the composite north american craton superior province deep crustal structure and mantle plume model of neoarchaean evolution abstract this paper aims to synthesize geological and geophysical data on superior province and in general archean crust of the composite north america craton relying on experience with deep geological and geophysical studies of composite east european craton by onset of the neoarchean archean provinces of the north american craton were parts of the single stable continent intensely reworked in the neoarchean crustal rocks were combined into a round oval domain wherein distribution of geophysical structural and metamorphic parameters displays concentric zoning the central zone occupies hudson bay basin located immediately above the lithospheric keel the internal zone is mainly composed of granulite facies plutonic volcanic and sedimentary rocks the external zone encompasses southern part of the superior province together with hearne and rae provinces this paper presents 3d crustal models of southern superior province obtained through synthesis of geological maps with a new geological interpretation of seismic sections along lithoprobe geotransects and of the eastern abitibi belt based on higher resolution reflection seismic images neoarchean evolution of the north american craton represents plate tectonic processes initiated by a superplume it is only one of series of similar phenomena recorded 2 75 ga ago in a number of regions the most important features repeated to a certain degree in tectonic units of this type are 1 synchronous formation between 2 79 and 2 58 ga 2 frequently repeated combination of high grade granulite and high temperature amphibolite facies and low or moderate grade greenschist and epidote amphibolite facies metamorphic rocks 3 lower crustal granulite basaltic layer reflectivity zone displayed in seismic images of crust formed or transformed at the final stage of endogenic activity

widespread neoarchean 2 7 2 6 ga magmatism of the yangtze craton south china as revealed by modern river detrital zircons abstract the kongling terrane which is the archean nucleus of the yangtze craton preserves paleoarchean proterozoic rocks as old as 3 45 ga however the dominant stage of formation of this archean terrane remains unclear in this paper u pb and lu hf isotopes of detrital zircons from two rivers and one stream in the northern part of the kongling terrane were studied by la icp ms and la mc icp ms respectively these zircons show complicated internal structures in cathodoluminescence images but the majority of them have linear or oscillatory zoning patterns indicating magmatic origins in general the detrital zircons from these three local rivers show similar u pb age distributions together they yield age peaks at 3 3 3 1 ga 5 3 0 2 8 ga 18 2 7 2 6 ga 30 2 6 2 2 ga 15 2 0 1 9 ga 27 1 9 1 7 ga 1 1 7 1 5 ga 2 and 1 0 0 8 ga 1 this age distribution implies that the north kongling terrane formed primarily during the meso to neoarchean 3 0 2 6 ga lu hf isotopic data reveal predominant subchondritic to chondritic e hf t values 11 3 to 0 for the mesoarchean zircons indicating the dominant role of crustal reworking during this period in comparison most neoarchean zircons exhibit near chondritic to suprachondritic e hf t values up to 7 4 suggesting juvenile crustal additions the 2 0 1 9 ga zircons have similar initial hf isotopic compositions as those of the 2 7 2 6 ga zircons suggesting a metamorphic recrystallization origin this observation further implies that the proportion of neoarchean ages might be underestimated previous studies have reported several 2 7 2 6 ga archean outcrops minor detrital zircons in precambrian sedimentary rocks and xenocrystal zircons in volcanic rocks from the yangtze craton combined with our data we propose that the 2 7 2 6 ga magmatism may have played an important role in forming the kongling terrane and have widely affected other parts of the yangtze craton as well similar to many other archean cratons worldwide the 2 7 2 6 ga may correspond to the initial stabilization of the archean nucleus of the yangtze craton thereafter this craton shows comparable 2 5 2 2 ga age records to the recently proposed nunavutia supercraton which could provide further clues to the early tectonic process of the yangtze craton

petrogenesis of intermediate volcanic assemblages from the shebandowan greenstone belt superior province evidence for subduction during the neoarchean abstract research on the petrogenesis of andesites and their implications for geodynamic setting are an important facet to understanding controversial tectonic processes during the archean the genesis of archean intermediate volcanic rocks and their relationship to mantle and crust derived melts can either support subduction dominated tectonic processes or plume crust interactions this study describes the lithogeochemistry and nd isotopic composition of intermediate volcanic assemblages in the shebandowan greenstone belt of the wawa abitibi terrane that were deposited prior to deformation and tectonic assembly of the superior province the intermediate rocks of the shebandowan greenstone belt are unique in that they are voluminous are relatively weakly deformed and are low metamorphic grade and contain significant amounts of magnesian andesites and adakites the major and trace element geochemistry of the intermediate assemblages share many of the geochemical characteristics of modern volcanic and continental arcs these features include enriched th nb and th la ratios steeply dipping rare earth patterns and pronounced negative ti and nb anomalies on primitive mantle normalized diagrams additionally these rocks also contain distinct positive pb and cs anomalies without evidence of major mobility of these elements during secondary processes neodymium isotopic analyses indicate the interaction with older crust and show good correlations with evolved nd isotopic values with other crustal contamination proxies such as th ce and ti sc ratios modeling mixing and assimilation fractional crystallization interactions between plume derived and crustal melts reveal that plume crust interactions cannot explain the compositional array obtained from the intermediate rocks in the shebandowan greenstone belt a compilation of u pb geochronology for the shebandowan greenstone belt reveals that tholeiitic komatiite dominated strata are slightly older than the intermediate dominated strata given that these domains are structurally separated it is very likely that the shebandowan greenstone belt was formed in two different geodynamic settings these different geodynamic settings have important implications for the metallogeny of the belt and explain the relative enrichment of au and ni cu mineralization in the shebandowan greenstone belt relative to other ca 2720 ma assemblages in the wawa abitibi terrane

geodynamic reconstruction of the winston lake greenstone belt and vms deposits new trace element geochemistry and u pb geochronology the ca 2720 ma winston lake greenstone belt of the wawa abitibi terrane superior province is located on the northern margin of the wawa subprovince and has important regional tectonostratigraphic implications for the development of the neoarchean superior province the metallogenic importance of the belt is evident from the presence of the winston lake pick lake and zenith volcanogenic massive sulfide vms deposits however the chemostratigraphic evolution has not been fully examined at the greenstone belt scale nor have the vms deposits been examined from the perspective of the belt scale petrogenesis and volcanic reconstruction this study 1 reviews the geology and vms mineralization in the winston lake greenstone belt and 2 reconstructs its geodynamic setting and its magmatic and metallogenetic history using new geological trace element and nd isotope data constrained by new u pb geochronology the belt is subdivided into two main lithotectonic assemblages the winston lake assemblage composed of tholeiitic to calc alkalic bimodal volcanic lithofacies and the big duck lake assemblage composed of alkalic to tholeiitic mafic flows and sills the contact between the winston lake and big duck lake assemblages is intruded by the differentiated zenith gabbro which hosts the 0 16 mt zenith vms orebody as a series of xenoliths the variably altered felsic lithofacies of the winston lake assemblage host the 3 1 mt winston lake main and 1 3 mt pick lake zn rich vms deposits the felsic volcanic lithofacies of the winston lake assemblage are fiii type rhyolites and have a bimodal distribution of zr ti indicating two distinct magmas formed by different temperatures of partial melting mafic lithofacies of the winston lake assemblage are typically calc alkalic to transitional between calc alkalic and tholeiitic in composition they have arc to back arc trace element contents with distinctive negative nb and ti anomalies enrichment in the light rare earth elements lree and higher th nb ratios than the mafic rocks from the big duck lake assemblage mafic strata from the big duck lake assemblage have trace element characteristics similar to mature back arc tholeiitic basalts and oib like or oceanic plateau alkalic basalts with high nb th and ti v ratios the chemostratigraphy of the winston lake greenstone belt is consistent with a geodynamic model where arc rifting is followed by back arc development nd isotope data indicate that strata from the winston lake and big duck lake assemblages are juvenile and show no evidence of contamination by significantly older crust therefore crustal contamination signatures in the mafic flows from the winston lake assemblage must have formed by contamination from crust that is of similar age and supports a comagmatic relationship between the mafic and felsic strata the mafic flows of the big duck lake assemblage represent a mature back arc setting with evidence for enriched plume generated basalts in the upper mafic unit the winston vms deposit formed during the early rifting of the arc and the timing is tightly constrained by the hydrothermally altered 2721 2 0 9 ma vms hosting felsic strata and younger postmineralization 2719 4 ma zenith gabbro the zn dominated vms mineralization suggests that temperature of the hydrothermal fluid during discharge was relatively cool 300 c and did not maintain sufficiently high temperatures to precipitate significant amounts of copper relative to zinc this is likely due to adiabatic cooling of the fluid because of a low pressure 0 5 kb sea floor environment and or permeable volcaniclastic strata a u pb age of emplacement of 2692 1 0 7 ma from the syndeformation quartz feldspar porphyry in the big duck lake assemblage indicate it is comagmatic with syndeformation magmatic events typical of other greenstone belts along the northern margin of the wawa abitibi terrane this event is interpreted to represent the timing of terrane accretion of the wawa abitibi terrane to the superior province

the role of granitic intrusions in the evolution of the abitibi belt canada abstract the abitibi greenstone belt canada constitutes a segment of a broad volcanic arc that experienced three distinct periods of volcanic activity peaking at 2730 2725 2718 2714 and 2705 2700 ma during all three volcanic episodes which were generated by subduction related processes mantle plume derived komatiitic volcanic rocks were also added to the composite abitibi arc plume activity resulted in local regional extension and restricted later volcanism within the belt north south shortening produced thrusts and folds followed by se trending dextral strike slip faults between 2700 and 2692 ma the principal belt wide deformation subsequent e w dextral strike slip faulting limited to the major faults continued to 2690 2680 ma destor porcupine fault and even to 2680 2670 ma cadillac larder fault extensional structures resulting from the thickened crust developed around 2660 ma granitic intrusions are associated with all the volcanic episodes and phases of deformation and have been used to constrain the age of many of these events their role in stabilizing and adding to the volcanic crust is perhaps less well appreciated large synvolcanic tonalite dominated intrusions of the tonalite trondhjemite granodiorite ttg suite are associated with the three major periods of volcanic activity of the abitibi greenstone belt and are genetically related to the volcanism these synvolcanic intrusions occur both as high level flat cauldron subsidence complexes and as batholithic masses at the base of the oceanic crust tonalite gneiss dated from 2714 to 2703 ma occurring in structural culminations is considered to be arc underplated material the gneiss crystallized at depth and in contrast to the other tonalites does not appear to have intruded volcanic rocks the gneisses also record the earliest compressional deformation numerous intrusions of island arc affinity both the ttg and monzodiorite mzd suites form stocks and small plutons emplaced during the late phases of major deformation these are more numerous than voluminous and most of the tonalite end members of the suites probably intruded deeper within the sequence retarded by the now thickened crust large tonalite intrusions dominate the uplifted deeper sections adjacent to the abitibi belt in the kapuskasing uplift and grenville parautochthon and presumably make up much of the deep crust of the abitibi belt imaged in lithoprobe seismic profiles small stocks and dyke complexes characterize the late localized strike slip deformational phase these intrusions are porphyritic syenites and related rocks including some alkaline compositions garnetiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2660 ma thus plutonic rocks are a volumetrically major overall component of the abitibi belt and were implicated in every phase of its development from early volcanic arc magmatic activity through the various stages of deformation to final stabilization as continental crust

