hf isotopic composition and age constraints on the evolution of the archean central uchi subprovince ontario canada the archean uchi subprovince located in the northwestern superior province comprises greenstone belts and granitoid complexes that formed in the course of a multistage history between about 3000 and 2690 ma the earliest volcanic plutonic complexes were assembled into a microcontinent which then progressively grew by the addition of magmatic arcs and tectonic slivers a major compressional orogeny concluded this evolution at around 2700 ma lu hf isotopic compositions for well dated zircon populations from volcanic and plutonic rocks of the central part of the subprovince have hf values that range from 6 1 to 1 4 these results show that the bulk of the province was derived predominantly from depleted mantle sources but there is a superimposed trend showing involvement of the oldest greenstone and crustal sequences in the generation of the youngest magmatic complexes the temporal and isotopic relationships of the uchi subprovince contrast with the situation observed in the southern abitibi greenstone belt which developed within a time span of less than 80 m y in an intraoceanic setting and exhibits quite uniform depleted mantle like isotopic values revealing only very subordinate degrees of recycling of older crust

lu hf systematics of 4 0 2 3 ga old zircons from the turee creek group pilbara craton w australia implications on the rise of atmospheric oxygen and global glaciation during the paleoproterozoic we investigated the hf isotopic systematics of detrital zircons in a succession of siliciclastic sediments and glacial diamictites from the early paleoproterozoic sequence of the boolgeeda iron formation hamersley group and overlying turee creek group of the pilbara craton western australia about 400 detrital zircons yielding 95 concordant u pb ages were analyzed for hf isotopes to constrain their magmatic sources 70 of the analyzed zircons display super chondritic initial hf isotopic compositions demonstrating crystallization in mantle derived magmas most of the data are comprised between model age lines at 2 5 and 3 2 ga which suggests a sub continuous crust generation by extraction from the depleted mantle during this time period a single grain yields a 4 0 ga age which represents the first hadean age for a zircon from the pilbara craton our results are significantly different from zircon hf isotope data of the glenburgh terrane in the southern border of the turee creek group or older successions of the pilbara kaapvaal and superior cratons but show overlap with some of the yilgarn craton this together with the occurrence of a hadean zircon crystal preserved in the boolgeeda glacial diamictite with similar hf isotopic signature than the jack hills zircons makes the yilgarn craton a possible source material for the boolgeeda glacial horizon alternatively the majority of the zircons analyzed show ages which are consistent with those of the underlying 2 45 2 78 ga hamersley and fortescue groups formed by sedimentary successions interleaved with thick subaerial volcanic sequences associated with the emplacement of large igneous provinces such subaerial volcanic rocks could account for the relatively juvenile character of the zircon analyzed a local provenance of the siliciclastic material delivered to the turee creek basin would support the role of large subaerial magmatic provinces as triggers of the rise of atmospheric oxygen and the onset of glaciations at the beginning of the proterozoic

zircon lu hf systematics and the evolution of the archean crust in the southern superior province canada a combined lu hf and u th pb isotopic study was made of 25 zircons and 2 whole rocks from the late archean crust 2 888 2 668 ma in the southern superior province canada the relative abundances of u th lu and hf in zircons from the low grade michipicoten and gamitagama greenstone belts show variable patterns which in part reflect the bulk compositional differences of their parent rocks zircons from the high grade lower crustal regions adjacent to these belts kapuskasing structural zone are distinguished from the low grade zircons by their strong depletions of lu and hf the low hf contents imply that the growth of metamorphic zircon involves a significant fractionation of the zr hf ratio

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

large scale erosion rates from in situ produced cosmogenic nuclides in european river sediments abstract we have calculated long term erosion rates of 20 100 mm kyr from quartz contained 10be in the bedload of middle european rivers for catchments ranging from 102 to 105 km2 these rates average over 10 40 kyr and agree broadly with rock uplift incision and exhumation rates historic soil erosion rates and erosion rates calculated from the measured sediment loads of the same rivers moreover our new erosion rate estimates correlate well with lithology and relief however in the regen neckar loire and meuse catchments cosmogenic nuclide derived erosion rates are consistently 1 5 4 times greater than the equivalent rates derived from measured river loads this may be due to the systematic under representation of high magnitude low frequency transport events in the gauging records which cover less than a century alternatively the discrepancy may derive from spatially non uniform erosion and preferential tapping of deeper sections of the irradiation profile a third explanation relates the high cosmogenic nuclide derived erosion rates to inheritance of an elevated pleistocene erosion signal uncertainties associated with the cosmogenic nuclide derived erosion rate estimates are not greater than the potential errors in conventional estimates therefore the cosmogenic nuclide approach is an effective tool for rapid catchment wide assessment of time integrated rates of bedrock weathering and erosion and we anticipate its fruitful application to the quaternary sedimentary record

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

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

plutonism in the southern abitibi subprovince a tectonic and petrogenetic framework studies of plutonic rocks in archean terranes such as the southern abitibi subprovince have been an important component in the development of an accretionary tectonic model for the superior province in the southern abitibi subprovince pretectonic approximately 2 75 2 70 ga low k tholeiitic calc alkaline dioritic to tonalitic plutons e g bourlamaque batholith flavrian pluton are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc alkaline basalt andesite pretectonic synvolcanic tholeiitic intrusions e g dundonald sill kamiskotia gabbroic complex 2 72 2 70 ga show in situ fractionation trends and locally have mixed magma textures with contemporaneous calc alkaline granitoids they are believed to be derived from a midocean ridge basalt morb like depleted mantle with little evidence for contamination by an older crustal component pre to syntectonic approximately 2 72 2 69 ga porphyry and tonalite plutons e g timmins porphyries round lake batholith have geochemical characteristics typical of archean tonalite trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades late to post tectonic 2 69 ga diorite monzonite and syenite plutons e g watabeag batholith lamaque stocks otto stock have large ion lithophile element enriched calc alkaline to sho shonitic compositions these intrusions and associated timiskaming type volcanic rocks are attributed to melting of large ion lithophile element enriched mantle in the late stages of subduction late to post tectonic biotite muscovite granite intrusions contain a significant contribution of evolved crust e g s type characteristics of the preissac lacorne batholith in the southern abitibi subprovince the tectonic setting of mantle and crust derived intrusions is broadly comparable to magmatic processes in phanerozoic arcs and accreted terranes the plutonic rocks record a transition from pre to synaccretionary subduction related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources this framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern abitibi subprovince which range from ni cu sulfides in layered synvolcanic tholeiitic intrusions e g kamiskotia gabbroic complex ontario to volcanogenic sulfides in early tholeiitic and calc alkaline sequences e g flavrian pluton noranda district quebec to the spatial association of gold with late large ion lithophile element enriched intrusions e g lamaque plugs val d or district quebec

