single crystal 40ar 39ar dating of the olorgesailie formation southern kenya rift single crystal laser fusion sup 40 ar sup 39 ar analyses and several conventional bulk fusion sup 40 k sup 40 ar dates have been used to determine the age of volcaniclastic strata within the olorgesailie formation and of associated volcanic and sedimentary units of the southern kenya rift in the principal exposures along the southern edge of the legemunge plain the formation spans the interval from approximately 500 to 1 000 ka deposition continued to the east along the ol keju nyiro river where a tuff near the top of the formation has been dated at 215 ka in these exposures the formation is unconformably overlain by sediments dated at 49 ka a possible source for the olorgesailie tephra the ol doinyo nyokie volcanic complex contains as ash flow dated at approximately 1 ma extending the known age range of this complex to encompass that of virtually the entire olorgesailie formation in the legemunge plain these geologic examples illustrate the importance of the single crystal sup 40 ar sup 39 ar dating technique whereby contaminant altered or otherwise aberrant grains can be identified and eliminated from the determination of eruptive ages for reworked or altered pyroclastic deposits the authors have presented a computer modeling procedure based on an inverse isochron analysis thatmore promotes a more objective approach to trimming sup 40 ar sup 39 ar isotope data sets of this type less

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

dispersion of channel sediment contaminants in distributary fluvial systems application to fluvial tephra and radionuclide redistribution following a potential volcanic eruption at yucca mountain abstract predicting the fluvial transport and mixing of channel sediment contaminants is necessary for assessing and mitigating heavy metal and nuclear waste contamination in rivers the dilution mixing model is widely used for this purpose in tributary channel systems that transport contaminants as bed material load without significant overbank sedimentation here a more general three dimensional 3d contaminant transport numerical model is developed and tested based on bed scour turbulent mixing of contaminant material with uncontaminated channel bed sediments and re deposition of the mixture by the cumulative effect of many flood events first the model is applied to a synthetic alluvial fan environment downstream from a localized contaminant source in order to illustrate the model behavior second the model is validated against measured tephra concentrations in channels downstream from the lathrop wells scoria cone volcano a localized source of basaltic tephra to downstream channels otherwise comprised of non basaltic sediments third the model is applied to the problem of predicting the concentration of radionuclide bound tephra in channels downstream from the proposed nuclear waste repository at yucca mountain nevada in the event of a volcanic eruption through the repository contaminated tephra is mobilized from the landscape in this model using threshold criteria for hillslope gradient and channel stream power mobilized contaminated tephra is mixed with uncontaminated channel bed sediments using the contaminant transport model and deposited in channels of the fortymile wash alluvial fan where the residents nearest to the proposed repository live the results of twenty monte carlo simulations of eruption fallout and post eruption redistribution corresponding to a range of wind conditions and eruption magnitudes provide information on the mean and variability of contaminated tephra concentrations to be expected in channels of the fortymile wash alluvial fan in the event of an eruption mean tephra concentrations are approximately 1 but vary from nearly zero to as high as 26 reflecting the combined effects of wind direction eruption magnitude and dilution of tephra with uncontaminated channel bed sediments during transport

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

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

the age of lathrop wells volcanic center an 40ar 39ar dating investigation the sup 40 ar sup 39 ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the lathrop wells volcanic center nevada yield eruption ages of 77 3 plus minus 6 0 and 76 6 plus minus 4 9 ka 2 sigma for the ql sub 1 and ql sub 2 flows respectively basaltic samples were analyzed in four to six replicate runs and yield normally distributed plateau ages miocene ash flow tuff xenoliths were collected from the basaltic units with the majority of samples coming from the ql sub 2 the sanidines from the ql sub 2 xenoliths yielded apparent plateau ages n 10 ranging from 69 plus minus 17 to 309 plus minus 11 ka with a strong cluster n 6 at approximately 77 ka because the diffusion dimension of sanidine is approximated by the physical crystal size cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and hf acid etching lead to anomalously old apparent ages these new ages for the ql sub 1 and ql sub 2 flows are analytically indistinguishable and agree well with published eruption ages determined by sup 3 he and sup 36 cl cosmogenic surface exposure dating methods we conclude that the ql submore 1 and ql sub 2 flows are temporally equivalent additionally the agreement between our eruption ages for these units and the published 76 plus minus 20 ka ital zreda ital et al 1993 sup 36 cl exposure age for the lathrop wells scoria cone qs sub 3 suggests that the cone may be temporally equivalent to the ql sub 1 and ql sub 2 flows we believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic copyright 1999 american geophysical union less

