basement terrane correlations and crustal recycling in the western superior province nd isotopic character of granitoid and felsic volcanic rocks in the wabigoon subprovince n ontario canada abstract the western superior province represents a collage of mesoarchean continental 3 2 2 8 ga and neoarchean continental arc and oceanic 2 8 2 66 ga crustal fragments nd isotope data presented here for 89 felsic volcanic and plutonic rocks have been combined with compiled data to generate a data set of over 240 analyses from the wabigoon and winnipeg river subprovinces these samples from both oceanic and continental environments have been used as crustal probes their nd isotopic character used to investigate the extent of basement terranes the ancestry of crustal fragments their interaction with each other and the degree of sialic crustal reworking the wabigoon and winnipeg river subprovinces include three isotopically distinct terranes 1 the winnipeg river terrane comprises the winnipeg river subprovince the north central wabigoon and the north eastern wabigoon subprovince this region is dominated by neoarchean granitoid rocks with mesoarchean vestiges of previous workers have suggested that the boundary between the western wabigoon and winnipeg river terranes represents a cryptic suture resulting from a collision at circa 2 71 ga the boundary between the western wabigoon and marmion terranes although obscured by late intrusions may be a continuation of this suture zone the boundary between the winnipeg river and marmion terranes in the central wabigoon is also cryptic isotopic data suggest that the marmion terrane is allochthonous with respect to the winnipeg river terrane common magmatism in these two terranes at 2 93 2 87 ga suggests that they may have collided during this time in the eastern wabigoon subprovince this collision zone is speculated to occur at the humboldt bay high strain zone a complex history of crustal recycling throughout the mesoarchean and neoarchean is revealed which demonstrates the importance of autochthonous development of granite greenstone terranes in a continental environment the data also support models of tectonic accretion during the archean indicated by the presence of granite greenstone terranes of allochthonous origin

assimilation and crystallization in basic magma chambers trace element and nd isotopic variations in the kerns sill nipissing diabase province ontario an investigation has been made of the trace element and nd isotopic effects of assimilation at the roof of the proterozoic kerns sill in the 2 2 ga nipissing diabase province in ontario the ratios

a punctuated late ordovician and early silurian deglaciation and transgression evidence from the subsurface of northern saudi arabia abstract the lower section of the lower silurian qusaiba member qalibah formation is characterized by regionally developed organic rich shales that have sourced many of the large paleozoic petroleum systems of saudi arabia in northern saudi arabia these high total organic carbon toc horizons are being assessed for their unconventional shale gas potential the initial phase of exploration drilling which resulted in a quadrupling of the number of penetrations in northern saudi arabia had the dual purpose of 1 assessing the high toc horizons as an unconventional resource play and 2 acquiring the fundamental data required to understand the geologic development of the zones of interest within the lower qusaiba the availability of numerous new cores from across northern saudi arabia enabled an extensive refinement of the existing biostratigraphy and enhanced integration between graptolite and palynomorph biozonation systems in cores from the study area four distinct sedimentary facies are recognized 1 pyritic siltstone 2 black mudstone 3 black chert and 4 gray shale representing distinct paleoenvironmental conditions related to the stepped latest ordovician and early silurian gondwanan deglaciation the failure of the gondwanan ice sheet was not a simple short lived consistent melting and associated flooding of a flat continental shelf this study highlights the complex interplay of sea floor topography ocean currents sediment supply and variations in the rate of melting of the ice sheet with the associated rising ambient temperatures there are 1 increasing clay concentrations associated with intensifying chemical weathering of the exposed land mass and 2 progressive lowering of the carbonate compensation depth as water temperatures rise enabling the preservation of carbonate shell material

provenance shallow to deep diagenesis and chemical mass balance in supermature arenites and pelites ordovician simpson group oklahoma and kansas u s a abstract this study considering original detrital compositions and diagenetic alterations relates sandstone and shale mineral and chemical composition to provenance and mass balance between the sedimentary assemblage and its provenance mature quartz rich sandstones and siltstones with clay rich shales in cores at depths of 60 m 5 2 km and burial temperatures of 150 c from over 50 wells were utilized in this work these provide the material for analysis of compositions and alterations in coastal and shallow marine clastic sedimentary rocks of the middle ordovician simpson group in oklahoma and kansas u s a mineral and geochemical data were obtained by optical petrography x ray diffraction scanning electron microscopy electron microprobe and whole rock x ray fluorescence and icp ms analyses results of the study show significant progressive diagenetic changes with increased burial depth and temperature mineralogically and chemically less altered rocks are comparable to modern quartz rich sands and kaolinitic muds in siliciclastic sediments developed during humid tropical weathering in crystalline basement shield provenance in africa and south america by analogy to modern tropical cases muds composed of major kaolinite with minor smectite and illite at deposition were diagenetically transformed to illite rich rocks by possible microbial action at surface to shallow 20 in sandstone buried 3 km during deep burial high temperature 110 c calcite cement from deep basin sources was precipitated in sandstones geochemical mass balance estimates physical and chemical evidence suggest quartz cement was sourced from within clastic rocks of the simpson group with minor external contribution of sio2 na is in low abundance due to loss in solution during weathering in the provenance rare earth elements ree have typical abundances and distributions in shales with enrichment in light ree littoral environment siltstones and matrix rich sandstones display enrichment of heavy ree in phosphatic brachiopod shell fragments

geophysical investigations and crustal structure of the north american midcontinent rift system abstract geophysical investigations have had a prominent role in studying the 1100 ma midcontinent rift system because of limited surface exposures and deep drilling of the rift rocks gravity and magnetic anomaly data delineate the rift system its central volcanic filled graben and overlying and adjacent post rift sedimentary basins and the major faults of the system over its 2000 km length in central north america crustal seismic studies indicate a high velocity crust that is thickened to as much as 55 km along the rift deeply penetrating seismic reflection data obtained across the rift show that the crust was thinned to less than one half of its original thickness during rifting and the present crust has been thickened by addition of up to a total of 30 km of mantle derived volcanic rocks within grabens and clastic sediments deposited in overlying sag basins as well as by intrusions into the crust and crustal underplating these data also confirm the occurrence of a post rift regional compressional event that locally caused broad folding of the rift rocks and reactivation of the graben normal faults as high angle reverse faults with vertical throws of several kilometers geophysical data indicate that the rifting event had a profound effect on the entire vertical range of the crust and that structures and thus the tectonic evolution are similar along the entire length of the rift