time integrated 3d approach of late quaternary sediment depocenter migration in the tagus depositional system from river valley to abyssal plain quantification of sediment volumes in continental to deep ocean basins is key to understanding processes of sediment distribution in source to sink depositional systems using our own and published data we present the first quantification of sediment volume changes in basins along the course of a major southwest european river during the deglaciation the salient points of this quantitative record in the tagus and equivalent north atlantic basins show crucial roles for sea level climate and land use in the distribution of sediments the bypass of sediments starved the tagus basins and subsequently sedimentation mainly occurred on the tagus abyssal plain during the sea level lowstand of the last glacial maximum the main sediment depocenter rapidly shifted via the continental shelf to the lower tagus valley during sea level rise in the deglaciation period finally the main sediment depocenter shifted further landward into the lower tagus valley during sea level high stand in the holocene during the high stand phase last 7 ky sediment flux increased up to 2 5 times due to climate and land use changes the average catchment denudation rate during the last 12 ky 0 04 0 1 mm y is in agreement with those of other european catchments our study clearly demonstrates the added value of detailed knowledge of 3d depocenter distribution size and chronology this allowed us to identify an increased sediment flux during the last 7 ky which was not identified using local observations from boreholes alone the uniqueness of the tagus depositional system lies in the combination of a large accommodation space in the bedrock confined lower tagus valley the steep lowstand surface gradient and the narrow continental shelf with canyons indenting the shelf break

late archean convergent margin tectonics and gold mineralization a new look at the norseman wiluna belt western australia on a world scale the most important hydrothermal gold deposits occur either in late archean ca 2700 ma greenstone belts or at late paleozoic to quaternary convergent plate boundaries in the latter environments epithermal and porphyry hosted deposits form during subduction related magmatism in volcanic arcs and deep level meso thermal deposits form during deformation in continental margin orogenic belts the mesothermal deposits are similar to archean gold deposits in greenstone belts such as the norseman wiluna belt in western australia we here suggest that this similarity exists because the norseman wiluna belt is an orogenic belt with a tectonic history similar to that of younger mineralized convergent margins such as the north american cordillera it is most likely that the norseman wiluna belt resulted from the interaction of lithospheric plates during the major period of continental growth and stabilization that occurred in the late archean the distribution of gold mineralization being controlled by convergent margin tectonics

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

environmental signal propagation in sedimentary systems across timescales abstract earth surface processes operate across erosionally dominated landscapes and deliver sediment to depositional systems that can be preserved over a range of timescales the geomorphic and stratigraphic products of this source to sink sediment transfer record signals of external environmental forcings as well as internal or autogenic dynamics of the sedimentary system here we evaluate environmental signal propagation across sediment routing systems with emphasis on sediment supply qs as the carrier of up system forcings we review experimental numerical and natural examples of source to sink sediment routing and signal propagation during three timescales 1 historic which includes measurement and monitoring of events and processes of landscape change and deposition during decades to centuries 2 centuries to several millions of years referred to as intermediate timescale and 3 deep time we discuss issues related to autogenic dynamics of sediment transport transient storage and release that can introduce noise lags and or completely mask signals of external environmental forcings we provide a set of conceptual and practical tools for evaluating sediment supply within a source to sink context which can inform interpretations of signals from the sedimentary record these tools include stratigraphic and sediment routing system characterization sediment budgets geochronology detrital mineral analysis e g thermochronology comparative analog approaches and modeling techniques to measure calculate or estimate the magnitude and frequency of external forcings compared to the characteristic response time of the sediment routing systems

trace element geochemistry of ore associated and barren felsic metavolcanic rocks in the superior province canada reply archaean felsic metavolcanic rocks in the superior province of the canadian shield may be divided into three major groups on the basis of trace element abundances and ratios 1 fi felsic metavolcanic rocks are dacites and rhyodacites characterized by steep ree patterns with weakly negative to moderately positive eu anomalies high zr y low abundances of high field strength elements e g hree y zr hf and high abundances of sr examples occur in the bowman subgroup and skead group in the abitibi belt in the kakagi lake lake of the woods shoal lake and sturgeon lake areas of the wabigoon belt and in the confederation lake area of the uchi belt none of these horizons as known hosts base metal sulphide deposits 2 fii felsic metavolcanic rocks are rhyodacites and rhyolites characterized by gently sloping ree patterns with variable eu anomalies moderate zr y and intermediate abundances of hfs elements and sr examples occur in the misema subgroup of the abitibi belt in the wabigoon lake a

evidence from neodymium isotopes for mantle contributions to phanerozoic crustal genesis in the canadian cordillera sm nd and rb sr isotopic studies of several phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked pre existing continental crust in contrast isotopic data collected from two of the largest terranes in the canadian cordilleran orogenic belt the alexander and stikine terranes show that these tectonic fragments are composed of mantle derived continental crust in many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some proterozoic crustal regions

