evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

crustal structure and surface zonation of the canadian appalachians implications of deep seismic reflection data in 1986 1181 km of marine seismic reflection data was collected to 18 20 s of two way traveltime in the gulf of st lawrence area the seismic profiles sample all major surface tectono stratigraphic zones of the canadian appalachians they complement the 1984 deep reflection survey northeast of newfoundland together the seismic profiles reveal the regional three dimensional geometry of the orogen three lower crustal blocks are distinguished on the seismic data they are referred to as the grenville central and avalon blocks from west to east the grenville block is wedge shaped in section and its subsurface edge follows the form of the appalachian structural front the grenville block abuts the central block at mid crustal to mantle depths the avalon block meets the central block at a steep junction that penetrates the entire crust consistent differences in the seismic character of the moho help identify boundaries of the deep crustal blocks the moho signature varies from uniform over extended di

crustal geometry of the abitibi subprovince in light of three dimensional seismic reflector orientations we provide precise estimates of reflector orientations beneath the archean abitibi subprovince using two distinct approaches based on lithoprobe seismic reflection data in the first we use the dip of reflections observed on intersecting profiles to establish the three dimensional orientation of reflective structures in the second the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth dip depth and medium velocity mid crustal reflectors define two areas with distinctive geometry reflectors beneath the southern abitibi belt are oppositely dipping and convergent at depth providing a v shape aspect to the greenstone rocks other reflectors beneath the northern abitibi belt are in general subparallel dipping at an average of 30 toward the north these north dipping reflectors are partly disrupted by a low reflectivity zone which is attributed to

contribution of mantle plumes crustal thickening and greenstone blanketing to the 2 75 2 65ga global crisis abstract assuming that the period 2 75 2 65 ga corresponds to a single but global geodynamic event we investigate through numerical experiments the mechanisms that could have led to the profound continental reworking that occurred at that time although the extent of the crisis at the earth s surface pledges in favour of the involvement of mantle plumes our numerical experiments suggest that the thermal impact of mantle plumes is unlikely to explain both the amplitude and timing of the thermal anomaly as observed in the superior province canada and the yilgarn craton australia similarly moderate crustal thickening can not lead to significant reworking of the continental crust within the observed time constraint crustal thickening with a factor 1 5 is also unlikely because it is not consistent with the moderate metamorphic grade observed at the surface of many archaean cratons burial of a radiogenic crust under a 10 km thick greenstone cover also falls short of explaining not so much the amplitude and the extent but the timing of the thermal anomaly in contrast the combination of the thermal anomaly related to the greenstone blanketing effect with the heat transfer from a plume head spreading at the top of the thermal boundary layer can adequately explain the amplitude the timing and the extent of the 2 75 2 65 ga crisis our favoured model involves a global rearrangement of convection cells in the deep mantle and formation of multiple mantle plumes the greenstones emplaced at the surface and the plumes that spread in the thermal boundary layer contributed to heat the crust from both above and below this produced massive crustal partial melting that reached its climax ca 40 myr after the emplacement of the plumes and associated greenstone cover rocks this led to gravitational instabilities in the crust as dense greenstone cover rocks began to sink into the thermally softened crust and granite domes rose in response the extraction of heat producing elements toward the upper part of the crust has contributed to the cooling and stabilisation of the cratons this succession of events which is not incompatible with plate tectonic processes may have profoundly changed the nature of the crust exposed at the surface and could explain the contrasting geochemical signatures of archaean and post archaean shales

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

high frequency magnetotelluric investigation of crustal structure in north central abitibi quebec canada summary thirty one audiomagnetotelluric amt soundings have been collected along a n s profile across the northern abitibi greenstone belt the recording frequency ranges of 10000 1 hz for 25 stations and 10000 0 001 hz for the other six stations were appropriate for defining the near surface geoelectric structures and for imaging major faults and shear zones down to approximately 10 km along the profile six time domain electromagnetic tem soundings were also collected near the southern end of the profile to provide a correction for static shift extensive static distortion analysis reveals that most of the amt data are not distorted by localized near surface structures however they are affected by static shift in order to remove this static shift a spatial filter was designed and applied to the data effectively reducing static shift as verified by tem soundings at the same sites extensive 2 d inversions of the filtered data were carried out the results suggest the presence of a thin overburden with both conductive and resistive blocks overlying a very resistive basement among the four geological deformation zones crossed by the profile only two zones are imaged as subvertical conductive crustal structures extending to the limit of resolution of amt these correspond to major breaks porcupine destor fault and casa berardi tectonic zone as evidenced by the seismic reflection data collected over the same area

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

applications of seismic pattern recognition and gravity inversion techniques to obtain enhanced subsurface images of the earth s crust under the central metasedimentary belt grenville province ontario summary project lithoprobe s abitibi grenville transect seismic reflection lines 32 and 33 traverse the exposed central metasedimentary belt cmb located in the grenville province of the precambrian shield of canada in southern ontario these seismic lines image a zone with a protracted deformational history spanning more than 300 myr detailed examination of the commercially processed stacked sections reveals a number of significant deficiencies in some important areas the image quality in these zones of reduced coherency needs to be enhanced to examine specific features and their relation to the surface geology examination of near vertical seismic data from lines 32 and 33 revealed that the signal to noise ratio was not improved by stacking due to misalignment of signals even after static normal moveout corrections and residual static corrections the presumed reason is that reflected seismic energy following long ray paths in heterogeneous media suffers from relative advances and delays in its propagation and hence arrives at slightly different times at the receivers tending to be poorly aligned relative to its theoretical traveltime curves a pattern recognition pr method for signal enhancement followed by energy stacking in moving time windows was used in this study to improve the images in spite of misalignments reprocessing has refined the geometry of the reflection profiles the objective of this paper is to use enhanced images of the seismic reflection data obtained by using a pr approach together with gravity data using 2 5 d forward and 3 d inversion routines to give an improved model of subsurface structure in the vicinity of lines 32 and 33 line 32 is dominated by southeast dipping reflectors soling into the lower crust the listric geometry of the strong reflection packages of the cmb boundary thrust zone is interpreted to represent a crustal scale ramp flat geometry that accommodated northwest directed tectonic transport of the cmb this interpretation is also supported by the gravity data

accretion of archean oceanic plateau fragments in the abitibi greenstone belt canada the arc arc collision model is often applied to the evolution of archean greenstone belts this model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic tholeiitic affinities toward calc alkalic affinities new geological data from the malartic composite block of the abitibi greenstone belt indicate that this volcanic pile rather than representing a conformable sequence is composed of four distinct mafic lithotectonic domains that are overlain by a calc alkalic sequence the mafic domains are thought to be examples of accreted archean oceanic plateau material that were deformed during their collage state the younger calc alkalic sequence represents extension related volcanism that erupted through the ac creted and deformed plateau material possibly as a consequence of ridge subduction in a regime of oblique convergence the relations described here may represent a common feature of archean terranes

