inertial currents estimated from surface trajectories of argo floats 1 we have developed an analysis method to determine the amplitude of inertial motion from surface trajectory data of argo profiling floats this method gives an estimate of inertial current speed and phase from satellite fixes transmitted while a float drifts at the surface over a time which is typically about one local inertial period the error of this method is about 3 4 cm sec for data records with at least 6 positions with accuracy of 500 m spanning longer than 68 of the inertial period whereas inertial amplitudes are as large as 40 cm sec with a median of about 12 0 cm sec in the japan east sea globally distributed argo floats could provide useful information on the co variability of wind forced inertial currents and mixed layer depths in the ocean

the metamorphic internal zone of wopmay orogen early proterozoic canada 30 km of structural relief in a composite section based on plunge projection in wopmay orogen an early proterozoic volcano sedimentary marginal prism and its underlying archean structural basement deformed together during the 1885 ma calderian orogeny are exposed in oblique view as a result of regional cross folding a composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic internal zone of the orogen a sole thrust separates archean basement and a thin mantle of autochthonous early proterozoic strata from the eastward transported calderian allochthon the allochthon is characterized by a transition from external thrust fold belt geometries in the east to polyphase isoclinal recumbent folds and thrusts in the west the center of the zone contains the hepburn batholith which has been been transported along with the deforming hot allochthon over the exposed basement both basement and allochthon are deformed by late large scale upright folds the metamorphic internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened thickened to over 30 km metamorphosed and thrust eastward while still hot over the relatively cool archean craton the basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large scale folding due to continued compression during uplift of the internal zone

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

deposition and alteration of volcaniclastic strata in two large early proterozoic iron formations in canada the gunflint iron formation of western ontario and the sokoman iron formation of the labrador quebec geosyncline both contain interbeds of coarse grained volcaniclastic detritus volcaniclastic beds in the gunflint are typically less than a metre thick and display normal grading and other physical structures typical of high and low density turbidites similar volcaniclastic beds are present in the sokoman as well as thicker accumulations with structures indicative of deposition from high density turbidity currents the volcaniclastic detritus in both iron formations consists largely of well sorted vitric ash and lapilli with accessory holocrystalline grains and solitary feldspar crystals internal textures of the vitric grains plus the presence of armored lapilli in the gunflint suggest they are products of hydroclastic eruptions however the clasts in most beds are heterogeneous and well rounded indicating they are sedimentary rather than eruptive deposits quench textures coalesced vesicles and d

trace element and stable isotope characteristics of algoma type sulfidic banded iron formations from the wutai complex central north china craton abstract the major archive of iron deposits in china occur within precambrian banded irons formations bifs here we investigate the wutai complex that represents an archean to paleoproterozoic subduction related arc system between the ordos and qianhuai micro blocks of the central north china craton ncc we report the carbon and oxygen isotopes of calcites from carbonate rich bifs and in situ trace elements la icp ms and sulfur isotopes ion probe of pyrites from sulfidic bifs and carbonate samples with an aim to characterize the nature and genesis of the iron deposits associated with these rocks the 13cv pdb 18ov pdb and 18ov smow of calcites range from 2 4 to 1 1 20 4 to 19 3 and 9 9 to 11 respectively displaying archean greenstone belt and marine carbonate settings pyrite from the bifs and carbonates are enriched ni as se te and pb their 34scdt values range from 8 8 to 7 9 with a peak at 0 0 comparing with the features of typical algoma type bifs our study provides insights into the depositional environment of the bifs in the wutai complex

tectonic systems and the deposition of iron formation abstract the iron formation group of chemically precipitated cherty iron rich sediments ranges in age from early precambrian to recent and is composed of a variety of lithological facies iron formations are classified as lake superior and algoma types to emphasize the contrasting conditions in the spectrum of sedimentary tectonic environments in which they formed thick stratigraphic units of lake superior type iron formation which form the most extensive iron ranges of the world are part of sedimentary tectonic systems that developed along the margins of cratons or continental platforms and were deposited over extended periods of time under relatively stable tectonic conditions the algoma type iron formations were deposited with volcanic rocks and greywacke in various tectonic volcanic systems formed under more dynamic tectonic conditions but are otherwise comparable to present day spreading ridges on the ocean floor the thick sequences of lake superior type iron formation deposited between 2 7 and 2 0 ga form part of major sedimentary tectonic systems that extended along the margins of continents direct biogenic factors and the composition of the atmosphere probably had only limited influence on the precipitation of these chemical sediments

upper ocean temperature structure inertial currents and richardson numbers observed during strong meteorological forcing abstract upper ocean currents and temperature in the northeastern pacific were measured during a 14 day period in november 1980 as part of strex velocities in the upper ocean are dominated by near inertial frequency oscillations thew oscillations are modified by wind stress variations associated with the passage of a strong cold front the change in the inertial currents both in the mixed layer and below is qualitatively consistent with linear internal wave dynamics if turbulent stresses during the storm are assumed to extend 10 20 m below the mixed layer the ratio of mean squared buoyancy frequency n2 and mean squared shear s2 computed over a 10 m interval defines an average richardson number r0 n2 s2 r0 is approximately 2 5 except in regions of high inertial shear in particular values as low as 0 7 are obtained in a 20 m thick region immediately below the base of the mixed layer the data are consistent with a model of the oecanic shear field consisting of a background shear corresponding to a va

manganese metallogenesis a review abstract metallogenesis of manganese in space and time is reviewed here in the light of the progressive development of the atmosphere the hydrosphere and the lithosphere attendant with the varied styles of tectonism economic deposits of manganese first appeared c 3000 ma ago postdating by at least 800 million years the oldest known geological sequence containing iron formation and base metal sulfide ores the development of manganese deposits in the archean as a whole vis a vis that of iron formation and stratiform massive sulfides was minor this is possibly a reflection of the composition of the then endogenic exhalations and or the character of the atmosphere and the hydrosphere the geologic setting of the manganese deposits of this age was always atypical of the archean period deposition of manganese was intensified with the advent of the proterozoic with the changing tectonic style leading to stabilization of the cratons and oxygenation of the hydrosphere and the atmosphere large to superlarge deposits were formed mainly through terrigenous input during this period the mesozoic era ushered in the supremacy of manganese deposition and the peak was reached in cenozoic time this was largely due to the formation of giant shallow water deposits in areas of marine transgression as well as deposition of manganese rich nodules and crusts in deep sea environments three major aspects of manganese metallogenesis stand out as most important but enigmatic these are a the extreme fractionation of iron and manganese in nature and their reverse trend of metallogenic development b the universal record of shallow water deposition of manganese in land based deposits in contrast with the deep sea milieu observed in modern basins and c the common evidence of biological activity in close association with manganese deposition which could be either causal or casual all these aspects merit further in depth study and metallogenic analysis in a broad spectrum

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

horizontal circulation and volume transports in the weddell gyre derived from argo float data argo floats provide both hydrographic and trajectory data affording the opportunity to investigate surface to mid depth ocean dynamics here argo float data are used to determine the absolute geostrophic velocity field of the upper 50 2000 m of the weddell gyre from which the overall circulation pattern is investigated the weddell gyre plays a pivotal role in the modification of climate by advecting heat towards the antarctic ice shelves and by modifying the water masses that feed into the lowest limb of the global ocean overturning circulation warm deep water the source water mass that delivers heat to the weddell gyre is conveniently located within the upper 2000 m domain covered by the floats we investigate its volume transport as it circulates the gyre full depth volume transports are estimated by applying a quadratic function to extrapolate the relative dynamic height field component to the full ocean depth using ctd profiles from ship based surveys to determine an extrapolation error major new insights are provided by this study there is an established double gyre structure to the circulation with a strong eastern cell and a weaker western cell regional variation of the baroclinic component of the flow field is revealed indicating a northeast to southwest reduction in the baroclinic flow along with a strong meridional gradient of baroclinic flow along the northern limb of the gyre especially east of 25 w the zonal mean gyre strength away from the shelf edge is 32 5 sv 1 sv 1 106 m3 s 1 of which 13 3 sv are associated with the advection of warm deep water there is a considerable amount of recirculation within the gyre interior where water does not traverse the full zonal extent of the gyre the recirculation is stronger in the eastern cell of the observed double gyre structure the interior circulation cells partly explain the large variations in previous gyre strength estimates we provide an extensive review of previous estimates in context of the new results obtained

regional o sr and pb isotope relationships in late cenozoic calc alkaline lavas of the andean cordillera quaternary recent volcanism in the andes has occurred in three regions 45 33 s 28 16 s and 2 s 5 n each of which has a distinct plate tectonic setting and contains volcanic suites with different chemical and isotopic characteristics isotope ratios of o and sr are lowest and those of pb least variable in the southern volcanic zone svz where medium k lavas have isotopic characteristics equivalent to volcanics from intraoceanic arcs where continental crust is absent the svz lavas were probably derived from an asthenospheric mantle source above a shallow benioff zone parental basaltic magmas rose largely unmodified through thin continental crust where differentiation occurred by low pressure crystal fractionation without concurrent modification of isotopic composition the slight enrichment of 206 pb in the northern volcanic zone nvz where the crust is thin and the benioff zone deep suggests a greater subduction zone component for parental medium k magmas the slight 18 o enrichment suggests a small amount of lower crustal interaction isotope ratios of o and sr are highest and those of pb most variable in the central volcanic zone cvz parental magmas in the cvz were probably generated within the same mantle source region as those in the svz and nvz subsequently during transit through the exceptionally thick continental crust of the central andes high k magmas were produced by a combination of bulk contamination in the lower crust and later low pressure fractional crystallization assimilation processes in the upper crust which altered both chemical and isotopic compositions