neoarchean crustal recycling and mantle metasomatism hf nd pb o isotope evidence from sanukitoids of the fennoscandian shield abstract the growing knowledge of the geological record reveals evidence of crustal recycling in the archean eon in order to assess the role of recycling of crust into the mantle we have analyzed radiogenic hf in zircon as well as whole rock nd and pb isotope compositions of mantle derived granitoids of the sanukitoid series in the neoarchean karelian province fennoscandian shield homogeneous initial hf and nd values suggest that sanukitoids are derived from well mixed source s limited scatter in the initial pb isotope data within intrusions as well as in other radiogenic isotopes point to effective mixing processes in the mantle o isotope data for zircon from karelian sanukitoids indicate a dominant mantle source 18 o 5 5 6 1 with minor contributions from sources with elevated 18 o 7 0 to 8 5 such as sediments and altered upper oceanic crust our results suggest that the parental melts of sanukitoids are mixtures of depleted mantle and recycled crustal components a slab breakoff at the end stage of subduction is a plausible trigger mechanism for sanukitoid magmatism and it explains variable contributions from different 18 o sources

the temporal evolution of sanukitoids in the karelian craton baltic shield an ion microprobe u th pb isotopic study of zircons abstract the u th pb isotopic systematics of zircons from six sanukitoid intrusions in the karelian craton has been investigated using the secondary ion mass spectrometer sims our results show that karelian sanukitoids formed between 2745 and 2705 ma ago and that there is an age difference of about 30 ma between sanukitoids in the eastern and western karelia results for intrusions made up of a number of magmatic phases show no internal age variation in the east of the study region zircons from a monzodiorite and two quartz monzonites from the panozero pluton were dated at 2737 11 2739 11 and 2741 8 ma in the hautavaara pluton a monzodiorite and a subalkaline granite were dated at 2743 8 and 2742 23 ma and a quartz diorite from the elmus pluton gave an age of 2741 7 ma in addition a lamprophyre dyke within the sjargozero pluton gave an age of 2742 14 ma in the west zircons from a quartz monzonite and a granodiorite in the njuk intrusion gave ages of 2709 10 and 2716 11 ma in the kurgelampi pluton zircons from a diorite and two granodiorites were dated at 2707 9 2719 6 and 2712 9 ma respectively zircons from the granodiorites and the alkaline granites contain older cores with 207 pb 206 pb ages greater than 2800 ma the absence of inherited zircons in the more mafic sanukitoids is attributed to their higher saturation levels for zirconium sanukitoid zircons have th u ratios above 0 5 and often higher whereas nearby tonalite tondhjemite granodiorites ttgs have th u ratios below 0 5 typically tonalites are older than sanukitoids by more than 100 ma and it is possible that tectonic processes associated with the collision of the belomorian mobile belt provided the heat necessary to initiate the generation of the sanukitoid melts

1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle 1 700 myr greenstone volcanic successions in southwestern north america and isotopic evolution of proterozoic mantle

neodymium isotopic evidence for the tectonic assembly of late archean crust in the slave province northwest canada the ca 2 7 2 5 ga slave province is a granitegreenstone terrane comprising deformed sedimentary and subordinate volcanic belts extensively intruded by granitoid rocks the nd isotopic data are reported for 58 samples of supracrustal and granitoid rocks exposed along a 400 km east west transect at 65 n across the structural grain of the province initial nd values reveal distinctly different crustal sources in the eastern compared to the western parts of the province as expected from tectonic assembly of the province through accretion of juvenile crust to older continental crust supracrustal sequences ca 2 71 2 65 ga from the central and eastern parts of the province have positive nd 1 values 0 3 to 3 6 consistent with juvenile sources and formation remote from significantly older crust syn to late deformation ca 2 63 2 60 ga mantle derived diorites and related tonalites type i from the central and eastern parts of the province have similar initial nd values 0 1 to 2 7 in contrast samples from the westernmost plutons which intrude exposed pre 3 1 ga crust have much lower nd 1 values 1 0 to4 6 suggesting contamination of these magmas by older crust the nd 1 values of post deformation granites s s type ii also vary systematically across the province values for granites west of longitude 110 30 w range from 0 2 to 5 3 those to the east range from 0 6 to 3 7 these data suggest mixed crustal sources dominated by mid to early archean material nd 2 6 to 17 at 2 6 ga for the western granitoid rocks and juvenile sources for the eastern granites the nd isotopic data are consistent with the geology of the province in that exposures of mid to early archean crustal rocks predating the principal 2 7 2 5 ga orogenic event are restricted to the western part of the province the asymmetric pattern defined by the nd isotopic data indicates the presence of distinct crustal rocks beneath the slave province similar isotopic variations observed across phanerozoic collisional orogens have been interpreted to reflect tectonic assembly of crust by accretion of juvenile crustal terranes to an older continental margin this process may also have been an important mechanism in the cratonization of the slave province

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

strike slip juxtaposition of ca 2 72 ga juvenile arc and 2 98 ga continent margin sequences and its implications for archean terrane accretion western superior province canada the north caribou terrane of the western superior province attained continental thickness 35 km by 2997 ma it records a subsequent 300 million years history of continental fragmentation arc magmatism and terrane accretion at lake winnipeg the 2978 ma lewis storey quartzite komatiite iron formation assemblage marks mesoarchean breakup unlike the relatively continuous 2980 2735 ma stratigraphic record of the red lake and birch uchi greenstone belts to the east little of this interval is recorded at lake winnipeg rather two belts of younger juvenile rocks are tectonically juxtaposed the black island assemblage of isotopically depleted 2723 ma basalt and calc alkaline andesite and rice lake greenstone belt of basalt calc alkaline andesite and dacite 2731 2729 ma collectively these terranes represent a short lived island arc back arc system that docked with the southwestern north caribou margin along a northwest trending dextral transpressive d1 suture this zone is marked by the highl

