u pb evidence for polymetamorphic history of huronian rocks within the grenville front tectonic zone east of sudbury ontario canada abstract this paper presents u pb geochronological data for zircon titanite monazite apatite allanite and rutile from street township east of sudbury that help unravel two main facets of the geology of the grenville front tectonic zone gftz they document the presence of extensive huronian supergroup magmatic ca 2480 2460 ma and sedimentary rocks inside the gftz and highlight the significance of bimodal magmatism in the genesis of the huronian supergroup the data also provide further insights into the complex orogenic evolution of the gftz showing in particular that amphibolite facies metamorphism migmatization and deformation during grenvillian compression at 995 980 ma overprinted but did not totally erase the memory of earlier metamorphic events at ca 1720 and 1470 ma from the point of view of u pb systematics the results emphasize the nearly ubiquitous state of isotopic disequilibrium in these rocks most of the minerals investigated comprise two or more generations and are generally in a state of partial resetting because of the superimposed effects of younger events the preservation of pre grenvillian ages in titanite and allanite demonstrates that processes related to final grenvillian collision and exhumation occurred at great speed only apatite and to some degree rutile yield ages that are clearly post tectonic thereby reflecting the final stages of exhumation and cooling of the region

upb geochronology of layered mafic intrusions in the eastern baltic shield implications for the timing and duration of paleoproterozoic continental rifting abstract paleoproterozoic layered mafic ultramafic intrusions are widespread in the eastern baltic fennoscandian shield and mark the initial stages of continental rifting and outpouring of voluminous flood basalts u pb zircon baddeleyite ages have been determined for seven layered mafic intrusions and a subvolcanic felsic unit associated with two paleoproterozoic volcanic sedimentary belts the pechenga imandra varzuga belt in the kola peninsula and the sumi sariola belt in karelia the layered intrusions in the sumi sariola belt show an age progression from 2449 0 1 1 ma for the burakovka complex located at the southeastern edge of the belt to 2442 1 1 4 ma and 2441 3 1 2 ma for the lukkulaisvaara and tsipringa intrusions of the olanga complex in northern karelia northeastern part of the belt these age results indicate a total duration of mafic magmatism along the ca 800 km belt in karelia to be less than 10 m y the pechenga imandra varzuga belt kola peninsula contains two distinct age groups of layered mafic intrusions and related rocks the younger of these overlaps with the ages obtained for the olanga intrusions and includes the 2441 0 1 6 ma imandra lopolith and a spatially associated felsic subvolcanic unit dated at 2442 2 1 7 ma the older group of mafic layered intrusions in the kola peninsula includes the 2501 5 1 7 ma pana tundra intrusion the 2504 4 1 5 ma monche pluton and the 2505 1 1 6 ma general skaya mountain intrusion the temporal and spatial distribution of the 2 50 2 44 ga rift related igneous activity in the baltic shield is consistent both with the model of starting mantle plume and with the model of passive rifting controlled by pre existing lithospheric weaknesses

fluvial process and morphology of the brahmaputra river in assam india abstract the brahmaputra river finds its origin in the chema yundung glacier of tibet and flows through india and bangladesh the slope of the river decreases suddenly in front of the himalayas and results in the deposition of sediment and a braided channel pattern it flows through assam india along a valley comprising its own recent alluvium in assam the basin receives 300 cm mean annual rainfall 66 85 of which occurs in the monsoon period from june through september mean annual discharge at pandu for 1955 1990 is 16 682 24 m3 s 1 average monthly discharge is highest in july 19 and lowest in february 2 most hydrographs exhibit multiple flood peaks occurring at different times from june to september the mean annual suspended sediment load is 402 million tons and average monthly sediment discharge is highest in june 19 05 and lowest in january 1 02 the bed load at pandu was found to be 5 15 of the total load of the river three kinds of major geomorphic units are found in the basin the river bed of the brahmaputra shows four topographic levels with increasing height and vegetation the single first order primary channels of this braided river split into two or more smaller second order channels separated by bars and islands the second order channels are of three kinds the maximum length and width of the bars in the area under study are 18 43 km and 6 17 km respectively the brahmaputra channel is characterised by mid channel bars side bars tributary mouth bars and unit bars the geometry of meandering tributary rivers shows that the relationship between meander wavelength and bend radius is most linear the brahmaputra had been undergoing overall aggradation by about 16 cm during 1971 to 1979 the channel of the brahmaputra river has been migrating because of channel widening and avulsion the meandering tributaries change because of neck cut off and progressive shifting at the meander bends the braiding index of the brahmaputra has been increasing from 6 11 in 1912 1928 to 8 33 in 1996 during the twentieth century the total amount of bank area lost from erosion was 868 km2 maximum rate of shift of the north bank to south resulting in erosion was 227 5 m year and maximum rate of shift of the south bank to north resulting in accretion was 331 56 m year shear failure of upper bank and liquefaction of clayey silt materials are two main causes of bank erosion

