geologic and geochemical relationships between the contact sublayer inclusions and the main mass of the sudbury igneous complex a case study of the whistle mine embayment more than half of the giant ni cu platinum group element pge sulfide ores of the sudbury igneous complex are associated with a discontinuous unit at the base of the main mass known as the sublayer the sublayer is comprised of two fragment rich members 1 a metamorphic textured footwall breccia and 2 an igneous textured contact sublayer the contact sublayer occurs as a thick unit in depressions at the base of the sudbury igneous complex termed embayments it is associated with disseminated to massive sulfides and contains a range of inclusion types such as diabase melanorite olivine melanorite metamorphosed melanorite wehrlite and dunite we present data for a case study of the whistle mine embayment and show that the igneous textured sublayer matrix is geochemically unlike the main mass norites quartz gabbros and granophyres for example the igneous textured sublayer matrix in the whistle mine has la sm 4 0 la nb 5 1 and th zr 0 02 and the main mass norites quartz gabbros and granophyres have ratios of 5 5 to 7 2 8 to 4 2 and 0 04 to 0 05 respectively this matrix contains partially digested hornfels diabase fragments and ghost textures of magnetite remaining from the melting of diabase the igneous textured sublayer matrix at the whistle mine can be modeled with small amounts of assimilation of local country rock granitoids approximately 10 large degrees of assimilation of diabase that are not derived from the immediate country rocks approximately 70 and small contributions from the main mass magma type approximately 20 mafic norite the amount of diabase assimilation would be too large for traditional assimilation models controlled by the available heat of the mafic magma even with a superheated magma 70 percent assimilation is too large a model is proposed where the melting of the target rocks initially produces a felsic melt sheet which is laden with mafic fragments these fragments sink to the base of the melt sheet in the crater and are concentrated and further melted to produce the mafic sublayer there are significant differences in the composition of the igneous textured sublayer matrix between different embayments which may reflect differing degrees of digestion of compositionally different protolith fragments the melanorite inclusions in the sublayer at the whistle mine have ratios of the incompatible trace elements essentially similar to those of the inclusions in the igneous textured sublayer matrix but they have similar high incompatible element concentrations e g olivine melanorites have 20 65 ppm ce in rocks with 15 21 wt mgo 1 to 10 percent interstitial sulfide up to 0 5 percent apatite 1 to 15 percent biotite and 1 85 ga age zircon and baddeleyite the mafic ultramafic inclusions are interpreted to be the broken up remnants of an earlier cumulate formed at depth from a main mass magma this parental magma has compositional traits which suggest that crystal accumulation took place from a magma which contains a large contribution from the diabase olivine compositional data for the mafic inclusions fo sub 71 79 500 3 800 ppm ni in the absence of reequilibration indicate that olivine crystallization both predated and postdated sulfur saturation of the magma cr rich spinels from these rocks confirm that the parental magma was especially cr rich and based on the olivine compositional data had a tholeiitic mg fe ratio

variability in cenozoic sedimentation along the continental rise of the bellingshausen sea west antarctica seismic reflection profiles bathymetric and magnetic data collected along and across the continental margin of the bellingshausen sea provide new constraints and interpretations of the oceanic basement structure and cenozoic glacial history of west antarctica evidence for tectonic boundaries that lie perpendicular to the margin has been identified on the basis of one previously unpublished along slope multichannel seismic reflection profile by combining several magnetic data sets we determined basement ages and verified the positions of possible fracture zones enabling us to improve previous tectonic and stratigraphic models we establish three main sediment units on the basis of one seismic along slope profile and by correlation to the continental shelf via one cross slope profile we interpret a lowermost unit be3 older then 9 6 ma as representing a long period of slow accumulation of mainly turbiditic sediments unit be2 from about 9 6 to 5 3 ma may represent a period of short lived ice advances on the continental shelf the uppermost unit be1 from about 5 3 ma to present apparently consists of rapidly deposited terrigenous sediment that we interpret as having been transported to the shelf edge by frequent advances of grounded ice listric faults are observed in be1 and indicate sediment instability due to interactions between different depositional processes correlation of the sediment classification scheme with the continental rise of the western antarctic peninsula shows obvious differences in sediment depositional patterns we estimate a very high sedimentation rate for unit be1 up to 295 m my which points to an increase in glacial sediment supply due to major glacial outlets that flowed to nearby parts of the shelf edge in pliocene and quaternary times this is in contrast to the situation at the adjacent antarctic peninsular margin and many other parts of the continental rise around antarctica

