late paleoproterozoic geon 18 and 17 reactivation of the neoarchean great lakes tectonic zone northern michigan usa evidence from kinematic analysis thermobarometry and 40ar 39ar geochronology abstract the neoarchean great lakes tectonic zone in the midcontinent of north america exerted a strong control during the proterozoic stabilization of the superior craton in northern michigan this zone underlies the paleoproterozoic marquette synclinorium a penokean compressional feature that was reactivated during post penokean orogenic collapse and gneiss dome formation early north side up motion along steep reverse faults under lower greenschist facies conditions was related to basin inversion of the marquette trough sediments during penokean orogenesis in the western portion of the marquette synclinorium these early penokean fabrics are overprinted by a high grade event recorded by synkinematic mineral growth in high grade shear zones these late shear zones accommodate s side up motion with archean basement uplifted relative to proterozoic supracrustal sequences activity related to the emplacement of the ca 1800 ma humboldt granite metamorphic amphiboles from the center of a circular pattern of metamorphic isograds the republic node record plateau ages of 1730 ma with progressively older ages across the marquette synclinorium away from the central metamorphic high model archean ages are preserved in the most distal amphibole samples north of the marquette syncline reflecting the preservation of amphibole crystallization in the biotite zone mica ages record complete resetting and a similar age progression with the youngest ages 1715 ma in the central metamorphic high and older ages 1806 ma towards the margin thermobarometry indicates peak temperatures of 620 c for the central sillimanite zone decreasing to 500 c for the garnet zone and maximum pressures of 300 400 mpa gasp and grips assemblages collectively these characteristics are interpreted to reflect two separate reactivation episodes of the gltz during penokean accretion marquette syncline formation and post penokean formation of granite cored gneiss domes collapse

calcite strains kinematic indicators and magnetic flow fabric of a proterozoic pseudotachylyte swarm minnesota river valley usa abstract near granite falls minnesota sub parallel pseudotachylyte mafic dikes and calcite veins crosscut archean granulite facies rocks in the minnesota river valley adjacent to the north dipping yellow medicine shear zone ymsz n80 e that separates the montevideo and morton tectonic terranes the docking of these two archean terranes occurred prior to intrusion of the 2 067 ga kenora kabetogama dike swarm as demonstrated by aeromagnetic anomalies correlated with field exposures that cross the ymsz without offset tectonic adjustments along the ymsz associated with the penokean orogeny 1 8 ga are likely responsible for pseudotachylyte formation pseudotachylyte is exposed in 22 sub parallel veins n80 e 90 each less than 2 cm wide across an outcrop width of 45 m the pseudotachylyte matrix is commonly banded and contains crystal fragments quartz plagioclase amphibole rutile apatite ilmenite ulvospinel magnetite microlites flow banding swirls amygdules filled with calcite ankerite and siderite collapsed vesicles and abundant lithic clasts pseudotachylyte formed in a number of phases kinematic reconstruction is complex utilizing winged porphyroclasts s c structures in the country rock and fault drag indicators along the pseudotachylyte zones dextral motion along the ymsz is the most common observation mechanically twinned calcite within amygdules in the pseudotachylyte preserves horizontal shortening normal to the pseudotachylyte strike calcite veins are apparently contemporaneous with the pseudotachylyte one set preserves twinning strains identical to the calcite amygdule strains and the second set contains a horizontal vein parallel n70 e shortening strain the pseudotachylyte contains a flow fabric as determined by ams techniques that is a proxy for vertical flow kmax is vertical the kenora kabetogama dikes identified geochemically are locally parallel to the pseudotachylyte and the adjacent ymsz tectonic suture and preserve a vertical to horizontal dike parallel ams fabric from east franklin to west granite falls hornblende andesite dikes 055 1 8 ga are not found south of the suture are not associated with pseudotachylyte and have a different paleopole and ams fabric

model for the origin of the troodos massif cyprus and other mideast ophiolites any comprehensive model for the origin of the troodos complex and other mideast ophiolite complexes must explain the geochemical evidence for subduction zone involvement the thin inferred oceanic crust the extensional environment indicated by sheeted dikes the existence of fault zones perpendicular to the inferred spreading axes and the discontinuous nature of ophiolite exposures around the arabian block the andaman sea region of the indian ocean may provide an actualistic model for the origin of these ophiolites there spreading takes place in short segments above a subduction zone in the region of active spreading the andaman sea model substantially accounts for the known geologic and geophysical data from the circum arabian ophiolite belt and it leads to some predictions about the structural and geophysical relationships to be expected in this important belt the model also predicts that andaman sea spreading center magmas may be similar in composition to those found in the mideast ophiolites

groundwater surfacewater interactions at the transition of an aquifer from unconfined to confined a three dimensional numerical model is developed to identify systematic changes in the interactions between the groundwater flow systems and streams that cross the transition zone where unconfined aquifers become confined a generalized model is designed that represents hydrologic conditions where gently dipping aquifer confining unit sedimentary sequences subcrop beneath a thin surficial aquifer and are transected by gaining streams flowing parallel to dip a series of simulations are performed to investigate the effect of different hydrogeologic characteristics of the edge of the confining unit where potential for facies changes or fluvial reworking results in variations in the material properties of the confining unit the results show an abrupt increase in groundwater discharge to the stream immediately upstream of the transition to confined conditions and a corresponding but not equal decrease in groundwater discharge along stream reaches downstream of the edge of the confining unit model results were compared to measured stream discharges at a location where the loosahatchie river of eastern tennessee usa crosses an unconfined confined transition in the upper mississippi embayment

new insights into the southern margin of the archean proterozoic boundary in the north central united states based on u pb sm nd and ar ar geochronology abstract new geophysical analysis of the precambrian basement in minnesota iowa wisconsin indicates that an archean proterozoic boundary spirit lake trend previously recognized in nw iowa can be continued eastward into central wisconsin and farther east as the spirit lake tectonic zone sltz to test the age of paleoproterozoic crust south of this structure several subsurface samples of precambrian basement from the north central united states have been analyzed or re examined using modern techniques of u pb sm nd and 40ar 39ar geochronology the results fill in a major data gap for the region and show that all u pb crystallization ages for samples south of the sltz are geon 17 1700 1800 ma bedrock core samples from eastern nebraska are ca 1760 1800 ma and two samples from se south dakota immediately south of the sltz yield ages of 1762 28 ma vermillion and 1733 2 ma elk point xenoliths from impact breccia in the manson structure in north central iowa yield a similar age of ca 1705 30 ma and metagabbro from se minnesota yields an age of 1760 9 ma farther to the northeast zircons from paleoproterozoic gneiss in the basement of manitoulin island only a few km south of the superior craton in ontario also yield a geon 17 age 1714 10 ma sm nd model ages tdm for samples immediately south of the sltz fall in the range 1 9 2 2 ga indicating limited involvement of archean crust in contrast sm nd tdm ages for samples north of the sltz typically range from 2 5 to 3 0 ga for both paleoproterozoic plutons and archean gneisses ion microprobe analyses of zircons from the elk point and manson samples also show the presence of geon 16 overgrowths indicating a strong regional thermal overprint during geon 16 accretion this is supported by mid geon 16 hornblende 40ar 39ar ages for samples from se south dakota and se minnesota although no u pb ages are available for juvenile basement beneath the ca 1760 ma granite rhyolite suite of southern wisconsin south of the sltz sm nd model ages are typically ca 1 9 2 0 ga consistent with basement to the rhyolites being geon 17 in age collectively the data require that most if not all of the paleoproterozoic crust immediately south of the sltz formed during geon 17 and probably represents eastward continuation from colorado through nebraska of the yavapai crustal province in the sw united states penokean geon 18 crustal rocks are limited mainly to northern and central wisconsin east central minnesota and northern michigan these results also show that medium grade 500 c tectonothermal effects of the subsequent geon 16 mazatzal orogeny to the south continue into the north central united states both terranes probably also continue eastward into ontario canada and farther east into protolith of the grenville province

crust enriched mantle derived tonalites in the early proterozoic penokean orogen of wisconsin the early proterozoic 1 82 to 1 89 ga penokean orogenic province represents the initial phase of a major period of crustal growth of the north american proterozoic craton in central wisconsin the orogeny is dominated by the intrusion of variably deformed and metamorphosed tonalite trondhjemite suites in part hosted by archean gneisses and migmatites the magmatism is calc alkaline and similar in composition to modern high k orogenic andesites moderate abundances of sr 600 750 ppm and ree in the most primitive tonalites are consistent with derivation from an eclogitic altered tholeiite rather than a crustal archean amphibolitic tholeiite or enriched mantle source existing nd sr isotopic data for other tonalites of the wisconsin penokean orogen also indicate a mantle origin but with some admixture of archean crustal component this study shows that an archean component most likely was incorporated during emplacement the tonalite to trondhjemite differentiation of these magmatic suites is not

proterozoic metamorphism and cooling in the southern lake superior region north america and its bearing on crustal evolution abstract metamorphism along the southern margin of the archean superior province has been historically attributed to the penokean orogeny a narrow corridor of amphibolite facies rocks north of the main suture does record 1 83 1 80 ga metamorphic monazite u th pb ages that mark the culmination of arc accretion however subsequent widespread amphibolite facies metamorphism and associated magmatism is recorded along the regions of greatest penokean crustal thickening the tectonically buried archean proterozoic continental margin in minnesota new monazite geochronology reveals a profound midcrustal metamorphic imprint caused by emplacement of the 1 775 ga east central minnesota batholith at moderate depths in northern wisconsin and upper peninsula michigan metamorphic monazite growth at 1 78 1 745 ga and far from geon 17 intrusions reflect a previously little recognized regional amphibolite facies metamorphic event associated with ca 1 76 ga yavapai interval accretion not solely penokean induced crustal collapse south of the penokean suture penokean terrane rocks were twice metamorphosed to upper greenschist facies first during yavapai accretion and again during geon 16 mazatzal accretion geon 16 overprinting also affected a small part of the continental margin in the northeast orogen the peavy metamorphic node south directed basement thrusts there likely accommodated substantial mazatzal foreland shortening suggesting thick skinned deformation mazatzal amphibolite facies metamorphism occurred throughout iowa and southernmost wisconsin south of the baraboo quartzite 40 ar 39 ar mineral cooling ages from eastern wisconsin reveal a limited metamorphic aureole associated with the intrusion of the 1 47 ga wolf river batholith in part reflecting its rapid emplacement at shallow crustal levels a local area of anomalously young 40 ar 39 ar mica ages away from known midcontinent rift exposures probably reflect shallow crustal level reheating during rifting not exhumation the pattern and degree of proterozoic metamorphism preserved across the northern interior of the north american midcontinent is highly variable and is a reflection of multiple accretionary and intrusive events between 1 87 and 1 47 ga collectively the geo and thermochronologic data reveal a progressive tectonic younging toward the south and overall waning of proterozoic metamorphism in this region these results demonstrate a changing tectonic environment from active margin to continental interior as laurentia grew southward and stabilized over a few hundred million years