fractional crystallization of anhydrous sulfide liquid in the system fe ni cu s with application to magmatic sulfide deposits abstract phase relations for coexisting monosulfide solid solution fe ni 1 xs mss and sulfide liquid have been determined in the fe rich portion of the anhydrous quaternary system fe ni cu s at 52 5 and 50 0 at s 850 1050 c and low pressure equations for the complex dependence of the mss solidus sulfide liquidus and distribution coefficients for exchange of ni cu and fe cu on temperature s content and proportion of metals have been derived by multiple regression analysis these equations may be used for geothermometry of pristine magmatic sulfides and prediction of liquid evolution paths but a limiting factor is uncertainty in the precise bulk composition of natural assemblages and particularly of s content intermediate solid solution cu fe s1 x iss is stable up to 900 950 c in ni poor compositions and a large field of a new ternary fe cu ni s solid solution intermediate between iss and mss exists at and below 850 c disseminated sulfides in komatiites are markedly depleted in cu relative to an origin by sulfide liquid immiscibility the compositions of magmatic sulfide ores from sudbury canada and the insizwa complex transkei are not consistent with derivation of the associated late cu rich ores by fractional crystallization of sulfide magma although the cu rich ores of the noril sk talnakh district russia could have evolved through a complex sequence of fractional crystallization events

fractional crystallization of sulfide ore liquids at high temperature tie line compositions of fe ni cu sub 1 x s monosulfide solid solution mss and fe ni cu s liquid in the presence of sulfur vapor have been quenched from temperatures between 1 050 degrees and 1 180 degrees c more than 80 bulk compositions on both sulfur rich and sulfur poor sides of the mss field were investigated by sealed silica tube techniques qualitative observations of wetting behavior suggest increasing mobility in a silicate host rock as sulfide liquids become more cu rich partition coefficients for copper or 0 25 d super mss hq cu are obtained from ni bearing experiments with approximately 2 wt percent cu and no quench phases the equation d super mss hq cu 0 0003 x t degrees c 0 0310 x wt s 0 0069 x wt cu 1 3450 describes partitioning observed in ni free experiments above 1 000 degrees c above 1 000 degrees c the ni distribution coefficient d super mss hq cu decreases with increasing temperature and or sulfur content of the liquid these results yield improved models describing the fractional crystallization of natural sulfide liquids major element fe ni cu s compositions of ores from the sudbury district are shown to be entirely consistent with fractional crystallization at temperatures above 1 000 degrees c with the possible exception of rare samples enriched in both ni and cu sulfide liquids fractionating ni fe rich hanging wall ores at sudbury ontario must have been less cu rich than previously thought at temperatures above 950 degrees c reduced sulfur activity in residual liquids can result in massive late stage bornite rich ores

geochemistry and petrology of 2 40 2 45ga magmatic rocks in the north western belomorian belt fennoscandian shield russia abstract widespread mafic and ultramafic rocks drusites and minor granites of 2 40 2 45 ga age occur within the archaean gneisses of the belomorian fold belt several of these mafic ultramafic and acid intrusions in the north western part of that belt have been studied in detail the mafic and ultramafic rocks are rich in si and mg and also show lree enrichment they have nd 2 45 values varying between 0 2 and 1 8 assimilation fractional crystallization afc and mixing models in terms of nd t la sm ratios have been evaluated for the origin of the mafic rocks the results show that an afc model is preferable it involves 20 30 crustal rock assimilation by undepleted picrite magma with fractional crystallization the granites are similar in composition to a type within plate granites the tupaya bay granite pluton is characterized by geochemical layering which supports an anorogenic origin the granites and mafic ultramafic rocks drusites in the belomorian belt are similar in composition to coeval granites acid and mafic volcanic rocks mafic dykes and mafic layered intrusions within the kola and the karelian terrains and can be interpreted as part of the palaeoproterozoic igneous province connected with ascent of a mantle plume the small sizes corona textures and evidence for ductile deformation in the drusites are connected with their deeper level of emplacement p 8 12 kbar than that of the mafic layered intrusions 2 7 kbar