global climatology of near inertial current characteristics from lagrangian observations 1 satellite tracked surface drifter data from 1999 2006 are used to compute global climatology of inertial current characteristics at seasonal scales the global mean near inertial current amplitude at 15m depth is 10 cm s 1 corresponding to mixed layer inertial energies of 300 j m 2 the southern ocean and the western north pacific and atlantic oceans are the most energetic in the near inertial frequency band whereas weaker inertial activity is observed in the subtropical and eastern boundary regions in every ocean basin inertial activity is higher during fall and winter associated with maximum storms activity and deeper mixed layers this study also shows that the mixed layer model developed by r t pollard and r c millard 1970 and forced by the qscat ncep blended wind product is too energetic in the tropics and not enough at high latitudes these discrepancies could question the previous estimates of the wind work to inertial motions based on those simulations

the energy flux from the wind to near inertial motions in the surface mixed layer abstract time series of wind stress computed from long term meteorological buoy data off north america are used to examine the forcing of surface inertial currents by the wind a simple damped slab model of the mixed layer is used to compute h the average flux of energy from the wind to mixed layer inertial currents in a mixed layer of fixed depth h the forcing of mixed layer inertial motions is highly intermittent most of the forcing occurs during the winter months with a few dozen events accounting for typically over half the total energy flux major forcing events are usually associated with translating cold fronts or small lows with scales of about 100 km the larger synoptic scale features have little energy at the inertial frequency and thus result in only weak forcing of inertial currents a strong seasonal signal exists in the inertial forcing at ows p 50 n 145 w 50 m is largest from october to february and is a factor of 12 above the june and july values if seasonally varying

formation age and genesis of the gongchangling neoarchean banded iron deposit in eastern liaoning province constraints from geochemistry and shrimp zircon u pb dating abstract the gongchang iron deposit a typical algoma type banded iron formation bif deposit located in the eastern block of the north china craton ncc is hosted in neoarchean metamorphic rocks including amphibolite biotite leptynite garnet biotite schist plagioclase hornblende schist garnet chlorite schist muscovite quartz schist and actinolite magnetite quartzite geochemical data suggest that protoliths of the amphibolites and biotite leptynite should be basalt and dacite respectively shrimp u pb dating of zircons from the plagioclase hornblende schist shows that the gongchangling iron deposit was formed at 2554 12 ma major elements and ree data of the ores indicate that the gongchangling bif precipitated from seawater and the ultimate iron and silica origins may have been related to nearby volcanic activity the 18o values of quartz from the bifs range between 9 1 and 12 9 suggesting that exotic high 18o fluids were involved during metamorphism the pyrite have 34s values of range between 3 5 and 14 7 suggesting that mineralization were originally from metamorphism comparison of the mineralization age of the gongchangling bif with those of other local bifs in north china and other cratons throughout the world indicates that the gongchangling bif formed coevally with the global bif mineralization at 2 5 ga taken together these data suggest that the gongchangling iron deposit was formed in a volcanic submarine environment in which hydrothermal fluids associated with volcanic activity supplied both iron and silica for the banded iron formations

recent volcanism in the puyehue cordon caulle region southern andes chile 40 5 s petrogenesis of evolved lavas

nomenclature for iron formations abstract this paper explains the terminology and a classification scheme which are used in an accompanying paper about the genesis of iron formations an iron formation is considered to be a stratigraphic unit which consists mostly of iron rich chemical sedimentary rock specifically rock with more than 15 fe the rock is called ironstone iron formations may be classified into six groups on the basis of the sedimentary environment in which they accumulated some previous authors have used the terms ironstone and iron formation in more restricted ways for example ironstone has been used to name one of the six environmental types of iron formations and iron formation has been used exclusively for chert rich iron formations these restrictions are avoided herein because they are not consistent with the simplicity of the names by analogy with limestone ironstone is simply an iron rich chemical sedimentary rock the word formation clearly refers to a stratigraphic unit and so iron formation is applied to any stratigraphic unit which is composed predominantly of ironstone alternative usages of the terms ironstone and iron formation have been sufficiently varied that none of the usages can be defended on the grounds of overwhelming support independent of semantic arguments

observations of near inertial surface currents off oregon decorrelation time and length scales 1 high resolution km in space and hourly in time surface currents observed by an array of high frequency radars off oregon are analyzed to quantify the decorrelation time and length scales of their near inertial motions the near inertial surface currents are dominantly clockwise with amplitudes of 9 12 cm s 1 however they appear asymmetric and elliptical as a result of counterclockwise inertial motions with magnitudes in a range of 2 5 cm s 1 the decorrelation time and length scales are computed from the decay slope of the near inertial peak and the spatial coherence in the near inertial frequency band respectively decorrelation time scales of clockwise near inertial motions increase from 2 days nearshore within 30 km from the coast to 6 days offshore and their length scales increase from 30 to 90 km seaward possibly due to coastal inhibition the local spatial coherence has an exponentially decaying structure for both clockwise and counterclockwise rotations and their phases propagate northwestward offshore for clockwise and northeastward onshore for counterclockwise rotations

formation age and tectonic setting of the shirengou neoarchean banded iron deposit in eastern hebei province constraints from geochemistry and sims zircon u pb dating abstract the shirengou iron deposit a typical bif deposit located in eastern hebei province in the north china craton is hosted in neoarchean metamorphic rocks the metamorphic rocks include hornblende plagiogneiss plagioclase amphibolite and magnetite quartzite we use contents and ratios of major elements for protolith reconstruction showing that protoliths of the hornblende plagiogneiss and plagioclase amphibolite should be dacite and basaltic andesite respectively based on geology and geochemistry of the hosting rocks we infer that the shirengou bif is an algoma type deposit formed in an arc related basin in the neoarchean constrains from sims u pb dating and oxygen isotopic analysis of zircon from wall rocks and interlayer of the shirengou iron bodies can be summarized as follow 1 igneous zircons from two samples of amphibolite and gneiss yield u pb ages of 2541 21 ma and 2553 31 ma respectively providing the age of the neoarchean protolith because the shirengou bif is an algoma type deposit determined u pb age 2541 2553 ma for zircons from hosting volcanic rocks can constrain the bif forming time 2 metamorphic overgrowth zircons from amphibolite and gneiss give two u pb ages of 2512 13 ma and 2510 21 ma respectively corresponding to the neoarchean matemorphism 3 the zircons with 18o values of 5 9 7 6 also indicate magmatic origin the zircons have 18o values of 6 8 9 9 suggesting that exotic high 18o fluids were involved during metamorphism based on the geology and geochemistry of bif and combing with zircon age and oxygen isotopic systematics we suggest that the shirengou iron deposit was formed in a submarine volcanic setting related to subduction of an oceanic slab

oxygenation of shallow marine environments and chemical sedimentation in palaeoproterozoic peritidal settings frere formation western australia the palaeoproterozoic frere formation ca 1 89 gyr old of the earaheedy basin western australia is a ca 600 m thick succession of iron formation and fine grained clastic sedimentary rocks that accumulated on an unrimmed continental margin with oceanic upwelling lithofacies stacking patterns suggest that deposition occurred during a marine transgression punctuated by higher frequency relative sea level fluctuations that produced five parasequences decametre scale parasequences are defined by flooding surfaces overlain by either laminated magnetite or magnetite bearing hummocky cross stratified sandstone that grades upward into interbedded hematite rich mudstone and trough cross stratified granular iron formation each aggradational cycle is interpreted to record progradation of intertidal and tidal channel sediments over shallow subtidal and storm generated deposits of the middle shelf the presence of aeolian deposits mud cracks and absence of coarse clastics indicate deposition along an arid coastline with significant wind blown sediment input iron formation in the frere formation in contrast to most other palaeoproterozoic examples was deposited almost exclusively in peritidal environments these other continental margin iron formations also reflect upwelling of anoxic fe rich sea water but accumulated in the full spectrum of shelf environments dilution by fine grained windblown terrigenous clastic sediment probably prevented the frere iron formation from forming in deeper settings lithofacies associations and interpreted paragenetic pathways of fe rich lithofacies further suggest precipitation in sea water with a prominent oxygen chemocline although essentially unmetamorphosed the complex diagenetic history of the frere formation demonstrates that understanding the alteration of iron formation is a prerequisite for any investigation seeking to interpret ocean atmosphere evolution unlike studies that focus exclusively on their chemistry an approach that also considers palaeoenvironment and oceanography as well the effects of post depositional fluid flow and alteration mitigates the potential for incorrectly interpreting geochemical data

