igneous and tectonic evolution of the batchawana greenstone belt superior province a u pb zircon and titanite study u pb isotopic dating of zircon and titanite from all the major litho tectonic units of the batchawana belt an archean greenstone belt of the abitibi subprovince of the superior province in canada shows that the belt evolved during a period of about 60 ma between about 2730 and 2670 ma ago the earliest date of 2729 3 2 ma was found in a predominantly tholeiitic metavolcanic sequence the base of which is intruded by a tonalite dated at 2716 2 ma younger ages of 2720 10 ma 2709 2ma 2711 2ma 2701 2ma and 2698 2 ma date rocks at increasingly higher stratigraphic levels of a mixed tholeiitic calc alkalic metavolcanic sequence which is itself overlain by a thick succession of clastic metasedimentary rocks subsequent deformation of the supracrustal sequences produced isoclinal folding and culminated in metamorphism ranging from lower greenschist to amphibolite facies and anatexis related to the intrusion of syn to late tectonic plutons four phases of which have ages of 2678 4 2ma 2677

late archean quetico accretionary complex superior province canada the 1200 km long quetico subprovince consists of monotonous metagraywacke with derived migmatite and granite in thrust and or transcurrent fault contact with the adjacent wabigoon and wawa metavolcanic subprovinces imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10 100 km wide stratigraphically north facing pile thermal relaxation in the 20 m y following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low pressure metamorphism

rock surface modeling as a tool to assess the morphology of inland notches mount carmel israel abstract inland notches are elongated c shape indentations carved into hard dense carbonate rocks of the mediterranean climate zone in recent years these unique features became of particular interest as geoindicators of denudation and slope erosion processes the study goals are to learn the processes acting upon the notches cavity by 1 examining the micromorphology of the backwall 2 identifying and classifying zones of erosion 3 learning the surface flow pattern and 4 proposing an erosion mechanism induced by exfoliation to this end a machine learning algorithm was designed using the microstructure texture streamlines formation and potential focal flow the workflow uses data from terrestrial laser scanning applies high resolution textural and curvature analyses and trains an artificial neural network to the different segments of the notch the reconstructed surface model shows division into three distinct belts within the cavity surface 1 the spalled belt sb 2 the transition zone belt tz and 3 the deposition belt db each belt owes its distinct microtopography implying on different geomorphic processes the newly developed methodology improves the recognition of streamline networks along the cavity backwall which mimic alluvial networks the organization of the reconstructed micro channels is similar to drainage basin systems where first order channels developed upstream converge into larger ones and diverge as distributaries as the gradient decreases depositing tufa along the notches base

archean gold mineralization in the wabigoon subprovince a product of crustal accretion evidence from u pb geochronology in the lake of the woods area superior province canada within the lake of the woods greenstone terrain in the wabigoon subprovince of northwest ontario gold is in faults shears and tension veins that developed in response to a late archean northwest directed contraction and emplacement of contemporaneous plutons nine rocks in this area have been dated using u pb analyses of zircon in order to obtain precise time constraints on gold mineralization and to compare its timing with the age of major crustal events in the region a mineralized dike at the duport mine shoal lake is 2716 pm 2 ma old whereas a later dike that cuts mineralized rocks has an age of 2709 pm 2 ma gold mineralization is therefore distinctly younger than calc alkaline and tholeiitic rocks dated at 2724 pm 2 ma in the shoal lake area and 2727 pm 2 ma in the high lake area to the north hydrothermal activity associated with mineralization at the duport mine overprints the amphibolite facies contact metamorphic aureole of the snowshoe bay batholith 2708 pm 7 ma

evolution of an archean subprovince boundary a sedimentological and structural study of part of the wabigoon quetico boundary in northern ontario a prograded clastic wedge represented by formation to group scale units of metasedimentary strata in the beardmore geraldton terrane developed in a transitional zone at the southern margin of a

stratospheric warmings during february and march 1993 two stratospheric warmings during february and march 1993 are described using ukmo analyses calculated pv and diabatic heating and n sub 2 o observed by the claes instrument on the uars the first warming affected temperatures over a larger region while the second produced a larger region of reversed zonal winds tilted baroclinic zones formed in the temperature field and the polar vortex tilted westward with height narrow tongues of high pv and low n sub 2 o were drawn off the polar vortex and irreversibly mixed tongues of material were drawn from low latitudes into the region between the polar vortex and the anticyclone diabatic descent was also strongest in this region increased n sub 2 o over a broad region near the edge of the polar vortex indicates the importance of horizontal transport n sub 2 o decreased in the vortex consistent with enhanced diabatic descent during the warmings 9 refs 5 figs

chronostratigraphic constraints on the genesis of archean greenstone belts northwestern superior province ontario canada abstract the paper reports new u pb zircon ages for supracrustal assemblages of the red lake mcinnes lake hornby lake north spirit lake and favourable lake greenstone belts of northwestern superior province and evaluates their significance within the regional stratigraphic and tectonic context the new data confirm and refine the existing picture of a multistage evolution lasting ca 300 m y initiated at ca 3000 ma by periods of coeval calc alkalic tholeiitic and komatiitic magmatic activity followed by the eventual accretion of a microcontinental nucleus by ca 2900 2800 ma and concluded by an intense period of extensive tholeiitic and calc alkalic volcanism and by widespread plutonism and tectonism between 2750 and 2680 ma volcanic sequences in the mcinnes lake and hornby lake greenstone belts of the berens river subprovince yield ages of 2974 2 2969 3 2928 2 and 2901 2 ma showing a correlation with the early stages of greenstone belt development in the region similarly a sedimentary unit in the northern red lake greenstone belt was probably deposited at ca 2900 ma as indicated by a range of detrital zircon ages between 2989 and 2916 ma and the absence of younger components by contrast a volcanic unit in the center of the red lake greenstone belt previously thought to be 2830 ma yields an age of 2745 7 4 ma which correlates with some of the latest volcanic episodes in the uchi subprovince a detrital zircon population in a sedimentary unit of the western favourable lake greenstone belt of the sachigo subprovince displays single grain ages ranging from 2843 to 2727 ma the youngest age corresponding to that of an unconformably underlying tonalite gneiss the geological and geochronological relationships suggest that the north spirit lake and favourable lake greenstone belts represent back arc basins that evolved between ca 2750 and 2730 ma in a transtensional setting within the older north caribou terrane sedimentation was initiated during opening and deepening of the basins and accompanied most of the concluding compression and imbrication stages the period between 2750 and 2680 ma was also characterized by widespread granitoid plutonism that enveloped the mcinnes lake and hornby lake greenstone belts and older tonalites and formed the bulk of the berens river subprovince

