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

the labradorian orogeny in the grenville province eastern labrador canada two minor intrusions within the hawke river terrane in the grenville province eastern labrador have upper intercept u pb ages as follows i for a megacrystic intermediate dyke and ii 1622 3 ma for a pegmatite dyke the upper intercepts date time of emplacement and the lower intercepts 508 and 320 ma respectively record the timing of early paleozoic events in the region a third sample a sillimanite bearing pelitic gneiss from the paradise metasedimentary gneiss belt in the hawke river terrane yielded a range of ages between 1647 and 1627 ma from seven single zircon analyses the zircons in the metasedimentary gneiss are interpreted as detrital because of their variation in morphology the range of ages obtained and their extreme variation in u content a detrital origin implies that the sedimentary protolith must have been deposited after 1627 ma and is therefore not the same age as morphologically similar metasedimentary gneiss that occurs as enclaves within pre 1670 ma migmatized quartz d

lithoprobe line 55 integration of out of plane seismic results with surface structure metamorphism and geochronology and the tectonic evolution of the eastern grenville province lithoprobe line 55 in the grenville province of eastern quebec provides unusually good control on the three dimensional 3 d geometry and structural relationships among the major lithological units there archean basement underlies the exposed proterozoic rocks along the entire seismic line and there is a lateral ramp in this basement immediately behind a lobate stack of thrust slices of high pressure metamorphic rocks comprising the manicouagan imbricate zone miz integration of the 3 d geometry with p t and geochronological data allows derivation of a tectonic model for the region the miz was buried to depths 60 km at 1050 ma preservation of its high pressure assemblages and the absence of metamorphism at 990 ma which is characteristic of lower pressure metamorphic rocks that tectonically overlie them indicates the miz rocks were rapidly unroofed early in the tectonic history there were two discrete pulses of crustal thickening during the grenvillian orogeny in this region the first inv

roots of the labradorian orogen in the grenville province in southeast labrador evidence from marine deep seismic reflection data marine deep seismic reflection data obtained as part of the eastern canadian shield onshore offshore transect ecsoot lithoprobe project from offshore southeast labrador are evaluated using geodynamic and gravity models and a tectonic interpretation is developed the geodynamic model explains the seismic reflection data in terms of doubly vergent tectonism resulting from southward directed underthrusting of mantle and lowermost crust middle and upper crustal seismic reflectors are considered to be proshears and retroshears and correlated through potential field data with major mylonite zones at terrane boundaries mapped onshore as there is little evidence of subsequent severe structural modification apart from strike parallel grenvillian dextral transpression which does not alter the crustal geometry along the line of the seismic transect the seismic reflectors are considered to represent labradorian structures inferred from onshore geochronological data to be mostly related to a 1665 ma labradorian accretionary event the southward directed underthrusting of the mantle and lowermost crust is consistent with existing models for southward labradorian subduction coast parallel gravity and magnetic anomalies are interpreted as an expression of a neoproterozoic to early phanerozoic extensional basin the existence of which had little effect on the underlying crystalline basement other than to downfault it relative to that exposed onshore

docking of the central metasedimentary belt to laurentia in geon 12 evidence from the 1 17 1 16 ga chevreuil intrusive suite and host gneisses quebec the chevreuil intrusive suite 1 17 1 16 ga represents a chronological field marker of regional extent that intruded the central metasedimentary belt in the western grenville province of quebec after peak metamorphism style and site of magma emplacement and extent of deformation of chevreuil plutons and dykes permit unravelling of the early grenvillian evolution of the belt with respect to cratonal north america the suite comprises a series of vertically layered gabbro stocks and monzonite diorite gabbro sheet intrusions and a swarm of microdiorite dykes that cut across gneisses the dykes display systematic variations in extent of deformation across the belt we targeted u pb geochronology on gneisses within the identified strain windows they preserve the record of a ca 1 20 ga high pressure temperature p t metamorphic event the sheet intrusions define magmatic corridors all along and concordant with the western northern and eastern tectonic boundaries of the belt the concordant and elonga