lithoprobe reflection studies of archean and proterozoic crust in canada abstract lithoprobe canada s national collaborative earth science research project established to develop a comprehensive understanding of the evolution of the north american continent is a multidisciplinary program spearheaded by seismic reflection studies five recently recorded seismic lines discussed in this paper are located in precambrian regions the mesoproterozoic grenville province the paleoproterozoic trans hudson orogen tho the paleoproterozoic archean basement of alberta and the archean superior province data acquired across the grenville orogen in eastern quebec show strong reflectivity throughout the crust upper crustal reflections can be correlated with exposed structural elements including extensional shear zones and packages of deformed high pressure rocks eclogites in a marine survey across the grenville orogen off southeastern labrador seismic images show variably dipping reflections and a structural high associated with a major gravity anomaly data acquired across central alberta show crustal scale thrust stacking and imbrication of the archean hearne craton to the east across the trans hudson orogen images of similar collisional features are observed geochronologic constraints indicate contemporaneity of tectonic activity between the two regions at 1 8 ga suggesting that collisional tectonic activity was coeval over a broad crustal region ca 1000 km across strike in the superior province seismic data across a collision zone involving the northern abitibi greenstone belt and the arc related opatica plutonic belt show spectacular crustal reflectivity and dipping reflections that extend 8 s 30 km into the mantle the latter feature is interpreted as representing a relict 2 69 ga old suture associated with subduction providing the first direct evidence that plate tectonics was active in the late archean these five examples supported by other lithoprobe results refute a number of generalizations about crustal reflectivity that have been made in the past and illustrate how reflection studies combined with other geoscience studies can lead to a better understanding of precambrian tectonics reflectivity persists throughout the crust there is no general separation into a poorly reflective upper crust and a reflective lower crust crustal reflectivity in archean and proterozoic regions is as pervasive as that in areas of more recent tectonism the precambrian reflection moho is generally well defined but shows a range of characteristics relative ages of reflectors can be discerned and tectonic significance can be attached to characteristic features of the crustal reflectivity

stratigraphy of the steep rock group northwest ontario a major archaean unconformity and archaean stromatolites the archaean steep rock group of northwest ontario situated in the wabigoon subprovince of the superior province canada comprises five formations wagita formation clastics mosher carbonate jolliffe ore zone dismal ashrock and witch bay formation metavolcanics reinvestigation of the geology of the group has shown that the basal clastics of the wagita formation 0 150 m unconformably overlie the marmion complex a massive tonalite tonalite gneiss terrane 3 ga old overlying the basal elastics is the mosher carbonate 0 500 m containing diverse stromatolite morphologies extensive zones of carbonate breccia occur adjacent to fault zones and mafic dykes stratigraphically above the mosher carbonate is the jolliffe ore zone 100 400 m which is divided into a lower manganiferous paint rock member and an upper goethite member within the jolliffe ore zone thin layers of buckshot ore occur these are horizons of haematitic pisolites and fragments set in a lighter ferruginous matrix of kao

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

teleseismic studies of the lithosphere below the abitibi grenville lithoprobe transect in the past decade the abitibi grenville lithoprobe transect has been the site of numerous geological and geophysical surveys oriented towards understanding the lithospheric evolution of the southeastern superior and adjoin ing grenville provinces among the different geophysical methods that have been employed earthquake seismology provides the widest range of information on the deep structures of the upper mantle this paper presents a review of studies both complete and ongoing involving teleseismic datasets that were collected in 1994 and 1996 along the transect a complete shear wave splitting analysis has been performed on the 1994 dataset as part of a comparative study on electrical and seismic anisotropies results suggest a correlation between the two anisotropies supported by xenolith data and favour a lithospheric origin for the seismic anisotropy the two anisotropies are believed to represent the fossilized remnants of archean strain fields in the lithospheric roots of the canadian shield preliminary splitting results for the 1996 experiment suggest that the s wave azimuthal anisotropy may be depth dependent and laterally varying ongoing receiver function analysis and traveltime inversion studies provide velocity models of the crust and upper mantle beneath the study area preliminary receiver function results reveal the presence of an s velocity increase at 90 100 km depth which appears to be laterally continuous over 200 km traveltime inversion models indicate the presence of an elongate low velocity anomaly beneath the southern portion of the 1996 array which strikes obliquely to major geological structures at the surface e g grenville front preliminary interpretation relates this anomaly to the same process e g fixed mantle plume continental rifting responsible for the emplacement of the monteregian hills igneous province

extended history of a 3 5 ga trondhjemitic gneiss wyoming province usa evidence from upb systematics in zircon abstract the beartooth bighorn magmatic zone bbmz and the montana metasedimentary province mmp are two major subprovinces of the archean wyoming province in the northwestern beartooth mountains these subprovinces are separated by a structurally lithologically and metamorphically complex assemblage of lithotectonic units that include 1 a strongly deformed complex of trondhjemitic gneiss and interlayered amphibolites and 2 an amphibolite facies mafic unit that occurs in a nappe that structurally overlies the gneiss complex zircons from a trondhjemitic blastomylonite in the gneiss complex yield concordant u pb ages of 3 5 ga establishing it as the oldest rock yet documented in the wyoming province two younger events are also recorded by zircons in this rock 1 an apparently protracted period of high grade metamorphism and or intrusion of additional magmas at 3 25 ga and 2 growth of hydrothermal zircon at 2 55 ga apparently associated with ductile deformation that immediately preceded structural emplacement of the gneiss although this latter event appears confined to areas along the bbmz mmp boundary evidence of 3 25 ga igneous activity is found in the overlying amphibolite 3 24 ga and throughout the mmp these data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3 25 ga the limited occurrences of 2 8 ga magmatic activity in the mmp suggest that it had a controlling influence on late archean magmatism as well

eclogitized gabbros from the eastern grenville province textures metamorphic context and implications undeformed lenses of olivine orthopyroxene gabbro belonging to the 1429 ma shabogamo intrusive suite parautochthonous belt of the eastern grenville province host the first occurrence of eclogite mineral assemblages reported from the grenville province these rocks preserve igneous textures overprinted by eclogite mineralogy and by textures related to subsequent unloading high pressure signatures include omphacite pseudomorphs after augite plagioclase replacement by garnet kyanite corundum and orthopyroxene omphacite garnet coronas between olivine and plagioclase transformations related to decompression include replacement of garnet kyanite corundum by spinel plagioclase growth of plagioclase collars between clinopyroxene and garnet and de jadeitization of omphacite clinopyroxenes with numerous plagioclase inclusions and garnets surrounded by clinopyroxene plagioclase symplectites in granoblastic mafic rocks from the same area are also evidence of decompression from high pressures these

u pb geochronology of the eastern abitibi subprovince part 2 noranda kirkland lake area u pb zircon ages for 15 volcanic and plutonic units in the noranda and kirkland lake areas help constrain the history of volcanism plutonism sedimentation and deformation in the south central part of the abitibi belt volcanism occurred over an interval of at least 50 ma beginning with the deposition of the volcanic and volcaniclastic units within the pacaud structural complex at 2747 ma following a period of apparent quiescence magmatism resumed at 2730 2725 ma with the eruption of volcanic rocks in the normetal and lac abitibi area from 2715 until about 2698 ma volcanism occurred sporadically throughout much of the area culminating in the eruption of the blake river group from 2703 to 2698 ma several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the blake river group plutons that are considered to have been emplaced during the kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the blake river group emphasizing

unconformity bounded upper paleozoic megasequences in the beishan region nw china and implications for the timing of the paleo asian ocean closure abstract the upper paleozoic sedimentary sequence in the beishan region northwest china provides a complete record of the complex amalgamation and extension history of the central asian orogenic belt caob for this part of china integrating newly obtained stratigraphic paleontological and radiometric data this paper reviews the upper paleozoic stratigraphic sequences of the beishan region the upper paleozoic strata here are divided into three megasequences ms they are in ascending order devonian mississippian guadalupian and lopingian megasequences ms1 ms2 and ms3 separated by two major unconformities each indicative of a huge sedimentary hiatus ms1 is made up of continental and epicontinental marine deposits in the pre rift stage ms2 of transgressive marine and volcanic strata in the syn rift stage and ms3 of continental sediments in the inversion stage the major unconformity between ms1 and ms2 is interpreted to represent the regional uplift and erosion caused by the final amalgamation of caob in the beishan region the syn rift ms2 represented by transgressive marine and volcanic strata in the beishan region can be well correlated with pennsylvanian guadalupian megasequences in inner mongolia where they rest on a major interregional unconformity this correlation suggests the closure of the paleo asian ocean in beishan and inner mongolia before the carboniferous followed by the development of a synchronous marine rift megasequence in the southern part of the caob

