magnetic field modelling and interpretation of the himalayan tibetan plateau and adjoining north indian plains abstract a prominent magnetic low is seen over the himalayan tibetan plateau and the northern indian shield region in satellite anomaly data here we model the sources for the observed magnetic low in the region the induced magnetic field is modelled based on a crustal magnetisation model computed in a geographical information system with horizontal layers containing susceptibility derived from the geology and a tectonic map of the region and global seismic crustal thickness model the modelling results indicate that the upper crust of the himalayan tibetan plateau is magnetised while much of the middle crust and the lower crust appears non magnetic thermal modelling corroborates the predictions placing the curie isotherm in the southern tibetan plateau no deeper than 10 km the results suggest that the sources of the magnetic low observed in the region are likely to be a thin magnetised crust because of the elevated curie isotherm in this region the indian shield region is believed to extend up to the foothills of the himalayas in the north but most of the shield is obscured by a thick sediment cover of the gangetic plains to the north the magnetic field modelling suggests a weakly magnetic crust in the region indicating either the indian shield may not extend up to the base of the himalayas or the lower crust in the region has been modified

holocene and pleistocene dissolution cycles in deep sea cores of baffin bay and davis strait palaeoceano graphic implications abstract calcium carbonate dissolution has been studied in several grab samples and piston cores from baffin bay and davis strait ranging in water depth from 200 to 2300 m the intensity of dissolution was determined from examining 1 the ratio of arenaceous to calcareous benthonic foraminifera 2 the ratio of benthonic to planktonic foraminifera 3 the relationships between the dissolution susceptible and dissolution resistant foraminifera 4 the degree of foraminiferal test fragmentation and 5 the relationships between plankton tows and the fauna in the surface sediments all core tops and grab samples from baffin bay surface sediments below 600 900 m water depth demonstrated intense dissolution of calcium carbonate sediments below 900 m were biogenic carbonate free indicating 100 foraminiferal loss due to dissolution possible causes of substantial undersaturation in calcium carbonate of baffin bay bottom water are very low temperatures and higher concentrations of carbon dioxide all cores also demonstrated intense dissolution during interglacial or interstadial isotope stages 1 3 5 7 and 9 similarities in planktonic foraminiferal assemblages suggested the presence of similar bottom water masses during these periods the preservation state of biogenic carbonate debris in glacial isotope stages 2 4 6 8 and 10 is similar to the assemblage found in the water at present

the nature age and petrogenesis of the cartier batholith northern flank of the sudbury structure ontario canada abstract the cartier batholith exposed about 50 km northwest of sudbury ontario canada is a late archean k rich granitic batholith along with the levack gneiss complex it forms the footwall of the north range of the sudbury igneous complex the cartier batholith dominantly monzogranitic to granodioritic is relatively homogeneous unfoliated and contains k feldspar megacrysts it shows several distinctive chemical features including high concentrations of zr th u and lree and low concentrations of nb ti and hree several discriminants suggest a post orogenic tectonic environment for the batholith this inference is consistent with a u pb age of 2642 1 ma obtained on zircon only slightly younger than the inferred timing of high grade metamorphism of the levack gneiss complex the cartier batholith could have formed as a result of approximately 33 partial melting of the levack gneiss complex

observation and inversion of surface wave group velocities across central india abstract surface wave group velocities across central india are measured from computer plots of frequency time analysis using multiple filter technique periods range from 5 to 56 sec for fundamental mode rayleigh wave 5 to 16 sec for first higher mode rayleigh wave and 5 to 49 sec for fundamental mode love wave the wave paths from koyna earthquakes to bokaro cross purely through the peninsular plateau the group velocities for these paths are found to be similar to those of surface waves recorded at poona from earthquake which lie to the northeast of bokaro in sub himalayan regions from this it is concluded that the shield structure of the peninsula extends to the sub himalayan regions to northeast of the peninsular plateu with a negligible part of sedimentary layer group velocities of fundamental mode rayleigh and love waves are also obtained for the paths from koyna earthquakes to shillong and are found to be similar to the above mentioned group velocities except at very low periods where group velocities are affected by the small part of each path through thick sedimentary layer of bengal basin northern limit of this basin sedimentary layer has been identified mean and standard deviation of observed group velocity at each period are obtained for each of rayleigh fundamental rayleigh first higher and love fundamental modes the group velocities closely agree with those obtained earlier for the path between new delhi and kodaikanal and are also found to be similar to those observed in some shield regions monte carlo inversion method is applied and crust and upper mantle structure of the indian shield across central india is obtained crustal structure obtained shows a marked resemblance to the results obtained earlier through body wave study

geochemical relationships in the sudbury igneous complex origin of the main mass and offset dikes petrological models relating the different rock types constituting the 1 85 ga sudbury igneous complex are constrained with extensive new geochemical data we show that the main mass felsic norite transition zone quartz gabbro and granophyre have similar ratios of the highly incompatible trace elements e g la sm 4 5 7 la nb 2 8 4 2 th zr 0 04 0 05 and that these variations are consistent with the crystallization and differentiation of magma types largely 80 derived from the upper crust with a smaller contribution from a mantle source although there is presently no conclusive proof that magma was generated in situ as a melt sheet produced by meteorite impact we find no principal reason why this model should be rejected however we propose that a small contribution of mantle derived picritic magma is required to explain the abundant ni cu and platinum group elements pge in the sudbury deposits as well as the compositions of the ultramafic inclusions mgo 12 36 wt fo sub 68 87 olivines with 450 3 700 ppm ni and abundant chrome rich spinel and the magnesian composition of the mafic norite 8 14 wt mgo and the sublayer 6 12 wt mgo we believe that the main mass of the sudbury complex achieved its present composition through incorporation of up to 20 percent mantle derived picritic magma emplaced along crustal fractures produced by the impact event these picritic magmas entered the melt sheet as a dense plume vigorously mixing with it and due to the marked compositional shift the mixed magma formed magmatic sulfides which sank through the magma column depleting the melt in ni cu and pge since both the felsic norite and granophyre have indistinguishable ratios of the incompatible trace elements we see no requirement to derive these units of rock by the crystallization of magmas derived from different sources rather the compositional difference between the felsic norite and granophyre is attributed to the in situ differentiation of the magma we show that the main mass has many compositional traits similar to those of most of the offset dike quartz diorites e g the parkin offset dike la sm 6 3 la nb 4 5 th zr 0 05 and of embayment related leucocratic norites from the whistle mine la sm 6 2 la nb 5 0 th zr 0 02 these rocks have compositions intermediate between the felsic norite and the granophyre and therefore crystallized from the same magma type arguably the unmineralized quartz diorites provide the best possible estimate of the original magma from which the sudbury complex crystallized in detail there are subtle variations in composition within and between offset dikes with the largest difference being between the north and south range offsets the north range offset dikes cut archean granitoids and gneisses and have elevated sr la yb la sm and gd yb and low tio 2 whereas the south range dikes cut early proterozoic sediments mafic volcanics and intrusions and have low sr la yb gd yb la sm and high tio 2 these differences may be caused by the assimilation of different country rocks during emplacement of the dike a strongly mineralized offset dike at the creighton mine has geochemical variations that are different when compared to the main mass and in the case of creighton are more similar to the local mineralized sublayer these data suggest that mineralized and barren quartz diorites have different geochemical compositions and that these traits may be of value in mineral exploration