tectonic controls on the endowment of neoarchean cratons in volcanic hosted massive sulfide deposits evidence from lead and neodymium isotopes review and analysis of initial lead isotope ratios from archean volcanic hosted massive sulfide vhms and lode gold deposits and of neodymium isotopes from igneous rocks from the geologic provinces that host these deposits identifies systematic spatial and temporal patterns both within and between the provinces the abitibi wawa subprovince of the superior province is characterized by highly juvenile lead and neodymium most other archean provinces however are characterized by more evolved isotopes although domains within them can be characterized by juvenile isotope ratios the analysis indicates that the endowment measured as the quantity of metal contained in geologic resources per unit surface area of vhms and komatiite associated nickel sulfide deposits is related to the isotopic character and therefore the tectonic history of provinces that host these deposits provinces with extensive juvenile crust have significantly higher endowment of vhms deposits possibly as a consequence of higher heat flow and extension related faults provinces with evolved crust have higher endowment of komatiite associated nickel sulfide deposits possibly because such crust provided either a source of sulfur or a stable substrate for komatiite emplacement in any case initial radiogenic isotope ratios can be useful in predicting the endowment of archean terranes for vhms and komatiite associated nickel sulfide deposits limited data suggest similar relationships may hold in younger terranes lead isotope data may provide constraints on supercraton reconstructions in the archean as lead isotope systematics appear to be highly provincial similar isotope systematics between the superior province and the eastern goldfields superterrane may indicate a linkage during the neoarchean consistent with the similar geologic and metallogenic histories of these provinces

microchemical and sr isotopic investigation of zoned k feldspar megacrysts insights into the petrogenesis of a granitic system and disequilibrium crystal growth k feldspar megacrysts kfm are used to investigate the magmatic evolution of the 7ma monte capanne mc monzogranite elba italy dissolution and regrowth of kfm during magma mixing or mingling events produce indented resorption surfaces associated with high ba contents diffusion calculations demonstrate that kfm chemical zoning is primary core to rim variations in ba rb sr li and p support magma mixing i e high ba and p and low rb sr at rims but more complex variations require other mechanisms in particular we show that disequilibrium growth related to variations in diffusion rates in the melt may have occurred as a result of thermal disturbance following influx of mafic magma in the magma chamber initial 87 sr 86 sr ratios isr obtained by microdrilling decrease from core to rim inner core analyses define a mixing trend extending towards a high isr rb sr melt component whereas the outer cores and rims display a more restricted range of isr but a larger range of rb sr lower isr at the rim of one megacryst suggests mixing with high k calc alkaline mantle derived volcanics of similar age on capraia trace element and isotopic profiles suggest 1 early megacryst growth in magmas contaminated by crust and refreshed by high isr silicic melts as seen in the inner cores and 2 later recharge with mafic magmas as seen in the outer cores followed by 3 crystal fractionation with possible interaction with hydrothermal fluids as seen in the rim the model is compatible with the field occurrence of mafic enclaves and xenoliths

geodynamic setting crustal architecture and vms metallogeny of ca 2720 ma greenstone belt assemblages of the northern wawa subprovince superior province the greenstone belts along the northern margin of the wawa subprovince of the superior province vermilion shebandowan winston lake manitouwadge formed at ca 2720 ma and have been interpreted to be representative of a rifted arc to back arc tectonic setting despite a common inferred tectonic setting and broad similarities these greenstone belts have a significantly different metallogeny as evidenced by different endowments in volcanogenic massive sulphide vms magmatic sulphide and orogenic gold deposits in this paper we examine differences in geodynamic setting and crustal architecture as they pertain to the metallogeny of each greenstone belt by characterizing the regional scale trace element and isotopic nd and pb geochemistry of each belt the trace element geochemistry of the vermilion greenstone belt vgb shows evidence for a transition from arc like to back arc mafic rocks in the soudan belt to plume driven rifted arcs in the ultramafic bearing newton belt the shebandowan greenstone

pericontinental crustal growth of the southwestern abitibi subprovince canada u pb hf and nd isotope evidence the abitibi subprovince is widely regarded as a classic example of neoarchean continental crust formed in an ensimatic setting however sialic crust as old as 2 93 ga occurs in the adjacent wawa subprovince and the two subprovinces are thought to have formed a contiguous block since the initiation of abitibi magmatic activity at 2 75 ga a detailed geochemical u pb geochronological and nd hf isotope study of plutonic and volcanic rocks from the southwestern abitibi provides evidence for the involvement of older crust during magma genesis near the abitibi wawa boundary kapuskasing structural zone inherited zircons dated at ca 2 85 ga occur both in 2747 ma tonalite and 2700 ma granodiorite of the rice lake batholith part of the larger kenogamissi plutonic complex igneous zircons from this batholith display large intrasample variations in 176 hf 177 hf which we attribute to mixing of magmas derived from mantle juvenile crust and older crustal sources nd isotope data for felsic volcanic and plutonic rocks in this region indicate a dominantly juvenile source however two older felsic volcanic units 2739 and 2729 ma show nd isotope evidence of significant crustal contamination post tectonic plutonic units that were emplaced at 2676 and 2662 ma also contain inherited zircons but none are older than 2 76 ga in contrast tonalitic units dated at 2744 and 2715 ma from the round lake plutonic complex located 150 km east of the kapuskasing structural zone lack zircon inheritance and display less intrasample hf isotope variability in zircons our data suggest that the influence of older sialic crust during abitibi volcanic and plutonic activity is restricted both in space and time similar evidence for an older crustal influence occurs elsewhere in the southwestern abitibi near the kapuskasing structural zone the data collectively suggest that an approximately 75 km wide belt of abitibi crust adjacent to this structure was underlain by older wawa crust from 2 75 ga onward this crustal geometry is consistent with development of the abitibi sub province at the edge of a wawa protocraton perhaps as the result of rifting of this older continental block

os isotope constraints on the origin of the 2 7 ga boston creek flow ontario canada abstract the re os isotopic systematics of the ca 2 7 ga boston creek flow of the abitibi greenstone belt of ontario are examined the approximately 100 m thick fe rich komatiite flow became differentiated as it crystallized and ranges from an olivine cumulate at the base to a gabbrodiorite near the top the melt from which this flow crystallized was enriched in feo and was likely derived from an fe rich reservoir within the archean mantle the melt was also enriched in light and heavy rare earth elements zr hf and many other incompatible trace elements whole rock samples believed to be most representative of parental melt compositions indicate that the parental melt contained 15 17 wt mgo and that concentrations of re and os were approximately 0 6 and 0 2 ppb respectively the re and os concentrations are somewhat lower than concentrations estimated for most komatiitic and picritic liquids as is also common for differentiated komatiitic picritic and basaltic flows there is a positive correlation between os concentration and mgo there is a weak inverse correlation between re and mgo the re os systematics of most samples appear to have remained closed since crystallization seven samples define an isochron with an age of 2708 13 ma 2 which is within uncertainty of the zircon age previously established for the rocks three of the samples plot well above this isochron and most likely indicate late stage addition of radiogenic 187 os from mineralized rocks the initial ratio of the isochron is 0 1044 6 2 equivalent to an initial os value of 3 8 0 5 the very depleted initial ratio compared to current models for upper mantle evolution of os requires derivation of the os from a long term re depleted source the re depletion in the source probably occurred 500 to 1000 ma prior to the generation of the flow the depleted initial ratio is consistent with derivation of the parental melt from early archean melt depleted subcontinental lithospheric mantle the fe and incompatible trace element enrichment may have occurred during a metasomatizing event that caused renewed melting at 2 7 ga alternatively the source of the flow could represent a portion of the mantle that retained some characteristics of early earth formation such as majorite fractionation from a magma ocean or an initially higher feo content in the latter instance an archean melt depletion event is still probably necessary to explain the depleted os isotopic composition

plutonism in the southern abitibi subprovince a tectonic and petrogenetic framework studies of plutonic rocks in archean terranes such as the southern abitibi subprovince have been an important component in the development of an accretionary tectonic model for the superior province in the southern abitibi subprovince pretectonic approximately 2 75 2 70 ga low k tholeiitic calc alkaline dioritic to tonalitic plutons e g bourlamaque batholith flavrian pluton are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc alkaline basalt andesite pretectonic synvolcanic tholeiitic intrusions e g dundonald sill kamiskotia gabbroic complex 2 72 2 70 ga show in situ fractionation trends and locally have mixed magma textures with contemporaneous calc alkaline granitoids they are believed to be derived from a midocean ridge basalt morb like depleted mantle with little evidence for contamination by an older crustal component pre to syntectonic approximately 2 72 2 69 ga porphyry and tonalite plutons e g timmins porphyries round lake batholith have geochemical characteristics typical of archean tonalite trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades late to post tectonic 2 69 ga diorite monzonite and syenite plutons e g watabeag batholith lamaque stocks otto stock have large ion lithophile element enriched calc alkaline to sho shonitic compositions these intrusions and associated timiskaming type volcanic rocks are attributed to melting of large ion lithophile element enriched mantle in the late stages of subduction late to post tectonic biotite muscovite granite intrusions contain a significant contribution of evolved crust e g s type characteristics of the preissac lacorne batholith in the southern abitibi subprovince the tectonic setting of mantle and crust derived intrusions is broadly comparable to magmatic processes in phanerozoic arcs and accreted terranes the plutonic rocks record a transition from pre to synaccretionary subduction related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources this framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern abitibi subprovince which range from ni cu sulfides in layered synvolcanic tholeiitic intrusions e g kamiskotia gabbroic complex ontario to volcanogenic sulfides in early tholeiitic and calc alkaline sequences e g flavrian pluton noranda district quebec to the spatial association of gold with late large ion lithophile element enriched intrusions e g lamaque plugs val d or district quebec