subprovince accretion tectonics in the south central superior province development of tectonic subprovinces as shear bounded granite greenstone and sediment dominated terranes during the late archaean is reviewed and interpreted from relationships between portions of

timing and kinematics of post timiskaming deformation within the larder lake cadillac deformation zone southwest abitibi greenstone belt ontario canada the larder lake cadillac deformation zone is one of several anastomosing zones of high strain within the abitibi greenstone belt in the kirkland lake area ontario the larder lake cadillac de

giant granulite terranes of northeastern superior province the ashuanipi complex and minto block the ashuanipi complex and minto block of the superior province are large regions that have been classified as high grade gneiss terranes on the basis of the presence of orthopyroxene bearing units like the granite greenstone and metasedimentary subprovinces of the southern superior province the two terranes consist predominantly of intrusive rocks but are distinguished by their primary magmatic orthopyroxene both high grade and gneiss are misnomers because granulite facies gneisses are only sparingly present and the regions consist dominantly of massive unmetamorphosed plutonic rock the ashuanipi complex consists of a deformed metamorphosed package of metasedimentary rocks and primitive early tonalite cut by widespread orthopyroxene garnet granodiorite diatexite as well as plutons of tonalite granite and syenite based on its lithological and chronological similarity and on strike position the complex appears to be the continuation of metasedimentary subprovinces such as the quetico its evolution involved deposition of immature greywacke in an accretionary prism early arc tonalitic magmatism and deformation followed by widespread intracrustal magmatism in the period 2700 2670 ma both metamorphic and igneous rocks record equilibration under granulite facies conditions 700 835 c 0 35 0 65 gpa a 0 3 and indicate exposure levels of 20 km the minto block at the latitude of leaf river consists of several north northwest trending domains of similar scale and diversity to the east trending subprovinces of the southern superior province the central goudalie domain is dominantly amphibolite facies tonalitic rocks including some with ages 3 ga with small belts of volcanic and sedimentary origin lake minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite clinopyroxene orthopyroxene granodiorite orthopyroxene biotite diatexite and granite the hornblende granodiorite suite constitutes most of the utsalik and tikkerutuk domains and is thought to represent continental arc magmatism on the basis of their distinct aeromagnetic and lithological character two additional domains are evident north of the leaf river area the inukjuak domain in the west and the douglas harbour domain in the east the northerly grain of the minto block appears to have been established in situ with respect to the easterly belts of the southern superior province i e no large scale block rotation during the same interval of time 3 0 2 7 ga modification of the tectonic framework for the superior province is required to explain minto arc magmatism in the interval 2730 2690 ma ago a continental magmatic arc built the berens river and bienville subprovinces and minto block on the southern and eastern edges respectively of a northern protocratonic foundation in the same period primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

a model of ocean crust accretion for the superior province canada abstract one of the keys to understanding the origin of archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones felsic to intermediate volcanic rocks and terrigenous sediments traditional models for greenstone belt evolution have been based on in situ stratigraphic relationships most of these models for example an oceanic island arc developed on oceanic basement back arc basins and the recently popular plume model predict concordant stratigraphic relationships among the various greenstone belt lithologies however rather than being depositional in nature several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic suggesting an allochthonous origin for most greenstone sequences all of these latter models make analogies to phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting in this paper we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the superior province where the accretion of oceanic material can be documented and direct comparisons are made to geological processes in phanerozoic accretionary complexes in the malartic area in the southeastern abitibi subprovince 3 to 4 km thick slices of komatiite and tholeiite with intercalated terrigenous sediment are tectonically imbricated and are overlain by calc alkaline volcanics which postdate tectonic stacking in both the larder lake region of the southwestern abitibi belt and in the beardmore geraldton belt at the south eastern limit of the wabigoon belt slices of iron rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment the malartic val d or area is considered to be an example of accretion of an archaean oceanic plateau while the larder lake and the beardmore geraldton regions are potentially typical of accretion of normal oceanic crust in an arc environment phanerozoic accretion of oceanic crust is accompanied by a step back in subduction and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the superior province asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late magmatism this scenario requires that magmas be erupted through previously accreted volcanic plutonic and sedimentary material furthermore later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano plutonic sequences the combined result of the plate tectonic scenario envisaged would result in the well described cyclic stratigraphy of many granite greenstone sequences

parallel evolution of archaean low and high grade terrane a view based on relationships between the abitibi wawa and kapuskasing belts abstract the kapuskasing structural zone ksz is a region of high grade archaean gneiss transverse to the regional east west structural trends of the adjacent lower grade abitibi wawa and quetico belts the structural zone is made up of granulite and amphibolite facies paragneiss tonalitic gneiss and anorthosite suite gneiss whereas adjacent regions are metavolcanic metasedimentary belts with gneissic and plutonic components foliations in the ksz strike northeast and dip moderately northwest to account for the development of contrasting lithological structural and metamorphic characteristics in adjacent areas a unique archaean history is postulated for the ksz psammitic sedimentary rocks possibly deposited in a fault bounded basin were intruded by anorthosite which may have had its erruptive equivalents in the tholeiitic volcanic rocks in the adjacent abitibi belt during the kenoran orogeny the ksz was metamorphosed and intruded at infrastructural levels while low grade regions outside underwent sub greenschist to amphibolite metamorphism at higher levels late ductile shear in the ksz reoriented gneissic structural elements into northeast directions proterozoic fault movements juxtaposed low and high grade regions

the complexity of the crust and moho under the southeastern superior and grenville provinces of the canadian shield from seismic refraction wide angle reflection data the major features of the individual velocity models poisson s ratio values and crustal complexity derived from the interpretation of seismic data sets from four long range seismic refraction wide angle reflection experiments are summarized the experiments were conducted from 1982 92 in the southeastern portion of the canadian shield in the conventional analysis of seismic refraction wide angle reflection data only the onset times and amplitudes of the major arrival phases are used to derive seismic velocity models of the region under study these models are over smoothed have a number of intermediate discontinuities are unable to explain the pg coda and bear very little resemblance to the models derived from the analysis of near vertical seismic reflection data in this paper some of the differences between seismic models derived from near vertical reflection analysis and those from refraction analysis are reconciled from an analysis of the wide angle reflection fields of the crustal coda wav

crustal velocity structure of the superior and grenville provinces of the southeastern canadian shield the 1992 lithoprobe abitibi grenville seismic refraction experiment was conducted using four profiles across the grenville and superior provinces of the southeastern canadian shield delay time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another with the largest velocity values occurring underneath the central gneiss and the central metasedimentary belts south of the grenville front the grenville front tectonic zone is imaged as a southeast dipping region of anomalous velocity gradients extending to the moho the velocity anomaly maps suggest an archean crust may extend horizontally 140 km beneath the northern grenville province near surface velocity anomalies correlate well with the known geology the most prominent of these is the sudbury structure which is well mapped as a low velocity basinal structure the tomography images also suggest underthrusting of the pontiac and quetico subprovinces