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

geochemical and u pb zircon geochronological constraints on the development of a late archean greenstone belt at birch lake superior province canada abstract this paper presents new high precision u pb zircon ages that place constraints on the timing of magmatic activity volcanic and plutonic deformation and gold mineralization within the late archean birch confederation lakes greenstone belt within the uchi subprovince of the superior province geochemical data including sm nd isotopic analyses place additional constraints on the nature of the magmatic activity three volcanic rocks from the birch lake area have similar ages 2742 2 1 8 1 7 2739 1 2 8 and 2741 7 2 3 2 2 ma and together with previously published data suggest that a major proportion of the volcanic rocks of the greenstone belt were erupted in a narrow time interval from 2742 to 2735 ma older volcanic rocks recognized to the south of the birch lake area are either absent or of limited extent plutonic rocks include the pre to syn tectonic horseshoe island stock 2725 0 2 5 ma the syn tectonic mainprize lake batholith 2724 3 1 8 1 5 ma and the late to post tectonic shabumeni lake stock 2710 8 1 9 1 3 ma and okanse lake pluton 2700 8 1 9 1 4 ma these ages and structural relationships indicate that the development of a d2 regional penetrative fabric at least in part post dates the emplacement of the horseshoe island stock this is consistent with the interpretation that d2 deformation developed synchronously with and perhaps in response to the emplacement of the mainprize lake batholith an earlier d1 penetrative deformational event as well as possible large scale subhorizontal structures d0 developed subsequent to the deposition of the youngest volcanic rocks and prior to d2 plutons are younger than and unrelated to volcanism in contrast to volcanic rocks plutons show no indication of having interacted with older felsic crustal rocks and are interpreted to be derived from partial melting of metasomatized mantle and subducted tholeiitic basalt the okanse lake pluton has affinities with the sanukitoid suite and is closely comparable in age to similar plutons over a wide area it is proposed that a 2700 ma sanukitoid magmatic event may reflect melting of previously metasomatized mantle in response to thermal re equilibration associated with the transition from an earlier subduction dominated regime to a later post collisional stage culminating in the stabilization of the northern superior province

geochronological constraints on the timing of magmatism deformation and gold mineralization in the red lake greenstone belt northwestern ontario the paper presents u pb ages on zircon baddeleyite titanite rutile and monazite from volcanic and plutonic rocks of the red lake greenstone belt in the northern superior province the purpose of the study was to refine a previously outlined time frame for the volcanic evolution of the belt define the time of major plutonism and deformation and constrain the age of formation of gold deposits volcanism spanned at least 270 ma between 3000 and 2730 ma new ages of 2989 3 and 2894 2 ma for units of a lower volcanic sequence and and 2744 1 ma for units of an upper volcanic sequence corroborate and refine the previously established relationships local gabbroic and felsic intrusions cutting through the lower sequence yield ages of 2870 15 and respectively volcanism was succeeded by major plutonism deformation alteration and metamorphism between about 2730 and 2700 ma these processes were interrelated and progressed as a relatively continuous sequence of events accentuated by major activity d

the evolution of the southern abitibi greenstone belt in light of precise u pb geochronology this paper summarizes u pb zircon ages for the archean southern abitibi greenstone belt and integrates them together with zircon and baddeleyite data for three new samples into a general framework for the evolution of the region new ages are reported for the ghost range gabbroic complex 2713 super 7 sub 5 ma and for a pyroclastic volcanic rock of the gauthier group 2700 or 3 ma a previously determined age of 2701 super 3 sub 2 ma for the felsic volcanic skead group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic component or 2720 ma indicating that the skead volcano was built on top of an older volcanic sequence the compiled ages show that the southern abitibi greenstone belt developed between 2750 and 2670 ma the earliest ages date remnants of volcanic complexes that occur at the periphery of the belt major preorogenic magmatism was confined mainly to the period 2720 to 2700 ma in general each episode was characterized by the essentially coeval formation of komatiitic tholeiitic and calc alkalic igneous rocks taking into consideration the known geologic geochemical and isotopic data this association is consistent with evolution in paired active are and back are systems older crust was absent from the region but it may have interacted with these arcs and rift systems farther to the west the extensional regime was followed by a period of compression that caused folding and thrusting emplacement of calc alkalic plutons and deposition of turbidites between 2700 and 2688 ma the magmatism became predominantly alkalic during the main timiskaming period between 2681 and 2676 ma this event was accompanied by the deposition of alluvial fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting probably between approximately 2676 and 2670 ma lamprophyre dikes were associated with these final stages of deformation postorogenic activity includes faulting and episodes of hydrothermal activity locally associated with gold mineralization ages in the range 2630 to 2580 ma for hydrothermal minerals e g titanite and rutile camflo suggest a correlation of these events with lower crustal magmatism and metamorphism as recorded for example in the adjacent kapuskasing zone

u pb zircon ages for magmatism in the red lake greenstone belt northwestern ontario u pb dating was carried out on nine volcanic rocks and two felsic intrusions from the red lake greenstone belt in order to establish an absolute time framework for the magmatic evolution of the area and yield first indications on the time of deformation and gold mineralization the data indicate a protracted period of igneous activity spanning at least 270 ma felsic volcanic rocks near the top of the tholeiitic to komatiitic sequence in the eastern part of the belt yield ages of and a third unit dated at contains inherited zircons older than 2982 ma which casts some uncertainty on the validity of the inferred intercept age rocks in the western part of the belt previously believed to form a relatively young calc alkalic sequence but now known to be dominantly tholeiitic are shown to be relatively old with ages of and these two dates also bracket the age of stromatolites occurring in chemical sediments that are under and overlain by the dated units another volcanic horizon in the centre of the b

single grain zircon u pb age constraints of the western and eastern sanukitoid zones in the finnish part of the karelian province abstract a single grain zircon u pb secondary ion mass spectrometer sims study of nine granitoid intrusions with sanukitoid geochemistry from the finnish part of the karelian province shows their time limited emplacement the age results indicate emplacement at 2715 and 2745 ma we observed also a significant number of relict cores in the zircons with 207 pb 206 pb ages up to 3 2 ga emphasizing the importance of inheritance processes in the petrogenesis of the sanukitoids compiled single grain age data of the finnish and russian bibikova et al 2005a parts of the karelian province confirm the occurrence of two temporally diverging sanukitoid zones throughout the province the western zone shows a younger average age 2718 ma and the eastern zone an older average age 2740 ma with an apparent age difference of 20 ma between the zones most of the sanukitoid intrusions in finland belong to the western sanukitoid zone but the southernmost intrusions can be regarded as a part of the eastern sanukitoid zone the boundary between the sanukitoid zones specifies the boundary of the western and central karelian terranes worldwide sanukitoid magmatism at 2 95 2 55 ga as reported from several archean blocks followed sharply after prolonged tonalite trondhjemite granodiorite ttg magmatism indicating a short lived tectonomagmatic phenomenon at the end of convergent accretion