os nd o and s isotope constraints on country rock contamination in the conduit related eagle cu ni pge deposit midcontinent rift system upper michigan abstract the eagle cu ni pge deposit is hosted by mafic to ultramafic intrusive rocks associated with the marquette baraga dike swarm in northern michigan sulfide mineralization formed in a conduit system during early stages in the development of the 1 1 ga midcontinent rift system the conduit environment represents a prime location for melt rock interaction in order to better assess the extent of country rock contamination in the eagle system a combined trace element nd os o and s isotope study of country rocks sulfide bearing igneous rocks and massive sulfide was undertaken both the eagle and the weakly mineralized east eagle intrusion show trace element patterns that are similar to those of picritic basalts that formed during early stages of rift development the trace element os nd and o isotopic values of the igneous rocks are consistent with os 1100 and e nd 1100 values for the magmas are 34 and 2 18 o values of pyroxene in feldspathic pyroxenite range from 6 5 to 6 6 and provide the only indication that bulk contamination may locally have exceeded 20 sulfur isotopic values of disseminated and massive sulfide in the eagle intrusion range from 0 3 to 4 6 the 34 s values are much lower than those that characterize most of the country rocks but could still be indicative of a contribution of s from country rocks of up to 50 33 s values of the disseminated and massive sulfides range from 0 10 to 0 09 indicating a source in paleoproterozoic country rocks semi massive sulfide in the eagle deposit has 34 s values between 2 2 and 5 3 and 33 s values show a broad range between 0 86 and 0 86 indicating a major contribution from an archean source isotopic data from the eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the ni cu pge mineralization and magmas which traversed variable paths through the mantle and crust were focused and utilized the same conduit at the level of the eagle deposit our results emphasize the fact that the sulfur isotopic values of immediate country rocks may not be appropriate as end member values for mixing calculations sulfur derivation from deeper in the conduit system may have involved rocks with very different sulfur isotope ratios in addition sulfur isotopic exchange reactions between passing magma and accumulated crystals or sulfide liquid in the conduit may have led to decreased sulfur isotope variability and 34 s values near those of mantle sulfur

the midcontinent rift in the lake superior region with emphasis on its geodynamic evolution abstract the midcontinent rift is a middle proterozoic continental rift which records about 15 m y of extension subsidence and voluminous volcanism in the period 1109 1094 ma in the central part of north america during that time the crust was nearly totally separated and as much as 25 km of subaerial basalts accumulated in a deep central depression following extension and volcanism a longer period of subsidence resulted in development of a post rift sedimentary basin in which as much a 8 km of fluvial and lacustrine clastic rocks were deposited partial inversion of the central depression occurred about 30 50 m y after extension to produce the current configuration of a central horst composed mostly of thick volcanic accumulations between shallower flanking basins

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

the uranium mineralization potential of the paleoproterozoic sioux basin and its relationship to other basins in the southern lake superior region abstract the paleoproterozoic sioux basin and the baraboo barron flambeau mccaslin and waterloo quartzites in northcentral usa are all supermature quartz arenites that are among the world s oldest redbeds these quartzites were deposited on a stable craton at ca 1750 ma following exhumation and uplift of penokean crust and major orogenic activity related to the amalgamation of archean microcontinents at ca 2 0 1 8 ga to form laurentia 40ar 39ar ages of 1615 1543 ma from diagenetic muscovite in the sioux basin provide the first geochronological evidence for mazatzal related deformation and low grade metamorphism of the quartzites during subsequent convergence on the southern margin of laurentia one muscovite sample from the sioux basin has a 40ar 39ar age of 1465 ma that likely corresponds with fluid and thermal resetting due to felsic magmatism in the region and is similar to 40ar 39ar ages for muscovite from the baraboo and waterloo quartzites thus the sioux basin and other quartzites in the region have experienced a similar tectonic and fluid history the sioux basin has similar sedimentologic characteristics as other paleoproterozoic basins in north america including the uranium rich athabasca basin canada that is host to some of the world s largest and highest grade unconformity type uranium deposits we compared key characteristics such as the stratigraphy character of diagenetic fluids paleoaquifer characteristics and composition of leachable pb in the sandstones from the sioux basin with the athabasca basin in order to evaluate the fluid history of the basin and the potential of the sioux basin to host unconformity type uranium mineralization based on the hydrogen and oxygen isotopic composition of muscovite m1 that formed during diagenesis at 150 200 c the pore fluids that were present during peak diagenesis in the sioux basin had 18o and d values of 0 5 to 5 2 and 32 to 46 respectively similar to those of the fluids associated with uranium mineralization and alteration in the athabasca basin however fluid flow in the sioux basin was likely restricted to relatively thin and discontinous proximal fluvial facies that could have been diagenetic paleoaquifers in contrast to the extensive paleoaquifers in the athabasca basin that promoted large scale diagenetic reactions and alteration that led to the formation of unconformity type uranium mineralization in addition the non radiogenic isotopic composition of leachable pb in the sioux basin sandstones indicates that the majority of the pb is from a normal crustal source inconsistent with mobilization of radiogenic pb from a uranium rich source such as a uranium deposit although the sioux basin shares similar general characteristics to the athabasca basin the results of this study suggest that it has a lower potential than the athabasca basin to host significant high grade unconformity type deposits

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

petrology of mariana trough and lau basin basalts the crust of actively opening back arc basins has close chemical and mineralogic similarities to normal mid ocean ridge basalts n morb but some subtle but distinctive differences exist these are largely due to differences in volatiles fe oxidation ratio abundances of large ion lithophile elements lil and some high field strength hfs elements isotope data for sr pb nd he and o are variable but broadly are morb like we present data contrasting mariana trough mtb and lau basin basalts lbb showing how they differ in certain trace element and isotopic signatures and compare both to n morb relative to n morb and lbb the mtb are enriched in alkalis alkaline earths light rare earth elements ree h2o and fe fe even for primitive samples while having morb type he 3 4 7 8 to 8 4 ra sr 87 86 mean 0 70293 and nd e 10 2 the lau basin is zoned with a central area having n morb characteristics for h2o lil ree nd e 8 1 to 9 0 and o but a depletion in hfs elements sr is more radiogenic mean 87 86 0 70347 and 3he 4he 3 4 7 2 to 11 0 ra is high relative to n morb the lau basin has a zone bordering the adjacent island arcs which is like mtb in both back arc basins relatively primitive unfractionated basalts with mg 100 mg fe 62 are found which resemble postulated primitive n morb calculated parental magmas obtained by adding xenocrystic olivine plot near or on 3 and 4 phase boundaries at 1 1 5 gpa for the ol di si system low p fractional crystallization and magma mixing explain most of the observed rock types possible explanations for differences between mtb and lbb include differences in the extent of previous melting episodes of their mantle sources and effects of source metasomatism through dehydration and partial melting of subducted lithosphere the long and complex history of mariana trough mantle sources led to a metasomatized mantle which gives rise to the distinctive mtb in contrast the lau basin is dominated by basalts derived from a source depleted in volatiles lil and hfs elements a previous melting history is implied mixing with a mantle component enriched in 87sr and 3he may be due to mantle counterflow driven by the subduction process

petrological and stable isotope studies of carbonate and sulfide minerals from the gunflint formation ontario evidence for the origin of early proterozoic iron formation abstract the 1 9 ga gunflint formation in the thunder bay district of ontario principally comprises iron formation overlain by an iron poor limestone dolostone member the upper limestone member rocks are virtually unmetamorphosed retaining primary textures except locally around diabase sills the gunflint presents an opportunity to study both unmetamorphosed and metamorphosed iron formation and to study the changes from iron rich to iron poor sedimentation of the carbonate minerals in iron rich units siderite formed at or near the sediment water interface with ankerite and calcite forming later during diagenesis and during metamorphism around sills heaviest 13c values of siderite and of dolomite in the upper limestone member are consistently near 0 throughout the stratigraphic section indicating that marine bicarbonate was the source of carbon a spread of values between 0 and 7 for unmetamorphosed rocks is the result of the incorporation of oxidized organic matter during diagenesis whereas 13c values more negative than 7 are the result of metamorphism the heaviest 18o values from unmetamorphosed rocks 5 3 pdb for siderite 6 1 for ankerite and 6 7 for calcite are considered to represent the original marine composition lighter values to 17 represent isotopic exchange reactions with pore fluids at higher temperatures and or isotopic exchange with 18o depleted meteoric water low s c ratios and a relatively narrow range of positive 34s values in pyrite throughout most of the formation are consistent with bacterial sulphate reduction of seawater containing sulphate at a significantly lower concentration than the modern ocean locally higher sulfide contents and a wider range of 34s values were the product of the introduction of sulfur after deposition it is proposed that the gunflint formation was deposited in a stratified water column with anoxic bottom water the depositional basin had restricted communication with the open ocean and was affected by distal volcanism hydrothermal activity associated with the volcanism provided a large source of dissolved iron and possibly silica which helped buffer o2 and sulphate in water to low levels low concentrations of sulphate limited the generation of h2s that would otherwise have restricted the solubility of iron during periods of increased hydrothermal activity the anoxic oxic water boundary moved upwards permitting transport of iron to the shallow shelf where it was precipitated as siderite iron hydroxides iron silicates or pyrite depending on physico chemical conditions the transition to the overlying limestone marks a decrease in hydrothermal activity with a contraction of the redox boundary

potassic and sodic metasomatism in the southern province of the canadian shield evidence from the paleoproterozoic serpent formation huronian supergroup canada abstract major element geochemistry petrographic observations and microprobe analyses of the paleoproterozoic serpent formation huronian supergroup canada suggest that these strata experienced both potassium and sodium metasomatism major element data plotted in al2o3 cao na2o k2o compositional space form an array nearly identical to data from sub huronian paleosols reported elsewhere suggesting that serpent fluvial systems captured materials from entire weathering profiles potassium metasomatism was concentrated in shales and represents the illitization of kaolinite which is a residual product of plagioclase weathering sodium metasomatism principally affected silt and sand sized particles microprobe analyses of plagioclase grains indicate that they are nearly pure albite ab96 corresponding whole rock major element data plot in bulk albite space on an al2o3 k2o cao na2o ternary diagram whereas basement rocks plot in bulk andesine and oligoclase space end member compositions and the absence of luminescence when bombarded with an electron beam indicate that the albites are of secondary origin extremely pure albite cements ab99 commonly fringe detrital grains there has been little replacement of detrital k feldspars despite the complete albitization of plagioclase grains the timing of metasomatism at least the last stage is constrained to the period between 1700 and 1750 ma based on geochronologic data reported elsewhere when the southern province of the canadian shield experienced considerable felsic plutonism possibly related to arc collision this plutonism may have resulted in the heating of deep basinal brines stresses associated with orogenic build up or gravity driven groundwater circulation working in concert with the plutonism expelled these brines through the serpent formation and the entire huronian supergroup thoroughly metasomatizing the region such processes associated with collisional tectonics are akin to the development of mississippi valley type pb zn mineralization in the mid continent of the united states

