the lower zone critical zone transition of the bushveld complex a quantitative textural study the lower zone critical zone boundary of the bushveld complex is an intrusion wide major stratigraphic transition from ultramafic harzburgite and pyroxenite in the lower zone to increasingly plagioclase richpyroxenitesandnoritesinthecriticalzone quantitative textural and compositional data for 29 samples through this transition show the following lower zone orthopyroxene grains are larger have higher aspect ratios are better foliated and have a lower trapped liquid component than those of the critical zone the larger grain size of the lower zone results in crystal size distribution plots that are rotated to lower slopes and intercepts relative to those in the critical zone although all rocks show differing amountsoffoliation mineral lineations are weak to absent these data are consistent with significant compaction driven recrystallization in the study section numerical modeling of concurrent compaction and crystallization provides a quantitative model of how the lower zone critical zone transition may have formed plagioclase is rare in the lower zone because compaction removes interstitial liquid before it reaches plagioclasesaturation however asthecrystalpilegrows plagioclase saturation is reached in the interstitial liquid before compaction is complete in more evolved pyroxenites producing more abundant but still modest amounts of plagioclase characteristic of the lower critical zone it is concluded that both the textures and the modal mineralogy are largely controlled by compaction and compactiondriven recrystallization primary magmatic textures are not preserved

filling the bushveld complex magma chamber lateral expansion roof and floor interaction magmatic unconformities and the formation of giant chromitite pge and ti v magnetitite deposits his mind was like a soup dish wide and shallow irving stone on william jennings bryan a compilation of the sr isotopic stratigraphy of the bushveld complex shows that the evolution of the magma chamber occurred in two major stages during the lower open system integration stage lower critical and lower main zone there were numerous influxes of magma of contrasting isotopic composition with concomitant mixing crystallisation and deposition of cumulates larger influxes correspond to the boundaries of the zones and sub zones and are marked by sustained isotopic shifts major changes in mineral assemblages and development of unconformities during the upper closed system differentiation stage upper main zone and upper zone there were no major magma additions other than that which initiated the upper zone and the thick magma layers evolved by fractional crystallisation the lower and lower critical zones are restricted to a belt that runs from steelpoort and burgersfort in the northeast to rustenburg and northam in the west and an outlier of the lower and lower critical zone up to the lg4 chromitite layer in the far western extension north of zeerust it is only in these areas that thick harzburgite and pyroxenite layers are developed and where chromitites of the lower critical zone occur these chromitites include the economically important c 1 m thick lg6 and mg1 layers exposed around both the eastern and western lobes of the bushveld complex the upper critical zone has a greater lateral extent than the lower critical zone and overlies but also onlaps the floor rocks to the south of the steelpoort area the source of the magmas also appears to have been towards the south as the mg chromitite layers degrade and thin northward whereas the lg layers are very well represented in the north and degrade southward sr and os isotope data indicate that the major chromitite layers including the lg6 mg1 and ug2 originated in a similar way extremely abrupt and stratigraphically restricted increases in the sr isotope ratio imply that there was massive contamination of intruding melt which hit the roof of the chamber and incorporated floating granophyric liquid which forced the precipitation of chromite kruger 1999 kinnaird et al 2002 therefore each chromitite layer represents the point at which the magma chamber expanded and eroded and deformed its floor nevertheless this was achieved by in situ contamination by roof rock melt of the intruding critical zone liquids that had an orthopyroxenitic to noritic lineage the main zone is present in the eastern and western lobes of the bushveld complex where it overlies the critical zone and onlaps the floor rocks to the south and the north where it is also the basal zone in the northern lobe the new magma first intruded the northern lobe north of the thabazimbi murchison lineament interacted with the floor rocks incorporated sulphur and precipitated the platreef along the floor rock contact before flowing south into the main chamber this exceptionally large influx of new magma then eroded an unconformity on the critical zone cumulate pile and initiated the main zone in the main chamber by precipitating the merensky reef on the unconformity the upper zone magma flowed into the chamber from the southern bethal lobe as well as the tml this gigantic influx eroded the main zone rocks and caused very large scale unconformable relationships clearly evident as the gap areas in the western bushveld complex the base of this influx which is also coincident with the pyroxenite marker and a troctolitic layer in the northern lobe is the petrological and stratigraphic base of the upper zone sr isotope data show that all the pge rich ores including chromitites are related to influxes of magma and are thus related to the expansion and filling of the magma chamber dominantly by lateral expansion with associated transgressive disconformities onto the floor rocks coincident with major zone changes these positions in the stratigraphy are marked by abrupt changes in lithology and erosional features over which succeeding lithologies are draped the outcrop patterns and the concordance of geochemical isotopic and mineralogical stratigraphy indicate that during crystallisation the bushveld complex was a wide and shallow lobate sill like sheet and the rock strata and mineral deposits are quasi continuous over the whole intrusion