source and timing of gold and other mineralization in the red lake area northwestern ontario based on lead isotope investigations the problems of the timing and source of gold mineralization and the sulfidation mechanism in the red lake area north western ontario have been addressed using more than 300 pb isotope analyses o

how and where do incised valleys form if sea level remains above the shelf edge sequence stratigraphic models predict that during relative sea level fall rivers will cut incised valleys across the exposed continental shelf incision is driven by the exposure of convex up topography and consequent downstream increases in stream power modern shelf profiles from passive margin and foreland basin settings show that profile convexities occur at the shelf edge and at the current highstand coastline the coastal prism in nonglacial times which form much of earth9s history it is unlikely that the shelf edge was subaerially exposed during such periods patterns of incision would have been primarily determined by the shape of the coastal prism on the basis of modern shelf profiles from passive margin and foreland basin settings river incision of as much as 20 to 70 m would be centered upon the preceding highstand coastline and would be restricted to the inner part of the shelf deeper incised valleys would only result from exposure of the shelf edge or from the processes involved in the formation of submarine canyons

autochthonous development of superior province greenstone belts abstract models for superior province greenstone belt development have changed over the last 30 years from ensimatic generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm models intermediate between these end members such as autochthonous terrane models have not been prominent within the superior province there is a secular variation from early ca 3 ga platformal assemblages with overlying rift related komatiite tholeiite units representing breakup of ca 3 ga continental fragments to late ca 2 7 ga pull apart basins autochthonous terranes should demonstrate some sort of logical order such as that described above whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order in the face of this situation this paper reviews evidence from throughout the superior province that suggests many but not all greenstone assemblages developed autochthonously or parautochthonously the earliest greenstone assemblages in the superior province are generally quartzite and carbonate bearing platform sequences in the north caribou terrane and the marmion terrane deposited unconformably upon granitoid or greenstone basement the evidence for autochthonous parautochthonous development includes the presence of unconformities between keewatin type greenstone units such as 1 basal unconformities between ca 3 ga granitoid or volcanic basement and slightly younger platformal assemblages 2 between ca 2 9 ga platformal assemblages and younger platformal and andean arc assemblages stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern central and southern superior province are considered suggestive of orderly autochthonous progression from platforms through rifting of continental fragments and late assembly during the kenoran orogeny petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the superior province suggests the presence of older basement implying autochthonous or parautochthonous development of many assemblages younger greenstone assemblages in the abitibi wawa subprovince and the north caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages this pattern has been interpreted to represent autochthonous greenstone development the current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect few actual unconformities between keewatin type greenstone assemblages have been recognized in the superior province to aid in future recognition of unconformities in keewatin type greenstone assemblages examples of unconformities of varying degrees of subtlety in archean sequences are reviewed archean unconformities ranging from sharp angular discordances in the northern superior province to subtle stratigraphic patterns to weathering related geochemical mobilization along inter formational contacts are described these styles of unconformity in archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between keewatin type assemblages in the superior province generally and the abitibi wawa subprovince specifically the most important disconformity suggested in the abitibi wawa subprovince is between the ca 2726 ma deloro and the ca 2708 ma tisdale assemblages the presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an autochthonous origin for many superior province greenstone belts this autochthonous origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place that is development of ca 3 ga platform sequences in selected locales followed by in place evolution of a succession of plume oceanic and arc assemblages culminating in late pull apart basin sequences implications of the autochthonous model for greenstone belts of the superior province particularly the abitibi wawa subprovince include the future ability to recognize unconformity bounded sequences potentially improving regional scale correlation

coupled zircon lu hf and u pb isotopic analyses of the oldest terrestrial crust the 4 03 ga acasta gneiss complex abstract the acasta gneiss complex of the northwest territories canada contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust forming processes on the early earth here we report the results of a combined lu hf and u pb isotopic study of zircons from predominantly felsic orthogneisses from the acasta gneiss complex that crystallized between 4 0 and 2 9 ga many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and hf isotopic domains to ensure the reliability of the analyses and of subsequent geologic interpretations we employed the laser ablation split stream lass technique to concurrently measure the lu hf and u pb isotopic systems in zircon our results confirm prior findings of precursor hadean crust 4 0 ga in the source of these rocks and the continued involvement of this reservoir until 3 6 ga we present evidence for the input of relatively more juvenile material at 3 6 ga which we suggest corresponds to a fundamental change in the source of the magmas this study extends the lower bound of the published acasta hf isotopic record from 3 6 ga to 2 9 ga and demonstrates that the 3 6 ga 2 9 ga interval is largely represented by reworking of relatively juvenile 3 6 ga crust and the diminution of the 4 0 ga crustal signal significantly there is no evidence that rocks within the acasta gneiss complex were derived from a strongly depleted mantle

accretion of the archean slave province continental rift models have long been applied to the archean slave province of northwestern canada a reassessment of these models shows them to be incompatible with observed geological relations and suggests that contractional tectonic models may be more appropriate than extensional ones regions composed of different rock suites e g orthogneisses vs mafic volcanics are separated by high strain zones recording large displacements it is proposed that the high strain zones separate four distinct terranes that have been juxtaposed during collisional orogenesis from west to east these include the anton terrane interpreted as an archean microcontinent the sleepy dragon terrane possibly an exhumed more eastern part of the anton terrane the contwoyto terrane a westward verging fold and thrust belt containing tectonic slivers of greenstone volcanics and the hackett river volcanic terrane interpreted as an archean island arc the contwoyto and hackett river terranes represent a paired accretionary prism and island arc system that formed above an east dipping subduction zone these collided with the anton microcontinent producing a basement nappe expressed as the sleepy dragon terrane during the main accretion event within the slave province the whole tectonic assemblage was intruded by late kinematic to postkinematic granitoids