rare earth element geochemistry of some archean iron formations north of lake superior ontario rare earth element ree compositions of iron formation from two archean terrains in western ontario have been determined in order to assess the possible influence of hydrothermal activity on the ree patterns of chemical sediments one terrain is characterized by sulfide facies iron formation in association with volcanic flows and volcaniclastic sediments whereas the other is dominated by oxide facies iron formation intercalated within submarine fan clastic sediments mineral separates of chert magnetite and pyrite from the iron formations have low ree concentrations 20 30 ppm and display moderate to strong positive eu anomalies relative to archean shale the positive anomalies and lack of negative ce anomalies indicate that archean sea water from which iron formation locally precipitated was reduced although to varying degrees the ree patterns of mineral separates from a given locality are almost identical but the patterns for various localities differ in detail a number of iron formation

dynamic sedimentation of paleoproterozoic continental margin iron formation labrador trough canada paleoenvironments and sequence stratigraphy abstract the paleoproterozoic sokoman formation ca 1 88 ga of the labrador trough eastern canada is a ca 100 m thick succession of interbedded iron formation and fine grained terrigenous clastic sedimentary rocks detailed examination of drill cores and outcrops indicates a dynamic paleoshelf where an oxygen stratified water column coastal upwelling of hydrothermally derived fe and si as well as tide and storm generated currents controlled lithofacies character vertical and lateral facies stacking patterns record deposition through two relative sea level cycles that produced seven distinct lithofacies comprising two unconformity bounded sequences sequence 1 reflects deposition of hematitic peritidal iron formation as deep as the upper shoreface sequence 2 is truncated by later erosion and encompasses the change to deeper water accumulation of magnetite and fe silicate rich iron formation the character and lateral distribution of redox sensitive facies indicate that iron formation accumulation was controlled as much by shelf hydraulics as oxygen levels the development of a suboxic surface ocean is interpreted to reflect photosynthetic oxygen production from a combination of peritidal stromatolites and cyanobacterial phytoplankton that flourished in nutrient rich upwelled waters offshore deposition of other continental margin iron formations also occurred on paleoproterozoic shelves that were favorably positioned for coastal upwelling variability between iron formations reflects intrinsic factors such as shelf profile fluvial contribution eolian input evaporation rates and coastal current systems which influenced upwelling dynamics and the delivery of fe si and nutrients aridity onshore was a primary depositional control since it governed the transport and type of diluting terrigenous clastics as well as evaporative precipitation along the coastline as in the phanerozoic unconformities and transgressive and maximum flooding surfaces frame iron formation sequences but with important differences the absence of trace and body fossils as well as lack of terrestrial vegetation can make the recognition of these surfaces difficult transgressive surfaces can also be easily mistaken for phanerozoic style maximum flooding surfaces since stratigraphic condensation was restricted to inboard environments during ravinement outboard the accumulation of fresh precipitates increased sedimentation to produce a maximum flooding surface not usually marked by a prominent depositional hiatus understanding these differences is essential for establishing an accurate sequence stratigraphic framework such context is the backdrop for properly interpreting the sedimentology oceanography microbial ecology and geochemistry of continental margin iron formations

neoarchean algoma type banded iron formations from eastern hebei north china craton shrimp u pb age origin and tectonic setting abstract algoma type bif hosted iron deposits occur in the shirengou xingshan erma lingshan and sijiaying areas of eastern hebei the deposits are hosted in archean rocks and their formation was related to volcanic and sedimentary processes in small volcano sedimentary basins supracrustal rocks of the qianxi and dantazi groups were metamorphosed from granulite facies to amphibolites facies available data indicate that these bif deposits mainly formed during paleoarchean and neoarchean times the paleoarchean bif hosted iron deposits are represented by the xingshan deposit that yielded a shrimp u pb zircon age of 3389 5 7 6 ma the other bif hosted iron deposits formed in the late neoarchean represented by the shirengou shuichang lingshan erma and sijiaying deposits that formed between 2605 ma and 2503 ma the late neoarchean bif iron deposits are considered to have formed under a mantle plume environment the eu anomalies of the ore samples from eastern hebei show that the bif ores formed in a hydrothermal related volcanic submarine environment where submarine hydrothermal effluents associated with volcanic activities may have supplied both the iron and silica for the bifs from eastern hebei

lead lead age of komatiitic lavas and limitations on the structure and evolution of the precambrian mantle abstract using lead isotopes we have studied komatiites and associated sulfides from three complexes at barberton the lead lead isochron gives an age of 3 46 0 07gy in good agreement with the sm nd age at munro township three distinct komatiitic flows yield significantly different ages pyke hill is 2 72 0 02gy theo s flow is 2 47 0 13gy and fred s flow is 2 58 0 02gy old 2 72 gy is the emplacement age while the alignment obtained on theo s flow and fred s flow clearly points out the effect of a metamorphism posterior to 2 7 gy at cape smith the age obtained by the lead lead method is 1 6 0 13gy this age is younger than that obtained by other methods in contrast the isochron obtained on sulfide gives an age of 1 88 0 17gy in agreement with sm nd methods these results together with others from the literature are used to describe the evolution of pb in the mantle the important result inferred is that the mantle 2 70 gy ago was very different from the primitive mantle according to both th u and u pb values indeed the latter are lower than the planetary values this evolution is the result of the formation of the continental crust

tidal currents and eddy statistics from iceberg trajectories off labrador extensive data on the trajectories of icebergs located off the coast of labrador has yielded information on tidal currents the effect of wind on iceberg motion and the properties of the low frequency eddies statistical properties of the data were shown to be related to theoretical ideas on two dimensional turbulence and to the connection between eulerian and lagrangian velocity statistics the results have led to an estimate of the cross shelf mixing rate and a statistical prediction scheme for iceberg trajectories

riverine mixing and fluvial iron formation a new type of precambrian biochemical sediment precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early earth the appearance of large economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean atmosphere system near the end of the archean exhalative iron formation contains a record of hydrothermal vent chemistry through time we introduce here fluvial iron formation a new type of fe rich microbial biochemical sediment that formed by mixing river discharge and seawater in coastal environments the paleoproterozoic chiall formation ca 1 8 ga earaheedy basin western australia contains laminated and granular hematitic iron formation in delta channel deposits where mixing occurred in adjacent peritidal settings laminated iron formation and hematitic oncoids formed because fluvial iron formations precipitated at the interface between terrestrial and marine realms the locus of known fe precipitation processes is shifted landward into paleoestuarine settings and reflects fe derived from both terrestrial weathering and coastal upwelling providing a new window into ocean atmosphere evolution

timing and tectonic setting of the sijiaying banded iron deposit in the eastern hebei province north china craton constraints from geochemistry and sims zircon u pb dating the north china craton ncc one of the oldest continental blocks in asia has a complicated evolutionary history with the age of the old crust up to 3 8 ga and records the important geological events of the earth the sijiaying bif the largest banded iron formation located in eastern hebei province in the east part of ncc records an important thermo tectonic event previous studies focused on the geological description but the timing and tectonic setting have been unclear so far the sijiaying bif is hosted in neoarchean metamorphic rocks which includes biotite leptynite hornblende plagioclase gneiss and biotite plagioclase gneiss using major element contents and ratios of the host rocks the protoliths of the hornblende plagioclase gneiss and biotite plagioclase gneiss are shown to be dacite whereas those of the biotite leptynites are sedimentary rocks based on geology and geochemistry of the host rocks we infer that the sijiaying bif is an algoma type deposit sims zircon u pb dating shows igneous zircons from the biotite plagioclase gneiss and hornblende plagioclase gneiss have u pb ages of 2535 8 ma and 2543 14 ma respectively zircons from the biotite leptynite have an age of 2537 13 ma we infer that the ages of 2543 2535 ma represent the time of the sijiaying bif paas normalized rey profiles of the ore samples are characterized by lree depletion and hree enrichment positive la and eu anomalies and an average y ho weight ratio of 32 indicating a mixture of submarine hydrothermal fluids and deep seawater ree data was combined with the 18o analyzed by previous researcher of the ore from individual magnetite bands to infer that the sijiaying bif precipitated from hydrothermal fluids discharging on the sea floor the hornblende plagioclase gneiss and biotite plagioclase gneiss that are enriched in lile and lree and depleted in high field strength elements hfse nb ta and ti have geochemical signatures that are similar to those of island arc volcanic rocks the biotite leptynite shows the characteristics of continental island arc based on the geology and geochemistry of associated wall rocks and ore samples we suggest that the sijiaying bif formed in a back arc basin in neoarchean

timing of deposition and tectonothermal events of banded iron formations in the anshan benxi area liaoning province china evidence from shrimp u pb zircon geochronology of the wall rocks abstract most of the algoma type bifs and associated volcanic suites in the north china craton ncc have undergone high grade metamorphism during the 2 5 ga tectonothermal event isotopic systems in high grade metamorphic rocks are often reset thereby losing the earlier records whereas zircons in low grade metamorphic rocks are potential candidates to evaluate earlier thermal events detailed textural relationship and internal structures of zircons and their age data for eight wall rock samples with metamorphic grades from greenschist to upper amphibolite facies of the algoma type bifs in the anshan benxi area are used to constrain the age of protolith zircons in the greenschist to lower amphibolite facies are generally metamorphism resistant whereas those in the upper amphibolite facies rocks show variable recrystallization shrimp zircon u pb dating of magmatic zircons in six samples constrains the peak of bif deposition at 2 56 2 53 ga which is consistent with 2 5 ga crustal growth in the ncc in addition the greenschist facies metamorphic rocks in the anshan area also record an event of volcanic activity at 2 72 2 65 ga we consider that low grade metamorphic wall rocks were not significantly affected by the 2 5 ga tectonothermal event and the 2 72 2 65 ga age group records another important magmatic event corresponding to 2 7 ga crustal growth the metamorphic zircons from these rocks record a thermal event during 1 88 1 84 ga