a reevaluation of crystal size distributions in chromite cumulates although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling it has not been demonstrated that crystal size distributions csds are consistent with such a process plots of crystal size fractions per unit volume vs crystal size for chromite grains from the stillwater complex show a log linear distribution with negative slope at larger sizes and a concave down distribution at smaller sizes the log linear portion of these trends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas however the lack of smaller grain sizes indicates that the original population was altered during a postnucleation crystalaging period that resulted in the loss of the smaller size fractions an interpretation consistent with textural and volume balance evidence the csd trends imply that cr was not concentrated by settling of chromite but was brought to the site of the growing chromite grains similarities between the stillwater data and csd trends for garnet in metapelites suggest that such trends are a characteristic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

a crystal size distribution study of the kiglapait layered mafic intrusion labrador canada evidence for textural coarsening crystal size distributions csd of plagioclase olivine and clinopyroxene from the kiglapait intrusion labrador were measured for a series of 13 samples spanning most of the exposed intrusion all csds lack small crystals and are slightly curved to the right concave up all csds of a mineral sweep out a fan as expected where there is relatively little variation in phase proportion csd dispersion can be replicated using the communicating neighbours model of textural coarsening ostwald ripening most of the textural variation in the intrusion is the result of variable degrees of coarsening it is proposed that coarsening occurred in the outer part of the crystallisation boundary layer where release of latent heat and changes in the intercumulus liquid composition resulted in a reduction in undercooling suppression of nucleation and coarsening particularly coarse grained areas reflect focusing of circulating magmatic fluids during coarsening the effects of crystal settling were not observed in the csds even within single well developed layers they were probably concealed by the pervasive textural coarsening csds do not suggest that mechanical compaction has played an important role in the formation of texture although it is very likely to have occurred however samples from the lower part of the intrusion show textural evidence of pressure solution compaction

grain growth in partially molten olivine aggregates grain growth rates in partially molten olivine aggregates were determined experimentally at a pressure of 1 gpa the aim of the experiments with fine grained starting material solution gelation derived fo90 olivine with a grain size of 1 m with 2 and 4 wt added basaltic glass was to determine growth rates during steady state grain growth a series of experiments was conducted at a fixed temperature of 1 350 c for durations from 2 to 700 h and a second series at fixed duration 168 h at temperatures from 1 250 to 1 450 c the resulting activation energy e 390 kj mol and growth exponent n 4 3 are comparable to melt free results nichols and mackwell 1991 the observed grain size distributions are broader than those predicted for normal grain growth or ostwald ripening and most similar to the log normal distribution the observed melt distribution is influenced by the continuous rearrangement of neighboring grains during growth the relatively large growth exponent observed in this study and by nichols and mackwell 1991 where secondary phases are present suggests that grain growth in the upper mantle is slower than predicted by karato 1989

compaction of igneous cumulates when initially deposited mafic igneous cumulates consist of a solid matrix of mineral phases plus 50 60 interstitial liquid due to its greater density the matrix compacts squeezing the interstitial liquid upward a numerical fluid mechanical model of this process indicates that if the cumulate deposition rate is constant compaction occurs in one of two distinct models for rapid deposition the system is permeability controlled with compaction occurring only at the base of the cumulate pile for slowly deposited cumulates an additional compaction zone that is partially viscosity controlled develops at the top of the pile if the extent of compaction and thickness of this zone are known for a real system the model can be used to estimate the permeability and effective viscosity of the compacting cumulates such calculations yield a permeability of 1 5 times 10 3 a 2 f 3 1 f 2 for olivine cumulates in the muskox intrusion reliable viscosity estimates await an empirical law relating ma

comment on characteristics and dynamic origin of the large scale jiaoluotage ductile compressional zone in the eastern tianshan mountains china by x w xu t l ma l q sun and x p cai journal of structural geology 25 2003 1901 1916

