rare earth thhfta and large ion lithophile element variations in metabasites from the proterozoic amphibolite granulite transition zone at arendal south norway abstract the proterozoic arendal metabasites are minor intrusions which possess wholly metamorphic textures and mineral assemblages data are presented for eight selected samples chondrite normalised ree patterns are relatively flat between 10 and 20 chondrite abundances they are characterised by 1 ce n yb n n nd n 1 normalised to primordial mantle values the incompatible elements reveal variable enrichment in the large ion lithophile elements except th and low ta they are similar to many destructive plate margin basalts on a th hf ta diagram the rocks plot in a sub linear trend reflecting variation in th hf samples with high th hf ratios come from the zone of highest metamorphic grade where the host gneisses are deficient in th consequently the patterns in the metabasites could not have been produced by any secondary metamorphic fractionation or by bulk contamination by the country rocks there is no evidence for any post magmatic mobility of any of the elements except rb which is depleted in the highest grade rocks relative to their lower grade counterparts the trace element distributions are largely reflecting those of the parental magmas but consideration of bulk distribution coefficients shows how they may have been modified in some aspects e g ce n nd n by the imposition of the metamorphic mineralogies the data are consistent with a model involving a derivation from a mantle source region which had been affected by subduction related hydrous metasomatism or partial melting and b crystallisation of the magmas directly under the high grade conditions

quantitative mineralogical and chemical assessment of the nkout iron ore deposit southern cameroon abstract the nkout deposit is part of an emerging iron ore province in west and central africa the deposit is an oxide facies iron formation comprising fresh magnetite banded iron formation bif at depth which weathers and oxidises towards the surface forming caps of high grade hematite martite goethite ores the mineral species compositions mineral associations and liberation have been studied using automated mineralogy qemscan combined with whole rock geochemistry mineral chemistry and mineralogical techniques drill cores saprolitic lateritic bif grab and outcrop samples were studied and divided into 4 main groups based on whole rock fe content and a weathering index the groups are enriched material em weathered magnetite itabirite wmi transitional magnetite itabirite tmi and magnetite itabirite mi the main iron minerals are the iron oxides magnetite hematite and goethite and chamosite the iron oxides are closely associated in the high grade cap and liberation of them individually is poor liberation increases when they are grouped together as iron oxides chamosite significantly lowers the liberation of the iron oxides automated mineralogy by qemscan or other similar techniques can distinguish between fe oxides if set up and calibrated carefully using the backscattered electron signal electron beam techniques have the advantage over other quantitative mineralogy techniques of being able to determine mineral chemical variants of ore and gangue minerals although reflected light optical microscopy remains the most sensitive method of distinguishing closely related iron oxide minerals both optical and electron beam automated mineralogical methods have distinct advantages over quantitative xrd in that they can determine mineral associations liberation amorphous phases and trace phases

neoproterozoic iron formation an evaluation of its temporal environmental and tectonic significance article neoproterozoic iron formation nif provides evidence for the widespread return of anoxic and ferruginous ba sins during a time period associated with major changes in climate tectonics and biogeochemistry of the oceans here we summarize the stratigraphic contextof neoproterozoic iron formation and its geographic and temporal dis tribution it is evident that most nif is associated with the earlier cryogenian sturtian glacial epoch although it is possible that some nif may be ediacaran there is no incontrovertible evidence to support this age assignment the paleogeographic distribution of nif is consistent with anoxic and ferruginous conditions occurring in basins within rodinia or in rift basins developed on its margins consequently nif does not require whole ocean anoxia simple calculations using modern day iron fluxes suggest that only models that invoke hydrothermal and or detrital sourcesofiron are capable ofsupplying sufficient ironto account forthe mass of the largernif occurrences this con clusion is reinforced by the available geochemical data that imply nif record is a mixture of hydrothermal and de trital components a common thread that appears to link most if not all nif is an association with mafi cv olcanics

zircon u pb age hf isotopes and geochemistry of shuichang algoma type banded iron formation north china craton constraints on the ore forming age and tectonic setting abstract the shuichang iron deposit is an algoma type bif in the north china craton the shuichang bif is interlayered with plagioclase hornblende gneiss indicating that these two lithologies are temporally closely associated we use the content of major elements and their ratios in order to define the protolith of the plagioclase hornblende gneiss sims u pb dating of zircons from the plagioclase hornblende gneiss suggests that the shuichang bif was formed at ca 2547 7 ma and was subjected to metamorphism at 2513 4 ma the major elements and ree data of the ore as well as individual magnetite and chert bands indicate that the shuichang bif precipitated from hydrothermal fluids discharging on the sea floor and that iron and silica may be associated with submarine volcanic activity in situ lu hf analysis of zircons from the plagioclase hornblende gneiss yielded initial 176 hf 177 hf ratios from 0 280903 to 0 281197 and e hf t values from 10 5 to 0 4 with a weighted mean value of 8 72 0 38 the single stage hf model age ranges from 2836 ma to 3249 ma we infer that the protolith of the plagioclase hornblende gneiss originated from the enriched mantle mixed with a small amount of subducted crust materials the shuichang bif formed at 2 5 ga and is coeval with other bifs in north china and in other cratons in the world we propose that the shuichang bif was formed in a subduction related back arc basin or intra arc basin based on the geochemical signatures of the wall rocks we further point out that the 2 5 ga global bif formation peak not only predates the amalgamation of the north china craton but probably also record an ancient supercontinent event

u pb zircon chronology geochemistry and isotopes of the changyi banded iron formation in the eastern shandong province constraints on bif genesis and implications for paleoproterozoic tectonic evolution of the north china craton abstract the changyi banded iron formation bif in the eastern north china craton ncc occurs within the paleoproterozoic fenzishan group the bif shows alternating quartz rich light and magnetite rich dark bands with magnetite 15 65 vol quartz 25 65 vol and amphibole 15 30 vol constituting the major minerals minor garnet epidote chlorite calcite biotite and pyrite occur locally the bif bands are interlayered with amphibolite hornblende gneiss biotite quartz schist garnet biotite schist biotite gneiss and leptynite and are intruded by granites la icp ms u pb dating on zircons separated from the bif bands and the wallrocks constrains the depositional age as 2240 2193 ma and metamorphic age as 1864 ma the dominant composition of sio 2 fe 2 o 3 t average value of 92 3 wt of the bif bands suggests their formation mainly through chemical precipitation however the widely varying contents of major elements such as al 2 o 3 0 58 6 99 wt mgo 1 00 3 86 wt cao 0 22 4 19 wt and trace elements such as rb 2 06 40 4 ppm sr 9 36 42 5 ppm zr 0 91 23 6 ppm hf 0 04 0 75 ppm cr 89 1 341 ppm co 2 94 30 4 ppm and ni 1 43 52 0 ppm clearly indicate the incorporation of clastics especially continental felsic clastics as also confirmed by the presence of ancient detrital zircons in the bif bands when normalized against post archean average shale paas the seawater like signatures of ree distribution patterns such as lree depletion positive la and y anomalies and superchondritic y ho ratios average value of 36 3 support the deposition in seawater strong positive eu anomalies eu eu paas 1 14 2 86 also suggest the participation of hydrothermal fluids in addition the sympathetic correlation between cr co and ni as well as the co ni cu vs ree and the al 2 o 3 vs sio 2 relations further indicates that the iron and silica mainly originated from hydrothermal fluids combined with regional geological investigation and protolith restoration of the wallrocks a continental rift environment is suggested for the changyi bif deposition the appearance of negative ce paas anomalies might suggest the influence of the great oxidation event at the time of deposition the changyi bif witnessed the major paleoproterozoic rifting collision events in the ncc and their unique distribution in the ncc contrasts with other examples elsewhere in the world