a regional perspective of the quetico metasedimentary belt superior province canada alternating greenstone granite and metasedimentary gneiss belts are a first order tectonic feature of the southern superior province the tectonic development of the quetico metasedimentary belt is reviewed with regard to depositional structural and metamorphic plutonic history over its 1200 km length the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite polyphase deformation has resulted in a steep easterly striking foliation and regional gently east plunging stretching lineation metamorphic grade varies in a low p facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite intrusive granite zone mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 mpa 2 5 kbar near the canada united states border to 600 mpa 6 kbar in granulites adjacent to the kapuskasi

new insights into archean crustal development from geochronology in the rainy lake area superior province canada precise isotopic analyses of zircon titanite and rutile from units in the rainy lake area of northwest ontario are combined with structural investigations to develop new stratigraphic interpretations in which lithotectonic sequences were juxtaposed by thrusting prior to steepening by wrenching ages for the oldest volcanic and plutonic rocks in the area range from 2725 pm 2 ma to 2728 4 3 ma the age of metagray wackes of the coutchiching group is bracketed between 2704 pm 3 ma and 2692 pm 2 ma by detrital zircon and a crosscutting intrusion alluvial conglomerate of the seine group is bracketed between 2696 5 3 ma the age of a clast and 2686 2 1 ma the age of the undeformed ottertail lake pluton the coutchiching group is younger than structurally overlying metavolcanic rocks and is therefore allochthon ous these data suggest that sedimentation and deformation were controlled by collisional plate interactions and subsequent transpression at the subprovince boundary combined with dat

u pb ages and tectonic significance of late archean alkalic magmatism and nonmarine sedimentation timiskaming group southern abitibi belt ontario the paper presents u pb ages for zircons of the calc alkalic to alkalic igneous suite and associated alluvial fluvial sedimentary rocks of the timiskaming group in the late archean abitibi greensto

zircon u pb and lu hf systematics of the major terranes of the western superior craton canada mantle crust interaction and mechanism s of craton formation abstract paleo to neoarchean granitoid gneisses ca 3 30 to 2 49 ga are well preserved in the western superior craton protoliths of these gneisses are mainly i type granitoids characterized by high sr y and la yb ratios and low mg consistent with archean tonalite trondhjemite granodiorites zircons from granitoid gneisses commonly contain three growth phases inherited cores zircon i magmatic rims zircon ii and outer rims that have undergone pb loss zircon iii the 3 12 ga to 2 86 ga zircon i represent early crustal material that was captured in younger zircons zircon ii preserve crustal re working and younger crustal additions that are constrained between 2 85 to 2 72 and 2 69 to 2 65 ga zircon ii contains both positive and negative ehf t values 6 3 to 8 1 with both depleted mantle and older crustal signatures half of the magmatic rims ii are characterized by depleted mantle signatures with positive ehf t values representing juvenile crust forming events whereas the other half are characterized by recycled crustal signatures with negative ehf t values ehf t results show that the north caribou and the island lake terranes and the northern uchi domain are isotopically more enriched than the southern uchi english river wabigoon and winnipeg river terranes suggesting the northern uchi margin represents a major terrane boundary based on mass balance calculations large volumes of juvenile material at circa 3 0 ga mixed with smaller amounts of older crust the vast majority of the granites were derived from a source with about 50 mantle material during the peak crust formation events after 2 8 ga the decline in the volume of felsic magmatism in the later archean is coeval with a reduced supply of both heat and material from depleted mantle sources combined with previously published geochemical geochronological and isotopic data this suggests an evolution in felsic magma sources consistent with crustal thickening

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

the history of a continent from upb ages of zircons from orinoco river sand and smnd isotopes in orinoco basin river sediments abstract we report shrimp u pb ages of 49 zircons from a sand sample from the lower orinoco river venezuela and nd model ages of the fine sediment load from the main river and tributaries the u pb ages reflect individual magmatic or metamorphic events the sm nd model ages reflect average crustal residence ages of the sediment sources together they allow delineation of the crust formation history of the basement precursors of the sediments the u pb ages range from 2 83 to 0 15 ga and most are concordant or nearly so discrete age groupings occur at 2 8 2 1 and 1 1 ga the oldest group contains only three samples but is isolated from its closest neighbors by a 600 ma age gap larger age groupings at 2 1 and 1 1 ga make up about a third and a quarter of the total number of analyses respectively the remaining analyses scatter along concordia and most are younger than 1 6 ga the 2 8 and 2 1 ga ages correspond to periods of crust formation of the imataca and trans amazonian provinces of the guyana shield respectively and record intervals of short but intensive continental growth these ages coincide with 2 9 and 2 1 ga nd model ages of sediments from tributaries draining the archean and proterozoic provinces of the guyana shield respectively indicating that the u pb ages record the geological history of the crystalline basement of the orinoco basin zircons with ages corresponding to the major orogenies of the north atlantic continents the superior at 2 7 ga and hudsonian at 1 7 1 9 ga were not found in the orinoco sample the age distribution may indicate that south and north america were separated throughout their history nd model ages of sediments from the lower orinoco river and andean tributaries are 1 9 ga broadly within the range displayed by major rivers and dusts this age does not coincide with known thermal events in the region and reflects mixing of sources with different crust formation ages the igneous and metamorphic history of these sources as recorded by the detrital zircons is that of the orinoco basin basement this implies that despite evidence of fast sedimentary recycling global similarities in nd crustal residence ages and the probability of cross continent mixing through continental drift the sedimentary material carried by individual rivers is mainly derived from the crystalline basement in the basin the global semblance in nd isotope ratios in major river sediments and atmospheric dusts results from the averaging effect of large scale sampling of the continents which are heterogeneous in age on smaller regional scales a large portion of the continental crust in the orinoco basin formed during the trans amazonian orogeny at 2 0 2 1 ga and smaller portions formed both earlier at 2 8 ga and later after 1 6 ga these observations which are consistent with the relative sizes of crustal age provinces in the orinoco basin indicate that sediments from the lower orinoco and andean tributaries contain 25 35 of material added to the crust since trans amazonian times nd model ages of these sediments underestimate the average crust formation age of the basement of the orinoco basin by only about 10 if this relationship holds in other river basins then nd model ages of major rivers and wind blown particulates indicate that the mean age of the continental crust is 1 9 2 ga