iron formations a global record of neoarchaean to palaeoproterozoic environmental history abstract iron formations if represent an iron rich rock type that typifies many archaean and proterozoic supracrustal successions and are chemical archives of precambrian seawater chemistry and post depositional iron cycling given that if accumulated on the seafloor for over two billion years of earth s early history changes in their chemical mineralogical and isotopic compositions offer a unique glimpse into environmental changes that took place on the evolving earth perhaps one of the most significant events was the transition from an anoxic planet to one where oxygen was persistently present within the marine water column and atmosphere linked to this progressive global oxygenation was the evolution of aerobic microbial metabolisms that fundamentally influenced continental weathering processes the supply of nutrients to the oceans and ultimately diversification of the biosphere and complex life forms many of the key recent innovations in understanding if genesis are linked to geobiology since biologically assisted fe ii oxidation either directly through photoferrotrophy or indirectly through oxygenic photosynthesis provides a process for if deposition from mineral precursors the abundance and isotope composition of fe ii bearing minerals in if additionally suggests microbial fe iii reduction a metabolism that is deeply rooted in the archaea and bacteria linkages among geobiology hydrothermal systems and deposition of if have been traditionally overlooked but now form a coherent model for this unique rock type this paper reviews the defining features of if and their distribution through the neoarchaean and palaeoproterozoic this paper is an update of previous reviews by bekker et al 2010 2014 that will improve the quantitative framework we use to interpret if deposition in this work we also discuss how recent discoveries have provided new insights into the processes underpinning the global rise in atmospheric oxygen and the geochemical evolution of the oceans

barotropic and baroclinic tidal currents in the eastern basins of the north atlantic data from a large scale moored array in the iberian and canary basins are used to determine the energies of barotropic and baroclinic m2 and s2 tides an analysis of time varying dynamical modes is performed the results for barotropic modes confirm the global surface tide model results of schwiderski 1980 for this region the barotropic modes dominate in the deep basins but increased baroclinic contributions are usually found over rough topography at three locations near the continental slope in the southern canary basin the baroclinic modes dominate the barotropic mode results from an array of three moorings at the northern part of the cape verde rise show an inverse behavior of barotropic and baroclinic energies such that the baroclinic energy is steadily enhanced while the barotropic energy is reduced towards the continental margin the increase in baroclinic energy is consistent with a generation of internal tides close to the shelf by surface tidal forcing over topography further evidence for this process is provided by the 2 week periodicity of the first order baroclinic mode at the slope corresponding to the spring neap cycle of the barotropic tide

influence of weathering and hydrothermal alteration on the ree and δ56fe composition of iron formation caue formation iron quadrangle brazil abstract continental margin superior type iron formation of the caue formation iron quadrangle brazil is composed of a laminated magnetite rich chemical meta mudstone that was altered during hydrothermal circulation of basinal fluids greenschist to amphibolite facies metamorphism and subsequent supergene enrichment of fe the absence of traction current deposits and storm generated structures suggests that deposition occurred on the distal shelf possibly as deep as the continental slope paragenesis based lithogeochemical analyses of two drill cores with contrasting alteration styles highlight the importance of hydrothermal metamorphic and supergene processes on the rare earth element plus yttrium ree y and 56fe compositions of iron formation the first drill core penetrates to a depth of 540 m in the ouro fino syncline ofs where lithofacies have experienced intense hydrothermal and supergene alteration the second core is from the gandarela syncline gs and contains 450 m of metamorphosed iron formation that has undergone later supergene enrichment of fe petrographically euhedral to subhedral magnetite in the ofs and gs is fabric destructive and depending on the style and degree of alteration can co occur with dolomite siderite goethite and or martite synsedimentary magnetite and other primary fe oxide phases are absent the effect of supergene enrichment and weathering is best captured along the depth profile of each core by the inverse relationship between sio2 and fe2o3 sio2 values are lowest and fe2o3 values highest near the surface the presence of goethite to 500 m records the maximum depth of these processes in the subsurface the ree y and y anomalies from the deepest and least altered lithofacies in both cores are typical of deposition in a redox stratified water column the weathered lithofacies show several changes in the ree y patterns ce anomalies and absence of y anomalies caused mainly by fluid percolation the change in fe isotopic composition with depth is more complex in drill core from the ofs negative 56fe values shift from 0 91 at depth to less negative values of 0 6 near the surface the most altered samples do not lie on this trend because they are characterized by positive values of 0 30 and 1 33 the 56fe values from the gs increase linearly from 0 11 to 1 23 towards the surface with the most negative values 0 7 and 1 16 near the top the most negative values from the ofs could reflect synsedimentary microbial dissimilatory fe reduction and the positive values from the gs may be related to partial fe2 oxidation between the hydrothermal fe source and fe oxyhydr oxides however when interpreted together with mineralogical and ree data these data likely record the various alteration processes that affected the caue iron formation such findings highlight the importance of coupling sedimentologic analysis with elemental and isotopic chemistry to understand iron formation deposition and fe ore forming processes

the role of sedimentology oceanography and alteration on the δ56fe value of the sokoman iron formation labrador trough canada abstract the sokoman formation is a ca 100 m thick succession of interbedded iron formation and fine grained siliciclastics deposited at 1 88 ga accumulation occurred on a dynamic paleoshelf where oxygen stratification coastal upwelling of hydrothermally derived fe and si microbial processes tide and storm currents diagenesis and low grade prehnite pumpellyite metamorphism controlled lithofacies character and produced complex associations of multigenerational chert hematite magnetite greenalite stilpnomelane and fe carbonate hematite rich facies were deposited along suboxic segments of the coastline where photosynthetic oxygen oases impinged on the seafloor hematitic cross stratified grainstones were formed by winnowing and reworking of freshly precipitated fe oxyhydr oxide and opal a by waves and currents into subaqueous dunes magnetite rich facies contain varying proportions of greenalite and stilpnomelane and record deposition in anoxic middle shelf environments beneath an oxygen chemocline minor negative ce anomalies in hematitic facies but prominent positive ce and eu anomalies and high lree hree ratios in magnetite rich facies imply the existence of a weakly oxygenated surface ocean above anoxic bottom waters the fe isotopic composition of 31 whole rock 0 46 56 fe 0 47 and 21 magnetite samples 0 29 56 fe 0 22 from suboxic and anoxic lithofacies was controlled primarily by the physical oceanography of the paleoshelf despite low grade metamorphism recorded by the 18 o values of paragenetically related quartz and magnetite the sokoman formation preserves a robust primary fe isotopic signal coastal upwelling is interpreted to have affected the isotopic equilibria between fe 2 aq and fe oxyhydr oxide in open marine versus coastal environments which controlled the fe isotopic composition of lithofacies unlike previous work that focuses on microbial and abiotic fractionation processes with little regard for paleoenvironment our work demonstrates that depositional setting is paramount in governing the fe isotopic composition of iron formations irrespective of what fe bearing minerals precipitated

banded iron formations through much of precambrian time similarities between proterozoic 1 8 2 5 gyr and archean 2 5 gyr banded iron formations are probably more significant than their differences the contrasts largely reflect differences in the tectonic settings of proterozoic and archean terrains archean banded iron formations are not as thick nor laterally as extensive as the major proterozoic iron formations nevertheless some archean iron formations have strike lengths of over 150 200 km and may have been quite extensive prior to the deformation that has affected most archean terrains stratigraphic sequences in which iron formations occur are highly variable and indicate that iron formations formed in many depositional environments sedimentary textures in the iron formations are dominated either by granules and oolites or laminations including microbanding reflecting differences in their physical conditions of deposition granular and oolitic textures are abundant in only three proterozoic depositional basins and most precambrian iron forma

age and genesis of the neoarchean algoma type banded iron formations from the dengfeng greenstone belt southern north china craton geochronological geochemical and sm nd isotopic constraints abstract banded iron formations bifs are important for understanding the composition and redox condition of paleo oceans as well as the evolution of the precambrian crust the neoarchean dengfeng greenstone belt ca 2 54 2 50 ga in the southern north china craton ncc consists of metamorphosed igneous and sedimentary assemblages including some bifs here we present mineralogical geochronological geochemical and sm nd isotopic data of the dengfeng bifs for the first time to constrain their age origins and source characteristics and then discuss their paleo environmental implications the bifs are mainly associated with metabasaltic rocks and quartz rich schists and can be classified as algoma type bifs zircon u pb dating of quartz rich schists interbedded with the bifs coupled with ages of wall rock metabasites demonstrate that the dengfeng bifs formed at 2 53 2 51 ga most bif samples have high contents of sio2 and fe2o3t and extremely low contents of al2o3 tio2 and high field strength elements e g zr hf th u indicating that they are detrital free deposits in the post archean australia shale normalized multi element diagram the detritus free bif samples are characterized by depleted light rare earth elements positive la and y anomalies and suprachondritic y ho ratios resembling geochemical features of modern seawater the absence of negative ce anomaly indicates an anoxic condition positive eu anomalies and relatively high concentrations of transition metal elements suggest that high temperature hydrothermal fluids contributed to the source of bifs isotopically all bif samples have positive end t values 0 12 to 4 53 consistent with the range of those 0 10 to 4 31 of ca 2 66 2 50 ga dengfeng metabasites and ttg gneisses suggesting input of possible juvenile crustal fluxes the elemental and isotopic characteristics of the dengfeng bifs collectively suggest that they precipitated from anoxic seawater with various input of hydrothermal fluids and oceanic crust derived fluxes