application of the ce anomaly as a paleoredox indicator the ground rules much attention has been paid in recent years to the potential application of the ce anomaly measured in various marine phases as a paleoceanographic indicator of widespread marine anoxia in this paper we present and discuss results from recent studies of present day rare earth element ree distributions and hence ce anomaly distributions in the marine environment which are particularly pertinent to paleoceanography subsequently we review and discuss the validity of the recent literature in which ce anomalies have been employed as paleoredox indicators

genetic modelling for banded iron formation of the hamersley group pilbara craton western australia abstract the banded iron formation bif of the hamersley group pilbara craton western australia particularly from the well studied dales gorge member is unique in its lateral stratigraphic and petrological continuity throughout an area exceeding 60 000 km 2 enabling reasonable estimates for the annual input of components to the depository in the model of this paper varying supply of materials for the medley of mesoband types particularly of iron and silica in the oxide bif can be accommodated by the interaction of two major oceanic supply systems 1 surface currents and 2 convective upwelling from mid oceanic ridge mor or hot spot activity both modified by varied input of pyrochastic material 1 the surface currents were saturated in silica and carried minimal iron due to photic precipitation but were periodically recharged by storm mixing precipitation from them gave rise to the banded chert rich horizons including the varves whose regular and finely laminated iron silica distribution resulted from seasonal meteorological influences 2 precipitation from convection driven upwelling of high iron solution from mor or hot spot activity periodically overwhelmed the delicate seasonal patterns of 1 to produce the iron dominated mesobands a wide range of intermediate mesoband types resulted where the deep water supply was modified by varied mor activity or by partial blocking of upwelling waters by surface currents such as by the present el nino during these periods of oxide dominated bif silica was deposited from saturated solution mainly by evaporative concentration and iron by oxidation due to photolysis and photosynthetically produced oxygen superimposed on these supply differences was the varying effect of fine aluminous ash from dominantly northern distal volcanic sources changing the meteorological and depositional conditions occasional input of extremely fi ash during bif precipitation produced mesoband cm scale variations involving increased carbonate silicate precipitation sustained volcanic periods resulted in s macroband deposition chert carbonate silicate bif with shale gradually returning to the dominant hematite magnetite chert bif as the volcanic input waned during volcanic periods the normally high capacity of sunlight to precipitate ferric iron directly by photolytic oxidation of ferrous iron and by photosynthetic production of oxygen was modified by turbidity in the atmosphere aerosols and dust and in the water colloids from reactive ash s surface precipitated ferric hydroxyoxide redissolved in the presence of decaying organic matter in the subphotic zone augmenting the iron content of the zone precursor ferrous carbonates and silicates were precipitated when the iron concentration of this sub photic zone exceeded their respective solubilities during volcanism the increased availability of nutrients particularly phosphorus to surface waters increased the organic contribution despite lower light values leading to an almost total absence of ferric iron oxides in the s macrobands i e no magnetite or hematite cooling of warm silica saturated sea water during these periods of olcanic winter increased the ratio of precipitation of silica to iron which however was still controlled by seasonal conditions intermediate concentrations of organic matter insufficient to totally convert the ferric compounds either during precipitation or diagenesis resulted in overgrowths of magnetite on hematite and eventually in the substantial conversion of hematite to magnetite where higher temperatures were achieved during low grade regional metamorphism changes in sea level to explain facies changes in bif are not required in this model but are not excluded the preferred conditions are for a very low oxygen to anoxic atmosphere a much higher level of mor activity than at present the presence of photosynthetic plankton the absence of si silica secreting organisms and a deep sea water temperature higher than 20 c however none of these conditions is essential to the model a narrow carbonate bank is postulated for part of the fortescue river valley area during marra mamba iron formation times basal hamersley group with bif precipitation on either side the reef is postulated to have grown northward becoming a major shallow water carbonate platform on the pilbara continent during upper marra mamba iron formation and wittenoom dolomite times but ceased to play an important role in subsequent periods

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

upb and smn edvidence for eburnian and pan african high grade metamorphism in cratonic rocks of southern cameroon abstract u pb analyses of zircons and sm nd mineral and whole rock analyses of samples from southern cameroon document three distinct rock forming or high grade metamorphic events 2900 ma 2050 ma and 600 ma zircons from the ebolowa charnockite in southern cameroon yield an age of 2896 7 ma consistent with published rb sr total rock data for the region sm nd crustal formation ages for these rocks are 3000 ma indicating that the granitoids and ntem series gneisses in the north end of the congo craton formed 2900 ma ago and do not contain major contributions from older archean crust a second metamorphic event of eburnian age is identified from re equilibration of sm and nd in gneisses and metadolerites of the ntem complex at 2050 ma zircons from the younger nyong series gneisses are a complex mixture of cores and overgrowths the overgrowths in most cases formed during a high grade metamorphic event between 2100 and 2000 ma and clearly postdate the ebolowa charnockite the ages of the cores are not precisely known but we believe most represent detrital zircons from 2900 ma protoliths sm nd crustal formation ages for samples of the nyong series gneisses are generally 2500 to 2900 ma indicating that their precursors were apparently derived mainly from the archean craton with variable amounts of younger component sm nd garnet total rock isochrons for metamorphosed mafic rocks in the nyong series yield ages of 2050 ma and zircons from one of them yield an upper intercept age of 2037 10 ma further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 ma we interpret the nyong series metasedimentary gneisses as post 2900 ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 ma eburnian orogeny we cannot define precise limits on the ages of sedimentation preliminary data on metamorphosed intrusive rocks associated with the yaounde series yield u pb concordant ages around 620 ma consistent with sm nd garnet whole rock ages from the same series this dates the pan african tectono metamorphic activity in the southern end mobile belt the pan african lower intercepts of zircons and sm nd garnet whole rock ages from thnyong series are the consequences of this event which is further represented by a 590 ma zircon age from a late syenitic pluton results from cameroon are consistent with recent results from the western part of the congo craton farther south and from the northeastern part of the sao francisco craton in brazil one interpretation of these results is that the congo and sao francisco cratons were joined during the ebumian trans amazonian orogeny 2100 ma and are bordered to the north by a late mesoproterozoic rift basin that was closed during the pan african brasiliano orogeny 600 ma

geochemical characteristics of archean cherts and other sedimentary rocks in the pilbara block western australia evidence for archean seawater enriched in hydrothermally derived iron and silica abstract geochemical and mineralogical characteristics of archean sedimentary rocks such as cherts banded iron formations bifs shales and sandstones collected from the pilbara block western australia were studied lithologic features of various types of chert occurring within the pilbara supergroup are closely related to their chemical compositions red and brown bands of banded cherts generally show a higher fe 2 o 3 total iron as fe 2 o 3 concentration than do other cherts whereas grey greenish grey and pale yellowish grey cherts are characteristic of relatively higher concentrations of al and ti white cherts are generally composed almost exclusively of sio 2 geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions evidence for the hydrothermal origin includes 1 low mno fe 2 o 3 values 2 low concentrations of heavy metals 3 positive euanomalies 4 low co zn and ni zn values and 5 high concentrations of mn and fe in carbonates positive correlations between fe and heavy metals such as ni and zn imply the intermittent precipitation of fe oxides hydroxides along with these heavy metals in the ancient ocean cherts except for grey greenish grey and pale yellowish grey cherts are highly depleted in detrital materials this may have been caused by rapid precipitation of hydrothermally derived fe and si the archean pilbara ocean is believed to have been enriched in fe and si derived from hydrothermal activity