sims oxygen isotopes indicate phanerozoic fluids permeated a precambrian gold deposit abstract the reef deposit is an anomalous au cu occurrence in the paleoproterozoic terranes of northern wisconsin better known as host to significant cu zn volcanogenic massive sulfide vms deposits previous work using lead isotopes and fluid inclusions has identified a protracted development of the mineralization from initial formation as the root zone veins of a vms deposit during the penokean orogeny 1 9 1 8 ga with the most recent mineralization remobilization activity associated with late paleozoic mississippi valley type mvt fluid flow haroldson et al 2018a 2018b here we use the oxygen isotope history of the reef deposit to verify and further examine the deposit s protracted development laser fluorination oxygen isotope measurements of primary mineralized quartz veins range in 18o from 6 8 to 10 0 vsmow and a trend is observed of increasing 18o values of parallel vein zones along a 400 m traverse from northwest to southeast likely from a temperature gradient during initial formation temperature estimates for a vms deposit setting 230 to 400 c are consistent with a hydrothermally shifted formation fluid using seawater 18o estimates during the penokean orogeny and evolving to higher 18o by incorporation of magmatic fluids or interaction with local crust in situ oxygen isotope measurement by secondary ion mass spectrometer sims of cross cutting quartz and carbonate range in 18o from 19 4 to 28 4 for quartz 25 3 to 28 1 for dolomite and 9 6 to 29 3 vsmow for calcite high 18o values 19 are measured in a crustiform textured quartz stockwork microveinlet in dolomite observed in late carbonate microveinlets and calcite in settings associated with the late quartz and dolomite and a separate calcite only setting directly linked with gold mineralization remobilization

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

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

the significance of partial melting processes in hydrothermal low sulfide cu ni pge mineralization within the footwall of the sudbury igneous complex ontario canada we present new results and review existing data highlighting different aspects of the genetic relationship between partial melting and hydrothermal mineralizing processes in the contact aureole of the sudbury igneous complex at the basal contact of the igneous complex in the footwall breccia crystallized partial melt pods and veins referred to as footwall granophyres are abundant and intrude all rock types including the breccia matrix as well as massive ni cu sulfide ore the final crystallization of these melts was accompanied by the segregation of high salinity fluids dominantly in the temperature range of 450 to 550 c as revealed by ti inquartz thermometry and studies on primary fluid inclusions in mineralized parts of the footwall breccia there is clear evidence that interaction of partial melts exsolved fluids and preexisting magmatic sulfides occurred in the deeper footwall of the sudbury igneous complex similar footwall granophyres occur as networks of veins intruding impact brecciated sudbury breccia country rocks and have an intimate spatial association with hydrothermal low sulfide assemblages highly enriched in pt pd and au with a unique assemblage of pgm including malyshevite and lisiguangite it is suggested that partial melting processes were widespread in the contact and proximal footwall environment of the igneous complex and they were important in providing high salinity magmatic fluids to a hydrothermal system responsible for redistribution of base and precious metals we describe observations which may evidence the migration of partial melts from the contact into the deeper footwall furthermore we suggest a cogenetic model for the precipitation of sulfide hydrous silicate assemblages in close spatial association to footwall granophyres we emphasize that the use of footwall granophyres in mineral exploration for cu ni pge ores in the footwall of the sudbury igneous complex is clearly justified mapping of footwall granophyre vein networks highlights areas that were proximal to the contact environment and point out structural pathways that may have been used by syn or postgenetic mineralizing fluids in a given area widespread partial melting appears to have occurred in the footwall of other large mafic ultramafic complexes such as bushveld and duluth and similar genetic relationships of partial melting and ore forming hydrothermal processes may have possibly existed also in these systems

magnetic and petrofabric studies in the multiply deformed thomson formation east central minnesota abstract two contrasting zones of deformation have been previously recognized in the proterozoic thomson formation of east central minnesota the northern zone consists of open upright e w trending folds with a well developed steeply south dipping axial planar cleavage the southern zone is marked by a pervasive subhorizontal cleavage s 1 crenulated by a second cleavage that is vertical to steeply south dipping s 2 paleomagnetic results and anisotropy of anhysteretic remanent magnetization aarm in these rocks indicate that the ferrimagnetic particles preserve a record of the field during deposition and primary compaction neither nrm nor aarm were much affected by tectonic deformation by contrast the low field magnetic susceptibility which is dominated by paramagnetic minerals particularly chlorite reflects the tectonic fabric of the rock anisotropy of magnetic susceptibility ams is characterized by minima normal to the cleavage in the northern zone and to the s 1 cleavage in the southern zone thus the s 2 deformation did not significantly affect the ams fabric march strains were obtained by measuring the 002 lattice preferred orientation of chlorite using an x ray pole figure goniometer the principal orientations of march strain match well those of ams and ams and march strain ellipsoids for selected samples in both northern and southern zones plot in the flattening field of a flinn diagram based on ams signature it is possible that the deformation in the northern zone occurred at the same time as the first deformation in the southern zone this could have involved simple shear on subhorizontal planes giving way to pure shear with horizontal shortening towards the north in this scenario the second deformation the result of regional late stage shortening was only marked by a cleavage s 2 in the southern zone due to the favorable orientation there of s 1

the goodman swell a lithospheric flexure caused by crustal loading along the midcontinent rift system rb sr biotite ages of archean and early to middle proterozoic crystalline rocks in northern wisconsin and adjacent upper peninsula of michigan describe a regionally systematic pattern related to differential uplift an age low occurs in northern wisconsin where values range from 1070 1172 ma for rocks with crystallization ages of 1760 to 1865 ma these values overlap with the main episode of mafic igneous activity 1090 to 1120 ma along the midcontinent rift system mrs we interpret these low biotite ages as registering closure due to cooling below the 300 c isotherm as a consequence of uplift and rapid erosion of an area that we are informally naming the goodman swell the position of the goodman swell is independent of the archean and early proterozoic regional geologic framework but closely linked with the curvilinear concave to the south mrs we interpret the swell to be a forebulge imposed on an elastic crust by loading of mafic igneous rocks along and within the axis of the mrs the southward concavity of load axes resulted in flexural interaction that localized the swell because of rapid uplift the swell may have been a source area for clastic sediments in the rift sequence

the chengwatana volcanics wisconsin and minnesota petrogenesis of the southernmost volcanic rocks exposed in the midcontinent rift the southernmost exposed rocks of the north american midcontinent rift system 1100 ma consist of 3000 m of mafic volcanic flows and minor interflow sediment exposed along the st croix river in minnesota and wisconsin the flows are mostly high fe tholeiitic basalt with plagioclase phenocrysts and ophitic to subophitic clinopyroxene abundant secondary chlorite epidote and actinolite indicate the group was metamorphosed to greenschist facies 350 c low sodium m4 site and tetrahedral aluminum aliv contents of actinolite indicate low pressure metamorphism 0 25 gpa and imply a geothermal gradient of 45 50 c km low magnesium mg 0 37 0 58 and ni contents 36 185 ppm indicate the basalts have undergone significant fractionation and are not primary mantle melts incompatible element abundances are inversely correlated with mg and most samples plot within either high or low trace element groups e g ti p zr the basalts are enriched in the light rare earth elements and th and are v

replacement of glass in the nakhla meteorite by berthierine implications for understanding the origins of aluminum rich phyllosilicates on mars a scanning and transmission electron microscope study of aluminosilicate glasses within melt inclusions from the martian meteorite nakhla shows that they have been replaced by berthierine an aluminum iron serpentine mineral this alteration reaction was mediated by liquid water that gained access to the glasses along fractures within enclosing augite and olivine grains water rock ratios were low and the aqueous solutions were circumneutral and reducing they introduced magnesium and iron that were sourced from the dissolution of olivine and exported alkalis berthierine was identified using x ray microanalysis and electron diffraction it is restricted in its occurrence to parts of the melt inclusions that were formerly glass thus showing that under the ambient physico chemical conditions the mobility of aluminum and silicon were low this discovery of serpentine adds to the suite of postmagmatic hydrous silicates in nakhla that include saponite and opal a such a variety of secondary silicates indicates that during aqueous alteration compositionally distinct microenvironments developed on sub millimeter length scales the scarcity of berthierine in nakhla is consistent with results from orbital remote sensing of the martian crust showing very low abundances of aluminum rich phyllosilicates

anisotropy of magnetic susceptibility studies in tertiary ridge parallel dykes iceland tertiary margin normal aishihik dykes yukon and proterozoic kenora kabetogama composite dykes minnesota and ontario abstract mafic dykes of different ages were collected from three different tectonic settings and analyzed using anisotropy of magnetic susceptibility ams as a proxy for magmatic flow during intrusion in iceland ridge parallel basaltic dykes were sampled on each side of the active tectonic boundary the dykes are k max orientation indicating vertical emplacement the eocene aishihik dyke swarm intrudes the yukon tanana terrane in the yukon province canada over an area 200 by 60 km these dykes were intruded normal to the accretionary margin are porphyritic andesites and have an intermediate geochemical signature based on major and trace element analyses ten dykes were sampled and 111 cores analyzed using ams and the dykes preserve a vertical k max orientation indicating intrusion was vertical through 30 km of continental crust the 2 06 ga kenora kabetogama dykes in northern minnesota and western ontario crosscut a variety of archean terranes thickness 50 km in a radiating pattern the unmetamorphosed basaltic dykes are 1 120 m wide 10 110 km in length are vertical in orientation and can be grouped as either being single intrusion or multiple intrusion composite dykes ams data preserve a vertical k max orientation for the southerly locations 2 dykes n 53 and horizontal k max for the remainder to the northwest 15 dykes n 194 maximum magnetic susceptibility axes 4 dykes n 92 for composite dykes are scattered and yield inconsistent flow directions with regard to the dyke margin almost all of our results are normal in that the magnetic foliation the plane containing k max and k int normal to k min is parallel to the dyke planes which gives us confidence that the magnetic lineations i e k max orientations are parallel to magmatic flow

on the origin of early proterozoic gneiss domes and metamorphic nodes northern michigan biotite 40 r 39 r cooling ages from medium pressure 500 600 mpa rocks in the watersmeet district northern michigan suggest significant cooling uplift and concomitant deformation during gneiss dome formation at 1755 ma well after the close of the 1870 1830 ma penokean orogeny however an 1822 ma hornblende plateau date indicates that the isograds surrounding the dome are penokean in age we attribute gneiss dome formation and doming of penokean aged isograds to an episode of orogenic collapse superimposed on an earlier history of crustal shortening this contrasts with the compressional origin for gneiss domes preserved in the low pressure 200 300 mpa republic district the different origins may reflect the fact that collapse was localized along the overthickened region of the orogenic belt in contrast to the watersmeet area hornblende and biotite 40ar 39ar ages obtained from the republic area are 1720 1680 ma given the relatively shallow depth of this region it is unlikely that temperatures re