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

genesis of the hanwang fe deposit in neoarchean granite greenstone succession of the eastern north china craton abstract research on history of the earth s continents and the genesis of mineral deposits has been one of the hotspots in geology but only limited studies have been completed on iron deposits hosted by banded iron formations bifs such as the large hanwang iron deposit located in the eastern north china craton ncc in this contribution we present the petrology zircon u pb geochronology lu hf isotopes and whole rock geochemistry on hanwang iron deposit hosted by the metamorphosed neoarchean to early paleoproterozoic taishan group and orthogneiss derived from tonalite trondhjemite granodiorite ttgs magnetite bearing amphibolite magnetite bearing quartz amphibolite and gneiss host the hanwang iron deposit which is characterised by subhedral to euhedral magnetite pyrite with minor galena and bif laminations gangue minerals include hornblende plagioclase and quartz with minor amounts of biotite chlorite epidote and microcline this mineral assemblage is indicative of amphibolite facies metamorphism with regression to greenschist facies the deposit is located in a transition zone between metamorphosed mafic to felsic volcanic rocks the age of the hanwang bif hosted iron deposit is late neoarchean lu hf isotope systematics indicate that the protolith for the biotite amphibolite is derived from a neoarchean mantle contaminated by the crust and orthogneiss is derived from the neoarchean crust the biotite amphibolite is characterised by high na fe and low k assays enrichment in light rare earth elements lree and relatively depletion in heavy rees hree without an obvious ce anomaly the geochemistry of the orthogneiss is characteristic of a volcanic arc granite although this must be accepted with caution given that the orthogneiss is metamorphosed and deformed at amphibolite facies it is proposed that hanwang iron deposit was deposited in a marine volcanic arc environment and derived from hydrothermally leached volcanic units and similar to the algoma type iron deposit most bifs occurring in neoarchean granite greenstone succession of the eastern ncc belong to the algoma type iron deposits globally this sequence appears to be coeval with the yilgarn craton in western australia

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

precambrian iron formations from the cauvery suture zone southern india implications for sub marine hydrothermal origin in neoarchean and neoproterozoic convergent margin settings abstract thick horizons of iron formations including banded iron formations bifs and banded silicate formations bsfs occur as e w trending bands in the eastern part of cauvery suture zone csz in the sothern granulite terrane of india some of these occur in close association with the neoarchean neoproterozoic suprasubduction zone complexes where as some others are associated with metamorphosed accretionary sequences including pyroxene granulites and other high grade rocks the iron formations are highly deformed and metamorphosed under amphibolite to granulite facies conditions and are composed of quartz magnetite hematite goethite garnet pyrite together with grunerite and pyroxene here we report the geochemical characteristics of twenty representative samples from the iron formations that reveal a widely varying composition with fe 2 o 3 t 22 65 wt as total iron total fe 2 o 3 tio 2 205 6532 mno tio 2 0 25 12 66 and sio 2 33 85 wt broadly representing the two types of iron formations these formations also show very low al al fe mn ratio 0 001 0 01 al 2 o 3 0 07 0 76 wt al 2 o 3 tio 2 ratio 2 7 21 mgo 0 01 4 41 wt cao 0 1 1 24 wt na 2 o 0 01 0 05 wt and k 2 o 0 01 wt together with low total ree 3 38 31 63 ppm the trace and ree elemental distributions show wide variation with high ni 274 ppm and zn contents up to 87 ppm when compared to mafic volcanics of the adjoining areas tectonic discrimination plots indicate that the iron formations of the cauvery suture zone are of hydrothermal origin their chondrite normalized patterns show slight positive eu anomaly eu eu up to 1 77 and relatively less fractionation of ree with slight lree enrichment compared to hree however the paas post archean average of australian sediments normalized ree patterns display significant positive eu anomaly eu eu up to 2 32 with well represented negative ce anomalies ce ce 0 66 1 28 the above results together with petrological characteristics and available geochronology of the associated lithologies suggest that the iron formations can be correlated to algoma type the fe and si were largely supplied by medium to high temperature sub marine hydrothermal systems in neoarchean and neoproterozoic convergent margin settings

lead isotopic study of early proterozoic wopmay orogen nw canada role of continental crust in arc magmatism lead isotopes are sensitive indicators of crustal interaction in arc magmatism because of the large contrast in lead concentration between mantle derived magmas and continental crust and the three independent decay paths in the u th pb system preserved within wopmay orogen 1 94 to 1 84 ga are the remnants of three magmatic arcs the isotopic composition of lead in alkali feldspars separated from a diverse suite of samples from all three arcs fall in a relatively tight cluster with 206 pb 204 pb 15 59 15 88 207 pb 204 pb 15 27 15 36 and 208 pb 204 pb 35 15 35 54 except for samples from the only plutonic suite within wopmay orogen observed to intrude archean crust plutons intruding archean crust have 206 pb 204 pb 15 29 15 49 206 pb 204 pb 15 23 15 31 and 208 pb 204 pb 35 21 35 30 lead isotopic compositions do not correlate with rock composition both diorites and peraluminous granites have the same lead isotopic composition this isotopic composition

aleutian lead isotopic data additional evidence for the evolution of lithospheric plumbing systems abstract lead isotopic ratios and concentrations have been measured in lavas from the aleutian volcanic centers of adak 12 and atka 12 whereas lavas from atka have very small pb isotopic ranges 206 pb 204 pb 18 78 18 86 207 pb 204 pb 15 55 15 62 208 pb 204 pb 38 31 38 52 those from adak have ranges nearly twice as large 206 pb 204 pb 18 34 18 86 207 pb 204 pb 15 47 15 63 208 pb 204 pb 37 81 38 56 on lead lead isotopic diagrams atka data form tight clusters whereas those from adaak define a well developed linear trend extending from the atka field toward less radiogenic values at a given 206 pb 204 pb the lavas from both islands plot in the morb trend for 208 pb 204 pb but atka lavas plot above the morb 207 pb 204 pb 206pb 204pb trend and those from adak fall both within and above the trend lead contents in lavas from atka increase four fold 5 21 ppm over the compositional range of the volcanic suite 49 68 sio2 in contrast actak lavas 49 65 have concentration levels of 0 6 13 ppm and display no simple correlation with sio2 the lead isotopic data alone can be explained by three different processes model 1 assigns lead isotopic differences to original magma source heterogeneity according to model 2 the isotopic ratios of a primary non radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab derived component in contrast model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non radiogenic lithospheric component during conduit formation because these models all adequately explain the lead isotopic data supplementary geologic petrographic geochemical and isotopic data must be used to select the most likely model careful consideration of the evidence suggests model 3 best explains our extensive lead isotopic data as well as other characteristics of aleutian lavas our study suggests detailed isotopic studies of individual volcanic centers arc or otherwise can be extremely useful in understanding the complex processes of magma generation extraction ascent and evolution

the joffre banded iron formation hamersley group western australia assessing the palaeoenvironment through detailed petrology and chemostratigraphy abstract the joffre member of the brockman iron formation is by volume the largest single known banded iron formation bif in the world here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this 2 45 billion year old unit oxide bif and silicate carbonate oxide bif dominate the lithology with minor amounts of interbedded stilpnomelane mudrock stilpnomelane rich tuffaceous mudrock and calcareous mudrock besides chert and magnetite the prominent mineralogy is riebeckite ankerite hematite stilpnomelane and crocidolite the bif is characterized by an average of 50 wt sio 2 and 44 5 wt fe 2 o 3 and an overall low abundance of al 2 o 3 2 sn of 0 24 the rey patterns also show a positive la sn anomaly no ce sn anomaly and a weakly developed positive y sn anomaly iron isotopes 56 fe with positive 56 fe values of 0 04 to 1 21 suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides no epiclastic grains have been found rather submarine hydrothermal fluids and fine grained volcanogenic detritus controlled bif chemistry the former source is reflected through a constant positive eu sn anomaly throughout the core average eu sn anomaly of 1 6 with a peak of 2 1 between 100 and 155 m depth while the latter source is best reflected through the stilpnomelane rich tuffaceous mudrock consisting of volcanic ash fall tuff with relict shards set in a stilpnomelane matrix the mudrock is overlain by well preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re worked volcanic ash formed either on the seafloor or in the water column prior to deposition an enriched hree to lree pattern a high iron content 30 wt and a 56 fe value of 0 59 collectively imply that the mudrock facies interacted with the fe rich seawater prior to deposition the tio 2 zr ratio of the bif and the associated mudrocks suggest a felsic only source related to the same style of volcanics as the slightly younger woongarra rhyolites given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics we speculate that the fine grained pelagic ash particles may have sourced bio available nutrients to the surface water this would have facilitated enhanced biological productivity including bacterial fe ii oxidation which is now recorded as the positively fractionated 56 fe iron oxide minerals in the joffre bif alongside submarine hydrothermal input to the basin the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways thereby making the joffre bif poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses

