the rooiberg group

problematic urn shaped fossils from a paleoproterozoic 2 2ga paleosol in south africa abstract small 0 3 1 8 mm long locally abundant urn shaped fossils within surface horizons of a paleosol in the 2 2 ga hekpoort formation near waterval onder south africa are here described and named diskagma buttonii retallack gen et sp nov the fossils are from fresh rock of a deep highway cutting and have been metamorphosed to upper greenschist facies like their matrix despite metamorphic alteration total organic carbon of the samples was 0 04 and its isotopic composition 13 c was 25 6 0 08 two standard deviations versus vienna pee dee belemnite standard organic outlines of the fossils are also accentuated by recystallized berthierine and opaque oxides the fossils are locally clumped within surface swales of a vertisol paleosol identified from characteristic penecontemporaneous deformation clastic dikes between swales of mukkara structure and from pronounced geochemical differentiation phosphorus and copper strain corrected mass depletion characteristic of an oxidized biologically active soil this paleosol s chemical composition is evidence of temperate humid climate mean annual temperature 11 3 4 4 c and mean annual precipitation 1489 182 mm associated paleosols indicate atmospheric co 2 of 6640 12 880 4293 ppm 0 6 and 0 9 5 atmospheric o 2 the best preserved examples of diskagma are shaped like an urn with a flared rim and closed below the flare observation of hundreds of specimens in thin section reveals substantial variation in growth elongation and decay shredding and deflation they had a hollow ellipsoidal interior that is unusually devoid of opaque debris unlike the matrix diskagma is superficially comparable with lichens such as cladonia ascomycota and geosiphon glomeromycota definitive reproductive structures remain unknown they predate the oldest other likely fossil eukaryotes 1 9 ga and fungi 1 5 ga and current molecular clock estimates for eukaryotes 1 6 ga and fungi 1 1 ga lichenized actinobacteria are plausible prokaryotic alternatives permitted by molecular clocks although biological affinities of diskagma are uncertain these fossils reveal the general appearance of paleoproterozoic life on land

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

calculation of partial melting equilibria in the system na2o cao k2o feo mgo al2o3 sio2 h2o nckfmash a thermodynamic model for haplogranitic melts in the system na2o cao k2o al2o3 sio2 h2o nckash is extended by the addition of feo and mgo with the data for the additional end members of the liquid incorporated in the holland powell 1998 internally consistent thermodynamic dataset the resulting dataset with the software thermocalc is then used to calculate melting relationships for metapelitic rock compositions the main forms for this are p t and t x pseudosections calculated for particular rock compositions and composition ranges the relationships in these full system pseudosections are controlled by the low variance equilibria in subsystems of nckfmash in particular the solidus relationships are controlled by the solidus relationships in nkash and the ferromagnesian mineral relationships are controlled by those in kfmash however calculations in nckfmash allow the relationships between the common metapelitic minerals and silicate melt to be determined in particular the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks particularly relating to open system behaviour of rocks under high grade conditions

terminology for layered intrusions the terminology of igneous cumulates was originally formulated on the concept that the rocks were concentrations of minerals accumulated by crystal settling it is now well established for several major intrusions that plagioclase that had been typed as cumulus should in fact have floated in its parental magmatic liquid nevertheless the terminology has been valuable in the classification of the rocks of these and other intrusions and it has been useful in certain aspects of their genetic interpretation the name cumulate is therefore redefined so that crystal settling is a possible but not essential process in the origins of the rocks to which it is applied the overall nomenclature of layered intrusions is then reviewed and definitions of names are refined and systematized to clarify their applications careful distinction is made between descriptive and genetic names a cumulate is defined as an igneous rock characterized by a framework of touching mineral crystals and grains that evidently were concentrated through fractional crystallization of their parental magmatic liquids a layer is a sheetlike unit of cumulate that is a distinctive entity in its compositional and or textural features a lamina is a thin 2 5 cm sharply defined layer igneous lamination is a general name for planar structure that is pervasive through cumulates on the scale of their grain size layering is the overall structure and fabric manifest through combinations of layers laminae and lamination the difference between layers and layering is essentially the difference between individuals and a population and the terms that describe one are not necessarily appropriate to the description of the other but the fundamental variants of all of layering lamination grading in layers and layer contacts can essentially be described by one or more of the four modifiers grain size modal textural and cryptic where the last refers to cumulus miner al compositions a system for subdividing layered intrusions is proposed in which distinction is made between general divisions ranked as series zones and subzones and prominent units that are called groups members rhythmic units and cyclic units applications to the skaergaard bushveld muskox stillwater and other intrusions are briefly described