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

regional shortening followed by channel flow induced collapse a new mechanism for dome and keel geometries in neoarchaean granite greenstone terrains the lateral flow and extrusive exhumation of ductile migmatitic gneisses due to a horizontal gradient in lithostatic pressure a process termed channel flow or lateral protrusion has previously been proposed as an important tectonic process in large hot orogens centrifuge simulation of i layer parallel shortening followed by ii collapse of a cover sequence during ductile flow of underlying layers in an analogous manner to channel flow suggests a new mechanism for the development of structures within neoarchaean granite greenstone terrains the centrifuge model incorporates an upper package of silicone modelling clay microlaminates that simulate a greenstone sequence that overlies slightly less dense ductile silicone putties whose rheological properties simulate migmatitic felsic gneiss a low viscosity low density layer along half of the infrastructure superstructure interface simulates the presence of granitoid melt during initial layer parallel shortening upright folds form in the upper greenstone package and the interface with underlying ductile layers is folded in basal silicone layers recumbent to overturned non cylindrical and upright folds form with increasing distance from the moving end wall removal of material parallel to fold hinges at the foreland end of the model i e furthest from the ram used to shorten models in several stages simulates erosion and the difference in gravitational loading thus created induces ductile flow and lateral extrusion of basal silicone layers models aim to reproduce features comparable to those developed during extrusive channel flow and focused exhumation of basement migmatitic gneiss in nature early formed recumbent to inclined folds are then accentuated during simulated channel flow while new recumbent isoclinal folds in basal layers develop broad antiforms and tight synforms similar to the dome and keel geometry that typifies many archaean granite greenstone belts are produced as a late feature in the model channel flow and collapse of a fold thickened crust is therefore proposed as a potential alternative mechanism for the formation of structures in some neoarchaean granite greenstone terrains by analogy with other physical and numerical models channel flow in the neoarchaean may be enhanced by impingement of an upper mantle wedge into the base of the crust our results imply that both lithospheric shortening and non diapiric gravitational instabilities may be responsible for the formation of some archaean dome and keel structures and may account for the juxtaposition of some granite greenstone and high grade gneiss terrains in the archaean model results also show similarities to structures produced during lateral flow and withdrawal of salt in fold belts

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

the early permian mafic ultramafic complexes in the beishan terrane nw china alaskan type intrusives or rift cumulates abstract the petrogenesis and geodynamic setting of the early permian mafic ultramafic complexes in the beishan terrane nw china have important bearing on the prospects of ni cu pge sulfide and ti fe oxide deposits as well as in understanding the history of evolution of the southern central asian orogenic belt caob here we present results from a detailed study on a representative suit of a mafic ultramafic rocks from the bijiashan complex in the beishan terrane the complex is composed of dunite troctolite olivine gabbro and gabbro without clinopyroxenite or hornblendite in addition to olivine and clinopyroxene orthopyroxene and plagioclase are also present in all these rock types in the absence of chromite or primary hornblende the minerals display marked compositional variations with the fo content in olivines ranging from 66 to 83 and the clinopyroxenes ranging from diopside to augite the whole rock geochemistry shows good correlations among the major elements and the trace elements are characterized by flat ree patterns with eu positive anomalies and hfse depletion relative to lile these features are comparable to other mafic ultramafic complexes in the beishan and eastern tianshan terranes but obviously deviate from the typical features of alaskan type intrusives the present study suggests that the beishan complexes were formed in a post orogenic extensional environment with potential for ni cu mineralization rather than in arc related setting with alaskan type pge mineralization our study confirms that the beishan area does not conform to an arc related setting but was located within a rift setting in permian probably related to permian mantle plume event suggesting that the subduction of the paleo asian ocean had culminated by this time and the southern caob witnessed a post orogenic extensional regime in late paleozoic

high speed video observation of stepwise propagation of a natural upward positive leader using a high speed video camera operated at 150 000 frames per second we have documented the upward propagation of an initial positive leader from a 325 m meteorological tower in beijing the stepwise development of the upward positive leader was clearly revealed during its self sustained propagation toward the cloud base providing the first optical evidence for the stepping processes of a positive leader the leader developed with definitive intermittent pauses and re establishment with abrupt jump of the leader top obvious brush like corona zone emitting outward from the leader top was identified in the frame of stepping and the luminosity waves propagated downward along the already formed channel from the leader top immediately following the stepping indicating that a current wave was generated at the leader top and subsequently traveled down the channel the channel luminosity decreased during the leader pause stage with the corona zone dimmed correspondingly the positive leader experienced an average interstep interval of 61 7 s ranging between 30 s and 120 s and an average 2 d speed of 8 1 104 m s the transient speed of the step jump was estimated to be larger than 7 3 105 m s with an average step length of 4 9 m the need of sufficient positive charge accumulation at the leader top could be the main cause for the stepping characteristics

moho reflectivity patterns a comparison of canadian lithoprobe transects abstract the complex nature of the continental moho may be strongly influenced by the tectonic and magmatic history of the lithosphere observations of moho variability from near vertical incidence seismic reflection surveys suggest that it may be possible to correlate moho reflectivity and lithospheric evolution as an initial step in attempting to determine such correlations we qualitatively compare seismic reflection data from canadian lithoprobe transects that sample regions of exceptionally diverse age and tectonic history the seven selected datasets were collected using consistent acquisition parameters facilitating effective and meaningful comparisons among transects the moho reflections exhibit considerable variability however in virtually all recent transects a distinct moho is observed even in regions where lower crust is considered to have stabilized in the early proterozoic or archean this suggests not only that relationships between moho reflectivity and tectonics can be extended into the archean but that tectonic age is poorly correlated with moho reflectivity the qualitative comparisons do not clearly link a specific tectonic process with a distinctive reflection pattern however well defined moho reflections are associated with lower crustal deformational processes including compression transpression and extension this indicates that ductile shear and the rheologies of the lower crust moho transition and upper mantle play important roles in influencing reflective character indistinct moho reflectivity may be associated with regions that have experienced little ductile deformation or with processes that have disrupted reflective laminations however the majority of incoherent moho reflections appear related to signal degradation associated with data acquisition the study demonstrates that the qualitative characteristics of moho reflectivity are difficult to correlate with tectonic history and thus use as an interpretive tool isolating quantitative reflection characteristics of the moho from distortions imposed by the overlying crust is a challenging problem but appears necessary to further understanding of the crust mantle transition

archean terrane docking upper crust collision tectonics abitibi greenstone belt quebec canada abstract the northern nvz and southern volcanic zones svz of the abitibi greenstone belt are separated by the major e trending destor porcupine manneville fault zone dpmfz the dpmfz is interpreted to be the locus of archean terrane docking between the older diffuse volcanic arc of the nvz 2730 2710 ma and the younger arc segments of the svz 2705 2698 ma two distinct evolutionary phases can be documented along the dpmfz of the abitibi greenstone belt and include 1 arc arc collision occurring between 2697 and 2690 ma and 2 arc fragmentation between 2689 and 2680 ma identification of these two events along the dpmfz is based on detailed structural studies sedimentary basin analysis and precise u pb age determinations the thrusting event representative of the arc arc collision phase is characterized by shallow north dipping foliations 20 40 and dip parallel stretching lineations in the eastern manneville segment of the dpmfz local overturned mafic pillowed units suggest recumbent folding late strike slip or transcurrent movement displayed in the late orogenic sedimentary duparquet basin records the arc fragmentation phase basin geometry e trending en echelon folds shallow e plunging stretching lineations and a late ne striking cleavage cross cutting the folds support a dextral shear sense along the western destor porcupine segment of the dpmfz the sedimentary facies observed in the basin are consistent with those of modern strike slip basins located along the east anatolian fault turkey hazar lake and the hope fault new zealand hanmer basin precise u pb zircon age determinations from porphyry stocks located at the northern and southern limits of the duparquet basin yielded 2681 1 ma and 2689 3 2 2 9 ma respectively these ages constrain the rapid change from thrusting to transcurrent movement it is apparent that once thrusting ceased the response to oblique subduction continued in the form of strike slip displacement modern fold and thrust belts commonly show this evolution the deformation pattern is the result of oblique convergence the abitibi greenstone belt is considered to be an archean analogue of modern subducting oceanic plates such as those found in the western pacific

archean crust as revealed in the kapuskasing uplift superior province canada in the central superior province of the canadian shield a 120 km wide transition from the low grade michipicoten greenstone belt to the high grade kapuskasing structural zone represents an oblique section through some 20 km of archean crust uplifted along a northwest dipping thrust fault the restored vertical section through the upper and middle crust consists of three megalayers with undulating boundaries 1 0 to 25 km a high grade heterogeneous gneissic assemblage in part older than the upper supracrustal succession

u pb age constraints for the magmatic and tectonic evolution of the pontiac subprovince quebec new u pb zircon titanite and monazite ages help constrain the history of magmatism and tectonism within the pontiac subprovince of western quebec the pontiac subprovince resembles other metasedimentary belts of the superior province however the stratigraphic relationships between the dominantly sedimentary rocks of the pontiac and the adjacent volcanic dominated abitibi belt to the north and west remain controversial volcanic rocks of the belleterre volcanic zone in the southern part of the pontiac subprovince have been interpreted by other workers as klippen of abitibi strata that were thrust southward onto the pontiac subprovince however volcanic rocks in the belleterre zone give crystallization ages of 2689 2682 ma which are younger than any extrusive rocks dated thus far from the abitibi belt single detrital zircon grains from pontiac sedimentary rocks give ages as young as 2683 ma indicating that the sediments are similar in age or younger than the volcanic units the volcanic rocks pro