crustal formation in the grenville province nd isotope evidence ninety eight new nd isotope analyses are presented for gneissic rocks from the grenville province when combined with over 250 published nd analyses and some unpublished analyses these data can be used to establish a first order crustal formation age map for most of the grenville parautochthon and allochthonous polycyclic belt in particular the geographic extents of a juvenile labradorian arc labradoria and a juvenile 1 5 ga arc quebecia are defined each with an area probably in excess of 100 000 km2 in addition pre 1 75 ga arcs are identified in ontario barilia and labrador makkovikia this work largely fills the last major gap in the continent wide crustal formation age map of the precambrian shield of north america

modeling long term contamination in river systems from historical metal mining heavy metal contamination of sediments within river systems is a major environmental problem around the world deposited as a by product of metal mining contaminated sediments are persistent and widespread frequently affecting large areas of floodplains which makes precise assessments of contamination levels and patterns difficult this paper describes findings from a new generic catchment sediment model called tracer which uses historical mining records to accurately predict present day and future levels and patterns of contamination this model provides detailed views of the extent of contami nation and demonstrates how contaminated sediments form into hot spots which in turn become secondary sources of pollution the exceptional longevity of the contamina tion is also revealed 70 of the deposited contaminants remain within the river system for 200 yr after mine closure simulations of the impact of future climate changes in northern england show that increased flood magnitudes are likely to decrease surface contamination through dilution by cleaner sediment from hillslopes unaffected by mining activity

northeast extension of proterozoic terranes of mid continental north america granite and rhyolite ranging in age from 1 8 1 34 m y are known to underlie much of mid continental north america west of the phanerozoic michigan basin u pb zircon ages of similar rocks exposed adjacent to the grenville province in ontario suggest that the mid continental terranes extend northeast beneath and beyond the michigan basin prevalence of similar u pb zircon ages obtained from metaplutonic rocks southeast of the grenville front and affected by the 1 2 1 0 ga grenvillian orogeny prompts the interpretation that deformed equivalents of the mid continental proterozoic rocks form the bulk of the crust in the central gneiss belt of the grenville province east of lake huron farther east pre 1 34 ga terranes appear to be tectonically occluded by 1 28 1 22 ga volcanic sedimentary and plutonic rocks of the central metasedimentary belt themselves deformed metamorphosed and intruded by younger grenvillian plutons

evidence for widespread approximately 1760 ma metamorphism and rapid crustal stabilization of the early proterozoic 1870 1820 ma penokean orogen minnesota

geomorphic controls on contaminant distribution along an ephemeral stream sediment borne contamination in a watershed can be highly variable as a result of uvial processes operating over a range of time scales this study presents a detailed analysis of the distribution of one contaminant along an ephemeral stream after 55 years of sediment transport deposition and exchange by ash oods wastewater containing plutonium was discharged into the pueblo canyon watershed from 1945 until 1964 and plutonium concentrations in uvial deposits vary over ve orders of magnitude these variations can be attributed to three primary factors time since contaminant releases particle size sorting and mixing of sediment from different sources the highest concentrations occur in ne grained sediment deposits near the source that date to the period of ef uent releases and concentrations are lower in younger deposits in coarser grained deposits and in deposits farther downstream the spatial distribution of plutonium is strongly affected by longitudinal variations in the size of sediment deposits of different age a major aggradation degradation cycle in the lower canyon overlapped with the period of active ef uent releases and a signi cant portion of the total plutonium inventory is contained within large coarse grained deposits below ll terraces that post date 1945 the spatial pattern of contamination is thus determined by the speci c geomorphic history of the watershed in addition to processes of mixing and sorting during transport that occur in all uvial systems copyright 2004 john wiley sons ltd