introduction to special issue of canadian journal of earth sciences the lake nipigon region geoscience initiative the lake nipigon region geoscience initiative lnrgi was a 3 year collaborative geoscience data acquisition and compilation project that focussed on lake nipigon in ontario and involved partnerships between mineral industry university and provincial and federal government geoscientists a major objective of the initiative was to collect new geoscience information to better understand the geological history of the nipigon region to inform land use planning identify areas of high mineral potential and attract mineral exploration the special issue on the lnrgi highlights the scientific successes of the initiative in the fields of geology petrology geochemistry geochronology stratigraphy and basin analysis mineral deposits and surficial geology and geochemistry

lithospheric thinning during the evolution of continental large igneous provinces a case study from the north atlantic tertiary province lithospheric thickness can act as an important physical control on the composition of asthenosphere derived melts by restricting the minimum depth at which decompression melting can occur a case study of plume related tertiary lavas from the isle of mull scotland has revealed the presence of three successive magma types which become progressively more depleted in the incompatible trace elements geochemical modeling suggests that melting initially took place within the garnet iherzolite stability field but subsequently involved more extensive melting 10 of spinel iherzolite at shallower mantle depths these results can be explained in terms of a progressively thinning lithosphere beneath the region during the evolution of the volcanic center the observed degree of lithospheric thinning cannot be explained by extension alone and it appears that some relatively rapid lower lithospheric erosion by the plume head is also required theoretical modeling suggests that lithospheric erosion can be a comparatively rapid process and this has important implications for the various starting plume models

the north american midcontinent rift beneath lake superior from glimpce seismic reflection profiling the midcontinent rift system is a 1 1 b y old structure extending from kansas through the lake superior region and into southern michigan the rift is filled with thick sequences of basaltic volcanic rocks and clastic sediments for most of its extent it is buried beneath paleozoic rocks but can be traced by its strong gravity and magnetic anomalies the rocks of the rift system are exposed only in the lake superior region and comprise the keweenawan supergroup much of the geology of the keweenawan is beneath lake superior and has only been inferred from potential field studies and seismic refraction studies and extrapolation from on shore geology seismic reflection surveys by the great lakes international multidisciplinary program on crustal evolution in 1986 imaged much of the deep structure of the rift beneath the lake in detail the reflection profiles across the rift reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented they show that in addition to crustal sagging documented by previous investigations normal faulting played a major role in subsidence of the axial region of the rift a sequence of volcanic and sedimentary rocks in places greater than 30 km thick fills the graben thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben near the axis the prerift crust is thinned to about one fourth of its original thickness apparently by a combination of low angle extensional faulting and ductile stretching or distributed shear the sense of asymmetry of the central graben changes along the trend of the rift documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments the location of the accommodation zones is inferred from abrupt disruptions in the bouguer gravity signature of the rift uplift of the central graben occurred when the original graben bounding normal faults were reactivated as high angle reverse faults with throws of 5 km or more in places the midcontinent rift has some striking similarities to some younger passive continental margins we propose that it preserves a record of nearly complete continental separation which had it not been arrested would have created a middle proterozoic ocean basin

calibrating the late ordovician glaciation and mass extinction by the eccentricity cycles of earth s orbit a process based sedimentological analysis of upper ordovician glacial marine rocks in africa suggests that full glaciation of the continental shelf started in the late extraordinarius zone of the hirnantian two cycles of ice sheet growth are represented during full glaciation initial terrestrial ice sheet growth in the early extraordinarius zone influenced the first event of the late ordovician mass extinction retreat of the ice sheet from the shelf ended by the persculptus zone when the second event of the late ordovician mass extinction began eccentricity controlled ice sheet growth is assumed periodicity 0 1 m y hence two cycles of full glaciation lasted 0 2 m y the duration of the extraordinarius zone is estimated as 0 5 m y therefore the minimum duration of the first extinction event was 0 3 m y

the igneous petrology and magmatic evolution of the midcontinent rift system klewin k w and shirey s b 1992 the igneous petrology and magmatic evolution of the midcontinent rift system in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 33 40 the 1100 ma midcontinent rift system of north america is one of the world s great igneous provinces containing over one million km3 of mafic volcanic and plutonic rocks volcanic rocks range from picrite to rhyolite although tholeiitic basalt predominates plutonic rocks range from peridotite to granite with plagioclase rich gabbros and troctolites being most abundant the igneous history of the rift can be divided into early and late stages flows of the early stage form a trend having low alumina high magnesia picrite as the primitive endmember whereas flows from the late stage form a trend having high alumina olivine tholeiite as the primitive endmember the early basalts have enriched nd isotopic compositions and were derived largely from old heterogeneous subcontinental lithosphere the late basalts have chondritic to depleted nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle apparently melting began initially in the subcontinental lithosphere as it was stretched subsequently a lower mantle plume which had entrained asthenospheric mantle during its ascent rose beneath the rift and melted at large degrees to yield the voluminous later basalts

geochemistry and geodynamic implications of the mesoproterozoic english bay granite rhyolite complex northwestern ontario the mesoproterozoic english bay complex consists of a granite rhyolite assemblage outcropping on the shores of lake nipigon in western superior province canada it intrudes neoarchean rocks and is disconformably overlain by a rift infracratonic basin sedimentary succession recording subsidence following a heating event the granites and rhyolites are characterized by light rare earth element lree enrichment la smn 2 8 5 1 and only weakly fractionated heavy ree hree gd ybn 1 1 1 6 the felsic igneous rocks are high k enriched in zr nb y and ree satisfying all the criteria for an a type suite trace element geochemistry particularly the absence of any negative nb anomalies indicates this melt did not originate in a suprasubduction zone setting unlike the st francois mountain complex to the south the english bay complex may record the northern portion of a mesoproterozoic plume track a plume that possibly led to earlier igneous activity and infracratonic basin formation to the north and would later interact with a suprasubduction zone margin to the south

are the frictional properties of creeping faults persistent evidence from rapid afterslip following the 2011 tohoku oki earthquake 1 geophysical observations and numerical studies have shown that creeping portions of faults have persistent rate strengthening frictional properties and can act as barriers to earthquake rupture propagation on the basis of gps data following the 2011 mw 9 0 tohoku oki earthquake in japan we find that the evolution of afterslip and postseismic shear stress on the plate interface is inconsistent with persistent rate strengthening frictional properties but is consistent with slip rate dependent frictional properties that exhibit less rate strengthening with increasing slip rate such slip rate dependent frictional properties tend to prevent creeping regions from acting as barriers to rupture propagation and therefore could be an important factor in determining the spatial extent of individual earthquakes