emplacement of a large igneous province as a possible cause of banded iron formation 2 45 billion years ago latest archaean and earliest palaeoproterozoic times from 2 6 to 2 2 billion years ago have generally been viewed as a largely quiescent period of earth history the geological record indicates the very slow deposition of pelagic and chemical sediments1 2 and bears only a limited record of magmatic and tectonic activity3 5 such quiescence is consistent with the contention that the earth s main banded iron formations bifs finely laminated chemical sedimentary rocks rich in iron oxide formed slowly as oxygen abundances in the oceans gradually increased thus reducing the capacity of sea water to retain dissolved iron6 10 here we show that a large igneous province comprising 30 000km3 of dolerite basalt and rhyolite accompanied deposition of a hamersley province bif 2 449 3 million years ago this observation indicates that hamersley bifs formed during a major tectono magmatic event and were deposited very much faster than previously thought at similar rates to or faster than modern pelagic sediments thus the largest palaeoproterozoic bifs rather than simply reflecting a gradual increase in the oxygen content of the oceans during a period of tectonic quiescence are more likely to have formed as a result of an increased supply of suboxic iron and silica rich sea water upwelling onto continental shelves during a pulse or pulses of increased submarine magmatic and hydrothermal activity

the scourie dyke suite petrogenesis and geochemical nature of the proterozoic sub continental mantle the petrogenesis of bronzite picrite olivine gabbro norite and quartz tholeiite dykes which make up the 2 39 b y scourie dyke swarm cutting the archaean lewisian gneisses of n w scotland is interpreted on the basis of their major and trace element geochemistry most of the dykes bear primary amphibole and or phlogopite and with one exception are all hypersthene or quartz normative apart from one tholeiite dyke which shows relative light rare earth element depletion all the dykes show enrichment in light rare earths and large ion lithophile elements they do not however show an equivalent enrichment in other incompatible high field strength ions such as nb and ta and in this respect resemble island arc and calc alkaline basalts the different dyke types have distinctive rare earth patterns and other trace element ratios which are maintained over a range of major element compositions

textural morphological and compositional varieties of modern arc sulfides a case study of the tolbachik volcano kamchatka abstract magma unmixing into separate sulfide and silicate melts is a key process in the formation of magmatic sulfide ore deposits sulfide inclusions in olivine preserve the original chemical composition of the segregated sulfide melt whereas the phase compositions of the solidified sulfide droplets and their textures record the final stages of sulfide evolution sulfide globules hosted in olivine phenocrysts of magnesian basalts from the tolbachik volcano kamchatka display a variety of textures ranging from homogeneous to fine grained to coarse grained and lamellar porous textures and textures associated with the oxidation and dissolution of sulfides were identified as separate categories the outer surfaces of sulfides vary significantly in shape which includes normal spherical or elliptical globules gravitationally layered inclusions thin sulfide foils and concave deformed sulfides in contact with fluid bubbles the estimated cooling rate of olivine phenocrysts responsible for observed crystallization textures varies by five orders of magnitude the fe ni cu ratio also exerts an influence on the resulting texture whereas the size of the globules does not have any significant effect the morphology and textural patterns of individual grains indicates that fine grained textures resulted from the rapid breakdown of a homogeneous solid phase which the sulfide melt solidified into during extremely rapid quenching the presence of large or abundant small pores in sulfides along with channels at the globule periphery indicates the separation of an appreciable amount of dissolved fluid from the sulfide melt during crystallization sulfides in the form of thin foils and planar swarms in healed cracks indicate cyclic pressure changes in the olivine environment which resulted in phenocrysts rupturing and healing the sulfide mineralogy is represented by high temperature sulfides mss monosulfide solid solution and iss intermediate solid solution as well as pyrrhotite pentlandite chalcopyrite cubanite bornite magnetite assemblages which formed at lower temperatures

