two magma series and associated ore deposit types in the permian emeishan large igneous province sw china the late middle permian 260 ma emeishan large igneous province in sw china contains two magmatic series one comprising high ti basalts and fe rich gabbroic and syenitic intrusions the other low ti basalts and mafic ultramafic intrusions the fe rich gabbros are spatially and temporally associated with syenites each series is associated with a distinctive type of mineralization the first with giant fe ti v oxide ore deposits such as panzhihua and baima the second with ni cu pge sulfide deposits such as jinbaoshan limahe and zhubu new shrimp zircon u pb isotopic data yielded 263 3 ma for the limahe intrusion 261 2 ma for the zhubu intrusion and 262 2 ma for a syenitic intrusion these new age dates together with previously reported shrimp zircon u pb ages suggest that all these intrusions are contemporaneous with the emeishan flood basalts and formed during a major igneous event at ca 260 ma the oxide bearing intrusions have higher al2o3 feo as total iron and total alkalis na2o k2o but lower mgo than the sulfide bearing intrusions all intrusions are variably enriched in lree relative to hree the oxide bearing intrusions display positive nb and ti anomalies and in certain cases negative zr hf anomalies whereas the sulfide bearing intrusions have obvious negative nb and ti anomalies a feature of crustal contamination individual intrusions have relatively small ranges of nd t values all the intrusions however have nd t values ranging from 3 9 to 4 6 and initial 87sr 86sr ratios from 0 7039 to 0 7105 the syenites have very low mgo 2 wt but highly variable fe2o3 2 5 to 13 wt with initial 87sr 86sr ratios ranging from 0 7039 to 0 7089 magmas from both series could have derived by melting of a heterogeneous mantle plume the high ti series from a fe rich more fertile source and the low ti series from a fe poor more refractory source in addition the low ti series underwent significant crustal contamination the two magma series evolved along different paths that led to distinct mineralization styles

liquid immiscibility and the evolution of basaltic magma this experimental study examines relationships between alternative evolution paths of basaltic liquids the so called bowen and fenner trends and silicate liquid immiscibility synthetic analogues of natural immiscible systems exhibited in volcanic glasses and melt inclusions were used as starting mixtures conventional quench experiments in 1atm gas mixing furnaces proved unable to reproduce unmixing of ferrobasaltic melts yielding instead either turbid opalescent glasses or crystallization of tridymite and pyroxenes in contrast experiments involving in situ high temperature centrifugation at 1000g g 1 4 9 8 m s did yield macroscopic unmixing and phase separation centrifugation for 3 4 h was insufficient to complete phase segregation and resulted in sub micron immiscible emulsions in quenched glasses for a model liquid composition of the middle zone of the skaergaard intrusion at super liquidus temperatures of 1110 11208c centrifugation produced a thin silicic layer 64 5 wt sio2 and 7 4 wt feo at the top of the main fe rich glass 46 wt sio2 and 21 wt feo the divergent compositions at the top and bottom were shown in a series of static runs to crystallize very similar crystal assemblages of plagioclase pyroxene olivine and fe ti oxides we infer from these results that unmixing of complex aluminosilicate liquids may be seriously kinetically hampered presumably by a nucleation barrier and thus conventional static experiments may not correctly reproduce it in the light of our centrifuge experiments immiscibility in the skaergaard intrusion could have started already at the transition from the lower to the middle zone thus magma unmixing might be an important factor in the development of the fe enrichment trend documented in the cumulates of the skaergaard layered series

mineral layering in the bushveld complex implications of cr abundances in magnetite from closely spaced magnetitite and intervening silicate rich layers the cr abundances in magnetite separates from the main magnetite layer 1 magnetite layers 1 and 2 and the intervening anorthositic horizons have been determined from two widely spaced localities in the eastern bushveld complex chromium abundance is greatest at the base of the main magnetite layer around 7 000 ppm and decreases rapidly with height through this layer and the overlying anorthositic horizon the base of layer 1 is associated with a high cr content about 1 500 ppm which is followed by a steady upward decrease the base of layer 2 is also characterized by a high cr abundance about 900 ppm and detailed studies indicate a slight rise in cr near the base of this layer chromium abundance profiles are examined in the light of several models which have been proposed for the origin of mineralogical layering the general features of the abundance profiles are compatible with a variety of models however it appears that the detailed features of these profiles are best accounted for by a model involving bottom growth with a slow and variable rate of supply of cr to the crystallization zone