evolution of palaeoproterozoic mafic intrusions located within the thermal aureole of the sudbury igneous complex canada isotopic geochronological and geochemical evidence abstract impact cratering and their resultant geological phenomena are recognised as significant factors in the lithological and biologic evolution of the earth age dating of impact events is critical in correlating cause and effects for these catastrophic processes the falconbridge and drury township twp intrusions were emplaced at the contact between neoarchaean basement and palaeoproterozoic volcanosedimentary rocks and also lie at the southeast and southwest edges of the sudbury igneous complex sic within its thermal contact aureole the falconbridge twp intrusion is dated at 2441 3 ma by u pb zircon with evidence of archaean inheritance from its host granitoids granitoids from the southernmost abitibi subprovince are dated here between 2670 11 ma for an undeformed algoman granite and 2696 18 ma for a foliated granitoid consistent with existing data from the abitibi greenstone belt and from the wawa subprovince major and trace element geochemical evidence common pb isotopic compositions and end2440 values between 0 and 1 are all consistent with a palaeoproterozoic origin for the falconbridge twp intrusion and support inclusion in the east bull lake type suite of leucogabbroic plutons and sills in contrast the drury twp intrusion gives a u pb zircon age of 1859 13 ma coincident with the date of sic emplacement while the major and trace element compositions are comparable to the falconbridge data the drury displays significant heterogeneity in end2440 with values ranging from 3 7 to 0 1 and contains more radiogenic pb isotopic compositions field geochemical and isotopic evidence clearly distinguishes this intrusion from constituents of the sic itself and indicates that the drury too is a palaeoproterozoic intrusion this requires that apparently unshocked undeformed magmatic looking zircon has been grown or reset in a postmagmatic setting this has significant implications for the identification of mantle derived magmas and crustal remelts associated with large impact craters a resetting mechanism involving aggressive hydrothermal alteration of zircon facilitated by halogen complexing is proposed inducing rapid postshock lead loss and subsequent annealing

impact induced perturbations of atmospheric sulfur abstract asteroids and comets that are vaporized during hypervelocity impact events can inject large masses of s into the stratosphere where it can potentially affect the radiation budget of the earth alter the chemistry of the ozone layer and eventually be converted to sulfuric acid rain relatively small carbonaceous asteroids 0 3 km in diameter contain 5 times more s than the entire modern stratosphere and these objects hit the earth at an average rate of 1 per 10 000 years larger impact events capable of injecting 10 15 g of s into the stratosphere occur at an average rate of 1 per 1 million years calculations indicate there is sufficient o and h in the vapor plumes of most impact events to convert the s to sulfuric acid aerosols if this conversion occurs then the larger impact events could depress mean surface temperatures by more than 2 c for 3 years or longer

shallow impact isotopic insights into crustal contributions to the sudbury impact melt sheet abstract the largest known terrestrial impact melt sheet occurs within the 1850 ma sudbury structure ontario in order to evaluate the relative contributions of different target lithologies to the melt sheet we have investigated the pb isotope compositions of feldspar separates from early formed quartz diorite magmas within offset dykes from around the impact structure the samples define a linear array on plots of age corrected 206 pb 204 pb versus 207 pb 204 pb samples from offset dykes hosted by the huronian supergroup south range have a range of 206 pb 204 pb 1850 from 15 424 to 17 255 and 207 pb 204 pb 1850 from 15 390 to 15 801 whilst those hosted by archean gneisses of the superior province north range cluster around 206 pb 204 pb 1850 14 8 and 206 pb 204 pb 1850 15 1 these values can be approximated by binary mixing between the two major groups of target lithologies a mix of 60 70 of superior province gneisses with 30 40 of huronian metasedimentary material closely matches the pb isotope compositions of north range offset dyke samples whereas in the south range the required huronian component is up to ca 80 these mixing proportions are consistent with sr nd and os isotope and trace element constraints a third minor component either locally exposed paleoproterozoic mafic rocks or the lower crust is also required however the isotopic trace element and ni cu platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component a major contribution of huronian supracrustal material which had a pre impact thickness of up to 12 km is required to explain the chemical characteristics of the impact melts which also have a strong upper crustal affinity e g eu sm 0 22 rb sr 0 2 0 35 as such a shallower level of melting is apparent than that predicted by many previous impact models for the sudbury event this can be accommodated by considering approach trajectories for the impactor oblique to the earth s surface in addition the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage and may not have been completely homogenised during crater formation our findings have significant implications for the nature of the sudbury impact event the evolution of the melt sheet and the crustal sources of metals contained in sudbury s world class ni cu pge sulphide ores

structural characteristics of the sudbury impact structure canada impact induced versus orogenic deformation a review orogenic deformation both preceding and following the impact event at sudbury strongly hinders a straightforward assessment of impact induced geological processes that generated the sudbury impact structure central to understanding these processes is the state of strain of the sudbury igneous complex the solidified impact melt sheet its underlying target rocks overlying impact breccias and post impact sedimentary rocks this review addresses 1 major structural metamorphic and magmatic characteristics of the impact melt sheet and associated dikes 2 attempts that have been made to constrain the primary geometry of the igneous complex 3 modes of impact induced deformation as well as 4 mechanisms of pre and post impact orogenic deformation the latter have important consequences for estimating parameters such as magnitude of structural uplift tilting of pre impact huronian strata and displacement on major discontinuities which collectively have not yet been considered in impact models in this regard a mechanism for the emplacement of offset dikes is suggested that accounts for the geometry of the dikes and magmatic characteristics as well as the occurrence of sulfides in the dikes moreover re interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact uncertainties still exist as to whether the sudbury impact structure was a peak ring or a multi ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification

seismic imaging of the enigmatic sudbury structure the sudbury structure on the southern margin of the superior craton was created by a catastrophic explosion in 1 85 ga and hosts one of the world s largest ni cu reserves as a unique terrestrial geological feature its genesis has been vigorously debated for more than a century in an effort to optimize the image from seismic reflection data acquired across the center of the sudbury structure we have developed a straight line common midpoint binning strategy and employed cross dip corrections these pseudo three dimensional seismic processing techniques coupled with standard processing methods have overcome limitations associated with conventional two dimensional seismic data processing and have substantially enhanced the seismic image warranting a more detailed and reliable structural interpretation the new image has revealed a major previously unrecognized zone of imbricated nw thrusts interpretation of these thrusts provides critical timing constraints relating the sudbury tectonic deformation to deposition of the sudbury basin sediments the onwatin argillites are penetrated by blind thrust faults whereas the overlying chelmsford turbidites are undeformed and most likely postdate the thrusting thus the uniform paleocurrent trends observed in the chelmsford reflect subsequent deformation history and may be dismissed as evidence against an impact origin based on our interpretation of the seismic image the original volume of the sudbury igneous complex likely an impact melt sheet is 1 104 km3 supporting the claim that the sudbury structure represents the eroded and tectonized remanent of one of the largest known impact structure 200 km on earth