provenance and depositional setting of paleozoic chert and argillite sierra nevada california abstract the shoo fly complex the remnants of an early to middle paleozoic subduction system contains the culbertson lake allochthon included in the culbertson lake allochthon are the post cambrian and pre upper devonian quartz hill toms creek and mcmurray lake cherts geochemical data derived from these three units plot on the fe2o3 tio2 versus al2o3 al2o3 fe2o3 and lan cen versus al2o3 al2o3 fe2o3 discrimination diagrams within either the continental margin island arc field or that part of the pelagic field overlapping the continental margin island arc field these relationships a e consistent with the presence of argillaceous turbidites interstratified with radiolarite and suggest a relatively distal continental island arc setting where muddy turbidity currents episodically interrupted pelagic deposition specimens from the quartz hill and toms creek cherts with al2o3 tio2 values 20 make up group i samples whereas specimens with al2o3 tio2 values 10 are classified as group ii samples the al2o3 tio2 ratios of group ii specimens suggest derivation from mafic rock mean th sc and th u values as well as lree enriched patterns are consistent with this interpretation and indicate a source area dominated by alkalic seamount or ocean island material the al2o3 tio2 ratios of group i samples indicate a source area with an average andesitic to rhyodacitic composition chondrite normalized ree distribution patterns as well as mean th sc and th u value support such an interpretation samples from the mcmurray lake chert have al2o3 tio2 values ranging between 19 and 28 and ree patterns characterized by lree enrichment negative eu anomalies and slightly fractionated hree patterns these features along with mean th sc and th u values indicate a source area dominated by old upper continental crust the location of the source area s supplying material to cherts and argillites of the culbertson lake allochthon is unknown however existing tectonic models commonly portray the source of old upper continental crustal material in the shoo fly complex as being located somewhere along the western north american continental margin whereas the source of island arc debris is portrayed as being located in a fringing arc system our data suggest that a relatively high standing seamount or ocean island probably resided on the subducting oceanic plate thus our work supports the idea that al fe ti th sc and the rees in chert and argillite deposited in proximal continental island arc settings can be used to assess the characteristics of sources supplying particulate matter to ancient t ench systems

continentally derived solutes in shallow archean seawater rare earth element and nd isotope evidence in iron formation from the 2 9 ga pongola supergroup south africa abstract the chemical composition of surface water in the photic zone of the precambrian ocean is almost exclusively known from studies of stromatolitic carbonates while banded iron formations ifs have provided information on the composition of deeper waters here we discuss the trace element and nd isotope geochemistry of very shallow water if from the pongola supergroup south africa to gain a better understanding of solute sources to mesoarchean shallow coastal seawater the pongola supergroup formed on the stable margin of the kaapvaal craton 2 9 ga ago and contains banded iron formations ifs that represent the oldest documented superior type iron formations the ifs are near shore pure chemical sediments and shale normalized rare earth and yttrium distributions reysn exhibit positive lasn gdsn and ysn anomalies which are typical features of marine waters throughout the archean and proterozoic the marine origin of these samples is further supported by super chondritic y ho ratios average y ho 42 relative to older isua ifs 3 7 ga from greenland and younger kuruman ifs 2 5 ga also from south africa the pongola ifs are depleted in heavy rare earth elements hree and appear to record variations in solute fluxes related to sea level rise and fall sm nd isotopes were used to identify potential sediment and solute sources within pongola shales and ifs the nd t for pongola shales ranges from 2 7 to 4 2 and nd t values for the coeval iron formation samples range 1 9 to 4 3 are generally indistinguishable from those of the shales although two if samples display nd t as low as 8 1 and 10 9 the similarity in nd isotope signatures between the shale and iron formation suggests that mantle derived rey were not a significant nd source within the pongola depositional environment though the presence of positive eu anomalies in the if samples indicates that high t hydrothermal input did contribute to their rey signature isotopic mass balance calculations indicate that most 72 of the nd in these seawater precipitates was derived from continental sources if previous models of fe nd distributions in archean ifs are applied then the pongola ifs suggest that continental fluxes of fe to archean seawater were significantly greater than are generally considered

geology of the gouap iron deposit congo craton southern cameroon implications for iron ore exploration abstract the ntem complex represents the northwestern edge of the archean congo craton in southern cameroon where numerous banded iron formation bif hosted iron deposits have been discovered the gouap iron deposit which is located on the western edge of the ntem complex is one of the recent discoveries of iron deposits in this area in this study geological mapping detailed drillcore logging and petrographical studies reveal that the gouap iron deposit comprises of two lithostratigraphic units namely the bif and the country rocks the bif unit consists of amphibole bif and amphibole pyrite bif whereas the country rocks unit is made up of gneisses garnet amphibole magnetite and amphibole garnet pyrite gneiss schists amphibole magnetite and garnet magnetite micaschist quartzites and dolerite sills the bifs are well interlayered and observed to be in sharp or gradational contacts to their associated country rocks the mineral assemblages of the bifs and wallrocks except the dolerites indicate that both units have been metamorphosed under amphibole facies conditions two main iron mineralization types are identified in the gouap deposit the soft iron type which is mainly located in the western and northern parts and the hard iron type mostly confined to the southern sector of the deposit the structural controls of the gouap iron deposit are better observed in drillcore samples other than on the outcrop scale there are three principal deformation events d1 d2 and d3 that led to the development of foliation folding and faulting respectively the iron mineralization are mostly concentrated on the f2 fold hinge zones and fractures that acted as passage ways for the circulation of meteoric water at surface we suggest that the following three major processes that led to the formation of the gouap deposit i the chemical precipitation of chert and iron oxide followed by sedimentation and diagenesis that formed chert magnetite bifs ii secondly the amphibolite facies metamorphism and deformation that formed quartz magnetite bifs which constitute the main iron ridge located on thicker zones of fold noses and iii finally the gouap bifs have been upgraded to medium grade iron ore due to supergene enrichment processes which are restricted to the modern land surface and locally reached deeper levels through fault and fracture zones the western and northern parts of the gouap deposit have been identified as potential exploration target areas for iron mineralization even though the iron formation processes of the gouap deposit are not yet completely understood and many questions remain unanswered regarding the genesis of the soft and hard iron mineralization these new insights have important implications on the exploration drilling strategies of this deposit

