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

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

synchronous vertical and horizontal tectonism in the neoarchean kinematic evidence from a synclinal keel in the northwestern superior craton canada the long and narrow carrot river greenstone belt in the northwestern superior craton is a synclinal keel between two broad granitoid domes its unique structural geometry and kinematics place significant constraints on the timing relationship between vertical and horizontal tectonism a high strain zone spatially coincident with the belt records evidence for synchronous greenstone down granitoid up dip slip and dextral strike slip components kinematic and regional considerations lead to the conclusion that the dip slip component was a result of vertical tectonism and the strike slip component was a result of horizontal tectonism and therefore that the vertical and horizontal tectonism were synchronous it is emphasized that vertical and horizontal tectonism do not have to act exclusively of each other it is not necessary to downplay the significance of vertical tectonism to validate horizontal tectonism and vice versa evidence for synchronous vertical and horizontal tectonism reported here makes it necessary that both processes as opposed to either one or the other and their potential interactions are considered in interpretations of archean terrains it is suggested that synchronous vertical and horizontal tectonism was a common process in the neoarchean and might represent a transition from dominant vertical tectonism in the mesoarchean and paleoarchean to dominant horizontal tectonism in the proterozoic and phanerozoic it might be an important process for gold mineralization at the late stages of archean cratonization 2005 elsevier b v all rights reserved

no heliotropism in neoproterozoic columnar stromatolite growth amadeus basin central australia geophysical implications an apparent sine wave pattern of columns in a single specimen of the stromatolite anabaria juvensis subsequently identified as kotuikania from a neoproterozoic dolomite unit originally assigned to the 850 ma bitter springs formation in the amadeus basin central australia was interpreted previously as recording heliotropic growth that is the non vertical growth of columns throughout the year controlled by averaged incident solar radiation vanyo j p awramik s m 1985 stromatolites and earth sun moon dynamics precambrian research 29 121 142 the model of heliotropic growth was used to estimate obliquity of the ecliptic earth s axial tilt and days year at 850 ma subsequent work however casts strong doubt on the heliotropic interpretation further field observations and the study of 11 additional specimens of anabaria kotuikania juvensis from the original locality confirm that the columns typically display strong branching which produces a common divergence and convergence of columns that is incompatible with heliotropic growth the rare apparent sinuosity of columns is seen as the fortuitous product of column irregularity and column branching moreover stratigraphic studies indicate that the host dolomite unit does not belong to the bitter springs formation but caps the younger cryogenian glaciogenic succession in the amadeus basin and hence is 600 ma the previous estimate of 435 range 409 485 days year based on extrapolated counts of laminae in the original specimen of a k juvensis conflicts with the figure of 400 7 days year indicated by high quality palaeotidal data obtained from the late cryogenian 640 600 ma elatina reynella tidal rhythmites in south australia we conclude that inferences concerning neoproterozoic obliquity and palaeorotation cannot be drawn from the non vertical growth patterns of the columnar stromatolite a k juvensis 2007 elsevier b v all rights reserved

biostratigraphical and ecostratigraphical significance of tremadoc ordovician graptolite faunas from the misty creek embayment and selwyn basin in yukon and northwest territories graptolite zones of early and late tremadoc ages are described from the elmer creek formation in the selwyn basin and from the duo lake formation in the misty creek embayment in ascending order these are staurograptus dichotomus anisograptus matanensis adelograptus cf a tenellus and aorograptus victoriae biozones the uppermost zone embraces the adelograptus antiquus biozone kiaerograptus pritchardi biozone and the paradelograptus kinnegraptoidesbiozone in the richardson trough twenty five species are recorded and fourteen species are described including the first record of the rhabdinopora fauna in the early tremadoc of northwest canada and kiaerograptus magnuswilliams and stevens and paratemnograptus isolatuswilliams and stevens in the late tremadoc when modifications to cooper s 1999 ecostratigraphic model are made based on data from yukon and northwest territories the upper slope biotope and lower slope ocean floor biotope become so similar that there is little basis for recognising both biotopes