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

intracrustal extension of an archean orogen revealed using single grain u pb zircon geochronology the metaplutonic wawa gneiss domain in the southern superior province is part of an uplifted block that was exhumed during the proterozoic exposing the lower crustal 30 km levels of the late archean 2720 to 2670 ma abitibi wawa orogen previous structural and geophysical studies of the midcrustal wawa gneiss domain reveal subhorizontal shear zones that formed during orogen parallel extension and correspond to gently dipping seismic reflectors present at depth elsewhere in the orogen in order to bracket the age of this extensional deformation the ages of metre scale igneous bodies that crystallized before and after extension have been determined in four outcrops using single grain u pb zircon dating techniques the results constrain the time of extension to the interval between 2660 and 2645 ma and show that deformation was coeval with pegmatite emplacement partial melting of tonalite gneisses as old as 2925 35 ma and regional circa 2660 ma granulite facies metamorphism of adjacent more deeply eroded crust this extension and associated high grade metamorphism in the wawa gneiss domain occurred 10 to 25 m y after the cessation of regional compression major transcurrent fault movement and sediment deposition in the upper structural levels of the orogen the middle and lower crust was therefore mechanically decoupled from the upper crust during this period of orogen parallel extension this intracrustal style of deformation is interpreted to be the consequence of gravitational forces working on a thermally softened middle crust the detachment of lithosphere from the orogen base is the favored mechanism for generating these forces and for introducing the heat that caused synextension metamorphism cyclic construction and delamination of orogen lithosphere could account for even younger metamorphic strain events evident in nearby exposures of lower crust

depositional setting of the late archean fe oxide and sulfide bearing chert and graphitic argillite in the shaw dome abitibi greenstone belt canada abstract interbedded chert rich exhalite and graphitic argillite are the only sedimentary rocks deposited in deep water settings during long lived hiatuses in mafic to ultramafic volcanism within the hart area of the shaw dome in the late archean abitibi greenstone belt in canada the fe oxide and sulfide bearing but predominantly cherty exhalite lithological unit in the hart area can be traced laterally to iron formation elsewhere in the shaw dome whole rock as well as fe and s isotope geochemistry suggest that the exhalite unit was formed as a result of direct precipitation from seawater distally from hydrothermal centres fractionation of fe isotopes through the precipitation of iron oxyhydroxides in a neutrally buoyant hydrothermal plume removed the heavier isotopes of fe resulting in the negative 56fe values observed in the exhalite in the hart area archean seawater is generally considered to be anoxic but moderate mn enrichments up to 1 87 wt mno in exhalite along with negative fe isotope values resulting from partial fe ii oxidation suggest the presence of oxygen in the upper part of the water column along the pathway of hydrothermal plumes from their source to the depositional site in the abitibi greenstone belt in contrast the graphitic argillite contains abundant pyrite nodules and bands that exhibit systematic negative fe isotope values but does not show mn enrichment this unit likely formed in a zone of upwelling of nutrient rich waters from deeper parts of the basin resulting in high organic productivity both exhalite and graphitic argillite have negative 33s values suggesting that sulfur was derived from seawater sulfate which is consistent with an anoxic atmosphere with sulfate aerosols produced by photochemical reactions combined our data indicates disequilibrium between anoxic atmosphere and partially oxygenated upper part of the water column during periods of volcanic quiescence in the 2 7 ga abitibi greenstone belt supporting the existence of oxidized oases within the archean ocean

review of archean supracrustal assemblages of the southern abitibi greenstone belt in ontario canada products of microplate interaction within a large scale plate tectonic setting abstract previously formulated plate convergent models account for many aspects of the large scale regularity and parallelism between different subprovinces of the archean superior province such models do not however appear to adequately explain the within subprovince evolution of 2 75 2 68 ga supracrustal assemblages of the late archean southern abitibi greenstone belt principal supracrustal assemblage classes include l assemblages dominated by tholeiitic metabasalt and metakomatiite with varying amounts of tholeiitic intermediate to felsic metavolcanic rocks these assemblages probably formed in extensional settings the ratio of basalt to komatiite and the ratio of basalt komatiite to intermediate felsic metavolcanic rocks may be a function of mantle temperature and crustal thickness at the site of extension respectively 2 assemblages dominated by intermediate to felsic fragmental and or effusive rocks are generally of calcalkalic affinity and likely formed in island arc settings 3 assemblages containing a substantial component of iron formation may represent older supracrustal assemblages that form structural and or stratigraphic basement to some of the younger supracrustal assemblages 4 assemblages consisting almost wholly of turbidite facies metasedimentary rocks appear to postdate volcanic activity related to the previous assemblages deposition of these sediments broadly coincides with the onset of regional deformation these assemblages may represent distal deposits related to an emerging collisional orogen 5 assemblages consisting of alluvial fluvial metasedimentary rocks with or without spatially associated alkalic metavolcanic rocks are associated with crustal scale shear zones and are the youngest assemblages within the greenstone belt alluvial fluvial assemblages likely represent the final emergence of the consolidating orogen geochronological data and assemblage distribution suggest that the supracrustal assemblages of the southern abitibi greenstone belt may represent the amalgamation of products of diverse and intricate microplate tectonic settings repeated in time and space prior to the impingement of the large scale plate tectonic regime that dictated the final large scale configuration of the archean superior province

architecture of the neoarchaean jaguar vhms deposit western australia implications for prospectivity and the presence of depositional breaks abstract the duration and nature of breaks in archaean volcanosedimentary packages recently re evaluated in the abitibi greenstone belt has been assessed at jaguar an archaean cu zn rich volcanic hosted massive sulphide vhms deposit in the teutonic bore volcanic complex tbvc the eastern goldfields of the yilgarn craton western australia the deposit is hosted by coherent volcanic facies and associated volcaniclastic facies with minor non volcanic facies the hypothesis that vhms ore lies on the intersection of volcanic centres and depositional breaks is found to be valid for the jaguar deposit with the site being an active volcanic complex at footwall time with further dome development close to the final ore location at mineralised package time and then reoccurring volcanic complexes andesitic and basaltic at several points in the hangingwall succession broadly tied into the ore position at jaguar the entire stratigraphic sequence faces west with no major structural complications making it ideal for stratigraphic analysis the footwall evidences prolific volcanism with no intervening sedimentation the mineralised package mp consists of intercalated dacite breccia conglomerate sandstone and mudstone the footwall volcanic rocks and the mp dacite appear to be differentiation products from the same magma chamber a major volcanic geochemical boundary is identified between the footwall mp tholeiitic to transitional affinities and the immediate hangingwall succession calc alkaline the hangingwall package includes four distinct volcanic units intercalated with mass flow deposits and graphitic argillites vhms mineralisation occurred across a distinct change in the geochemistry of erupted volcanic facies at a time of increased volcaniclastic sedimentation and extension this break is comparable to member level hiatuses seen in the abitibi gb i e the most brief of breaks because there is no silicification of sedimentary unit tops by silicic seawater exposure and there is minimal development of black shales a third time sensitive criteria was the development of peperite on the base of hanging wall hw andesite lavas the mp closely resembles breaks within interpreted deeper water domains of the abitibi gb for instance the deloro tisdale assemblage interface south of timmins the recognition of the change in the rate of emplacement of volcanic units and particularly the change in chemistry of volcanic rocks are critical exploration lithologic guides to use in the absence of chemical sediment deposits exhalites although crustal extension and basin subsidence were factors in the shaping of the succession their effects were too subtle to use as an exploration tool a corollary of the recognition of a member level vhms bearing interval in the teutonic bore volcanic complex is that like the kidd munro assemblage stratigraphy abitibi gb where several ore bearing depositional breaks are known below the great kidd creek deposit there is scope for the discovery of further potentially larger deposits on higher formation or group level breaks in the tbvc

nd and sr isotope systematics for the kamiskotia montcalm area implications for the formation of late archean crust in the western abitibi subprovince canada geochemistry and nd isotopic compositions are used to characterize mantle and crustal sources and to provide constraints on petrogenetic models for tholeiitic calc alkalic and lamprophyric suites in the kamiskotia montcalm area the kamiskotia gabbroic complex kgc and cogenetic bimodal volcanic rocks have end t 2 2 to 2 6 consistent with a direct derivation from a long term light rare earth element lree depleted mantle the montcalm gabbroic complex has decreasing end t upsection from 2 8 to 1 0 consistent with contamination by long term lree enriched with respect to the long term lree depleted abitibi mantle crust during fractionation two calc alkalic lamprophyre samples characterized by large ion lithophile element lile and lree enrichment and high mgo ni and cr contents have end t of 2 5 and 2 8 indicating a derivation from a depleted mantle source that had undergone recent trace element enrichment a different lamprophyre suite is extremely lile and lree enriched and h

accretion of archean oceanic plateau fragments in the abitibi greenstone belt canada the arc arc collision model is often applied to the evolution of archean greenstone belts this model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic tholeiitic affinities toward calc alkalic affinities new geological data from the malartic composite block of the abitibi greenstone belt indicate that this volcanic pile rather than representing a conformable sequence is composed of four distinct mafic lithotectonic domains that are overlain by a calc alkalic sequence the mafic domains are thought to be examples of accreted archean oceanic plateau material that were deformed during their collage state the younger calc alkalic sequence represents extension related volcanism that erupted through the ac creted and deformed plateau material possibly as a consequence of ridge subduction in a regime of oblique convergence the relations described here may represent a common feature of archean terranes

geology lithogeochemistry and significance of porphyry intrusions associated with gold mineralization within the timmins porcupine gold camp canada the timmins porcupine gold camp abitibi greenstone belt is host 60 moz of au with many gold deposits spatially associated with porphyry intrusions and the porcupine destor deformation zone pddz

central superior province geology evidence for an allochthonous ensimatic southern abitibi greenstone belt published u pb geochronological geological and petrochemical data suggest that there are late archean ensialic greenstone belts gb michipicoten gb and possibly the northern abitibi gb ensima