consequences of crystal shape and fabric on anisotropic permeability in magmatic mush crystals that form an interconnected porous network can become preferentially oriented both prior to and during compaction of magmatic mush this introduces anisotropy in the melt pore space that can reduce permeability in the direction of compaction and in turn decrease melt flux and compaction rate using a number of grain scale numerical models the consequences of end member magmatic fabrics on the directional dependence of permeability are tested over a range in melt fraction from 22 to 77 as the crystal aspect ratio i e ratio of long to short axis length increases from 2 to 10 isotropic permeability decreases by a factor of 2 and 5 for randomly oriented prolate and oblate shaped crystals respectively at a melt fraction of 22 with a flattening fabric permeability is reduced in the compaction direction no more than approximately a factor of 2 relative to the isotropic permeability at the same melt fraction and crystal shape for both oblate and triaxial prisms however permeability is enhanced in directions orthogonal to the compaction direction for example permeability is enhanced up to a factor of 11 relative to the isotropic permeability at a melt fraction of 22 for oblate prisms with a ratio of the long to short axis length of 10 anisotropy in permeability increases as the melt fraction decreases and the crystal aspect ratio increases ratios of the principal permeabilities are sufficiently large based on the realistic crystal shapes tested here to warrant including anisotropic permeability into macroscale melt segregation models including those for compaction

experimental analysis of heaving phenomena in sandy soils seepage erosion phenomena can arise when particles of soil are transported underneath any hydraulic structure by the seepage flow as a consequence of the difference in the surface elevation upstream and downstream of the hydraulic structure to assess the stability of hydraulic structures against seepage erosion computational models are available varying from simple empirical rules to advanced models in this paper the safety coefficient is assessed by comparing the exit gradient with the critical gradient the former represents the hydraulic gradient at the exit point along the downstream ground surface whereas the latter is the gradient at which upward drag forces on the soil particles equal the submerged weight of the soil particles and a quicksand condition originates an experimental research was carried out to analyze different thickness sheet piles embedded in sandy soils resulting in agreement between experimental data and theoretical values if the thickness of the sheet pile and the ground surface deformation are properly taken into account

combining quantitative textural and geochemical studies to understand the solidification processes of a granite porphyry shanggusi east qinling china igneous rock textures reflect the cooling history of the parental magma combined with chemical data they can provide physical and chemical information about the evolution of a magma body the petrographic textures and chemical compositions of 21 coarse and fine grained granite samples along an 250 m horizontal outcrop of the shanggusi granite porphyry are presented in this case study the coarse grained granite porphyry is an early intrusion and the fine grained granite dykes mostly intruded into the granite porphyry are later intrusions the studied samples have nearly homogeneous major element bulk rock and mineral compositions but show large variations in their trace element compositions and textural characteristics the trace element data suggest the influence of hydrous fluids possibly enriched in co2 f and cl in the evolution of the plutonic body textural analysis of the coarse grained granite porphyry indicates that the crystal size distribution csd slopes intercepts and total numbers of groundmass decrease from the center to the margin of the intrusion in contrast to the maximum diameter of the crystals lmax average length of the four largest quartz crystals for each sample however most fine grained samples and the groundmass of the coarse grained samples show concave down csds indicating textural coarsening quartz csds in the coarse grained samples are kinked with a steep sloped log linear section representing small crystals 5 1mm and a shallow sloped log linear section representing large crystals 4 1mm these two crystal populations are interpreted as resulting from a shift in cooling regime the straight csds of two fine grained samples may be due to a different cooling history in general the spatial variation of the csd patterns can be attributed to various degrees of overgrowth and mechanical compaction the quartz phenocrysts in several coarse grained samples exhibit a high degree of alignment which may be the result of magmatic flow by integrating the field geology geochemistry and quantitative textural data from the horizontal profile of the shanggusi granite porphyry it is suggested that hydrous fluids at the top of the intrusion not only controlled the fractionation of elements but also affected its cooling history fluid migration controlled undercooling can explain the solidification processes in the shanggusi intrusion and may also be prevalent in other fluid rich shallow intrusions quantitative integration of textural and geochemical data for igneous rocks can contribute to our understanding of the relationships between physical and chemical processes in a magma system and provide relatively comprehensive insights into the petrogenesis of granites