order or chaos origin and mode of emplacement of breccias in floors of large impact structures abstract breccias in the crater floor of large impact structures are pseudotachylites sensu largo authigenic monomict and polymict clastic matrix breccias so called footwall breccias and impact melt breccias pseudotachylite bodies in the center of large impact structures e g vredefort dome south africa appear to have a random distribution and orientation but most dip steeply or vertically large bodies of pseudotachylite in the more distal sectors of the 200 km diameter sudbury structure have been interpreted as ring and terrace collapse features in the vredefort dome networks of randomly distributed pseudotachylite veins accompany large mother lode pseudotachylite dikes in general pseudotachylites in the floors of central parts of impact craters may form through explosive transfer of thermal shock energy in a process that could be termed flash replacement melting whereas pseudotachylites at large distances from the centers of large impact structure are believed to have formed through friction leading to partial or complete melting similar to the formation of tectonic pseudotachylites in smaller structures e g ries and slate islands clastic matrix breccias instead of pseudotachylites occur as the most common breccias in the crater floors they have a chaotic distribution pattern their dips are commonly also steep to vertical melt breccia dikes in the target rocks of the crater floor are associated with melt sheets that fill the lower part of the excavation cavity at vredefort erosion has removed the coherent melt sheet but melt breccia dikes vredefort granophyre in the crater floor are preserved they are characterized by a remarkably homogeneous chemical composition and are believed to represent the initial undifferentiated impact melt near the vredefort collar the granophyre forms more or less concentric dikes in the more central parts of the dome their orientation is more random but in places may be controlled by the archean fabric of the crater basement the offset dikes of the sudbury structure are associated with the sudbury igneous complex that represents in total or in part a differentiated impact melt sheet the dikes in many aspects are similar to vredefort granophyres but their interpretation as undifferentiated bulk melt is problematic on geochemical and structural grounds chemically these dikes are not homogeneous and in places they contain massive sulfide deposits that are unlikely components of an undifferentiated impact melt differentiation and precipitation of massive sulfides are slow processes compared with the presumably high energy and fast emplacement of a supposedly undifferentiated melt pseudotachylites of the central impact crater apparently are mainly produced in the compression phase of the impact process whereas clastic matrix breccias form during the uplift and crater modification phases impact melt breccia dikes contain pseudotachylite bearing inclusions and cut across pseudotachylite bodies such impact melt breccia dikes were probably emplaced during uplift and crater modification

chalcophile and platinum group element pge concentrations in the sulfide minerals from the mccreedy east deposit sudbury canada and the origin of pge in pyrite magmatic sulfide deposits consist of pyrrhotite pentlandite chalcopyrite pyrite and platinum group minerals pgm understanding the distribution of the chalcophile and platinum group element pge concentrations among the base metal sulfide phases and pgm is important both for the petrogenetic models of the ores and for the efficient extraction of the pge typically pyrrhotite and pentlandite host much of the pge except pt which forms pt minerals chalcopyrite does not host pge and the role of pyrite has not been closely investigated the ni cu pge ores from the south range of sudbury are unusual in that sulfarsenide pgm rather than pyrrhotite and pentlandite are the main carrier of pge probably as the result of arsenic contribution to the sulfide liquid by the as bearing metasedimentary footwall rocks in comparison the north range deposits of sudbury such as the mccreedy east deposit have as poor granites in the footwall and the ores commonly contain pyrite our results show that in the pyrrhotite rich ores of the mccreedy east deposit os ir ru rh ipge and re are concentrated in pyrrhotite pentlandite and surprisingly in pyrite this indicates that sulfarsenides which are not present in the ores were not important in concentrating pge in the north range of sudbury palladium is present in pentlandite and together with pt form pgm such as ptpd tebi 2 platinum is also found in pyrite two generations of pyrite are present one pyrite is primary and locally exsolved from monosulfide solid solution mss in small amounts 2 wt together with pyrrhotite and pentlandite this pyrite is unexpectedly enriched in ipge as pt and the concentrations of these elements are oscillatory zoned the other pyrite is secondary and formed by alteration of the mss cumulates by late magmatic hydrothermal fluids this pyrite is unzoned and has inherited the low concentrations of ipge and re from the pyrrhotite and pentlandite that it has replaced