millimeter wave spectroscopic measurements over the south pole 1 a study of stratospheric dynamics using n2o observations millimeter wave measurements of n2o and o3 cheng et al 1995 along with several other trace gases have been made nearly continuously from february 1993 through early january 1994 at the amundsen scott station south pole in order to separate chemical and dynamical effects this paper uses the observations of the long lived tracer n2o to study stratospheric dynamics the main emphasis is on the synoptic evolution of the polar vortex over an entire winter period and quantitative results are given for various times and altitudes diabatic descent rates derived for different altitude levels showed the strongest descent in austral fall at high altitudes agreeing fairly well with model predictions by rosenfield et al 1994 subsidence was observed to continue until late october well after polar sunrise the breakdown of the vortex occurred first in the upper stratosphere marked in the intrusion of n2o rich air at these altitudes consistent with trajectory calculations our calculated descent rates are not consistent with the idea that the polar vortex is a flowing processor but instead should be viewed as an isolated system

u pb ages for late magmatism and regional deformation in the shebandowan belt superior province canada five precise u pb zircon and titanite ages from different lithologic units of the shebandowan greenstone belt in the western wawa subprovince of the superior province put tight constraints on the time of late archean magmatism and of two major deformation events a porphyry sill from the older supracrustal sequence has an age of 2733 3 ma another porphyritic rock a trondhjemite occurring as a clast in a conglomerate of the unconformably overlying timiskaming type supracrustal sequence formed 2704 2 ma ago and defines a maximum age for the deposition of the timiskaming type sequence an alkalic volcanic rock from this sequence has been directly dated at in accord with the above constraint and with another probable maximum age of deposition given by the date of 2696 2 ma for the shebandowan lake pluton a first deformation event d1 was related to a predominantly vertical tectonic regime and occurred during or before intrusion of the shebandowan lake pluton at 2696 2 ma the second deformatio

nd isotopic evidence for early to late archean 3 4 2 7 ga crustal growth in the western superior province ontario canada abstract new sm nd analyses of archean metavolcanic plutonic gneissic and metasedimentary rocks from sachigo and berens river subprovinces north caribou terrane and winnipeg river and wabigoon subprovinces in conjunction with previously published data are used to trace the crustal evolution of the western superior province wsp from ca 3 4 to 2 7 ga the extensive wsp geochronological database which contains 3 17 2 55 ga plutonic rocks allows the calculation of precise e t nd values and the identification of the main periods of new crustal addition at 3 4 0 1 3 02 2 92 and 2 76 2 69 ga mass balance calculations indicate that the western superior province represents a collage of archean crusts comprising 7 44 and 48 wt of crust created at 3 4 3 0 and 2 7 ga respectively although these calculations depend on the isotopic signatures of the mantle and those of the average crust at different times several important observations can be made 1 the importance of pre 2 7 ga terranes recycled in the 2 7 ga orogen 2 the existence of time intervals of 150 300 ma dominated by intra crustal processes resulting in a large range of crystallization ages u pb zircon ages that do not reflect new additions from the mantle 3 much of the new crustal additions at 2 7 ga were contaminated by crustal material subducted into the source region ttg suites or were contaminated during intrusion into the crust sanukitoids

simulations of the february 1979 stratospheric sudden warming model comparisons and three dimensional evolution abstract the evolution of the stratospheric flow during the major stratospheric sudden warming of february 1979 is studied using two primitive equation models of the stratosphere and mesosphere the united kingdom meteorological office stratosphere mesosphere model smm uses log pressure as a vertical coordinate a spectral entropy coordinate version of the smm entropy coordinate model or ecm that has recently been developed is also used both models produce similar successful simulations through the peak of the warming capturing the splitting of the vortex and the development of small scale structures such as narrow baroclinic zones the ecm produces a more realistic recombination and recovery of the polar vortex in the midstratosphere after the warming due mainly to better conservation properties for rossby ertel potential vorticity in this model another advantage of the ecm is the automatic increase in vertical resolution near baroclinic zones comparison of smm simulations with forecasts perf

observations of ozone structures in the arctic polar vortex lidar and balloon measurements at the new observatory astro 80 05 n 86 42 w near eureka in the canadian arctic have revealed laminations inside the ozone layer both inside and outside the polar vortex these observations have been conducted by the lidar group of the institute for space and terrestrial science during february march 1993 and the winters of 1993 1994 and 1994 1995 observations of the vortex edge region were obtained as it passed over eureka revealing ozone profiles rich in structure this paper discusses the observed ozone structures and their relationship to vortex filaments the vortex edge structure low ozone pockets motion of the vortex edge region in the ozone profiles and observed temperatures complementing the observations are back trajectories potential vorticity maps and lagrangian domain filling potential vorticity trajectories the relations of magnitude and thickness of the laminations to their positions relative to the vortex are revealed by presenting data as a function of potential vorticity mechanisms for the formation and transport of the laminae are discussed as well as their role as a possible exchange mechanism of air masses across the polar vortex boundary from observations of the laminations and their relation to potential vorticity it is suggested that motion of the vortex sometimes associated with warmings is involved in the formation of laminations which are subsequently advected as filamentary structures