strain variations across the proterozoic penokean orogen usa and canada abstract strata in the huron 2 5 2 0 ga and animikie 2 2 1 85 ga basins were deposited on the southern margin of the archean superior province these rocks were deformed during the penokean orogeny 1850 ma followed by subsequent accretionary orogens to the south at 1750 ma yavapai and 1630 ma mazatzal strain patterns are unique to each orogenic belt with no far field effect archean wawa terrane rocks in the penokean foreland preserve deformation associated with archean accretion with no younger penokean yavapai or mazatzal strain overprint the penokean orogeny deformed huron animikie basin sediments into a north vergent fold and thrust belt with no yavapai or mazatzal strain overprint yavapai orogen strains sw ne margin parallel shortening are unique when compared to the younger mazatzal shortening n20 w shortening with no strain overprint penokean deformation is characterized by shortening from the south including uplifted archean gneisses and a northerly thin skinned fold and thrust belt with north vergent nappes and a gently dipping foreland our study of finite and calcite twinning strains n 60 along 1500 km and across 200 km the penokean belt indicate that this orogeny was collisional as layer parallel shortening axes are parallel across the belt or parallel to the tectonic transport direction n s penokean nappe burial near the margin resulted in vertical shortening strain overprints some of which are layer normal the sudbury impact layer 1850 ma is found across the animikie basin and provides a widespread deformation marker with many local unique strain observations we also report new geochronology u pb zircon and apatite for the gneiss mafic dike rocks at wissota chippewa falls wi and arbutus black river falls wi dams respectively which bears on penokean yavapai deformation in the archean marshfield terrane which was accreted during the penokean orogen pseudotachylite formation was common in the superior province archean basement rocks especially along terrane boundaries reactivated by contemporaneous penokean trans hudson cape smith and new quebec deformation in the hinterland south the younger yavapai orogen 1750 ma n 8 deformation is preserved as margin parallel horizontal shortening sw ne in yavapai crust and up to 200 km to the north in the penokean thrust belt as a strain and barrovian metamorphic overprint mazatzal deformation 1630 ma n 16 is preserved in quartzites on yavapai and penokean crust with layer parallel and layer normal shortening strains oriented n20 w

the penokean orogeny in the lake superior region abstract the penokean orogeny began at about 1880 ma when an oceanic arc now the pembine wausau terrane collided with the southern margin of the archean superior craton marking the end of a period of south directed subduction the docking of the buoyant craton to the arc resulted in a subduction jump to the south and development of back arc extension both in the initial arc and adjacent craton margin to the north a belt of volcanogenic massive sulfide deposits formed in the extending back arc rift within the arc synchronous extension and subsidence of the superior craton resulted in a broad shallow sea characterized by volcanic grabens menominee group in northern michigan the classic lake superior banded iron formations including those in the marquette gogebic mesabi and gunflint iron ranges formed in that sea the newly established subduction zone caused continued arc volcanism until about 1850 ma when a fragment of archean crust now the basement of the marshfield terrane arrived at the subduction zone the convergence of archean blocks of the superior and marshfield cratons resulted in the major contractional phase of the penokean orogeny rocks of the pembine wausau arc were thrust northward onto the superior craton causing subsidence of a foreland basin in which sedimentation began at about 1850 ma in the south baraga group rocks and 1835 ma in the north rove and virginia formations a thick succession of arc derived turbidites constitutes most of the foreland basin fill along with lesser volcanic rocks in the southern fold and thrust belt tectonic thickening resulted in high grade metamorphism of the sediments by 1830 ma at this same time a suite of post tectonic plutons intruded the deformed sedimentary sequence and accreted arc terranes marking the end of the penokean orogeny the penokean orogen was strongly overprinted by younger tectonic and thermal events some of which were previously ascribed to the penokean principal among these was a period of vertical faulting in the archean basement and overlying paleoproterozoic strata this deformation is now known to have post dated the terminal penokean plutons by at least several tens of millions of years evidence of the penokean orogen is now largely confined to the lake superior region comparisons with more recent orogens formed by similar plate tectonic processes implies that significant parts of a once more extensive penokean orogen have been removed or overprinted by younger tectonic events

polyorogenic reworking of ore controlling shear zones at the south range of the sudbury impact structure a telltale story from in situ u pb titanite geochronology

determining the extent and nature of mazatzal related overprinting of the penokean orogenic belt in the southern lake superior region north central usa abstract twenty one hornblende and mica 40ar 39ar dates from central and northwest wisconsin usa provide important information on the timing spatial extent and intensity of mazatzal age metamorphism and deformation which overprinted the paleoproterozoic 1870 1820 ma penokean orogenic belt in the southern lake superior region 1760 1750 ma mica plateau ages from bedrock beneath undeformed 1750 1630 ma quartzites are interpreted as the time of lower temperature 300 350 c cooling and crustal stabilization after the penokean orogeny six mica ages from bedrock underlying deformed paleoproterozoic quartzites cluster around 1600 ma 1576 1614 ma the complete absence of 1760 1750 ma mica ages beneath regions of deformed quartzites suggests widespread heating to temperatures above 300 350 c during mazatzal related deformation and metamorphism the 1600 ma mica ages are interpreted to date the cooling phase of this metamorphism two anomalously young biotite dates are interpreted to indicate partial resetting associated with mesoproterozoic rifting at 1100 ma ten hornblende 40ar 39ar dates obtained in this study address the higher temperature overprinting effects in the southern lake superior region one latest archean age of 2503 18 ma and two ages of 1853 and 1830 ma are interpreted as remnant evidence of archean and penokean age amphibolite metamorphic events respectively the majority of hornblende ages are younger than the penokean orogeny scattering between 1796 and 1638 ma microtextural analysis indicates that similar microstructures exist in samples yielding highly discordant hornblende ages this suggests that shearing and recrystallization did not play an important role in the retention or loss of argon the 1638 ma hornblende age is concordant with the mazatzal orogeny to the south and is interpreted as representing complete thermal or fluid related resetting associated with that event six other post penokean ages scatter over a 70 million year interval 1796 1723 ma and probably reflect variable retention of radiogenic argon they are interpreted to indicate variable degrees of partial intermediate temperature 350 500 c resetting of hornblende argon systematics at 1650 1630 ma collectively these data suggest that the effects of the mazatzal orogeny in the southern lake superior region involved 350 500 c metamorphism and penetrative deformation

evolution of the oceanic sulfur cycle at the end of the paleoproterozoic here we present new measurements of 32 s 33 s 34 s and 36 s in sedimentary sulfides and couple these measurements with modeling treatments to study the sulfur cycle of a late paleoproterozoic marine basin we target the transition in ocean chemistry from the deposition of paleoproterozoic iron formations gunflint formation biwabik formation trommald formation and mahnomen iron formations to the inferred sulfidic ocean conditions recorded by overlying shale rove formation the data suggest that certain features of the global sulfur cycle such as a control by sulfate reducing prokaryotes and low mm concentrations of oceanic sulfate were maintained across this transition this suggests that the transition was associated with changes in the structure of the basin scale sulfur cycle during deposition of these sediments sulfide data from the iron formations are interpreted to reflect sedimentary sulfides formed from microbial reduction of pore water sulfate that was supplied through steady state exchange with an overlying oceanic sulfate reservoir the sulfide data for the euxinic rove formation shales reflect the operation of a sulfur cycle that included the loss of sulfide by a rayleigh like process we suggest that the prevalence of large and variable heavy isotope enrichments observed in rove formation sulfide minerals reflect a sustained and significant net loss of sulfide from the euxinic water column either as a result of a shallow chemocline and degassing to the atmosphere or as a result of a water column pyrite sink the inclusion of 36 s measurements in addition to 32 s 33 s and 34 s illustrates the mass dependent character of these sedimentary environments ruling out contributions from the weathering of archean sulfides and pointing to at least modest levels of sustained atmospheric oxygen 10 5 present atmospheric levels of o2

oblique subduction footwall deformation and imbrication a model for the penokean orogeny in east central minnesota the penokean orogeny was a major early proterozoic 1875 1825 ma tectonic event in the great lakes region in east central minnesota it is marked by multiply deformed and highly metamorphosed supracrustal rocks of the early proterozoic denham and thomson formations structural features similar to those in the supracrustal rocks also exist in the basement archean 2700 ma mcgrath gneiss such features are here explained in a tectonic model consisting of southward directed oblique subduction along the great lakes tectonic zone intense deformation occurred in the footwall of the major thrust which marked the boundary between downgoing and overriding plates during a type continental subduction sedimentary rocks thomson formation deposited on the footwall during loading caused by thrusting eventually became incorporated into the deformation zone early formed structures related to footwall deformation are a dominantly well developed foliation in the gneiss and isoclinal recumbent folds with a bedding subparallel foliation in the denham and lower thomson formations progressive metamorphism during subduction reached the garnet zone of the amphibolite facies various deformation inversions show that this early phase of deformation involved extreme flattening with z vertical and large amounts of extension in both the north south and east west directions footwall deformation was followed by imbrication and accretion onto the hanging wall during uplift associated with continued compression and isostatic rebound later formed structures associated with imbrication and deformation within the hanging wall consist of folding of the foliation and development of shear zones in the mcgrath gneiss and open to close upright to overturned folds in the denham and thomson formations the peak metamorphic event represented by staurolite occurred after the later deformation at temperatures of about 470 530 c and a minimum pressure of 3 4 kbar minimum depth of 12 4 km increasing temperature associated with decreasing pressure uplift is explained by conductive relaxation caused by crustal thickening and erosion this tectonic model may have more widespread implications for explaining similar structural features found in many precambrian terranes worldwide

the age of the gunflint formation ontario canada single zircon u pb age determinations from reworked volcanic ash the gunflint formation a paleoproterozoic chemical clastic sedimentary assemblage outcropping to the immediate northwest of lake superior became famous in 1954 as containing the oldest fossil assemblage known at that time older microfossils have since been discovered but the gunflint procaryotes remain one of the most diverse precambrian fossil communities the finding of possible multicellular organisms in correlative lithic units in michigan has recently added to the need for an exact age of the formation zircons were extracted from rainout and storm reworked volcanoclastic beds in the upper portion of the gunflint formation a euhedral zircon population has yielded a 1878 3 1 3 million years bp u pb age believed to be nearly synchronous with the depositional age this not only provides a precise age for the community of organisms but also strongly supports a back arc extensional setting for the animikie basin rather than a foreland trough resume la formation de gunflint un assemblage sedimentaire chimique et clastique datant du paleoproterozoique affleurant immediatement au nord ouest du lac superieur est devenue celebre en 1954 car elle contenait les plus an ciens assemblages fossiliferes connus a cette epoque de plus anciens microfossiles ont ete decouverts depuis mais les procaryotes de gunflint demeurent l une des communautes fossiles precambriennes les plus diversifiees la decouverte