osmium isotope evidence for a crustal origin of platinum group elements in the sudbury nickel ore ontario canada abstract sulphide ores from the international nickel company s inco creighton mine sudbury were analysed for osmium isotope ratios by icp ms rhenium and osmium abundances were determined by isotope dilution using a mixed spike in solid solution in a nickel sulphide matrix calculated initial 187 os 188 os ratios at 1 85 ga the emplacement age of the sudbury complex cluster around 0 60 the occurrence of less radiogenic compositions is attributed to post emplacement open system behaviour of the re os system the creighton results strongly overlap the initial ratios of published osmium data from two other sudbury mines suggesting a narrow range of isotope ratio in the original ore within the range of estimated osmium isotope compositions in the country rock at 1 85 ga using published and new data therefore the sulphide ores of the sudbury complex can have an entirely crustal source without a mantle derived contribution this conclusion is consistent with published neodymium isotope data for the complex and supports the meteorite impact hypothesis since this is the most effective means of fusing the large quantity of crustal rocks necessary to generate the complex the extraction of platinum group elements pge from this silicate melt by a nickel sulphide liquid was probably analogous to the laboratory fire assay procedure

depth of crustal conducting layer and asthenosphere in the pannonian basin determined by magnetotellurics abstract on the basis of numerical modelling and statistical investigations of the horizontal electrical inhomogeneities narrow fractures change of conductance of the surface sediments min magnetotelluric mt sounding curves were found to be less distorted and therefore they can be used for the determination of deep conducting formations in the crust and upper mantle carefully selected mt curves indicate the crustal conductive layer at depths of 7 13 km and the conductive asthenosphere between 50 and 80 km the relatively shallow position of the asthenosphere corresponds to the high heat flow q 80 mw m 1 in the pannonian basin according to the empirical relation derived by adam 1978

isotopic heterogeneity in the sudbury impact melt sheet abstract a unique terrestrial large impact melt sheet is preserved in the 1850 ma sudbury structure ontario we have undertaken a pb isotope investigation of the southern limb of the melt sheet termed the south range main mass the model initial pb isotope ratios 207 pb 204 pb m vary stratigraphically through the predominantly quartz monzogabbroic lower unit varying from 15 40 to 15 45 at the base to ca 15 35 at the top of the sequence lateral variations of similar range occur in basal lower unit samples over scales of less than 5 km the range of these variations is similar to those of locally exposed upper crustal target rocks and it is evident that the melt sheet has efficiently preserved inherited variability during the violent phases of crater formation superheated impact melts are expected to be well mixed mechanically therefore significant post impact melting of target rocks fallback material and entrained clasts is required to explain such heterogeneity the sudbury structure hosts world class ni cu pge sulphide ore deposits systematic variation in 207 pb 204 pb m occurs throughout sulphide ores within the creighton embayment from massive 15 42 15 45 to interstitial ca 15 40 15 41 and disseminated ca 15 39 sulphide linking the pb isotope composition of these ores to the immediately overlying lower unit stratigraphy a protracted sulphide segregation history is apparent massive sulphides segregated early prior to or during initial silicate crystallisation although the total time involved in sulphide accumulation spanned much of the crystallisation of the lower unit it is also shown that lateral variations in ni depletion throughout the main mass correlate with pb isotopes those segments with the strongest chalcophile element depletion signatures reflecting the accumulation of significant basal sulphides have high initial pb isotope values consistent with early sulphide segregation the characterisation of pb isotopic heterogeneity has therefore provided insights into the evolution and scales of mixing of the melt sheet with the identified chemical variability between melt cells having a significant influence on ore forming processes

two high precision u pb zircon ages for the sudbury nickel irruptive two independent u pb zircon ages were measured using norite samples from the north and south ranges of the sudbury nickel irruptive ages of and ma were obtained with a probability of fit within experimental error of 51 and 57 respectively previous determinations by the rb sr whole rock method gave ages between about 1700 and 2000 ma with errors of the order of 100 ma the difference in precision is inherent in the two methods zircons from the norite are highly radiogenic and almost concordant whereas rb sr whole rock ages are determined by very small increments in radiogenic 87sr the two zircon ages have completely different discordia so that all simple models for lead loss by continuous diffusion can be excluded

morokweng south africa a large impact structure of jurassic cretaceous boundary age geophysical petrographical and geochemical data indicate the presence of a large impact structure in the area around morokweng northwest province south africa possibly up to 340 km in diameter drill cores from the center of the structure show a thick layer of impact melt rocks with high abundances of cr ni co and the platinum group elements consistent with the presence of up to 5 of a chondritic component ion probe dating of zircons extracted from the impact melt yielded a 206 pb 238 u age of 146 2 1 5 ma and a 208 pb 232 th age of 144 7 1 9 ma these are indistinguishable from the age of the jurassic cretaceous boundary following the identification of the chicxulub impact structure of cretaceous tertiary boundary age the discovery of a second large impact structure at a previously established major chronostratigraphic boundary strengthens suggestions that large impact events have been major factors in the evolution of the earth

tectonic influences on the morphometry of the sudbury impact structure implications for terrestrial cratering and modeling impact structures developed on active terrestrial planets earth and venus are susceptible to pre impact tectonic influences on their formation this means that we cannot expect them to conform to ideal cratering models which are commonly based on the response of a homogeneous target devoid of pre existing flaws in the case of the 1 85 ga sudbury impact structure of ontario canada considerable influence has been exerted on modification stage processes by late archean to early proterozoic basement faults two trends are dominant 1 the nnw striking onaping fault system which is parallel to the 2 47 ga matachewan dyke swarm and 2 the ene striking murray fault system which acted as a major paleoproterozoic suture zone that contributed to the development of the huronian sedimentary basin between 2 45 2 2 ga sudbury has also been affected by syn to post impact regional deformation and metamorphism the 1 9 1 8 ga penokean orogeny which involved nnw directed reverse faulting uplift and transpression at mainly greenschist facies grade and the 1 16 0 99 ga grenville orogeny which overprinted the se sector of the impact structure to yield a polydeformed upper amphibolite facies terrain the pre syn and post impact tectonics of the region have rendered the sudbury structure a complicated feature careful reconstruction is required before its original morphometry can be established this is likely to be true for many impact structures developed on active terrestrial planets based on extensive field work combined with remote sensing and geophysical data four ring systems have been identified at sudbury the inner three rings broadly correlate with pseudotachylyte friction melt rich fault systems the first ring has a diameter of 90 km and defines what is interpreted to be the remains of the central uplift the second ring delimits the collapsed transient cavity diameter at 130 km and broadly corresponds to the original melt sheet diameter the third ring has a diameter of 180 km the fourth ring defines the suggested apparent crater diameter at 260 km this approximates the final rim diameter given that erosion in the north range is 6 km and the ring faults are steeply dipping impact damage beyond ring 4 may occur but has not yet been identified in the field one or more rings within the central uplift ring 1 may also exist this form and concentric structure indicates that sudbury is a peak ring or more probably a multi ring basin these parameters provide the foundation for modeling the formation of thisrelatively large terrestrial impact structure