trace element geochemistry and petrogenesis of felsic volcanic rocks associated with volcanogenic massive cu zn pb sulfide deposits volcanogenic massive cu zn pb sulfide vms deposits occur primarily in subaqueous rift related environments e g oceanic fore arc arc back arc continental margin or continental are hosted primarily by bimodal mafic felsic volcanic successions and are typically associated with felsic volcanic rocks with specific geochemical characteristics fi alkalic dacites and rhyodacites despite being abundant in the rock record are typically barren some fii calc alkalic rhyodacites and rhyolites host vms deposits but most are barren fiii tholeiitic and fiv depleted rhyolites and high silica rhyolites are much less abundant in the rock record but commonly host vms deposits regardless of age and fiii rhyolites appear to host the largest deposits most petrogenetic models proposed for the formation of fii and fiii fiv felsic volcanic rocks link felsic magma genesis to fractionation processes in high level magma chambers now represented by associated subvolcanic intrusions where the magma is also interpreted to have supplied the heat and or metals required to generate and sustain the vms forming convective hydrothermal system however the relatively constant compositions of fii and fiii fiv felsic volcanic rocks within individual areas the high eruptive temperatures at or above liquidus of fiii rhyolites and the bimodality of vms hosting volcanic successions indicate that fractional crystallization within subvolcanic intrusions could not have generated or significantly modified the compositions of fii and fiii fiv magmas this coupled with detailed geological geochemical and geochronological studies indicates that many of these subvolcanic intrusions were emplaced in multiple phases and that the later most voluminous phases often cut ore associated hydrothermally altered rocks a reassessment of the physical conditions responsible for producing the geochemistry of ore associated fii and fiii fiv felsic volcanic rocks and a review of the compositions of felsic volcanic rocks associated with vms deposits that range in age from mesoarchean to cenozoic provide important constraints on models for vms associated felsic volcanic rocks and their relationship to mineralization the compositions of felsic volcanic rocks may be explained by low to moderate degrees of partial melting of mafic sources at a range of depths within rift environments where the mineralogy and composition of the source regions modes and degrees of partial melting pressure and temperature of melting and to a lesser extent subsequent fractionation processes account for the compositional variations from fi through fii to fiii fiv long lived enhanced heat flow and structural permeability of rift environments that allows partial melting to form some fii rhyolites at midcrustal levels 10 15 km and fiii fiv rhyolites at shallow crustal levels 10 km both within the zone of brittle fracture permeability are essential to sustain the high temperature convective hydrothermal systems that are required to form large vms deposits and camps rift environments contain long lived thermal magmatic and structural corridors that focus magma ascent heat flow high temperature convective hydrothermal systems and emplacement of subvolcanic intrusions that are favorable environments for the formation of vms deposits and fii and fiii fiv felsic volcanic rocks

geochemistry of metamorphosed volcanic rocks in the neoarchean qingyuan greenstone belt north china craton implications for geodynamic evolution and vms mineralization abstract the qingyuan greenstone belt qgb situated in the northeastern section of the north china craton ncc is considered to be the unique metallogenic province in china that hosts both banded iron formations bifs and volcanogenic massive sulfide vms deposits traditionally the qgb is divided into the hunbei and hunnan terranes separated by the hunhe fault and grouped into the lower shipengzi middle hongtoushan and upper nantianmen formations which have undergone amphibolite to granulite facies metamorphism exposed supracrustal rocks are composed of plagioclase amphibolite and biotite plagioclase gneiss with minor amounts of bif marble and schist in this study detailed field and petrographic observations lithogeochemical u pb geochronological as well as whole rock sm nd isotope analyses were conducted on the metamorphosed mafic and felsic volcanic rocks i e plagioclase amphibolite and biotite plagioclase gneiss respectively in the qgb in order to discuss implications for tectonic evolution and comitant vms mineralization the supracrustal rocks of both hunbei and hunnan regions were formed synchronously from 2 57 to 2 53 ga and then had undergone 2 52 2 48 ga regional metamorphism specifically the lower section of the shipengzi formation is composed dominantly of transitional tholeiitic basalts with negative nb anomalies suggesting that they were likely generated by partial melting of a mantle source that was previously altered by variable amounts of slab derived fluids or melts in contrast the lithologic assemblages grade upward into interlayered bimodal volcanic rocks i e n morb type tholeiitic basalts and fi to fii type dacites of transitional to calc alkalic affinities representing a change in tectonic regime related to subduction from compression to extension among them all the basalts are characterized by consistently positive initial nd compositions end t 2 7 to 3 4 further indicate a depleted mantle source whereas those dacites possess negative to positive initial nd values end t 4 6 to 3 0 suggesting they were formed by partial melting of depleted mafic source likely lower mafic crust and accompanied with variable addition of old crustal material significantly vms deposits are genetically linked to this extensional event and closely associated with the fii type dacites that were generated by higher melting degree at high temperature conditions and shallow crustal level likely 10 15 km in all geochemical features e g negative nb anomalies and end value together with contamination from continental crusts for tholeiitic mafic rocks and inherited zircons from these volcanic rocks imply a continental arc setting where the qgb was evolved

u pb geochronology of the eastern abitibi subprovince part 2 noranda kirkland lake area u pb zircon ages for 15 volcanic and plutonic units in the noranda and kirkland lake areas help constrain the history of volcanism plutonism sedimentation and deformation in the south central part of the abitibi belt volcanism occurred over an interval of at least 50 ma beginning with the deposition of the volcanic and volcaniclastic units within the pacaud structural complex at 2747 ma following a period of apparent quiescence magmatism resumed at 2730 2725 ma with the eruption of volcanic rocks in the normetal and lac abitibi area from 2715 until about 2698 ma volcanism occurred sporadically throughout much of the area culminating in the eruption of the blake river group from 2703 to 2698 ma several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the blake river group plutons that are considered to have been emplaced during the kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the blake river group emphasizing

lode gold deposits and archean mantle plume island arc interaction abitibi subprovince canada abstract in combination with seismic interpretations and geochronological constraints the association of juvenile arc type low ti tholeiitic basalts with komatiites in the southeastern abitibi subprovince canada supports a history of subduction step back following late archean mantle plume island arc interaction the resulting paired collision zones preserved abundant komatiites and numerous massive sulphide deposits and established the critical metallogenic features to concentrate the majority of canada s precambrian gold resources in a small area of the southern abitibi subprovince

deep structure of an archaean greenstone terrane more than half of the continental crust was formed before about 2 500 million years ago1 yet there is still considerable uncertainty regarding the structure and evolution of the ancient archaean cratons2 7 modern analogues proposed for the setting of archaean greenstone belts include aborted intra cratonic rifts continental margins and various oceanic settings2 7 alternatively archaean tectonics may have been quite different from the plate tectonics we see today2 4 6 the abitibi belt in canada is the largest archaean greenstone ternane3 and has been a touchstone for many models because its surface geology is relatively well known8 10 it contains significant gold and base metal deposits10 and it is less deformed and has suffered less erosion than most archaean ter ranes here we present deep seismic reflection data from part of the abitibi belt which reveal extensive subhorizontal and gently dipping reflection zones truncated by steeply inclined discontinuities which project to major fault zones at the surface our preliminary model of the abitibi belt comprises a mostly juvenile magmatic crust similar to that exposed in the nearby kapuskasing uplift11 12 dissected and imbricated by deep penetrating discontinuities associated with thrust and strike slipe faulting in view of the similarity of greenstone belts worldwide2 7 13 our seismic sections provide new constraints on the general processes that have affected archaean metavolcanic terranes

ramp flat geometry within the central kapuskasing uplift evidence from potential field modeling results potential field data constrained by seismic reflection seismic refraction geologic and physical properties data have been used to study the deep crustal structure of the central kapuskasing uplift the val rita block is interpreted as a thrust sheet of granulites detached at mid crustal depths and uplifted along a ramp producing an arched double granulite layer a thin thrust sheet of southward thickening granulites best describes the groundhog river block whereas simple thrusting along a ramp characterizes the chapleau block from this interpretation a sequential evolution of the kapuskasing uplift is proposed at about 2450 ma northwest southeast compression 1 resulted in variable thrusting along the ivanhoe lake fault zone this was followed by a period of normal faulting resulting in the juxtaposition of high and low pressure rocks across the lepage and saganash lake faults with possible truncation of the val rita block arch further rotation of 1 to an east west direction resulted in the

hadean origins of paleoarchean continental crust in the central wyoming province the scarce remnants of earth s earliest history make it challenging to describe the crust forming processes that operated during that time and all evidence that survived subsequent tectonism and recycling deserves to be studied closely we present geologic petrologic geochemical and isotopic descriptions of paleoarchean gneisses in the central wyoming province we identify two groups of gneisses a bimodal suite of amphibolite and tonalite trondhjemite granodiorite ttg layered gneisses 3385 3450 ma and a suite of massive trondhjemite and granite gneisses 3300 3330 ma here 3 82 ga inherited zircon components are present in several samples negative bulk rock initial e nd values also indicate that older crust was involved oxygen isotopic compositions of zircon mainly fall within the range of mantle zircon 18 o values but several analyses extend up to 6 5 initial hf isotopic compositions of 3 82 ga zircon are negative and require derivation from hadean crustal sources our data reveal the record of a 3 82 ga differentiation event during which hadean crust was partially melted and zircon crystallized from those melts hadean crust must have persisted in the central wyoming province until 3 45 3 37 ga when mafic crust partially melted to form the ttg layered gneisses which also incorporated the 3 82 ga zircons subsequent intracrustal recycling at 3 33 3 30 ga produced calc alkalic granites the central wyoming province provides a significant addition to the sparse record of hadean crust being magmatically reworked to form the abundant quartzofeldspathic gneisses common in archean terrains worldwide which effectively transformed earth s evolving continental crust from mafic to felsic in composition