quantifying the timing and rate of crustal evolution global compilation of radiometrically dated detrital zircon grains abstractmultiple models steady state episodic and early growth followed by crustal reworking have been postulated to explain the evolution of earth s continental crust an independent assessment of these models is now possible as a result of the massive numbers of detrital zircon grains that have been dated individually over recent years using u pb and hf isotopes a compilation of 200 000 published zircon dates reveals a global age distribution u pb with six prominent statistically significant peaks 3 2 3 0 2 7 2 5 2 0 1 7 1 2 1 0 0 7 0 5 and 0 3 0 1 ga in most cases these peaks are recorded on all seven continents most of the peaks are detectable also in modern sediments a comparison of grains with both u pb ages and hf isotope model ages available for 5100 detrital zircons suggests that the age frequency distribution of detrital grains reflects predominantly episodic crustal recycling rather than crustal growth the zircon age distributions and associated data provide consistent ev

tectono stratigraphic evolution of the early proterozoic wisconsin magmatic terranes of the penokean orogen the early proterozoic penokean orogen developed along the southern margin of the archean superior craton the orogen consists of a northern deformed continental margin prism overlying an archean basement and a southern assemblage of oceanic arcs the wisconsin magmatic terranes the south dipping niagara fault suture zone separates the south facing continental margin from the accreted arc terranes the suture zone contains a dismembered ophiolite the wisconsin magmatic terranes consist of two terranes that are distinguished on the basis of lithology and structure the northern pembine wausau terrane contains a major succession of tholeiitic and calc alkaline volcanic rocks deposited in the interval 1860 1889 ma and a more restricted succession of calc alkaline volcanic rocks deposited about 1835 1845 ma granitoid rocks ranging in age from about 1870 to 1760 ma intrude the volcanic rocks the older succession was generated as island arcs and or closed back arc basins above the south dipping subducti

tectonic setting of the midcontinent rift system abstract the continental lithosphere which hosts the midcontinent rift mcr system comprises a crustal assemblage ranging in age from 3 6 to 1 5 ga the most voluminous igneous activity along the mcr occurred within late archean 2 7 ga crust presumably due to magmatism associated with a rising mantle plume two major arms of the rift extend to the southeast and the southwest where rifting magmatism and sedimentation were confined to a 2000 km long rift that cuts through the various crustal provinces ranging in age from 3 6 to 1 5 ga tensional reactivation of pre existing crustal discontinuities may have influenced the location of parts of the mcr however direct cause and effect are difficult to distinguish for want of sufficient exposures the most active phases of magmatism and rifting occurred between 1109 and 1087 ma a u pb age of 1097 ma for the southern end of the mcr shows that magmatic activity was synchronous throughout its length there is evidence of rifting and magmatism prior to 1109 ma but there is little evidence of magmatism after 1087 ma rifting may have effectively ceased by that time rifting may have ceased as a result of compressional forces being exerted on the foreland during the grenville collisional event spanning 1030 970 ma and compressional reactivation of normal faults may have resulted in partial inversion of the mcr chemical data and u pb and sm nd isotopic data for igneous rocks along the rift are similar to those for mcr rocks in the lake superior area which are best explained in terms of a mantle plume model however since the distal arms of the mcr were developed in proterozoic lithosphere and did not involve nearly as much magmatism as in the lake superior area mantle plume sources may not have been as important in formation of the distal magmas data from the buried arms are also consistent with the mafic magmas being derived by decompression melting of proterozoic lithosphere during rifting

age of volcanic rocks and syndepositional iron formations marquette range supergroup implications for the tectonic setting of paleoproterozoic iron formations of the lake superior region a rhyolite in the hemlock formation a mostly bimodal submarine volcanic deposit that is laterally correlative with the negaunee iron formation yields a sensitive high resolution ion microprobe shrimp u pb zircon age of 1874 9 ma but also contains inherited archean zircons as old as 3 8 ga this precise age determination for the classic paleoproterozoic stratigraphic sequence of northern michigan the marquette range supergroup mrs necessitates modification of previous depositional and tectonic models our new data indicate that the menominee group previously ascribed to continental rifting during early pre collision phases of the penokean orogenic cycle is coeval with arc related volcanic rocks now preserved as accreted terranes immediately to the south and is more aptly interpreted as a foredeep deposit we interpret these to be second order basins created by oblique subduction of the continental margin rather than basins formed on a rifting margin along with a recently reported age for the gunflint formation in ontario of 1878 2 ma our data suggest that an extensive foredeep in the western lake superior region was the locus of iron formation deposition during arc accretion from the south further we interpret the lower mrs chocolay group a glaciogenic and shallow marine succession that lies atop archean basement to be equivalent to the upper part of the huronian supergroup of ontario and to represent the original continental rifting and passive margin phase of the penokean cycle the upper mrs baraga group represents deeper marine basins dominated by turbidites and lesser volcanic rocks resulting from increased subsidence and continued collision a stitching pluton which cuts correlatives of the hemlock formation in a thrust sheet yielded a u pb zircon age of 1833 6 ma consistent with other post tectonic plutons in wisconsin and northern michigan indicating that penokean convergence lasted no longer than 40 million years

revisiting the paleoproterozoic baraboo interval in southern wisconsin evidence for syn depositional tectonism along the south central margin of laurentia abstract the statherian baraboo quartzite 1710 1650 ma is present in the baraboo ranges wisconsin usa it is one of several isolated quartzites outcropping across the southern lake superior region collectively these quartzites are known as baraboo interval quartzites and they record a history of basin development followed by regional deformation along the south central laurentian margin baraboo interval stratigraphy above the baraboo quartzite in the baraboo ranges is unresolved limiting understanding of the style and history of basin formation sedimentation and deformation as well as correlation between regional quartzites new precambrian geologic mapping within the core of the baraboo syncline baraboo ranges wi confirms early interpretations of local stratigraphy the baraboo interval section includes two upward fining sequences separated by an angular unconformity the lower sequence comprises the baraboo quartzite seeley slate and freedom formation the upper sequence comprises the dake quartzite and rowley creek slate the depositional age of the upper sequence is poorly constrained but could be nearly 200 million years younger than the underlying units u pb detrital zircon ages for the lower conglomeratic baraboo quartzite and the dake quartzite are dominated by nearby basement ages between 1800 and 1750 ma and 1900 1835 ma suggesting a restricted drainage system tapping proximal basement sources subsidence modeling of the lower sequence demonstrates at least 1 28 km of subsidence is required to generate the accommodation necessary to deposit the baraboo quartzite seeley slate and freedom formation these results support a model where deposition of the lower sequence between ca 1710 1650 ma occurred along a tectonically active plate margin the lack of coeval magmatism in the baraboo interval suggests deposition did not occur in a convergent arc setting however coeval orthoquartzite deposition in colorado arizona and new mexico along the southwestern margin of laurentia 1700 1650 ma occurred synchronous with arc volcanism requiring a component of plate margin convergence to the southwest to reconcile these differences a model where the entire southern margin of laurentia between 1710 and 1650 ma was dominated by strike slip motion is favored varying from highly oblique convergence in the southwest to dominantly strike slip motion in the southern lake superior region

rb sr biotite and whole rock data from the kapuskasing uplift and their bearing on the cooling and exhumation history rb sr isotopic ratios were measured on biotite and whole rocks from a suite of 21 granitoid rocks from the southern kapuskasing uplift results of 18 of the whole rock analyses fall on an isochron with an age of 2 677 0 057 ga and sri 0 70080 biotite model ages range from 2 50 to 1 93 ga in a general spatial relationship of decreasing age with depth in the structural section as calibrated with the igneous hornblende barometer for tonalitic rocks and garnet pyroxene barometers for granulites near the michipicoten belt in the west biotite ages of 2 ga reflect disturbances also recorded by paleomagnetic and whole rock rb sr systems to the east in a 70 km long northwest southeast transect toward the ivanhoe lake fault zone ages decrease from 2 50 to 1 95 ga in a southern transect ages are in the range 2 30 2 38 ga to within 15 km of the fault reflecting consistent erosion levels corresponding to 0 53 mpa but decrease to 1 93 ga to the east in concert with paleopressures in the 0 7 1 0 gpa ra

revisiting thrusting reverse faulting and transpression in the southern sudbury basin ontario abstract the synformal sudbury igneous complex sic is the deformed relic of a large impact melt sheet understanding the geometry and kinematics of deformation of the sic is paramount for mineral resource deposit exploration of this complex constraining its pre impact configuration and understanding paleoproterozoic tectonism at the southern margin of the archean superior province in particular it is uncertain to what extent thrusting on the south range shear zone srsz a ductile shear zone transecting the southern portion of the sic is coeval with folding of the sic geometric properties of 1090 discontinuities mapped from air photos and geometry of metamorphic l s fabrics revealed the presence of two spatially and kinematically distinct deformation regimes in the southern sic the so called south range nw directed thrusting on the srsz dominates in the western south range whereas dextral transpression prevails in the eastern south range the two deformation regimes are geometrically and kinematically related to the srsz we propose that the srsz transects the south range south of the se lobe a tight fold of the sic this is at variance with previous hypotheses in which the srsz continues into the hinge zone of the se lobe and in which dextral transpression is spatially confined largely to this lobe the curvature of the srsz and its mechanical coupling to its footwall rocks imparted dextral transpressive deformation to the eastern south range dextral component shear of this deformation was associated with shortening of the sic oblique to its contacts collectively this led to map view corrugation of sic contacts and thickness variations of the sic that culminated in the formation of the se lobe our study suggests that thrusting on the srsz is kinematically linked and thus coeval with folding of the sic

crustal scale thrusting and origin of the montreal river monocline a 35 km thick cross section of the midcontinent rift in northern michigan and wisconsin a structurally simple 35 km thick north facing stratigraphic succession of late archean to middle proterozoic rocks is exposed near the montreal river which forms the border between northern wisconsin and michigan this structure the montreal river monocline is composed of steeply dipping to vertical sedimentary rocks and flood basalts of the keweenawan supergroup middle proterozoic along the south limb of the midcontinent rift and disconformably underlying sedimentary rocks of the marquette range supergroup early proterozoic these rocks lie on an archean granite greenstone complex about 10 km of which is included in the monocline this remarkable thickness of rocks appears to be essentially structurally intact and lacks evidence of tectonic thickening or repetition tilting to form the monocline resulted from southward thrusting on listric faults of crustal dimension the faults responsible for the monocline are newly recognized components of a well known regional fault system that partly closed and inverted the midcontinent rift system resetting of biotite ages on the upper plate of the faults indicates that faulting and uplift occurred at about 1060 20 ma and followed very shortly after extension that formed the midcontinent rift system