geochemical stratigraphy of the huronian continental volcanics at thessalon ontario contributions of two stage crustal fusion the 1500 m thick sequence of huronian continental volcanics at thessalon ontario is subdivided into 4 volcanic cycles each of which includes abundant early mafic end members central intermediate flows and late rhyolite units major and trace element concentrations are dominated by extensive gabbroic fractionation trends that ultimately produced two types of felsic flows 1 rhyolites with high light rare earth element lree and relatively low large ion lithophile element lile concentrations high lree low lile rhyolites and 2 following late separation of ree rich accessory phases rhyolites depleted in lree low lree high lile rhyolites mafic end members of individual volcanic cycle are progressively less siliceous and less enriched in lile and lree with height in the stratigraphic section ti zr ratios gradually rise from 35 in early mafic flows to stabilize at about 85 in late units while average sio2 contents decrease from 56 to about 50 and mg rises from about 48 to 52 nd values are consistently negative indicating variable degrees of pre fractionation crustal contamination of the end member magmas during their uprise through the crust mixing models are consistent with up to 50 contamination by crustal material of tonalitic hornblende gneiss composition a progressive increase in nd from about 5 0 to 0 5 upward in the volcanic succession reflects a decreasing degree of crustal contamination due to development of insulating layers along margins of the feeder system detailed stratigraphic variations suggest that successive magmas batches were intercepted by a progressively fractionating periodically replenished magma source giving rise to open system magmatism despite the prevalence of crustal assimilation in the huronian lavas la sr n ratios are too low in least contaminated end members to be explained by contamination of tholeiitic magmas the late basalts resemble instead modern island are basalts and it is suggested that the subcontinental mantle source was enriched by subduction related processes during crustal formation within individual volcanic cycles gabbroic fractionation trends systematically deviate from calculated factors toward compositions characteristic of hornblende gneiss such relations suggest that further crustal contamination of the magmas occurred simultaneous with crystal fractionation probably within undulating sills at upper crustal levels quantitative analysis suggests assimilation fractional crystallization a fc ratios of about 0 45 as a result of extensive two stage contamination rhyolites from the initial volcanic cycle incorporate a total of over 60 of crust

seismic and electromagnetic evidence of dehydration as a free water source in the reactivated crust according to recent estimates the continental mid crust contains 35 40 per cent amphibolites heating of the crust by an underlying mantle plume for example beneath continental rifts high plateaus and areas of intraplate volcanic activity releases water dehydration of amphibole bearing rocks at depths of 20 40 km occurs mainly in the temperature range 650 700 c and this releases about 0 4 wt per cent of water seismic tomography studies of the crust in the kirgyz tien shan range where the age of the tectonic activity is less than 30 ma revealed a low velocity zone in the mid crust the velocity of p waves was 0 4 km s 1 lower than in normal crust mt sounding data in the region show the existence of a low resistivity layer with an average resistivity of about 25 m at the depth of the low velocity layer the spatial correlation of the observed anomalous layers and calculated effect of fluid phase on seismic and electric parameters of rocks suggests the presence of aqueous fluids released by the heating of the mid crust

re os isotope systematics of ni cu sulfide ores sudbury igneous complex ontario evidence for a major crustal component sudbury igneous complex sublayer ores from the levack west falconbridge and strathcona mines were analyzed for their re and os concentrations and os isotopic compositions the re os isotope systematics of three ores from the different mines give isochron ages of1 840 60 ma 1770 60 ma and 1780 110 ma suggesting that the re os system became closed at the time of or soon after the 1850 1 ma crystallization age of the complex the os isotopic compositions of different portions of the complex at the time of crystallization varied considerably with initial187os 186os1850 ranging from 4 64 at levack west to 7 55 at strathcona these heterogeneities require that the os and probably also the other platinum group elements contained in the ores were derived from at least two sources in addition the high initial187os 186os ratios indicate that the os was derived predominantly from ancient crust previous studies have suggested that the complex either crystallized from a mixture of mantle derived basaltic melt and ancient continental crust or was derived exclusively from the fusion of ancient continental crust resulting from a meteorite impact results of modelling suggest that if a contemporaneous mantle derived basaltic melt was involved in the origin of the sic it likely contributed 50 of the os to all three ores the large percentage of ancient crust involved in the production of the ores is most consistent with an interpretation of substantial crustal fusion resulting from meteorite impact

paleoclimatic control on the composition of the paleoproterozoic serpent formation huronian supergroup canada a greenhouse to icehouse transition unlike other feldspathic arenites in the 12 km thick huronian supergroup strata of the 250 350 m thick serpent formation contain abundant plagioclase deposited in a distal alluvial setting the serpent formation consists of arenites with minor shale and siltstone the plagioclase abundance could reflect unroofing of a unique source relative to other huronian units or could be due to less intense chemical weathering major trace and rare earth element ree geochemistry of the serpent formation are used to evaluate these two possibilities serpent formation source rocks form part of the archean superior province which mainly consists of tonalitic granitic and supracrustal assemblages and their metamorphosed equivalents in al2o3cao na2ok2o acnk space shales define a linear array which indicates that the unweathered source contained plagioclase and k feldspar in the ratio of 5 1 this particular array indicates that serpent fluvial systems tapped a variably weathered carapace developed on the superior province rather than purely fresh bedrock a modelled mixture of 80 tonalite and 20 granite group rocks reproduces ree and trace element characteristics of the serpent formation sandstones and shales average eu eu 0 87 lan smn 4 4 gdnybn 1 8 and th sc 1 65 mass balance considerations indicate that less than 5 supracrustal group rocks can be added at the expense of the granite group a unique source for serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition plagioclase k feldspar ratios in serpent sandstones range from 2 1 to 1 2 which achieves mass balance among source mudstone and sandstone compositions when plotted in acnk space achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles these findings indicate that plagioclase preservation in the serpent formation is related to less intense compared with other huronian arenites paleoweathering conditions probably a harbinger of the widespread continental glaciation recorded in the overlying gowganda formation