a new interpretation of the structure of the sept iles intrusive suite canada the layered mafic intrusion at sept iles canada is one of the largest intrusions in the world a new interpretation of its structure is proposed based on a review of its geology and a comparison with the skaergaard intrusion greenland several different magmatic components are recognized hence the name sept iles intrusive suite siis is proposed emplacement of the suite may have been preceded by eruption of flood basalts the first magmas of the suite rose in the crust to accumulate beneath the density filter afforded by the basalts the largest component is the sept iles mafic intrusion simi the lower series of the simi is dominated by leucotroctolites and leucogabbros above it lie the layered series which is largely comprised of gabbro and troctolite both these units are unchanged from earlier interpretations the anorthosites s l gabbros and monzogabbros formerly called the transitional series are now considered to be the upper border series developed by floatation of plagioclase common autoliths in the layered series are parts of the hydrothermally altered upper border series from towards the interior of the intrusion which have foundered and settled through the magma the contamination of the magma that accompanied this event oxidised iron in the magma and led to the precipitation of magnetite around the periphery of the intrusion the subsequent depletion of fe3 and or increase in sio2 cao and p2o5 may have induced apatite saturation and accumulation to form two layers rich in apatite near the base and at top of the layered series granitic magma was developed by fractional crystallisation and was emplaced along the roof of the chamber where it acquired large quantities of xenoliths these were probably derived from the flood basalts their evolved members and fragments of mafic dykes chilled by the granitic magma accumulations of monzonite pillows in this unit testify to another magmatic event and a floor to the granitic magma chamber indicating lateral transport of magma chemically distinct syenites in the upper part of the intrusion are part of the point du criade intrusion a large late composite sill diabase and leucogabbro components show a close link with the simi and all the acidic magmas may have originally formed by differentiation of the main magma in cupolas towards the centre of the intrusion a series of late gabbro intrusions that cut the simi may represent a rejuvenation of magmatism the border zone is a mass of fine grained rocks that occurs along the border of the simi it may be another magmatic component or just the lateral border series of the simi

evidence for sulfide and fe ti p rich liquid immiscibility in the duluth complex minnesota apatite and oxide ilmenite magnetite rich dunite pyroxenite peridotite melatroctolite and gabbro occur as transgressive bodies in the boulder lake area of the southern duluth complex nelsonite apatite ilmenite is locally found as vein or dikelike bodies whose geometric form resembles veinlike massive cu ni sulfide mineralization in the babbitt deposit massive sulfide mineralization at babbitt has been attributed to the separation of an immiscible sulfide magma from a tholeiitic parent nelsonite contains assessory minerals such as zn rich hercynitic spinel biotite zircon baddeleyite and sulfide minerals massive sulfide mineralization at babbitt is characterized by the presence of minor 1 10 vol amounts of euhedral apatite as well as fe ti oxides and the same assemblages of accessory minerals that is found in the nelsonite textures developed in the nelsonite together with the unusual composition are supportive of an origin involving fe ti p and s liquid immiscibility apatite from the nelsonites and related rocks contain very low ree and cl contents relative to that in the massive sulfide these data are thought to reflect the evolution of a cl and ree bearing fluid from the magma that gave rise to the nelsonite and a later separation of an fe ti p rich immiscible liquid in comparison to the early separation of an immiscible sulfide rich liquid from a basaltic magma in the babbitt area sulfur oxygen and hydrogen isotope data are not supportive of an origin for the apatite oxide rich rocks that is linked to assimilation of metasedimentary country rock however elevated delta 18 o values up to 13 2 in plagioclase in many of the apatite oxide rich rocks together with anomalously high an content of plagioclase suggest that subsolidus interaction with a ca rich fluid derived from dehydration reactions in the pelitic rocks of the virginia formation has been important although an origin involving immiscibility of an fe ti p rich liquid is strongly indicated for the nelsonites the origin of spatially associated apatite oxide rich rock types is problematic the sio 2 content of most of the crosscutting bodies is less than 30 wt percent and derivation by extreme closed system fractional crystallization of a tholeiitic parent is considered unlikely due to predicted early saturation in fe ti oxides the oxideapatite rich rocks may themselves be products of liquid immiscibility reflecting a continuum in compositions with nelsonite as an end member