plate tectonics and island arcs the plate tectonic concepts that developed rapidly in the late l960s made possible the understanding of island arcs before that time mobilistic concepts evolved slowly hindered particularly in the united states by an obstructionist geoscience establishment the volcanic belts of island arcs form about 100 km above subducting plates convergent plate boundaries evolve complexly with time and at any one time vary greatly along their lengths seismicity defines positions but not trajectories of descending slabs which sink more steeply than they dip and are overridden by advancing upper plates subduction occurs beneath only one side at a time of an internally rigid plate and the common regime in an overriding plate behind a surficial accretionary wedge is extensional except where a collision is underway back arc basin lithosphere is built behind or by migrating island arcs which lengthen and increase their curvatures a collision can involve two active arcs in which case intervening lithosphere sinks beneath both of them or an active margin and a passive one either type of collision generally is followed by the breaking through of new subduction beneath the composite mass of light crust from a new trench on the outside of the aggregate conversely a new subduction system commonly is a by product of collision a strip of back arc basin crust is in many cases left attached to the aggregate in front of the new trench and becomes the basement for a fore arc basin the leading edge of which is raised as melange is stuffed under it sedimentation in trenches is predominantly longitudinal and can be from distant sources accretionary wedges are dynamic being thickened at both toes and bottoms by tectonic accretion and thinned by gravitational forward flow melange is largely a product of tectonic imbrication and flowage driven by these conflicting processes not of submarine sliding high pressure metamorphic rocks form beneath overriding plates not within wedges in front of them arc magmas incorporate much material from the lithosphere through which they rise and vary correspondingly with the evolving composition of that lithosphere arc crust is inflated into geanticlines by intrusive rocks and thermal expansion submarine island arc volcanic rocks are widely spilitized with na enrichment and ca depletion by hydrothermal reaction with sea water the lower crust of mature island arcs consists of granulite facies rocks of mafic intermediate and felsic intermediate compositions the mohorovicic discontinuity may be primarily a constructional boundary representing the shallow limit off crystallization of voluminous rocks of ultramafic composition or plagioclase free mineralogy

the southern uplands controversy report of a discussion meeting held at burlington house on 12 february 1986 the idea for a discussion meeting followed from remarks made by professor c h holland 1986 in his presidential address to this society in 1985 when he expressed concern about the uncritical application of the accretionary prism model to the southern uplands of scotland it has since become apparent that divergent views are held both among those against the model and those in favour of it this meeting served to air some of these different opinions even if it has not immediately resolved many of the arguments most of the contributors to the meeting see the society newsletter for january 1985 have produced written papers for this volume and so far only one or two have modified their views between writing their abstract and submitting their paper the concept of an accretionary prism model developed gradually a comparison of scotland with burma suggested that the southern uplands were like western burma where trench and oceanic sediments are being underthrust by subduction processes mitchell mckerrow 1975 with the development of modern analogues from the pacific seeley et al 1974 karig sharman 1975 the model was applied to the southern uplands with more emphasis mckerrow et al 1977 leggett et al 1979 a b 1982 moseley 1977 1978 was one of the first to express doubts about this model he believed that the iapetus ocean had closed by the end of the ordovician and thus like many others

rb sr and u pb ages of volcanism and granite emplacement in the michipicoten belt wawa ontario the michipicoten greenstone belt at wawa ontario is typical of archean volcanic belts in the superior province the supracrustal rocks are divisible into lower middle and upper metavolcanic sequences which are separated by iron formation and clastic metasedimentary rocks these are intruded by granitic stocks and embayed by granitic batholiths this study reports whole rock rb sr and zircon u pb ages for the lower and upper metavolcanics for the granitic rocks that are physically within the greenstone belt internal granites and for the granitic rocks that embay the greenstone belt external granites the apparent rb sr ages for the lower metavolcanics are 2530 90 2285 70 and 2680 490 ma the u pb ages are 2749 2 and 2744 10 ma the internal granites give an rb sr age of 2560 270 ma and a u pb age of 2737 6 ma the external granite at hawk lake indicates an rb sr age of 2550 175 ma and a u pb age of 2747 7 ma it is possible that this unit contains elements older than 2812 ma as

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

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

the development of narrow baroclinic zones and other small scale structure in the stratosphere during simulated major warmings numerical simulations of stratospheric major warmings with a multi level primitive equation model of the stratosphere and mesosphere reveal several examples of processes giving rise to small scale structure in the simulated flow for a simulation of the warming of 1984 85 the following examples are cited 1 the maintenance of the aleutian high by injections of air with low values of potential vorticity 2 the erosion of the main cyclonic vortex as it is stripped of air with high values of potential vorticity 3 the merger of a pair of anticyclones at the height of the warming 4 differential advection of potential vorticity in a vertically sheared flow and 5 the generation in the stratosphere of narrow baroclinic zones with strong temperature gradients in the horizontal and in the vertical for a simulation of the warming of 1979 the example given is of the in situ generation in the stratosphere of inertio gravity waves near a developing baroclinic zone the dynamics involved in the formation of narrow baroclinic zones in the simulated stratosphere is studied in some detail it is shown that the ageostrophic flow mainly the vertical motion provides a positive feedback on the frontogenetic effect of the geostrophic large scale flow similarities and differences with frontogenesis at the ground and in the upper troposphere are enumerated