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

archean and proterozoic tectono magmatic activity along the southern margin of the superior province in northwestern iowa united states a precambrian igneous body of ultramafic and mafic rocks named the otter creek layered igneous complex occurs within the basement of northwestern iowa united states it is marked by a circular magnetic anomaly one of several that lie north and west of an inferred suture between the archean superior province and early proterozoic juvenile crust sm nd whole rock analyses for several rock types from the otter creek complex yield an isochron age of 2890 90 ma with an end t of 0 9 2 4 a block of older banded iron formation itself intruded by lamprophyre dikes is contained within the layered sequence the iron formation lamprophyre block has undergone high temperature metamorphism followed by a retrograde event a quartz monzodiorite gneiss with a u pb age of 2523 5 ma occurs near the layered complex but the contact relations are not known the layered series is overlain by proterozoic keratophyre with a u pb age of 1782 10 ma these felsic pyroclastic rocks are extremely depleted in k

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

early proterozoic central plains orogen a major buried structure in the north central united states a major early proterozoic orogen central plains more than 1000 km long and at least 500 km wide has been delineated in the subsurface of nebraska kansas and missouri as a result of a new compilation of precambrian basement drill data in the northern midcontinent the orogen is composed of metamorphic and granitoid rocks in the ringe 1 63 1 80 ga and it is interpreted on the basis of isotopic ages rock types and structure as reflected by magnetic and gravity anomalies as being the eastward extension of the early proterozoic foldbelt exposed in basement uplifts in colorado and southeastern wyoming it truncates the southern margin of the superior archean craton and its marginal early proterozoic orogens the penokean 1 83 1 90 ga and the trans hudson 1 85 1 95 ga the pattern of intersecting proterozoic mobile belts marginal to archean cratons is comparable to that in other continents such as western australia and southern africa

paleoproterozoic rocks of central colorado accreted arcs or extended older crust paleoproterozoic 1770 1735 ma bimodal volcanic rocks in central colorado have been considered to represent southward growth of laurentia by arc accretion although the bimodality of these rocks suggests an extensional continental setting rather than continental or oceanic arcs there has been little evidence for pre 1800 ma crust south of the wyoming craton other than the 1840 1 ma elves chasm pluton in the upper granite gorge of the grand canyon arizona we report shrimp u pb ages of inherited zircons from metarhyolites and plutons in central colorado that are latest archean earliest proterozoic 2520 2000 ma and trans hudson penokean 1878 1814 ma associated quartzites contain detrital zircons with mean ages of 1735 ma indicating only local derivation a meta arkose however contains detrital zircons of trans hudson penokean and archean ages we believe it likely that the 1900 1800 ma trans hudson penokean orogens including archean enclaves extended farther south and west than is currently thought and were the source of the bimodal volcanic rocks and associated plutons during the period 1770 1700 ma

interpretation of monazite ages obtained via in situ analysis abstract monazite grains from nepal and vietnam were compositionally analyzed with an electron microprobe and dated th pb with an ion microprobe five sources of uncertainty explain age distributions from single samples that appear inconsistent with a single population 1 pb loss due to diffusion 2 dissolution reprecipitation reactions along a retrograde path 3 analytical uncertainties 4 analyses of overlapping age domains and 5 episodic monazite growth the influence of these factors is sample dependent but can be evaluated 1 using peak metamorphic conditions and x ray element maps to assess potential polymetamorphism or retrogression 2 obtaining other geochronologic data including previous work or dating other minerals in the sample 3 evaluating any method related uncertainty including counting statistics for electron microprobe analyses or calibration reproducibility for ion microprobe analyses and 4 ascertaining the potential growth mechanism of the monazite grain including dissolution of detrital grains or production from rare earth element ree oxide or allanite chemical contents of monazite grains analyzed in this study fail to reflect timing information or mineral growth mechanisms instead of relying on monazite chemical composition major mn fe mg ca and minor y element garnet zoning patterns and peak p t conditions should be used to facilitate age interpretation this thermobarometric data records the sample s thermal history changes in garnet growth rate and mechanisms and accessory mineral breakdown

the timing of partial melting barrovian metamorphism and granite intrusion in the naxos metamorphic core complex cyclades aegean sea greece abstract the relationship between partial melting and granite intrusion in a classic barrovian metamorphic terrane has been assessed thirteen samples were dated by shrimp u pb zircon geochronology from the island of naxos greece one of the aegean metamorphic core complexes the effect of partial melting during peak barrovian metamorphism on naxos is recorded by fine