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

nd isotope systematics of 2 7 ga adakites magnesian andesites and arc basalts superior province evidence for shallow crustal recycling at archean subduction zones abstract an association of adakite magnesian andesite ma and nb enriched basalt neb volcanic flows which erupted within normal intra oceanic arc tholeiitic to calc alkaline basalts has recently been documented in 2 7 ga wawa greenstone belts large positive initial nd values 1 95 to 2 45 of the adakites signify that their basaltic precursors with a short crustal residence were derived from a long term depleted mantle source it is likely that the adakites represent the melts of subducted late archean oceanic crust initial nd values in the ma 0 14 to 1 68 nb enriched basalts and andesites neba 1 11 to 2 05 and normal intra oceanic arc tholeiitic to calc alkaline basalts and andesites 1 44 to 2 44 overlap with but extend to lower values than the adakites large tightly clustered nd values of the adakites together with th ce and ce yb systematics of the arc basalts that rule out sediment melting place the enriched source in the sub arc mantle accordingly isotopic data for the ma neba and normal arc basalts can be explained by melting of an isotopically heterogeneous sub arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late archean subduction zones up to 200 ma prior to the 2 7 ga arc if the late archean wawa adakites ma and basalts were generated by similar geodynamic processes as their counterparts in cenozoic arcs involving subduction of young and or hot ocean lithosphere then it is likely that late archean oceanic crust and arc crust were also created and destroyed by modern plate tectonic like geodynamic processes this study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the archean upper mantle in addition the results of this study indicate that the nd isotope compositions of archean arc and plume derived volcanic rocks are not very distinct whereas phanerozoic plumes and intra oceanic arcs tend to have different nd isotopic compositions

seismic evidence for preservation of the archean uchi granite greenstone belt by crustal scale extension abstract in the westernmost superior province of canada the east west alignment of granite greenstone belts and the adjacent highly deformed gneiss belts led to the first proposals that plate tectonics existed before 2 5 ga ago with the belts thrust against one another by east west oriented subduction zones here we present seismic reflection data which demonstrate that in this region the present juxtaposition of the uchi granite greenstone belt and the north caribou gneiss terrane occurred along a late southeast dipping extensional shear zone that extends from the surface into the lower crust the preservation of the uchi belt and probably the english river metasedimentary belt is directly related to their dropping along extensional shear zones which limited subsequent erosion the relative lateral transport of these greenstone rocks implies that they were neither derived from the immediately underlying crust nor preserved by vertical crustal movements as might occur in the absence of plate tectonics extension may have been associated with the emplacement of mantle derived magmas at 2700 ma which has been linked to slab break off or lithospheric delamination making the extension approximately coeval with local gold mineralisation since crustal scale faults can facilitate the circulation of gold bearing fluids we suggest that greenstone rocks preserved in the hanging walls of syn to post accretion extensional shear zones may preferentially host archean lode gold deposits in the westernmost superior province our seismic observations imply that some of the late structures in the well developed belts defined by surface mapping arose through the collapse of a collage of laterally accreted terranes

patterns of total and incremental strain in subsiding troughs experimental centrifuged models of inter diapir synclines the experimental technique of centrifuge modelling has been used to elucidate patterns of total strain and progressive deformation associated with troughs that form by gravitational subsidence of a dense cover into a less dense basement model structures are analogous to synclines found between mantled gneiss domes and the models provide data with which to test the hypothesis that archean greenstone belts are products of vertical gravity tectonics experimental results include the following 1 strain within the main body of subsiding belts is dominated by strong horizontal contraction and vertical extension in natural structures this strain pattern would produce vertical foliation and steeply plunging mineral lineation 2 within cover units near trough margins the flattening plane trends parallel with or at a low angle to the cover basement interface at all levels in the trough 3 patterns of strain within subsiding troughs at least as expressed in surface deformation reflect the geometry of the

structural analysis of the miniss river and related faults western superior province post collisional displacement initiated at terrane boundaries mountain building in the western part of the archean superior province culminated with the formation of regional strike slip faults this paper reports on the kinematics and timing of several major faults at the juncture between the uchi english river winnipeg river and western wabigoon subprovinces sinistral oblique mylonitization along the northeast striking miniss river fault occurred at 2681 ma this involved 40 km of sinistral offset and a scissor like motion whereby vertical displacement increased southwestward toward a restraining bend near sioux lookout to the north the miniss river fault is intersected by the east striking dextral strike slip sydney lake lake st joseph fault the latter merges along strike with the pashkokogan fault restoration of respective displacements indicates that the faults formed sequentially not simultaneously in response to tectonic indentation dextral strike slip motion along the sydney lake lake st joseph pashkokogan fault was instigated at 2670