geochemistry and pb pb geochronology of the neoarchean nkout west metamorphosed banded iron formation southern cameroon the nkout west iron deposit of the northern edge of the congo craton in southern cameroon forms part of the northwest plunging isoclinal mega fold composed of metamorphosed banded iron formations bifs which have been deformed and metamorphosed resulting in recrystallized magnetite quartz and silicates here we present whole rock geochemistry together with pb pb isotopic compositions and dating of metamorphosed bif with the aim to constrain the genesis and elucidate the metamorphic resetting age based on the mineral composition two types of bif magnetite bif garnet bif have been distinguished the magnetite bif shows high sio2 and fe2o3 and low al2o3 tio2 hfse and ree contents similar to detritus free chemical sediments in contrast their high concentrations in garnet bif indicate clay rich contaminated input the co zn ratios are consistent with trace metals of hydrothermal sources 0 14 for magnetite bif or with minor hydrogenous component 0 27 for garnet bif the paas normalized ree y patterns of the studied metamorphosed bifs show lree depletion relative to hree and positive eu anomalies the magnetite bif exhibits positive y anomaly y y paas 1 3 which is the signature inherited from sea water but its y ho 39 93 ratio suggests a mixture of hydrothermal solution with sea water the garnet bif shows positive eu negative y anomalies and chondritic y ho ratio 27 49 which is consistent with hydrothermal solution with significant clastic contamination whole rock pb concentration is roughly on average of 2 0 6 0 ppm suggesting a hydrothermal pb source the pb pb dating yielded an age of 2679 ma interpreted as the neoarchean metamorphic event in the ntem complex

fluctuations in late neoproterozoic atmospheric oxidation cr isotope chemostratigraphy and iron speciation of the late ediacaran lower arroyo del soldado group uruguay oxygenation of the earth s atmosphere occurred in two major steps near the beginning and near the end of the proterozoic eon 2500 to 542 ma ago but the details of this history are unclear chromium isotopes in iron rich chemical sediments offer a potential to highlight fine scale fluctuations in the oxygenation of the oceans and atmosphere and to add a further dimension in the use of redox sensitive tracers to solve the question regarding fluctuations of atmospheric oxygen levels and their consequences for earth s climate we observe strong positive fractionations in cr isotopes 53 cr up to 5 0 in iron rich cherts and banded iron formation horizons within the arroyo del soldado group ediacaran uruguay that can be explained by rapid effective oxidation of fe ii rich surface waters these fluctuations are correlated with variations in ratios of highly reactive iron fe hr to total iron fe tot which indicate a predominance of anoxic water columns fe hr fe tot 0 38 during the onset of oxidation pulses we favor the scenario by which isotopically heavy cr vi entered the basin after pulses of oxidative weathering on land and in which fe ii accumulated in the water column neodymium isotopes reveal that these oxygenation pulses were followed by increased influxes to the basin of continental crust derived detrital components of paleoproterozoic nd t dm model ages 2 1 2 2 ga provenance typical of the rio de la plata craton the association of positive 53 cr ferruginous fe hr fe tot 0 38 stratigraphic intervals with low diversity acritarch assemblages dominated by bavlinella faveolata strongly support models postulating a stratified eutrophic neoproterozoic ocean thus even within a few million years of the precambrian cambrian boundary paleoceanographic conditions resembled more those of paleoproterozoic oceans than phanerozoic and present oceans this highlights the sheer magnitude of ecological changes at the precambrian cambrian transition changes which ultimately led to the demise of the precambrian world and the birth of the metazoan dominated phanerozoic

the gongchangling bifs from the anshan benxi area ne china petrological geochemical characteristics and genesis of high grade iron ores abstract the gongchangling bifs are located in the anshan benxi area northeastern part of the north china craton of which the no 2 mining area is the largest and most typical high grade iron ore producing area in china in this study the mineralogy petrology and chemistry of iron ores bifs and high grade iron ores and their wall rocks amphibolite and garnet chlorite schist from the gongchangling no 2 mining area were examined to constrain the genesis of high grade iron ore the bifs are composed essentially of magnetite and quartz in addition to actinolite in a few samples while the high grade iron ores are mainly composed of magnetite and the quartz is relatively rare the low contents of al 2 o 3 tio 2 and hfse in the bifs and high grade iron ores indicate that they are marine chemical precipitates with little input of terrigenous clastic sediments the magnetite and quartz from bifs and high grade iron ores exhibit the similar chemical composition the rey patterns of bifs and high grade iron ores suggest their precipitation after mixing of hydrothermal component with seawater in certain proportions under relatively low oxygen level and the obvious positive eu anomalies indicate that high temperature hydrothermal fluids which percolate and dissolve the submarine volcanic rock may supply the ore forming material to the gongchangling iron deposit the amphibolite as interlayer of bifs recorded the metamorphic temperature of 567 25 c moreover the garnet from wall rock of high grade iron ore exhibits the growth zoning produced during prograde metamorphism indicating that metamorphic hydrothermal fluids played an important role in the genesis of high grade iron ore it was proposed that the gongchangling iron deposit was formed in an arc related tectonic setting at the late neoarchean then it underwent lower amphibolite facies metamorphism and the high grade iron ore is the reformed product of bifs by metamorphic hydrothermal fluids

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

geology and geochemical constrains on the origin and depositional setting of the kpwa atog boga banded iron formations bifs northwestern congo craton southern cameroon abstract the kpwa atog boga kab area lies within the paleoproterozoic nyong unit which defines the northwestern edge of the archean congo craton in southern cameroon rock types in the area are mainly comprises of garnet gneiss associated with banded iron formations bifs the bifs and its host rocks were subjected to amphibolite facies metamorphism the rare earth elements ree patterns and the discrimination ratios e g al2o3 tio2 cr th th sc la sc of the garnet gneiss suggest that they are metasedimentary rocks derived from a felsic parent sources that have undergone intensive weathering the kab bifs exhibit significant concentration of tfe2o3 sio2 93 wt and lower al2o3 and tio2 contents and high field strength elements e g hf nb ta and zr this involves little contamination with continental detritus at their depositional environment the post archean australian shale paas normalized ree patterns of the kab bifs show enrichment in heavy ree and the depletion in light ree with positive la anomalies which imply that these bifs precipitated in ambient seawater moreover the contribution of low temperature hydrothermal fluids during the deposition of the studied bifs were highlighted through their high superchondritic y ho ratio mean 31 48 weak positive eu eu sn ratio mean 1 83 and the low contents of transition metals this suggests that the geochemical characteristics of the kab bifs and those of the paleoproterozoic superior type bifs are almost similar in summary low temperature hydrothermal fluids mixed with seawater contributed to the deposition of the kab bifs on the continental margin this depositional setting similar to its host rocks is consistent with the paleoproterozoic geological history recorded within the nyong unit this preliminary study reveals that the kab bifs are associated with low grade gold

geochemistry of sandstones from the upper miocene kudankulam formation southern india implications for provenance weathering and tectonic setting abstract petrographic major trace and rare earth element compositions of sandstones from the upper miocene kudankulam formation southern india have been investigated to determine their provenance tectonic setting and weathering conditions all sandstone samples are highly enriched in quartz q but poor in feldspar f and lithic fragments l the major element concentrations of these sandstones reveal the relative homogeneity of their source geochemically the kudankulam sandstones are classified as arkose subarkose litharenite and sublitharenite the cia values chemical index of alteration mean value 44 5 for these sandstones and the a cn k diagram suggest their low weathering nature similarly their fe2o3 mgo mean 2 7 al2o3 sio2 0 09 k2o na2o 2 2 ratios and tio2 contents 0 3 are consistent with a passive margin setting the eu eu 0 5 la lu cn 21 la sc 5 9 th sc 1 9 la co 5 7 th co 1 8 and cr th 5 3 ratios support a felsic source for these sandstones chondrite normalized ree patterns with lree enrichment flat hree and negative eu anomaly also are attributed to felsic source rock characteristics for kudankulam sandstones total ree concentrations of these sandstones reflect the variations in their grain size fractions the source rocks are probably identified to be proterozoic gneisses charnockites and granites of the kerala khondalite belt which must have been exposed at least since the late miocene finally the unusual ni enrichment in the kudankulam sandstones unaccompanied by a similar enrichment in cr co and v may be related to either the presence of pyrite in the sandstones or more likely the fractionation of garnet from the source rocks during transportation