turbidites and iron formations beardmore geraldton ontario application of a combined ramp fan model to archaean clastic and chemical sedimentation sandy turbidites grain flows conglomeratic mass flows and oxide facies iron formation are present in the late archaean beardmore geraldton terrain a metasedimentary belt which extends for at least 80 km in an e w direction the marine portion of this basin contains four lithofacies associations la 1 thinbedded iron formation clastic sediment association this association represents a continuum of deposit types containing iron formation subtypes are defined on the basis of bedding attributes and the proportion of iron formation to sand silt 2 thin bedded turbidite dominated association these sediments consist mostly of silt sand beds which either show no vertical trends or thin and fine upwards over a few metres 3 medium bedded turbidite dominated association most of these sediments are medium to coarsegrained vertically unstructured sand sequences with occasional structured intervals 4 thick bedded association this is dominated by poorly graded sands up to 7 8 m in thickness sand beds are characterised by a thin basal zone of coarse sand and pebbles a large central interval containing a mixture of medium and coarse sand and a thin upper zone of fine sand silt the overall depositional system was initiated by transport of sediment by braided streams to the strand area where it accumulated in distributary mouth bars we infer a nearshore break in slope locally with large channels la 4 extending from close to the strand line across deltaic surfaces to the deeper portions of submarine fans structured portions of la 3 however many deltaic surfaces probably were not tapped by major channels but merged downslope into a submarine ramp sediment was transported across the ramp by slump events and sheet like grain flows unstructured portions of la 3 iron formation and la 2 sediments probably accumulated both in upper mid ramp areas with low sediment delivery rates and distal to fan ramp successions as major streams on the braid plain changed position associated submarine channels and slump fed ramp deposits also would have shifted laterally this produced overlap of different facies associations in both the fan and ramp environments which may explain why observed vertical trends in bedding are limited to several metres we suggest that on narrow active archaean cratonic margins additional non regular processes such as variations in sediment supply related to periods of heightened pyroclastic activity and seismic activity associated with the arc also contributed to the lack of vertically structured sequences turbidite sequences in such environments in general may contain important contributions from both submarine ramp and submarine fan sedimentation

oxygenation of the earth s atmosphere ocean system a review of physical and chemical sedimentologic responses abstract the great oxidation event goe is one of the most significant changes in seawater and atmospheric chemistry in earth history this rise in oxygen occurred between ca 2 4 and 2 3 ga and set the stage for oxidative chemical weathering wholesale changes in ocean chemistry and the evolution of multicelluar life most of what is known about this important event and the subsequent oxygenation history of the precambrian earth is based on either geochemistry or data mining published literature to understand the temporal abundance of bioelemental sediments bioelemental sediments include iron formation chert and phosphorite which are precipitates of the nutrient elements fe si and p respectively because biological processes leading to their accumulation often produce organic rich sediment black shale can also be included in the bioelemental spectrum thus chemistry of bioelemental sediments potentially holds clues to the oxygenation of the earth because they are not simply recorders of geologic processes but intimately involved in earth system evolution chemical proxies such as redox sensitive trace elements cu cr v cd mo u y zn and ree s and the ratio of stable isotopes 56fe 53cr 97 95mo 98 95mo 34s 33s in bioelemental sediments are now routinely used to infer the oxygenation history of paleo seawater the most robust of these is the mass independent fractionation of sulfur isotopes mif which is thought to have persisted under essentially anoxic conditions until the onset of the goe at ca 2 4 ga since most of these proxies are derived from authigenic minerals reflecting pore water composition extrapolating the chemistry of seawater from synsedimentary precipitates must be done cautiously paleoenvironmental context is critical to understanding whether geochemical trends during earth s oxygenation represent truly global or merely local environmental conditions to make this determination it is important to appreciate chemical data are primarily from authigenic minerals that are diagenetically altered and often metamorphosed because relatively few studies consider alteration in detail our ability to measure geochemical anomalies through the goe now surpasses our capacity to adequately understand them in this review we highlight the need for careful consideration of the role sedimentology stratigraphy alteration and basin geology play in controlling the geochemistry of bioelemental sediments such an approach will fine tune what is known about the goe because it permits the systematic evaluation of basin type and oceanography on geochemistry this technique also provides information on how basin hydrology and post depositional fluid movement alters bioelemental sediments thus a primary aim of any investigation focused on prominent intervals of earth history should be the integration of geochemistry with sedimentology and basin evolution to provide a more robust explanation of geochemical proxies and ocean atmosphere evolution

deriving inertial wave characteristics from surface drifter velocities frequency variability in the tropical pacific two techniques have been developed for estimating statistics of inertial oscillations from satellite tracked drifters these techniques overcome the difficulties inherent in estimating such statistics from data dependent upon space coordinates that are a function of time application of these techniques to tropical surface drifter data collected during the norpax epocs and toga programs reveals a latitude dependent statistically significant blue shift of inertial wave frequency the latitudinal dependence of the blue shift is similar to predictions based on global internal wave spectral models with a superposition of frequency shifting due to modification of the effective local inertial frequency by the presence of strongly sheared zonal mean currents within 12 of the equator

depositional environment and origin of the lilaozhuang neoarchean bif hosted iron magnesite deposit on the southern margin of the north china craton the neoarchean lilaozhuang iron magnesite deposit is located in the middle of the huoqiu banded iron formation bif ore belt in anhui province on the southern margin of the north china craton the huoqiu bif is the unique one that simultaneously develops quartz type silicate type and carbonate type magnetite in the region the lilaozhuang deposit is characterized by magnesium rich carbonate magnesite in magnetite ores the bif hosted iron ores include mainly of silicate type and carbonate type with a small amount of quartz type which chiefly exhibit banded and massive structure with minor disseminated structure the magnesite ores occur as crystal like bright white and exhibits massive structure the y ho ratio and rey pattern of both iron and magnesite ores are similar to that of seawater while eu shows positive anomaly which is the sign of seafloor hydrothermal mixture these features suggest that ore forming materials of iron and magnesium in the lilaozhuang deposit are mainly from the mixture of seafloor hydrothermal and seawater both ores do not exhibit negative ce anomaly which indicates that the deposit was formed in an environment showing a lack of oxygen c o isotopic compositions indicate that magnesite ore has been reformed by metamorphism of low amphibolite facies and later hydrothermal alteration based on the comprehensive analysis authors suggest that iron and magnesite ores in the lilaozhuang deposits formed in a confined sea basin on continental margin and was influenced by later complex geological processes

petrography and geochemical behaviour of trace element ree and precious metal signatures of sulphidic banded iron formations from the chikkasiddavanahalli area chitradurga schist belt india abstract the 1 2 km long and 250 m wide chikkasiddavanahalli c s halli hill range consists of mixed sulphidic oxide banded iron formations bifs and fe rich phyllites carbonaceous which overlie carbonated schistose and massive meta volcanics in stratigraphic succession the lithologies represent the ingaldhal formation and are an integral part of the chitradurga schist belt in the western dharwar craton the general strike at c s halli varies from n s to 340 with vertical to steep dips towards east and west the sulphides oxides and silicates exhibit intergrowth replacement textures developed during regional greenschist and amphibolites facies metamorphism the bifs show mesobands of recrystallised cherts and iron sulphides such as pyrite arsenopyrite and silicates such as subordinate grunerite hornblende chlorite muscovite actinolite and minor carbonates such as ankerite calcite and magnesian siderite chemical data indicate depletion in ti mn co cu cr and ni in these iron formations most chondrite normalized ree patterns of the iron formation show moderate lree and hree enrichment coupled with strong positive eu anomaly the mineralized portions exhibit characteristic negative ce and eu anomalies eu eu 0 21 to 3 00 the total ree abundance varies correlates well with the iron contents of the bifs and similar to those exhibited by hydrothermal plumes chown e h dah e n mueller w g 2000 the relation between iron formation and low temperature alteration in a archean volcanic environment precambrian research 101 263 275 trace and ree data suggest that primary mantle derived hydrothermal solutions were added to the pore fluids of sediments of the chitradurga basin and supplied chemical constituents such as feo sio 2 and ree oxidation of feo to fe 2 o 3 was caused by the photosynthesis of primitive stromatolite building cyanobacteria geochemical data suggest a model involving epigenetic gold mineralisation in close association with shear zone deformation quartz calcite vein activity and sulphidation in the mixed sulphide oxide facies bif and associated iron phyllites in the c s halli area western dharwar craton india