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy implications for crystal transfer and magma mingling in s type granites microsampling of cm scale feldspar crystals within an s type granite from the lachlan fold belt of southeastern australia has revealed complex internal sr and nd isotopic variations the observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle like magmas the latter stages of which are now represented in modified form by microgranular enclaves similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation feldspar rims have higher 87sr 86sri and lower nd i than adjacent whole rock analyses but match those of mineral separates from the surrounding enclave matrix this suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high 87sr 86sr low nd i component such as metasedimentary wall rocks isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma our results reveal major isotopic heterogeneities on a single crystal and also inter mineral scale in a pluton which shows well constrained evidence for magma mingling these results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale

quaternary exhumation history of the ne tibetan plateau revealed by peculiar distributions of polycyclic aromatic hydrocarbons in core extracts from the sanhu depression eastern qaidam basin

constraints on the nature of the kapuskasing structural zone from the study of proterozoic dyke swarms petrochemical paleomagnetic and structural studies on early proterozoic dyke swarms show that the rocks of the kapuskasing structural zone ksz central superior province were upthrust about 10 15 km along the ivanhoe lake fault after intrusion of 2 04 ga kapuskasing dykes this uplift was part of a more widespread deformation in flanking terranes that involved sinistral motion along north north northwest trending faults dextral displacement along east northeast northeast trending faults and a distortion of the 2 45 ga matachewan swarm to the west and northwest of the ivanhoe lake fault the most spectacular demonstration that the ksz is largely a product of proterozoic deformation is that matachewan dykes change their magnetic polarity on crossing the zone a consequence of remanence acquisition at deep crustal levels being delayed until after a reversal of the earth s magnetic field and prior to uplift of the dykes matachewan and kapuskasing dykes within the amphibolite to granulite grade rocks o

komatiite flooding of a rifted archean rhyolitic arc complex geochemical signature and tectonic significance of the stoughton roquemaure group abitibi greenstone belt canada the 0 2 2 km thick archean stoughton roquemaure group srg in the northern volcanic zone of the abitibi greenstone belt quebec canada is composed of tholeiitic basalt komatiitic basalt and komatiite the mafic and ultramafic rocks are pillowed brecciated and massive columnar jointed flows the srg conformably overlies the 2730 ma hunter mine group a volcanic complex dominated by calc alkaline felsic rocks the tholeiitic basalts of the srg resemble morb the komatiitic basalts and komatiites have positive nd values overlapping those of the tholeiitic basalts komatiitic basalts with low al2o3 tio2 ratios 10 and fractionated heavy ree patterns are similar to al depleted komatiites in contrast the komatiites have high al2o3 tio2 20 unfractionated heavy ree patterns and resemble al undepleted munro type komatiites the al depleted komatiitic basalts occur at the base of the srg whereas the al undepleted komatiites are prevalent higher up in the stratigraphy the association of calcalka

magmatic and tectonic emplacement of the pukaskwa batholith superior province ontario canadathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh the neoarchean pukaskwa batholith consists of pre syn and post tectonic phases emplaced over an interval of 50 million years pre tectonic phases are broadly synvolcanic and have a high al tonalite trondhjemite granodiorite ttg affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths minor syn tectonic phases slightly post date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic mantle source component magmatic emplacement of pre and syn tectonic phases occurred in the midcrust at paleopressures of 550 600 mpa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development subsequent to emplacement of the syntectonic phases and likely at approximately 2680 ma the pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral t

nd isotope evolution of archaean plutonic rocks in southeastern superior province the opatica plutonic belt abitibi greenstone belt and pontiac subprovince represent a major proportion of the southeastern superior province which was formed and accreted rapidly between approximately 2 9 and 2 8 ga plutons in these belts are grouped into four types i trondhjemite tonalite granodiorite ttg suite 2 82 2 69 ga ii monzodiorite mzd suite 2 697 2 669 ga iii late alkaline granitoid alk suite 2 68 2 67 ga and iv anatectic granite and monzonite ana suite 2 69 2 64 ga the four suites are represented in all belts and show similar petrography and geochemistry in terms of nd isotope composition the ttg mzd and alk suites are typical of destructive plate margin magmatism and have 1 4 initial end 3 7 values which are very similar to that of the abitibi mantle end 2 5 the lower values for the ana suites end 0 1 to 2 4 result from recycling of crustal components in the opatica belt the ana granitoids fall on the nd isotope evolution curve defined b

geochemistry of the archean kam group yellowknife greenstone belt slave province canada abstract the geochemistry and isotope systematics of archean greenstone belts provide important constraints on the origin of the volcanic rocks and tectonic models for the evolution of archean cratons the kam group is a 10 km thick pile of submarine tholeiitic mafic and subordinate felsic volcanic rocks erupted between 2712 and 2701 ma that forms the bulk of the yellowknife greenstone belt in the dominantly granite metasedimentary slave province mafic rocks range from normal mid ocean range basalt like basalts to slightly light rare earth element enriched lree enriched but nb depleted basaltic andesites and andesites whereas dacitic to rhyodacitic felsic rocks are strongly lree enriched and highly depleted in nb the documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcomma

structural correlation within the kapuskasing uplift comparison of progressive deformation and metamorphic history within and between the tectonic domains of the kapuskasing uplift indicates significant variation in age and style of deformation across this large segment of the central superior province multiple stages of tonalite and granitoid intrusion melt generation polyphase diachronous deformation and likely rapid deep burial of supracrustal rocks collectively produced the complex character of this example of archean mid to deep crust at least four archean deformation phases are recognized although not all are of regional extent dated structural chronology suggests that the locus of the earliest recorded deformations migrated to deeper crustal levels with time pre 2680 ma deformation local d d within high level tonalites is correlated with deformation in the michipicoten supracrustal belt the apparent earliest deformational fabrics at deeper crustal levels in the granulite terrane of the kapuskasing structural zone occurred between 2660 and 2640 ma archean third and fourth phase deformation phases 2667 to 2629 ma are present at mid crustal and deeper levels and deform post 2667 ma metaconglomerate these resulted in large scale folding and subhorizontal ductile shear zones which seem to represent an important transitional zone that separated a passive upper crust from continued ductile strain at deeper levels subsequent uplift of the high grade rocks was accomplished in multiple stages initiated prior to 2 45 ga and likely culminated around 1 9 ga although continued movement occurred as late as 1 14 ga the ivanhoe lake fault zone along which much of the uplift must have occurred exhibits some evidence of ductile deep thrust related fabrics but most of the observed structures are brittle to brittle ductile and steeply inclined a broad zone of pervasive cataclasis and brittle ductile shear zones is a characteristic feature of the fault zone throughout its length and both dextral and sinistral offset are locally present clear ground evidence for major transcurrent or thrust displacements however has not been recognized

variability of nb u and th la in 3 0 to 2 7 ga superior province ocean plateau basalts implications for the timing of continental growth and lithosphere recycling extensive volcanic sequences of tholeiitic basalts having near flat ree patterns and spatially associated with komatiites occur in many archean superior province greenstone belts they are considered to be fragments of intraoceanic volcanic plateaus derived from a plume basalts from the 2 9 3 0 ga lumby lake greenstone belt carefully screened for minimum alteration have variable nb u ratios of 36 to 58 least altered basalts from the abitibi and wawa greenstone belts also have variable nb u ratios of 25 to 50 and 28 to 42 respectively compared to an average value of 47 for modern ocean basalts in the abitibi suite nb u correlates positively with nb lapm but negatively with th lapm alteration can be ruled out as the cause of nb u variation as there are no correlations of nb u with loi or eu eu and nb u correlates with nb th in all three suites of basalts numerous lines of evidence indicate that crustal contamination can be eliminated as the cause of nb u variability especially for samples with nb u 36 high nb u ratios can be explained by recycling ocean crust processed through a subduction zone high nb u and nb lapm low th lapm into the mantle source of the basalts whereas low nb u ratios can be accounted for by recycling complementary subarc mantle lithosphere or continental crust low nb u and nb lapm high th lapm into the mantle source for the entire population of basalts th lapm ratios generally 1 may result from recycling the residue of slab derived archean type tonalites having high th la ratios nb u ratios as high as the 47 10 range of modern ocean basalts have been found in a 2 7 ga volcanic belt of the yilgarn craton taken together the results signify very early growth of the continental crust rather than episodic growth over several ga

constraints on the geodynamic evolution of the southern superior province u pb la icp ms analysis of detrital zircon in successor basins of the archean abitibi and pontiac subprovinces of ontario and quebec canada abstract to investigate the tectonic development of the southern superior province new u pb laser ablation inductively coupled mass spectrometry analysis of detrital zircon was conducted on graywacke samples from successor basins of the abitibi and pontiac subprovinces the three successor basin successions investigated are the 2690 2685 ma porcupine assemblage and the 2679 2669 ma timiskaming assemblage of the abitibi subprovince and the 2682 ma sedimentary rocks of the pontiac subprovince predominately neoarchean zircon grains 80 95 of the total zircon population reflect local sources in the abitibi and pontiac subprovinces the successor basins were deposited within 100 ma of the majority of the detrital zircon grains primary crystallization ages which is consistent with patterns observed at modern convergent margin settings porcupine assemblage samples contain 5 mesoarchean zircon while pontiac subprovince and timiskaming assemblage samples contain 18 and 13 respectively no local mesoarchean sources exist in the abitibi or pontiac subprovinces suggesting input from an adjacent hinterland the higher proportion of mesoarchean zircon in the timiskaming assemblage relative to the porcupine assemblage suggests that detritus from the hinterland was more prevalent during the later stages of collision probably as a result of progressive uplift and denudation of the hinterland the high proportion of mesoarchean zircon in the pontiac subprovince sedimentary rocks may reflect an additional mesoarchean source comparison to previously published geochronology data for the superior province indicates that the hinterland was to the northwest and is in part composed of the winnipeg river marmion and opatica subprovinces patterns of contractional deformation successor basin formation and felsic plutonism progressively young to the south in the southern superior province and are interpreted in terms of accretionary processes detrital transport across subprovince or terrane boundaries progressive hinterland emergence and foreland directed propagation of the deformational front recorded by successor basins is similar to that observed in modern style collisional or accretionary orogens