u pb age and nd isotopic evidence for archean terrane development and crustal recycling in the south central wabigoon subprovince canada u pb geochronology is presented for 19 volcanic and associated plutonic rocks from nine greenstone belts within the southern part of the central wabigoon subprovince cws western superior province data indicate that early felsic volcanism 3 014 2 999 ma was coeval with emplacement of the marmion batholith at 3 002 ma this was followed by periods of mafic volcanism with minor felsic and komatiitic compositions between 2 963 and 2 898 ma across the south cws plume driven mafic and komatiitic volcanism occurred in the lumby lake and lac des mille lacs belts at ca 2 828 ma eruption of komatiitic tuffs in the steep rock belt at 2 780 ma was proceeded by more widespread mafic and minor intermediate to felsic volcanism at 2 729 2 726 ma zircon inheritance and nd isotope data indicate that the cws south of the garden lake belt comprises a juvenile 3 0 ga terrane the marmion terrane subsequent magmatism was ensialic occurring in a continental although submerged setting recycling of the 3 0 ga marmion basement during successive volcanic events although somewhat cryptic was widespread and represents a significant archean process 2 726 ma rhyolite in the garden lake belt has a 3 22 ga nd model age and lies at the southern boundary of the distinct north cws which has a 3 3 3 2 ga crustal prehistory the north cws may represent a heavily reworked extension of the adjacent 3 3 ga winnipeg river subprovince and was probably separate from the marmion terrane at 3 0 ga these two recycled terranes differ significantly from the juvenile neoarchean volcanic dominated western wabigoon although the boundaries between these terranes are in most places cryptic they may indeed represent suture zones which were fundamental to the development of the wabigoon subprovince

u pb zircon systematics at gruinard bay northwest scotland implications for the early orogenic evolution of the lewisian complex the gruinard bay area of the mainland lewisian complex comprises a metamorphosed suite of archaean trondhjemites and minor granites enclosing remnants of older tonalitic gneiss and mafic to ultramafic enclaves the u pb zircon dating yields ages of 2731 14 ma and 2728 2 ma for two trondhjemite and 2732 4 ma for one granite sample also revealing the presence of large amounts of inherited xenocrystic zircons although the region has been pervasively overprinted by retrogressive events in amphibolite to greenschist facies the textural relations between biotite hornblende quartz and titaniferous minerals indicate that these minerals are pseudomorphs of pyroxene and high ti amphibole formed in hornblende granulite facies structural relations link this metamorphism to a steep northeast trending fabric coeval with the intrusion of the trondhjemites dated at 2730 ma dating of zircon in amphibolite and tonalite enclaves yields complex internal isotopic relations with apparent ages ranging from 2825 to 2740 ma this age range reflects new growth during the 2730 ma metamorphic metasomatic events superimposed on older zircon phases which include combinations of xenocrystic cores and magmatic and or metamorphic growth phases whose mode of formation cannot clearly be resolved by imaging techniques e g cathodoluminescence alone a pegmatitic vein that escaped the d3 strain and related isotopic disturbances yields a precise age of 2792 2 ma which constrains to some degree the earliest orogenic events in the area age relationships displayed in the central block at scourie badcall and in the gruinard bay area indicate that petrogenetic events in both areas were comparable about 2800 ma and that both areas underwent trondhjemitic magmatism about 2730 ma in contrast at gruinard bay there is no isotopic evidence for a period of high grade metamorphism and magmatism at 2490 2480 ma that drastically affected the scourie block indicating that at this stage the two regions occupied different levels of the crust

importance of the lu hf isotopic system in studies of planetary chronology and chemical evolution the 176lu 176hf isotope method and its applications in earth sciences are discussed greater fractionation of lu hf than sm nd in planetary magmatic processes makes 176hf177hf a powerful geochemical tracer in general proportional variations of 176hf177hf exceed those of 143ndl44nd by factors of 1 5 3 in terrestrial and lunar materials lu hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time and this is the principal importance of lu hf new data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in 176hf177hf 87sr86sr and 143ndl44nd 87sr86sr diagrams 176hf177hf and 143nd144nd display a single linear isotopic variation in the suboceanic mantle these discordant 87sr86sr relationships may allow with the acquisition of further hf nd sr isotopic data a distinction between processes such as mantle metasomatism influence of seawater altered material in the magma source or recycling of sediments into the mantle in order to evaluate the hf nd isotopic correlation in terms of mantle fractionation history there is a need for measurements of hf distribution coefficients between silicate minerals and liquids and specifically for a knowledge of hf behavior in relation to rareearth elements for studying ancient terrestrial hf isotopic variations the best quality hf isotope data are obtained from granitoid rocks or zircons new data show that very u pb discordant zircons may have upwardly biased 176hf177hf but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial 176hf177hf until the controls on addition of radiogenic hf to zircon are understood combined zircon whole rock studies are recommended lu hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples but because it offers little advantage over existing methods is unlikely to find wide application in pure chronological studies

a critical assessment of neoarchean plume only geodynamics evidence from the superior province abstract criticisms of archean plate tectonic models rarely offer detailed descriptions of alternative paradigms the catalytic delamination driven model for the coupled genesis of archean crust and sub continental lithospheric mantle described by bedard 2006 is an exception in that a relatively comprehensive but essentially 2 d model is described although this novel model is frequently cited as an example of the diversity in geodynamic thought for the archean it is rarely assessed with reference to events in a specific craton such as the superior province or a particular greenstone belt such as the abitibi when this type of assessment is undertaken it becomes clear that the apparent simplicity of the model one of its supposed virtues constitutes a critical weakness although bedard 2006 developed the catalytic model to overcome the perceived melt productivity deficit of archean plate tectonic models the catalytic process at the heart of the model is likely to be inefficient and provides insignificant improvements to crustal generation rates associated with subduction and non catalytic plume tectonics the model implies a near global craton in the mesoarchean to neoarchean but fails to address that fate of this craton during and after crustal rigidification seismic reflectors observed beneath archean cratons are characterized as failed delamination features but their global abundance suggests that syn to post tectonic ttg suites could not have been generated as a result of crustal restite delamination the brief descriptions of suggested mechanisms for the generation of minor rock types such as sanukitoids mask the complexity inherent in the overall model when considered in terms of the evolution of the superior province as a whole it is evident that many of these suggestions are not only implausible they are inconsistent with the timing and distribution of geological criteria used to mark the onset of crustal rigidification