trace element and sr nd isotopic geochemistry of the pge bearing xinjie layered intrusion in sw china abstract the xinjie layered intrusion is one of a number of major ultramafic mafic bodies hosting fe ti v deposits and cu ni pge sulfide in the pan xi area of the sichuan province sw china the intrusion is a 7 5 km long 1 to 1 5 km wide and 1200 m thick ultramafic mafic sill which is associated with permian plume related emeishan flood basalts the intrusion is characterized by three lithological cycles each representing a sequence from ultramafic to basic felsic composition the basal part of the intrusion is composed of three lithological units namely the marginal peridotite and pyroxenite units two major pge mineralized layers occur within the marginal and pyroxenite units the marginal unit mu is characterized by marked nb ta sr and ti enrichments and obvious th depletion while the peridotite unit peu shows significant sr slight zr depletion and pronounced positive ti anomalies the pyroxenite unit pyu is depleted in nb and zr but significantly enriched in sr and ba with lower ratios of nb la and zr sm compared to those of the underlying two units the pge enriched layer in the marginal unit has elevated and highly variable rb la and ba th values and the lower part of the pyroxenite unit which includes a pge enriched layer exhibits high and decreasing rb la and ba th ratios upwards sr nd isotopic compositions of samples from the three units ca 90 m thick were systematically analyzed and fall in a range of 0 7056 to 0 7074 for 87sr 86sr i and of 5 26 to 2 79 for end i within the marginal and peridotite units high 87sr 86sr i and low end i values are demonstrated by contrast an enriched sr isotopic 87sr 86sr i 0 7074 and slightly depleted nd isotopic end i 2 5 signature occurs in the basal pyroxenite unit 8 m below the pge enriched layer in this unit the marginal unit is interpreted as the product of contamination of a plume component e g end i 5 and 87sr 86sr i 0 704 by continental crust the peridotite unit has undergone decreasing degrees of crustal contamination at the bottom of the pyroxenite unit a new magma input contaminated by upper crust is proposed where its slight nd sr decoupling may be attributed either to the much higher sr content than the nd content of the contaminant or the selective contamination of sr rather than nd the pge mineralization in the pyroxenite unit was caused by magma mixing between the new magma and the resident magma whereas the pge enriched layer in the marginal unit resulted from wallrock contamination however the displacement of the isotopic pulse from the pge enriched layer in the pyroxenite unit is due to later sulfide saturation being achieved in both end members both of the pge enriched layers are linked directly to the segregations of the fe ti oxides and sulfides

field investigation of love waves in near surface seismology we examine subsurface conditions and survey parameters suitable for successful exploitation of love waves in nearsurface investigations love wave generation requires the existence of a low shear velocity surface layer we examined the minimum thickness of the near surface layer necessary to generate and record usable love wave data sets in the frequency range of 5 50 hz we acquired field data on a hillside with flat lying limestone and shale layers that allowed for the direct testing of varying overburden thicknesses as well as varying acquisition geometry the resulting seismic records and dispersion images were analyzed and the love wave dispersion relation for two layers was examined analytically we concluded through theoretical and field data analysis that a minimum thickness of 1 m of low velocity material is needed to record usable data in the frequency range of interest in near surface love wave surveys the results of this study indicate that existing guidelines for rayleigh wave data acquisition such as receiver interval and line length are also applicable to love wave data acquisition

abundant fe ti oxide inclusions in olivine from the panzhihua and hongge layered intrusions sw china evidence for early saturation of fe ti oxides in ferrobasaltic magma abundant fe ti oxide inclusions in cumulus olivine fo77 81 from the panzhihua and hongge intrusions emeishan large igneous province sw china document the first evidence for early crystallization of fe ti oxides in ferrobasaltic systems in nature the intrusions also contain significant stratiform fe ti v oxide ores the oxide inclusions are sub rounded or irregular range from 5 to 50 m in diameter and are dominated by either titanomagnetite or ilmenite the fact that the inclusions are either titanomagnetite or ilmenite dominant suggests that they are trapped crystals instead of immiscible oxide melt formed during growth of the host olivine the absence of other silicate phases in the inclusion bearing olivine is difficult to reconcile with a possible xenocrystic origin of the oxide inclusions these oxide inclusions are thus interpreted to be cumulus minerals crystallized together and trapped in olivine from the same parental magma in addition to fe ti oxides some inclusions contain amphibole biotite fluorapatite that might have formed by reaction of trapped hydrous liquid with the host olivine numerical modeling of high ti emeishan basalts using the melts program successfully simulates early crystallization of olivine fo81 and fe ti spinel in the presence of a moderate amount of h2o 1 5 wt under pressure and fo2 conditions generally pertinent to the panzhihua and hongge intrusions the modal mineralogy of the oxide inclusions is in good agreement with the bulk compositions of the ore as inferred from whole rock data in a given intrusion this is consistent with the interpretation that the stratiform oxide ores in the intrusions formed by accumulation of fe ti oxide crystals that appeared on the liquidus with olivine and clinopyroxene