convergent margin tectonics central wabigoon subprovince superior province canada abstract best known for its neoarchean greenstone granite terranes in the east and west the wabigoon subprovince of the superior province features a central transverse corridor of meso and neoarchean gneissic granitoid and minor supracrustal rocks resembling an eroded intermontane plateau structural analysis and u pb geochronology in the north central wabigoon subprovince reveal a complex history of plutonism supracrustal rock deposition polyphase deformation and amphibolite facies metamorphism between 3075 and 2670 ma the geometric and kinematic character of d 1 and d 2 deformational events recorded in 2890 2718 ma tonalitic gneisses remain cryptic as a result of magmatic and structural d 3 5 overprinting but the early structures are bracketed in age between 2725 and 2713 ma by a polydeformed gneiss and tonalite dyke these events occurred deep within an active continental magmatic arc and correspond in age to a collision between the juvenile wabigoon and continental winnipeg river subprovinces recognized to the west clastic sediments deposited after 2701 ma are the youngest rocks involved in subsequent regional deformation a d 3 event is responsible for rapid burial of supracrustal units to 15 km within warm arc crust and imposition of a regional penetrative s 3 foliation that accommodated some relative uplift prior to 2697 ma the age of a discordant granodiorite dyke this event corresponds to variably preserved steep north trending d 1 structures recognized in adjacent greenstone belts subsequent d 4 ductile deformation defines the map pattern in the deep crustal rocks of the central wabigoon subprovince upright east trending f 4 folds developed synchronously with d 2 dextral transpressive structures that formed throughout the wabigoon subprovince prior to 2698 ma the final d 5 sinistral shearing event occurred during emplacement of mantle derived granodiorites of sanukitoid affinity 2688 ma and crust derived granites 2685 ma this event appears to have triggered rapid cooling reflected in 2680 2670 ma titanite dates and late brittle increments of displacement

unscrambling the stratigraphy of an archean greenstone belt a upb geochronological study of the favourable lake belt northwestern ontario canada abstract new zircon and baddeleyite u pb ages show that the major units of the archean favourable lake greenstone belt in the northern superior province were formed by episodic magmatism spanning more than 250 ma and demonstrate that the original stratigraphy is disrupted by thrusts that juxtaposed older supracrustal rocks on top of younger ones the oldest rocks of the area are a 2950 ma gneissic tonalite and a 3000 2960 ma granodiorite clast from a ca 2725 ma conglomerate five distinct volcanic and sedimentary groups formed during presumably short lived episodes at 2925 ma i 2870 ma ii 2858 ma iii 2734 ma iv and 2725 ma v the youngest group contains the thickest sedimentary unit a turbiditic and alluvial fluvial sequence compression caused thrusting that placed groups i and v on top of iv iii on v and ii on iii the thrust complex was subsequently isoclinally folded compression was accompanied by major plutonism that emplaced the bulk of the bounding batholiths between 2732 and 2711 ma ago the late tectonic mo mineralized setting net lake stock in the centre of the belt has an age of about 2708 ma and was overprinted by younger hydrothermal events that produced monazite 2706 ma titanite ca 2695 2690 ma and rutile the youngest rutile at 2657 ma similar late hydrothermal pulses are recorded by secondary titanite elsewhere in the belt and within the batholiths the protracted magmatic evolution of the belt is typical of that observed in a number of greenstone belts of the northern superior province but is uncommon farther to the south in contrast the structural complications and put of sequence stratigraphy appear to be a quite common tectonic characteristic of greenstone belts of the whole superior province

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

thermo tectonic evolution of an archean accretionary complex u pb geochronological constraints on granulites from the quetico subprovince ontario canada abstract u pb geochronological data have been obtained for a granulite zone in the quetico subprovince an archean accretionary complex of the superior province north of manitouwadge ontario canada this granulite zone represents the culmination of a low pressure high temperature regional metamorphism in the quetico subprovince and an apparent continuation of a northward increase in metamorphic grade in the wawa subprovince and across the quetico wawa boundary a u pb zircon age of 2666 1 ma from a concordant magnetite bearing granitic pegmatite in the quetico granulite zone is consistent with the inferred timing of regional metamorphism in amphibolite facies zones 2671 2665 ma and indicates that the low pressure high temperature metamorphism in the quetico subprovince commenced during the main compressional deformation titanite in a calc silicate rock from low leucosome fraction layered migmatites that might represent incipient melting gave a 207pb 206pb age of 2658 1 ma the timing of the granulite facies metamorphism is constrained by u pb zircon ages of 2650 2 and 2651 3 ma from mafic granulite and garnet bearing tonalitic leucosome respectively that are similar to reported u pb ages for mafic granulites in the upper levels of the kapuskasing structural zone the present u pb data suggest a diachronous development of peak metamorphic conditions of the regional metamorphism in the quetico and wawa subprovinces this diachronous development and the zonal distribution of the regional metamorphism are probably related to oblique collision at a convergent plate margin and differential rates of erosional uplift away from the quetico wawa boundary

evolution of the archean abitibi greenstone belt and adjacent terranes new insights from geochronology geochemistry structure and facies analysis

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

archean tectonic and metallogenic evolution of the superior province of the canadian shield abstract the superior province of the canadian shield has mainly experienced an archean tectonic history comprising several cycles of volcanism plutonism and tectonism although the rocks of the superior province span more than 500 million years of archean time most mineral deposits formed in an interval of 100 million years or less corresponding to a period of accretion of oceanic and pre existing continental fragments in the late archean at 2 7 ga in accord with recent tectonic syntheses which argue for the accretionary origin of the superior province the metallogenic history of the superior province can also be interpreted in the same context provided secular anomalies in the early history of the earth are acknowledged for relatively unique deposit types such as komatiitic nickel deposits and voluminous iron formations volcanogenic massive sulphide iode gold and granitoid associated deposits as well as ni cu pge cr and ti deposits associated with mafic ultramafic intrusions are readily interpreted in terms of the construction and accretion of late archean island arcs

a reply to how many arcs can dance on the head of a plume by jean bedard precambrian research 2012 abstract bedard s 2013 comment on wyman 2013 itself an outgrowth of a 5ias workshop debate has clarified several points concerning the catalytic delamination driven model of bedard 2006 many of the references to archean plate tectonic models however are expressed in terms of an improbable uniformitarian viewpoint that could not be applied to the modern earth whereas differences between archean and phanerozoic plateau basalts have been attributed to a less depleted mantle source any similar arguments from advocates of archean plate tectonics are characterized as special pleading bedard 2013 does not address key issues raised by wyman 2013 and when the paper does attempt to resolve apparent dilemmas related to craton development it undermines the new refinements made to the non plate tectonic scenario that bedard favors