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

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

vitric compositions in the onaping formation and their relationship to the sudbury igneous complex sudbury structure the 1 85 ga sudbury structure is the largest well preserved exposed example of a terrestrial impact crater it contains world class ni cu platinum group element pge ores associated with the igneous component a new understanding of the geology and geochemistry of the vitric crater fill sequence the onaping formation provides constraints on the evolution of 1 85 ga igneous rocks in the sudbury structure the onaping formation has a well defined mappable stratigraphy composed of three members seven stratigraphic units and two types of intraformational dikes that are overprinted by an impact generated hydrothermal system stratigraphic and alteration mapping provided a basis for well constrained sampling and allowed the definition of the geochemical characteristics of the less altered rocks from each unit including vitric bombs and blocks and intraformational vitric dikes significantly the identification of a least altered composition of the vitric onaping formation enabled an investigation of the link between the early formed vitric melt phase in the impact crater fill sequence and the sudbury igneous complex and offset dikes the least altered vitric onaping formation represented by the massive xenolith poor cores of vitric bombs blocks and intraformational dikes is andesitic in composition consistent with a crustally derived melt the melt composition 61 6 wt sio 2 4 28 mgo 60 ppm ni is remarkably uniform throughout the onaping formation with a typical enrichment of large ion lithophile and light rare earth elements and pronounced negative nb and ti anomalies incompatible trace element ratios and bulk concentrations of the onaping formation coincide with the main mass norite and offset quartz diorite compositions of the underlying sudbury igneous complex but differ distinctly from the granophyre and quartz gabbro root mean squared differences in composition of the vitric onaping formation with the units of the sudbury igneous complex and quartz diorite offset dikes show that the vitric onaping formation does not directly match any of these igneous units but more closely resembles the composition of the ministic and manchester offset dikes the quartz diorite offset dikes have been proposed as representative of the bulk composition of the original melt of the sudbury igneous complex in detail there are small compositional differences between north and south range offset dikes and between these offset dikes and the concentric manchester offset dike however such differences are lacking in the onaping formation we propose that the least altered vitric composition of the onaping formation represents the best estimate of the bulk composition of an initial quenched melt or shock melt formed early in the evolution of the sudbury structure and is a logical starting liquid to model the evolution of the sudbury main mass and offset dikes the offset dike compositions can be derived from onaping formation vitric shock melt by assimilation of progressively greater amounts of mafic material at deeper levels in the crater this mafic contaminated melt or impact melt developed at the base of the melt sheet and was injected into the offset dikes both prior to and contemporaneously with the earliest stages of sulfide precipitation from the sudbury complex fractionation of the mafic contaminated melt by plagioclase and pyroxene accumulation produced the north range felsic and mafic norite and the south range norites the chalcophile metal content of the onaping formation vitric phase closely reflects the overall metal ratios of the magmatic sulfide ores the vitric onaping formation and unmineralized offset dikes have low liquidus temperatures 1 115 1 125 c and if these units represent the source of metals in the sulfide deposits unusually high mass distribution coefficients for cu and ni and high r factors are necessary to account for the metal endowment