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

a tale of two sutures cocorp s deep seismic surveys of the grenville province in the eastern u s midcontinent a pair of oppositely dipping crustal scale shear zones imaged within grenville basement beneath the paleozoic cover of ohio can be correlated via geopotential lineaments with similarly oriented geologic and seismically imaged structures hundreds of kilometres to the northeast and southwest suggesting a relatively simple structural framework for the eastern midcontinent region an east dipping zone extending from lake huron through western ohio and possibly farther southwest marks the western edge of the grenville province perhaps of greater consequence to an understanding of grenville tectonics is the discovery of a west dipping zone underlying the appalachian basin from northern alabama to new york within the grenville province correlation of this feature with the seismogenic clarendon linden fault in western new york and a boundary between terranes containing magmatic arc rocks exposed in canada suggests that it could mark the site of an intra grenville province suture zone implications of this interpretation are that the precambrian foundation of the eastern u s midcontinent comprises a relatively simple assemblage of laterally extensive terranes or belts of coeval terranes accreted by familiar plate tectonic processes and that deep seismic profiling is an effective tool for mapping the three dimensional distribution of these terranes

the 3 d crustal structure in the manicouagan region new seismic and gravity constraints lithoprobe line 55 represents an oblique seismic crossing of tectonic domains in the manicouagan region of the grenville province in eastern quebec that has provided a basis for construction of a 3 d model of crustal structure reprocessing of the seismic profile presented here has enhanced the imaging of prominent out of the plane reflection zones that originate from high pressure metamorphic rocks northwest of the line we show that after conventional migration the apparent dips of out of plane reflections can be compared with regional foliation trends but the reflection orientations in themselves provide only weak constraints on the orientation of out of plane of reflectors we describe and implement several graphical techniques to estimate reflector orientation near large bends in the profile using a prestack processing technique we find poorly resolved but shallow dips and approximately eastward strike directions for reflections from berthe hart jaune and tshenukutish terranes near relais gab

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

seismic images of eclogites crustal scale extension and moho relief in the eastern grenville province quebec seismic images from a 250 km lithoprobe reflection profile in the interior of the eastern grenville province provide important new constraints on the crustal architecture of this part of the orogen prominent upper crustal reflections can be correlated with exposures of high pressure metamorphic rocks in the manicouagan shear belt providing the first direct evidence for eclogite reflectivity in the grenville province the eclogites are cut by major late grenvillian normal faults that penetrate the deep crust and preserve evidence of extensional collapse of the overthickened orogen north to south crustal thinning indicated by a change in moho reflection time from 16 to 13 s correlates well with regional bouguer gravity trends and is accompanied by a dramatic increase in the reflectivity of the lower crust these features underscore the significance of recently recognized along strike variations in tectonic style within the grenville province and point to the internal complexity that characterizes the root zones of collisional orogens

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

southern abitibi greenstone belt archean crustal structure from seismic reflection profiles lithoprobe seismic reflection profiles from the 2 75 2 68 ga southern abitibi greenstone belt reveal subhorizontal reflection zones of regional extent in the upper 15 km of crust that contrast with the steep dips of rock units and structures at the surface the shallowest 6 12 km regional subhorizontal reflection zone is interpreted to be the base of the abitibi greenstone belt supracrustal assemblage which may be an intrusive and or tectonic contact shallowly dipping subparallel reflectors in the upper crust are interpreted as low angle faults consistent with out of sequence stratigraphy revealed by u pb geochronology truncations of reflectors suggest that regional steeply dipping gold mineralized fault zones penetrate to depths of 15 km the profiles are consistent with the kapuskasing crustal model of the central superior province wherein greenstone belts are underlain by middle crustal felsic intrusions and gneiss and a lower crustal zone of mafic gneiss and intrusions the profiles support tectonic models of deformation of the abitibi greenstone belt that invoke either folding and thrusting followed by regional steep faulting or folding and thrusting linked to the steep faults