age and source of detrital zircons from the missi formation a proterozoic molasse deposit trans hudson orogen canada the missi formation in the flin flon basin forms part of a discontinuous series of molasse type sediments found throughout the early proterozoic trans hudson orogen in northern saskatchewan and manitoba the flin flon basin contains a sequence of proximal fan to braided stream fluvial conglomerates and sandstones which unconformably overlie subaerially weathered amisk group volcanic rocks stratigraphic way up indicators have been preserved even though these rocks have undergone greenschist facies metamorphism and polyphase deformation the sedimentary rocks are crosscut by intrusive rocks which provide a minimum age of sedimentation of 1840 7 ma detrital zircons from each of the six stratigraphic subdivisions of the flin flon basin were analyzed using the single zircon pb evaporation technique euhedral to slightly rounded zircons dominate each sample and these zircons give ages of between about 1854 and 1950 ma the missi sediments were thus deposited between 1840 and 1854 ma possible sources for

geological evolution of the manitouwadge greenstone belt and wawa quetico subprovince boundary superior province ontario constrained by u pb zircon dates of supracrustal and plutonic rocks fifty million years of archean evolution is recorded in the manitouwadge greenstone belt and the wawa quetico boundary region from ca 2720 ma volcanism and subvolcanic plutonism associated with m

geochronological constraints on the tectonic evolution of a late archaean greenstone belt wabigoon subprovince northwest ontario canada abstract u pb isotopic age analyses have been carried out on zircon and titanite from rocks near the contact between the kakagi lake savant lake volcanic belt ksvb within the wabigoon subprovince and the english river subprovince in northwest ontario canada two tonalite gneisses intruded by mafic dykes at tannis lake and caribou lake have minimum zircon ages of 3051 ma and 3075 ma respectively this contrasts with a 2775 1 ma age on zircon from a felsic tuff located within a lower predominantly mafic volcanic sequence in the interior of ksvb these data combined with previous studies indicate that ksvb is presently flanked by older sialic crust in the sioux lookout and kashaweogama lake areas lower mafic volcanic sequences are unconformably overlain by sedimentary units that are in sheared contact with overlying calc alkaline volcanic units titanite and zircon analyses from clasts within a basal conglomerate indicate a maximum age for deposition of 2698 4 ma this late age is confirmed by a 2703 2 ma zircon age for an unconformably underlying porphyry intrusion one clast gives a zircon age of 2903 16 ma suggesting that the sediments were at least partly derived from older material in the english river subprovince data from previous studies show that the structurally overlying calc alkaline volcanic sequences are at least 30 ma older than the sediments the absence of zircon inheritance in all but the youngest ksvb sequences as well as geological and geochemical arguments suggest that the greenstone belt evolved largely in an ensimatic environment in isolation from older sialic material the intervening oceanic crust must therefore have been consumed by subduction which led to arc magmatism and eventual collision between the arc ksvb and continental material english river subprovince the sediments unconformably overlying the lowermost mafic volcanic sequences were deposited in a partly subaerial continental environment since chemical and textural evidence suggests the lower mafic volcanic sequences were deposited in an oceanic environment they must have been obducted onto the edge of a continental block this was perhaps followed by nappe and thrust emplacement on top of the sediments of calc alkaline sequences derived from earlier arc magmatism continued compression resulted in isoclinal folding of the supracrustal sequences into their present vertical positions

isentropic mixing in the arctic stratosphere during the 1992 1993 and 1993 1994 winters dynamic isolation of the winter arctic circumpolar vortex during 1992 1993 and 1993 1994 the second and third northern hemisphere winters of the uars mission is studied using quasi horizontal isentropic trajectories ejection of vortex air and entrainment of mid latitude air into the vortex are quantified and compared with climatological values obtained from the analysis of 16 arctic winters a number of unusual features of both winters are discussed the most notable features are the anomalous isolation experienced by the vortex during december 1992 and the unusual degree of isolation and persistence of the vortex during february and march of both years the 1992 1993 winter season is the most consistently isolated vortex on record only during january 1993 when entrainment is large is this pattern of extreme isolation broken 14 refs 3 tabs

geochronology of the lumby lake greenstone belt a 3 ga complex within the wabigoon subprovince northwest ontario the lumby lake greenstone belt lies within the central predominantly metaplutonic terrane of the western wabigoon subprovince it is conformable with gneiss of the marmion lake tonalite batholith u pb analyses of zircons from two felsic tuffs and a quartz porphyry within the greenstone belt give a scattered lead loss pattern indicative of early disturbance the ages of the oldest concordant fractions agree however and define an age of 2999 1 m y for emplacement of these felsic units zircons from a tonalite gneiss within the marmion lake batholith are also somewhat disturbed the oldest abraded fraction is concordant and has an age of 3003 5 m y indicating that the gneiss is probably close in age to the supracrustal rocks u pb analyses of rutile from one of the felsic units give an age of 2690 5 m y and record the effect of late archean metamorphism three titanite fractions from the tonalite gneiss agree within error and have an age of 2809 1 m y the age and consistency of the titanite fractions indicate that titanite in this rock was formed as a result of a metamorphic event and was never subsequently disturbed this age is therefore a minimum estimate for high grade metamorphism of the gneiss the presence of a pre 2809 ma fabric in a gneiss of regional extent suggests that an early metamorphic deformational event affected this area these data constitute the first firm evidence for extensive 3 0 ga crust in the southern superior province and give the first evidence of a regional deformational event which predates cratonization of the superior province

shrimp dating of zircons from high grade metasediments of the schwarzwald sw germany and implications for the evolution of the moldanubian basement four metasedimentary zircon populations from different tectonometamorphic units of the central and the northern schwarzwald variscan belt sw germany were investigated using sem cathodoluminescence and shrimp dating despite partially strong modifications of primary internal morphologies during variscan metamorphism at amphibolite 750 c 0 4 0 6 gpa and granulite facies conditions 950 1 000 c 1 4 1 8 gpa many grains show well preserved protolith ages the detritus indicates a northern gondwana origin and different palaeozoic episodes of sediment deposition and consolidation two of the studied sediments were deposited in cambrian early ordovician times and consolidated in positions close to northern gondwana late ordovician and rare devonian detritus from sediments of two other tectonometamorphic units indicates much later sedimentation close to the leading edge of gondwana or a terrane assemblage during northern drift towards laurussia subsolidus growth of new zircon due to variscan granulite facies metamorphism of one of the tectonometamorphic units is precisely dated at 335 2 ma