lead isotopic fingerprinting of tectono stratigraphic terranes east central alaska common lead isotopic compositions have been determined on feldspars from meta igneous rocks from nine tectono stratigraphic terranes or subterranes in east central alaska most of the terranes have distinct and well defined signatures in terms of isotopic composition thus most can be distinguished on conventional lead isotopic diagrams lead isotopic ratios provide evidence for 1 possible sources for the igneous rocks 2 time of metamorphism 3 correlation of terrane fragments and 4 delineation of juxtaposed terranes determination of lead isotopic ratios from igneous rocks can be useful in characterizing tectono stratigraphic terranes as to mantle or crustal origin and in correlation particularly where terrane relationships are enigmatic

global distribution of the decay timescale of mixed layer inertial motions observed by satellite tracked drifters 1 the decay timescale of mixed layer inertial amplitudes has been estimated from satellite tracked drifter trajectories from 1990 to 2004 as the e folding timescale of the temporal correlation functions the decay timescales increase with latitude in all basins except the north atlantic a beta dispersion model shows that dephasing leads to meridional variations of the decay timescale in the north pacific and the southern ocean but meridional variations of the buoyancy structure in the north atlantic act to compensate the beta effect leading to a lack of meridional variation of the decay timescale in that ocean

iron formation in neoarchean deltaic successions and the microbially mediated deposition of transgressive systems tracts abstract neoarchean deltaic deposits present in superior province canada contain an excellent record of shallow water siliciclastic and iron formation if deposition the deltas were fed by braided fluvial systems which commonly resulted in the development of small active lobes that repeatedly cycled through progradation and abandonment although a thick progradational deltaic assemblage was also investigated magnetite rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore proximal fluvial channels and distributary mouth bars of the braid delta deposits magnetite laminae are also interbedded with some distributary mouth sediments draping reactivation surfaces in ripples to barforms thicker accumulations of if up to 73 m form portions of transgressive systems tracts where chemical sedimentation dominated offshore equivalents of these successions do not contain chemical sediments sedimentologic data imply that if deposition in this well mixed nearshore water mass was not compatible with slow rates of constant iron compound precipitation in the world ocean combined with siliciclastic sediment starvation the model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore stimulating photosynthetically induced oxidation of fe2 under these conditions elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts

global statistics of inertial motions statistics of inertial currents in the north pacific north atlantic and southern ocean are derived for the period 1997 2003 from surface trajectories of argo profiling floats by employing a method developed recently by park et al 2004 the distribution of inertial amplitudes is non gaussian in the range 0 80 cm sec with an average value of 13 7 cm sec the inertial amplitude in the mid latitude 30 45 n band exceeds those in both the low 15 30 n and high 45 60 n latitude bands in three basins the amplitude in summer is greater than that in winter by 15 25 the inertial energy in the mixed layer reaches approximately 1100 j m2 1165 j m2 globally the spatiotemporal patterns of inertial amplitudes in the north atlantic mostly depend on the mld which exhibits strong meridional gradients however the small meridional gradient of the mld in the north pacific does not contribute significantly to the spatial distribution of inertial amplitudes the total inertial energy estimated from the floats is consistent with the theory that inertial energy could contribute significantly to sustaining the meridional overturning circulation

genesis of banded iron formations a series of experimental simulations abstract a series of experimental simulations of the genetic mechanism of banded iron formations has been conducted using a suite of self designed glassware to model the hypergene circulation of surface water the simulation process involved two parts in the first part the weathering and leaching of basalt by strongly acidic meteoric water during early precambrian volcanism would have resulted in amorphous silica precipitation and strongly acidic ore bearing solutions ph 2 saturated meteoric water without contamination of other volcanic gases would increase the ph value of the solution when the ph increased to 1 25 5 50 ferric hydroxides and amorphous silica would alternately precipitate owing to periodic heating and continuous addition of high ph basalt leaching solutions this is a possible explanation of the genetic mechanism of the algoma type banded iron formations

reply to comment by q zheng on can near inertial internal waves in the east sea be observed by synthetic aperture radar 1 sar images of the sea surface are very useful to detect numerous phenomena which are driven by oceanic as well as atmospheric processes johannessen et al 1994 we think that the sar images of d j kim et al 2005 are not exceptions as they show regular patterns of various scales in the east sea in particular we investigated the specific signals denoted by a b and c in figure 1 which is the same as figure 2 by d j kim et al 2005 except that it is enlarged to show feature clearly 2 zheng 2005 argued that the wave phase speed which was derived by d j kim et al 2005 from two sar images was not a real phase speed but related to a group speed this argument is based on an assumption that these wave like patterns a b and c are footprints of atmospheric coastal lee waves however it should be noticed that interpretations by d j kim et al 2005 are based on characteristics of near inertial internal waves which were clearly observed by an ocean buoy as well the relationship between the phase velocity and group velocity of near inertial internal waves is shown in figure 2 the phase velocity and group velocity are perpendicular and the horizontal directions of two velocities are the same but vertical directions are opposite thorpe 1999 the horizontal directions of the two velocities are shoreward because the crest of a b and c moves to a0 b0 and c0 toward the coast the vertical direction of group velocity of near inertial waves is downward because the phase of the east west currents measured at the esrob propagates upward in the depth 5 15 meters during 140 142 days see d j kim et al 2005 figure 5 the movement of the wave crests between the a a0 b b0 and c c0 on the two successively acquired sar images is about 12 km indicating that the horizontal phase speed not group speed of the waves is about 0 3 m s in average we can not estimate the group velocity with sar images only because the movements of the near inertial internal wave group can not be detected directly on the sar image

banded iron formations in proterozoic greenstone belts call for further studies abstract banded iron formations occur in greenstone belts in which volcanic rocks are predominant greenstone belts are not restricted to the archaean 2500 ma as is commonly perceived but they continued to form albeit in lesser abundance in the proterozoic thus banded iron formations which are closely associated with volcanic sequences occur in several well documented early mid proterozoic greenstone belts examples are the yavapai belts at jerome in arizona the trans amazonian belts in guiana and the dalma belts of the singhbhum region of ne india stratigraphic and sedimentological studies are needed to establish the similarities and differences of these iron formations with those in archaean greenstone belts and with the banded iron formations which were common in cratonic shelf environments in the early mid proterozoic

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

genesis and evolution of the ore deposits in the early precambrian greenstone belts and adjacent high grade metamorphic terrains of peninsular india a study in similarity and contrast abstract karnataka and the adjacent areas of tamil nadu and andhra pradesh in peninsular india are characterized by sublinear greenstone belts set in a tonalite trondhjemite dominated gneissic terrain mainly 3400 3000 ma skirted by a high grade metamorphic amphibolite granulite facies belt in the south and the east the greenstone belts are divided into the sedimentary volcanic sargur holenarsipur nuggihalli type 3000 ma the volcanic dominated kolar ramagiri hutti type 3000 ma and the sediment dominated dharwar type 3000 2600 ma arranged in order of increasing areal coverage the granulite facies metamorphism of the peninsular gneiss and associated supracrustals took place 2500 2600 ma ago the first type of greenstone belt is poor in ore content these belts contain a few lean base metal deposits a few chromite deposits and ti and v bearing magnetite deposits in places locally banded magnetite deposits are also present the second type of belt is known for its au content the dharwar type of belt of course contains rich deposits of fe and mn and rare small deposits of cu ore deposition in the high grade metamorphic terrain is also poor as in the sargur type of greenstone belt whatever metallizations occur there are confined to the supracrustal enclaves differential metallization in the different segments is explained in terms of the differential development of the crust the atmosphere and the biosphere

near surface frontal zone trapping and deep upward propagation of internal wave energy in the japan east sea the full depth current structure in the japan east sea was investigated using direct velocity measurements performed with lowered and shipboard acoustic current doppler profilers rotary spectral analysis was used to investigate the three dimensional energy distribution as well as wave polarization with respect to vertical wavenumbers yielding information about the net energy propagation direction highly energetic near inertial downward propagating waves were found in localized patches along the southern edge of the subpolar front between 500 and 2500 m depth the basin average energy propagation was found to be upward with the maximum of relative difference between upward and downward propagating energy lying at about 1500 m depth this difference was most pronounced in the southeastern part of the basin

structure of velocity and temperature in the northeast pacific as measured with lagrangian drifters in fall 1987 abstract in october 1987 49 lagrangian surface drifters tristar ii were released in a 200 km 200 km square area southeast of ocean station papa as part of the ocean storms experiment the drifters measured temperature at the drogue level and reported their position through argos approximately 11 times per day thirty one of the drifters retained drogues for longer than three months and data from those instruments are used to describe the evolving fall 1987 pattern of current and temperature structures at 15 m in the area between 46 and 49 n 142 w and 132 w time variable currents were dominated by mesoscale eddies of anticyclonic rotation with horizontal radii of 53 86 km and rotational speeds of 10 20 cm s 1 these eddies persisted for at least 90 days as evidenced by successive drifter trajectories through the eddies currents with periods longer than 1 day had a mean to the east of 4 4 cm s 1 and a mean to the north of 0 7 cm s 1 background eddy kinetic energy levels were 40 cm s 2 thus eddy