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

integrated potential field and seismic constraints on the structure of the archean metasedimentary english river belt western superior craton canada the english river subprovince is a prominent belt of metasedimentary rocks in the archean western superior province the structure of its western half was investigated by using techniques of enhancement and automatic interpretation of magnetic data and integration of magnetic derived information with seismic and gravity data the results indicate that a suite of exposed felsic plutons that intruded the belt at ca 2698 ma extends under most of the metasedimentary rocks that are exposed at the surface the thickness of the metasedimentary rocks is interpreted to be less than 1 km in areas where it is underlain by the members of this intrusive suite in other areas the metasedimentary rocks attain thicknesses of 3 4 km and appear to be underlain by rocks similar to the gneissic rocks that are exposed in the adjacent metaplutonic winnipeg river subprovince the integration of enhanced magnetic data with gravity data indicates that the large gravity anomaly that extends along the english river belt correlates well spatially and morphologically with the extensive suite of felsic intrusions that underlies the belt suggesting that the crustal component of the gravity anomaly is related to this suite of intrusions we interpret the source of the gravity anomaly as a dense unit comprising anhydrous mineral assemblages that formed within these felsic intrusions in response to low pressure high temperature metamorphism that affected the belt at ca 2691 ma on the basis of geochronological geological and geophysical constraints we propose that this metamorphic episode is linked to the continuation of magmatism at depth after the emplacement of the ca 2698 ma felsic plutons being ultimately related to the advection of mantle heat into the crust during a period of regional extension 2005 elsevier b v all rights reserved

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

growth of granite greenstone terranes at convergent margins and stabilization of archean cratons abstract archean granite greenstone terranes represent juvenile continental crust formed in a variety of plate tectonic settings and metamorphosed through a complex series of structural and magmatic events most archean granite greenstone terranes appear to have acquired their first order structural and metamorphic characteristics at convergent plate margins where large accretionary wedges similar in aspect to the chugach makran and altaids grew through offscraping and accretion of oceanic plateaux oceanic crustal fragments juvenile island arcs rifted continental margins and pelagic and terrigenous sediments buoyant slabs of parts of archean oceanic lithosphere may have been underplated beneath these orogens forming thick crustal roots characterized by interleaving between the depleted slabs and undepleted asthenosphere back stepping of the subduction zones after accretion of plateaux and arcs caused the arcs magmatic fronts to migrate trenchward through the accretionary wedges dehydration of the subducting slabs hydrated the mantle wedges below the new arcs and generated magmas sanukitoid suite in the mantle wedge whereas other magmas tonalite trondhjemite granodiorite or ttg suite appear to have been generated by melting of hot young subducted slabs eventual collision of these juvenile orogens with other continental blocks formed anatectic granites then thickened the crust beyond its ability to support its own mass which initiated gravitational collapse and decompressional release of syn to late tectonic granitoids from wedges of fertile mantle trapped between underplated oceanic lithospheric slabs and aided in the cratonization of the granite greenstone terranes deeply penetrating structural discontinuities such as shear zones and sutures provided pathways for fluids and granitoids to migrate into the mid and upper crust forming ore deposits and plutons most preserved granite greenstone terranes have been tectonically stable since the archean and form the cratonic interiors of many continents