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

listric normal faulting during postorogenic extension revealed by 40ar 39ar thermochronology near the robertson lake shear zone grenville orogen canada the robertson lake shear zone is a major plastic to brittle extensional shear zone in the grenville orogen that bounds the mazinaw and sharbot lake domains and provides information on the style of late extension and the unroofing history of the orogen argon isotope data were collected from hornblende and micas to determine 40ar 39ar ages constrain the temperature time histories of these two domains and infer the unroofing history of the region hornblende cooling ages across the mazinaw domain footwall show little variation indicating uniform unroofing of the footwall since 950 ma phlogopite muscovite and biotite cooling ages of the footwall are 924 to 890 ma the cooling history of the mazinaw domain is characterized by slow cooling after peak metamorphism circa 1000 ma accelerated cooling 4 5 c m y from 950 ma to 890 ma and an average cooling rate of 1 c m y to circa 590 ma when these rocks were at or near the surface the cooling history of sharbot lake hanging wall domain is drastically different than that of the mazinaw domain hornblende and biotite cooling ages in the central portion of the domain are 1009 and 969 ma respectively indicating a cooling rate of 5 c m y after slow cooling from metamorphic temperatures biotite and phlogopite cooling ages determined from samples located at different distances from the shear zone do not lie along the same cooling curve indicating that the cooling history varied across the domain cooling rates in the hanging wall adjacent to the shear zone are low 2 c m y a biotite cooling age 1029 ma and preservation of an amphibole growth age 1205 ma in the hanging wall adjacent to the shear zone reflect shallow crustal levels for this sample since 1205 ma these data indicate that the hanging wall away from the shear zone was unroofed from deeper crustal levels faster and much later than the hanging wall adjacent to the shear zone the varied cooling histories across the region are resolved by listric normal faulting that lead to uniform unroofing of the footwall and differential unroofing across the hanging wall due to rotation during fault displacement

crustal evolution along a seismic section across the grenville province western quebec results of deep seismic reflection survey along a 375 km long transect of the grenville province in western quebec are combined with a review of geological observations and published isotopic ages the seismic profile offers a remarkably clear image of the crust mantle boundary and a good definition of the various crustal blocks crust about 44 km thick beneath the grenville front zone thins abruptly to ca 36 km southeastward perhaps the result of extension on southeast dipping surfaces extending to the moho other zones of relatively thin crust although less pronounced occur where proterozoic crust overlies archean crust and beneath the morin anorthosite complex the thickest crust is found at the extreme southeast of the transect east of the morin anorthosite from northwest to southeast three main crustal subdivisions are 1 deformed archean rocks with southeast dipping reflectors in the grenville front zone 2 an archean parautochthon with northwest dipping reflectors extending to the lower

the deep structure of the grenville front a new perspective from western quebec the grenville front is a major tectonic boundary exposed on the canadian shield the front is defined as the northwestern limit of grenvillian deformation on the basis of geochronological and metamorphic data this boundary is also evident in some geophysical data sets the lithoprobe abitibi grenville transect crosses the grenville front near lac temiscamingue in western quebec a new 114 km long deep seismic reflection line shows a crustal structure quite different from that seen on previous surveys across the grenville front the archean foreland pontiac subprovince has a pattern of reflectivity similar to that seen in most of the superior province this pattern continues for some 30 km south of the surface exposure of the grenville front there is no evidence for a band of dipping reflectors truncating the horizontal pontiac reflectors in fact the leading edge of the grenville province is difficult to identify on the seismic section the moho is well defined and reveals that the crust thins under

contrasting tectonic styles in the northern grenville province implications for the dynamics of orogenic fronts in western labrador tectonic elements of the parautochthonous belt of the northern grenville orogen include a regional mid crustal thin skinned metamorphic fold thrust belt structurally overlain by a thick skinned thrust sheet with a high pressure metamorphic signature in central ontario the parautochthonous belt is composed of a major reverse sense shear zone along which deep crustal rocks have been uplifted and exhumed tectonic models for both areas involve crustal shortening by delamination in the lower crust and tectonically driven exhumation up a crustal scale ramp the fold thrust belt northwest of the ramp in labrador is a result of the development of critical topography in the thrust wedge at the top of the ramp and the accretion of thrust slices to the base of the wedge during its propagation onto the foreland the fold thrust belt is absent in ontario because syntectonic erosion of the thrust wedge prevented the development of critical topography

metamorphic constraints on the tectonic evolution of the allochthonous monocyclic belt of the grenville province western quebec the tectono metamorphic evolution of grenvillian monocyclic terranes in western quebec has been constrained along a northwest southeast transect from reservoir cabonga to the morin anorthosite by t