the influence of sediment recycling and basement composition on evolution of mudrock chemistry in the southwestern united states this paper reports systematic changes in mudrock composition through time on a single con tinental cmstal block the changes reflect both sediment recycling processes and changes through time in the composition of crystalline material being added to the sedimentary system and are related to tectonic evolution as the block matures from a series of accreted arc terranes to a stable craton the major and trace element distributions reflect different aspects of the provenance of the mudrocks in this study major elements record sediment recycling processes as well as changing proportions of sedi mentary and first cycle source rocks with the exceptions of kzo which tends to increase and sioz and a o3 which show no trend most major oxides tend to decline in relative abundance in younger mud rocks patterns shown by the index of compositional variability fe03 kro naao cao mgo mno tiojialro and by k20 a1203 indicate that the major oxide trends are due to decreasing proportions of nonclay silicate minerals and a concomitant increase in the proportion of clay minerals probably due to decreasing input of first cycle detritus coupled with recycling of sedimentary material excursions from progressive trends marked by increases in mgo k20 and cao reflect episodes of large scale input of nonclay first cycle minerals from crystalline source rocks due to large scale basement uplift the chemistry of low solubility trace elements in contrast is not sensitive to recycling effects and reflects the composition of first cycle input incompatible elements are progressively enriched relative to compatible elements in younger mudrocks and values for chondrite normalised rare earth elements also increase in addition the eu anomaly becomes systematically more negative in younger samples these trends cmnot be explained by diagenetic or weathering processes and therefore indicate that the proportion of fnc tionated granitic first cycle detritus being added to the sedimentary system becomes greater with time these results confirm the importance of tectonic setting in controlling mudrock chemistry and also demonstrate that there is a dynamic relationship between the tectonic evolution of a continental block and the composition of its sedimentary mantle

ree geochemistry of carbonates from the guanmenshan formation liaohe group ne sino korean craton implications for seawater compositional change during the great oxidation event abstract the worldwide 2 33 2 06 ga positive 13 c carb excursion has been correlated with the great oxidation event goe and termed as the lomagundi event the 2 3 1 85 ga guanmenshan formation in the liaohe group of the northeastern sino korean craton is characterized by positive 13 c carb excursion and is a potential candidate to evaluate the lomagundi event using rey rare earth element and yttrium ree y chemical fingerprints here we present major and trace element analysis of 42 samples from the guanmenshan formation which are pure marine chemical sediments and use the data to trace the seawater composition during 2 3 1 85 ga 15 least altered dolomicrite samples 600 m strata have ree values of 0 739 4 175 ppm 2 414 1 184 ppm and the y ho ratios of 34 5 56 6 44 1 5 7 they show uniform positive la sn la sn 1 04 0 27 and gd sn gd sn 1 64 0 40 anomalies and notable lree depletions indicated by nd sn yb sn values of 0 24 0 92 average 0 56 0 19 these features are consistent with the geochemistry of well oxygenated shallow ambient seawater and suggest that these samples provide a robust record of the primary rey signature of seawater during the lomagundi event the rey patterns of 15 silicified dolomites marbles locally with veinlets from the pb zn mining camps in the region with average eu cn eu cn 1 56 0 95 are identical to those of high temperature hydrothermal fluids 250 c characterized by a flat pattern and marked positive eu anomalies indicating that these rocks were subjected to metasomatism by hydrothermal fluids the guanmenshan formation shows average ce sn ce sn of 0 93 0 09 and sm cn yb cn of 1 which are higher than those of the archean 2 33 ga chemical sediments generally f o 2 and p co 2 the rey in the dolomicrite were mainly sourced from fluxes of solutes from terrestrial weathering and also from seafloor hydrothermal processes on a subordinate scale the guanmenshan dolomicrites have eu sn eu sn values of 1 34 2 55 i e around 1 53 indicating that they were deposited during 2 33 2 06 ga as the eu sn eu sn 1 53 can be used as a proxy for the 2 33 2 06 ga marine chemical sediments our study shows that the guanmenshan formation was formed at a critical turning point in earth history when the global atmosphere hydrosphere system witnessed a dramatic change from reducing to oxidizing conditions

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

main eddy vertical structures observed in the four major eastern boundary upwelling systems in the four major eastern boundary upwelling systems ebus mesoscale eddies are known to modulate the biological productivity and transport near coastal seawater properties toward the offshore ocean however little is known about their main characteristics and vertical structure this study combines 10 years of satellite altimetry data and argo float profiles of temperature and salinity and our main goals are i to describe the main surface characteristics of long lived eddies formed in each ebus and their evolution and ii to depict the main vertical structure of the eddy types that coexist in these regions a clustering analysis of the argo profiles surfacing within the long lived eddies of each ebus allows us to determine the proportion of surface and subsurface intensified eddies in each region and to describe their vertical structure in terms of temperature salinity and dynamic height anomalies in the peru chile upwelling system 55 of the sampled anticyclonic eddies aes have subsurface intensified maximum temperature and salinity anomalies below the seasonal pycnocline whereas 88 of the cyclonic eddies ces are surface intensified in the california upwelling system only 30 of the aes are subsurface intensified and all of the ces show maximum anomalies above the pycnocline in the canary upwelling system 40 of the aes and 60 of the ces are subsurface intensified with maximum anomalies extending down to 800 m depth finally the benguela upwelling system tends to generate 40 50 of weak surface intensified eddies and 50 60 of much stronger subsurface intensified eddies with a clear geographical distribution the mechanisms involved in the observed eddy vertical shapes are discussed

elemental distributions and neodymium isotopic compositions of silurian metasediments western maine usa redistribution of the rare earth elements abstract pelitic schists from the lower garnet to lower staurolite zones from the rangeley perry mountain and smalls falls formations of western maine were analyzed for major elements trace elements and neodymium isotopes these formations were derived from highlands created during the taconian orogeny deposited into a trough and metamorphosed during subsequent orogenic events most major and trace element abundances relative to al 2 o 3 were statistically identical between zones of the same formation as well as between formations although the average major element composition of these formations are the same there are systematic variations in some elements notably plots of sio 2 vs al 2 o 3 and k 2 o vs al 2 o 3 suggest that most of the variation could be produced by mixing of a fairly constant ratio of clay minerals and feldspar with varied amounts of quartz due to sorting in the sedimentary system different amounts of these minerals should not influence the shape of the ree patterns of the metapelites but higher amounts of quartz and feldspar may dilute the rees and most elemental abundances of the clay minerals and lead to lower elemental abundances the major difference between the samples within the perry mountain formation are different lree and mree abundances relative to al 2 o 3 which are not correlated to differences in major element or other trace element abundances relative to al 2 o 3 the samples in the perry mountain with higher lree and mree abundances have for example 42 3 8 3 ppm and those with low abundances have 5 6 3 6 ppm the samples with the high ree abundances of the perry mountain formation are similar in abundances and ree patterns to those of the rangeley and smalls falls formations typical of mudstones derived from granitoids another difference between the low and high ree abundance samples are the calculated t dm model ages the high ree abundance samples of the perry mountain formation show t dm similar to the samples of the rangeley formation with ages of about 1 7 1 8 ga the perry mountain samples with low ree abundances however give unrealistically old t dm s between 2 5 and 5 3 ga these unrealistically old t dm s are due to the relatively high sm nd ratios compared to crustal values which are characteristic of samples of the perry mountain formation with lower ree abundances we therefore suggest that these samples may be indicators for open system behavior of the neodymium isotopic system the timing of this disturbance of the neodymium isotope system is difficult to determine and cannot be tied to weathering or a definite postdepositional event the complexities of the data suggest more than one resetting event the most likely event that could have produced much of the movement of the lrees and mrees could have been due to small scale migration between anoxic hemipelagites and turbidite mudstones during diagenesis but some migration may have continued during metamorphism in order to reconcile the neodymium isotopic data