east west extension and holocene normal fault scarps in the hellenic arc examination of surface fault traces with spot images and in the field corroborates the inference that the active tectonics of southern peloponnesus and crete are dominated by approximately north south normal faulting and approximately east west extension the heights of holocene normal fault scarps yield first order regional estimates of fault slip rates between 0 1 and 2 3 mm yr most of the surface scarps probably ruptured during past earthquakes such as that which destroyed sparta in 464 b c on the sparta fault the holocene average slip rate and the recurrence time of large earthquakes may be 1 mm yr and 3000 yr respectively the regional pattern of quaternary faulting suggests that the east west extension near the hellenic subduction zone is fast about 5 10 m y the change from north south to east west extension in the late pliocene 2 4 ma implies that the aegean is starting to collide with the northern margin of africa

new textural and mineralogical constraints on the origin of the hongge fe ti v oxide deposit sw china the hongge magmatic fe ti v oxide deposit in the panxi region sw china is hosted in a layered mafic ultramafic intrusion this 2 7 km thick lopolith like intrusion consists of the lower middle and upper zones which are composed of olivine clinopyroxenite clinopyroxenite and gabbro respectively abundant fe ti oxide layers mainly occur in the middle zone and the lower part of the upper zone fe ti oxides include cr rich and cr poor titanomagnetite and granular ilmenite cr rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1 89 to 14 9 wt cr2o3 and 3 20 to 16 2 wt tio2 whereas cr poor titanomagnetite typically occurs as net textured and massive ores in the upper middle and upper zones and contains much lower cr2o3 0 4 wt but more variable tio2 0 11 to 18 2 wt disseminated cr rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene whereas cr poor titanomangetite of the net textured and massive ores is mainly interstitial to clinopyroxene and plagioclase the lithology of the hongge intrusion is consistent with multiple injections of magmas the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas cr rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of cr poor titanomagnetite granular ilmenite and apatite in the net textured ores of the middle and upper zones are thought to have formed from an fe ti p rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility the dense fe ti p rich melt percolated downward through the underlying silicate crystal mush to form net textured and massive fe ti oxide ores whereas the coexisting si rich melt formed the overlying plagioclase rich rocks in the intrusion

silicate liquid immiscibility within the crystal mush late stage magmatic microstructures in the skaergaard intrusion east greenland late stage microstructures developed during the last stages of solidification of the skaergaard intrusion comprise a wide array of reactive and non reactive intergrowths reactive microstructures ascribed to open system behaviour include serrated grain boundaries between pyroxene and plagioclase primocrysts fish hook pyroxenes and mafic symplectites they form by the addition of fe and ca and removal of alkalis and silica other microstructures include those formed by internally generated redox reactions between olivine fe ti oxides and pyroxene non reactive microstructures include closely spatially associated granophyric intergrowths and previously undescribed ilmenite rich intergrowths that are interpreted as a consequence of crystallization of separated conjugate immiscible liquids the open system reactive microstructures occur predominantly in the cumulates on the chamber floor appearing in lzb and disappearing to be replaced by a granophyre ilmenite rich intergrowth association in the upper zone they are not common in the marginal border series their distribution mirrors that of efficient expulsion of trapped liquid and can be attributed to the gravitationally driven loss of a si rich immiscible component from the interstitial liquid the loss of the si rich component causes the remaining fe rich liquid to react with the primocrysts bulk rock major element data are consistent with little or no preferential loss of the si rich liquid from the mush but the resolution of the available data is not sufficient to assess the effect of this relative movement on the liquid line of descent of the bulk magma the first appearance of the paired conjugate non reactive intergrowths in mz marginal border series points to the early onset of immiscibility in the bulk liquid

geochemistry petrogenesis and metallogenesis of the panzhihua gabbroic layered intrusion and associated fe ti v oxide deposits sichuan province sw china the panzhihua gabbroic layered intrusion is associated with the 260ma emeishan large igneous province in sw china this sill like body hosts a giant fe ti v oxide deposit with 1333 million ton ore reserves which makes china a major producer of these metals the intrusion has a marginal zone of fine grained hornblende bearing gabbro and olivine gabbro followed upward by lower middle and upper zones the lower and middle zones consist of layered melanogabbro and gabbro composed of cumulate clinopyroxene plagioclase and olivine these zones also contain magnetite layers the upper zone consists chiefly of leucogabbro composed of plagioclase and clinopyroxene with minor olivine most rocks in the body show variable scale rhythmic modal layering in which dark minerals primarily clinopyroxene dominate in the lower parts of each layer and lighter minerals primarily plagioclase dominate in the upper parts the oxide ores occur as layers and lenses within the gabbros and are concentrated in the lower parts of the intrusion ore textures and associated mineral assemblages indicate that the ore bodies formed by very late stage crystallization of v rich titanomagnetite from an immiscible oxide liquid in a fluidrich environment the rocks of the panzhihua intrusion become more evolved in chemistry upward and follow a tholeiitic differentiation trend with enrichment in fe ti and v they are enriched in light rare earth elements relative to heavy rare earth elements and exhibit positive nb ta and ti anomalies and negative zr and hf anomalies the silicate rocks and oxide ores of the panzhihua intrusion formed from highly evolved fe ti v rich ferrobasaltic or ferropicritic magmas the textures of the ores and the abundance of minor hydrous phases indicate that addition of fluids from upper crustal wall rocks induced the separation of the immiscible oxide melts from which the fe ti v oxide ore bodies in the lower part of the intrusion crystallized