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

nd isotope evolution of archaean plutonic rocks in southeastern superior province the opatica plutonic belt abitibi greenstone belt and pontiac subprovince represent a major proportion of the southeastern superior province which was formed and accreted rapidly between approximately 2 9 and 2 8 ga plutons in these belts are grouped into four types i trondhjemite tonalite granodiorite ttg suite 2 82 2 69 ga ii monzodiorite mzd suite 2 697 2 669 ga iii late alkaline granitoid alk suite 2 68 2 67 ga and iv anatectic granite and monzonite ana suite 2 69 2 64 ga the four suites are represented in all belts and show similar petrography and geochemistry in terms of nd isotope composition the ttg mzd and alk suites are typical of destructive plate margin magmatism and have 1 4 initial end 3 7 values which are very similar to that of the abitibi mantle end 2 5 the lower values for the ana suites end 0 1 to 2 4 result from recycling of crustal components in the opatica belt the ana granitoids fall on the nd isotope evolution curve defined b

u pb geochronology of the opatica tonalite gneiss belt and its relationship to the abitibi greenstone belt superior province quebec u pb ages for zircon monazite and titanite from samples collected from two transects across the opatica tonalite gneiss belt in the superior province of quebec indicate that the belt contains rocks that are significantly older than those in the adjacent low grade northern abitibi belt tonalite and tonalite gneiss which make up most of the belt formed over an interval of 100 ma from pre 2800 to 2702 ma five samples have ages of ca 2807 13 2773 23 and 2702 3 ma zircon growth at 2718 2721 ma in the two oldest samples may record an early period of high grade metamorphism in the belt hornblende diorite monzodiorite and tonalite plutons were intruded at 2693 2696 ma particularly along the southern boundary with the abitibi belt these include the canet pluton at ma the lac ouescapis pluton at 2693 2 ma and the barlow pluton at 2696 3 ma pink granite plutons and dykes are the youngest intrusive rocks in the belt three samples have yielded zircon ages of 2690 2 and 2686 4 ma

crustal structure of the grenville front and adjacent terranes under the auspices of the great lakes international multidisciplinary program on crustal evolution approximately 320 km of deep seismic reflection data were collected in lake huron along a profile that extends east from the manitoulin terrane across the grenville front to the interior of the grenville orogen the manitoulin terrane is characterized by a series of gently east dipping reflections at about 20 km depth that separate a highly reflective lower crustal layer from a markedly less reflective upper layer imaged by strong reflections at the western end of a spectacular band of moderately east dipping reflections the grenville front clearly truncates manitoulin terrane structures to the west these data are interpreted in terms of a speculative two stage model involving 1 creation of a major decollement during northward collision of an allochthonous terrane with the southern superior cratonic margin 1 83 1 89 ga penokean orogeny and 2 northwest directed stacking of microterranes at the southeastern margin of the craton followed by crust penetrating ductile imbrication under high pressure high temperature conditions leading to the ramping of deeply buried rocks to the near surface 1 0 1 3 ga grenvillian orogeny

source and tectono metamorphic evolution of mafic and pelitic metasedimentary rocks from the central quetico metasedimentary belt archean superior province of canada abstract a study of the sedimentary rocks in the jean lake area of the quetico metasedimentary belt superior province canada was conducted in order to evaluate the origin source and evolution of rocks including mafic rocks previously mapped as ultramafics rocks bulk compositions of these sedimentary rocks show a mixing between two end members quartzo feldspathic sedimentary rock and komatiitic basalt high cao and mgo contents of the rocks suggest a proximal source of the komatiitic basalt the rocks in the study area record a pressure temperature p t path with three tectono metamorphic stages the first stage formed staurolite 500 700 c under medium p t mp mt metamorphic conditions shortly after the sedimentation the second stage yielded the biotite sillimanite garnet assemblage under the peak conditions of 0 6 0 1 gpa and 700 70 c during transpressional deformation the third stage low p medium t lp mt 0 25 0 11 gpa 540 80 c metamorphism was associated with regional south southeast compression and its timing is constrained by a new u th pb monazite age of 2667 20 ma combining the regional deformation events we suggest the burial metamorphism of sediments up to mp mt conditions during the d1 deformation at 2698 2689 ma they attained the peak metamorphic conditions during the regional transpressive d2 d3 deformation 2689 2671 ma and retrograded to lp mt condition during the south southeast compression regional d4 at 2671 2667 ma a short time span between the sedimentation and mp mt metamorphism accompanied by d1 deformation suggest that the quetico sedimentary rocks formed in an acretionary prism the studied rocks in the jean lake area deposited close to the wabigoon subprovince transported towards the south and buried up to 20 km by rapid underthrusting to amphibolite facies conditions subsequent dextral transpressive deformation d2 d3 during the oblique docking of the quetico belt to the wabigoon subprovince to the north and wawa greenstone belt from the south resulted in the displacement of the sedimentary rocks to the west from the original depositional site the p t time path of the quetico sedimentary rocks is similar to that in modern arc accretion prisms except for a high geothermal gradient of 30 c km recorded in the former compared to 10 c km in modern counterparts the high temperature gradient in the archean accretionary prism explains the lack of high pressure metamorphic rocks such as blueschist that are common in modern accretionary prisms

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

moho reflectivity and seismic signal penetration abstract analysis of amplitude decay and frequency change is applied to determine limits of seismic signal penetration on deep seismic reflection data collected by lithoprobe and the consortium for continental reflection profiling cocorp changes in spectral frequency content are efficiently monitored with averaged instantaneous frequency and bandwidth results from montana show pronounced increases in average frequency and slight drops in amplitude associated with a die out of reflections at the moho results from quebec show amplitude and frequency change occurring through the moho on lines with and without moho reflections these contrasting observations confirm both adequate signal penetration and that the differences in moho reflectivity are representative of geologic differences