a flower like glomerophyric diorite porphyry from central china constraints on the unusual texture abstract an unusual texture with plagioclase phenocrysts clustered as flower like glomerocrysts has been discovered in the wulong diorite porphyry at the southern margin of north china craton the rock consists mainly of plagioclase and amphibole with the former as dominant phenocryst two groups of the plagioclase phenocrysts have been distinguished flower like glomerocrysts fg type and single isolated phenocrysts sp type chemical compositions of both fg and sp type plagioclases are similar in anorthite contents an35 45 which are slightly higher than the matrix an25 35 calculation of the plagioclase crystallization time based on crystal size distribution csd program shows that the fg type crystals have been formed within a longer timescale 226 2782 years than the sp type crystals 98 1910 years the concave down csds and lack of small crystals for fg type plagioclases suggest that coarsening may have been involved in contrast a concave up csd within the size of 3 10 mm plagioclase indicating that a rejuvenation kinetic process after coarsening a possible formation history for the unusual flower like plagioclase glomerocrysts was proposed as follows 1 initial nucleation and growth of single crystals in a confined environment 2 ascent of early crystals with magma upwelling and radial coarsening at the roof layer of magma chamber 3 remobilization of dense crystal mush and final emplacement at shallow level of crust 4 rapid cooling and extensive nucleation and growth of matrix 5 post magmatic alteration and metamorphism it is likely that the morphologic diversity of plagioclase glomerocrysts are largely influenced by variation of undercooling degrees the second step is the most important for the formation of the flower like glomerophyric texture which requires an environments of relatively low undercooling degree

origin of anorthosite by textural coarsening quantitative measurements of a natural sequence of textural development the textures of plagioclase crystals within large olivine oikocrysts preserve the sequence of the formation of anorthosite such textures have been quantified in a troctolite anorthosite from the proterozoic lac st jean anorthosite complex canada crystal size distributions csds indicate that initially plagioclase nucleated and grew in an introduction environment of linearly increasing undercooling producing a straight the textures of plutonic rocks are one of their most line csd during this phase latent heat of crystallization was striking aspects but they have been little used to elucidate largely removed by circulation of magma through the porous crystal petrological processes as compared with chemical and mush by about 25 solidification the crystallinity was such that isotopic methods where used discussion has tended it reduced but did not eliminate the circulation of magma resulting to be qualitative in contrast to quantitative chemical in the retention of more latent heat within the crystal pile the methods hence reducing the weight of this approach temperature rose until it was buvered by the solution of plagioclase indeed it is not possible to verify physical models if close to the liquidus temperature of plagioclase nucleation of quantitative data are not available quantitative textural plagioclase was inhibited and conditions were suitable for textural studies of volcanic rocks are becoming more common coarsening of both plagioclase and olivine to occur ostwald and these methods are applied here to the development ripening in this process small crystals were resorbed whereas of anorthosite larger crystals grew from both material recycled by the resorption of quantitative textural studies of plutonic rocks really crystals smaller than the critical size and new material brought in started with jackson s studies of the stillwater complex by the circulating magmatic fluid the results of this process resemble in 1961 jackson 1961 he measured crystal size disthose of high temperature metasomatism but there is no necessity for tributions but did not record the area that he measured a magmatic fluid with a diverent composition the shapes of the hence his data cannot be completely compared with plagioclase csds fit better the communicating neighbours equation of modern data his pioneering work was not really followed textural coarsening rather than the classic lifshitz slyozov wagner up and since then most studies have looked only at equation as do other examples drawn from the literature if olivine average or typical crystal sizes these data are much started to nucleate at a higher temperature than plagioclase then less revealing of petrological processes than crystal size during the textural coarsening phase olivine would have been more distributions partly because they only give one parameter undercooled than plagioclase and would have had a higher maximum for each sample the few existing studies of crystal size growth rate in these conditions olivine would coarsen more rapidly distributions in plutonic rocks have not treated than plagioclase and engulf it hence the order of crystallization plagioclase determined from the textures would be the reverse of the order of one problem especially pertinent to quantitative texfirst nucleation of the two phases from equilibrium phase diagrams tural studies is the way that many igneous petrologists maintenance of the temperature near the plagioclase liquidus may look at rocks they commonly try to reconstruct how a rock has formed by examining the final fully crystallized also inhibit the nucleation and growth of other phases