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

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

core complex model proposed for gneiss dome development during collapse of the paleoproterozoic penokean orogen minnesota gneiss domes preserved throughout the paleoproterozoic 1870 1820 ma penokean orogenic belt are commonly viewed as collisional features however 1700 ma mica 40ar 39ar ages from archean rocks of the mcgrath gneiss dome minnesota are in marked contrast with 1750 1760 ma mica ages obtained everywhere outside the dome suggesting the dome may have formed well after compression had ceased we hypothesize that the bimodal age pattern is a result of crustal excision between archean basement and paleoproterozoic cover during extensional collapse of the overthickened orogen removal of structural section and progressive unroofing may have led to the development of a domed basement cover shear zone in this respect the mcgrath gneiss dome would represent one of the oldest analogues of a metamorphic core complex postorogenic collapse of the penokean orogen was apparently long lived with significant pulses occurring at 1755 ma and at 1700 ma the sudden and widespread crustal fusion at 1770 1760 ma that preceded the proposed collapse suggests that rapid subcrustal lithospheric thinning may have triggered collapse as has been proposed recently for several phanerozoic collapsed orogens

reinterpretation of paleoproterozoic accretionary boundaries of the north central united states based on a new aeromagnetic geologic compilation abstract the paleoproterozoic crust in the north central u s represents intact juvenile terranes accreted to the rifted archean superior craton a new tectonic province map based on the interpretation of a new aeromagnetic compilation published geologic maps and recent geochronologic data shows progressive accretion of juvenile arc terranes from ca 1900 1600 ma contrary to earlier models geon 18 penokean interval crust is primarily confined to a 2100 ma tectonic embayment of the rifted superior craton the newly defined spirit lake tectonic zone characterized by a sharp magnetic discontinuity that marks the southern limit of archean and penokean interval rocks is here interpreted to represent an eastern analog of the cheyenne belt suture zone in southern wyoming south of this boundary geon 17 yavapai interval rocks form the basement upon which 1750 ma rhyolite and succeeding quartzite sequences were deposited substantial portions of the penokean and yavapai terranes were subsequently deformed during the 1650 1630 ma mazatzal orogeny the northern boundary of the mazatzal terrane is obscured by abundant 1470 1430 ma anorogenic plutons that stitched the suture with the older yavapai terrane rocks these data reveal a progressive tectonic younging to the south as the laurentian craton grew southward and stabilized during the proterozoic late mesoproterozoic rift magmatism produced pronounced geophysical anomalies indicating strong but localized crustal modification in comparison to the western u s little tectonism has occurred here in the last 1 billion years providing a uniquely preserved record of the precambrian evolution of the continental u s lithosphere

age and significance of precambrian basement samples from northern illinois and adjacent states geochronologic results are reported for 12 localities where deep drilling has penetrated precambrian basement in iowa illinois indiana michigan and kentucky for the northern illinois deep drill holes uph 1 and uph 3 a zircon u pb age of 1463 10 m y was obtained rb sr data for these samples indicate mild disturbance of this system but are not interpreted in terms of any specific younger event u pb ages for other localities fall in the interval 1450 1510 m y and indicate that this was a major period of igneous activity throughout the southern great lakes region rb sr results indicate that alteration was ubiquitous and that most ages in the range 1100 1400 m y probably represent disturbed systems the results obtained so far tend to support interpretations in which kentucky indiana and illinois are primarily underlain by a rhyolite and epizonal granite terrane that is 1450 1500 m y old older proterozoic rocks may be absent or more deeply buried

u pb zircon ages from athapuscow aulacogen east arm of great slave lake n w t canada athapuscow aulacogen is an early proterozoic intracratonic basin located in the east arm of great slave lake between the slave and northwest churchill provinces athapuscow aulacogen comprises three stratigraphic sequences the wilson island group the great slave supergroup and the et then group new u pb zircon ages provide constraints on the development of the aulacogen the blachford lake intrusive suite consists of an older alkaline phase hearne channel granite dated at 2175 7 ma and a younger peralkaline phase thor lake syenite dated at 2094 10 ma confirming the suggestion that the two phases may not be related a felsite from the wilson island group has an age of 1928 11 ma the wilson island group is intruded by epizonal granites butte island intrusive suite one of which has an age of 1895 8 ma the wilson island group and the butte island instrusive suite are entirely allochthonous with respect to the slave craton rocks of the great slave supergroup overlie mylonitized wilson isl