crystallization sequence and magma chamber processes in the ferrobasaltic sept iles layered intrusion canada the sept iles layered intrusion quebec canada 564 ma is a large plutonic body with a diameter of 80 km and a thickness of 6 km made up from its base to top of a layered series with troctolite and gabbro and an upper border series with anorthosite capped by cupolas of a type granite chilled margin compositions suggest a ferrobasaltic parental magma close to that of the skaergaard intrusion but much richer in iron and titanium samples from drill cores and surface sampling of the 4 7 km thick layered series reveal a succession of massive troctolites and layered gabbros that contain 24 fe ti oxide layers cm to m thick and many anorthositic autolithic blocks the sequence of crystallization in the layered series is plagioclase an72 34 and olivine fo75 21 followed by magnetite and ilmenite then ca rich pyroxene and finally apatite an olivine gap is observed between fo66 and fo59 the saturation of fe ti oxides before ca rich pyroxene is interpreted to be the result of the high feot and tio2 contents and the low cao content of the parental magma contamination by old continental crust has occurred during crystallization of the layered series as indicated by sr isotopic compositions 87sr 86sr564 0 70360 0 70497 the differentiation trend of the intrusion is interrupted by two large and many small reversals to more primitive compositions of cumulus phases cr content of magnetite and lower sr isotope ratios these reversals and the intermittent disappearance of some phases are interpreted as resulting from magma chamber replenishments by undifferentiated primitive basaltic magma and mixing with the resident magma the author 2010 published by oxford university press all rights reserved for permissions please e mail journals permissions oxfordjournals org

the role of fe ti oxide crystallization in the formation of a type granitoids with implications for the daly gap an example from the permian baima igneous complex sw china abstract the baima igneous complex bic is a member of the permian emeishan large igneous province sw china and is composed of a layered gabbroic unit and an isotropic syenitic unit the gabbroic unit consists of four distinct zones 1 lower cumulate zone 2 massive fe ti v oxide ore zone 3 olivine gabbro zone and 4 upper gabbro zone shrimp zircon u pb dating results yield an age of 261 2 ma for the gabbroic unit contemporaneous with the spatially associated syenitic unit the gabbroic rocks show slightly lree enriched patterns with la yb n values between 1 6 and 12 1 and positive eu anomalies eu eu 1 1 3 6 in contrast the syenites display stronger enrichments of lree la ybn 6 8 51 3 and negative eu anomalies eu eu 0 3 1 1 rocks of the syenitic unit are chemically similar to the ferroan alkalic a type granitoids k2o na2o 0 58 0 80 feot feot mgo 0 80 0 91 and have a within plate geochemical signature both units have similar trace element ratios zr nb th u ni co cu ni hf ta and zn ga resembling an ocean island basalt source the end t values for the gabbroic unit end t 1 6 4 2 and syenitic unit end t 2 5 3 2 are within the range of the high ti emeishan flood basalts the geochemical and geochronological data indicate that the gabbroic and syenitic units are comagmatic and cogenetic geochemical modeling suggests that the two units originated by fractional crystallization of a common parental magma that resembles the high ti emeishan flood basalts the formation of the two units by fractional crystallization was likely responsible for the enrichment of fe ti v oxide minerals in the gabbroic unit and thus the development of the daly gap the results of this study indicate that silica saturated peralkaline a type granitoids can be formed by fractionation of mantle derived mafic magmas

plagioclase content of cyclic units in the bushveld complex south africa the upper critical zone of the bushveld complex south africa has been divided into so called cyclic units ideally they should consist of from the base upward layers of chromitite pyroxenite norite and anorthosite some hypotheses for their origin suggest that they result from the addition and mixing of a magnesian magma having orthopyroxene or olivine as liquidus mineral these models predict that the bulk cumulate composition in a cyclic unit formed from such magma ought to contain excess orthopyroxene to plagioclase relative to their cotectic proportions an hypothesis tested here all rocks investigated here from the upper critical zone at impala platinum mine are mixtures of cumulus plagioclase and orthopyroxene except for the chromitites with very minor intercumulus minerals mainly clinopyroxene hence the proportion of plagioclase can be determined by measurement of the density of a sample the density of every piece of continuous bore core from the upper group 2 ug2 and merensky cyclic units has been determined and the proportion of plagioclase calculated for the bastard cyclic unit estimates of the proportion of plagioclase have been made based on published modal and chemical analyses vertical summation of the proportion of plagioclase indicates that these three cyclic units ug2 merensky and bastard respectively contain 64 74 and over 80 plagioclase by weight for the merensky and bastard cyclic units such proportions exceed that of the plagioclase orthopyroxene cotectic and could not have been produced by processes involving magnesian magma