petrology of the keweenawan mamainse point lavas ontario petrogenesis and continental rift evolution the mamainse point formation forms a 5250 m thick sequence of flood basalts ranging from high mgo picrites and basalts to basaltic andesites that formed in the keweenawan 1100 ma midcontinent rift of north america this section is the most continuous and complete sequence of keweenawan lavas and records almost the entire igneous history of the rift the section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics a variety of petrogenetic processes including partial melting of different sources at decreasing depth periodically replenished tapped and fractionated magma chambers crustal contamination and polybaric fractionation were responsible for the evolution of the magmas the dominant petrogenetic process changes with stratigraphy which suggests that the physical evolution of the rift controlled which process operated at any time the initial fracturing of the crust presumably allowed high mgo liquids generated at depth to be erupted the early stage of rifting was apparently slow and allowed the formation of long lived magma chambers in which complex processes such replenishment mixing and assimilation contamination could occur the late stage of rifting however was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted these conclusions from the chemostratigraphy agree with the physical stratigraphy which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation

clingfilm preservation of spiraliform graptolites evidence of organically sealed silurian seafloors the early silurian graptolites spirograptus turriculatus and sp guerichi are typically preserved in condensed graptolite shale lithologies as entirely flattened outlines all whorls are visible and little or no sediment infills the originally cone shaped rhabdosomes as though the graptolites had been sealed in clingfilm or plastic wrap prior to burial and compaction by contrast the rhabdosomes of graptolites transported in turbidity currents typically are filled with sediment the most likely reason for the clingfilm mode of preservation is encasement or covering of the graptolite rhabdosomes by marine snow and or microbial mats prior to burial by clastic sediment and compaction on an anoxic seafloor experimental evidence reported herein supports this suggestion the organic material that mediated the preservation of such graptolites was likely akin to but probably physically stronger than the delicate benthic flocculation layer of the current black sea floor like the latter it probably formed significant microtopography at and just below the sediment water interface and mediated geologic processes such as early diagenesis in graptolite shale lithologies

silt 2 62 μm 9 4φ the particles in finer grained detrital sediments are usually composed of quartz they fall into two size grades sand 2 mm 62 m and silt 62 m 2 m these size gradings conceal important geological processes since there are geological controls on both the quartz sand and silt populations sand nature is largely controlled by geochemical reactions for example in cooling granite from which it is eventually released by weathering action the quartz is among the last part of the rock system to solidify and the eutectic like reaction which occurs ensures that the quartz exists as small crystalline units each suffering considerable cooling and recrystallisation stresses silt is broken quartz and has traditionally been considered to lack a specific geological control the situation is confused by the large range of materials that fall into the size category but there do seem to be distinguishable modes and possibly comminution limits within the 60 m span the controls operating in this size range are probably the critical concentration of moss defects in the quartz particles moss postulated the formation of specific crystal defects in the quartz formed in granites these affect sand formation and it is possible that they also control the mode size of silt particles within the silt range there may be several usefully definable populations as moss proposed for sand the quaternary appears to be a silt rich period due to tectonic and glacial activity but silt production is apparent throughout the sedimentary record very long term silt producing processes are required to produce silt in nature on a large scale very energetic processes are required many processes that are believed to generate silt particles have been listed however large scale production is essentially due to glacial grinding or to intense weathering processes in high cold tectonically active mountain regions the region of high asia of crustal overlap is a major generator of silt particles these form the productive alluvial soils of north india most of the nation of bangladesh the loess deposits along the syr darya and amu darya rivers in central asia and the great loess deposits of north china some claims have been made for silt production in hot deserts large amounts of very fine aerosolic dust are produced but in terms of loess sized particles this is a small scale process and leads to modest disputed deposits the loess deposits around the sahara usually have smaller mode sizes e g nigeria or larger mode sizes e g libya tunisia than the true chinese mode at 25 m this paper aims to introduce the distinguishing properties of sand and silt discuss in detail the possible processes of silt formation in relation to the evidence of what is produced describe the major silt populations and conclude on the most likely formation process es in particular three silt ranges are discussed in relation to the r size diagram c 2 5 m d1 20 30 m and d2 60 m these can represent canadian quickclay qq chinese loess and the ideal loughborough loess ll and north african loess nal it is intended that this paper will put into context the diverse work in the area and in so doing shed light on what is an often overlooked subject

long range transport of giant aeolian quartz grains linkage with discrete sedimentary sources and implications for protective particle transfer abstract several articles have documented the presence of large particles 62 5 m that have been transported great distances 1000 km through the atmosphere at present we do not understand the mechanisms that lead to mineral grains being carried aloft far beyond the limits imposed by conventional gravitational settling models typically marine geologists refer to ice rafting and or advective transport by ocean currents as the dominant transport processes that deliver large mineral grains to deep sea sediments textural analyses of giant quartz grains from six sedimentary deposition events collected from various sources including a single site in the north pacific ocean and sites in the british isles and france strongly support the idea that in each case all of the grains were initially eroded from a discrete sediment type and often a discrete point source and subsequently transported via the atmosphere to our sampling sites up to 10 000 km away these findings suggest that at least at times erosional events fuelling long distance atmospheric transport are exceedingly specific and operate upon geologically distinctive assemblages and are able to effect long distance transfer of large grains with no grain to grain collisions this work also suggests that atmospheric transport should be included as a possible means of delivery for significant quantities of giant sedimentary particles to remote marine sites such as the lord howe rise

chemostratigraphy of the neoproterozoic alona bay lavas ontario a basal sequence of flood basalt lavas associated with the neoproterozoic midcontinent rift system crops out in alona bay along the southeastern shore of lake superior in ontario the alona bay lava succession is about 1200 m thick and lies just north of the well studied contemporaneous mamainse point formation detailed chemostratigraphy of the alona bay lavas suggests they are grossly correlative with the basal portion of the mamainse point formation for instance like the basal part of mamainse point formation the alona bay section contains numerous high mgo lavas and can be subdivided into 4 5 groups with distinct chemical characteristics chemical variations within the alona bay groups are largely the result of fractional crystallization likely at moderate pressures one small group of alona bay lavas also carries the compositional imprint of crustal contamination the remaining inter group chemical distinctions at alona bay are the consequence of temporal changes in partial melting and source ch