stochastic theory of particle trajectories through alluvial valley floors abstract temporary storage of sediment within alluvial valley floors modulates the long term transport of sediment through landscapes the fate of weathering minerals or sediment bound constituents in fluvial environments depends on the relative time scales of constituent degradation and particle residence time within valleys particles follow a set of trajectories through valley floors some particles pass directly through the channel reaching the basin outlet rapidly after being introduced to the fluvial system others remain for long periods in deposits such as flood plains traditional sediment routing theory based on the principle of sediment mass conservation along reaches of channel does not account for exchanges of sediment with temporary sediment storage reservoirs outside the channel such as flood plains deltas and alluvial fans this article formalizes a theory that incorporates the role of such exchanges in the migration of sediment through river systems by computing the probabilistic s

evaluating suitability of a tephra dispersal model as part of a risk assessment framework in volcanic risk assessment it is necessary to determine the appropriate level of sophistication for a given predictive model within the contexts of multiple sources of uncertainty and coupling between models a component of volcanic risk assessment for the proposed radioactive waste repository at yucca mountain nevada usa involves prediction of dispersal of contaminated tephra during violent strombolian eruptions and the subsequent transport of that tephra toward a hypothetical individual via surface processes we test the suitability of a simplified model for volcanic plume transport and fallout tephra deposition ashplume coupled to a surface sediment transport model far that calculates the redistribution of tephra and in light of inherent uncertainties in the system the study focuses on two simplifying assumptions in the ashplume model 1 constant eruptive column height and 2 constant wind speed and direction during an eruption variations in tephra dispersal resulting from unsteady column height and wind conditions produced variations up to a factor of two in the concentration of tephra in sediment transported to the control population however the effects of watershed geometry and terrain which control local remobilization of tephra overprint sensitivities to eruption parameters because the combination of models used here shows limited sensitivity to the actual details of ash fall a simple fall model suffices to estimate tephra mass delivered to the hypothetical individual

early archaean component 3 5 ga within a 3 05 ga orthogneiss from northern nigeria upb zircon evidence abstract conventional u pb analyses on selected single zircon grains and on zircon fractions from a granodioritic orthogneiss at kaduna northern nigeria show that the nigerian basement contains archaean components that have not been recognized until now a distinct u rich zircon group records an age of 3040 60 ma which is interpreted as the age of emplacement and crystallization of the granodioritic magma the occurrence of a magmatic event at ca 3000 ma is confirmed by the addition of core free abraded grains 3050 23 ma these ages are identical to the less precise value 2980 90 ma given by zircon fractions these results also demonstrate that zircons underwent pb loss during the pan african orogeny at 612 23 ma which is well supported by an identical age on a single monazite grain 618 8 ma moreover u pb analyses of abraded u poor core bearing crystals clearly identify the presence of source material of early archaean age 3500 ma the oldest age for crust initiating processes ever found in this area in contrast to earlier rb sr geochronology no evidence for an eburnean 2000 200 ma or kibaran 1100 200 ma orogeny is present in the u pb systems of these zircons

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

gneiss domes and orogeny many gneiss domes record positive feedback between decompression and partial melting of orogenic middle crust exhumed orogens are riddled with gneiss domes cored by migmatites that underwent dehydration melting during decompression the decreasing buoyancy associated with increasing melt fraction drives further decompression at near isothermal conditions as the partially molten crust rises diapirically this combination of processes may explain the generation and retention of large volumes of crustally derived melt recorded in many deep seated migmatite terranes and inferred for active orogens in exhumed orogens the signature of the rapid ascent of partially molten crust is a gneiss dome cored by migmatite granite the large volume of material involved in the vertical transfer of partially molten crust indicates that the formation of gneiss domes is an efficient mechanism for heat advection during orogenesis