sudbury breccia canada a product of the 1850 ma sudbury event and host to footwall cu ni pge deposits abstract the sudbury structure formed by meteorite impact at 1850 ma consists of three major components 1 the sudbury basin 2 the sudbury igneous complex which surrounds the basin as an elliptical collar and 3 breccia bodies in the footwall known as sudbury breccia in general the breccia consists of subrounded fragments set in a dark fine grained to aphanitic matrix a comparison of the chemical composition of host rocks clasts and matrices indicates that brecciation was essentially an in situ process sudbury breccia forms irregular shaped bodies or dikes that range in size from mm to km scale contacts with the host rocks are commonly sharp the aspect ratio of most clasts is approximately 2 with the long axes parallel to dike walls the fractal dimension dr 1 55 although there appears to be some concentration of brecciation within concentric zones small sudbury breccia bodies within and outside these zones have more or less random strikes and steep dips sudbury breccia bodies near an embayment structure tend to be subparallel to the base of the sudbury igneous complex sudbury breccia occurs as much as 80 km from the outer margin of the sudbury igneous complex in an inner zone 5 to 15 km wide breccia comprises 5 of exposed bedrock with an increase in brecciation intensity in embayment structures sudbury breccia may be classified into three types based on the nature of the matrix clastic pseudotachylite and microcrystalline clastic sudbury breccia the dominant type in the southern province is characterized by flow surface structures possibly a sudden rise in pore pressure caused explosive dilation and fragmentation followed by fluidization and flowage into extension fractures pseudotachylite sudbury breccia mainly confined to archean rocks apparently formed by comminution and frictional melting microcrystalline sudbury breccia formed as a result of the thermal metamorphism of the north range footwall by the sudbury igneous complex this produced a zone approximately 1 2 km wide wherein the matrix of the breccia either recrystallized or locally melted an overprint of regional metamorphism obliterated contact effects in the south range footwall the ni cu pge magmatic sulphide deposits may be classified into four types based on structural setting sudbury igneous complex footwall contact footwall offset and sheared deposits sudbury breccia is the main host for footwall deposits e g mccreedy east victor lindsley sudbury breccia locally hosts mineralization in radial e g parkin and copper cliff and concentric e g frood stobie offset dikes

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

evolution of the sublayer of the sudbury igneous complex geochemical sm nd isotopic and petrologic evidence abstract the mineralized sublayer at the base of the sudbury igneous complex sic consists of two variants the noritic contact sublayer and radial and concentric quartz dioritic offset dykes both are characterized by the presence of significant quantities of ni cu pge sulphides and by a prominent population of recrystallized diabasic textured and melanocratic to ultramafic fragments the two variants of the sublayer contain compositionally distinct inclusion populations and inclusion bearing matrices contact sublayer and offset dykes hosted by north range granitoid footwall can be distinguished from those hosted by south range basaltic and metasedimentary footwall environments the compositional variation in sic rocks can be described in terms of contributions from exposed crustal rocks and differentiation of the resultant melt s the basaltic inclusion population is characterized by hornfels recrystallization of the plagioclase and is geochemically and isotopically identified with huronian basalts which comprise the south range footwall with ce yb n ratios of around 2 5 and e nd 1850 between 2 to 5 the melanocratic inclusions in the sublayer are typically coarse grained and undeformed with incompatible element contents and radiogenic isotopic compositions intermediate between those of the basaltic inclusions and those of the melt sheet which has ce yb n ratios of around 10 and e nd 1850 around 9 calculated crystallization models are consistent with derivation of the ultramafic inclusions by crystallization from a magma produced by mixing of molten basaltic footwall with basal melt sheet it is proposed that the sublayer appeared as the marginal facies of a meteorite impact melt sheet as a result of footwall melting following the impact this basal layer was progressively enriched in sulphides and mafic cumulates from above through differentiation during cooling offset dykes were emplaced and the magmas effectively removed from the system subsequently continued evolution of the marginal facies produced the more mafic inclusions in the contact sublayer no extracrustal e g mantle component is envisioned in this model to explain the silicate compositional distributions in the sic and mafic crustal rocks in the target zone are implicated as the metal source for the sic deposits

geology mineralization and emplacement of the whistle parkin offset dike sudbury the whistle parkin dike is a 12 km long radial offset dike located in the northeast sector of the 1 85 ga sudbury impact structure the dike is connected to the sudbury igneous complex via a 0 5 km long 250 m wide embayment the whistle segment of the dike narrows to a width of approximately 30 m and extends for about 1 5 km from the complex where it is cut by the post creek fault zone this fault displaces the dike 2 km to the northwest beyond which it continues as the parkin portion of the dike the parkin segment extends a further 10 km north northeast the dike and genetically associated embayment are comprised of numerous rock types these include the sublayer radial breccia mafic sulfide bearing breccia inclusion rich quartz diorite and inclusion poor quartz diorite economic ni cu pge mineralization is found in association with the sublayer mafic sulfide bearing breccia and inclusion bearing quartz diorite phases rare earth element ree data reveal that the dike lithologies are representative of bulk sudbury igneous complex melt we propose a multistage emplacement mechanism for the dike the breccia units including the inclusion bearing quartz diorite phase were forcefully injected laterally into an impact generated radial crack during the excavation stage of crater formation subsequent early modification processes facilitated the intrusion of inclusion poor quartz diorite particularly along the dike margins inclusion poor quartz diorite emplacement may have been gravitationally driven from the overlying impact melt sheet later stage modification i e final transient cavity collapse caused decoupling of the whistle and parkin dike segments via faulting this occurred concurrently with the settling of the sublayer unit and associated economic sulfides into the embayment and main dike segments