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

enhancing base metal exploration with seismic imaging commencing in 1988 and continuing for 5 years lithoprobe acquired a series of high resolution seismic experiments within and near base metal mining camps in canada including the abitibi subprovince of quebec and ontario the world class sudbury ni cu mining district the buchans mine in newfoundland and the thompson ni belt in manitoba this work undertaken in close cooperation with the geological survey of canada and major canadian mining companies stimulated an intensive and broadened series of followup studies with the common objective of assessing potential applications of multichannel seismic mcs imaging for deep mineral exploration and mine development this research was motivated by a widely recognized disparity between the depths from which ores can be profitably mined up to 2 km or more and the resolving depths typically 500 m of commonly used geophysical methods for mineral exploration initial rock property studies established that the expected contrast in acoustic impedance between

a three dimensional perspective on the evolution of archaean crust lithoprobe seismic reflection images in the southwestern superior province in 1990 1991 the lithoprobe project completed 450 km of seismic reflection profiles across the late archaean crust of the southwestern superior province the results define a broad three fold division of crust upper crust in the abitibi greenstone belt is non reflective and is a 6 8 km veneer of volcanic and plutonic supracrustal rocks whereas in the sediment gneiss dominated pontiac subprovince upper crust comprises shallow northwest dipping turbidite sequences mid crust in both the abitibi and the pontiac subprovinces is interpreted as imbricate sequences of metasedimentary and metaplutonic rocks lower crust in both subprovinces has a horizontal layer parallel strycture which may represent interleaved mafic intermediate gneisses the seismic signature of the northern abitibi greenstone belt may be represented in an exposed 25 km crustal section in the kapuskasing stuctural zone preliminary tectonic models based on the seismic data are consistent with a plate tectonic scenario involving oblique subduction and imbrication of sedimentary plutonic and volcanic sequences the northern abitibi supracrustal sequences either represent an allochthon or overlie an allochthonous underthrust metasedimentary and plutonic sequence which may be equivalent to a metasedimentary subprovince such as the pontiac or quetico seismic velocities have yet to be defined however crustal thicknesses are relatively constant at 35 40 km the thinnest crust is adjacent to the grenville front where moho is very well defined

nd model age mapping of the southeast margin of the archean foreland in the grenville province of ontario nd model ages are used to map the southeast margin of superior southern province basement within the grenville province of ontario this boundary located 60 km southeast of the grenville front is tentatively identified as a collisional suture between an archean foreland to the northwest and a proterozoic island are of penokean age to the southeast

features of upward positive leaders initiated from towers in natural cloud to ground lightning based on simultaneous high speed videos measured currents and electric fields original simultaneous records of currents close electric field and high speed videos of natural negative cg lightning striking the tower of morro do cachimbo station are used to reveal typical features of upward positive leaders before the attachment including their initiation and mode of propagation according to the results upward positive leaders initiate some hundreds of microseconds prior to the return stroke while a continuous uprising current of about 4 a and superimposed pulses of a few tens amperes flow along the tower upon leader initiation the electric field measured 50 m away from the tower at ground level is about 60 kv m the corresponding average field roughly estimated 0 5 m above the tower top is higher than 0 55 mv m as in laboratory experiments the common propagation mode of upward positive leaders is developing continuously without steps from their initiation unlike downward negative leaders upward positive leaders typically do not branch off though they can bifurcate under the effect of a downward negative leader s secondary branch approaching their lateral surface the upward positive leader s estimated average two dimensional propagation speed in the range of 0 06 106 to 0 16 106 m s has the same order of magnitude as that of downward negative leaders apparently the speed tends to increase just before attachment

tectonic evolution of the northern volcanic zone abitibi belt quebec the archean abitibi subprovince has been divided formally into a northern volcanic zone nvz including the entire northern part of the subprovince and a southern volcanic zone svz on the basis of distinct volcano sedimentary successions related plutonic suites and precise u pb age determinations the nvz has been further formally subdivided into i a monocyclic volcanic segment mvs composed of an extensive subaqueous basalt plain with scattered felsic volcanic complexes 2730 2725 ma interstratified with or overlain by linear volcaniclastic sedimentary basins and ii a polycyclic volcanic segment pvs comprising a second mafic felsic volcanic cycle 2722 2711 ma and a sedimentary assemblage with local shoshonitic volcanic rocks a sequence of deformational events d1 d6 over a period of 25 ma in the nvz is consistent with a major compressional event north south shortening was first accommodated by near vertical east trending folds and with continued deformation was concentrated along ma

crustal scale shortening and extension across the grenville province of western quebec a deep seismic reflection survey shot in 1993 crosses almost the full width of the grenville province in western quebec the seismic transect provides a very clear image of the crust mantle boundary and the most precise definition to date of the various grenvillian terranes the crust is around 44 km thick beneath the grenville front but thins rapidly to 36 km some 60 km to the southeast it is also notable that the greatest crustal thickness of 50 km occurs at the southeast end of the transect far from the inferred location of the main grenvillian collision the grenville front zone in which nw directed thrusting at about 1 ga was followed by se directed extension is defined by discontinuous se dipping reflections which extend down to the moho to the southeast the overlying migmatitic archean parautochthon almost half of the transect is characterised instead by nw dipping reflectors extending into the lower crust these reflectors are in turn truncated to the southeast by a 12 km thick zone of intense se dipping reflections the baskatong crustal ramp the base of the allochthonous terranes allochthon boundary thrust is likely located at this ramp which flattens out at around 30 km depth into the base of the relatively transparent intermediate crust a highly reflective upper crustal deck corresponding to rocks of the mont laurier terrane was thrust over the baskatong crustal ramp and is represented further to the northwest by the klippelike cabonga allochthon the synformal transparent morin anorthosite charnockite complex belongs to the same upper crustal level ramp anticlines and the overriding basal thrust of the upper allochthons demonstrate the nw directed propagation of tectonic transport during the grenvillian orogeny involving deformation and displacement along the baskatong ramp with a relay into the grenville front zone postaccretional extension appears to have been primarily accommodated along the latter two crustal discontinuities thinning the crust immediately south of the grenville front and affecting the crust up to 350 km away from the front

archaean subduction inferred from seismic images of a mantle suture in the superior province plate tectonics provides the basis for the interpretation of most current terrestrial tectonic activity and is widely accepted as having been active over much of the earth s history1 yet the timing of initiation of this process is subject to debate2 9 so far the earliest seismic evidence for plate tectonics has come from a fossil mantle suture in the svecofennian orogen 1 89 gyr ago 10 and from inferred plate convergence subduction and accretion in the trans hudson orogen 1 91 1 79 gyr ago 11 as yet seismic data from archaean areas have been able to demonstrate only the importance of compression in the construction of the continental crust12 15 here we present seismic data from a collision zone in the superior province of canada involving the abitibi granite greenstone subprovince and the plutonic arc related opatica belt we interpret dipping seismic reflections that extend 30km into the mantle as representing a relict 2 69 gyr old suture associated with subduction although crustal structure lithospheric thicknesses and convergence rates may have differed from those seen today these seismic data provide direct evidence that plate tectonics was active in late archaean times 800 myr earlier than indicated by previous seismic reflection surveys

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

seismic reflection constraints from lithoprobe line 29 on the upper crustal structure of the northern abitibi greenstone belt detailed reprocessing of east west lithoprobe seismic reflection line 29 includes cross dip analysis to improve the continuity of the reflectors and median filtering to attenuate shear wave refractions the interpretation provides additional constraints on the tectonic models of northern abitibi but cannot be used to invalidate either of the two models recently presented for the area however the seismic profile defines a large east dipping faulted contact between the brouillan tonalite and the volcanic rocks exposed west of the intrusion the moderate dip of the reflectors and their extensive lateral continuity combined with geological information provide evidence for an east dipping thrust sequence and suggest an allochthonous origin for the brouillan tonalite the maximum thickness of the volcanic sequence in the northern abitibi greenstone belt is 8 km but could be as low as 4 km if opatica orthogneisses are considered to underthrust northern abitibi the mid crustal reflections confirm the east west continuity of south vergent imbrications also observed on a north south reflection line 28 through northern and central abitibi breakage and displacement of some mid crustal reflectors may define a west to east thrust sequence of sense opposite to that which thrust the brouillan pluton over the volcanic rocks