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

basalt geochemistry and tectonic discrimination within continental flood basalt provinces abstract continental flood basalts are usually regarded as a single tectonomagmatic entity but frequently quoted examples exhibit a variety of tectonic settings in one well studied classic flood basalt province the mesozoic karoo province of southern africa magmatism occurred in the following tectonic settings a continental rifting leading to ocean floor spreading in the south atlantic ocean etendeka suite of namibia b stretched continental lithosphere and rifting not leading directly to ocean floor formation lebombo suite of southeastern africa and c an a tectonic within plate continental setting characterized by an absence of faulting or warping lesotho highlands and karoo dolerites of south africa by means of spidergrams of the elements rb ba th nb k la ce sr nd p hf zr sm ti tb y v ni and cr uncontaminated tholeiites from c above i e the lesotho type continental flood basalts ltcfb are compared with mid ocean ridge basalts morb ocean island tholeiites oit and tholeiites and calc alkali basalts from subduction environments the comparison reveals the ltcfbs are geochemically distinct the differences are reflected in relative enrichments or depletions of the more incompatible elements rb ce to less incompatible elements ce y i e the overall slope of the spidergrams and in anomalous enrichments or depletions of one or more of the elements th k nb sr ti hf and zr the distinctive geochemical character of the lesotho ltcfbs is interpreted in terms of a lithospheric mantle source for the basalts this is supported by isotopic data there are no major geochemical differences between lesotho cfbs and basalts of the rift related etendeka and lebombo suites although the latter are somewhat enriched in rb ba and k however unlike the lesotho basalts the lebombo and etendeka basalts are associated with voluminous silicic volcanics or intrusive centres and late stage dolerites having morb oit i e asthenospheric geochemical characteristics the flood basalt silicic magmatism late stage dyke swarm association is characteristic of several rift or thinned lithosphere environments e g ethiopia skye eastern greenland but in many of these the flood basalts have ocean island basalt oit geochemical characteristics the lesotho type cfb geochemistry is exhibited by the grande ronde basalt of the columbia river group a possible subduction related flood basalt province and the basic rocks associated with mesozoic rifting in the north and south atlantic basalt geochemistry alone is unhelpful in determining the tectonic setting of cfbs although the rift related environments may be identified by the petrology and geochemistry of the whole igneous suite a two source model is proposed for the mantle derived basic rocks in rift related cfb provinces early enriched basalts are derived from the lithosphere and following pronounced lithospheric attenuation or rifting later morb like melts are emplaced from the rising asthenosphere the presence of both lesotho and oit type geochemical patterns in rift related cfbs suggests that the lithosphere exhibits different styles of enrichment

geochronology and petrology of migmatites from the proterozoic damara belt importance of episodic fluid present disequilibrium melting and consequences for granite petrology abstract the oetmoed granite migmatite complex ogmc central damara orogen namibia consists of grt and crd bearing s type granites and hbl and titanite bearing a type granites that intruded into crd sil kfs bearing metasedimentary rocks stromatic migmatites and nebulites stromatic migmatites formed by limited in situ partial melting of metapelites under h2o saturated conditions at 700 c and 4 5 kbar this partial melting event took place close to the peak of regional metamorphism at 510 ma as revealed by sm nd garnet whole rock ages as well as u pb monazite ages the newly formed melt remained more or less at the site of origin melanosomes of the stromatic migmatites do not resemble true residues instead they represent reaction zones between in situ melt and the metasedimentary host rock leucosomes from the stromatic migmatites are lree and hfse depleted which is typical for the low melt fractions generally observed in migmatite terranes mass balance calculations suggest that these leucosomes may be disequilibrium melts similar 18 o whole rock values between 13 and 14 observed in leucosomes and corresponding melanosomes suggest homogenization of oxygen isotopes between the different rock types via the melt and an internal fluid phase during melting other leucosomes with a major element chemistry similar to the leucosomes from the stromatic migmatites but higher ree contents suggest significant incorporation of lree enriched phases monazite from the country rocks for these leucosomes sm nd garnet whole rock ages are 473 ma indicating a second phase of melting nebulites mainly result from injection of granitic melts into the country rocks but their residual chemistry indicates that partial melting and limited melt removal must have occurred monazites from these nebulites record concordant u pb ages between 540 ma and 470 ma indicating episodic migmatization during high grade regional metamorphism intrusive peraluminous granites are likely generated by partial melting of pelitic sources in the lower crust and different source rocks are probably not the controlling factor for the different chemistry of leucosomes and granites therefore different melting conditions fluid absent vs fluid present and different modes of entrainment and solubilities of accessory phases control the elemental budgets of the leucosomes large scale migmatite granite complexes are a substantial part of some high grade terrains but the distinctive geochemical composition of some leucosomes make these rocks unlikely precursors of large scale granitic bodies