evidence of anoxic to oxic atmospheric change during 2 45 2 22 ga from lower and upper sub huronian paleosols canada paleoweathered zones in four underground drill holes in quirke ii mine and one drill hole by denison mines in elliot lake area ontario canada were studied for petrography mineralogy and whole rock chemistry these paleosols formed beneath 2 45 ga old lower huronian formations are characterized by upward depletion of fetotal and fe3 in the whole rock by upward depletion of fetotal fe mg ratios in chlorites and fine grained muscovite and by the presence of fe2 minerals e g pyrite pyrrhotite ilmenite in contrast the paleosols underlying upper huronian strata 2 22 ga old exhibit an upward increase in fe3 and fetotal and contain hematite and goethite uraninite and pyrite bearing paleoplacers occur in lower huronian strata whereas the upper huronian formations contain red beds and are devoid of detrital uraninite and pyrite these differences between the sub huronian paleoweathering profiles and mineralogical characteristics of the lower huronian strata overlying the paleosols support the previously postulated oxyatmoversion theory and restricts the time interval for its occurrence to between 2 45 and 2 22 ga assuming that few tens of millions of years were required for the deposition of lower huronian groups oxyatmoversion likely occurred at about 2 4 ga

crustal block of the western ganga basin a fragment of oceanic affinity abstract three component long period new delhi records of two earthquakes occurring near multan central pakistan are used to determine group velocities of multimode surface waves in the period range of 5 to 57 sec inversion of these data has provided insight regarding the crustal shear velocity structure in the western ganga basin in the northwestern wedge of the indian subcontinent the shear velocity is less than 3 10 km sec within the topmost 10 km of its 40 km thick crust the estimated velocity in the 10 to 20 km depth range is 3 80 0 09 km sec in the lower half crust the velocity is as high as 3 98 0 08 km sec unlike the continental shield crust in the new delhi shillong region the crustal structure in this western part of the ganga basin is strongly reminiscent of certain oceanic plateaus this suggests that the crustal block underlying the sediments is not part of the stable core or the craton of the indian shield the available data do not allow the determination of fine velocity structure in the upper mantle the average velocity in the uppermost 80 km of the mantle is 4 52 km sec this value appears to be characteristic of the entire northern indian subcontinent and may be indicative of active tectonism in the mantle south of the himalayas

conductivity structure of the lithosphere asthenosphere boundary beneath the eastern north american margin tectonic plate motion and mantle dynamics processes are heavily influenced by the characteristics of the lithosphere asthenosphere boundary lab yet this boundary remains enigmatic regarding its properties and geometry the processes involved in rifting at passive margins result in substantial alteration of the lithosphere through the transition from continental to oceanic lithologies here we employ marine magnetotelluric mt data acquired along a 135 km long profile offshore martha s vineyard new england usa to image the electrical conductivity structure beneath the new england continental margin for the first time we invert the data using two different mt 2 d inversion algorithms and present a series of models that are obtained using three different parameterizations fully unconstrained unconstrained with an imposed lab discontinuity and a priori constrained lithosphere resistivity this suite of models infers variability in the depth of the lab with an average depth of 115 km at the eastern north america passive margin models robustly detect a 350 m lithospheric anomalous conductivity zone lacz that extends vertically through the entire lithosphere our preferred conductivity model is consistent with regional p to s receiver function data shear wave velocity gravity anomalies and prominent geological features we propose that the lacz is indicative of paleo lithospheric thinning either resulting from kimberlite intrusions associated with rifting and the new england great meteor hotspot track or from shear driven localized deformation related to rifting

structural control on along strike variation in the seismicity of the northwest himalaya an overview of seismicity along the northwest himalaya exhibit along strike segmentation intricately controlled by the subsurface topographic ridges on the underthrusting indian plate as well by rift and nappe structures in the overriding wedge of the himalaya the segmentation exists for thrust dominated large magnitude earthquakes m 6 seated on the active detachment beneath the outer and lesser himalaya segmentation also prevails for moderate and small magnitude earthquakes concentrated in a narrow himalaya seismic belt hsb straddling northern lesser himalaya and southern higher himalaya numerical calculations of stress distribution favour that the degree of seismicity in the hsb is a good proxy to the presence of mid crustal ramp connecting the locked section of active detachment to aseismically slipping detachment beneath higher himalaya further gap or diffused pattern in the concentrated seismicity in the hsb in agreement with mapped high electrical conductive structure suggests ramp structure may be absent where underthrusting delhi hardwar ridge interacts with the himalaya arc in the nappe dominated tectonics the accommodation of the accumulating strains on the listric thrust faults produces increased frequency of moderate magnitude earthquakes and thereby possibly influences stress level on the detachment this may explain relatively less frequent occurrences of large earthquake in the kangra chamba region compared to that in the garhwal himalaya the low level seismicity in the simla region may also be influenced by the active kaurik chango rift in the higher himalaya as the former casts a stress shadow on the latter

crustal geochemistry in the wawa foleyet region ontario fifty three rock samples from the kapuskasing structural zone ksz and 56 from the wawa domal gneiss terrane wgt ontario have been analysed for major elements 32 trace elements and 18o d was measured in a few samples average chemical compositions for the ksz and wgt regions have been calculated from map unit averages weighted by regional abundance compared with estimates of the composition of the upper continental crust ucc the ksz is enriched in al fe mg ca p transition elements sc and sr depleted in si b most rare earth elements zr hf nb ta li na k u th ba and rb but similar in composition to estimates of the lower continental crust the wgt is closer in composition to the ucc these data support the interpretation on geophysical and petrological grounds that the crust here is layered and has been uplifted the wgt and ksz regions representing progressively deeper levels igneous rock and orthogneiss 18o values in the ksz and wgt show good correlation with the weigh

origin of the sudbury complex by meteoritic impact neodymium isotopic evidence samarium neodymium isotopic data on whole rocks and minerals of the sudbury complex in canada gave an igneous crystallization age of 1840 21 x 10 6 years the initial epsilon neodymium values for 15 whole rocks are similar to those for average upper continental crust falling on the crustal trend of neodymium isotopic evolution as defined by shales the rare earth element concentration patterns of sudbury rocks are also similar to upper crustal averages these data suggest that the sudbury complex formed from melts generated in the upper crust and are consistent with a meteoritic impact

surface wave and lithospheric structure across the northwestern part of the indian peninsula group velocities of fundamental mode rayleigh and love waves have been obtained across the central highlands in the northwestern part of the indian peninsula and across the plateau region of the peninsula such data across the latter region were also obtained earlier and revealed model lithosphere ip11 for this region data of the two regions are similar and show that the lithospheric model ip11 continues from the plateau region to the central highlands the model ip11 has been verified by comparing observed love wave forms with synthetics ones

origin and emplacement of offset dykes in the sudbury impact structure constraints from hess the hess offset is a steeply dipping dyke located 12 15 km north of the 1 85 ga sudbury igneous complex sic within the 200 250 km diameter sudbury impact structure it is up to 60 m wide and strikes subconcentrically to the sic for at least 23 km the main phase of the dyke is granodioritic but it conforms with what is locally referred to as quartz diorite a term used for all the offset dykes of the sudbury impact structure rare earth element data shows that the hess offset is genetically related to the sic hess is most closely affiliated with an evolved felsic norite component of sic and not bulk impact melt this indicates that hess was emplaced during fractionation of the impact melt sheet rather than immediately following impact the main quartz diorite phase of the dyke comprises a quartz plagioclase hornblende biotite clinopyroxene orthopyroxene assemblage critically the hess offset occupies a concentric fault system that marks the northern limit of a pseudotachylyte rich shatter cone bearing annulus about the sic this fault system was active during the modification stage of the impact process