metaconglomerate preserves evidence for kimberlite diamondiferous root and medium grade terrane of a pre 2 7ga southern superior protocraton article i nfo we studied heavy minerals extracted from a diamondiferous metaconglomerate that formed 2697 2701 ma in a successor basin within the michipicoten greenstone belt mgb of the wawa abitibi terrane southern superior craton the conglomerate is metamorphosed in the greenschist facies and contains mainly locally derived igneous mafic to felsic detritus but also very minor components of medium grade metamorphic minerals diamonds and paragenetic diamond indicator minerals comparison of the size distribution resorb tion and n aggregation of diamonds in nearby wawa lamprophyres and the metaconglomerate diamonds confirms that the latter were not derived from the proximal lamprophyric source the heavy minerals in the metaconglomerate include diopside olivine corundum chromite almandine pyrope with kelyphitic rims picroilmenite amphibole and anorthite low abundances of the heavy minerals several grains per 4 70 tons of the metaconglomerate are in part explained by their complete or partial replacement by the greenschist mineral assemblage detrital almandine and amphibole are inferred to originate in amphibolite facies rocks cr diopside olivine chromite and anorthite were sourced from mafic ultramafic anorthosite and chromitite bearing layered complexes mapped in the mgb the presence of pyrope with more than 6 wt cr2o3 suggests derivation from a cratonic root picroilmenite has compositions typical of kimberlite and unlike that of ultramafic lamprophyres and other unconventional diamondiferous volcanics the wawa metaconglomerate therefore should be considered analogous to the witwatersrand successor basin conglomerate in recording indirect evidence for archean kimberlites the tight localization of the diamondif erous conglomerate in time and space was controlled by a quick 3 ma erosion of the source kimberlite body the location of the kimberlite bearing n2 7 ga superior protocraton was inferred from the provenance of the metaconglomerate detrital material the clasts could have originated as close as the northern wawa abitibi terrane or as distant as the opatica terrane the pre 2 7 ga diamonondiferous cratonic root below the southern superior was removed in the neoarchean proterozoic the existence of archean kimberlites and deep diamondiferous roots below smaller pre 2 7 ga protocratons emphasizes the similarity of neoarchean and phanerozoic mantle processes

plume and arc magmatism in the abitibi subprovince implications for the origin of archean continental lithospheric mantle abstract based on eclogitic mantle xenoliths that appear to be the residue of slab melts it is commonly argued that the formation of distinctive lithospheric keels beneath archean cratons occurred by the tectonic accretion of oceanic crust these models are complicated by evident buoyancy of the keels the varied compositions of the mantle xenolith suite as a whole and a compositional constraint that a large portion of the keels originated in mantle plume melting events resolution of these issues is complicated by the long development history of most cratons 100 s of m y and the multiple cycles of magmatism they represent in contrast the single 50 m y period of arc volcanism in the abitibi subprovince and the immediate onset of orogeny and craton stabilisation avoids many of these uncertainties chemical and isotopic data demonstrate that syn volcanic to syn tectonic adakitic abitibi granitoids were derived via slab melting deep seismic reflection profiles image singular slab like features along the moho these are interpreted as the remnants of oceanic slabs emplaced and torn during orogeny nonetheless their presence directly contradicts subcretion models of keel formation because they occupy regions that should correspond to the top of an earlier formed subcreted keel post adakite shoshonitic granitoids and lamprophyres also attest to the presence of a metasomatised mantle wedge throughout the entire period of abitibi arc magmatism diamond bearing lamprophyres in the abitibi and adjacent wawa subprovince are compositionally equivalent to paleoproterozoic diamondiferous rocks and characteristic of adakite generating shallow subduction rather than evidence of pre existing sub cratonic keels abitibi mantle plume and subduction related volcanism also demonstrates the continuous presence of shallow asthenospheric mantle beneath the belt over a 50 m y period komatiites and then boninites erupted along the central part of the belt halfway through its evolution and rifted arc style volcanism dominated the belt within a few m y of orogeny none of these events are consistent with the development of a progressively thickened continental lithospheric keel the geodynamic evolution of the abitibi belt demonstrates a fact recognised in some xenolith studies the characteristic association of deep keels and archean cratons was not developed gradually during greenstone belt volcanism rather it was initiated by the coupling of shallow imbricated plume and arc related crust with the buoyant residue of mantle plume melting that rose from depths of 80 km to 400 km subduction was active along the southern and eastern margin of the superior province craton before and after ascent of the abitibi plume renewal of subduction in the abitibi as marked by the occurrence of the 2716 ma boninitic volcanic suite delayed the ascent of the deep keel and its coupling to the composite crust final ascent of the plume residue occurred following orogeny and slab loss and may have included entrainment of fragmented slab residue into the rising diapir the coupling of shallow crust and deep keel occurred soon after and the keel s presence beneath the subprovince is recognised by phanerozoic diamond deposits xenolith chemistry and geophysical data

the evolution of the southern abitibi greenstone belt in light of precise u pb geochronology this paper summarizes u pb zircon ages for the archean southern abitibi greenstone belt and integrates them together with zircon and baddeleyite data for three new samples into a general framework for the evolution of the region new ages are reported for the ghost range gabbroic complex 2713 super 7 sub 5 ma and for a pyroclastic volcanic rock of the gauthier group 2700 or 3 ma a previously determined age of 2701 super 3 sub 2 ma for the felsic volcanic skead group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic component or 2720 ma indicating that the skead volcano was built on top of an older volcanic sequence the compiled ages show that the southern abitibi greenstone belt developed between 2750 and 2670 ma the earliest ages date remnants of volcanic complexes that occur at the periphery of the belt major preorogenic magmatism was confined mainly to the period 2720 to 2700 ma in general each episode was characterized by the essentially coeval formation of komatiitic tholeiitic and calc alkalic igneous rocks taking into consideration the known geologic geochemical and isotopic data this association is consistent with evolution in paired active are and back are systems older crust was absent from the region but it may have interacted with these arcs and rift systems farther to the west the extensional regime was followed by a period of compression that caused folding and thrusting emplacement of calc alkalic plutons and deposition of turbidites between 2700 and 2688 ma the magmatism became predominantly alkalic during the main timiskaming period between 2681 and 2676 ma this event was accompanied by the deposition of alluvial fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting probably between approximately 2676 and 2670 ma lamprophyre dikes were associated with these final stages of deformation postorogenic activity includes faulting and episodes of hydrothermal activity locally associated with gold mineralization ages in the range 2630 to 2580 ma for hydrothermal minerals e g titanite and rutile camflo suggest a correlation of these events with lower crustal magmatism and metamorphism as recorded for example in the adjacent kapuskasing zone

comparative reos smnd and rbsr isotope and trace element systematics for archean komatiite flows from munro township abitibi belt ontario abstract a series of komatiite flows from munro township ontario have re and os concentrations that range from 0 545 to 1 27 ppb and 1 00 to 2 20 ppb respectively the187re 186os in the suite range from 14 05 to 52 37 and the187os 186os range from 1 49 to 3 22 giving a crystallization age of2726 93ma and an of initial187os 186os of0 873 0 035 this age is in good agreement with the crystallization ages that have been reported for these rocks using other isotopic systems the re os isochron indicates that the system was resistant to the low grade metamorphism that altered the rocks the initial os isotopic composition is consistent with the derivation of the melts from a mantle source s that originated with a chrondritic 187os 186os i and evolved with an approximately chondriticre os sm nd data for the same suite cluster about anend value at 2 72 ga of 2 to 3 rb sr isotope data indicate significant mobilization of these elements by the greenschist grade metamorphism that has altered the rocks comparisons of re and os concentrations with those of mgo feo ni cr and s suggests that re was incompatible d 1 os concentrations were likely controlled by the presence or precipitation of trace metallic or sulfide phases both during melting and crystallization and not by the modally predominant silicate minerals

hf isotope evidence for effective impact melt homogenisation at the sudbury impact crater ontario canada abstract we report on the first zircon hafnium oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets the differentiated melt sheet at the 1 85 ga originally ca 200 km in diameter sudbury impact crater ontario canada yields a tight range of uniform zircon hf isotope compositions e hf 1850 of ca 9 to 12 this is consistent with its well established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised we propose that the heterogeneity in other isotopic systems such as pb in early emplaced impact melt at sudbury is associated with volatility related depletion during the impact cratering process this depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post impact assimilation of country rock whereas the systems of more refractory elements preserve initial homogeneities zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks however they display a marked offset approximately one third up the melt sheet stratigraphy that is interpreted to be a result of post impact assimilation of 18 o enirched rocks into the base of the cooling impact melt given that impact cratering was a more dominant process in the early history of the inner solar system than it is today and the possibility that impact melt sheets were sources of ex situ hadean zircon grains these findings may have significance for the interpretation of the early zircon hf record we speculate that apparent e hf time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust we also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself

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

autochthonous development of superior province greenstone belts abstract models for superior province greenstone belt development have changed over the last 30 years from ensimatic generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm models intermediate between these end members such as autochthonous terrane models have not been prominent within the superior province there is a secular variation from early ca 3 ga platformal assemblages with overlying rift related komatiite tholeiite units representing breakup of ca 3 ga continental fragments to late ca 2 7 ga pull apart basins autochthonous terranes should demonstrate some sort of logical order such as that described above whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order in the face of this situation this paper reviews evidence from throughout the superior province that suggests many but not all greenstone assemblages developed autochthonously or parautochthonously the earliest greenstone assemblages in the superior province are generally quartzite and carbonate bearing platform sequences in the north caribou terrane and the marmion terrane deposited unconformably upon granitoid or greenstone basement the evidence for autochthonous parautochthonous development includes the presence of unconformities between keewatin type greenstone units such as 1 basal unconformities between ca 3 ga granitoid or volcanic basement and slightly younger platformal assemblages 2 between ca 2 9 ga platformal assemblages and younger platformal and andean arc assemblages stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern central and southern superior province are considered suggestive of orderly autochthonous progression from platforms through rifting of continental fragments and late assembly during the kenoran orogeny petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the superior province suggests the presence of older basement implying autochthonous or parautochthonous development of many assemblages younger greenstone assemblages in the abitibi wawa subprovince and the north caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages this pattern has been interpreted to represent autochthonous greenstone development the current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect few actual unconformities between keewatin type greenstone assemblages have been recognized in the superior province to aid in future recognition of unconformities in keewatin type greenstone assemblages examples of unconformities of varying degrees of subtlety in archean sequences are reviewed archean unconformities ranging from sharp angular discordances in the northern superior province to subtle stratigraphic patterns to weathering related geochemical mobilization along inter formational contacts are described these styles of unconformity in archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between keewatin type assemblages in the superior province generally and the abitibi wawa subprovince specifically the most important disconformity suggested in the abitibi wawa subprovince is between the ca 2726 ma deloro and the ca 2708 ma tisdale assemblages the presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an autochthonous origin for many superior province greenstone belts this autochthonous origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place that is development of ca 3 ga platform sequences in selected locales followed by in place evolution of a succession of plume oceanic and arc assemblages culminating in late pull apart basin sequences implications of the autochthonous model for greenstone belts of the superior province particularly the abitibi wawa subprovince include the future ability to recognize unconformity bounded sequences potentially improving regional scale correlation