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

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

a late miocene pliocene origin for the central himalayan inverted metamorphism abstract perhaps the best known occurrence of an inverted metamorphic sequence is that found immediately beneath the himalayan main central thrust mct generally thought to have been active during the early miocene however in situ 208 pb 232 th dating of monazite inclusions in garnet indicates that peak metamorphic recrystallization of the mct footwall occurred in this portion of the central himalaya at only ca 6 ma the apparent inverted metamorphism appears to have resulted from activation of a broad shear zone beneath the mct which tectonically telescoped the young metamorphic sequence this explanation may resolve some outstanding problems in himalayan tectonics such why the mct and not the more recently initiated thrusts marks the break in slope of the present day mountain range it also renders unnecessary the need for exceptional physical conditions e g high shear stress to explain the generation of the himalayan leucogranites

the effects of accretion erosion and radiogenic heat on the metamorphic evolution of collisional orogens petrological and thermochronological data provide our best record of the thermal structure of deeply eroded orogens and in principle might be used to relate the metamorphic structure of an orogen to its deformational history in this paper we present a two dimensional thermal model of collisional orogens that includes the processes of accretion and erosion to examine the p t evolution of rocks advected through the orogen calculated metamorphic patterns are similar to those observed in the field metamor phic temperatures depths and ages generally increase with distance from the toe of the orogen p t paths are anti clockwise with rocks heating during burial and early stages of unroofing followed by cooling during late stage unroofing the results indicate that peak metamorphic temperatures within the core of a collisional orogen and the distance from the toe of an orogen to the metamorphic core can be related to the relative rates of accretion erosion and plate convergence model orogens displaying high metamorphic temperatures 600 c are associated with low ratios of accretion rate to plate convergence velocity and with high heat flow through the foreland model orogens with metamorphic cores far from the toe of the orogen are associated with high ratios of accretion rate to erosion rate calculated metamor phic gradients mimic steady state geotherms and inverted thermal gradients can be preserved in the metamorphic record suggesting reconsideration of the concept that the metamorphic record does not closely reflect geothermal gradients within an orogen

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

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

growth and retrograde zoning in garnets from high grade metapelites implications for pressure temperature paths a previously unrecognized pattern of garnet zoning has been preserved in garnets from the upper amphibolite facies britt domain ontario grenville province some large garnets contain discrete regions of high grossular content as well as variations in the mg mg fe and mn nearly parallel to grain boundaries the latter are superimposed on and cut across regions of high grossular content such crosscutting zoning patterns have not been previously recognized in garnets the ca zoning involves changes of 4 to 10 mol grossular and is interpreted to represent growth zoning whereas the decreasing mg mg fe and increasing mn from core to rim are believed to be a diffusional retrograde effect the ca zoning indicates a pressure drop of ca 5 kbar at 700 50 c

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

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

a calculated petrogenetic grid for the system k2o feo mgo al2o3 sio2 h2o with particular reference to contact metamorphosed pelites a petrogenetic grid is presented for the system kfmash k o feo mgo a1 0 sio h o including biotite muscovite k feldspar chlorite chloritoid staurolite cordierite garnet ortho amphibole orthopyroxene spinel andalusite sillimanite kyanite quartz and corundum with h o in excess which was calculated using the computer program thermocalc and the powell and holland internally consistent thermodynamic dataset by removing the normal constraint of having quartz in excess both quartz bearing and quartz absent equilibria are shown quartz absent equilibria are particularly relevant at high t and low p conditions because of their common occurrence at these conditions the calculated mineral assemblage and mineral compositional variations in terms of femg and fe mg siall exchange vectors are broadly compatible with observations on natural rocks particularly when non kfmash components are taken into account