a synopsis of mineral deposits in the archean and proterozoic rocks of the lake nipigon region thunder bay district ontario a variety of metallic and non metallic mineral deposit types occur within archean and proterozoic rocks in the area encompassing the lake nipigon region geoscience initiative archean deposit types include algoma type banded iron formation hosted iron e g lake nipigon iron range volcanogenic massive sulphide copper zinc e g onaman tashota belt ultramafic intrusion hosted chromium e g puddy chrome lakes mafic to ultramafic intrusion hosted copper nickel platinum group element pge e g lac des iles and pegmatite hosted deposits of rare metals li ta be uranium and molybdenum e g georgia lake field black sturgeon lake and anderson lake respectively mesothermal lode gold deposits are prominent in the beardmore geraldton camp superior type iron formation occurs in paleoproterozoic gunflint formation red bed copper occurs in mesoproterozoic midcontinent rift related osler group volcanic and interflow sedimentary rocks native copper and copper sulphides occur in mesoprote

reviewing the configuration and extent of the midcontinent rift system abstract uncertainty exists in the configuration and extent of the midcontinent rift system mrs because of deficiencies in geophysical data and limited information from outcrops and basement drill holes additional ambiguity is caused by misunderstanding the definition of continental rifts six principal problematic regions in mapping the mrs are described gravity and magnetic data supported by drill hole and seismic reflection data show that the eastern lake superior rift segment of the mrs continues south from lake superior and connects to a much narrower rift in northern lake michigan the eastern margin of this transition is ill defined because of the lack of definitive anomalies and supporting seismic and drill hole data but is interpreted to occur near the u s canada border the rift segment in southeastern michigan intersects the grenville front and likely continues eastwards in modified form to near the boundary with canada cross cutting gravity and magnetic signatures may reflect grenvillian overthrusts near the terminus of the mrs in michigan the proposed southerly extensions of both branches of the rift system into oklahoma and ohio are based primarily on positive gravity anomalies but neither postulated extension appears to be associated with rifted troughs rather the gravity anomalies of the western branch are related to intrusive mafic rocks and those of the eastern branch are most likely related to deep crustal metamorphic rocks thrust into juxtaposition with less dense crust by grenville orogenesis recent paleomagnetic investigations in conjunction with high resolution radiometric dating imply that the mrs developed during the rapid southward movement of laurentia during a quiescent period along its eastern continental margin massive magmatic activity accompanying the rifting was likely due to rising mantle material that was displaced by subducted lithosphere along the southern margin the heated crust was made more ductile fostering rifting due to extensional stresses the nipigon embayment remains as a possible candidate for an early third branch of the mrs but current evidence is insufficient to include the fort wayne rift as part of the mrs future studies of the mrs would be well served by new age dating and high resolution seismic studies of the lithosphere

depositional controls on mudstone lithofacies in a basinal setting implications for the delivery of sedimentary organic matter deep water mudstones from ancient epicontinental settings are significant repositories for organic matter but the detailed temporal variations of and controls on the abundance and type of organic matter om is little studied using micro petrographic and geochemical data from late mississippian mudstones of the widmerpool gulf uk the processes that delivered fine grained sediment to this basin during a glacioeustatic sea level cycle are interpreted from detailed lithofacies analysis seven primary lithofacies are identified from core which show specific and systematic variations in total organic carbon toc content and bulk carbon isotope composition of organic material 13corg during sea level highstands thin bedded carbonate bearing mudstones are the dominant facies deposited contain up to 6 6 toc average 4 6 1 3 and have mean 13corg of 28 5 0 9 during phases of lower sea level thin bedded silt bearing clay rich mudstones with up to 4 1 toc average 2 3 0 8 mean 13corg 28 2 1 0 were interbedded with more organic lean graded silt bearing mudstones and sand bearing silt rich mudstones average toc 1 7 0 6 derived from turbidity currents the latter mean 13corg 26 2 0 7 are closely linked to significant proportions of terrestrial plant material while some rare plant debris and sand bearing mudstones produced from debris flows have more than 7 0 toc and 13corg 26 0 the 13c values of wood fragments ranged from 27 1 to 24 0 and therefore the 13corg is interpreted as a function of the ratio of marine and terrestrial organic matter more negative values in the carbonate bearing and the clay rich mudstones indicate marine planktonic algae whereas the least negative values reflect greater contribution of terrestrial plant material the data suggest that the marine conditions prevailed and supported marine planktonic algae throughout different sea level stages marine om was delivered to the sea floor by continuous hemipelagic settling whereas terrestrial om was delivered by sediment density flows variations in bioproductivity and dilution by siliciclastics influenced the burial rate of marine om organic rich mudstones preserved in these marine basins are potential hydrocarbon source rocks especially as unconventional shale gas reservoirs detailed microtextural and compositional analysis coupled with rigorous geochemical parameters as used in this study are important for the understanding of the source rock potential of basinal mudstones and of fine grained organic rich sediments in general

mud re distribution in epicontinental basins exploring likely processes abstract fine grained clastic sediments are very common in the interior deposits of ancient epicontinental seas not only do they make up the gross lithology in these basins but they can also be traced for more than 1000 km offshore from basin margins given that epicontinental seas were overall shallow and in many parts most likely less than 100 m deep basin floor slopes can safely be expected to be in the 0 01 to 0 001 range for much of depositional history known processes that bring muds to the basin margin and beyond are hypopycnal river plumes hyperpycnal fluvial discharge events storm setup relaxation flows and gravity driven fluidized muds with the exception of river plumes all of these processes require the presence of sufficient slope for sustained movement due to that constraint these processes combined might in the majority of situations have been able to move muddy sediments on the order of 100 km offshore whereas this is sufficient to distribute mud across marginal shelf seas it becomes problematic in the case of much larger epicontinental seas for example those of upper devonian or upper cretaceous times extended in places for thousands of kilometers and thus a process is needed that can move muddy sediments the rest of the way flume studies of the bedload transport of mud combined with observations from the rock record suggest that wind or tide induced bottom current circulation was most likely essential for moving muddy sediments from the periphery of epicontiental seas to their interiors remobilization of seafloor muds during frequently recurring lowering of sea level is likely to have aided in this process

eolian derived siltstone in the upper permian phosphoria formation implications for marine upwelling previously described organic rich shale facies of the meade peak member of the phosphoria formation at soda springs idaho consist of well sorted laminated siltstone containing little or no clay the planar parallel fabric preserved in many of the siltstone beds suggests suspension settling onto the outer shelf or upper slope most likely under low oxygen conditions evidence for transport of silt to the depositional site by sediment gravity flows is absent as are sandstone beds we propose that meade peak siltstone facies record subaqueous deposition of windborne silt that was transported southward from central montana as such they provide the first direct geologic evidence of a wind regime favorable for marine upwelling the process commonly postulated to have localized the deposition of permian phosphatic sediments they may also provide indirect evidence of upwind eolian sand transport at that time