uranium lead zircon systematics in the sudbury impact crater fill implications for target lithologies and crater evolution the sudbury impact structure is one of only a few terrestrial impact craters capable of providing insight into large impact processes however despite more than a century of study no consensus exists regarding its depths of excavation and melting this study presents 3920 u pb zircon dates for target lithologies and the crater fill as well as new pb isotope data for target lithologies in an attempt to further constrain these depths and understand the behaviour of zircon in large impacts only 1 5 of the crater fill zircons have dates within uncertainty of the 1 85 ga impact event with most seeming to preserve their pre impact u pb systematics the preponderance of undisturbed zircon in the crater fill suggests that this mineral is an effective means of tracing target lithologies in impact basins a significant fraction of crater fill zircon was dated between 2 50 and 2 61 ga which is negligible amongst known target lithologies and therefore identifies previously unrecognized target rocks the pb isotope systematics are compatible with the proposal that the 2 6 ga rock represents typical mid to lower crustal basement and could have been an important contributor to the melt sheet the combined data argue against a shallow melting scenario with regard to hadean terrestrial zircon the lesson from sudbury is that zircon can be recycled through multiple large impact events and still preserve the age of the original crust supplementary materials sample information trace element and pb isotope data for granitoid and gneiss feldspar pb modelling parameters u pb zircon data concordia diagrams and airborne radiometric images are available at http www geolsoc org uk sup18881

new u pb geochronology from timiskaming type assemblages in the shebandowan and vermilion greenstone belts wawa subprovince superior craton implications for the neoarchean development of the southwestern superior province abstract timiskaming type assemblages in the wawa abitibi terrane of the superior craton and other archean cratons worldwide are important because of their spatial and temporal association with world class orogenic gold mineralization especially in the kirkland lake area of ontario these mainly locally derived sedimentary assemblages are deposited in transpressional pull apart basins that developed during the final stages of terrane accretion and are commonly associated with calc alkalic to alkalic volcanic and plutonic rocks the shebandowan and vermilion greenstone belts sgb vgb in the wawa subprovince have significantly less economic base and precious metal mineralization than the greenstone belts of the abitibi subprovince and the timing of deformation and magmatism in the sgb and vgb is less well constrained this study presents new u pb geochronology from the sgb and vgb that demonstrating that the sedimentary and volcanic rocks of the lake vermilion formation in the vgb are coeval with similar rocks in the sgb and their interpreted depositional environment are consistent with these units being timiskaming type assemblages analyses of magmatic zircons in a dacitic tuff breccia from the gafvert lake sequence the volcanic member of the lake vermilion formation provided a high precision u pb age of 2689 7 0 8 ma using thermal ionization mass spectrometry detrital zircon geochronology using la icp ms confirms that the source of the detritus in the lake vermilion formation was locally derived from the gafvert lake metavolcanic lithofacies a similar temporal pattern was observed in many of the timiskaming type sedimentary rocks that were sampled in the sgb the locally derived alluvial fluvial to shallow marine depositional environment in long narrow basins with rapid lateral facies changes described in the sgb and vgb are consistent with deposition in structurally controlled pull apart basins however one sample from the sgb had detrital zircons with ages that were more akin to the age range found to the north in the wabigoon subprovince which is suggestive of trans terrane transport of detritus in a foreland type basin geochemically the timiskaming type volcanic rocks in the vgb and sgb are less enriched in incompatible elements and are comparatively less alkalic than the volcanic rocks in the timiskaming assemblage of kirkland lake this may indicate that the structures that formed the basins within the sgb and vgb in the wawa were not as deeply penetrating as those in the abitibi which may explain the paucity of economic gold mineralization associated with the former the timiskaming type deposits of the northern wawa abitibi terrane are 20 ma older than the timiskaming assemblage in the abitibi subprovince and co depositional with the flysch like deposits of the porcupine assemblage in the southern part of that terrane

u pb geochronology of the eastern abitibi subprovince part 1 chibougamau matagami joutel region u pb zircon titanite and baddeleyite ages are reported for 20 volcanic and plutonic rock units in the chibougamau matagami and joutel areas of the northern abitibi belt pretectonic magmatism occurred in three main pulses volcanic rocks of the obatagamau formation in the chibougamau area were erupted at 2759 ma following a possible hiatus in igneous activity volcanism accompanied by the emplacement of layered mafic intrusions occurred in the chibougamau matagami and joutel areas in the period of 2730 2722 ma in the chibougamau area a slightly younger period of plutonism between 2720 and 2712 ma is thought to be comagmatic with felsic volcanic rocks in the uppermost part of the roy group syntectonic intrusive rocks in the northern abitibi belt range in age from 2701 to 2693 ma these ages are broadly comparable to ages obtained for syntectonic plutons elsewhere in the abitibi belt suggesting that tectonism associated with the kenoran orogeny occurred at approximately the same time throughout

enriched komatiitic basalts from newton township ontario their genesis by crustal contamination of depleted komatiite magma lree enriched komatiitic basalts are commonly found in association with lree depleted komatiites this association is found in a sequence of late archaean lavas from newton township ontario the komatiitic lavas at newton township differ from most late archaean examples in that both the komatiites and the komatiitic basalts are depleted in al and in the hree the close association and distinctive al and hree depletions of the two lava types strongly suggest a common origin despite their contrasting lree patterns a model is proposed whereby the lree depleted komatiites represent the parental magma to the lree enriched komatiitic basalts the two being linked by a combination of crystal fractionation and crustal assimilation the composition of the contaminant is estimated by comparing the lree enriched komatiitic basalts with the evolved part of a thick layered komatiite flow that has similar major element chemistry the contaminant composition coincides closely with taylor mclennan s estimate of the composition of the archaean upper crust it is concluded that lree enriched komatiitic basalts can be produced from lree depleted komatiite parent magmas by combined assimilation and fractionation and that such a process best explains the geochemistry and nd isotopic features of most komatiitic basalts

importance of the lu hf isotopic system in studies of planetary chronology and chemical evolution the 176lu 176hf isotope method and its applications in earth sciences are discussed greater fractionation of lu hf than sm nd in planetary magmatic processes makes 176hf177hf a powerful geochemical tracer in general proportional variations of 176hf177hf exceed those of 143ndl44nd by factors of 1 5 3 in terrestrial and lunar materials lu hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time and this is the principal importance of lu hf new data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in 176hf177hf 87sr86sr and 143ndl44nd 87sr86sr diagrams 176hf177hf and 143nd144nd display a single linear isotopic variation in the suboceanic mantle these discordant 87sr86sr relationships may allow with the acquisition of further hf nd sr isotopic data a distinction between processes such as mantle metasomatism influence of seawater altered material in the magma source or recycling of sediments into the mantle in order to evaluate the hf nd isotopic correlation in terms of mantle fractionation history there is a need for measurements of hf distribution coefficients between silicate minerals and liquids and specifically for a knowledge of hf behavior in relation to rareearth elements for studying ancient terrestrial hf isotopic variations the best quality hf isotope data are obtained from granitoid rocks or zircons new data show that very u pb discordant zircons may have upwardly biased 176hf177hf but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial 176hf177hf until the controls on addition of radiogenic hf to zircon are understood combined zircon whole rock studies are recommended lu hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples but because it offers little advantage over existing methods is unlikely to find wide application in pure chronological studies

u pb zircon and monazite ages from the kapuskasing uplift age constraints on deformation within the ivanhoe lake fault zone a decade of u pb dating of zircon and monazite from high grade metamorphic rocks in the kapuskasing uplift has identified a series of magmatic and metamorphic events between 2700 and 2585 ma and indicates that the onset of regional granulite metamorphism took place at mid crustal levels of the southern superior craton ca 2660 ma new u pb ages for zircon and monazite have been used to constrain the age of ductile deformation fabrics at two sites in the ivanhoe lake fault zone the structure along which the granulite facies kapuskasing structural zone was uplifted these results suggest that the fault zone was probably active in the late archean as young as 2630 ma and again at approximately 2500 ma

a critical assessment of neoarchean plume only geodynamics evidence from the superior province abstract criticisms of archean plate tectonic models rarely offer detailed descriptions of alternative paradigms the catalytic delamination driven model for the coupled genesis of archean crust and sub continental lithospheric mantle described by bedard 2006 is an exception in that a relatively comprehensive but essentially 2 d model is described although this novel model is frequently cited as an example of the diversity in geodynamic thought for the archean it is rarely assessed with reference to events in a specific craton such as the superior province or a particular greenstone belt such as the abitibi when this type of assessment is undertaken it becomes clear that the apparent simplicity of the model one of its supposed virtues constitutes a critical weakness although bedard 2006 developed the catalytic model to overcome the perceived melt productivity deficit of archean plate tectonic models the catalytic process at the heart of the model is likely to be inefficient and provides insignificant improvements to crustal generation rates associated with subduction and non catalytic plume tectonics the model implies a near global craton in the mesoarchean to neoarchean but fails to address that fate of this craton during and after crustal rigidification seismic reflectors observed beneath archean cratons are characterized as failed delamination features but their global abundance suggests that syn to post tectonic ttg suites could not have been generated as a result of crustal restite delamination the brief descriptions of suggested mechanisms for the generation of minor rock types such as sanukitoids mask the complexity inherent in the overall model when considered in terms of the evolution of the superior province as a whole it is evident that many of these suggestions are not only implausible they are inconsistent with the timing and distribution of geological criteria used to mark the onset of crustal rigidification

u pb zircon geochronology of the kapuskasing structural zone and vicinity in the chapleau foleyet area ontario geological development of the archean abitibi and wawa greenstone granite subprovinces of the superior province occurred in three main stages 1 a volcanic sedimentary depositional stage 2750 2696 ma ago 2 a metamorphic tectonic event 2700 2685 ma ago and 3 a late plutonic event at 2685 2668 ma the area between wawa in the west and ivanhoe lake in the east represents an oblique section through some 20 km of the crust where geochronological relationships can be examined at different structural levels much of the eastern wawa subprovince is characterized by tonalite granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high level metavolcanic rocks near lake superior and the deep seated gneisses of the kapuskasing structural zone a tonalitic phase of the complex gneiss of the wawa subprovince has a minimum u pb zircon date of 2707 ma but the fine zircons are partly reset possibly in response to lit par lit injections of gneissic to foliat

mantle derived archaean monozodiorites and trachyandesites the 2 680 2 750 myr old1 5 intrusive granite greenstone terranes6 8 in the rainy lake region ontario and in and adjacent to the vermilion district minnesota of the superior province in north america include a wide variety of volcanic and intrusive rocks with initial pb sr and nd isotope ratios close to those for the mantle4 5 9 12 the isotopic constraints require the production of large volumes of silica oversaturated magmas from silica undersaturated mantle within 100 200 myr we present here geochemical data on monzodiorites and trachyandesites from the rainy lake area which are strongly enriched in large ion lithophile elements lile we conclude that they are derived by direct melting of the mantle at depths of 50km in either anhydrous or hydrous conditions their rare earth element ree abundances and initial 143nd 144nd ratios suggest that their mantle sources were enriched in lile shortly before melting the monzodiorites trachyandesites and their granodioritic derivatives may comprise up to 20 of the exposed igneous rocks in the rainy lake region and vermilion district if these rocks are as abundant in other archaean terranes a significant part of the early continental crust could have formed by direct melting of lile enriched mantle