palaeoproterozoic sedimentation on the singhbhum craton global context and comparison with kaapvaal the singhbhum craton has a limited palaeoproterozoic supracrustal record which suggests a three part history comprising a long period of high freeboard and palaeosol formation on granitoids subsequent rift related mafic ultramafic volcanism and subordinate sedimentation c 2 25 2 1 ga dhanjori and jagannathpur basin fills possibly also simlipal malangtoli and ongarbira basin fills which overlapped locally with mafic soil formation and a major regression at around2 0 ga followinga longhiatus theapproximately 1 6 gadhalbhum dalmasuccession was laid down probably under continental conditions this rather truncated record stands in con trast to the chronologically and geographically much more widespread supracrustal basin fills of the kaapvaal craton and there appears to be an overall poor comparison between these two early precambrian crustal blocks however on kaapvaal three analogous events are identified widespread approximately 2 2 ga mafic volcanism followed by a well developed palaeosol and a major transgression prior to 2 05 ga the three shared events between the two cratons are com patible with the postulate of a global approximate 2 45 2 2 ga shutdown of magmatic and tec tonic geodynamics with the origin of the triumvirate directly reflecting its resumption again after about 2 2 ga we recognize here that a large diversity of views on singhbhum s geodynamic history exists predicated on a lack of precise geochronology and commonly poor outcrops and the current hypotheses are presented with these factors in mind

o isotope study of the bushveld complex granites and granophyres constraints on source composition and assimilation the 2 05 ma bushveld magmatic event culminated in the production of490 000 km 3 of granite and granophyres in these granitic rocks high temperature equilibrium o isotope fractionations are generally preserved between quartz and zircon but not between quartz and feldspar or between biotite and amphibole quartz separated from four granite samples shows no significant difference in core and rim d 18 o values which indicates that quartz is not significantly zoned and provides further evidence that it is unaffected by alteration quartz can therefore be used as a proxy for the magma d 18 o value leading to estimates of 6 9o for both the granites assuming quartz magma 1 11o and granophyres assuming quartz magma 0 62o similar magma d 18 o values 6 6o were obtained using zircon d 18 o values assuming zircon magma 1 3o the initial nd isotope ratio of the granitic rocks ranges from 0 509676 to 0 509822 with an average value of 0 509655 n 12 this corresponds to average end values of 5 9 and 4 8 for the granites and granophyres respectively the similarity in isotope composition between the granites and granophyres and between the granitic rocks from each of the three major lobes of the bushveld complex is consistent with a common origin the d 18 o values of the granitic rocks suggest derivation from mantle derived magmas by either fractional crystallization or partial melting but this hypothesis is incompatible with their crustal end values average 5 5 the associated rustenburg layered suite rls rocks have average d 18 o values of 7 1o which is within error of the average estimate for the bushveld granitic rocks and similar end values however granitic magma derived from the same paretal magmas that produced the rls would have had an average magma d 18 o of about 7 9o 1 o higher than observed we therefore suggest that the granitic magmas were produced by fractional crystallization of rls magma or by partial melting of solidified rls magma at depth followed by assimilation at a shallower level of a significant quantity of hydrothermally altered low d 18 o material from the since eroded volcanic edifice

the 190pt 186os decay system applied to dating platinum group element mineralization of the bushveld complex south africa abstract we evaluate the 190pt 186os decay system measured by laser ablation mc icpms as a useful geochronometric tool for direct dating of platinum group minerals pgm in large mafic ultra mafic intrusions by analysing mineralised rocks from the geochronologically well constrained bushveld complex s africa mixed pgm laurite cooperite laurite platarsite sperrylite and pt fe alloy from the merensky reef yield a pt os isochron age of 1995 50 ma 2 n 48 mswd 1 16 initial 186os 188os 0 119819 0 000006 while this is 59 myr younger than the u pb zircon age for the merensky reef scoates and friedman 2008 it is consistent with recent re os ages for the platreef considering the younger age recorded by both re os and pt os systems we propose that the pt os isochron age records a late low temperature hydrothermal event affecting pgm and sulphides in the platreef and merensky reef pt rich phases may also yield single grain 190pt 186os model ages since initial 186os 188os can be well constrained a pt os model age of 2024 ma 101 ma was calculated for a merensky cooperite a pt fe alloy from the onverwacht pipe and a tweefontein hill sperrylite give pt os model ages of 2113 106 ma and 2042 102 ma respectively a pt os isochron derived from all bushveld units gives an age of 2012 47 ma 2 186os 188osi 0 119818 0 000006 with an mswd of 1 19 hence pge mineralisation in different stratigraphic horizons of the complex cannot be distinguished temporally using the pt os chronometer nonetheless the potential for the pt os system in dating events in pge mineralised systems is evident and the agreement of single grain model ages in high pt os phases with the pt os and re os isochron ages indicates their usefulness in the geochronology of these systems