differentiation and emplacement of the worthington offset dike of the sudbury impact structure ontario the offset dikes of the 1 85 ga sudbury igneous complex sic constitute a key topic in understanding the chemical evolution of the impact melt its mineralization and the interplay between melt migration and impact induced deformation the origin of the melt rocks in offset dikes as well as mode and timing of their emplacement are still a matter of debate like many other offset dikes the worthington is composed of an early emplaced texturally rather homogeneous quartz diorite qd phase at the dike margin and an inclusion and sulfide rich quartz diorite iqd phase emplaced later and mostly in the centre of the dike the chemical heterogeneity within and between qd and iqd is mainly attributed to variable assimilation of host rocks at the base of the sic prior to emplacement of the melt into the dike petrological data suggest that the parental magma of the worthington dike mainly developed during the pre liquidus temperature interval of the thermal evolution of the impact melt sheet 1200 c based on thermal models of the cooling history of the sic the two stage emplacement of the worthington dike occurred likely thousands to about ten thousand years after impact structural analysis indicates that an alignment of minerals and host rock fragments within the worthington dike was caused by ductile deformation under greenschist facies metamorphic conditions rather than flow during melt emplacement it is concluded that the worthington offset dike resulted from crater floor fracturing possibly driven by late stage isostatic readjustment of crust underlying the impact structure

isotopic evidence for distinct crustal sources of north and south range ores sudbury igneous complex lead isotope ratios were measured on fe cu ni sulphide ores and associated silicate phases from nine mines on the north and south ranges of the sudbury igneous complex sic these data are compared with the lead isotope signatures of country rocks and shock melted sudbury breccias around the complex both ore and feldspar data from the south range fall along a 1 85 ga reference line the age of the igneous complex and indicate a magmatic origin for lead in the ore data from the north range are also consistent with a magmatic origin but are displaced to lower 206pb 204pb and 207pb 204pb ratios these two suites fall close to the lead isotope composition of country rocks adjacent to the south and north ranges respectively furthermore there is a close correspondence between lead signatures of the sic and sudbury breccia on each side of the complex the evidence suggests an origin for the sic by shock melting of different crustal units on the two sides of the complex this is consistent with the impact of a meteorite on the southward thickening apron of huronian strata which overlay the margin of the superior craton at 1 85 ga

a u pb age for mineralized nipissing diabase gowganda ontario concordant baddeleyite and rutile analyses from mineralized nipissing diabase at gowganda s castle mine yield a u pb age of baddeleyite is a primary phase of the diabase therefore the age dates the magmatic emplacement the genesis of rutile is less certain rutile could have formed during an episode of alteration veining and ag mineralization in which case the u pb age would also date the secondary event and demonstrate that ag mineralization was related to the nipissing magmatism however a primary magmatic origin of the rutile cannot be completely excluded for now thus the u pb age may not constrain the time of ag mineralization further research is needed to clarify this point

pre emplacement re os ages from ultramafic inclusions in the sublayer of the sudbury igneous complex ontario abstract re os isotope analyses have been performed on a suite of 14 melanorite inclusions from the sublayer of the sudbury igneous complex sic at whistle mine 30 km nne of sudbury ontario canada six s poor olivine melanorites have relatively unradiogenic present day 187 os 188 os ratios of 0 16 1 2 and define an imprecise age of 1 97 ga their 187 os 188 os 1 85 ratios of 0 107 0 237 are compatible with derivation either from the mantle at the time of sic formation 1 85 ga ago or from pre existing crustal rocks with low re os ratios and unradiogenic os isotope compositions in contrast the relatively s rich inclusions define much older ages of 2 46 ga melanorites and 2 67 ga olivine melanorites while 187 os 188 os 1 85 ratios of 0 41 1 80 unequivocally demonstrate that their os was largely derived from pre existing crustal lithologies the os isotope compositions and s abundances of the inclusions are consistent with mixing at 1 85 ga between an unradiogenic silicate fraction defined by the s poor suite and a radiogenic sulphide rich component with re os ratios similar to those of pre existing crustal lithologies attempts to evaluate the source of the ni cu s and pge in the sic through mass balance calculations are not definitive because of the difficulties in estimating accurately both present day and pre sic crustal abundances however crude estimates suggest that the overall levels of ni and cu in the pre impact crust were probably sufficient to account for the budget of these elements in the present day sic but do not rule out the possibility of significant late archaean early proterozoic sulphide mineralisation in the pre existing sic target rocks

origins of outliers of the huronian supergroup within the sudbury structure abstract the spatial relations between outliers of huronian metasediment and the current outline of the sudbury igneous complex sic have been used previously to constrain the original size of the meteorite impact crater despite uncertainty regarding their structure and origin the outliers are tight to isoclinal gently plunging synclines with northeast southwest trending axes slightly overturned to the northwest major deformation of the huronian supergroup preceded the 1849 ma sudbury meteorite impact event by sufficient time to permit peneplanation of the region impact occurred in a foredeep related to the subduction of the laurentia plate under the wisconson magmatic terranes faults truncate the northwest margins of most of the outliers where examined at several localities the northwest margin bounding faults are invariably occupied by pseudotachylite structural considerations provide evidence for thousands of meters of slip along these surfaces each of which was apparently accomplished in

thick sedimentary sequence around bahraich in the northern part of the central ganga foreland basin abstract we present the results of a magnetotelluric study along a 285 km long profile between hamirpur and rupadia nepal border across the central ganga basin the electrical resistivity image obtained by combining 1 d occam inversion models for 39 sites reveals a significant contrast in the subsurface structure from south to north along the profile at the southern end the bundelkhand massif is delineated as a high resistivity block buried beneath 250 300 m thick sediments the thickness of sediments gradually increases to about 500 600 m at kanpur and to about 1 2 km at lucknow here the basement depth increases to more than 2 5 km within a profile distance of 20 km which could be attributed to the lucknow fault the underlying rocks also have moderate resistivity and possibly represent the vindhyans the sedimentary sequence at the northern end of the profile around bahraich is more than 9 km thick integrating the resistivity image with a published seismic velocity structure from the region and the lithology from the 3927 m deep matera i well reveals that the top 4 km succession is constituted of highly conductive oligocene and younger rocks of the matera formation and the siwaliks and recent sediments whereas the underlying 5 km section is composed of sedimentary rocks of the bahraich group overlying the archean basement the high conductivity of sediments in conjunction with the low seismic velocity and large vp vs obtained by receiver function analysis implies poor consolidation of sediments and thus high seismic hazard potential the present results have implications for hydrocarbon exploration hazard potential scenario of the central ganga basin and flexural strength of the indian plate