compaction of igneous cumulates part ii compaction and the development of igneous foliations quantitative petrofabric analysis of 66 samples from olivine bearing zones iii and iv of the middle banded series of the stillwater complex montana was conducted to allow direct comparison among textural and chemical variations systematic variations in foliations with stratigraphic height are observed in both zones statistical analysis of the orientations of inequant grains was used to calculate an alignment factor af corresponding to the extent of the development of the foliation both petrofabric and petrographic evidence indicates that the foliation is best explained by compaction of the crystal pile comparison of afs with estimates of the amount of compaction based on whole rock compositions shows approximate agreement variations in mole fraction cl of interstitial apatite mimic the observed trends in the af indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations we conclude that 1 differing amounts of co

the cumulate paradigm reconsidered the cumulate terminology outlined by wager and coworkers provided a logical system for describing the textures of the rocks of layered intrusions but recent work shows that this model fails to accommodate all the processes responsible for the petrographic features of coarse grained rocks textures of adcumulates and orthocumulates in the skaergaard intrusion cannot be correlated with concentrations of incompatible elements representing trapped liquid compaction and open system percolative processes have altered their textures modes and even bulk compositions although a rock may be a cumulate in the absence of other evidence it can be misleading to refer to it in terms of the cumulate nomenclature

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

sr and pb isotopic disequilibrium between coexisting plagioclase and orthopyroxene in the bushveld complex south africa microdrilling and progressive leaching evidence for sub liquidus contamination within a crystal mush progressive leaching of plagioclase for sr iso topes and microdrilling for sr and pb isotopes from grains of plagioclase and orthopyroxene from the critical zone and the lower zone indicates that these minerals are not in isotopic equilibrium leaching suggests critical zone plagioclase either lost rb or had a more radiogenic sri rim relative to the core whereas plagioclase from an upper zone sample is isotopically homogeneous for sri microdrilling analyses of plagioclase from the lower and critical zones consistently have a higher initial 87 sr 86 sr sri and a less radiogenic modeled 238 u 204 pb composition l2 than coexisting ortho pyroxene the range of calculated sri for plagioclase and orthopyroxene is 0 70506 0 70662 34 and 0 70290 0 70654 36 respectively the average difference in sri between mineral pairs was 0 00095 the range of calculated l2 for plagioclase and orthopyroxene is 9 42 10 30 average 9 7 and 9 83 15 75 average 10 1 respectively the range of measured 208 pb 206 pb for plagioclase and orthopyroxene is 34 757 36 439 33 and 36 669 41 845 85 respectively one orthopyroxenite without evidence for more than one population of crystal size distribution nonetheless had sri 0 70654 36 with calculated l2 of 10 32 for larger grains as compared with sri 0 70290 32 and calculated l2 of 9 97 for smaller grain size fractions isotopic results from this study demonstrate that whole rock isotopic data may not provide the appropriate level of detail necessary to address some processes in the bushveld complex however systematic changes have the potential to elucidate the timing of contamination with regard to other processes crystal aging compaction driven recrystallization and mineral exsolution occurring within a slowly cooled crystal liquid vapor mush system

textures of orthopyroxenites from the burgersfort bulge of the eastern bushveld complex republic of south africa a suite of 30 samples was collected from the burgersfort bulge area of the eastern bushveld complex for a detailed quantitative textural study the studied section represents a stratigraphic column approximately 1 000 m thick of mainly orthopyroxenites from the lower zone lz and lower critical zone crystal size distribution csd plots and quantitative fabric analyses reveal a history of crystal aging and compaction as evident from the loss of smaller sized crystals and the development of foliation with little evidence of mineral lineation compared with the thicker section from jagdlust 80 km north the lz orthopyroxenites at burgersfort are consistently finer grained foliation is equally well developed at both locations and is consistent with a relatively thin compaction zone at the top of the mush column grain size is largely a function of local cooling history with a grain size growth exponent estimated to be between 2 0 and 3 4