petrologic consequences of rift propagation on oceanic spreading ridges abstract the production of anomalously differentiated lava compositions at several mid ocean spreading centers can be attributed to magmatic processes associated with propagating rifts the degree of differentiation attained by magmas beneath oceanic spreading ridges depends mainly on the balance between cooling rate and the supply rate of new magma to shallow chambers low supply rates and moderate cooling rates allow advanced degrees of closed system fractionation to occur high supply rates result in open systems in which magma compositions are buffered by frequent replenishment with new hot magma propagating rift tips are a special class of ridge transform intersection in which the balance between cooling and supply rates is conducive to the development of advanced degrees of differentiation over an expanded length of ridge this balance is affected by the spreading rate the propagation rate of the rift the length of the bounding transform and proximity to hotspots maximum compositional variability and maximum degree of differentiation occur within 50 km of propagating rift tips and subsequently diminish with increasing distance rifts that propagate through plates in directions approximating their absolute motion relative to the lower mantle are characterized by the presence of anomalously differentiated lavas over longer ridge segments than are rifts that propagate against their absolute motion geochemical anomalies may persist though changing in degree and extent for several million years on ridge segments that stop propagating the concept of magnetic telechemistry is generally supported by our study but in the vicinity of hotspots magnetic anomaly amplitude may be controlled more by bathymetric and or thickened magnetic layer effects than by geochemistry

nd and sr isotopic geochemistry of mafic layered intrusions in the eastern baltic shield implications for the evolution of paleoproterozoic continental mafic magmas nd and sr isotopic data are presented for the 2449 2441 ma olanga and burakovka layered mafic complexes in the eastern baltic shield these complexes have similar tectonic position but differ in two aspects the age of the enclosing crust and the post crystallization metamorphic history the sm nd isotopic results for the kivakka and lukkulaisvaara intrusions olanga complex are consistent with the model of closed system crystallization of a single magma without significant wallrock assimilation the rb sr systems of minerals were disturbed by late rb addition during 1 75 1 50 ga metamorphism the nd and sr isotopic systems in the burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct components isotopic variations in the burakovka complex can be explained by a 4 20 per cent contamination of a primary komatiitic or picritic magma with a mesoarchean crust similar to that exposed in the region a similar model applied to the olanga complex using a neoarchean crustal isotopic composition cannot reproduce the observed isotopic signature the nearly uniform initial nd values between 1 and 2 3 observed in the kivakka and lukkulaisvaara intrusions of the olanga complex as well as in the other 2 50 2 44 ga layered mafic intrusions throughout the eastern baltic shield are better explained by a mantle plume model with small amounts of crustal contamination and minor involvement of asthenospheric material this model is also consistent with the geological observations and the temporal distribution of the paleoproterozoic mafic magmatism in the eastern baltic shield as an alternative the enriched isotopic characteristics may be explained by melting of a metasomatically modified lithospheric mantle source

platinum group element gold silver and base metal distribution in compositionally zoned sulfide droplets from the medvezky creek mine noril sk russia concentrations of ag au cd co re zn and platinum group elements pge have been determined in sulfide minerals from zoned sulfide droplets of the noril sk 1 medvezky creek mine the aims of the study were to establish whether these elements are located in the major sulfide minerals pentlandite pyrrhotite chalcopyrite and cubanite to establish whether the elements show a preference for a particular sulfide mineral and to investigate the model which suggests that the zonation in the droplets is caused by the crystal fractionation of monosulfide solid solution mss nickel cu ag re os ir ru rh and pd were found to be largely located in the major sulfide minerals in contrast less than 25 of the au cd pt and zn in the rock was found to be present in these sulfides osmium ir ru rh and re were found to be concentrated in pyrrhotite and pentlandite palladium and co was found to be concentrated in pentlandite silver cd and zn concentrations are highest in chalcopyrite and cubanite gold and platinum showed no preference for any of the major sulfide minerals the enrichment of os ir ru rh and re in pyrrhotite and pentlandite exsolution products of mss and the low levels of these elements in the cubanite and chalcopyrite exsolution products of intermediate solid solution iss support the mss crystal fractionation model because os ir ru rh and re are compatible with mss the enrichment of ag cd and zn in chalcopyrite and cubanite also supports the mss fractionation model these minerals are derived from the fractionated liquid and these elements are incompatible with mss and thus should be enriched in the fractionated liquid gold and pt do not partition into either iss or mss and become sufficiently enriched in the final fractionated liquid to crystallize among the iss and mss grains as tellurides bismithides and alloys during pentlandite exsolution pd appears to have diffused from the cu rich portion of the droplet into pentlandite