crustal evolution by arc magmatism arc magmas generated at depths near 100 km by dehydration of subducting slabs are olivine rich melabasalts but the magmas that reach the surface in mature continental magmatic arcs have an average composition near that of rhyodacite depth varying fractionations and equilibrations profoundly modify the initial magmas plate tectonics has operated throughout proterozoic and phanerozoic time much as it does now and so many deeply eroded terrains must expose products of arc magmatism most exposed middle and deep continental crust consists of igneous rocks plus older rocks equilibrated at magmatic temperatures variably deformed and metamorphosed subsequently each crustal level displays a typical assemblage of magmatic rock types which are here deduced to be products mostly of arc magmatism pre arc mantle consists of depleted dunite and harzburgite rising arc magmas precipitate much additional olivine and add the equivalent of 10 or 15 of basalt in clinopyroxene orthopyroxene garnet and spinel the seismic m discontinuity may be controlled primarily by the shallow limit of the depth zone in which crystallization is mostly of ultramafic components and the fractionated magmas that reach the crust are mostly of gabbroic to granodioritic compositions the lower crust is characterized by differentiated layered complexes which often contain mafic or ultramafic basal cumulates medial floated plagioclase anorthosite and upper quartz poor pyroxene mesoperthite granites and by other magmatic rocks magmatic rocks are more voluminous than pre existing rocks supracrustal rocks are in middle granulite facies and much granitic material has been melted from them by magmatic heat the magmatically modified middle crust consists primarily of migmatites in lower granulite facies in the deeper part and upper amphibolite facies in the shallower part much dissociation of hydrous mineral assemblages is caused by the magmatic heat producing water rich aluminous magmas assimilation and anatexis the high water contents restrict rise of the equilibrated magmas voluminous pegmatites are expelled into the wall rocks and crystallization is forced sheets of two mica granites characterize the upper part of the middle crust the comparatively dry magmas that rise into the upper crust are mostly tonalite to adamellite these magmas spread out in steep sided batholiths above the migmatites erupt as ash flow sheets from calderas and produce voluminous far travelling volcanic ash inverted metamorphic gradients and outward verging structures are produced beneath the spreading batholiths magmatic arcs are extensional at all crustal levels

the geochronology of uranium deposits in the great bear batholith northwest territories the oldest uranium mineralisation found in the great bear batholith during this study may be hydrothermal pitchblende hematite veins at hottah lake their apparent age of 2058 34 ma can also be explained by the contamination of deposits only 440 57 ma old which is the age of pitchblende veins nearby numerous pendants of metamorphosed uraninite bearing black sand placers in a north trending belt west of the wopmay fault are 1860 20 ma old the age of the granites that intrude them mineralisation at echo bay is from 1500 10 to 1424 29 ma old and extends up to 30 km north and 40 km south of echo bay the jd claims contain small quartz vein deposits dated at 535 164 and 1092 115 ma at mountain lake pitchblende in helikian sandstones overlying the batholith is 1076 96 ma old polymetallic veinlets at mazenod lake are 457 26 ma old pitchblende in a giant quartz vein at the rayrock mine is 511 86 ma old small pitchblende veins east of the batholith along the coppermine river are be

satellite tracked drifter measurement of inertial and semidiurnal currents in the northeast pacific we examine current variability in the open northeast pacific using 3 hourly sampled trajectories from satellite tracked drifters drogued at depths of 15 and 120 m within the wind mixed layer and permanent pycnocline respectively findings show the following 1 motions were dominated by inertial currents which accounted for 58 35 of the total variance measured by the shallow deep drifters semidiurnal motions accounted for roughly 10 of the variance at both depths there were detectable currents at twice the inertial frequency 2f 3 cpd and negligible diurnal currents 2 inertial currents attained root mean square rms speeds of around 50 cm s in the mixed layer and 10 cm s in the pycnocline speeds in the semidiurnal band were similar 10 cm s at both depths 3 inertial and semidiurnal currents were strongly clockwise rotary while low frequency currents 0 95 occurred for drifter separations r 500 km semidiurnal currents were uncorrelated beyond several tens of kilometers 6 inertial currents appear to be linked to the passage of atmospheric lows and semidiurnal currents to the passage of remotely generated baroclinic tides the spectral peak near 2f is indicative of nonlinear inertial wave interactions

nd isotopic evidence from wopmay orogen for 2 0 2 4 ga crust in western north america abstract nd isotopic data from the 1 9 ga wopmay orogen n w t canada strongly suggest the involvement of crust in the range of 2 0 2 4 ga remnants of three compositionally diverse magmatic arcs 1940 1902 ma 1890 1880 ma and 1875 1840 ma are preserved in wopmay orogen each of the three arcs originated west of the limit of exposed archean crust nineteen samples of plutonic and volcanic rocks from the three arcs yield a narrow range of initial e nd from 1 8 to 0 3 there is no apparent correlation between e nd and geographic distribution nor with whole rock chemistry sio 2 54 75 two basalt samples from west of the archean have initial e nd s between 2 and 3 two 1 87 ga granites which intrude through archean crust have much lower initial e nd s of 5 9 and 8 6 the data from the three magmatic arcs are best interpreted in terms of a source region which had a very limited range of e nd at 1 9 ga which we interpret as 2 0 2 4 ga crust presumably hottah terrane basement wopmay fault zone is therefore the surface manifestation of a suture between 2 0 2 4 ga hottah terrane and the archean slave craton there is a growing body of data which suggests that much of western north america may be underlain by 2 0 2 4 ga crust geochronologic and isotopic data from other precambrian cratons also suggest that 2 0 2 4 ga crust may be much more widespread than previously thought models of crustal growth which interpret 1 9 1 7 ga crust as mixtures of archean crust and proterozoic depleted mantle may be overestimating the amount of juvenile material added to the crust in this time period

hepburn intrusive suite peraluminous plutonism within a closing back arc basin wopmay orogen canada within the hepburn metamorphic plutonic internal zone of the wopmay orogen northwest territories canada there are two chronologically and petrologically distinct plutonic associations the more voluminous of the two the older 1 885 ga hepburn intrusive suite includes rocks ranging in composition from gabbro to granite peraluminous granite dominating the younger 1 855 ga neighboring bishop intrusive suite also gabbro to granite represents the waning stages of a well documented calc alkaline arc the great bear magmatic zone the petrological distinctions between the two suites are all late acquired features imposed primarily by contrasting environments of emplacement hepburn magmas were intruded within a closing dominantly sedimentary back arc basin magma emplacement was synchronous with crustal imbrication regional metamorphism and translation of the basin fill units onto archean crust significant assimilation of sedimentary host rocks by the rising hepburn magmas occurred whereas the postregional metamorphism emplacement of the bishop magmas precluded similar assimilation the gabbroic contribution observed in the hepburn intrusive suite is interpreted to reflect a mantle derived precursor inherited from the back arc rifting event that immediately preceded emplacement of the suite

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

the composition and genesis of the mesoarchean dagushan banded iron formation bif in the anshan area of the north china craton abstract the dagushan bif hosted iron deposit in the anshan benxi area of the north china craton ncc has two types of iron ore quartz magnetite bif fe 2 o 3 t 2 o 3 t 90 wt chlorite quartz schist and amphogneiss border the iron orebodies and are locally present as interlayers with bifs chlorite quartz schist and bifs are enclosed by amphogneiss in some locations the quartz magnetite bifs are enriched in hrees heavy rare earth elements with positive la eu and y anomalies indicating their precipitation from marine seawater with a high temperature hydrothermal component moreover these bifs have low concentrations of al 2 o 3 tio 2 and hfses high field strength elements e g zr hf and ta suggesting that terrigenous detrital materials contributed insignificantly to the chemical precipitation the high grade iron ores exhibit similar geochemical signatures to the quartz magnetite bifs e g ree patterns and y ho ratios implying that they have identical sources of iron however these ores have different ree rare earth element contents and eu eu values and the magnetites contained within them exhibit diverse ree contents and trace element concentrations indicating that the ores underwent differing formation conditions and the high grade ores are most likely the reformed product of the original bifs the chlorite quartz schist and amphogneiss are characterized by high sio 2 and al 2 o 3 contents and exhibit variable abundances of rees enrichment in lrees light rare earth elements negative anomalies in hfses e g nb ta p and ti and positive anomalies in liles large ion lithophile elements e g rb ba u and k a protolith reconstruction indicates that the protoliths of the chlorite quartz schist are felsic volcanic rocks sims and la icp ms zircon u pb dating indicate that this schist formed at approximately 3110 to 3101 ma which could represent the maximum deposition age of the dagushan bif however two groups of zircons from the amphogneiss are identified 3104 to 3089 ma zircons that are most likely derived from the chlorite quartz schist and 2997 to 2995 ma zircons which are interpreted to represent the time of protolith crystallization thus the dagushan bif most likely formed before 2997 to 2995 ma the 3 1 ga zircons yield e hf t values of 8 07 to 5 46 whereas the 3 0 ga zircons yield e hf t values of 3 96 to 2 09 these geochemical features suggest that the primitive magmas were derived from the depleted mantle with significant contributions of ancient crust