strontium isotope disequilibrium of plagioclase in the upper critical zone of the bushveld complex evidence for mixing of crystal slurries we report in situ sr isotope data for plagioclase of the bushveld complex we found disequilibrium sr isotopic compositions on several scales 1 between cores and rims of plagioclase grains in the merensky pyroxenite the bastard anorthosite and the ug1 unit and its noritic footwall 2 between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the merensky unit the footwall anorthosite of the merensky reef and the footwall norite of the ug1 chromitite the data are consistent with a model of co accumulation of cumulus plagioclase grains that had crystallized from different magmas followed by late stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile we propose that the rocks formed through slumping of semi consolidated crystal slurries at the top of the critical zone during subsidence of the center of the intrusion slumping led to sorting of crystals based on density differences resulting in a layered interval of pyroxenites norites and anorthosites

pyroxene oikocrysts in troctolitic cumulates evidence for supercooled crystallisation and postcumulus modification studies of pyroxene oikocrysts in plagioclase olivine cumulates crystallised from high alumina olivine tholeiite magmas in layered intrusions inferred to represent periodically replenished magma chambers show that small plagioclases enclosed within the oikocrysts differ significantly from plagioclases in the host rock plagioclases within oikocrysts are very elongate with a high surface area show extensive mutual attachment at high angles resembling skeletal growth and tend to be randomly orientated some are rounded and embayed are reversely zoned and more sodic by an 5 10 than some of the plagioclase in the host rock plagioclase crystal centres tend to be closer spaced in the oikocrysts than in the host rock in contrast plagioclases in the host rock are discrete coarser grained less elongate strongly aligned and locally polygonised

multiple reinjections and crystal mush compaction in the beacon sill mcmurdo dry valleys antarctica the beacon sill a member of the jurassic ferrar group of the transantarctic mountains south victoria land is a 150 m thick tholeiitic diabase intrusion uniform on a field and macroscopic scale it displays chemical and textural variability indicative of a sustained and complex history of emplacement and differentiation emplacement of the sill consisted of at least two and probably four discrete massive injections of magma each averaging 35 m thick over a time span of 100 years the final injection event is marked by a 30 m thick interval of significantly finer grained rock at the sill center these fine grained textures can be successfully reproduced by combining the results of thermal and crystal growth models but only if the sill was a multiple intrusion with the exception of a minor reinjection event captured in the chilled margin of the sill earlier reinjection events are not evident texturally most probably because of textural overprinting during prolonged cooling after initial crystallization the dominant process involved in the postemplacement differentiation of the beacon sill was compactiondriven redistribution of interstitial liquid transfer of residual liquid from the compacting lower solidification front to the dilating upper solidification front resulted in characteristic chemical and mineralogical effects such as the depletion of the lower half of the sill and the enrichment of the upper half of the sill in incompatible elements e g tio2 zr and modal granophyre based on thermodynamic models most of the compaction occurred at a crystallinity of roughly 33 geochemical profiles are distinctly segmented suggesting that the sill was repeatedly split and reinjected with fresh magma near the center after compaction had redistributed the interstitial liquid within the partially solidified magma the recognition of evidence for reinjection and compaction in a macroscopically uniform relatively quickly cooled sill suggests that these processes may be common in the construction of sills and other mesoscale igneous intrusions in general

origin of k feldspar megacrysts in rhyolites from the emeishan large igneous province southwest china abstract silicic rocks occur in the uppermost units of the longest volcanic succession 5000 m thick in the binchuan area of the permian emeishan flood basalt province of sw china they are predominantly rhyolites and to a lesser extent trachytes both containing potassium feldspar megacrysts as the dominant phenocryst phase up to approximately 20 mm in size these megacrysts contain domains of albite arranged in vein like networks likely formed by post magmatic alteration crystal size distributions csd suggest that these megacrysts grew in a stable magmatic system consistent with relatively uniform core to rim compositional k 2 o 14 16 wt and isotopic profiles 208 pb 206 pb ratios 2 06 2 08 0 005 both whole rock trace elements and pb isotope ratios of these silicic rocks are similar to the emeishan basalts suggesting a common source for both mafic and felsic units and a limited role of crustal melting in genesis of the felsic units major and trace element models further indicate that these rocks could not have formed exclusively by re melting of old crust or solidified basaltic rock but must have formed through crystal fractionation from the flood basalts or possibly partial melting of basaltic rock followed by fractional crystallization k feldspar bearing rhyolites are also observed in the last stages of other large igneous provinces we suggest that they represent final melt fractions and their appearance in the magmatic system coincides with waning of voluminous magmatic activities