thermal regime of the lithosphere in the canadian shieldthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geothermal studies were conducted within the framework of lithoprobe to systematically document variations of heat flow and surface heat production in the major geological provinces of the canadian shield one of the main conclusions is that in the shield the variations in surface heat flow are dominated by the crustal heat generation horizontal variations in mantle heat flow are too small to be resolved by heat flow measurements different methods constrain the mantle heat flow to be in the range of 12 18 mw m 2 most of the heat flow anomalies high and low are due to variations in crustal composition and structure the vertical distribution of radioelements is characterized by a differentiation index di that measures the ratio of the surface to the average crustal heat generation in a province determination of mantle temperatures requires the knowledge of both the surface heat flow and di mantle temperatures increase with an increase in surface heat flow but decrease with an increase in di stabi

the age and provenance of metasedimentary rocks in the quetico subprovince ontario from single zircon analyses implications for archean sedimentation and tectonics in the superior province abstract u pb isotopic analyses have been performed on single detrital zircon grains from greywacke samples collected along the northern boundary of the quetico subprovince in northwest ontario secondary lead loss was virtually eliminated by choosing crack free grains and removing the outer surface using air abrasion thus the 207 pb 206 pb age can be considered a good approximation to the age of crystallization of each grain zircons in the quetico metasediments show a range of ages from 2698 3 ma to 3009 4 ma deposition and metamorphism of the metasediments is bracketed within a 10 5 ma time span between crystallization of the youngest detrital zircon grain and a late tectonic intrusion the blalock pluton which cuts the sediments and has a zircon age of 2688 4 ma the most easterly metasediment sample contains zircons with ages which cluster around 3007 6 ma which is within error of the age of a batholith dated in the adjacent marmion lake area of the wabigoon subprovince this along with the fact that most detrital zircons were only slightly abraded by sedimentary transport suggests that the source of many zircons may have been proximal from the north there is also a significant cluster of young zircon ages in the range 2698 2709 ma surprisingly only a few ages correspond to the time of most intense arc type felsic volcanism 2710 2745 ma in the main greenstone belt of the western wabigoon subprovince sedimentary deposition occurred at a time of extensive deformation in the adjacent wabigoon subprovince the pattern of detrital grain ages indicates that this involved uplift of a complex older sialic landmass petrographic evidence for the presence of substantial felsic volcanic detritus as well as the fact that many of the zircons only slightly pre date deposition indicates that much of the sediment was probably derived from erosion of active felsic volcanic centers these data combined with regional geochronological and field studies favour deposition in a basin formed by late orogenic collisional events

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define

investigation of the archaean crust by single grain dating of detrital zircon a greywacke of the slave province canada u pb dating of single zircon grains from a graywacke of the point lake area of the archaean slave structural province in canada indicates that this sediment is derived from two sources 1 the ancient shield parts of which are at least 3100 ma old which is the basement of the slave province supracrustal rocks and 2 younger rocks about 2700 ma old this implies the creation of large amounts of acidic to intermediate rocks during the 2700 ma event it also indicates that magma generation and intracontinental sedimentary basin formation and deepening are strongly related

crustal structure and implications for the tectonic evolution of the archean western superior craton from forward and inverse gravity modeling 1 the distribution of mass anomalies within the crust of the archean western superior province has been investigated with forward and inverse gravity modeling routines the gravity models indicate that in most of the western superior province significant mass anomalies occur only within the top 10 km of the crust where they are generally related to dense metavolcanic rocks and low density granitoid plutons and at the crust mantle boundary where they are linked to undulations of this interface an exception is the region encompassing the central wabigoon subprovince and the segments of the quetico and wawa belts south of it where implied large intracrustal mass anomalies are associated with a segment of denser lower crust and with overlying depressions of the interfaces between the main crustal layers we interpret the predominant lateral mass homogeneity observed at deep crustal levels as evidence of mass redistribution processes associated with a major postaccretionary episode of thermal softening of the western superior crust the incongruent presence of a denser lower crustal segment may reflect the subsequent modification of the lower crust through magmatic intra and underplating possibly in relation with the mesoproterozoic midcontinent rifting event

a structural reappraisal of the beardmore geraldton belt at the southern boundary of the wabigoon subprovince ontario and implications for gold mineralization the beardmore geraldton belt occurs along the southern margin of the archean wabigoon subprovince superior province ontario the belt consists of shear bounded interleaved metasedimentary and metavolcanic units the units were imbricated from 2696 to 2691 ma during d1 thrusting and accretion of the wabigoon quetico and wawa subprovinces post accretion d2 deformation produced regional f2 folds that transposed lithological units parallel to the axial plane s2 cleavage of the folds during d3 deformation the folds were overprinted by a regional s3 cleavage oriented anticlockwise of f2 axial planes and lithological contacts and s2 cleavage were reactivated as planes of shear within dextral regional shear zones that generally conform to the trend of the belt d3 is a regional dextral transpression event that also affected the quetico and wawa subprovinces south of the beardmore geraldton belt gold mineralization at the leitch and macleod cockshutt mines the two richest past producing gold mines in the beardmore geraldton belt is associated with d3 shear zones and folds overprinting regional f2 folds the plunge of the ore zones is parallel to f3 fold axes and to the intersection of d3 shear zones with f2 and f3 folds