nd and pb isotopic evidence for provenance and post depositional alteration of the paleoproterozoic huronian supergroup canada neodymium model ages for fine grained formations of the paleoproterozoic huronian supergroup mckim pecors gowganda gordon lake range from 3 00 to 2 55 ga and indicate a provenance dominated by the late archean superior province to the north and west the stratigraphically highest unit gordon lake formation has a distinctive nd isotopic composition with tdm being 100 400 ma younger than underlying mudstones this suggests that the provenance changed consistent with a previously documented change towards more negative eu anomalies in the gordon lake lead isotopes are consistent with a superior province provenance and in addition provide evidence for two episodes of regional post depositional disturbance of the u pb system lower huronian mckim pecors samples align along 207pb 204pb 206pb 204pb slopes equivalent to 2170 58 ma mswd 92 n 9 and 2212 92 ma mswd 9 1 n 5 respectively these ages are at the minimum age limit on sedimentation and within uncertainty of the nipissing diabase 2219 4 ma a ubiquitous regional feature parts of which may have intruded while much of the huronian was unconsolidated these pb pb ages are interpreted to represent widespread diagenetic processes possibly associated with an early phase of nipissing intrusion 206pb 204pb varies mostly from 19 to 34 up to 59 whereas implied 232th 238u are mostly between 2 and 4 only slightly below the upper crustal value of 4 changes in 206pb 204pb imply changes of 238u 204pb by factors of 2 and up to 12 5 values for are also in the range 2 4 and accordingly pb loss appears to dominate this disturbance elevated 206pb 204pb correlates with post depositional addition of potassium in the gordon lake and possibly the gowganda formations k metasomatism has been demonstrated previously in the underlying serpent formation and in paleosols developed at the unconformity beneath the huronian supergroup the latter being dated at 1690 1730 ma widespread metasomatism resulting in k addition and pb loss may have been related to northerly directed basin wide fluid movement in response to post tectonic intrusions and erosion of the ca 1 85 ga penokean orogen to the south since pb isotope systematics in the lower huronian mudstones appear unaffected apart from natural conduits provided by unconformity surfaces and possibly lower huronian sandstone aquifers that also may have been affected by k metasomatism it appears that fluid movement was more pervasive in the upper levels of this sedimentary sequence

proterozoic anorogenic magmatism and its orogenic connections there is a major belt of proterozoic so called anorogenic igneous rocks that extends from the baltic shield through southern greenland and eastern canada to southern usa all rocks are situated within a zone of 1 9 1 6 ga arc accretionary orogens that consist of juvenile mantle derived material especially accretionary prism sediments that provided the fertile source for abundant granitic crustal melts there were two main periods of formation of the anorogenic rocks 1 76 1 55 ga rocks are mainly rapakivi granites and rhyolites that formed about 60 200 ma after the last compressional events in the svecofennian ketilidian and penokean orogens the time lag being caused by the slow thermal relaxation of thickened lower crust that had commonly been depleted by the extraction of earlier granitic crustal melts 1 45 1 4 ga anorogenic rocks are mainly rhyolites and peraluminous granites in central southern usa and anorthosites in eastern canada that formed in the continental margin of the grenvillian ocean the subduction of which led to the grenville orogen these anorogenic igneous rocks formed in extensional regimes during the formation and breakup of a 1 5 ga supercontinent

fold axes oblique to the regional plunge and proterozoic terrane accretion in the southern lake superior region abstract the niagara fault is believed to represent the suture between a magmatic arc terrane and a passive margin assemblage which were tectonically juxtaposed about 1 85 ga ago during the penokean orogeny accretion related deformation is concentrated within an 12 km wide belt that straddles the suture a distinctive structural feature of this belt is the rotation of early fold axes toward parallelism with the direction of bulk extension resulting in a great circle stereonet distribution of fold axes within a vertical axial plane rocks of both arc and passive margin assemblages crop out in this more highly strained belt but there has been no material transfer between them across the fault structural style and metamorphic assemblages demonstrate deep level beneath the brittle ductile transition erosion of the terrane accretion boundary in the southern lake superior region there is some evidence for pure shear during the accretion event but we are unable to evaluate the amount or sense of simple shear

penokean tectonics along a promontory embayment margin in east central minnesota abstract recent geologic investigations in east central minnesota have utilized geophysical data test drilling and high resolution geochronologic dating to produce a significantly improved map of a poorly exposed part of the 1880 1830 ma penokean orogen these investigations have elucidated major changes in the structure of the orogen as compared to its counterparts in northern michigan and northwestern wisconsin foreland basin fold and thrust belt and magmatic terrane components that are recognized to the east extend into east central minnesota but they appear to be deflected southwards and truncated in proximity to archean rocks of the minnesota river valley mrv subprovince in contrast the interior of the mrv subprovince to the southwest shows little sign of penokean tectonism in addition the magmatic and metamorphic rocks of the internal zone of the orogen in east central minnesota are extensively invaded by ca 1785 1770 ma granitic rocks the east central minnesota batholith whereas post orogenic granites of this age occur sparingly to the east these differences in orogenic structure may be related to their location near the juncture of an embayment becker embayment and a promontory mrv promontory that formed the pre penokean continental margin in this scenario the mrv promontory which at the surface consists chiefly of high metamorphic grade mesoarchean gneisses would have formed competent high standing crust that resisted deformation and did not host significantly thick continental margin sequences in contrast the part of the becker embayment adjoining the promontory would have involved relatively weak low standing crust that favored deposition of continental margin sequences and during penokean collision would have accommodated tectonic loading of the cratonic margin through thin skinned deformation thrusting of thick embayment sequences and possibly a block of archean crust marshfield terrane onto the embayment margin may have produced a greatly thickened crust that subsequently promoted crustal melting and generation of the geon 17 granites preliminary gravity and magnetic model studies of the present day crust imply that rocks of the fold and thrust belt may sole out at 5 8 km depth whereas magmatic and high metamorphic grade rocks associated with the internal zone of the orogen could extend to mid crustal depths the tectonic model proposed here implies that a collision between an embayment and an impinging continental mass may enhance tectonic thickening and subsequent generation of post orogenic magmas this and other hypotheses regarding the penokean orogen need to be investigated further in the third dimension of depth which will require a comprehensive suite of geophysical studies

age and origin of authigenic k feldspar in uppermost precambrian rocks in the north american midcontinent authigenic k feldspar occurs widely in the north american midcontinent in an alteration profile developed on uppermost precambrian rocks in the study areas of west central wisconsin and southeast missouri the alterationprofiles are kaolinitic and interpreted as former saprolite formed by weathering prior to deposition of upper cambrian sandstone k feldspar in granitic alteration profiles of west central wisconsin occurs either as coarse grained red crystals and veins replacing kaolinite or as overgrowths and fine grained crystals in pore space of partly altered granite in a granitic profile of southeast missouri k feldspar replaces primary feldspar and probably kaolinite in a diabasic profile k feldspar is disseminated and forms veins in saprolite and it replaces the outer 3 5 cm of core stones dating was conducted with the 4 0 ar 3 9 ar method using incremental heating observed spectral types are plateaus 6 near plateaus 3 undulatory 14 and stair step 5 undulatory patterns are attributed chiefly to recoil and integrated ages of these are usually within the error of plateau ages where present indicating a balanced enrichment depletion of 3 9 ar due to recoil stair step spectra are attributed to severe recoil plateau 4 0 ar 3 9 ar ages of 446 427 ma late ordovician and silurian were obtained from coarse grained k feldspar replacing kaolinite in wisconsin suggesting a duration of the alteration episode of 19 m y however potentially accurate nonplateau age measurements integrated and near plateau ages suggest that alteration continued for a considerable time thereafter possibly to ca 399 395 ma early devonian integrated 4 0 ar 3 9 ar ages of ca 362 ma late devonian and or early mississippian were obtained from fine grained k feldspar plateau ages of ca 450 late ordovician were obtained from k feldspar of the diabase profile in missouri duration of k feldspar alteration in this region was at least 16 m y and may have been as long as 52 m y the late devonian early mississippian k feldspar formed at temperatures of 100 c from saline fluid averaging 10 nacl equivalent and having a 1 8 o value of 3 5 this brine may have originated from the michigan basin east central iowa basin and or the illinois basin coincident with the acadian orogeny the middle ordovician to early devonian dates of k feldspar do not generally correspond in time with orogenic activity at the continental margin and the mechanism s for forming the k feldspar are uncertain fluid sources proposed for some or all of the k feldspar include the michigan basin taconic mountains reelfoot rift illinois basin and midcontinent epeiric seas middle ordovician k feldspar of missouri very likely formed at elevated temperatures but evidence of fluid temperature is lacking for other pre late devonian k feldspar

paleoproterozoic crustal history of the southwestern grenville province evidence from nd isotopic mapping nd isotopic mapping in the north bay area of the central gneiss belt southwestern grenville province has revealed the precise trend of a tdm model age line developed between the uplifted southern margin of the archean superior craton tdm 2 7 ga and a paleoproterozoic allochthon tdm 1 9 ga separating these two crustal blocks is a narrow zone of gneisses with intermediate tdm ages these transitional gneisses are interpreted to reflect a remnant fault or ductile shear zone of uncertain age along which crustal material from both blocks mechanically mixed during their juxtaposition accordingly the nature of the tdm line in the north bay area is interpreted to be tectonic in the temiscaming area widespread exposures of mature metasedimentary gneisses are shown by their tdm ages to be dominantly of paleoproterozoic provenance these results are consistent with the existing detrital zircon geochronology inferring a maximum depositional age of 1 7 ga the anorogenic chemistry of the north bay o

two paleoproterozoic statherian siliciclastic metasedimentary sequences in central wisconsin the end of the penokean orogeny and cratonic stabilization of the southern lake superior region abstract statherian siliciclastic sedimentary rocks in the southern lake superior region are key to understanding the evolution and stabilization of paleoproterozoic crust in the midcontinent deposited during an important period of significant crustal growth controversy exists regarding the conditions of sandstone deposition and the relations between magmatism and sedimentation in central wisconsin the hamilton mounds inlier although restricted in areal extent is significant in that it reveals the existence of two separate statherian units an 1800 1760 ma immature meta arkose and a post 1760 ma baraboo interval supermature quartzite the meta arkose is crosscut by 1762 ma granite and contains abundant penokean age detrital monazite grains the younger quartzite contains detrital monazite as young as 1750 ma and was deposited unconformably upon these older units all units experienced 1630 ma low grade recrystallization and isotopic disturbance associated with the mazatzal orogeny in addition the supermature quartzite experienced k metasomatism and brecciation at 1470 ma ascribed to regional scale hydrothermal alteration related to the influence of the nearby 1470 ma wolf river batholith collectively these data suggest an early statherian episode of arkosic deposition and magmatism during yavapai orogenesis followed by a longer interval of deposition of supermature quartz arenite associated with crustal stabilization the existence of two separate statherian units reconciles apparent conflicts in depositional models and provides a greater understanding of the paleoproterozoic evolution of the midcontinent