bottomset and foreset sedimentary processes in the mixed carbonate siliciclastic upper jurassic lower cretaceous vaca muerta formation picun leufu area argentina abstract the upper jurassic lower cretaceous vaca muerta formation at the picun leufu area constitutes bottomset and foreset marine deposits comprising open bay siliciclastic basin and mixed carbonate siliciclastic shelf facies detailed sedimentological ichnological and sequence stratigraphic analysis of six stratigraphic sections allows establishing two depositional sequences depositional sequence 1 ds1 begins with lowstand eolian deposits included in the quebrada del sapo formation and continues with thin retrograding transgressive open bay facies of the vaca muerta formation encompassing coastal sand sheet and dune marginal marine bay margin and distal bay facies above the transgressive deposits the regressive hemicycle of ds1 consists of bottomset deposits representing a siliciclastic basin facies association lobe and lobe fringe hyperpycnal flow sedimentation alternated with hemipelagic basinal deposition in an oxygen deficient environment a relative sea level fall generated a sequence boundary which coincides with an angular unconformity that marks the base of depositional sequence 2 ds2 this sea level fall triggered the formation of an extensive lowstand channel fill and lobe complex at the base of the slope followed by retrogradational lobe facies during the subsequent transgression the regressive hemicycle of ds2 represents foreset mixed carbonate siliciclastic shelf facies forming a slope mud belt and slope sand bodies near the top sedimentation in a foreset topset transition resulted in the accumulation of bioturbated mixed slope and sandy shoal deposits sand bars and lagoonal facies occur on top of all sections and are truncated by a sequence boundary the combined analysis provides insights into the sedimentary processes affecting bottomset and foreset deposition underscoring the role of wave influenced hyperpycnal flows and the effect on organic matter dilution in one of the most important unconventional reservoirs from argentina

proterozoic and archean geology of the nipigon embayment implications for emplacement of the mesoproterozoic nipigon diabase sills and mafic to ultramafic intrusions the nipigon embayment is underlain by archean rocks of the english river wabigoon and quetico subprovinces and intruded along the west side by late to post tectonic mafic to ultramafic intrusions the early mesoproterozoic ultramafic to felsic badwater intrusion and felsic english bay complex are located in the northwest corner of the nipigon embayment three mafic to ultramafic intrusions the disraeli seagull and hele intrusions are located south of lake nipigon and the kitto intrusion is located east of the lake a number of mafic to ultramafic bodies jackfish island shillabeer kama hill nipigon bay have only limited outcrops the gabbroic nipigon diabase sills intrude all other rocks in the nipigon embayment and generally have a consistent mineralogy and geochemistry except for the inspiration sill s and the mcintyre sill geological and geophysical data suggest emplacement of the ultramafic intrusions by mechanisms similar to those controlling emplacement of the saucer shaped diabase sills these mechanisms are partially dependent on a series of pre existing north northwest and northeast trending faults formed prior to keweenawan magmatism the presence of sills rather than dykes indicates that the nipigon embayment was not extensional during the keweenawan midcontinent rift suggesting that the nipigon embayment is not a classic failed arm

crustal contamination identified in keweenawan osler group tholeiites ontario a trace element perspective in the lake superior area voluminous middle proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the midcontinent rift and overlie archean and older proterozoic basement the 3000 m thick section of gently dipping 10 sse basalts on the black bay peninsula is divided into three suites based on the relationship between stratigraphy petrography and geochemical variations e g mg number a1 2 o 3 la sm and gd yb the lower suite consists of augite olivine porphyritic lavas with high mg number sio 2 tio 2 and gd yb but low la sm the overlying central suite flows are either aphyric or plagioclase porphyritic and have lower mg number sio 2 la sm and gd yb decline upward through the central suite the upper suite lavas are either aphyric or plagioclase porphyritic and element ratios plot on slightly different trends compared to the central suite the lower suite has a narrow compositional range the average of the four most primitive lavas is used

sedimentology and microfacies of a mud rich slope succession in the carboniferous bowland basin nw england uk a paucity of studies on mud rich basin slope successions has resulted in a significant gap in our sedimentological understanding in these settings here macro and micro scale analysis of mudstone composition texture and organic matter was undertaken on a continuous core through a mud dominated slope succession from the marl hill area in the carboniferous bowland basin six lithofacies all dominated by turbidites and debrites combine into three basin slope facies associations sediment starved slope slope dominated by low density turbidites and slope dominated by debrites variation in slope sedimentation was a function of relative sea level change with the sediment starved slope occurring during maximum flooding of the contemporaneous shelf and the transition towards a slope dominated by turbidites and then debrites occurring during normal or forced shoreline progradation towards the shelf margin the sediment starved slope succession is dominated by type ii kerogen whereas the slope dominated by low density turbidites is dominated by type iii kerogen this study suggests that mud dominated lower slope settings are largely active depositional sites with consistent evidence for sediment traction additionally the composition and texture of basin slope mudstones as well as organic content vary predictably as a function of shelf processes linked to relative sea level change

icp ms a powerful tool for high precision trace element analysis in earth sciences evidence from analysis of selected u s g s reference samples abstract inductively coupled plasma mass spectrometry icp ms is a powerful analytical technique with considerable application to geochemistry in this paper we demonstrate the power of icp ms as a tool in petrogenetic studies data for 28 elements in seven u s g s basalt and andesite reference samples agv 1 bcr 1 bhvo 1 bir 1 dnc 1 w 1 and w 2 are presented accuracy and precision standard deviation for 26 of these elements is either excellent a detailed description of the icp ms technique we have developed is given a simple hf hno 3 dissolution in screwtop teflon bombs was used for sample preparation icp ms instrumental sensitivity was measured by external solutions with surrogate calibration for 4 elements matrix correction was made by use of standard addition naturally occurring internal standards rb y ce and pb were used to correct for instrumental drift between sample and spiked sample measurements we report data for the elements li cs rb sr ba zr hf nb ta y sc u th pb and all 14 of the ree in addition the procedure acquired data for be mo w tl and bi many elements in this array are crucial for the evaluation of petrogenetic models and source tracing herein lies the real power of icp ms one technique one dissolution small sample size good limits of detection good to excellent accuracy and good precision for many geochemically important elements

geochemistry of the mesoproterozoic intrusive rocks of the nipigon embayment northwestern ontario evaluating the earliest phases of rift development the intrusive rocks of the nipigon embayment comprise a series of four mafic to ultramafic intrusions and a number of laterally extensive diabase sills that are among the oldest expression of the