mantle derived archaean monozodiorites and trachyandesites the 2 680 2 750 myr old1 5 intrusive granite greenstone terranes6 8 in the rainy lake region ontario and in and adjacent to the vermilion district minnesota of the superior province in north america include a wide variety of volcanic and intrusive rocks with initial pb sr and nd isotope ratios close to those for the mantle4 5 9 12 the isotopic constraints require the production of large volumes of silica oversaturated magmas from silica undersaturated mantle within 100 200 myr we present here geochemical data on monzodiorites and trachyandesites from the rainy lake area which are strongly enriched in large ion lithophile elements lile we conclude that they are derived by direct melting of the mantle at depths of 50km in either anhydrous or hydrous conditions their rare earth element ree abundances and initial 143nd 144nd ratios suggest that their mantle sources were enriched in lile shortly before melting the monzodiorites trachyandesites and their granodioritic derivatives may comprise up to 20 of the exposed igneous rocks in the rainy lake region and vermilion district if these rocks are as abundant in other archaean terranes a significant part of the early continental crust could have formed by direct melting of lile enriched mantle

reflected turbidity currents on an ordovician basin floor canadian appalachians sedimentary structures associated with turbidity current deposits show either partial or complete bouma ta e sequences1 that suggest deposition of sediment from a waning decelerating unidirectional flow2 bathymetric highs and basin margins can cause turbidity currents to reflect back along their original flow paths a rare situation in both ancient3 4 and modern5 marine basins the internal structures of such contained or reflected turbidites have only been described in one case the contessa bed ref 4 pp 268 300 our data from 1 6 m thick graded mud dominated turbidites from the quebec appalachians using mainly sole marks megaripple ripple and grain fabric data suggest that during the deposition of these beds the turbidity currents were reflected several times from either internal basin highs or basin slopes the recognition of such beds not only provides an insight into the depositional environment deep water basin floor but also allows detailed bed by bed correlations over tens of kilometres of coastal sections since the beds are reliable chronostratigraphical datum lines

formation of the neoarchean bad vermilion lake anorthosite complex and spatially associated granitic rocks at a convergent plate margin superior province western ontario canada abstract the bad vermilion lake anorthosite complex henceforth the bvla complex in western ontario is one of the well exposed anorthosite bearing archean layered intrusions in the superior province canada this study presents new whole rock major and trace element data for the various units of the complex oxygen isotope data for the anorthosite and major and trace element data for the spatially associated granitic rocks intruding the bvla complex to constrain their petrogenetic and geodynamic origin zircons from granitic rocks have yielded a 207pb 206pb age of 2716 18 ma constraining the minimum intrusion age of the complex despite deformation and greenschist facies metamorphism primary igneous textures are locally well preserved in the bvla complex its whole rock major and trace elemental compositions and the oxygen isotopic systematics appear not to have been substantially modified by deformation and metamorphism mantle like oxygen isotope signatures and major and trace element compositions are inconsistent with significant crustal contamination of the bvla complex during its emplacement the existence of primary calcic igneous plagioclase coherent negative nb anomalies nb nb 0 08 0 88 and geochemical similarities between gabbros from the bvla complex and gabbros from cenozoic arcs collectively suggest an intra oceanic subduction zone geodynamic setting for the complex near flat ree patterns in the various units of the bvla complex suggest that they were derived from melting of a shallow source beneath a subarc mantle wedge trends in immobile major e g mgo and trace e g zr element data indicate that the mineralogical composition of the complex can be explained by fractional crystallization and accumulation of olivine orthopyroxene clinopyroxene plagioclase and possibly amphibole compositionally the bordering granitic rocks are a2 type and strongly enriched in th and ree 100 times chondrite and depleted of ba sr eu and ti we suggest that they formed in a post collisional extensional tectonic regime following emplacement of the bvla complex in an oceanic arc

u pb geochronology of the central uchi subprovince superior province u pb zircon and titanite ages for rocks of the central uchi subprovince in northwestern ontario indicate a late archean magmatic and tectonic development spanning over 200 ma an early period at 2900 2800 ma formed volcano plutonic complexes presumably linked to 3 1 2 8 ga terrains of the northwestern superior province a later period of southward growth by magmatic and tectonic accretion occurred at 2750 2710 ma and was concluded by large scale compression and plutonism at 2700 ma the oldest 2890 2860 and 2840 2820 ma components occur in the pickle lake and meen dempster greenstone belts and as gneisses in the seach achapi and the lake st joseph batholiths in northern and central sectors of the region together with distinct 2750 2740 ma volcano plutonic complexes they form a collage assembled by multiple episodes of tectonic juxtaposition and magmatic accretion plutons of 2730 2710 ma age are intrusive into these older northern domains whereas their volcanic counterparts compose the lake st joseph and miminiska fort hope greenstone belts to the south late tectonic to posttectonic granitoid rocks intruded a region extending from the northern berens river subprovince to the southern english river subprovince at 2700 ma these plutons were cut by regional scale faults formed by residual north northwest directed shortening the timing of this movement seems to be recorded by titanite ages of 2690 2670 ma reactivation of the same faults may account for proterozoic pb loss observed in some of the zircon populations the age patterns are consistent with crustal growth along a continental margin in a north dipping subduction environment