experiments on liquid immiscibility along tholeiitic liquid lines of descent crystallization experiments have been conducted on compositions along tholeiitic liquid lines of descent to define the compositional space for the development of silicate liquid immiscibility starting materials have 46 56 wt sio2 11 7 17 7 wt feotot and mg number between 0 29 and 0 36 these melts fall on the basaltic trends relevant for mull iceland snake river plain lavas and for the sept iles layered intrusion where large scale liquid immiscibility has been recognized at one atmosphere under anhydrous conditions immiscibility develops below 1 000 1 020 c in all of these compositionally diverse lavas extreme iron enrichment is not necessary immiscibility also develops during iron depletion and silica enrichment variations in melt composition control the development of silicate liquid immiscibility along the tholeiitic trend elevation of na2o k2o p2o5 tio2 promotes the development of two immiscible liquids increasing melt cao and al2o3 stabilizes a single liquid field new data and published phase equilibria show that anhydrous low pressure fractional crystallization is the most favorable condition for unmixing during differentiation pressure inhibits immiscibility because it expands the stability field of high ca clinopyroxene which reduces the proportion of plagioclase in the crystallizing assemblage thus enhancing early iron depletion magma mixing between primitive basalt and fe ti p rich ferrobasalts can serve to elevate phosphorous and alkali contents and thereby promote unmixing water might decrease the temperature and size of the two liquid field potentially shifting the binodal solvus below the liquidus leading the system to evolve as a single melt phase

origin of fe ti oxide ores in mafic intrusions evidence from the panzhihua intrusion sw china economicconcentrationsof fe ti oxidesoccuras massive conform able lenses or layers in the lower part of the panzhihua intrusion emeishan large igneous province sw china mineral chemistry textures and quilfequilibria indicate that oxides in rocks of theintrusion were subjected to extensive subsolidus re equilibration andexsolution the primary oxide reconstructed from compositions oftitanomagnetite in the oresand associated intergrowths isan alumi nous titanomagnetite usp

anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust abstract the sept iles layered intrusion quebec canada is dominated by a basal layered series made up of troctolites and gabbros and by anorthosites occurring 1 at the roof of the magma chamber 100 500 m thick and 2 as cm to m size blocks in gabbros of the layered series anorthosite rocks are made up of plagioclase with minor clinopyroxene olivine and fe ti oxide minerals plagioclase displays a very restricted range of compositions for major elements an 68 an 60 trace elements sr 1023 1071 ppm ba 132 172 ppm and sr isotopic ratios 87 sr 86 sr i 0 70356 0 70379 this compositional range is identical to that observed in troctolites the most primitive cumulates of the layered series whereas plagioclase in layered gabbros is more evolved an 60 an 38 the origin of sept iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts the density of the feo rich tholeiitic basalt parent magma first increased from 2 70 to 2 75 g cm 3 during early fractionation of troctolites and then decreased continuously to 2 16 g cm 3 with fractionation of fe ti oxide bearing gabbros plagioclase an 69 an 60 was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite with further differentiation plagioclase 60 became negatively buoyant and anorthosite stopped forming blocks of anorthosite autoliths even fell downward to the basal cumulate pile the presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and or minor plagioclase nucleation within the main magma body dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid the processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust with gabbro noritic rocks at the base and anorthositic rocks at the top we propose that the lunar crust mainly crystallized bottom up this basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian suite without kreep contamination while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust

cyclicity in the main and upper zones of the bushveld complex south africa crystallization from a zoned magma sheet the major element composition of plagioclase pyroxene olivine and magnetite and whole rock sr sr data are presented for the uppermost 2 1 km of the layered mafic rocks upper main zone and upper zone at bierkraal in the western bushveld complex initial sr sr ratios are near constant 0 7073 0 0001 for 24 samples and imply crystallization from a homogeneous magma sheet without major magma recharge or assimilation the 2125 m thick section investigated in drill core comprises 26 magnetitite and six nelsonite magnetite ilmenite apatite layers and changes up section from gabbronorite an72 plagioclase mg 74 clinopyroxene to magnetite ilmenite apatite fayalite ferrodiorite an43 mg 5 clinopyroxene fo1 olivine the overall fractionation trend is however interrupted by reversals characterized by higher an of plagioclase higher mg of pyroxene and olivine and higher v2o5 of magnetite in the upper half of the succession there is also the intermittent presence of cumulus olivine and apatite these reversals in normal fractionation trends define the bases of at least nine major cycles we have calculated a plausible composition for the magma from which this entire succession formed forward fractional crystallization modeling of this composition predicts an initial increase in total iron near constant sio2 and an increasing density of the residual magma before magnetite crystallizes after magnetite begins to crystallize the residual magma shows a nearconstant total iron an increase in sio2 and decrease in density we explain the observed cyclicity by bottom crystallization initially magma stratification developed during crystallization of the basal gabbronorites once magnetite began to crystallize periodic density inversion led to mixing with the overlying magma layer producing mineralogical breaks between fractionation cycles the magnetitite and nelsonite layers mainly occur within fractionation cycles not at their bases in at least two cases crystallization of thick magnetitite layers may have lowered the density of the basal layer of melt dramatically and triggered the proposed density inversion resulting in close but not perfect coincidence of mineralogical breaks and packages of magnetitite layers

large scale silicate liquid immiscibility during differentiation of tholeiitic basalt to granite and the origin of the daly gap the dearth of intermediate magmatic compositions at the earth s surface referred to as the daly gap remains a major issue in igneous petrology the initially favored explanation invoking silicate liquid immiscibility during evolution of basalt to rhyolite has lost support because of the absence of any firm geological evidence for separation of fe and si rich liquids in igneous rocks this work presents a record of large scale magmatic differentiation due to immiscibility in the tholeiitic sept iles intrusion canada one of the largest layered plutonic bodies on earth gabbroic cumulate rocks from the critical zone of this intrusion show a bimodal distribution in density and p2o5 content despite identical major element chemistry of the principal magmatic phases immiscibility is supported by the presence of contrasting fe rich and si rich melt inclusions trapped in cumulus apatite phase diagrams and well documented occurrences of small scale immiscibility confirm that liquid liquid unmixing and the separation of fe rich and si rich liquids may contribute significantly to the daly gap along the tholeiitic liquid line of descent

silicate liquid immiscibility within the crystal mush evidence from ti in plagioclase from the skaergaard intrusion a key target in the study of layered intrusions is to constrain the liquid line of descent of the magma however the evolution of the interstitial liquid is rarely considered and its liquid line of descent is often assumed to be equivalent to that of the bulk magma because of extensive sub solidus and diffusional changes that occur in slowly cooled rocks clues to the composition of the interstitial liquid can only be obtained using very slowly diffusing trace elements and components this study uses the ti concentrations and anorthite contents of interstitial plagioclase to consider the compositional evolution of the interstitial liquid in the skaergaard intrusion ti xan zoning of interstitial plagioclase does not follow the same cryptic variations that develop in plagioclase primocrysts as a function of stratigraphic height demonstrating that the bulk and interstitial liquid lines of descent are not equivalent after fe ti oxides start to crystallize ti concentrations decrease in both primocryst and interstitial plagioclase as a result of decreasing melt ti however in the interstitial plagioclase within a single thin section divergent trends develop adjacent to fine grained interstitial pockets containing diverse mineral assemblages which are interpreted to represent the crystallized products of late stage immiscible liquids these trends vary systematically as a function of stratigraphic height and spatial location within the intrusion the distribution and compositions of these plagioclase zoning trends are used to comment on the spatial distribution and differential movement of interstitial immiscible liquids within the intrusion the author 2010 published by oxford university press all rights reserved

mineralogy and geochemistry of fe ti oxide and apatite nelsonite deposits and evaluation of the liquid immiscibility hypothesis modal mineralogy determined for 32 fe ti oxide and apatite rocks from localities in virginia new york quebec norway and sweden largely supports the 2 1 oxide apatite ratio suggested as a eutectic mixture by philpotts 1967 a fairly consistent suite of accessory minerals including biotite clinoamphibole spinel zircon and sulfides is present some silicate accessory minerals tend to form glomeroporphyritic intergrowths in an equigranular matrix of nelsonite suggesting that the accessories are xenocrysts that have been trapped in an oxide apatite liquid this liquid may represent an extreme case of partitioning of high charge density cations into a low silica phosphorus enriched immiscible melt temperature and oxygen fugacity estimates for oxide pairs in nelsonites range from 600 degrees to 1 000 degrees c and 10 super 20 to 10 super 11 atm f sub o 2 and bracket an equilibration path that is close to the fmq buffer curve several stages of exsolution are evident in fe ti oxides of some deposits nelsonites contain fluorapatite that is enriched in light lanthanides the occurrence of nelsonite dikes in roseland anorthosite and associated rocks of the roseland piney river district of virginia is explained by the infilling of fractures with nelsonite liquid that has settled out of an overlying ferrodiorite pluton