estimation of dust fallout rates in the northern arabian gulf the penetration of dust clouds over a sea surface is modelled as a sediment transport diffusion problem the model is used to determine the steady state concentration field of two dimensional dust clouds during offishore dust storms the rates of dust deposition into the sea can then be calculated from the obtained concentration fields the model was applied to offshore dust storms blowing over the arabian gulf yielding estimated dust sedimentation rates of 0 8 mm yr 1 this is substantially higher by several orders of magnitude than dust fallout estimates in most seas of the world the calculated fallout rates are in line with the results of existing field measurements

aeolian sediment transport and deposition in a modern high latitude glacial marine environment aeolian sand and dust in polar regions are transported offshore over sea ice and released to the ocean during summer melt this process has long been considered an important contributor to polar sea floor sedimentation and as a source of bioavailable iron that triggers vast phytoplankton blooms reported here are aeolian sediment dispersal patterns and accumulation rates varying between 0 2 g m 2 yr 1 and 55 g m 2 yr 1 over 3000 km2 of sea ice in mcmurdo sound south west ross sea adjacent to the largest ice free area in antarctica sediment distribution and the abundance of southern mcmurdo volcanic group derived glass show that most sediment originates from the mcmurdo ice shelf and nearby coastal outcrops almost no sediment is derived from the extensive ice free areas of the mcmurdo dry valleys due to winnowed surficial layers shielding sand sized and silt sized material from wind erosion and because of the imposing topographic barrier of the north south aligned piedmont glaciers southerly winds of intermediate strength ca 20 m sec 1 are primarily responsible for transporting sediment northwards and offshore the results presented here indicate that sand sized sediment does not travel more than ca 5 km offshore but very fine sand and silt grains can travel 100 km from source for sites 10 km from the coast the mass accumulation rate is relatively uniform 1 14 0 57 g m 2 yr 1 three orders of magnitude above estimated global atmospheric dust values for the region this uniformity represents a sea floor sedimentation rate of only 0 2 cm kyr 1 well below the rates of 9 cm kyr 1 reported for biogenic dominated sedimentation measured over much of the ross sea these results show that even for this region of high windblown sediment flux aeolian processes are only a minor contributor to sea floor sedimentation excepting areas proximal to coastal sources

coal derived rates of atmospheric dust deposition during the permian despite widespread evidence for atmospheric dust deposition prior to the quaternary quantitative rate data remains sparse as dust influences both climate and biological productivity the absence of quantitative dust data limits the comprehensiveness of models of pre quaternary climate and biogeochemical cycles here we propose that inorganic matter contained in coal primarily records atmospheric dust deposition to test this we use the average concentration of inorganic matter in permian coal to map global patterns and deposition rates of atmospheric dust over pangea the dust accumulation rate is calculated assuming permian peat carbon accumulation rates in temperate climates were similar to holocene rates and accounting for the loss of carbon during coalification coal derived rates vary from 0 02 to 25 g m 2 year 1 values that fall within the present day global range a well constrained east west pattern of dust deposition corresponding to expected palaeoclimate gradients extends across gondwana with maximum dust deposition rates occurring close to arid regions a similar pattern is partially defined over the northern hemisphere patterns are consistent with the presence of two large global dust plumes centred on the tropics the spatial patterns of dust deposition were also compared to dust cycle simulations for the permian made with the community climate system model version 3 ccsm3 key differences between the simulations and the coal data are the lack of evidence for an antarctic dust source higher than expected dust deposition over n and s china and greater dust deposition rates over western gondwana this new coal based dust accumulation rate data expands the pre neogene quantitative record of atmospheric dust and can help to inform and validate models of global circulation and biogeochemical cycles over the past 350 myr

geochemical stratigraphy of the keweenawan midcontinent rift volcanic rocks with regional implications for the genesis of associated ni cu co and platinum group element sulfide mineralization the development of the north american midcontinent rift can be understood in the context of a tectonic and magmatic event in which the far field effects of continental drift influenced magmatism produced by a mantle plume we report and interpret new data for samples collected from stratigraphically controlled sequences of basaltic rocks in the osler volcanic group and the mamainse point volcanic group in ontario canada we confirm that the earliest phase of rift development involved primitive melts that produced basalts in the lower formation of the osler volcanic group with fo74 80 olivine 1 490 2 280 ppm ni the earliest basalts are primitive in both the mamainse point volcanic group and osler volcanic group sections with elevated mgo cr and ni these rocks also have high tio2 and high gd yb which are features that indicate low degrees of melting of a garnet bearing lherzolite mantle the central formation a and b basalts of the osler volcanic group that overlie the lower formation lavas record an upward increase and then decrease in the degree of contamination of basaltic rocks with normal ni and platinum group element pge abundance levels that require no sulfide saturation event to accompany contamination the upper formation of the osler volcanic group is characterized by a limited number of pge depleted flows although these basalts are highly fractionated and typically develop textures with stellate to laminated plagioclase feldspar they are usually not significantly contaminated the lower and upper formations of the mamainse point volcanic group show a diversity in geochemistry consistent with contributions from different mantle sources but contamination is restricted to a small number of flows with normal ni and pge abundance levels there is no indication of a regional metal depletion signature on the scale of the nadezhdinsky formation at noril sk but there is evidence that local s saturation events were triggered in the unexposed conduits feeding some of the upper sequence flows in response to fractionation of the magma massive to heavy disseminated ni cu pge sulfide mineralization in the midcontinent rift is hosted by small primitive intrusions that were formed early in the evolution of the rift and are typically associated with transtensional structures in the cratonic blocks adjacent to the main rift examples include the eagle deposit in michigan the tamarack mineralization in minnesota and the current lake complex in ontario the american examples are associated with splays of the great lakes tectonic zone and the canadian example is associated with the quetico fault zone the intrusions share the geochemical fingerprints of the more primitive lowermost volcanic rocks of the midcontinent rift and they are broadly contemporaneous with the development of the lower stratigraphy of the osler volcanic group and mamainse point volcanic group although the low grade but large tonnage cu ni pge deposits of the duluth were formed significantly later than the small primitive intrusions their host rocks share the same lithophile trace element signature as these early intrusions the parental magmas that produced all of these intrusions were derived from deeper mantle depths where garnet was a stable phase than the magmas that produced the central formation basalts and some of the upper formation basalts investigated in this study