the basal onaping intrusion in the north range roof rocks of the sudbury igneous complex the 1 85 ga sudbury impact structure is one of the largest impact structures on earth igneous bodies the so called basal onaping intrusion occur at the contact between the sudbury igneous complex sic and the overlying onaping formation and occupy 50 of this contact zone the basal onaping intrusion is presently considered part of the onaping formation which is a complex series of breccias here we present petrological and geochemical data from two drill cores and field data from the north range of the sudbury structure which suggests that the basal onaping intrusion is not part of the onaping formation our observations indicate that the basal onaping intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the basal onaping intrusion planar deformation features within quartz clasts of the basal onaping intrusion are indicators for shock metamorphism and together with the melt matrix point to the basal onaping intrusion as being an impact melt rock by definition importantly the contact between granophyre of the sic and basal onaping intrusion is transitional and we suggest that the basal onaping intrusion is what remains of the roof rocks of the sic and thus is a unit of the sic and not the onaping formation we suggest henceforth that this unit be referred to as the upper contact unit of the sic

chemical variations and genetic relationships between the hess and foy offset dikes at the sudbury impact structure offset dikes are found concentrically around and extending radially outward from the sudbury igneous complex sic which represents an 3 km thick differentiated impact melt sheet the dikes are typically composed of an inclusion rich so called quartz diorite iqd in the center of the dike and an inclusion poor quartz diorite qd along the margins of the dike new exposures of the intersection between the concentric hess and radial foy offset dikes provide an excellent opportunity to understand the relationship between the radial and concentric offset dikes and their internal phases the goal was to constrain the timing of the dike emplacements relative to the impact and formation of the sic results herein suggest that 1 the timing between the emplacement of the qd and iqd melts was geologically short 2 the hess and foy dikes coexisted as melts at the same time and the intersection between them represents a mixture of the two 3 the foy dike has a slightly more evolved chemical composition than the hess dike and 4 the iqd melt from the foy dike underwent some degree of chemical fractionation after its initial emplacement

the sudbury igneous complex viscous emulsion differentiation of a superheated impact melt sheet the sudbury igneous complex of ontario canada is the remnant of a voluminous melt sheet produced in a few minutes by impact of a massive meteorite into continental crust 1 85 ga ago the transient cavity and melting zone reached the moho and instantly 2 min relaxed to form a more familiar large shallow crater holding a thick superheated 1700 c melt sheet covered by 2 km of breccia there is little about the resulting bimodal igneous complex that resembles crystallization of well known sheet like bodies of similar composition yet the norite and granophyre exhibit a remarkable similarity in isotopic and trace element compositions suggesting an intimate common parentage from the surrounding crust this petrogenetic enigma is explained here as a natural unavoidable consequence of the impact process in the rapid formation of a superheated magmatic emulsion which we take as the high temperature equivalent of breccia a wide spectrum of viscously discrete interdispersed parcels of mafic and felsic liquids reflecting the compositional heterogeneity of the target crustal materials formed the emulsion within days to months the emulsion components separated according to their relative densities into a bimodal norite granophyre assemblage that formed the basic structure of the present sudbury igneous complex there is clear evidence of this emulsion in the earliest dikes i e offsets which likely give the earliest state of the nascent melt sheet immediately following emulsion separation the strongly superheated bimodal melt sheet underwent vigorous thermal convection in each layer these convective motions homogenized and rapidly cooled the magma to the liquidus temperatures whereupon convection ceased the pattern of convection was pinned in place by the embayment topography of the crater floor which in turn played a pivotal role in directing sulfide deposition into the embayments all further cooling was by conduction of heat through the upper and lower boundaries during which time solidification fronts were established and propagated inward from the upper and lower margins there is clear evidence of solidification from the floor upward and the roof downward minimal differentiation and compositional modification took place throughout cooling and solidification nevertheless during the solidification stage granitic rock fragments on the crater floor and rafts of fallback breccia from the thick overlying onaping formation became unstable and entered the melt sheet and the partially melted remnants collected at the interface between the norite and granophyre some interstitial melt from the norite also percolated upward and altogether the blocks and melt produced the distinctive chemical and physical characteristics of the unusual transition zone the sudbury melt sheet is in essence a full scale magmatic experiment the conditions of formation relative to any other large terrestrial magma are precisely known thus the clear lack of any significant modal layering the overall homogeneous nature of each unit and the lack of any significant chemical differentiation through crystal fractionation establish sudbury as a valuable example of what does not happen under the initial conditions long assumed to prevail at the formation of most large magma chambers