paleomagnetism and u pb geochronology of diabase dyke swarms of minto block superior province quebec canada in the first collaborative study of paleomagnetism and precise u pb geochronology in the minto block of the superior province mafic dyke swarms with three widely divergent paleomagnetic signatures and isotopic ages have been identified the 2505 2 ma ptarmigan dykes trend north to northeast and have a virtual geomagnetic pole at 42 s 220 e similar to that of 2473 2446 ma matachewan dykes of the southern superior province the ca 2230 ma maguire dykes trend west to northwest and yield a paleopole at 9 s 267 e similar to those for 2216 8 4 ma senneterre dykes and 2217 2210 ma nipissing sills of the southern superior and southern provinces respectively the 2209 1 ma klotz dykes trend west northwest but do not carry a consistent magnetization direction finally 1998 2 ma minto dykes of west northwest to northwest trend identical in age to the 1998 ma 2 ma purtuniq ophiolite of the cape smith belt have a paleopole at 38 n 174 e the similarity of paleopoles for the ca 2 23 2 21 ga maguire

paleomagnetism of 2 44 ga mafic dykes in russian karelia eastern fennoscandian shield implications for continental reconstructions abstract results of a paleomagnetic study of paleoproterozoic dykes and archean host rocks in the eastern fennoscandian shield are reported here most of the dykes are related to ca 2440 ma layered intrusions and have isotopic ages between 2349 and 2476 ma the dykes form an extensive nw se and e w trending swarm the majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature the high temperature remanence component d which is thought to represent the primary 2 44 ga magnetization of the dykes has a paleomagnetic pole at 20 s 278 8 e a95 6 0 n 11 sites another d like component d with shallower inclination vgp 9 6 n 256 2 e eight samples at one site from baked archean host rock may also represent the primary remanence of the dykes the commonest remanence component a records an extensive svecofennian partial overprint and is interpreted to be ca 1 84 ga old it yields a paleomagnetic pole at 52 6 n 226 8 e a95 2 7 n 23 the fourth component b has a paleopole at 53 9 n 169 6 e a95 6 3 n 16 and is inferred to be ca 1 75 ga on the basis of isotopic dating although ages of 2 1 1 3 and 0 3 ga are also possible baked contact tests for the 2 44 ga dykes are negative as shown by the occurrence of component d in archean host rocks beyond one dyke width it is proposed that the negative baked contact test indicates regional reheating and remagnetization of the archean basement at ca 2 44 ga ago due to the spatially vast occurrence of coeval 2 44 ga mafic dykes layered intrusions volcanics and granites in the area based on this hypothesis it is suggested that component d in the dykes may be primary and acquired at ca 2 44 ga ago the similarity of pole d with poles from coeval dykes and layered intrusions elsewhere in the karelian province and kola peninsula suggest that within the archean domain of the fennoscandian shield no large scale block movements have taken place since 2 44 ga at 2 44 ga the fennoscandian shield was located at the latitude of ca 30 s which is different to that at 1 84 and 1 75 ga when fennoscandia was located at the latitudes of ca 30 n a continental reconstruction based on pole d and the pole from the 2 45 ga matachewan dykes of the superior craton in laurentia places the kola peninsula adjacent to southern greenland reconstructions based on recent geological interpretations and a reconstruction yielding pole d are also discussed