rare earth element systematics of the chemically precipitated component in early precambrian iron formations and the evolution of the terrestrial atmosphere hydrosphere lithosphere system irrespective of provenance age and metamorphic grade the chemically precipitated component in early precambrian 2 3 ga iron formations ifs displays smyb cn 1 which reflects the corresponding ratios of contemporaneous seawater in conjunction with end if end shale jacobsen and pimentel klose 1988a b this rare earth element ree signature reveals that the ree distribution in early precambrian ifs must be explained by mixing between a marine bottom and a surface water component and that the rees and by analogy the fe cannot be derived from weathering of a continental source eusm n ratios of detritus free ifs are controlled by the marine bottom water component where eusm cn 1 results from ree input from high temperature high t hydrothermal fluids which received their ree signature during high t alteration of oceanic crust at spreading centres decreasing eusm cn of ifs with decreasing depositional age reflect the vaning importance of high t compared to low t 200 c hydrothermal fluids hence oceanic crust alteration for the ree budget of marine bottom waters and are related to decreasing upper mantle temperatures smyb n ratios of detritus free ifs are controlled by the marine surface water component whose ree distribution is affected by the same processes which operate today in the entire ocean mixing calculations reveal that smyb cn in early precambrian marine surface waters was significantly lower than it is today to explain this difference two mechanisms are discussed on the basis of higher pco2 and lower po2 levels of the precambrian atmosphere 1 1 higher co32 in precambrian river and coastal waters and 2 2 redox cycling of the rees at an oceanwide chemocline which separated anoxic and oxic water masses however these processes are synergetic and could have operated in unison the ree distribution in precambrian ifs is described as a result of mixing in a multicomponent system where high t and low t hydrothermal fluids contributed to the marine bottom water and ree input from the dissolved ree pool in river waters after some modification in estuaries dominated the ree distribution in marine surface waters after mixing of bottom and surface waters their combined ree load was quantitatively scavenged onto precipitating iron oxihydroxides or iron carbonates and deposited with the chemically precipitated component after addition of variable amounts of clastic detritus this eventually resulted in the ree distribution observed in precambrian ifs

rare earth geochemistry of arenig cherts from the ballantrae ophiolite and leadhills imbricate zone southern scotland implications for origin and significance to the caledonian orogeny rare earth element ree data from low to mid arenig cherts are used to test competing models for the early ordovician evolution of the laurentian margin in the northern british isles cherts from the ballantrae ophiolite complex have chondrite normalized ree patterns typical of continental margin settings with lree enrichment a slight negative eu anom and shale and chondrite normalized la yb values of 0 97 1 41 and 7 78 11 4 respectively this pattern together with a large positive chondrite normalized ce anom 1 44 1 70 is virtually identical to that found in radiolarian chert of the gascoyne abyssal plain in the timor sea cherts from the raven gill formation within the leadhills imbricate zone northern belt southern uplands have typical continental margin ree patterns chondrite normalized ce anom 0 9 1 21 and eu anom 0 61 0 79 values indicating that they formed closer to the continental margin than those from ballantrae shale and chondrite normalized and la yb values of 0 95 1 27 and 4 92 13 88 respectively confirm this interpretation it is concluded that the ballantrae ophiolite formed in a rifted arc basin above a northwards dipping intra oceanic subduction zone the modest depth of burial of the raven gill formation precludes it being part of a marginal basin which was subsequently trapped as the ballantrae ophiolite was obducted in the late arenig the arenig rocks of the leadhills imbricate zone represent an allochthonous terrane accreted to the western extension of the midland valley in ireland in pre caradoc times here it formed the basement to the southern uplands basin palaeontological evidence places this basin adjacent to pomeroy co tyrone in the early caradoc sinistral strike slip faulting from the late ashgill transported the southern uplands terrane to its present location a distance of less than 250 km

rare earth element variations in mid archean banded iron formations implications for the chemistry of ocean and continent and plate tectonics abstract abundances of major and rare earth elements rees are reported for mid archean 3 3 3 2 gyr sedimentary rocks including banded iron formations bifs and ferruginous siliceous mudstone from the cleaverville area in the pilbara craton western australia geological lithological and geochemical lines of evidence indicate that these sedimentary rocks preserve a continuous record of depositional environments ranging from that typical of mid oceanic spreading centers to convergent plate boundary settings a range of environments most likely caused by plate movements except for the mudstone the ree content of these sedimentary rocks changes gradually from the lower to upper stratigraphic horizons europium anomalies decrease up section eu eu values normalized to nasc change from 3 5 to 1 1 as the ree contents and lree hree ratios increase the striking similarity in these ree signatures of bifs and modern hydrothermal sediments leads us to propose that the bifs were in situ hydrothermal precipitates near a mid ocean ridge significant amounts of terrigenous materials contributed to the siliceous and ferruginous mudstone of the uppermost horizon the observation that the source of the sediments shifted from proximal hydrothermal through distal hydrothermal to terrigenous suggests that plate tectonics dominated by horizontal movement was already operating in the mid archean distal hydrothermal sediments without a eu anomaly when normalized to nasc suggest that mid archean seawater had already been strongly influenced by a riverine flux from an upper continental crust and that this component bore no eu anomaly i e it had a negative eu anomaly when normalized to chondrite in addition to an absence of a eu anomaly mid archean seawater did not have a ce anomaly suggesting less oxic conditions in the mid archean than in the modern ocean

a mixed seawater and hydrothermal origin of superior type banded iron formation bif hosted kouambo iron deposit palaeoproterozoic nyong series southwestern cameroon constraints from petrography and geochemistry abstract the kouambo iron deposit contains banded iron formations bifs and is located in the northwestern margin of the congo craton the bifs are hosted in palaeoproterozoic nyong series a dominantly metasedimentary formations which were metamorphosed into greenschist to granulite facies the kouambo bifs are medium to coarse grained banded rocks consisting of alternation of si rich and fe rich mesobands and belong to oxide facies iron formations geochemistry analyses reveal that these iron formations are composed of 96 wt fe2o3 and sio2 and have low concentrations of al2o3 tio2 and trace hfse suggesting chemical precipitates of silica and iron moreover these bifs have low concentrations of al2o3 tio2 and trace hfses high field strength elements e g zr hf ta pb and th suggesting that terrigenous detrital materials contributed insignificantly to the sedimentation the post archean australian shale paas normalized ree y patterns display seawater like profile minor lree depletion and hree enrichment positive y anomalies however they display positive eu and negative ce anomalies and low y ho ratio average 29 which suggest the influence of the hydrothermal fluids the weak positive eu eu paas ratio average 1 5 associated with the low v 17 5 ppm co 6 1 ppm and ni 27 5 ppm contents similar to other superior type bifs worldwide are consistent with the deposition of the kouambo bifs in continental marginal sea or back arc basin environment in summary the kouambo bifs show a seawater like ree y signature however the positive eu anomalies and reduced y ho ratios relative to seawater indicates a possible mixing with hydrothermal fluids 0 5