sub million year age resolution of precambrian igneous events by thermal extraction thermal ionization mass spectrometer pb dating of zircon application to crystallization of the sudbury impact melt sheet a new approach to zircon dating is described that potentially offers a considerable improvement in the accuracy of 207 pb 206 pb ages on a wider variety of samples and at less cost than that generally available from conventional techniques zircon is first preheated in a vacuum to evaporate pb from altered domains leaving predominantly pb with isotopes that preserve the primary age of the sample refractory pb is removed from the zircon by thermal extraction at higher temperature directly into a silica melt ionization activator within a thermal ionization mass spectrometer this produces strong ion beams that allow 207 pb 206 pb isotope ratios to be measured to high precision 10 4 with almost negligible contamination from common pb isotope fractionation appears to be more reproducible than with conventional solution analysis allowing routine age determinations with precision of 0 2 m y for the precambrian as a test application zircon from a noritic boundary phase of the sudbury impact melt gives 1849 53 0 21 ma while a phase from several hundred meters higher in the noritic layer is resolvably younger at 1849 11 0 19 ma 95 confidence errors the enhanced precision and ease of application of this method should greatly increase the scientific power and availability of zircon dating

u pb geochronology of the sublayer environment sudbury igneous complex ontario cu ni sulfide deposits of the sudbury igneous complex are hosted by the inclusion rich sublayer which occurs discontinuously between the footwall and main mass of the sudbury igneous complex by brecciated footwall in the vicinity of the sudbury igneous complex and by offset dikes protruding for tens of kilometers into the country rocks the genesis of mafic ultramafic inclusions in the sublayer and the relationships between the sublayer main mass and offset dikes have long been at the center of debate on sudbury geology this paper reports u pb isotope data on zircon and baddeleyite from samples representing norite matrix melanorite and olivine melanorite pods a metapyroxenite inclusion and diabase from the sublayer at the whistle embayment these samples yield ages ranging from 1848 1 or 1 8 to 1849 1 or 1 1 ma similar to a u pb age of 1849 8 or 2 0 ma obtained for zircons in quartz diorite from the copper cliff offset and indistinguishable from the previously published average age of 1850 or 1 ma for the main mass of the sudbury igneous complex none of the samples yield evidence for inherited zircon components the isotopic data prove that the exotic mafic ultramafic inclusions and pods in the sublayer are not xenoliths scavenged from older layered complexes but require the existence of primitive moderately mg rich magmas that crystallized in the early stages of development of the sudbury igneous complex such mg rich cumulates are unlikely to have crystallized from melted average upper crust

emplacement of sulfide deposits in the copper cliff offset dike during collapse of the sudbury crater rim evidence from magnetic fabric studies quartz diorite offset dikes in the country rocks surrounding the 1 85 ga sudbury impact structure were emplaced within fractures formed during hypervelocity impact providing conduits for the injection of quartz diorite impact melts the offset dikes host major ni cu pge deposits with the radial copper cliff dike hosting 15 percent of the known sudbury ore the anisotropy of magnetic susceptibility of the copper cliff dike was studied to determine the nature of the fabrics that developed during emplacement of the quartz diorite and the associated sulfide bodies the anisotropy of magnetic susceptibility reveals a predominantly northwest southeast striking steeply dipping foliation and a steeply northwest to vertically plunging lineation the foliation along both the eastern and western sides of the dike is counterclockwise oblique to the north south to north northwest south southeast striking vertically dipping dike margins the magnetic fabrics in the dike are consistent with a component of strike slip displacement during emplacement of the quartz diorite melt the data are interpreted to indicate that the copper cliff dike acted as a melt lubricated dextral transfer fault during collapse of the inferred inner rim of the central peak ring to accommodate ultra high strain rate displacements on previously documented pseudotachylite rich normal fault zones collapse may have caused injection of sulfide rich melts into the copper cliff dike leading to the formation of cu ni pge deposits collapse of the inner rim of the central peak ring is thought to have occurred too soon after the impact to allow for fractionation of the sulfides from the impact melt sheet therefore it is suggested the sulfide rich impact melts were derived principally from the target rocks