coral oxygen isotope records of interdecadal climate variations in the south pacific convergence zone region 1 the south pacific convergence zone spcz a region of high rainfall is a major feature of subtropical southern hemisphere climate and contributes to and interacts with circulation features across the pacific yet its past temporal variability and forcing remain only partially understood here we compare coral oxygen isotopic 18o series spanning a d 1997 1780 and a d 2001 1776 from two genera of hermatypic corals in fiji located within the spcz to examine the fidelity of these corals in recording climate change and spcz interdecadal dynamics one of these coral records is a new 225 year subannually resolved 18o series from the massive coral diploastreaheliopora diploastrea s use in climate reconstructions is still relatively new but this coral has shown encouragingly similar interannual variability to porites the coral genus most commonly used in pacific paleoclimate studies in fiji we observe that interdecadal 18o variance is also similar in these two coral genera and diploastrea contains a larger amplitude interdecadal signal that more closely tracks instrumental based indices of pacific interdecadal climate change and the spcz than porites both coral 18o series record greater interdecadal variability from 1880 to 1950 which is consistent with the observations of folland et al 2002 who reported higher variability in spcz position before 1945 these observations indicate that diploastrea will likely provide a significant new source of long term climate information from the spcz region

persistent drainage migration in a numerical landscape evolution model 1 numerical landscape evolution models driven by uniform vertical uplift often develop a static drainage network and a precise balance between uplift and erosion small scale physical models of uplifting drainage basins however continually reorganize by ridge migration and do not reach an ideal steady state here i show that the presence or absence of persistent drainage migration in a bedrock numerical landform evolution model depends on the flow routing algorithm used to determine upstream contributing area the model version that uses steepest descent routing achieves an ideal steady state while the model version that uses bifurcation routing results in continually evolving drainage basins even under conditions of uniform vertical uplift bedrock erodibility precipitation and landsliding threshold this result suggests that persistent drainage migration can occur by erosional processes alone this result has important implications for numerical modeling methodology our understanding of the natural variability of landform evolution and the interpretation of thermochronological data

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

relict colluvial boulder deposits as paleoclimatic indicators in the yucca mountain region southern nevada early to middle pleistocene boulder deposits are common features on southern nevada hillslopes these darkly varnished ancient colluvial deposits stand but in stark contrast to the underlying light colored bedrock of volcanic tuffs and they serve as minor divides between drainage channels on modern hillslopes to demonstrate the antiquity of these stable hillslope features six colluvial boulder deposits from yucca mountain nye county nevada were dated by cation ratio dating of rock varnish accreted on boulder surfaces estimated minimum ages of these boulder deposits range from 760 to 170 ka five additional older deposits on nearby skull and little skull mountains and buckboard mesa yielded cation ratio minimum age estimates of 1 38 ma to 800 ka an independent cosmogenic chlorine 36 surface exposure date was obtained on one deposit which confirms an estimated early to middle quaternary age these deposits have provided the oldest age estimates for unconsolidated hillslope deposits in the southwestern united states we suggest that the colluvial boulder deposits were produced during early and middle pleistocene glacial pluvial episodes and were stabilized during the transition to drier interglacial climates by comparison to modern periglacial environments winter minimum monthly temperatures of 3 to 5 c were necessary to initiate freeze thaw conditions of such vigor to physically weather relatively large volumes of large boulders from the upper hillslopes of the yucca mountain area these conditions imply that early and middle pleistocene glacial winter temperatures were at least 1 to 3 c colder than existed during the last pleistocene glacial episode and 7 to 9 c colder than present we conclude that at least several early and middle pleistocene glacial episodes were colder and perhaps wetter than glacial episodes of the late pleistocene in the southern great basin geomorphic processes necessary to form these colluvial boulder deposits are not active on modern hillslopes in the southern great basin in addition the lack of young relatively unvarnished colluvial boulder deposits on these hillslopes suggests that boulder forming conditions did not exist during the late pleistocene in this region modern semiarid hillslope processes primarily erode colluvium during infrequent high intensity storms the preservation of old thin hillslope deposits and the less than 2 m incision by hillslope runoff adjacent to these deposits however indicate that extremely low denudation rates have occurred on resistant volcanic hillslopes in the southern great basin during quaternary time

geochemistry and evolution of the early proterozoic post penokean rhyolites granites and related rocks of south central wisconsin u s a

copper dispersion in ephemeral stream sediments sediment transport related parameters in ephemeral streams may be used to model and delineate 1 average dispersion patterns of copper laden sediments 2 differences in dispersion of copper in bedload and suspended sediments and 3 variability in the copper sediment dispersion patterns a model that effectively describes dispersion of copper in ephemeral stream sediments in a simple mixing model where cr is the resultant concentration beneath the confluence of the main channel with a tributary ct is the concentration of metal in sediments of the tributary cm is the metal concentration in main channel sediments and xm and xt are the basin areas or sediment yields of the main channel and tributary channel at their confluence variability in metal concentrations about values predicted by this model may be due to the different responses of bedload and suspended load to changes in stream hydraulics the dynamics of bedload transport the spatial and temporal variability rainfall within the drainage basin and chemical mobility of the copper