pb isotope constraints on the provenance and diagenesis of detrital feldspars from the sudbury basin canada abstract residues after hf leaching of detrital feldspar grains from the early proterozoic chelmsford formation of the sudbury basin ontario record isotopic compositions very close to the initial pb isotopic composition of their late archean superior province source these grains have been recrystallized to albite and k feldspar residues of chelmsford feldspar grains lie within the range of reported 206 pb 204 pb compositions of feldspars from plutons of the adjacent abitibi subprovince of the superior province with a steep 207 pb 206 pb slope of 0 35 0 15 3 7 0 7 ga leaches are more radiogenic than residues but leach residue pairs do not record in situ u and th decay since either the late archean time of feldspar crystallization in the superior province or the early proterozoic time of recrystallization in the chelmsford formation instead leach residue pairs lie along an average 207 pb 206 pb slope of 0 250 0 036 representing mixing trends most whole rock samples from the chelmsford formation lie close to a 2 7 ga slope collinear with detrital feldspar compositions we interpret the pb data from the leaches as recording the passing of altering fluids these fluids likely carried pb derived from rocks or minerals with relatively high u pb th pb and th u within the sedimentary basin itself the residues appear to record very close to initial values because they lie along the very steep trend defined by feldspars of the superior province

lead isotopic heterogeneities within alkali feldspars implications for the determination of initial lead isotopic compositions abstract the mobility of uranium during metamorphism and alteration places constraints on the use of lead isotopic compositions as petrogenetic tracers in older rocks the problem of uranium mobility may be partially circumvented by analysis of alkali feldspar which because of its low u pb ratio should have a lead isotopic composition similar to the initial lead isotopic composition of the rock previous leaching and volatilization experiments however have demonstrated that alkali feldspars contain radio genic lead which can be removed preferentially to nonradiogenic lead this paper reports on the results of progressive acid leaching of a single alkali feldspar phenocryst and on albitic perthites separated from the phenocryst this experiment suggests that a number of distinct lead isotopic reservoirs are present within the alkali feldspar including a nonradiogenic reservoir approximating the initial lead isotopic composition of the system a radiogenic reservoir corresponding to albitic exsolution lamellae within the alkali feldspar loosely bound labile lead presumably associated with uranium and thorium occupying fractures and finally a radiogenic component corresponding to unidentified uranium thorium rich phases the commonly observed decrease in radiogenicity with progressive leaching of alkali feldspars is interpreted to be largely the result of preferential leaching of the albitic perthites relative to the host alkali feldspar containing the nonradiogenic reservoir the results of this experiment confirm that initial lead isotopic compositions may be estimated from alkali feldspars through severe acid leaching however this experiment also illustrates the complexity of the lead isotopic systematics of alkali feldspars the determination of the initial lead isotopic composition of an alkali feldspar requires experimental procedures which recognize and compensate for the presence of multiple lead reservoirs within typical alkali feldspars

impact and volcanism a momentum scaling law for erosion the controversy that raged through the 1950s 1970s over similarities and differences between meteorite impact and volcanic processes is revisited in this paper we propose that there are quantitative similarities in erosion caused by high speed ejecta produced by either impacts or volcanic processes field and petrographic data from the manicouagan impact crater canada are used to demonstrate that during the emplacement of the impact melt sheet erosion occurred at a rate of 2562 kg m 2 s 1 field data for the mount st helens lateral blast of may 18 1980 suggest an erosion rate of 2 kg m 2 s 1 and field data for a small pyroclastic flow on august 7 1980 suggest an erosion rate of 14 kg m s 1 it is proposed that these three rates were determined dominantly by the momentum of the ejecta and a quantitative formulation based on lofting theory is given a new application of the monte carlo approach to analysis provides minimum most likely and maximum estimates for both the field and the theoretical analyses the substantial erosion that occurs by fast moving flows results in mixing of stratigraphic components over large distances and to very fine scales

comparative geochemistry of pleistocene and paleoproterozoic huronian glaciogenic laminated deposits relevance to crustal and atmospheric composition in the last 2 3 ga abstract comparison of the major and trace element geochemistry of glaciogenic laminated deposits of pleistocene and paleoproterozoic age from an area north of lake huron ontario has revealed differences and similarities that may provide insight into the erosional history of part of the archean superior province of the canadian shield during the last 2 ga and some aspects of the evolution of the earth s atmosphere both the major element and trace element compositions of the ancient and recent glacial deposits are very similar the paleoproterozoic mudstones are however enriched in fe and u characteristics that may reflect the oxygen content of the archean early paleoproterozoic atmosphere hydrosphere system strong ca depletion is exhibited by the paleoproterozoic glaciogenic argillites but is absent from the pleistocene samples the proterozoic argillites have slightly higher values for a chemical index of alteration than those of pleistocene muds these characteristics may reflect the acid natur

deep electrical conductivity structure beneath the southern part of the indo gangetic plains analysis of magnetotelluric data near the southern boundary of the indo gangetic basin reveals a clear two dimensional structure two dimensional numerical modelling of the data has been carried out the e polarization response is found to be very similar to the local one dimensional response a study has been carried out to remove the screening effect on the magnetotelluric data caused by the top layer sediments after the removal of the top layer sediments a best fit model of the averaged magnetotelluric field data has been found which has a very thick resistive upper crust and a conducting lower crust the conductance of the lower crust is found to be much less than the values of lower crustal conductance obtained in eastern siberia in contrast the upper mantle below the indo gangetic plains has a resistivity significantly greater than that of the upper mantle below eastern siberia

the formation of the sudbury breccia in the north range of the sudbury impact structure abstract in the north range of the ca 1 85 ga sudbury impact structure massive bodies of sudbury breccia cut across the archean levack gneiss complex and early proterozoic matachewan diabase dykes the complex underlies the sudbury igneous complex sic which represents an impact melt sheet that lined the floor of the impact crater the breccia occurs as 1 100 m wide irregular bodies that contain rounded clasts of diabase and tonalitic and dioritic levack gneiss within a fine grained to aphanitic black to dark grey matrix mineral chemical analyses and rietveld analyses based on x ray diffraction patterns show that the breccia matrix contains microclasts of magnesiohornblende oligoclase and quartz surrounded by metamorphic andesine laths actinolite kfeldspar albite and chlorite which crystallized during cooling of the sic on binary oxide sio2 diagrams breccia values plot on a compositional mixing line between those of the more felsic tonalitic gneiss and the more mafic dioritic gneiss and diabase the breccia has concentrations in rare earth elements rees intermediate between those of the dioritic and tonalitic gneisses and a ree pattern with a negative slope that is less pronounced than those of the gneisses this is due to the addition of a minor diabase component that has a slightly negative ree slope suggesting that the breccia is a mix of comminuted tonalitic gneiss dioritic gneiss and minor diabase comminution and cataclasis of these source rocks occurred along pre existing anisotropies and fractures that formed and were reactivated during the growth and collapse of the transient crater the comminuted materials mixed together as large bodies of breccia that were injected in dilational sites that opened during the upward expansion and collapse of the crater