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

the significance of titanite and apatite u pb ages constraints for the post magmatic thermal hydrothermal evolution of a batholithic complex berens river area northwestern superior province canada abstract titanite apatite and in one case rutile from a suite of twenty seven samples representing the major phases of an extensive and well dated archean plutonic domain of the northwestern superior province have been dated by u pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers most samples contain 1 dark brown titanite with 350 20 ppm u showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area 2 pale titanite with 30 2 ppm u showing textural evidence for a secondary origin by breakdown of biotite hornblende oxide or older titanite and yielding ages that in general post date the major plutonic and metamorphic events in the area 3 apatite containing 30 5 ppm u occasionally up to 150 ppm u showing igneous shapes and textural relationships but yielding post magmatic ages similar to those of coexisting pale titanite 4 rutile in one sample postdating the ages of coexisting titanite and apatite the common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of pb diffusion during slow cooling in this case has no general validity the distribution of primary dark titanite ages however could be explained by late closure to diffusion or alternatively as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity the ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity the correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite on a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes the most protracted activity is recorded in northern domains of the berens river area whereas the gold mineralized red lake area in the south underwent a very rapid decline of the thermal hydrothermal activity these data also highlight important differences between the red lake and other gold camps of the superior province such as hemlo and val d or where hydrothermal activity locally associated with mineralization occurred as much as 30 80 m y after the major orogenic developments

a special issue devoted to base metal and gold metallogeny at regional camp and deposit scales in the abitibi greenstone belt preface the abitibi greenstone belt contains some of the world s largest cu zn and au deposits and significant amounts of ni cu pge mineralization discoveries early in the 20th century were made by classic prospecting with subsequent discoveries commonly resulting from diamond drilling of targets identified by electromagnetic surveys the next generation of deposits will be found at greater depth based on geological and geophysical models principally beneath the extensive overburden that covers most of the abitibi greenstone belt thus exploration will require improved knowledge of the geologic characteristics of the existing deposits particularly the relationship between deposit and regional scale features including the stratigraphic plutonic structural and metamorphic architecture of the region and will rely more heavily on expensive geophysical and geochemical techniques verified by diamond drilling the papers in this special issue are part of the discover abitibi initiative a geoscience program funded by federal provincial and municipal governments in collaboration with the mining industry many of the results reported here are from projects funded by phase ii of the discover abitibi initiative which commenced in 2003 and was focused on the timmins kirkland lake region fig 1 a high mineral potential portion of the abitibi greenstone belt that has been the source of exceptional mineral wealth as part of the discover abitibi initiative the greenstone architecture project was undertaken by the mineral exploration research center at laurentian university to bridge the gap between academic and company information at the scale of individual deposits and the broad based knowledge derived from government mapping the project was designed as a multidisciplinary approach involving faculty postdoctoral fellows and graduate students in collaboration with geoscientists from the ontario geological survey the geological survey of canada consultants and company geologists as can be seen in the papers in this special issue the program had both geological and

evolution and tectonic significance of intermediate to felsic plutonism associated with the hemlo greenstone belt superior province canada abstract plutonic rocks from the hemlo area in the southern superior province span a period from ca 2720 to 2677 ma early ttg plutons intruded into lower basaltic volcanic sequences and were probably derived from melting of basaltic crust at upper mantle depths they predate major regional deformation and span ages from 2719 to 2697 ma felsic to intermediate volcanism 2697 2688 ma partially overlaps ttg plutonism but is mostly slightly younger and lacking plutonic equivalents regional deformation and metamorphism of the greenstone belt and ttg plutons progressed rapidly following deposition of the youngest supracrustal rocks syn to late tectonic plutons emplaced between 2684 and 2677 ma have a distinctive dual primitive evolved character that is interpreted to reflect their derivation by direct melting of long term depleted mantle that was metasomatically enriched in a wide range of lile and hfse this secular evolutionary change in the nature of intermediate to felsic plutonism at hemlo and its relationship to regional deformation and metamorphism parallels that occurring more regionally within the superior province and is interpreted to be the consequence of a transition in the nature of large scale tectonic processes intermediate to felsic magmatism within a protracted period of volcanism and ttg plutonism originated primarily by melting of subducted basaltic crust probably in an oceanic arc related environment the termination of this style of magmatic activity and initiation of regional deformation and metamorphism at ca 2688 ma is interpreted to be a consequence of collisional orogeny the petrogenetic characteristics of the latest period of magmatism at 2684 2677 ma requires that the melting regime migrate into sub arc lithospheric mantle that was metasomatized during the earlier magmatic stage slab breakoff following terrane accretion is suggested as one possible mechanism for the abrupt termination of ttg magmatism and ensuing rapid and progressive introduction of large amounts of heat into sub arc mantle and basal portions of the crust this pattern of development is seen in many superior province greenstone belts but is older in the north therefore it is probably an inevitable consequence of the tectonic processes that led to accretion of the superior province

seismic reflection profiles across deep continental crust exposed in the kapuskasing uplift structure seismic imaging of precambrian cratons holds the promise of deep structural mapping and interpretation of fundamental crustal construction processes deep structures may be identified by tracing reflections to the surface1 particularly in exposed crustal cross sections2 elevated along deeply penetrating faults but many such cross sections have structural orientations too steep for successful reflection imaging like the ivrea zone3 or occur in plate boundary settings not representative of continental interiors by contrast the archaean greenstone terrain exposed in the kapus kasing uplift of canada4 5 has gently dipping structures and an intracratonic setting within the superior province thus providing an opportunity to examine the third dimension of continental crust down to 25 30 km palaeodepth6 here we present new seismic reflection data which although generally supportive of structural models based on surface geology4 gravity modeling5 and seismic refraction studies7 8 also indicate that the kapuskasing structure is a relatively thin thrust sheet containing seismically reflective sequences of high grade rock which may be analogues of seismi cally reflective in situ lower crust

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

evolution of the southern abitibi greenstone belt based on u pb geochronology autochthonous volcanic construction followed by plutonism regional deformation and sedimentation abstract new mapping and u pb zircon geochronological results in the southern abitibi greenstone belt sagb support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages rather than the collage of allochthonous terranes proposed in recent publications based on lithological and geochronological criteria these supracrustal assemblages also encompass the swayze shining tree and montcalm greenstone belts indicating that despite separation by large granitic intrusions they are in fact part of one extensive greenstone belt in conjunction with the mapping the geochronological results support coherent upward facing stratigraphic sections with about 20 of the samples containing inherited zircons with ages similar to those found in underlying assemblages the seven oldest assemblages represent semi continuous volcanism from 2750 to 2697 ma the volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc alkaline mafic to felsic lavas intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section regional fault control on the distribution of the volcanic assemblages provides evidence for early dip slip movement associated with volcanic extension dating back to at least 2725 ma the two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults the earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 ma and the youngest one of subaerial conglomerates fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 ma the sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons regional folding and reactivated movement on the regional faults probably related to accretion of the abitibi to the superior province craton autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic komatiitic assemblages and two dominantly calc alkaline assemblages as well as the extensive intermingling of these different magma clans in a number of the assemblages appears to be unique to the archean it suggests complex large scale and long lived interaction between mantle plumes and subduction zone magmas the nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with end 2 5 0 5 demonstrating a lack of contamination by ancient enriched components

petrogenesis of mantle derived lile enriched archean monzodiorites and trachyandesites sanukitoids in southwestern superior province reply in southwestern superior province diorite monzodiorite and trachyandesite sanukitoids occurring within syn to post tectonic intrusive complexes and within greenstone belts have the following chemical characteristics 55 60 wt sio2 mgo 6 wt mg 0 60 ni and cr both 100 ppm na2o k2o 6 wt sr and ba both 600 1800 ppm and rare earth element ree patterns that are strongly light rare earth element lree enriched cen 80 250 ybn 4 10 and show no eu anomalies sanukitoids and their granodioritic derivatives constitute at least 5 of the exposed crust in the study area the sanukitoids cannot be derived by melting fractionation or crustal contamination of basalts or lamprophyres that are coeval with the sanukitoids crustal contamination of komatiites fails to explain the high large ion lithophile element lile contents of the sanukitoids rather we suggest that the sanukitoids were derived by hydrous melting of lile enriched mantle peridotite at pressures between 10 and

provenance of proterozoic garnet biotite gneiss recovered from elan bank kerguelen plateau southern indian ocean at elan bank of the kerguelen plateau in the southeast indian ocean leg 183 of the ocean drilling program recovered clasts of garnet biotite gneiss in a fluvial conglomerate intercalated with basalt flows u pb and pb pb dates of zircons and monazites in these clasts and an overlying sandstone range from 534 to 2547 ma which is much older than the surrounding indian ocean seafloor these dates show that old continental crust resides in the shallow crust of the oceanic kerguelen plateau and that the breakup of gondwana dispersed continental fragments into the nascent indian ocean lithosphere

numerical simulation of elastic wave propagation in fractured rock with the boundary integral equation method this paper describes a boundary integral equation method for simulating two dimensional elastic wave propagation in a rock mass with nonwelded discontinuities such as fractures joints and faults the numerical formulation is based on the three dimensional boundary integral equations that are reduced to two dimensions by numerical integration along the axis orthogonal to the plane of interest the numerical technique requires the assembly and solution of the coefficient matrix only for the first time step resulting in a significant reduction in computational time nonwelded discontinuities are each treated as an elastic contact between blocks of a fractured rock mass across such an elastic contact seismic stresses are continuous and particle displacements are discontinuous by an amount which is proportional to the stress on the discontinuity and inversely to the specific stiffness of the discontinuity simulations demonstrate that such formulated boundary element method successfully models elastic wave propagation along and across a single fracture generated by a line source

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