single zircon age constraints on the tectonic juxtaposition of the archean abitibi greenstone belt and pontiac subprovince quebec canada abstract zircons from metasediments and granitoids in the high grade lacorne block within the low grade archean abitibi greenstone belt have been dated by the single zircon pb evaporation technique yielding 207pb 206pb minimum ages detrital zircons in the mature clastic metasediments of the lacorne block display a range of ages from 2691 8 ma to 3042 6 ma the younger zircon ages 2691 8 2695 4 2715 16 and 2719 4 ma thus impose an upper limit for deposition and indicate that the high grade lacorne block is not basement to the abitibi supracrustal sequence 2747 2680 ma existence of abundant 69 older detrital zircons 2750 ma suggests in turn that the abitibi supracrustal rocks are not the source of the lacorne sediments two generations of granitoids occur in the lacorne block an early monzodiorite monzonite granodiorite syenite series mmgs 2671 4 to 2675 24 ma and a younger s type garnet muscovite granite series gmg 2630 20 to 2655 8 ma this contrasts with granitoid magmatism in the abitibi greenstone belt which ended at 2675 ma the pontiac subprovince to the south of the abitibi greenstone belt shares all of the above features of the lacorne block including detrital zircon ages as well as the composition and timing of granitoid magmatism this is interpreted to indicate that the lacorne block was originally part of the same tectonic terrane as the pontiac subprovince after development of the mmgs magmatism 21670 2680 ma the pontiac subprovince locally underthrust the abitibi greenstone belt and crustal thickening promoted partial melting of underthrust pontiac metasediments to form the 2644 13 ma s type granites later differential uplift exposed the lacorne block as a high grade tectonic window in the abitibi greenstone belt

a link between deformation history and the orientation of reflective structures in the 2 68 2 83 ga opatica belt of the canadian superior province true three dimensional reflector orientations can be derived from prestack seismic reflection data where a seismic profile is particularly crooked this is accomplished by estimating a measure of coherency along travel time trajectories defined by the azimuth dip and depth of a reflector and the medium velocity results from lithoprobe line 48 located in the opatica belt of the archean superior province differentiate reflectors with two distinct orientations which coincide with the attitudes of two deformational fabrics mapped at surface assuming a connection between reflectivity and strain induced by tectonic processes the reflectors with nne strikes and shallow dips toward the east are correlated with surface evidence for early west vergent thrusting in the opatica belt other reflectors which strike ene wsw and dip shallowly to the north and to the south indicate that most of the reflectors in the southern opatica and beneath the abitibi greenstone belt at middle and lower crustal levels formed during a later approximately n s shortening event mantle reflections previously interpreted as a relict suture of an archean subduction zone dip to the north at around 30 45 and are also associated with this n s event the distribution of reflector orientations estimated at the crooked parts of line 48 indicates that much of the opatica crust was reworked during the n s shortening event although a region of the middle and lower crust characterized by the earlier d1 reflectors is preserved in the central part of the belt

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

deep into the chibougamau area abitibi greenstone belt structure of a neoarchean crust revealed by seismic reflection profiling copper gold magmatic hydrothermal systems dominate in the chibougamau area of the neoarchean abitibi subprovince greenstone belt of the superior province craton whereas orogenic gold mineralization is more common in the rest of the abitibi understanding differences in metal endowment within the abitibi greenstone belt requires insights into the geodynamic evolution of the chibougamau area this was addressed by imaging the crust using seismic reflection profiling acquired as part of the metal earth project seismic reflection sections display shallowly south dipping reflectors located within the upper crust e g a possible deep continuation of the barlow fault and a northward dipping midcrust imbricated with older crust opatica subprovince to the north multiple reflectors characterize the upper part of the midcrust interpreted as faults superimposed on a major lithological boundary these structures likely formed during terrane accretion prior to craton stabilization combining the new seismic data with known stratigraphic structural and magmatic records we propose that the study area was initially a normal i e thick archean oceanic crust that formed at or before 2 80 ga and that evolved through terrane imbrication at 2 73 2 70 ga shortening caused rapid burial devolatilization and partial melting of hydrated mafic rocks to produce tonalite magmas that may have mixed with mantle derived melts to produce the diorite tonalite suite associated with observed cu au magmatic hydrothermal mineralization

mafic and ultramafic amphibolites from the northwestern pontiac subprovince chemical characterization and implications for tectonic setting in the northwestern pontiac subprovince metavolcanic rocks are exposed within a metagraywacke sequence that is intruded by metamorphosed mafic dykes the metavolcanics are al undepleted komatiites la sm n 0 3 tb yb n 0 9 and tholeiitic fe basalts la sm n 0 8 and tb yb n 0 8 the nearly flat chondrite normalized distributions of high field strength elements hfse ti and p the constant zr y nb th ti zr and ti p ratios and the lack of depletion of hfse relative to rare earth elements ree in both ultramafic and mafic metavolcanics imply that crustal assimilation and magma mixing with crustal melts were not significant during differentiation and argue against the presence of subduction related magmatic components contemporaneous volcanism and sedimentation in the northwestern pontiac subprovince are unlikely the metavolcanics do not show any evidence of crustal contamination and likely represent a structurally emplaced disrupted assemblage chemically similar to early volcanics o

structure of the high grade opatica belt and adjacent low grade abitibi subprovince canada an archaean mountain front plutonic gneisses in the archaean opatica belt in canada record crustal scale wsw vergent i e longitudinal high temperature d1 ductile shearing similar structures occur in adjacent greenstone belts but only in narrow amphibolite facies zones near the base metavolcanics in the greenschist facies portions do not record penetrative d1 structures the greenstone belts are interpreted as allochthonous on the gneisses they form the uppermost d1 thrust sheets sse vergent i e approximately transverse d2 thrusting imbricated the opatica belt and formed a large antiform either a culmination or an antiformal stack in the southern opatica the imbricated opatica belt wedged under the northern part of the abitibi subprovince and created a tight syncline with nnw vergent d2 thrusts interpreted as backthrusts on its northern limb the opatica belt abitibi subprovince contact therefore resembles some phanerozic mountain fronts further southwards propagating imbrication of the crust beneath the abitibi subprovince created a s vergent foreland fold and thrust belt in the overlying greenstones high grade opatica gneisses and adjacent low grade abitibi greenstones contain complementary structural patterns and may therefore represent the internal and foreland parts of an archaean mountain belt similar in many respects to phanerozoic examples

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

the 1982 cocrust seismic experiment across the ottawa bonnechere graben and grenville front in ontario and quebec summary during the summer of 1982 the canadian consortium for crustal reconnaissance using seismic techniques cocrust conducted a major long range seismic refraction and wide angle reflection experiment across the grenville province of the canadian shield three seismic lines each approximately 300 km in length were located i along the ottawa bonnechere graben ii perpendicular to the graben and iii perpendicular to the crenville front geological evidence indicates that the graben may have originated from a failed arm of the st lawrence rift system and the grenville front marks the boundary between the grenville province and the much older superior province other geological features of the grenville province that were traversed by the profiles were the central metasedimentary belt and the central gneiss belt analysis of the data involved conventional traveltime procedures coupled with the use of synthetic seismogram analysis using programs that were written to handle laterally heterogeneous structures results from the survey indicate a variation in near surface seismic velocity

seismic reflection profiles across the mine series in the noranda camp of the abitibi belt eastern canada the abitibi grenville lithoprobe project completed a regional line 21 and a high resolution line 21 1 seismic survey in the noranda central volcanic complex of the blake river group abitibi quebec line 21 provides a regional framework in which the archean crust is divided into three layers two of which are discussed here the uppermost layer which corresponds to the blake river group is the least reflective and lies above 4 s 12 km and the mid crustal layer which is composed of a complex pattern of generally east northeast dipping reflectors and lies between 4 and 8 s within the regional data the mine series of the central volcanic complex is imaged as a semitransparent series of reflectors overlying a highly reflective east facing structure interpreted as the subvolcanic flavrian pluton the high resolution data line 21 1 were collected in the vicinity of the ansil mine the seismic images in this region can be controlled by surface geology and extensive drill hole data and the projec