precambrian basement geology of north and south dakota combined analysis of drill hole gravity and magnetic data indicates that the buried precambrian basement rocks of the dakotas can be divided into several lithotectonic terranes eastern north dakota and northeastern south dakota are underlain by archean gneiss except for the black hills region of south dakota where archean rocks are also exposed the western third of both dakotas is underlain mainly by early proterozoic gneiss and metasedimentary rocks part of this region is underlain by archean crust with an early proterozoic tectonic overprint a broad transition zone of strongly overprinted archean crust occurs between the proterozoic rocks to the west and the archean rocks to the east south central south dakota is underlain by an early proterozoic batholith early proterozoic felsic volcanic rocks occur in southeast south dakota the bootheel portion of south dakota contains a diverse assemblage of basement rocks that are partly archean in age churchill province rocks of the trans hudson foldbel

geochronology of proterozoic metamorphism in the deformed southern province northern lake huron region canada abstract metamorphic geochronology and thermochronology of the deformed southern province north of lake huron in ontario canada reveals a complex and protracted tectonothermal history for the proterozoic southern laurentide margin that is distinct from that of the lake superior region to the west while penokean interval deformation is well constrained in the lake superior area a well defined geon 18 geochronometric signature is lacking in the huronian vicinity the oldest metamorphic ages revealed by in situ monazite u th total pb electron microprobe analyses are ca 1800 ma with a strong geon 17 yavapai interval and local geon 14 15 magmatic influences furthermore conventional 40 ar 39 ar incremental step heating of hornblende indicates paleoproterozoic heating through 500 c occurred only adjacent to geon 17 intrusions regional analysis of mica cooling ages suggests that mesoproterozoic thermal overprinting of primary geon 17 magmatism and metamorphism is limited to south of the murray fault the penokean orogeny long considered the dominant paleoproterozoic event in the great lakes region is now recognized as only the first of several accretionary events that impinged on the southern laurentide margin however the penokean orogen is restricted to the 2100 ma embayment of the superior province rifted margin moreover subsequent tectonothermal pulses yavapai mazatzal and ca 1450 ma recorded in the rocks of the southern province in ontario mimic that documented across much of the central and western united states based on the distinct lack of penokean interval ages and the strong geon 17 and geon 14 total pb and 40 ar 39 ar geochronometric signature the tectonothermal history of southern province more closely resembles that of the southwest united states than the lake superior region

the role of geomorphic processes in the transport and fate of mercury in the carson river basin west central nevada the historic processing of precious metal ores mined from the comstock lode of west central nevada resulted in the release of substantial but unquantified amounts of mercury contaminated mill tailings to the carson river basin geomorphic and stratigraphic studies indicate that the introduction of these waste materials led to a period of valley floor aggradation that was accompanied by lateral channel instability the combined result of these geomorphic responses was the storage of large volumes of mercury enriched sediment within a complexly structured alluvial sequence located along the carson river valley much of the contaminated sediment is associated with filled paleochannels produced by the cutoff and abandonment of meander loops and their subsequent infilling with contaminated particles geochemically these deposits are characterized by variations in mercury levels that exceed three orders of magnitude continued lateral instability coupled with an episode of channel bed incision followed the decline of comstock mining and has reexposed contaminated debris within the banks of the river erosion of bank sediments reintroduces mercury enriched particles to the modern channel bed it is suggested on the basis of geochemical and sedimentological data that during the bank erosion process much of the mercury associated with fine 63 valley fill deposits are carried downstream without being incorporated to any appreciable extent within the channel bed sediments in contrast mercury associated with larger and denser particles particularly mercury gold silver amalgam grains are accumulated in the channel bed sediments as the river traverses polluted reaches of the carson river valley concentration patterns developed along the modern channel indicate that the valley fill is the primary source of mercury to the river today thus these data imply that efforts to reduce the influx of mercury to the aquatic environment should examine methods for reducing bank erosion rates