u pb systemstics in single zircons from the pontiac sediments abitibi greenstone belt twenty six detrital zircon grains from the late archaean pontiac sediments of the abitibi greenstone belt were dated individually by the u pb method in order to identify possible elements of an ancient pre 2 7 ga shield the results show that at least two components were present at the source areas of these sediments 1 materials of about 2700 2725 ma which were most likely derived from contemporaneous acid igneous rocks and 2 significantly older materials parts of which are at least 2940 ma old which were either part of an uplifted sedimentary basin or a sialic basement complex these results can be used to suggest a close link between igneous activity and basin subsidence and that the development of the abitibi volcanic belt was closely associated with the presence of some older sialic blocks

bimodal volcanism in archean greenstone belts exemplified by greywacke composition lake superior park ontario in the archean gamitagama lake greenstone belt a metagreywacke metasiltstone formation at least 3 6 km thick largely had an easily eroded unconsolidated subaerial pyroclastic felsic volcanic provenance the remains of this provenance are now represented by a 0 600 m thick felsic formation sandwiched between mafic flow formations in an adjacent and laterally equivalent 4 5 km thick metavolcanic sequence the felsic formation includes flows plus various pyroclastic units including crystal rich ash flow tuff volcanism erosion transportation and sedimentation appear to have been essentially contemporaneous with subaerially erupted ash that was being transported rapidly by rivers to the sedimentary basin where it was deposited in a submarine fan angular to rounded framework sand grains form 39 of the greywacke in the proportion 49 plagioclase 36 quartz and 15 lithic clasts dominantly felsic volcanic despite the moderately high quartz content a felsic pyroclastic volcanic provenance can be sho

accretion of japanese island arcs and implications for the origin of archean greenstone belts the present day region of arc arc collision between the izu bonin arc and mainland japan honshu arc in the western pacific called the izu collision zone or icz provides a useful kinematic model for the development of many archean greenstone belts the compositional assemblage thermal structure and structural style of the crust in the icz are very similar to those inferred for many archean greenstone belts in both cases thick young and flaky arc crust with a high heat flux is in collision with another crustal block the icz has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms many archean greenstone belts show similar geological features and we suggest that the incremental imbrication of the crust was a common tectonic style in some archean convergent margins

the behavior of wave 2 in the southern hemisphere stratosphere during late winter and early spring abstract a detailed analysis is presented of the behavior of the zonal wavenumber 2 component of the flow wave 2 for july through october in the southern hemisphere stratosphere wave 2 in the stratosphere is characterized by a broad meridional structure peaking between 55 and 65 s and regular eastward propagation with periods ranging from 5 to 40 days maximum geopotential height amplitudes for a year range from approximately 600 to 1000 m examination of vertical structure suggests that during episodes of largest growth wave 2 propagates upward from the upper troposphere regular eastward propagation is however evident only within the stratosphere there are also episodes of wave 2 growth that do not appear connected with the troposphere in general the wave 2 amplitude is not as large in these cases there are several years when wave 1 and wave 2 amplitudes are strongly anticorrelated in time during september and october there are also years with strong positive correlation during august and

oxygen isotope ratios of archean plutonic zircons from granite greenstone belts of the superior province indicator of magmatic source zircons commonly preserve primary magmatic oxygen isotope ratios even through high grade metamorphic events unlike quartz which more readily loses its primary 18o value by subsolidus exchange intrusive bodies from the superior province canada have an average 18o zircon of 5 8 0 6 n 59 rocks those from granite greenstone belts have an average 18o zrc 5 8 0 6 n 55 in contrast to those from metasedimentary belts that average 18o zrc 6 7 0 2 n 4 the 18o zircon values from granite greenstone belts indicate a maximum allowable input of 14 supracrustal high 18o material into archean magmas of the superior province relative to mantle 18o values igneous bodies of the superior province have been separated into two groups based on geochemistry unenriched plutons and sanukitoids magmatic oxygen isotope ratios inferred from these zircons indicate up to a subtle but significant division of 1 between the two geochemical sets of rocks this distinction is apparent only in the 18o zrc and not in more commonly analyzed less refractory minerals such as quartz and feldspar oxygen isotope data correlate with geochemical modeling of sanukitoids the average 18o zrc for sanukitoids is 6 4 0 2 in comparison to 5 5 0 4 for unenriched plutons there is almost no overlap in values the elevation of sanukitoid 18o zrc indicates input from a high 18o source such as fluids from the subducting oceanic crust melts of subducted sediments or continental crust all of the plutons analyzed in this study have been dated by u pb zircon techniques and the combination of u pb ages and 18o zrc indicates little variability of 18o versus age during archean plutonism in the superior province therefore recycling of sediments and other high 18o sources was not a dominant process during crustal growth from 3003 to 2670 ma in the superior province

potential vorticity in the stratosphere derived using data from satellites maps of ertel s potential vorticity q are computed on an isentropic surface in the middle of the stratosphere they are derived using data from stratospheric sounding units on board noaa satellites the reliability of the maps is demonstrated mainly by conservation of q that is shown in successive analyses and by the close agreement obtained using independent data from two satellites the maps are used to follow the movement of material during the course of strong disturbances which occurred in december 1981 they clearly show the breaking of planetary waves in the stratosphere an inherently nonlinear process the effect of wave breaking on the structure of the westerly vortex is considered any mixing of q on isentropic surfaces during wave breaking is shown to be far from complete in the examples studied the behaviour of diagnostics based on zonal averaging such as the eliassen palm flux is accounted for by local changes in the distribution of q the implications of our findings for the study of stratospheric dynamics are discussed

u pb geochronology of the grenville orogen of ontario and new york constraints on ancient crustal tectonics based on lithological structural and geophysi cal characteristics the proterozoic grenville orogen of southern ontario and new york has been divided into domains that are separated from each other by ductile shear zones in order to constrain the timing of meta morphism u pb ages were determined on metamorphic and igneous sphenes from marbles calc silicate gneisses amphibolites granitoids skarns and pegmatites in addi tion u pb ages were obtained for monazites from meta pelites and for a rutile from an amphibolite these mineral ages constrain the timing of mineral growth the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed grenville orogen based on the ages from metamorphic minerals regional and contact metamorphism occurred in the following intervals central granulite terrane