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

models of convergent boundary tectonics implications for the interpretation of lithoprobe data physical models of convergent boundary processes can provide insights into compressional orogens such as those studied by lithoprobe we summarize the qualitative tectonic style of a series of mode

late paleozoic tectonic amalgamation of northwestern china sedimentary record of the northern tarim northwestern turpan and southern junggar basins sedimentary rocks contained in basins adjacent to the tian shan provide a long and complex record of the late paleozoic continental amalgamation of northwestern china complementing that provided by rocks preserved within the range this record which comprises dramatic changes in sedimentary facies sediment dispersal patterns sandstone provenance and basin subsidence rates broadly supports previous interpretations of a two part evolution of the tian shan late devonian to early carboniferous collision of the tarim continental block with the central tian shan followed by collision of this combined block with island arcs in the north tian shan and bogda shan in late carboniferous early permian times the first collision resulted in widespread angular unconformities within the tarim basin continued convergence following the collision created a long lived flexural foredeep along the northern margin of the tarim block which received at least 2000 m of lower carboniferous through lower permian fluvial and marine sediment derived from the interior of tarim subsequent early permian continental extension of the northern tarim basin resulted in the deposition of interbedded nonmarine siliciclastic sedimentary rocks and mafic to felsicvolcanic rocks sandstone within this interval was derived from the paleo tian shan and is composed predominantly of lithic volcanic grains similar to the rhyolite in contrast to the tarim basin calc alkaline volcanic rocks and volcanogenic sedimentary rocks dominated carboniferous and permian sedimentation in the northern turpan and northwestern junggar basins volcanic arcs remained active in the north tian shan and bogda shan through the early late carboniferous depositing a kilometers thick interval of deep marine sediment gravity flows in the northwestern junggar basin major arc magmatism ceased in the late carboniferous in response to closure of the oceanic basin between the combined tarim central tian shan block and the north tian shan bogda shan arcs upper carboniferous through lower permian rocks in the northwestern junggar basin compose the sedimentary fill of a bathymetric basin of oceanic depth on the northern side of the volcanic arcs culminating in a 1000 m thick marine regressive sequence middle to upper permian sandstones were derived from the uplifted paleo tian shan and bear the distinctive provenance imprint of granitic rocks presently exposed within the range late permian subsidence of the junggar basin accommodated 5 km of nonmarine sediments however the cause of this subsidence and its relationship to regional tectonic events remain controversial

tectonic evolution of a greenstone sequence in northern zimbabwe sequential early stacking and pluton diapirism structural metamorphic relationships in the 2 71 2 64 ga harare shamva greenstone belt in northern zimbabwe demonstrate that the archean stratigraphy of the zimbabwe craton can not be regarded as a coherent and nonrepetitive autochthonous succession of ultra mafic and felsic greenstones the greenstone belt shows an early set of amphibolite grade d1 m1 structures and associated synkinematic metamorphic assemblages that were formed during an episode of layer parallel shearing at about 2 64 ga this resulted in sequential west directed imbricate stacking and recumbent folding of distinct tectonostratigraphic greenstone sequences and includes the development of a syntectonic sedimentary succession d2 m2 fabrics strain patterns and contact aureoles overprint d1 geometries and are related to the diapiric emplacement of the chinamora batholith the structural data indicate that granite gneiss terrains can be subdivided into two types 1 structurally emplaced terrains the madziwa gneiss terrain and 2 diapiric domes the chinamora batholith the history of the harare shamva greenstone belt indicates that the late archean 2 7 2 6 ga evolution of the zimbabwe craton must allow for accretion of diverse crustal fragments including oceanic and back arc mafic crust volcanic arc felsic crust continental crust and related sedimentary sequences the process of accretion was probably diachronous across the craton and may have involved concomitant diapirism strike slip faulting magma intrusion and sedimentation

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

geochemical evolution of the minto block a 2 7 ga continental magmatic arc built on the superior proto craton abstract a 400 km long by 100 km wide geological transect across the minto block of northeastern superior province reveals five plutonic suites and minor amphibolite to granulite grade supracrustal remnants the oldest regionally significant plutonic rocks include 3 0 0 1 ga foliated to gneissic tonalite characterized by high na k ratios high al contents light ree enrichment and initial gend 1 8 the leaf river plutonic suite is the most widespread with crystallization ages of about 2725 5 ma across the transect the suite comprises massive to foliated pyroxene and hornblende bearing granodiorite tonalite granite diorite and minor gabbro pyroxenite and syn plutonic mafic dykes the rocks are i type calc alkaline have variable na k ratios moderate to high ba and sr contents and ree patterns with variable light ree enrichment depletion of heavy rees with increasing silica and neutral negative or positive eu anomalies initial nd values for the suite range from 0 5 to 1 3 mean 0 4 and initial 87 sr 86 sr ratios isr are 0 7020 the diatexite plutonic suite 2713 ma occurs within a 150 km wide zone and consists of orthopyroxene garnet granodiorite containing 25 50 inclusions of granulite grade mafic supracrustal and tonalitic inclusions the variable major and trace element chemistry of the suite with generally high ni and cr contents reflects heterogeneous source materials and the presence of non liquid components initial nd values are from 0 1 to 1 7 monzogranite and orthopyroxene granite dykes plugs and plutonic masses both 2690 ma comprise the last important plutonic suites both biotite monzogranite and orthopyroxene granite are light ree enriched but the former have negative eu anomalies and the latter positive initial nd values range from 0 7 to 3 2 for monzogranite isr 0 7016 to 0 7067 and 0 7 to 0 2 for orthopyroxene granite isr 0 701 to 0 702 the initial nd and isr values of the 2 7 ga plutonic suites are lower and higher respectively than estimatesf of the contemporaneous depleted mantle suggesting significant and wide scale reworking of older tight ree enriched high rb sr lithosphere early tonalite orthogneiss from goudalie domain may be derived by partial melting of mafic crust at high pressures the lithological and geochemical diversity of the 2725 ma leaf river plutonic suite indicate a range of parental magma compositions and petrogenetic processes but fractional crystallization of mantle derived basaltic melts which had assimilated small amounts of significantly older lower crust may have been the dominant mechanism crustal inclusion charged granodioritic diatexite 2713 ma was probably derived by partial melting of heterogeneous lower crustal materials under relatively dry conditions late biotite monzogranite was generated by partial melting of older sialic crust and orthopyroxene granite by deeper melting or fractional crystallization of more juvenile source materials with garnet as a residual mineral we infer from the presence of coeval mafic dykes in all plutonic suites that heat for melting was ultimately derived from intrusion of basaltic magmas at depth the 2 7 ga magmatism in the minto block occurred during a period of active production and accretion of juvenile oceanic terranes in the southern superior province the northwest directed subduction regime inferred from the southern superior province arc terranes may also have been responsible for the magmatism that acted on 3 ga lithosphere in the northern superior province melting within the mantle wedge beneath the proto cratonic lithosphere and in the lower crust occurred within a tectonic setting similar to that of modem continental magmatic arcs

crustal velocity models for the archean abitibi greenstone belt from seismic refraction data we present velocity models for two seismic wide angle refraction profiles across the archean abitibi greenstone belt and the pontiac subprovince the seismic profiles are 210 and 220 km long traveltime inversion and amplitude forward modelling were used to obtain two dimensional velocity structure and interface geometry the main features of the velocity models include 1 three crustal layers 2 variable velocities 5 6 6 4 km s in the upper crust 0 12 km with the higher velocities generally associated with mafic metavolcanics and the lower velocities with metasediments and granitic plutons 3 a relatively uniform middle crust 12 30 km with velocities ranging from 6 4 to 6 6 km s 4 a velocity increase of 0 3 km s across the middle crust lower crust boundary 5 a lower crust 30 40 km with velocities increasing from 6 9 km s at the top to 7 3 km s at the base 6 an average upper mantle velocity of 8 15 km s 7 depth to moho of about 40 km in the north central abitibi belt decreasi