geology mineralization and emplacement of the foy offset dike sudbury impact structure the foy offset dike is the longest of the known radial dikes associated with the 1 85 ga sudbury igneous complex the foy dike extends for at least 30 km from the complex into the north range archean and proterozoic footwall rocks of the sudbury impact structure the dike ranges in width from 400 m where it connects to the sudbury igneous complex to 50 m at its most distal known exposure we divide the dike into a proximal segment and a distal segment based on where it intersects the concentric hess offset dike three main dike lithologies are distinguished all referred to as varieties of quartz diorite inclusion rich pyroxene quartz diorite occurs in the embayment adjacent to the base of the main mass of the sudbury igneous complex this constitutes the sublayer northward the sublayer gives way to amphibole biotite inclusion bearing quartz diorite and amphibole biotite inclusion poor quartz diorite the dike is mineralized and deposits in the proximal segment have been mined mineralization is associated with the inclusion rich dike phase at the nickel offset mine the dike contains inclusion bearing massive ni cu sulfides that also host platinum group minerals the orebodies occurred as strike parallel sheets within the central parts of the dike in association with inclusion bearing quartz diorite the geochemistry of the dike lithologies indicates that both the inclusion poor and inclusion bearing quartz diorite varieties are close in composition to the postulated bulk composition of the sudbury igneous complex this implies that the dike was emplaced prior to any significant fractionation of the main melt body and hence very early in the evolution of the sudbury complex we envisage dike emplacement rapidly following rebound and central uplift formation as part of the impact process early dike emplacement implies that the ore was also introduced early in the evolution of the melt sheet when the inclusion poor quartz diorite phases were still unconsolidated uplift would have caused extension in the target rocks and so dilated the footwall fractures and faults that then became hosts primarily via gravitationally driven intrusion to the primitive polymict impact melt breccias and sulfide ores the chilled margin compositions of the foy inclusion poor quartz diorite are considered good indicators of the original bulk composition of the sudbury igneous complex this idea is supported by comparison with calculated estimations of the bulk composition a shift in geochemical trends a grain size increase and a decrease in inclusion content occurs 22 km along the length of the dike which we believe corresponds to a fault system that downfaults to the north given that the dense ore would have preferentially settled toward the base of the foy dike fracture system postimpact downfaulting of the distal segment of the dike may have displaced any mineralized zones present to deeper levels relative to the proximal segment

geology of the giant sudbury polymetallic mining camp ontario canada a new 1 100 000 scale compilation bedrock geology map provided with this issue presents a geologic synthesis of canada s most prolific mining camp the world class sudbury structure with total past production and current reserves of over 1 7 billion tonnes of ni cu co pt pd au ag ore lydon 2007 the polymetallic ore is hosted within one of earth s largest preserved impact craters the new map of the ni cu pge sudbury district after ames et al 2005 is included in a pocket at the back of this issue map 1 and is linked to mineral deposit commodity and geochronology data in tables 1 and 2 of this paper view this table table 1 list of mineral deposits and occurrences of the sudbury map area map 1 view this table table 2 u pb isotope data across the map area since 1891 six compilation maps of the geology of sudbury have been published bell 1891 collins 1937 cooke 1946 card 1969 dressler 1984 ames et al 2005 fig 1 some of the ni cu pge mines have operated for over a century whereas new ore deposits discovered as recently as 2004 are already in production or are in the process of being developed i e advanced prospects in the last 15 years sudbury has been the training ground for 60 postgraduate geoscience students in north america fig 1 from 1903 to 1990 only 23 graduate theses were completed on sudbury related topics eight of them in the 1970s after the discovery of shatter cones dietz 1964 fig 1 number of graduate theses of north american geoscientists trained on sudbury geology per decade showing the minor peak in training after the discovery of shatter cones and the recent education boom with over 50 geoscience graduate theses completed since 1990 the dashed lines represent the regional geologic maps of the sudbury area