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

florence niagara terrane an early proterozoic accretionary complex lake superior region u s a the florence niagara terrane is an elongate fault bounded block 10 km wide and 80 km long composed of miogeoclinal assemblage lithologies bounded on the north by a generally coherent miogeocline extending to the superior craton and on the south by a probably exotic magmatic arc terrane the florence niagara terrane is composed of a collage of eight fault bounded packets all fault packets are characterized by faults that trend west to west northwest with a dominant west northwest striking deeply dipping foliation elongation lineations plunge down the dip of foliations in all cases except in packet 1 where bulk extension locally parallels shallowly plunging fold axes a probable product of superimposed strains fold axes are subhorizontal or girdled in the mean foliation plane or plunge steeply packets containing girdled fold axes probably were subjected to greater strain than were packets with subhorizontal fold axes steeply plunging fold axes are not easily explained but they were perhaps formed by noncoaxial deformation one of the packets contains a younger generation of folds formed during east west shortening another packet contains folds associated with later strike slip faulting convergence between the magmatic terrane and the miogeocline probably resulted in accretion of miogeoclinal sediments in the forearc area of the magmatic terrane the florence niagara terrane may represent such a zone of forearc accretion formed prior to and during collision and thus it may be referred to as an accretionary complex

physical and chemical evidence of the 1850 ma sudbury impact event in the baraga group michigan an ejecta layer produced by the sudbury impact event ca 1850 ma occurs within the baraga group of northern michigan and provides an excellent record of impact related depositional processes this newly discovered 2 4 m thick horizon accumulated in a peritidal environment during a minor sea level lowstand that punctuated a period of marine transgression common ejecta clasts include shock metamorphosed quartz grains splash form melt spherules and tektites accretionary lapilli and glassy shards suggesting sedimentation near the terminus of the continuous ejecta blanket sedimentologic and geochemical data indicate that primary fallout from a turbulent ejecta cloud was reworked to varying degrees by an impact generated tsunami wave train observed platinum group element anomalies ir rh and ru within the sudbury ejecta horizon are sufficient to suggest that the impactor was a meteorite documenting and interpreting the detailed characteristics of the sudbury ejecta horizon in michigan have yielded a fingerprint to identify this chronostratigraphic marker in other paleoproterozoic basins for the first time a foundation exists to assess the consequences of the sudbury impact on precambrian ocean chemistry and early life

middle proterozoic uplift events in the dunbar dome of northeastern wisconsin usa isotopic ages of granitic and metamorphic rocks exposed in the dunbar structural dome of northeastern wisconsin identify a protracted series of tectonic and hydrothermal events that culminated in major regional uplift during middle proterozoic keweenawan ca 1 100 ma continental rifting and volcanism the major rock forming events and the structural development of the dome occurred during the interval 1 862 4 ma to 1 836 6 ma whole rock rb sr ages are partly reset in response to a widely recognized but cryptic event in wisconsin and michigan at about 1 630 ma the scale and systematic character of the whole rock resetting strongly suggests the presence of a fluid phase derived in situ from water dissolved in the silicates or externally from a subthrust plate of low grade metamorphic rocks the regional nature of the 1 630 ma disturbance possibly indicates that it is related to a major tectonic event such as an active plate margin far to the south rb sr biotite ages for the dunbar dome this study the southern complex of the marquette district van schmus and woolsey 1975 and the felch trough area aldrich and others 1965 provide a remarkably coherent pattern that reflects multiple episodes of differential uplift younger events superimposed on a regional 1 630 ma imprint are recorded at 1 330 ma and 1 140 ma the 1 330 ma disturbance could reflect stabilization following intrusion of the wolf river batholith at 1 485 ma the 1 140 ma uplift event occurred during keweenawan rifting and volcanism as a result of stresses imposed on a mosaic of fault bounded blocks with possible subcrustal influence the remarkably small variance in the 1 140 ma biotite age peak argues for rapid uplift and cooling and hence rapid erosion detritus from the uplift probably was being shed into nearby tectonic basins most of which did not survive subsequent uplift and erosion

late paleoproterozoic climate tectonics and metamorphism in the southern lake superior region and proto north america evidence from baraboo interval quartzites abstract red supermature quartzites of the baraboo interval of the lake superior region contain detrital zircon that ranges in age from 1782 to 1712 ma deposition clearly occurred after the geon 18 penokean orogeny these late paleoproterozoic sedimentary rocks consist largely of quartz kaolinite or pyrophyllite and hematite detrital feldspar and muscovite are rare or absent their chemical index of alteration ranges from 96 8 to 98 6 among the most chemically mature clastic sediments in the geological record the quartzites are underlain by mature feldspar free paleosols accounting for the absence of feldspar in the overlying sediments and indicating the presence of first cycle quartzose detritus such physical and chemical characteristics imply that late paleoproterozoic deposition in the lake superior region occurred in a stable tectonic setting with subdued topographic relief in a warm humid climate folding and low grade metamorphism of the quartzites is thought to reflect 1630 ma foreland

geoscience impact a synthesis of studies of the sudbury structure geophysical probing results are synthesized into a three dimensional framework necessary for understanding the genesis of the sudbury structure based primarily on seismic reflection results centred on the sudbury igneous complex remnants of crustal melting from a catastrophic meteorite impact are superimposed on the juxtaposition of mid crustal rocks exhumed during the archean against deformed paleoproterozoic sedimentary rocks sedimentation metamorphism deformation and metasomatic overprints are all part of the post impact history of sudbury and tend to dominate the geophysical response of the structure pre impact deformation although certainly preserved in some aspects of sudbury geology is not clearly expressed in the geophysical data nor are any elements of impact induced deformation geophysical views of the sudbury igneous complex are thus somewhat biased in representing mostly the post impact but pre grenvillian history of the region with the exception of igneous events establishing th

evidence for episodic oxygenation in a weakly redox buffered deep mid proterozoic ocean abstract over the last two decades popular opinion about prevailing conditions in the mid proterozoic deep ocean has evolved from fully oxygenated to globally euxinic sulfidic to a more heterogeneous stratified water column with localized pockets of euxinia existing in predominantly iron rich ferruginous deep waters the animikie basin in the lake superior region has been essential in shaping our view of marine redox evolution over this time period in this study we present a multi proxy paleoredox investigation of previously unanalyzed strata of the late paleoproterozoic animikie basin using drill cores through the 1 85 ga stambaugh formation paint river group in the iron river crystal falls district of the upper peninsula of michigan usa based on previous tectonic reconstructions and analysis of sedimentary regimes the iron river crystal falls section captures strata from among the deepest water facies of the animikie basin in contrast to previous work on sedimentary rocks in this basin we find evidence from iron speciation trace metal and mo isotope data for episodes of at least local deep water oxygenation within a basin otherwise dominated by ferruginous and euxinic conditions while trace metal enrichments and iron speciation data suggest predominantly anoxic conditions the occurrence of mn rich intervals up to 12 3 wt mno containing abundant mn fe carbonate and a wide range of mo isotope data with extremely negative values 98 95mo 1 0 to 1 1 record the shuttling of mn oxides from surface waters through oxic or suboxic waters to the sediment water interface we propose that such conditions are analogous to those of locally restricted modern and holocene basins in the baltic sea which receive episodic inflow of oxygenated water producing similar geochemical signatures to those observed for the animikie basin we argue that the mid proterozoic was characterized by a lack of a strong redox buffer low sulfide ferrous iron and oxygen contents and thus was vulnerable to dramatic and at least local redox shifts including briefly oxygenated bottom waters a refined view of the mid proterozoic ocean is emerging one that was still predominantly anoxic but marked by regional heterogeneities and short term redox variability that may in part reflect a transitional state between prevailingly anoxic archean and predominantly oxic phanerozoic oceans

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

nd isotopes and the origin of 1 9 1 7 ga penokean continental crust of the lake superior region nd isotopic data on 26 samples demonstrate the origin of the early proterozoic crust of the penokean orogen the three major components are 1 the marquette range supergroup a predominantly miogeoclinal continental margin sequence deposited before 1 85 ga on archean basement 2 ca 1 88 ga felsic metavolcanic rocks of the wisconsin magmatic terrane to the south of evolved island arc affinity and 3 1 87 1 76 ga granitoids that intrude the metavolcanic rocks initial nd values for the metavolcanic rocks of the magmatic terrane range between 0 0 and 2 4 and t dm ages between 1 9 and 2 2 b y the volcanic rocks primarily represent new crustal material that had only a limited archean input probably through mixing of subducted sediments into the magma source area nd t values for sedimentary rocks in the lower part of the marquette range supergroup indicate an archean source most likely the 2 7 ga superior province to the north on the other hand the sedimentary rocks in the upper part of the marquette range supergroup graywackes of the upper michigamme formation have initial nd values between 0 8 and 1 5 indicative of an early proterozoic source these graywackes probably are foredeep deposits derived from the volcanic rocks of the wisconsin magmatic terrane to the south thus the time of deposition of the upper michigamme formation dates the final convergence of the wisconsin magmatic terrane with the continental margin the granitoids of the wisconsin magmatic terrane have a wide range of nd t values from 4 5 to 4 0 they represent mixtures of variable amounts of new crustal material and recycled archean detritus the more negative nd t values occur in samples close to the northern boundary of the wisconsin magmatic terrane which is marked by a mylonitic shear zone the niagara fault zone we suggest that the miogeoclinal lower marquette range supergroup rocks having an archean nd signature became involved in magma genesis close to the collisional boundary whereas increasingly lesser amounts of this older material were available for mixing farther away from the suture the 1 9 1 7 ga penokean events involved major growth of new crust from the mantle

sm nd isotopic evidence for crustal contamination in the ca 1750 ma wanapitei complex western grenville province ontario the wanapitei complex consists of a variably metamorphosed gabbronorite lying immediately adjacent to the grenville front boundary fault u pb zircon geochronology indicates a crystallization age of for a noritic component with both grenville aged ca 1000 ma metamorphism and minor older inheritance indicated geochemical evidence is consistent with plagioclase pyroxene fractionation but indicates additional open system behaviour data indicate contamination of a depleted mantle by light rare earth element enriched material during the penokean this was followed during emplacement by extensive contamination of the then isotopically near chondritic magma with variable amounts of evolved archean crustal material on the order of 40 in extreme cases generating end 1 75 values between 0 and 7 5