keweenaw hot spot geophysical evidence for a 1 1 ga mantle plume beneath the midcontinent rift system the proterozoic midcontinent rift system of north america is remarkably similar to phanerozoic rifted continental margins and flood basalt provinces like the younger analogues the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad asthenospheric thermal anomaly which the authors call the keweenaw hot spot great lakes international multidisciplinary program on crustal evolution seismic reflection profiles constrain end member models of melt thickness and stretching factors which yield an inferred mantle potential temperature of 1 500 1 570c during rifting combined gravity modeling and subsidence calculations are consistent with stretching factors that reached 3 or 4 before rifting ceased and much of the lower crust beneath the rift consists of relatively high density intruded or underplated synrift igneous material the isotopic signature of keweenawan volcanic rocks presented in a companion paper by nicholson and shirey this issue is consistent with the model of passive rifting above an asthenospheric mantle plume

early history of the midcontinent rift inferred from geochemistry and sedimentology of the mesoproterozoic osler group northwestern ontario the mesoproterozoic 1108 1105 ma osler group a3k m thick succession of basaltic flows and sedimentary units on the north shore of lake superior is among the oldest expressions of the midcontinent rift basal sediments of the simpson island formation new name deposited by braided fluvial systems record westward transport of debris eroded from local archean and proterozoic rock units strata deposited by this fluvial system are intercalated with and overlain by ocean island basalt oib like basalts which become increasingly contaminated up section end 1100ma 0 3 to 5 3 the light rare earth element lree enriched la smn 1 5 3 9 and heavy ree hree fractionated gd ybn 1 5 3 7 subaerial flows are divisible into two units that correlate with other sections of the osler group to the east but simple correlations with more distant sequences are difficult the volcanic rock dominated portion of the succession is overlain by a thin 25 m thick conglomerate sandstone assemblage representing southeast progradation of an alluvial fan in a semi arid climatic setting clast lithologies and geochemistry indicate no extra rift detritus was delivered from the hinterland of the fan various lines of evidence in both volcanic and sedimentary rocks support a scenario where early pre 1108 ma subsidence along a north south axis from the western arm of the rift to the nipigon embayment was replaced by subsidence along the east west rift axis between 1108 and 1105 ma resume le groupe d osler mesoproterozoique 1108 1105 ma une succession de coulees basaltiques et d unites sedimentaires d une epaisseur de 3 km sur la rive nord du lac superieur est l une des plus anciennes expressions du rift mi continental les sediments a la base de la formation simpson island nouveau nom deposes par un systeme fluvial anastomose enregistrent le transport vers l ouest de debris erodes des unites rocheuses locales archeennes et proterozoiques des basaltes de points chauds recouvrent et sont intercales avec les strates deposees par ce systeme fluvial ces basaltes sont de plus en plus contamines en remontant la section end 1100ma 0 3 to 5 3 les ecoule ments sub aeriens de terres rares legeres enrichies la smn 1 5 3 9 et de terres rares lourdes enrichies fractionnees gd ybn 1 5 3 7 peuvent etre divises en deux unites qui correspondent a d autres sections du groupe d osler vers lst mais de simples correlations avec des sequences plus eloignees sont difficiles la portion de la succession dominee par des roches volcaniques est recouverte d un mince 25 m assemblage conglomerat gres qui represente la prograda tion vers le sud est d un delta alluvionnaire dans un environnement climatique semi aride la lithologie des clastes et la geochimie indiquent qu aucun detritus hors du rift ne provient de l arriere pays du delta diverses preuves a la fois dans les roches sedimentaires et les roches volcaniques soutiennent un scenario selon lequel une subsidence precoce avant 1108 ma le long d un axe nord sud a partir du bras ouest du rift vers la baie nipigon a ete remplacee par de la subsidence le long de l axe est ouest du rift entre 1108 et 1105 ma

the late ordovician soom shale lagerstatte an extraordinary post glacial fossil and sedimentary record fossils of the late ordovician soom shale lagerstatte are characterized by exceptional preservation of their soft tissues in clay minerals the low diversity community lived in an unusual cold water setting dominated by anoxic bottom waters in the immediate aftermath of the hirnantian glaciation giant conodonts represented by complete tooth sets and one with trunk musculature and liver preserved unarmoured jawless fish lobopods and enigmatic taxa are some of the more important fossils furthermore this lagerstatte also preserves biomineralized ordovician taxa such as brachiopods orthoconic nautiloids and trilobites it is important in capturing the only known examples of many taxa extending temporal ranges of others and providing a unique glimpse of a post glacial refugium at a time when other lagerstatten are unknown

eolian input into the late ordovician postglacial soom shale south africa the soom shale member of the cedarberg formation south africa is a key early paleozoic lagerstatte that directly overlies glacigenic deposits of the late ordovician glacial maximum it was deposited on the gondwanan craton during deglacial transgression we show that it contains a substantial coarse silt to fine sand component that occurs in discrete laminae intimately associated with plankton derived organic material we interpret this component as loess sourced from glacially derived debris and blown into the sea either directly or across seasonal sea ice as happens today on the mcmurdo ice shelf antarctica falling through the water column this material likely stimulated production of phytoplankton which then sank as sediment loaded aggregates in marked sedimentary partitioning these silt and or organic laminae alternate with fine organic poor mud laminae that likely represent river derived nepheloid plumes and that sporadically thicken into centimeter scale mud turbidites sustained eolian input directly into the surface water may have been key to maintaining the high productivity of the soom sea and then via eutrophication and anoxia to the exceptional preservation of its biota

the geochemistry geochronology and paleomagnetism of dikes and sills associated with the mesoproterozoic midcontinent rift near thunder bay ontario canada abstract the midcontinent rift mcr of north america comprises a series of basaltic sheets flows and intrusive rocks emplaced in the lake superior region during the mesoproterozoic the mafic rocks preserved on the northern flank of lake superior represent the older portions of the rift sequence and offer insights into the early development of the rift new geochronological geochemical and paleomagnetic data are presented for the dikes and sills located in and south of thunder bay ontario three sill suites are recognized within the study area an earlier spatially restricted ultramafic unit termed the riverdale sill the predominant logan sills and nipigon sills in the north of the study area in addition three dike sets are recognized the north east trending pigeon river swarm the north west trending cloud river dikes and the mt mollie dike the geochemical data demonstrate that the majority of sills south of thunder bay are of logan affinity and distinct from those of broadly similar age in the nipigon embayment to the north the pigeon river dikes that intrude the sills are geochemically coherent but distinct from the logan sills and could not be feeders to the sills the new age of 1109 2 4 2 ma for the cloud river dike and its r polarity are consistent with published magnetostratigraphy the mt mollie dike age 1109 3 6 3 ma indicates that it is not coeval with the spatially associated crystal lake gabbro as previously thought the complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south the geochemistry and geochronology of the intrusions suggest a long lived and complex magmatic history for the midcontinent rift