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

long lived transpression in the archean bird river greenstone belt western superior province southeastern manitoba abstract the archean bird river greenstone belt brgb is located on the southwestern edge of the superior province between the 3 2 ga old winnipeg river subprovince to the south and the metasedimentary belt of the english river subprovince ersp to the north this position between two major subprovinces makes the brgb a primary target for investigating the geodynamic and kinematic evolution of a major structural boundary new structural and geochronological data have allowed us to present an evolutionary framework for the southern boundary of the north caribou superterrane the brgb underwent 3 main deformation phases the d1 event took place ca 2698 ma and displays a north side up shearing the d2 event occurring at ca 2684 ma in a transpressive context presents a complex structural pattern mixing vertical tectonics in the brgb and strike slip tectonics along the boundaries of the greenstone belt with other subprovinces between the brgb and the ersp the 2832 2858 ma old maskwa batholith acted as a rigid passive block during the collision and marks the boundary between pure dextral strike slip tectonics along his northern boundary with the ersp and vertical south side up motion in the brgb the brgb can be considered as a pop up structure with anastomosed shear zones displaying different horizontal offset according to the orientation of the shear zones the southern boundary with the winnipeg river subprovince is represented by a sinistral south side up shear zone the same pattern is found at the regional scale where major shear zones acted as a conjugate set in the horizontal plane at ca 2640 ma the d3 event occurred in a general dextral transpressive tectonic regime coeval with the emplacement of rare elements pegmatitic plutons in a still hot 400 500 c country rock the geodynamical and mechanical significance of the partitioning between pure strike slip tectonics in the english river subprovince and vertical motion in the brgb can be explained by the rheological behaviour of a hot and weak lithosphere undergoing transpressive strain the structural framework of the brgb is the result of strong interactions between hot and weak domains coeval with widespread plutonism and a rigid older domain maskwa batholith during the d2 transpressive event

structural evolution of the cross lake greenstone belt in the northwestern superior province manitoba implications for relationship between vertical and horizontal tectonism evidence for both vertical and horizontal movement is well preserved in the cross lake greenstone belt in the northwestern superior province the vertical movement components are concentrated in high strain zones along pluton greenstone contacts and are characterized by pluton side up or greenstone side down movement and the geometry kinematics and strain distribution are consistent with a vertical tectonic model involving diapirism and sagduction the horizontal components are concentrated in major east southeast trending dextral high strain zones and in subordinate northeast trending sinistral antithetic high strain zones and can be readily explained by a horizontal tectonic model involving dextral transpression results of a detailed structural analysis indicate that the vertical and horizontal tectonism were more or less synchronous and there was a transition from dominantly vertical tectonism at the early stages to dominantly horizontal tectonism at the late stages the cross lake group consist

two linear granite belts in the central western north china craton and their implication for late neoarchaean palaeoproterozoic continental evolution abstract precambrian granitoid rocks are widely distributed in the north china craton ncc the dominant granitic rocks in the central western ncc are trondhjemitic tonalitic and granodioritic ttg gneisses as well as some calc alkaline granitic gneisses and minor charnockites two linear granite belts in this area are different from other early precambrian granitic rocks in the ncc in terms of their occurrence and chemistry the liang tu lt garnet granite belt and the sanggan sg granite belt the lt belt occurs along two large scale ductile shear zones in metamorphosed al rich sedimentary country rocks khondalitic series in the central part of the lt belt are metagabbroic granulite lenses that were originally mafic dykes in the outer part of the belt garnet granites grade into khondalitic rocks lt belt granites have geochemical characteristics of s type granites the sg granite belt is 400 km long and composed of many deformed gneissic granitic bodies and granitic sills and veins with without gneissosity they contain various lenses including high pressure granulites and retrograded eclogites the sg granite belt trends east northeast parallel to the regional gneissosity of the country rocks major and trace element geochemistry of these granites shows characteristics of syn collisional and post collisional granite the country rocks beside the sanggan granite belt are different including the huai an fengzhen terrain hft to the northwest and the wutai fuping terrain wft to the southeast the lt and sg granite belts formed within an old continental craton and an old orogenic zone between two blocks respectively

structure of the archean english river subprovince implications for the tectonic evolution of the western superior province canada the english river subprovince is one of two metasediment dominated terranes in the western superior province it has been interpreted as an accretionary complex a foreland or a fore arc basin that developed and was subsequently deformed between the metavolcanic rich uchi subprovince and the orthogneiss and metaplutonic dominated winnipeg river subprovince during a prolonged transpressive orogeny to test these hypotheses we combined a satellite image aeromagnetic image and lithoprobe reflection seismic profile interpretation with detailed structural mapping to better characterize the internal geometry and significance of structural features in the western part of the subprovince in ontario northward directed subduction and collision of the winnipeg river subprovince with the uchi subprovince at ca 2713 2698 ma can account for the deposition of the sedimentary rocks initial metamorphism and the main phase of deformation in the subprovince whereas the subduction of wabigoon crust generated exten