changes in holocene paleoseismic activity in the hula pull apart basin dead sea rift northern israel abstract paleoseismic analysis of the azaz fault segment of the dead sea fault northern israel indicates that this region was subjected to intermittent periods of strong seismicity and quiescence during the latest pleistocene and the holocene the azaz fault forms the eastern margin of the hula valley a pull apart basin between two left stepping segments of the dead sea fault the fault is currently inactive but it displaces a 25 30 ka travertine in the northeastern corner of the hula valley and near surface marsh sediments inside the valley two trenches excavated across a 15 20 m high fault scarp near kefar szold expose fluvial and colluvial sequences showing clear evidence of recent tectonics one trench exposes well sorted bedded alluvial sediments comprising two upward fining units each capped by a weakly developed paleosol the sediments were deposited on the down faulted block and are bounded in the east by the fault scarp the second trench uncovers fine colluvial sediments cross cut by a wide fault parallel fissure filled by collapsed sediments in both trenches the lower sedimentary unit is capped by a coarse colluvium containing boulders up to 1 5 m in size the colluvium shows no clear bedding and has a weakly developed paleosol on top optically stimulated luminescence dating of the lower fluvial sequence shows that its age ranges between 12 ka at the base to 6 ka at the top while the middle part of the overlying colluvium is ca 4 8 ka the relation between the lower fluvial units and the fault indicates that at least two large scale earthquakes occurred in the early holocene each one resulting in a 1 1 5 m high fault scarp during this seismically active period no coarse colluvial sediments were deposited along the fault trace and thus the region must have had low tectonic relief the clear contact with the overlying coarse colluvium reflects a middle holocene rapid change to the present high steep relief this change was achieved by frequent strong seismic events which triggered colluvial processes and prevented stratification and soil formation on the slope the soil on the present slope reflects a quiescent period that has lasted at least several hundred years during which stress has accumulated and seismic risk increased

mesoscale eddies in the northwestern subtropical pacific ocean statistical characteristics and three dimensional structures mesoscale eddy properties in the northwestern subtropical pacific ocean are investigated by analyzing 22 567 cyclonic eddies ces and 26 365 anticyclonic eddies aes detected from 19 year altimetric sea level records eddy occurrence frequency and kinetic energy are prevailingly high in the subtropical countercurrent zonal band between 19 degrees n and 26 degrees n and further elevated near the luzon taiwan coast a general superiority of aes is observed at most latitudes except between 19 degrees n and 22 degrees n where the ce number is larger the modal radius and mean lifespan of the ces aes are 134km and 11 2weeks 121km and 10 9weeks respectively after generation most eddies propagate westward with a mean speed of 7 2cms 1 and deflect northward following the kuroshio along the luzon taiwan coast three dimensional eddy structures are further explored with composite eddy images in five subregions constructed by surfacing argo temperature salinity data into altimeter detected eddy areas due to the existence of mode waters in the main thermocline eddy induced temperature anomaly exhibits a double core vertical structure which is especially evident in ce images because of the vertical water mass distribution salinity anomaly features a sandwich like pattern which is more evident in ae images also revealed is the significant structure difference in these five subregions eddies are greatly intensified as they approach the western boundary inducing larger temperature and salinity anomalies and influencing deeper ocean along the luzon taiwan coast aes are preferentially strengthened by the northward background flow

the nature of mesoarchaean seawater and continental weathering in 2 85 ga banded iron formation slave craton nw canada abstract banded iron formations bif have been extensively used as proxies to infer the chemical composition of ancient bulk seawater however their proximity to ancient crust suggests that they might also be used to reveal the composition of emergent continental landmass at the time of their deposition here we use the combination of geochemistry and sm nd isotopes on a layer by layer basis to interpret the relative contributions of hydrothermal hydrogenous and terrestrial input to one of the oldest documented superior type bif in the world the 2 85 ga central slave cover group bif is deposited within a rift basin related to a continental margin and is found associated with basement gneisses as well as shoreline and shallow shelf type facies such as fuchsitic quartzite and pebble to cobble conglomerate that confirm a near shore depositional setting for the bif the bif ranges from a pure chemical oxide magnetite silicate grunerite actinolite sediment with low al 2 o 3 2 o 3 10 wt and the occurrence of ferro hornblende biotite and garnet the silica bands have low trace metal content e g ni low ree average of 6 ppm and a shale normalized rare earth and yttrium rey pattern that is hree to lree enriched with pr yb sn values reaching sn of 0 5 during active rifting of the basement excess of eu 2 impacted the basin yielding seawater with eu anomalies eu eu sn as high as 3 85 average 2 75 larger than similarly aged bif high resolution geochemistry shows that there is more silica 19 4 wt sio 2 in the iron bands than iron 8 7 wt fe 2 o 3 in the silica bands implying that dissolved fe 2 came to the bif site in pulses and that silica likely represents background deposition consistently radiogenic e nd t values for the iron bands average 1 7 show that the dissolved rey in the source water during ferric iron precipitation was provided by submarine hydrothermal fluids with relatively uniform 143 nd 144 nd the silica bands by contrast reveal high variation in seawater 143 nd 144 nd as evident from the bimodal e nd t distribution with one segment exhibiting negative e nd t values averaging 1 1 and another with positive e nd t values averaging 2 5 this suggests input of dissolved rey into the upper seawater from weathering of isotopically different crustal components in the source region collectively we speculate that the low rey in the upper seawater and the overall low ni content implies a highly weathered crustal surface that was unable to contribute a significant dissolved load to the shelf environment

hydrothermal plumes and the delivery of iron to banded iron formation most of the iron in banded iron formation bif was deposited during the period 2 7 1 8 ga within this period fe deposition rates were at least as high as 10 12 gm y according to new age models for the largest bifs peak deposition occurred between 2 4 2 3 ga it is likely that such high deposition rates would not be met by a solely continental source of iron unless significant quantities were remobilized in marine sediment estimates of global heat loss for the late archean early proterozoic suggest a hydrothermal cycling rate approximately three times greater than at present so hydrothermal water rock exchanges were a more important source of fe to the oceans than today hydro thermal plumes depleted in o 2 and enriched in fe were generated at mid ocean ridge crests with substantially shallower depths than present assuming hydrothermal effluent dilution factors of 10 4 plumes transported iron which had a dominantly hydrothermal source into the upper water column and transferred it to

tectonic environment of the devonian gramscatho basin south cornwall framework mode and geochemical evidence from turbiditic sandstones the portscatho formation within the allochthonous unit of the middle and upper devonian gramscatho group is a thick sequence of deep water sandstones and interbedded slates deposited by southerly derived turbidity currents into the gramscatho basin of south cornwall throughout an approximately 3 5 km thick sequence the portscatho formation is petrographically and chemically coherent except that the upper section shows a higher proportion of metamorphic clasts high but variable cr and low uniform zr abundances complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition similar to average upper continental crust but with an admixture of minor intermediate basic material flysch deposition took place in a fore arc setting the presence of an arc to the south of cornwall during the devonian implies that there was subduction at the margin of the gramscatho basin whose ultimate closure was accommodated by the northward stacking of flysch ophiolite nappes