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

variation in metamorphic style along the northern margin of the damara orogen namibia the northern margin of the inland branch of the pan african damara orogen in namibia shows dramatic along strike variation in metamorphic character during convergence between the congo and kalahari cratons m3 metamorphic cycle low p contact metamorphism with anticlockwise p t paths dominates in the western domains ugab zone and western northern zone and high p barrovian metamorphism with a clockwise p t path is documented from the easternmost domain eastern northern zone the sequence of m3 mineral growth in contact aureoles shows early growth of cordierite porphyroblasts that were pseudomorphed to biotite chlorite muscovite at the same time as an andalusite biotite muscovite transposed foliation was developed in the matrix the peak t metamorphic assemblages and fabrics were overprinted by crenulations and retrograde chlorite muscovite the kfmash p t pseudosection for metapelites in the ugab zone and western northern zone contact aureoles indicates tight anticlockwise p t loops through peak metamorphic conditions of 540 570 c and 2 5 3 2 kbar these semi quantitative p t loops are consistent with average p t calculations using thermocalc which give a pooled mean of 556 26 c and 3 2 0 6 kbar indicating a high average thermal gradient of 50 c km in contrast the eastern northern zone experienced deep burial high p moderate t barrovian m3 metamorphism with an average thermal gradient of 21 c km and peak metamorphic conditions of c 635 c and 8 7 kbar the calculated p t pseudosection and garnet compositional isopleths in kfmash appropriate for the metapelite sample from this region document a clockwisep t path early plagioclase kyanite biotite parageneses evolved by plagioclase consumption and the growth of garnet to increasing xfe xmg and xca and decreasing xmn compositions indicating steep burial with heating the developed kyanite garnet biotite peak metamorphic parageneses were followed by the resorption of garnet and formation of plagioclase moats indicating decompression which was followed by retrogressive cooling and chlorite muscovite growth the clockwise p t loop is consistent with the foreland vergent fold thrust belt geometry in this part of the northern margin earlier formed 580 570ma pervasive matrix foliations m2 were overprinted by contact metamorphic parageneses m3 in the aureoles of 530 3ma granites in the ugab zone and 553 514ma granites in the western northern zone available geochronological data suggest that convergence between the congo and kalahari cratons was essentially coeval in all parts of the northern margin with similar ages of 535 530ma for the main phase of deformation in the eastern northern zone and northern platform and 538 505ma highgrade metamorphism of the central zone immediately to the south consequently nne ssw directed convergent deformation and associated m3 metamorphism of contrasting styles are interpreted to be broadly contemporaneous along the length of the northern margin of the inland branch in the west heat transfer was dominated by conduction and externally driven by granites whereas in the east heat transfer was dominated by advection and internally driven radiogenic heat production the ultimate cause was along orogen variation in crustal architecture including thickness of the passive margin lithosphere and thickness of the overlying sedimentary succession

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

high geothermal gradient metamorphism during thermal subsidence the burial of a basement sequence enriched in heat producing elements during thermal subsidence following rifting produces two concomitant changes in the thermal structure of the crust firstly the burial of the enriched layer produces high geothermal gradients in the overlying sedimentary succession with the high gradients propagating down into but not through the enriched basement sequence secondly the lithospheric thickening that drives thermal subsidence reduces the heat flowing into the deeper crust from the mantle because the process of thermal subsidence promotes burial it naturally increases the depth extent of the high geothermal gradients in the upper crust potentially inducing significant temperature increases in the mid upper crust during burial the lowering of the thermal gradients in the deep crust accompanying burial severely limits the temperature changes affecting the moho potentially allowing moho cooling while the mid upper crust heats these effects can promote high geothermal gradient 40 c km metamorphism in the mid upper crust without inducing significant melting in the lower crust providing the basement heat production contributes 70 mw m 2 to the surface heat flow and that the horizontal length scale for the basement heat production anomaly is 50 km these conditions appear to be met in several australian intermediate to high temperature low pressure metamorphic terranes where the thermal causes of metamorphism have hitherto remained enigmatic one of these terrains the mt painter province in the northern flinders ranges south australia is used to illustrate some of the attributes of the model

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

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

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