u pb baddeleyite ages for the scourie dyke swarm scotland evidence for two distinct intrusion events precise and accurate radiometric ages for continental mafic dyke swarms are a prerequisite for global correlation of mafic magmatic events calibration of apparent polar wander paths and deciphering mechanisms of dyke emplacement precambrian dyke swarms such as the scourie dyke swarm in north west scotland are invaluable time markers so that a precise and accurate knowledge of their emplacement age is critical when unravelling the complex geological evolution of many terrains precise dating is often difficult however because magma interaction with country rock and subsequent metamorphic events can severely perturb some isotopic systems recent advances in u pb geochronology1 3 combined with the discovery that some mafic dykes contain trace amounts of uranium bearing minerals such as baddeleyite zro2 and or zircon have made it possible to obtain u pb ages for these rocks with a precision typically on the order of 1 2 myr4 5 some of the first detailed u pb studies of precambrian mafic dyke swarms5 6 have shown that large volumes of mafic magma 100 000 km3 were emplaced into the continental crust in surprisingly short periods of time 2 myr here we report the results from a u pb study of three members of the scourie dyke swarm and find at least two periods of dyke emplacement at 2 418 and 1 992 myr bp we speculate on a potential global correlation of early proterozoic mafic magmatism and hence on the origin of dyke swarms

late palaeoproterozoic monazite and titanite u pb ages in the archaean suomujarvi complex n finland abstract the paper documents a late palaeoproterozoic episode of metamorphism and anatexis in the suomujarvi complex an archaean enclave in the palaeoproterozoic central lapland granitoid complex of northern finland u pb ages were obtained from monazite and titanite in four archaean granitoid rocks one quartzite and a massive granite the results show that growth of monazite and titanite in these rocks spanned a period of about 15 million years between 1780 and 1765 ma presumably reflecting complex processes of metamorphic re crystallisation invasion by granitic dykes veins and fluids and post peak metamorphic retrogression the timing of this activity corresponds to post orogenic granitoid magmatism throughout the baltic shield in this region between the lapland kola and that of svecofennian orogens it may represent a retarded response to their coeval ca 1 9 ga convergent collision the results support a proposed affinity of the suomujarvi complex with the belomorian rather than the archaean karelian province

shrimp u pb zircon ages for the palaeoproterozoic kuruman iron formation northern cape province south africa evidence for simultaneous bif deposition on kaapvaal and pilbara cratons abstract stilpnomelane rich tuffaceous mudrocks are an important component of the palaeoproterozoic kuruman and griquatown iron formations in the northern cape province they document volcanic activity during the deposition of these iron formations and the zircon populations that they contain record syn depositional volcanism resedimentation and post depositional isotopic disturbance the trace element signatures of tuffaceous samples suggest that although zircons are felsic volcanogenic in origin mixing of felsic ash with basaltic ash and or muds in the water column was also likely in addition these rocks record minor terrigenous input and some samples indicate mixing with iron rich clays oxides and smectites moreover they reflect a shift from mafic volcanism e g mafic tuffs in the underlying gamohaan formation towards more felsic volcanism that is a trend also recorded for tuffaceous rocks in the brockman iron formation the best estimate of the depositional age for the top of the riries member and hence the kuruman iron formation is 2460 ma where most of the zircon populations were derived from volcanism coeval with bif deposition ages obtained in this study that are older than the 2465 5 ma age for the lower riries member probably represent resedimented pyroclastic deposits i e give eruptive ages from earlier eruptions that contain an unimodal zircon population and as such only provide maximum depositional ages although a compacted sedimentation rate csr for the entire kuruman iron formation is 3 4 m per million years a csr of about 22 m per million years is deduced for the riries member suggesting that there are cryptic gaps within the kuruman stratigraphic record the latter is comparable to the csr of 33 m per million years calculated for hamersley province bifs ages of the tuffaceous horizons in the kuruman iron formation are comparable to those from the brockman iron formation of the hamersley province the 2460 5 ma age for the top of the riries member corresponds to the 2459 3 ma age for the base of the joffre member and the 2463 5 ma whaleback shale member of the brockman iron formation although ca 2468 2478 ma ages reflect detrital or syn depositional zircon populations they are comparable to the 2470 4 ma s13 and the 2479 3 ma s9 macrobands of the dales gorge member brockman iron formation moreover the zircon age spectra and pb loss histories of both the kuruman and brockman iron formation tuffaceous sediments are strikingly similar without doubt the kuruman and brockman iron formations were deposited synchronously whether they were a response to global magmatic events or were deposited in the same basin are questions still being debated