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

preservation and paleoclimatic significance of eolian dust in the ancestral rocky mountains province upper paleozoic pennsylvanian permian silty strata of the northern pedregosa basin southern ancestral rocky mountains province exhibit distinctive features that suggest origin of the silt as eolian dust prior to deposition in marine environments studies of mechanisms controlling modern eolian dust accumulation imply that combined marginal marine epeiric sea environments constitute an ideal sink for eolian dust accordingly pedregosa siltstones may reflect the prevalent mode of eolian dust preservation downwind from coeval ergs and significant volumes of eolian dust may be preserved within analogous epeiric strata because eolian dust reflects conditions of aridity and windiness it bears on paleoclimatic and paleogeographic reconstructions recent studies within several basins of the ancestral rocky mountains suggest an eolian derivation for many siltstones and thus corroborate inferences of increasing aridity and concomitant monsoonal conditions for western pangea further regional analysis may provide valuable insights for refining concepts of the zonal to monsoonal transition that accompanied pangean assembly

radiogenic isotope characteristics of the mesoproterozoic intrusive rocks of the nipigon embayment northwestern ontario the ca 1110 1120 ma diabase sills of the nipigon embayment represent the oldest phase of the midcontinent rift recognized to date the sills can be subdivided into five geochemically distinct suites nipigon mcintyre inspiration shillabeer and jackfish sm nd rb sr and pb pb isotope data for the sills can be used to evaluate the source regions and emplacement histories of these sills the endt values for all the sill suites are consis tently negative 0 5 to 6 6 but show coherent variations both within and between suites the negative endt values can be interpreted as the result of contamination of a plume related mantle source by older crustal material the initial strontium sri values 0 7032 0 7068 for the sills indicate that at least two distinct contaminants are required 1 a source with strongly negative endt and lower sri likely archean metasediment or granite and 2 one with elevated sri likely sedimentary rocks of the proterozoic sibley group the pb data for the nipigon and inspiration sills form distinct linear arrays consistent with variable contamination of a source comparable to that of other igneous suites of the midcontinent rift the radiogenic isotopes preserve a complex contamination history best interpreted as the result of 5 contamination by a variety of archean and proterozoic sources the ultramafic jackfish sill shows the least ev idence of contamination and is interpreted to have been emplaced along a crustal scale fault with little interaction with crustal material in contrast the other sill suites have undergone complex contamination histories requiring variable crustal residence times and assimilation of material both at depth in large magma chambers and during emplacement

petrology of middle proterozoic diabases and picrites from lake nipigon canada mafic rocks at lake nipigon provide a record of rift related continental basaltic magmatism during the keweenawan event at 1109 ma the mafic rocks consist of an early volumetrically minor suite of picritic intrusions varying in composition from olivine gabbro to peridotite and a later suite of tholeiitic diabase dikes sheets and sills the diabase occurs primarily as two 150 to 200 m thick sills with a textural stratigraphy indicating that the sills represent single cooling units compositional variation in the sills indicates that they crystallized from several magma pulses

further refinement to the timing of mesoproterozoic magmatism lake nipigon region ontario the midcontinent rift in the lake superior region of north america is one of the best preserved examples of an aborted precambrian intercontinental rift one that hosts a diverse suite of rock types in addition to the well studied and voluminous rift fill flood basalts although there is a growing database of high precision age information for the main volcanic packages and the largest mafic intrusions there is relatively little information available on the absolute timing of mafic ultramafic intrusions dyke swarms and alkaline complexes especially in the ontario portion of the rift we report new high precision u pb ages for 29 samples primarily collected in the lake nipigon area ontario from these new age results it is now possible to expand the known distribution of geon 15 magmatism in the region confirm an early stage of midcontinent rift mafic magmatism between 1150 and 1130 ma provide evidence that significant mafic ultramafic magmatism occurred in the lake nipigon region slightly earlier

sulfur isotope and mineralogical studies of ni cu sulfide mineralization in the bovine igneous complex intrusion baraga basin northern michigan the bovine igneous complex referred to as the bic intrusion occurs within the marquette baraga dike swarm in the upper peninsula of michigan the intrusion is likely related to the 1 1 ga midcontinent rift system based on chemical similarities with the nearby eagle mine and other midcontinent rift related picrites the lack of deformation fabric and well developed hornfels that overprints penokean deformation fabrics however there is currently no age date for the bic intrusion to confirm that it is a rift related intrusion the bic is a relatively small layered intrusion with dimensions of 1 100 400 732 m thick units include feldspathic wehrlite olivine melagabbros feldspathic olivine clinopyroxenite and gabbro the little bic a satellite intrusion located 100 m from the main intrusion is primarily feldspathic wehrlite ni cu sulfide mineralization occurs in both intrusions with a higher relative proportion in the little bic intrusion sulfide assemblages are comprised predominantly of pyrrhotite chalcopyrite and pentlandite textural types vary from disseminated through globular to semimassive net textured and locally massive the 34s values of sulfide minerals in the bic intrusion range from 6 0 to 1 5 with 80 of the values between 0 and 1 5 the proterozoic michigamme formation country rocks show 34s values between 3 0 and 9 8 the 34s values of the semimassive sulfides in the little bic intrusion are similar to those in country rock 3 8 6 7 whereas the massive sulfide values are similar to those of the sulfide minerals in the bic intrusion although many values are near those of mantle derived sulfur the presence of strongly negative values in both intrusions indicates that crustally derived sulfur was involved in the mineralization process in addition the distinctly positive values of disseminated sulfides in the little bic intrusion indicate that a large fraction of the sulfur involved in the mineralization was of country rock origin and that at least two pulses of sulfide saturated liquids were involved in its genesis the 33s values for sulfide minerals in the bic and the little bic intrusions range from 0 33 to 0 35 indicating that a component of archean sulfur was also involved in the mineralization olivine fo and ni contents are consistent with rock types being related via fractional crystallization but this process cannot account for the wide range of sulfide zones found throughout both intrusions multiple pulses of magma several sulfide saturated and with distinct sulfur isotope compositions were involved in the formation of the bic and the little bic intrusions both sulfide concentration and isotopic composition suggest that sulfide saturation was promoted via the addition of country rock derived sulfur

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

the midcontinent rift system one of the most prominent features on gravity and aeromagnetic maps of the united states is a series of major generally linear anomalies that extend from central kansas to lake superior and then turn southward into central michigan figures 1 2 this system of anomalies clearly reflects major geological features and as such it must represent a very significant episode in the history of the north american continental lithosphere the geologic features associated with these anomalies particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension indicate that the episode was probably one of incipient rifting