precambrian terrane of north central wisconsin an aeromagnetic perspective a shaded relief magnetic map covering most of the region of exposed precambrian rocks of north central wisconsin shows the structural grain and many lithologic units with clarity and comprehensive

involvement of variably sourced fluids during the formation and later overprinting of paleoproterozoic au cu mineralization insights gained from a fluid inclusion assemblage approach abstract au cu mineralized quartz veins of the reef deposit wisconsin usa originally formed prior to or early in the paleoproterozoic penokean orogeny in central north america as either the root zone of a gold rich vms deposit or in an orogenic gold setting nearly 400 m y later magmatic hydrothermal fluids associated with a continental scale anorogenic magmatic event remobilized mineralization within the veins and still later during the paleozoic remobilization and perhaps upgrading of au and other metals occurred in response to circulation of fluids associated with mississippi valley type deposits found in overlying supracrustal rocks fluid constraints on the formation of the reef deposit and later overprinting over a 1 5 b y timeframe are examined using the fluid inclusion assemblage fia approach distinctive assemblages are examined on individual merit each offering individual stories to aid in our understanding of the protracted development of the reef deposit presumably primary fluid inclusions in au and cu bearing quartz sulfide veins that formed during the paleoproterozoic event show h2o co2 ch4 nacl compositions based on microthermometry and laser raman spectroscopy possibly primary h2o nacl and secondary h2o co2 ch4 nacl fia formed prior to regional deformation and are recognized as discrete relict and neonate clusters the intersection of isochores from the highest density neonate inclusions of either fluid composition indicates re equilibration at lower amphibolite conditions during regional deformation quartz hosted secondary fluid inclusion assemblages characterized by h2o ch4 and co2 ch4 compositions formed at high temperatures and have not been re equilibrated and are interpreted to post date regional metamorphism these fia occur along healed fractures with chalcopyrite mineralization these inclusions are likely associated with fluid circulation driven by emplacement of the adjacent wolf river batholith ca 1 45 ga that altered pyrite to pyrrhotite and remobilized or emplaced chalcopyrite at temperatures as high as 700 c this reduced fluid was in equilibrium with locally observed graphitic sediments secondary lower temperature

role of kilometer scale weak circular heterogeneities on upper crustal deformation patterns evidence from scaled analogue modeling and the sudbury basin canada the sudbury basin is a non cylindrical fold basin occupying the central portion of the sudbury impact structure the impact structure lends itself excellently to explore the structural evolution of continental crust containing a circular region of long term weakness in a series of scaled analogue experiments various model crustal configurations were shortened horizontally at a constant rate in mechanically weakened crust model basins formed that mimic several first order structural characteristics of the sudbury basin 1 asymmetric non cylindrical folding of the basin 2 structures indicating concentric shortening around lateral basin termini and 3 the presence of a zone of strain concentration near the hinge zones of model basins geometrically and kinematically this zone corresponds to the south range shear zone of the sudbury basin according to our experiments this shear zone is a direct mechanical consequence of basin formation rather than the result of thrusting following folding overall the models highlight the structurally anomalous character of the sudbury basin within the paleoproterozoic eastern penokean orogen in particular our models suggest that the basin formed by pure shear thickening of crust whereas transpressive deformation prevailed elsewhere in the orogen the model basin is deformed by thickening and non cylindrical synformal buckling while conjugate transpressive shear zones propagated away from its lateral tips this is consistent with pure shear deformation of a weak circular inclusion in a strong matrix the models suggest that the sudbury basin formed as a consequence of long term weakening of the upper crust by meteorite impact

troodos extrusive series of cyprus a comparison with oceanic crust the extrusive section on the northern flank of the troodos ophiolite is 1 000 to 1 500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia some pillowed units which average 100 to 200 m in thickness vary systematically from lava tubes up to 6 m in diameter at the base through common pillows less than 1 m in diameter to small pillows and extrusive breccias at the top these assemblages appear to be individual volcanoes suggesting episodic volcanic accretion the upper 100 to 200 m of the section is pervasively altered and oxidized but fresh volcanic glass is preserved throughout most of the extrusive section the lavas are subdivided on geochemical evidence into a lower differentiated andesite to dacitic andesite series resembling evolved island arc tholeiites and an upper series of picrite mafic basalt and basaltic andesite very low in tio2 k2o p2o5 nb rb and ba the highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process the occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion located above a subduction zone

effects of paleoweathering and supergene activity on volcanogenic massive sulfide vms mineralization in the penokean volcanic belt northern wisconsin michigan and east central minnesota usa implications for future exploration abstract three cycles of paleoweathering and supergene activity have been identified within the penokean volcanic belt pvb of northern wisconsin michigan and east central minnesota usa these cycles are responsible for significant subaerial weathering erosion oxidation and most importantly local supergene au ag and cu enrichment that has a major economic impact upon volcanogenic massive sulfide vms style mineralization within the pvb the paleoweathering profile regolith saprolite that developed on the exposed pvb surface now marks an angular unconformity with stratigraphically overlying late paleoproterozoic baraboo interval quartzites and late cambrian sandstones differential uplift of the pvb surface is a direct result of the 1850 ma compressional tectonic event that occurred during the penokean orogeny the uplifted pvb paleosurface experienced its first weathering cycle in the mid to late paleoproterozoic vms mineralization near the paleosurface was exhumed and affected during this protracted cycle which ceased upon deposition of baraboo interval quartzites 1788 1714 ma further uplift and subsequent unroofing of the quartzites during the mazatzal orogeny 1650 1630 ma initiated a second paleoweathering cycle that culminated within the neoproterozoic cambrian transition during this cycle of accelerated and uninterrupted paleoweathering new and surviving supergene au ag cu enriched profiles geochemically matured over exhumed mineralized sections in terrains with favorable fault block movements supergene systems that developed during both cycles were highly acidic environments low protolith reactivity and high hypogene pyrite concentrations promoted acidity of supergene fluids thus facilitating enhanced cu au and ag mobility in those systems that lacked sufficient carbonate zn and pb were lost to surrounding aqueous regimes the second paleoweathering cycle ended with the deposition of overlying late cambrian sandstone 497 485 4 ma erosional retreat of the overlying paleozoic sequence exposed many of these supergene deposits to a third possibly brief paleoweathering cycle before the onset of quaternary continental glaciation vms profiles not protected by sandstone outliers or within down dropped fault blocks were glacially eroded of the 14 or more known vms deposits and occurrences in the belt four have preserved or partially preserved paleoweathering profiles the most significant are at the flambeau and back forty deposits where au ag enriched indigenous gossans with extensive or local underlying supergene enriched cu sulfide zones have developed over hypogene massive sulfide horizons these profiles demonstrate efficient cumulative or multicyclic au ag and cu enrichment to ore grades although commonly small 2 million tonnes such supergene enriched deposits are generally shallow allowing for development by low cost open pit methods and production of exceptionally high grade direct shipping copper and or gold ores as such the buried pvb unconformity is considered a prime exploration target that is highly prospective for new discoveries of commercially viable supergene enriched vms resources

transpressive tectonism in the eastern penokean orogen canada consequences for proterozoic crustal kinematics and continental fragmentation abstract results of a structural analysis of archean basement huronian cover and various igneous rocks in the sudbury area ontario and regional tectonometamorphic correlations suggest that structures in the eastern penokean orogen formed dominantly by dextral transpression bulk horizontal shortening started with the ca 2 4 to 2 2 ga blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism during the 1 89 to 1 83 ga penokean orogeny horizontal dextral shear parallel to the curved axis of the eastern penokean orogen gained in importance and is most likely related to docking of proterozoic magmatic arc terranes to the superior province margin this tectonic activity led to distributed heterogeneous deformation of archean basement rocks and exhumation of deep crustal rocks in the sudbury area and the kapuskasing structural zone but may have failed to accrete paleoproterozoic terranes in the lake huron area the lack of evidence for horizontal extension following blezardian and penokean contraction suggests that crustal fragmentation of the archean supercontinent started at 2 45 ga finally the great lakes tectonic zone and the murray fault master dislocations in the penokean orogen may have acted as crustal weakness zones which influenced the orientation and development of 1 1 ga midcontinent rift segments

upb dating of a carbonate subaerial exposure event ten samples of the dolomite fraction from the lower ordovician oneota formation directly below the sauk tippecanoe unconformity in wisconsin define a 238u 206pb age of 504 14 ma mswd 190 206pb204pb 27 85 238u204pb 84 795 which is in agreement with the 500 4 ma depositional age based on conodont biostratigraphy the u pb age is interpreted as recording u enrichment of the carbonate during the early stages of subaerial exposure shortly following deposition variable initial pb isotope ratios prolonged interaction with meteoric fluids and later recrystallization associated with the influx of fluids that produced the permian upper mississippi valley pb zn sulfide deposits are likely factors in producing scatter about the isochron however the substantial u enrichment that can be associated with subaerial exposure of carbonates apparently allows precise u pb ages to be obtained despite some isotope mobility that occurs during later diagenetic events dolomite samples have 232th238u 0 3 much lower than that of average silicate crust consistent with the low solubility of th in low temperature systems and the high solubility of u in oxidized fluids 204pb206pb 208pb206pb variations in dolomite and oxide fractions from the oneota formation are linearly correlated 208pb206pb 38 955 204pb206pb 0 050 r2 0 9996 and are interpreted as indicating that the u and common pb were derived from penokean 1 8 ga crust exposed to the north i e the wisconsin dome or from its derivative sediments

p wave teleseismic traveltime tomography of the north american midcontinent the remains of the 1 1 ga midcontinent rift mcr lie in the middle of the tectonically stable portion of north america previous and ongoing studies have imaged strong heterogeneity associated with the mcr in the crust but have not imaged such within the mantle it is unclear whether this is due to the absence of rift related mantle structures or these studies had insufficient resolution to image them to address this issue we measured 46 374 teleseismic p wave delay times from seismograms recorded by the usarray transportable array superior province rifting earthscope experiment and surrounding permanent stations we included these and 54 866 delay times from prior studies in our tomographic inversion we find that high velocity anomalies are widespread in our study area but there are also prominent low velocity anomalies two of these are coincident with high bouguer gravity anomalies associated with the mcr in iowa and the minnesota wisconsin border at 50 to 150 km depth extensive resolution testing shows that these anomalies could be the result of downward vertical smearing of relatively low velocities from rift related material that underplated the crust although we cannot exclude that the subcrustal mantle lithosphere beneath the mcr is anomalously enriched hydrated or warm other anomalies occur at syntaxes of the penokean orogen one with the superior province and marshfield terrane in southern minnesota and another with the yavapai and mazatzal terranes both at 100 to 250 km depth in the midmantle we image two linear high velocity anomalies interpreted as subducted fragments of the farallon and kula plates