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record

u pb geochronology of the wolf river batholith north central wisconsin evidence for successive magmatism between 1484ma and 1468ma abstract single grain zircon u pb age determinations were completed on individual phases of the mesoproterozoic anorogenic wolf river batholith wrb of wisconsin and the nearby wausau syenite complex in order to a define the age more precisely and accurately b determine whether there is a resolvable age difference within units of the wrb and c determine whether the wausau syenite complex is coeval with any or all parts of the wrb u pb results for many individual zircon grains from the wrb plot near concordia and yield ages ranging from 1468 4 ma to 1484 2 ma plutons of the northeastern lobe of the wrb peshtigo monzonite hager porphyry belongia granite and high falls granite have a weighted average crystallization age of 1470 3 1 2 ma which is younger than the main complex plutons wolf river granite wiborgite porphyry red river granite and waupaca granite which yields a weighted average crystallization age of 1475 6 1 7 ma excluding one sample of wolf river granite from hogarty which yields an age of 1484 2 ma two samples from the main complex also contain zircons with younger apparent ages of 1470 1471 ma these younger grains may reflect late stage alteration of some zircons coeval with emplacement of the northeastern lobe the wausau syenite and the nine mile granite of the nearby wausau syenite complex 1522 ma and 1506 ma respectively are significantly older than the wrb and definitely not coeval or comagmatic the syenite complex may be a precursor to the larger wrb magmatism sm nd t dm ages for most wrb samples indicate crust formation ages from 1 8 ga to 1 9 ga compatible with formation of the magmas from late paleoproterozoic penokean crust one unit which locally intrudes archean gneiss has a small component of that older rock involved in its evolution the two wausau complex units have younger tdm ages and appear to have incorporated slightly greater amounts of juvenile constituents

identification of 2 0 to 2 4 ga nd model age crustal material in the cheyenne belt southeastern wyoming implications for proterozoic accretionary tectonics at the southern margin of the wyoming craton a detailed survey was conducted of the nd isotope compositions of precambrian metamorphic rocks both within and adjacent to the cheyenne belt a metamorphosed crustal segment in southeastern wyoming that defines the proterozoic suture zone between the archean wyoming craton and accreted proterozoic island arc s the results of the study indicate that proterozoic miogeoclinal sedimentary rocks exposed north of the cheyenne belt and deposited at the southern margin of the wyoming craton contain only archean crust derived material and no detritus from proterozoic island arcs approaching from the south similarly proterozoic crust exposed south of the cheyenne belt contains no crustal material derived from archean sources however within the cheyenne belt paragneiss of the barber lake block has 2 0 to 2 4 ga nd model ages and is interpreted as metamorphosed intermixed sedimentary detritus from both archean and proterozoic sources the paragneiss is thought to represent remnants of oceanic trench or foredeep sediments deposited in close proximity to both the archean craton and the proterozoic island arcs similar to sedimentary rocks with 2 0 to 2 4 ga nd model ages exposed in the penokean orogen in wisconsin the overall lack of 2 0 to 2 4 ga modal age crustal material in the vicinity of the cheyenne belt may reflect the fact that much of this crustal material was overthrust onto the wyoming craton during accretion and then eroded during postorogenic rebound of the continental margin

quimby iowa scientific drill hole definition of precambrian crustal features in northwestern iowa geophysical data and sparse subsurface samples suggest that the spirit lake trend slt in the aeromagnetic fabric of northwest iowa is the southern boundary of the archean superior craton several small diameter positive aeromagnetic anomalies south of the slt have been interpreted as middle proterozoic anorogenic magnetite bearing granites that intruded juvenile early proterozoic crust a scientific drill hole 609 m deep was drilled into a 1200 gamma aeromagnetic anomaly near quimby iowa to test these interpretations precambrian basement was reached at a depth of 510 m and 99 m of undeformed megacrystic magnetite bearing granite core was collected petrographic and chemical data are consistent with derivation of the granite magma by melting of preexisting tonalitic continental crust a u pb zircon age of 1433 4 ma shows that this granite is part of the older anorogenic granite rhyolite suite of the united states sm nd data yield a depleted mantle separation age of 1860 ma which confirms that the source for the pluton magma was early proterozoic crust all data are consistent with the hypothesis that the aeromagnetic anomalies are caused by middle proterozoic anorogenic plutons and with interpretation of the slt as an archean early proterozoic crustal boundary