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

an isotopic study of the island lake greenstone belt manitoba crustal evolution and progressive cratonization in the late archean three new u pb zircon ages are reported for units within the island lake greenstone belt in northern manitoba the jubilee island dacite 2761 12 ma the chapin bay tonalite 2748 3 ma and t

u pb geoehronology and evolution of the english river subprovince an archean low p high t metasedimentary belt in the superior province u pb isotopic data for zircon monazite and titanite are used to assess the evolution of the late archean english river subprovince ers and adjacent domains of the winnipeg river subprovince in the northern superior province ontario the ers consists of turbiditic metasedimentary rocks deposited during the final stages of magmatic and tectonic accretion within the greenstone granite uchi subprovince to the north at around 2720 to 2710 ma the sedimentary rocks were intruded by a suite of calc alkalic plutons at 2698 ma major regional deformation amphibolite to granulite facies metamorphism anatexis and emplacement of an extensive peraluminous granitic suite culminated at 2691 ma late episodes of metamorphism metasomatism and emplacement of pegmatites occurred locally at circa 2680 and 2669 ma in contrast the winnipeg river subprovince to the south consists of widespread tonalitic suites formed mainly between 3200 and 2800 ma and granodioritic to granitic intrusions formed between 2710 and 2690 ma metamorphism occurred at around 2710 ma in southern domains and a distinct granulite facies event at around 2680 ma affected central and northern domains of the winnipeg river subprovince the rapid sequence of events recorded in the ers supports the notion that metamorphism was caused by thermal perturbations related to injection of granitic magmas these melts were generated through partial melting of the lower crust by a thermal anomaly presumably caused by mantle upwelling related to collapse of the inactivated subducting plate the short lived nature of the metamorphic events and their cyclicity follow the behaviour predicted by recent models linking the evolution of strain rates to episodes of segregation and upward magma transfer in the course of prolonged orogenic events

seismic triggering of landslides and turbidity currents offshore portugal 1 sediments in deep water basins often include turbidites that record sediment input from adjacent continental margins in seismically active areas where turbidity currents are triggered by earthquakes the basinal turbidite sequence may thus contain a record of palaeoseismicity which can be used to infer the frequency of earthquakes affecting the margins of the basin this is particularly useful where large earthquakes have a recurrence interval than is greater than the historical record however turbidity currents can be triggered by several processes and it is often difficult to trace individual turbidites to their precise source areas and to assign a definite trigger to a particular turbidite here we demonstrate that turbidites emplaced at 6600 and 8300 cal yr bp in the tagus abyssal plain off portugal correlate with erosional hiatuses in two submarine canyons on the continental margin the turbidites are sourced from simultaneous landsliding in both canyons requiring regional triggers interpreted as earthquakes an earthquake recurrence interval for the continental margin of 4000 years is estimated by extrapolation to deeper turbidites in the basin sequence however the example of the 1755 earthquake which caused widespread devastation in southwest iberia shows that palaeoseismic interpretations must be made with caution the 1755 earthquake had a magnitude 8 5 and yet the associated turbidite in the abyssal plain is typically 5 cm thick while older turbidites can be 1 m thick given the large 1755 earthquake magnitude the difference in turbidite thickness is unlikely to be related to the relative size of triggering earthquakes instead we suggest that the offshore location of the 1755 earthquake coupled with low sedimentation rates during the holocene may have limited the size of the associated turbidite

recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin abstract recent sediment transport and deposition in the lisbon setubal and cascais submarine canyons portuguese continental margin were investigated on the basis of water column profiles of suspended particulate matter records of near bottom currents and settling fluxes of particulate matter obtained with benthic landers and analysis of surface sediments the results show that fine grained predominantly lithogenic sediment derived from adjacent shelf areas accumulates in the upper reaches of the canyons sediment deposited further down in the middle and lower reaches of the canyons is essentially similar to the hemipelagic sediment found on the adjacent continental slope indicating that down canyon transport of sediment from the upper to the lower canyon is limited tidal currents measured at various depths in the lisbon setubal canyon appear sufficiently strong to resuspend and transport sediment but net up canyon flow of the bottom water may retain shelf derived lithogenic sediment in the upper canyon sediment gravity flows which in the nearby nazare canyon are an effective mechanism for down canyon sediment transport appear rare in the lisbon setubal canyon and probably also in the cascais canyon turbidity current events recorded in the sedimentary record of the canyons may correspond to seismic events of 1969 and 1755 ad

contrasting types of late archean plutonic rocks in northwestern ontario implications for crustal evolution in the superior province abstract the late archean aulneau and lount lake batholiths have contrasting geochemical characteristics that are interpreted to be a consequence of derivation from different sources the aulneau batholith which was intruded into low grade metavolcanic rocks of the wabigoon subprovince is characterized by high na2o and low k2o rb and y together with enriched light rare earth elements lree near chondritic heavy rare earth elements hree negligible eu anomalies and mantle type initial sr and nd isotopic compositions these geochemical characteristics are consistent with derivation from the partial melting of metamorphosed tholeiitic basalt in contrast the lount lake batholith which was intruded into older 2 83 3 17 ga tonalitic plutonic rocks of the winnipeg river subprovince has relatively high k2o rb y and ree low na2o and prominent eu anomalies the initial sr and nd isotopic compositions of the lount lake batholith are consistent with a crustal source at least 200 400 ma older than the crystallization age of the batholith these geochemical characteristics indicate that the lount lake batholith originated from the partial melting of deeper level equivalents of the tonalitic metaplutonic rocks 2 83 3 17 ga which they intrude these results suggest that reworked 3 0 ga crust may be extensive in the winnipeg river subprovince and that the geochemical and isotopic signature of late archean plutonic rocks may be a useful reconnaissance tool in the identification of areas of extensive older crust the results can be interpreted within a uniformitarian model diachronous ensimatic magmatic arcs represented by the winnipeg river and western wabigoon subprovinces developed independently collision of these arcs at 2707 8 ma marks the cessation of subduction and related magmatic activity and the initiation of high grade metamorphism and melting in tectonically thickened portions of the crust