experiments on silicate melt immiscibility in the system fe2sio4 kalsi3o8 sio2 cao mgo tio2 p2o5 and implications for natural magmas the effect of cao and mgo with or without tio2 and p2o5 on the two melt field in the simplified system fe2sio4 kalsi3o8 sio2 has been experimentally determined at 1 050 1 240 c 400 mpa despite the suppressing effect of mgo cao and pressure on silicate melt immiscibility our experiments show that this process is still viable at mid crustal pressures when small amounts 0 6 2 0 wt of p2o5 and tio2 are present our data stress that the major element partition coefficients between the two melts are highly correlated with the degree of polymerisation nbo t of the sio2 rich melt whatever temperature pressure or exact composition experimental immiscible melt compositions in natural systems at 0 1 mpa from the literature lunar and tholeiitic basalts plot on similar but distinct curves compared to the simplified system these relations between melt polymerisation and partition coefficients which hold for a large range of compositions and fo2 are extended to various volcanic and plutonic rocks this analysis strengthens the proposal that silicate melt immiscibility can be important in volcanic rocks of various compositions from tholeiitic basalts to lamprophyres however the majority of proposed immiscible compositions in plutonic rocks are at least not coexisting melts but may have suffered accumulation of early crystallized minerals

dual origin of fe ti p gabbros by immiscibility and fractional crystallization of evolved tholeiitic basalts in the sept iles layered intrusion we present a detailed study of two ca 200 m thick apatite bearing ferrogabbro horizons of the sept iles layered intrusion canada these rocks are the most evolved cumulates of the megacyclic units mcu i and ii and mark the transition between basaltic and silicic magmatism they are made up of plagioclase an55 34 olivine fo66 21 clinopyroxene mg 75 55 ilmenite magnetite apatite pigeonite and are a significant source of fe ti p ore ferrogabbros have relatively uniform bulk rock compositions in mcu i but are bimodal in mcu ii the liquid lines of descent for major elements in equilibrium with cumulates of mcu i and ii have been calculated using a forward model formalism both trends evolve towards sio2 enrichment and feot depletion after saturation in fe ti oxides however because of magma mixing in mcu ii they do not follow the same path evolved liquids from mcu ii are shown to enter the experimentally determined two liquid stability field while mcu i liquids do not immiscibility in mcu ii and its absence in mcu i are supported by the presence of contrasted reactive symplectites in cumulate rocks apatite bearing ferrogabbros in mcu ii have crystallized from distinct immiscible fe rich and si rich silicate melts which have physically segregated in the slow cooling magma chamber two different types of cumulate rocks are thus produced leucocratic and melanocratic gabbros this is consistent with the presence of si rich and fe rich melt inclusions in apatite in contrast homogeneous ferrogabbros from mcu i were produced by simple fractional crystallization of a homogeneous liquid our data suggest that immiscibility could also explain the large geochemical variability of ferrogabbros in the upper zone of the bushveld complex south africa

geochemistry of archean metasedimentary rocks from west greenland abstract archean metasedimentary rocks occur as components of the isua supracrustals akilia association and malene supracrustals of southern west greenland primary structures in these rocks have been destroyed by metamorphism and deformation their chemistry and mineralogy is consistent with a sedimentary origin but other possible parents e g acid volcanics altered pyroclastic rocks cannot be excluded for some of them there is little difference in the composition of metasedimentary rocks from the early archean isua supracrustals and probable correlative akilia association both have a wide range in rare earth element ree patterns with la n yb n ranging from 0 61 5 8 the ree pattern of one akilia sample with low la n yb n compares favourably with that of associated tholeiites and it is likely that such samples were derived almost exclusively from basaltic sources other samples with very steep ree patterns are similar to felsic volcanic boulders found in a conglomeratic unit in the isua supracrustals samples with intermediate ree patterns are best explained by mixing of basaltic and felsic end members metasedimentary rocks from the malene supracrustals can be divided into low silica 55 sio2 and high silica 77 sio2 varieties these rocks also show much variation in la n yb n 0 46 14 0 and their origin is explained by derivation from a mixture of mafic volcanics and felsic igneous rocks the wide range in trace element characteristics of these metasedimentary rocks argues for inefficient mixing of the various source lithologies during sedimentation accordingly these data do not rigorously test models of early archean crustal composition and evolution the systematic variability in trace element geochemistry provides evidence for the bimodal nature of the early archean crust