tectonic stratigraphic and geochemical comparisons between ca 2500 2440ma mafic igneous events in the canadian and fennoscandian shields abstract palaeoproterozoic mafic igneous rocks 2500 2440 ma are exposed in the form of layered intrusions dykes and volcanic rocks in the southern canadian shield and the fennoscandian shield both igneous events were associated with episodes of continental rifting the tectonic and regional geological evolution of the two regions during the palaeoproterozoic are strikingly similar further comparisons of the mineralogical and stratigraphic characteristics of the layered intrusions the east bull lake suite in the canadian shield and the fennoscandian suite in the fennoscandian shield show additional similarities earlier investigations have identified possible parental magmas from mafic dykes for each of the layered intrusion suites these include the so called streich dykes associated with the east bull lake suite and the viianki dykes with the fennoscandian suite the viianki dykes are more primitive in composition mg 0 66 0 78 relative to the streich dykes mg 0 62 0 65 and probably represent primary mantle derived magmas geochemical modelling suggest that these two parental magmas cannot be related through fractional crystallization the differences in ree characteristics la yb cn 3 6 9 1 for viianki dykes 3 6 3 7 for streich dykes and selected incompatible element ratios can however be accounted for by either differential amounts of upper crustal contamination or different degrees of partial melting from a similar garnet bearing mantle source the data suggest that the viianki dyke parental magmas may have been produced by a degree of partial melting that was more than a factor of two lower than that which gave rise to the more primitive parent of the streich dyke magmas we conclude that these early palaeoproterozoic igneous events in the southern canadian shield and the fennoscandian shield could have been derived from a similar mantle source that was characterized by partial melt extraction of amorb and enrichment by subduction like lree enriched fluids during the archaean

petrogenesis of the early proterozoic matachewan dyke swarm canada and implications for magma emplacement and subsequent deformation major minor and rare earth element analyses of fine grained marginal material from 249 dykes of the matachewan giant radiating swarm show that the range in compositions cannot be produced by any simple fractionation model instead a two stage process is required whereby melts were modified by both fractionation and assimilation of lower crustal material before rising and ponding at shallower crustal levels 15 20 km maximum where a combination of fractional crystallization plagioclase phenocryst formation and periodic melt replenishment took place these shallow crustal chambers acted as feeders for the numerous dykes one hypothesis compatible with these observations involves an extensive rising plume that would provide heterogeneous melts over a large area accompanied by stresses that could produce a radial pattern of dykes in a search for indications of lateral magma flow a comparison of longitudinal variations in composition along four sample traverses across the western subswarm showed no t

the 2 40ga ringvassoy mafic dykes west troms basement complex norway the concluding act of early palaeoproterozoic continental breakup abstract a north to northwest trending mafic dyke swarm of gabbronoritic and gabbroic composition makes up a significant part of the archean basement on the island of ringvassoy in northern norway u pb geochronology of zircon and baddeleyite in a gabbronorite provides an age of emplacement of 2403 3 ma metamict zircon in a plagioclase phyric dyke yield data that are discordant but consistent with the age of the gabbronoritic dyke titanite indicates a metamorphic overprint at 1768 4 ma the two types of dyke show some distinct chemical characteristics they are both tholeiitic enriched in lrees and lile elements but depleted in hfs elements including nb their nd isotopic composition yields nd values of 1 5 to 1 8 for gabbronorites and 0 4 to 1 3 for the plagioclase phyric dykes the chemical and isotopic constraints are typical of continental basalts the ringvassoy mafic dykes correlate broadly with a global palaeoproterozoic magmatic event that formed extensive bimodal intrusive and extrusive suites in most archaean cratons including the northeastern fennoscandian shield in detail the 2403 3 ma ringvassoy dykes postdated most episodes of magmatism at this time on the regional scale there is a distinct trend from a 2505 2490 ma suite present in the kola peninsula over a second 2460 2440 ma suite present both in kola and further south in karelia to the 2403 ma dykes on ringvassoy this variation suggests that the locus of maximum extension and magmatic activity may have been shifting with time