images of a lower crustal oceanic slab direct evidence for tectonic accretion in the archean western superior province the archean western superior province in canada is the type area for proposed archean plate tectonics seismic images from this region provide direct evidence for assembly of the craton by terrane accretion and for a large slab of remnant oceanic crust preserved at the base of the crust this slab with inferred garnet amphibolite composition adds a critical piece of evidence to previous suggestions that archean subduction was at a shallow angle and that some neoarchean tonalite trondhjemite granodiorite suites distinct from most modern day suprasubduction magmas are melts primarily derived directly from subducted slabs

evolution and tectonic significance of intermediate to felsic plutonism associated with the hemlo greenstone belt superior province canada abstract plutonic rocks from the hemlo area in the southern superior province span a period from ca 2720 to 2677 ma early ttg plutons intruded into lower basaltic volcanic sequences and were probably derived from melting of basaltic crust at upper mantle depths they predate major regional deformation and span ages from 2719 to 2697 ma felsic to intermediate volcanism 2697 2688 ma partially overlaps ttg plutonism but is mostly slightly younger and lacking plutonic equivalents regional deformation and metamorphism of the greenstone belt and ttg plutons progressed rapidly following deposition of the youngest supracrustal rocks syn to late tectonic plutons emplaced between 2684 and 2677 ma have a distinctive dual primitive evolved character that is interpreted to reflect their derivation by direct melting of long term depleted mantle that was metasomatically enriched in a wide range of lile and hfse this secular evolutionary change in the nature of intermediate to felsic plutonism at hemlo and its relationship to regional deformation and metamorphism parallels that occurring more regionally within the superior province and is interpreted to be the consequence of a transition in the nature of large scale tectonic processes intermediate to felsic magmatism within a protracted period of volcanism and ttg plutonism originated primarily by melting of subducted basaltic crust probably in an oceanic arc related environment the termination of this style of magmatic activity and initiation of regional deformation and metamorphism at ca 2688 ma is interpreted to be a consequence of collisional orogeny the petrogenetic characteristics of the latest period of magmatism at 2684 2677 ma requires that the melting regime migrate into sub arc lithospheric mantle that was metasomatized during the earlier magmatic stage slab breakoff following terrane accretion is suggested as one possible mechanism for the abrupt termination of ttg magmatism and ensuing rapid and progressive introduction of large amounts of heat into sub arc mantle and basal portions of the crust this pattern of development is seen in many superior province greenstone belts but is older in the north therefore it is probably an inevitable consequence of the tectonic processes that led to accretion of the superior province

age and isotopic composition of late archean leucogranites implications for continental collision in the western superior province u pb and sm nd isotopic data are presented for leucogranites and pegmatites from the boundary zones of the english river winnipeg river and wabigoon subprovinces in the late archean superior prov

lead diffusion in apatite and zircon using ion implantation and rutherford backscattering techniques abstract ion implantation was used to introduce pb into the minerals zircon and apatite diffusion profiles were obtained using rutherford backscattering and the results were fitted with a model to determine the diffusion coefficients this approach is both simple and useful in studying diffusion over a temperature range of geologic interest without inordinate annealing times results for apatite over the temperature range 600 900 c are in good agreement with earlier results obtained for higher temperatures watson et al 1985 and are described by the following arrhenius law d 1 27 10 4 cm 2 s 1 exp 54 6 1 7 kcl mol 1 rt this suggests that radiation damage induced by ion implantation has little effect on pb diffusion in this case due to rapid annealing of induced damage at low temperatures the activation energy stated above is somewhat smaller than that previously determined 70 kcal mol 1 reflecting the larger range in 1 t sampled by the present results closure temperatures calculated with these diffusion parameters are in good agreement with those inferred from geochronologic data diffusion coefficients for pb in zircon determined in this study are greater than those obtained from a single experimental measurement and estimates of pb diffusion based on geochronologic data and u pb zoning in natural zircons the differing results for zircon and apatite obtained in this study appear to be related to each mineral s ability to anneal the radiation damage induced by ion implantation

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