depositional environment and tectonic implications of the paleoproterozoic bif in changyi area eastern north china craton evidence from geochronology and geochemistry of the metamorphic wallrocks abstract the changyi banded iron formation bif in the eastern north china craton ncc occurs within the paleoproterozoic fenzishan group three types of metamorphic wallrocks interbedded with the bif bands are identified including plagioclase gneisses and leptynites garnet bearing gneisses and amphibolites protolith reconstruction suggests that the protoliths of the plagioclase gneisses and leptynites are mainly graywackes with minor contribution of pelitic materials the garnet bearing gneisses are fe rich pelites contaminated by clastics and the amphibolites are tholeiitic rocks trace elements of la th sc and zr of the plagioclase gneisses and leptynites and the garnet bearing gneisses support that these meta sedimentary rocks were probably derived from recycling of archean rocks with felsic and mafic materials differentiated into different rock types 207 pb 206 pb ages of detrital zircons from the meta sedimentary rocks concentrate at 2 7 3 0 ga confirming their derivation from the archean rocks the presence of several paleoproterozoic detrital zircons 2240 to 2246 ma however also suggests minor involvement of paleoproterozoic materials the archean detrital zircons have e hf t values varying from 0 7 to 7 6 which mainly fall between the 3 0 ga and 3 3 ga average crustal evolution lines on the age vs e hf t diagram further illustrating that the rocks providing materials for the meta sedimentary rocks mainly originated from partial melting of a mesoarchean crust this is strongly supported by their crust like trace element distribution patterns such as nb ta p and ti depletion and ancient nd depleted mantle model ages t dm 2 9 3 4 ga in addition the remarkably high e hf t values 7 5 to 9 3 of the paleoproterozoic detrital zircons constrain the paleoproterozoic materials to originate from a depleted mantle the amphibolites show low sio 2 46 5 to 52 8 wt and high mgo 5 68 to 10 9 wt contents crust like trace element features and low e nd t values 4 5 to 0 3 suggesting that these ortho metamorphic rocks were mainly derived from subcontinental lithospheric mantle with some contamination by archean crustal materials since an intra continental environment was required for the formation of the above metamorphic rocks these rocks not only confine the depositional environment of the changyi bif to be an intra continental rift but also support the rifting processes of the eastern ncc during paleoproterozoic

nature of ore forming fluid and formation conditions of bif hosted gold mineralization in the archean amalia greenstone belt south africa constraints from fluid inclusion and stable isotope studies abstract orogenic gold mineralization in the amalia greenstone belt is hosted by oxide facies banded iron formation bif hydrothermal alteration of the bif layers is characterized by chloritization carbonatization hematization and pyritization and quartz carbonate veins that cut across the layers the alteration mineral assemblages consist of ankerite ferroan dolomite minerals siderite chlorite hematite pyrite and subordinate amounts of arsenopyrite and chalcopyrite information on the physico chemical properties of the ore forming fluids and ambient conditions that promoted gold mineralization at amalia were deduced from sulfur oxygen and carbon isotopic ratios and fluid inclusions from quartz carbonate samples associated with the gold mineralization microthermometric and laser raman analyses indicated that the ore forming fluid was composed of low salinity h 2 o co 2 composition 3 wt nacl equiv the combination of microthermometric data and arsenopyrite pyrite geothermometry suggest that quartz carbonate vein formation gold mineralization and associated alteration of the proximal bif wall rock occurred at temperature pressure conditions of 300 30 c and 2 kbar thermodynamic calculations at 300 c suggest an increase in f o 2 10 32 10 30 bars and corresponding decrease in total sulfur concentration 0 002 0 001 m that overlapped the pyrite hematite magnetite boundary during gold mineralization although hematite in the alteration assemblage indicate oxidizing conditions at the deposit site the calculated low f o 2 values are consistent with previously determined high fe fe mg ratios 0 7 in associated chlorite absence of sulfates and restricted positive 34 s values in associated pyrite based on the fluid composition metal association and physico chemical conditions reported in the current study it is confirmed that gold in the amalia fluid was transported as reduced bisulfide complexes e g au hs 2 at amalia gold deposition was most likely a combined effect of increase in f o 2 corresponding to the magnetite hematite buffer and reduction in total sulfur contents due to sulfide precipitation during progressive fluid rock interaction the epigenetic features coupled with the isotopic compositions of the ore forming fluid 34 s s 1 8 to 2 3 18 o h2o 6 6 to 7 9 and 13 c c 6 0 to 7 7 at 300 330 c are consistent with an externally deep sourced fluid of igneous signature or and prograde metamorphism of mantle derived rocks

rare earth element patterns in archean high grade metasediments and their tectonic significance abstract metasediments from two contrasting types of archean high grade terrains are interpreted as being derived from distinct tectonic settings the kapuskasing structural zone canada represents the deep roots of a typical greenstone belt whereas the limpopo province southern africa and western gneiss terrain australia mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism upper amphibolite to granulite fades paragneisses from the kapuskasing structural zone have ree patterns similar to those of greenstone belt sediments except where partial melting has occurred forming restites with eu enrichment and melts with eu depletion except in this latter instance metamorphism has not affected the primary ree patterns ree patterns in archean upper amphibolite to granulite facies metasediments from the central limpopo province and western gneiss terrain show wide differences ranging from patterns which resemble those in post archean terrigenous sediments to typical archean sedimentary rock patterns the diversity in ree patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances samples with post archean patterns displaying eu depletion are interpreted as being derived from k rich granitic plutons which were portions of small stable early archean terrains precursors of the widespread late archean proterozoic episode of major cratonic development

resetting of neodymium isotopes and redistribution of rees during sedimentary processes the early proterozoic chelmsford formation sudbury basin ontario canada abstract the rees abundances and neodymium isotope compositions of sandstones and siltstones from the early proterozoic chelmsford formation reveal that the rees in some samples were redistributed during the early proterozoic by surficial processes and that prior to redistribution the nd isotope system was partially reset the chelmsford formation is a coarse grained turbidite sequence the youngest of three clastic units that fill the sudbury basin in ontario it is constrained to have been derived from nearby late archean superior province rocks by petrography major and trace element chemistry and pb isotopic compositions of whole rocks and feldspar although ree patterns and nd model ages for samples from around the base of the formation reflect their derivation from undisturbed superior province rocks tdm 2800 ma five samples from higher in the stratigraphic section show evidence that up to 60 of their la has been removed and sm nd has increased by up to 60 these samples also have anomalously high nd model ages tdm 3400 to 4400 ma the presence of pronounced positive ce anomalies in the samples with anomalous tdm indicates the influence of surficial processes on the ree composition the data scatter about a vertical array on a plot of f sm nd vs ndt 1800 nd calculated at 1800 ma approximately the depositional age with values of f sm nd between 0 45 and 0 14 mixing of nd from other sources is difficult to reconcile with the petrography major and trace element geochemistry and nd and pb isotopes removal of an lree enriched phase from the sedimentary system provides a viable mechanism to produce the ree and nd isotopic characteristics of the chelmsford formation samples neodymium isotopes require that this phase was partially isotopically equilibrated with the matrix sediment near the time of sedimentation this study shows the dangers of indiscriminate use of nd model ages in provenance studies and demonstrates that the simultaneous examination of sm nd and nd isotopes on f sm nd vs nd or isochron plots is required for the recognition of sedimentary redistribution of the ree