homogeneous impact melts produced by a heterogeneous target sr nd isotopic evidence from the popigai crater russia the 35 7 0 2 ma old popigai crater siberia with a diameter of about 100 km is one of the best preserved large terrestrial impact structures the heterogeneous target at the impact site consists of archean to lower proterozoic metamorphic rocks of the crystalline basement upper proterozoic quartzites and other clastic deposits as well as cambrian to cretaceous clastic sediments and sedimentary rocks including carbonate rocks moreover proterozoic and permo triassic dolerite dykes are found in the target area we report major element sr and nd isotope data for 13 of these target rocks and for various types of impactites the 15 analysed impactite samples include tagamites impact melt rocks suevites and impact glass from small veins furthermore two impact breccias and two impact glass coated gneiss bombs were analysed we discuss the relation of these impactites to the target lithologies and evaluate on the basis of literature data the relation of microkrystites and associated microtektites in upper eocene sediments to the popigai event the impactites have sio2 abundances ranging from 59 to 66 wt and show significant variations in the content of fe ca and ti they have present day 87sr 86sr ratios between 0 7191 and 0 7369 their sr model ages tsrur range from 1 9 to 2 3 ga the 143nd 144nd ratios for the impactite samples cluster between 0 5113 and 0 5115 the nd model ages tndchur range from 1 9 to 2 1 ga in an echur nd eur sr diagram the impactites and upper eocene microkrystites and associated microtektites plot in a field delimited by popigai target lithologies the impactites are restricted to the field of crystalline basement rocks and upper proterozoic quartzites but they show different isotopic signatures in different crater sectors impactites and upper eocene microkrystites plot in different only partly overlapping clusters the leucocratic microkrystites and microtektites have a higher affinity to the post proterozoic rocks in the target area than the impactites seemingly the melanocratic microkrystites originated mostly from crystalline basement this data alignment supports the assumption that popigai is the source crater for all three types of ejecta for the first time clear relations are established of the geochemically variable upper eocene microkrystites and associated microtektites to specific target lithologies at popigai crater finally the observed range in sr and nd isotope parameters determined for impact melt lithologies that originated during the popigai event show a much higher variability than known from other craters this result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias was incomplete at the time of ejecta dispersal

rare earth element characteristics and origin of the sudbury igneous complex ontario canada abstract systematic geochemical studies have been carried out on the sudbury igneous complex sic and its country rocks the two main rock types of the sic norite and granophyre show contrasting chemical features the norite contains 54 58 wt sio2 whereas the granophyre has a sio2 content of 65 71 wt all rocks are characterized by high absolute rare earth element contents ree 63 314 ppm the granophyre having more than twice the ree contents of the norite and by lree enriched chondrite normalized distribution patterns huronian volcanic rocks and huronian plutonic granites have the highest ree contents and hree enrichment among the country rocks archean rocks are characterized by very steep ree patterns and positive eu anomalies nipissing diabase contains the lowest ree contents and has a flat ree pattern within the sic the ree values are more or less constant through the norite but the abundances of some elements such as ce and yb decrease slightly upward with stratigraphy ree contents increase sharply in passing from norite to granophyre and the eu eu ratio decreases from around unity to 0 66 the contrasting ree contents of the norite and granophyre combined with their distinctive major and trace element contents are considered to indicate that they resulted from two separate batches of magma probably emplaced at about the same time the previous models in which the sic was considered to originate from a single magma either contaminated mantle magma or crustal melt are not adequate a new hypothesis is developed based on the chemical features observed in this study the sic is considered to have comprised two independent intrusions the norite was derived from a heavily contaminated mantle magma 30 60 the main contaminant being archean granulite facies rocks of the lower crust the granophyre represents a crustal melt 75 archean rocks and 25 huronian granite induced by meteorite impact the new hypothesis better accounts for the large proportion of granophyre in relation to norite and for the ni cu sulfide mineralization associated with the sic it is also favored by the rb sr isotopic data

differentiation and source of the nipissing diabase intrusions ontario canada major and trace element data are presented for 2 2 ga proterozoic diabase sills from across the nipissing magmatic province of ontario in situ differentiation of the magma coupled with assimilation of huronian supergroup roof sediments is responsible for the variation in composition between quartz diabase and granophyric diabase seen within many of the differentiated intrusions uniform trace element and isotope ratio signatures such as la sm 2 8 3 7 and endchur 2 7 to 5 9 characterize chilled margins and undifferentiated quartz diabases these chemical signatures suggest the existence of a single magma source that was parental to intrusions throughout the magmatic province this magma has higher la sm and lower ti y than primitive mantle and is displaced towards the composition of shales most chilled diabases and quartz diabases have a similar mg 0 64 and 0 60 and ni content 98 and 127 ppm and it is argued that the magma differentiated at depth and was emplaced as a uniform low mg magma

190pt 186os and 187re 187os systematics of the sudbury igneous complex ontario abstract we measured by negative thermal ionization mass spectrometry ntims re os and 186 os 188 os and 187 os 188 os in 26 samples of 18 ni cu sulfide ores from the falconbridge mccreedy west and strathcona mines at sudbury ontario at mccreedy west and falconbridge the isochron re os ages are 1835 70 ma and 1827 340 ma and the initial 187 os 188 os ratios 0 514 0 019 and 0 550 0 024 respectively the ages agree with the canonical value of 1850 1 ma for the sudbury igneous complex sic for hangingwall and deep zone ores at strathcona the age of 1780 7 ma may reflect resetting by dyke activity the high initial 187 os 188 os of 0 934 0 005 in these ores is distinct from those at mccreedy west and falconbridge strathcona deep copper zone ores have highly radiogenic os giving a mean model age of 1883 54 ma that is similar to ages at mccreedy west and falconbridge but distinct from other strathcona sulfides initial 186 os 188 os in two strathcona ores with low 190 pt 188 os average 0 119 826 0 000 009 n 3 and 0 119 827 0 000 004 n 3 respectively with a grand mean of 0 119 827 0 000 003 this ratio may be slightly lower than the chondritic value at that time similar ores at falconbridge and mccreedy west show more scatter averaging 0 119 855 0 000 008 n 6 and 0 119 867 0 000 020 n 3 respectively these values are substantially suprachondritic the re os isotope systematics of sudbury ores are clearly of crustal origin and may be derived from a binary mixture of superior province and huronian metasedimentary rocks with strathcona falconbridge and mccreedy west ores containing respectively 55 16 and 12 of os from superior sediments the suprachondritic 186 os 188 os at mccreedy west and falconbridge may be due to admixture of archean or paleozoic mafic rocks with 190 pt 188 os 0 1 no trace of the asteroid that produced the sudbury structure has been reported at the whistle mine s poor olivine melanorite inclusions with high ni and os and low 187 os 188 os may contain the signature of a magmatically fractionated asteroidal core contributing 1 to 2 5 metal the s poor melanorite ni and os data are equally well explained by admixture of 40 mantle peridotite however