geochemical evolution of a 10 m thick intrusive body the south brenterc h diabase dyke western armorican massif france the south brenterc h 9 5 m thick diabase dyke belongs to the intermediate ti quartz normative tholeiitic magmatism which has intruded the western end of britanny france near the trias lias boundary it has been previously identified as a simple dyke resulting from a single magmatic injection the present study is based on ten samples collected along a half transverse section of this dyke geochemical analyses and some textural parameters have been used to point out two flow related differentiation processes partially masked by alteration these late magmatic mechanisms have induced subtle geochemical variations among samples which sometime counteract each other the processes pointed out here are i expulsion of phenocrysts from the borders during flow differentiation likely by the way of an hydrodynamic grain dispersive pressure and ii emplacement of a more evolved magma in the central part of the intrusion probably due to a viscosity segregation mechanism during the flow

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

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

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

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

paleomagnetically determined rotations in the western sicilian fold and thrust belt new paleomagnetic data from the west sicilian fold and thrust belt indicate large scale clockwise rotations during tectonic transport for the platform and seamount paleogeographic units of the fold and thrust belt panormide and trapanese units there is a general consistency in the magnitude of rotation along strike suggesting the existence of large coherent structural units for the panormide platform clockwise rotations in the range of 90 to 140 relative to the underlying iblean parautochthon are recorded along 125 km of outcrop strike in the case of the trapanese unit clockwise rotations in the range of 47 to 70 relative to iblei are recorded along about 90 km of outcrop strike intervening basinal paleogeographic units imerese and sicanian units show clockwise rotations of similar magnitude although few reliable data are available from these units palinspastic reconstruction of the thrust belt incorporating the rotations and arguments for the present orientation of the margins of the paleogeographic units implies that the panormide platform and trapanese seamounts were originally elongated more or less north south relative to the present orientation of iblei the two units apparently coalesced to the south and were discontinuous to the north they and their intervening basins were part of a passive continental margin which occupied a large part of the area that was subsequently rifted to form the tyrrhenian sea

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

the sudbury igneous complex a differentiated impact melt sheet the sudbury structure ontario is the remnant of a 1 85 ga old impact crater which originally had a diameter of 200 to 250 km the sudbury igneous complex occurs within the sudbury structure the sudbury igneous complex is a 2 5 to 3 0 km thick 60 27 km elliptical igneous rock body which consists of four major lithologies from top to bottom traditionally termed granophyre quartz gabbro norite and contact sublayer sulfide and inclusion bearing noritic rock with the exception of the latter all these lithologies are continuous across the structure modal analyses reveal that following the iugs system of nomenclature quartz gabbro samples are in fact quartz monzogabbros a few of the norite samples are quartz gabbros and most norite samples are quartz monzogabbros in view of these observations and in order to clarify the nomenclature an updated terminology is proposed from top to bottom upper unit middle unit lower unit and contact sublayer the bulk composition of the sudbury igneous complex from north range data is granodioritic continuous and gradational mineralogical and geochemical variations between the lithological units are evidence that the complex behaved as a single melt system all the sudbury igneous complex lithologies have the same light to heavy rare earth element ree ratio and an overall pattern of increased light ree and depleted heavy ree the occurrence of primary hydrous minerals hornblende and biotite deuteric alteration and abundant micrographic and granophyric intergrowths demonstrate that the melt was rich in h2o moreover the granophyric and other far from equilibrium textures are most likely due to rapid crystallization triggered by exsolution of a volatile phase the sudbury igneous complex differs from traditional layered mafic complexes in the following aspects it has an overall intermediate composition a hydrous nature a crustal isotopic signature normative corundum and an unusually large volume of granophyre the sudbury complex differs from known terrestrial impact melt sheets only by its great thickness and the presence of chemical and therefore mineralogical layering reported here for the first time and similar to those found in impact melt rocks elsewhere are the occurrences of plagioclase xenocrysts with complex twinning and zoning patterns and planar deformation features in quartz xenocrysts the well known ore deposits of the sudbury region are directly related to the genesis of the sudbury igneous complex some ores precipitated from the sudbury melt whereas others were concentrated by hydrothermal fluids that percolated through the crystallized complex it is concluded that the sudbury igneous complex is the best exposed and only well documented to date terrestrial impact melt sheet to have differentiated