experimental evaluation of liquid immiscibility in a portion of the system fe ni cu s using high gravitational acceleration experimental results reported by ballhaus et al 2001 suggest there may be some natural sulfide liquid compositions in equilibrium with primitive mafic magmas that encounter a field of immiscibility at superliquidus conditions in this communication we present results of static and high acceleration experiments done using one of the bulk compositions for which immiscibility is reported their ms 11 as well as mixtures of the alleged conjugate melts at all conditions investigated melts quenched to a homogeneous distribution of quench phases with no evidence for compositional separation over the length scale of the sample these results indicate that the ms 11 composition produced a homogeneous liquid during the experiment and therefore we could not verify the two phase field reported by ballhaus et al 2001 we suggest caution when evoking sulfide liquid immiscibility as a differentiation mechanism during magmatic sulfide evolution

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

precise u pb ages for grenvillian and pre grenvillian thrusting of proterozoic and archean metamorphic assemblages in the grenville front tectonic zone canada while the position of the grenville front is placed at a structural feature associated with the onset of high grade rocks when approached from the north the time of first metamorphism in these rocks is in general pre grenvillian and highly variable at killarney amphibolite to granulite grade rocks were metamorphosed lineated and foliated to their present front parallel configuration mainly by 1453 6 ma north of north bay amphibolite grade migmatites have been subjected to a pre 2650 ma metamorphism granulite grade gneisses south of the grenville front near val d or record a 2640 ma metamorphic event south of chibougamau mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 ma in central labrador a 1650 ma foliated granite is present south of the grenville front and at least some of the banding is this old at the atlantic coast granulite facies metamorphism in front parallel gneisses predates a 1790 1662 ma deformation in contrast each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies nw directed thrusting or crustal loading near killarney on georgian bay titanite records an episode of 977 2 ma partial resetting the degree of which increases with distance into the grenville province here grenvillian deformation where present is manifest as feldspar porphyroclastic fabric with strong se plunging lineations near north bay archean pegmatites in the grenville front zone were similarly deformed at 988 10 ma and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 2 ma a 991 4 ma cooling age for titanite in this melt is analytically identical to the zircon growth age hence rapid cooling occurred south of chibougamau new zircons grew in high pressure coronite in response to tectonic loading at 995 6 ma and in central labrador melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 2 ma titanite in the granitic host rock has an analytically identical age of 992 4 ma again indicating rapid cooling this second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 5 ma to 988 5 ma two age mixing line grenville resetting at about 1037 19 ma in the grenville front zone on the labrador coast is indicated but major thrusting is pre 1662 ma u pb isotopic results indicate that the grenville front zone is characterized along its length by partially reset u pb systems high pressure assemblages local melting and rapid cooling crustal loading and shearing with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling are required

toward a quantitative model for the formation of gravitational magmatic sulfide deposits abstract a preliminary quantitative model for the formation of magmatic sulfide deposits through gravitational sulfide droplet settling is developed the model incorporates thermodynamic consideration of the oversaturation of sulfide liquid from a silicate melt and the coupled growth kinetics and settling dynamics of sulfide droplets the conditions for the sulfide droplets to have enough time to grow and settle to form a sulfide liquid layer at the bottom of a magma chamber are referred to as the necessary criteria for sulfide ore formation simulations are carried out for dry etna basaltic melt because their melt viscosity and sulfur diffusivity have been measured so that the simulations are more realistic rather than parametric furthermore the effects of empirical nucleation rate and condition and of the variation of solubility viscosity and diffusivity have been parametrically evaluated the simulations show that for a given magma containing about 0 1 to 0 3 wt sulfur sulfide melt layer can form for realistic magma body size with cooling time of 1000 yr or more and realistic solubility viscosity and diffusivity the ability of the sulfide melt to collect ore elements depends critically on the diffusivity of individual ore elements when the diffusivity of an ore element is similar to or greater than that of sulfur near equilibrium partitioning is reached when the diffusivity of an ore element is much smaller than that of sulfur which is often the case for trivalent and tetravalent ions the concentration of the element in the sulfide melt would be far below the equilibrium concentration it is hoped that the model will help both researchers and exploration geologists