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

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

lateral ramps as an aid to the unroofing of deep crustal rocks seismic evidence from the grenville province we suggest that lateral ramps in the basement to orogenic wedges may be important agents in the rapid unroofing of deeply buried continental crust our suggestion is based on a study of the distribution of high pressure metamorphosed continental rocks in the mesoproterozoic grenvillian orogen in eastern quebec these rocks were buried to depths of more than 50 km through crustal thickening during a continental collision and were then rapidly unroofed they are currently exposed in a 100 km 60 km lobate embayment north of the manicouagan reservoir using a combination of surface structural data and seismic reflection data from lithoprobe line 55 we reconstruct the three dimensional form of the high pressure rocks our reconstruct ion methods are based on demonstrable correlations between the attitudes of gneissic layering at the surface and the attitudes of reflectors on the seismic profile they make extensive use of the fact that much of the seismic reflectivity on line 55 comes from outside the vertical plane of the seismic line because of the sinuous nature of the line and its obliquity to tectonic strike the validity of the reconstructions is demonstrated by the generation of a synthetic seismogram from the interpreted geometry that is very similar to the observed seismogram our reconstruction shows that a steep lateral ramp extends to the base of the crust immediately behind the broad lobe of high pressure rocks at the surface the lateral ramp probably provided a favorable pathway for the extrusion of deep seated rocks from the deep crust and their transport onto the foreland

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

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

orogeny and metamorphism a model for deformation and pressure temperature time paths with applications to the central and southern appalachians a model is presented for deformation uplift and exhumation in convergent orogens based on the following premises 1 foreland basin subsidence monitors the distribution in time and space of overthrust loads added to an orogen 2 the presence of an inherited rifted margin plays an important role in the subsequent history of an orogen by forming a basement ramp or buttress that must be surmounted in order for thrusts to advance on the craton on the seaward side of this transition up to 20 km of overthrusts can be accumulated without creating topography 3 critical shape theory can be applied to orogens as a whole allowing predictions to be made concerning dynamic balance between mass addition and loss uplift exhumation and growth of topography and 4 pressure temperature time p t t data can be used to reconstruct particle paths within the deforming orogen three states of overthrust orogens can be defined based on the relative rates of mass loss and mass addition as monitored by the foreland rapid exhumation of metamorphic terranes is most likely in the steady state where erosion balances accretion a constructive state where mass addition is greater than mass loss results in crustal thickening and initially little exhumation a destructive state where mass loss outweighs mass addition results in slow exhumation or subsidence of metamorphic rocks in the orogen depending on whether mass is removed by erosion or by extension p t t data on peak metamorphism pluton emplacement uplift exhumation and cooling can be used in conjunction with data from the foreland to distinguish among these various states the topography and strength of the inherited rifted margin play an important role in localizing zones of exhumation and uplift within the orogen application of this model to the central and southern appalachians suggests that during the constructive phases of the taconian and acadian orogenies overthrust loads accumulated mainly on the seaward side of the ocean continent transition as marked by the bouguer gravity gradient between the acadian and alleghanian orogenies the appalachian orogen was probably in a steady state with uplift and exhumation of rocks in the orogen but little subsidence of the foreland or advance of thrusts toward the craton the constructive stage of the alleghanian orogeny achieved the critical topography necessary to drive crystalline thrust sheets over the craton this resulted in widespread uplift and westward transport of older metamorphic rocks post orogenic unroofing of the southern appalachians is compatible with the predictions of flexural models the post orogenic unroofing history of the central appalachians and ouachitas probably reflects mesozoic extension in these areas

crustal architecture and tectonic assembly of the central gneiss belt southwestern grenville province canada a new interpretation the central gneiss belt southwestern grenville province is characterized by parautochthonous crust in the north and allochthonous lithotectonic domains in the south despite nearly two decades of study the basal decollement to allochthonous domains transported from the southeast known as the allochthon boundary thrust has not been precisely located throughout much of the belt between lake nipissing and georgian bay where its surface trace is known it separates 1 24 ga sudbury metadiabase in the footwall from eclogite remnants and 1 17 1 15 ga coronitic olivine metagabbro confined to its hanging wall on the premise that this relationship can be used to trace the allochthon boundary thrust elsewhere in the central gneiss belt we have sought to extend the known distribution of these mafic rock types making use of field petrographic and geochemical criteria to identify them new occurrences of all three mafic types are identified in a region extending from south of lake nipissing to western quebec

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

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

mantle temperature and lithospheric thinning beneath the midcontinent rift system evidence from magmatism and subsidence the tectono magmatic history of the midcontinent rift system can be explained by the presence of a mantle plume bringing elevated temperature mantle beneath the rift system at about 1110 ma huge volumes of extrusive and intrusive igneous rocks were generated as abnormally hot mantle decompressed beneath the lithospheric rift geochemical and isotopic data from the keweenawan volcanics show that the earliest melts were derived from small degree melting of primitive plume mantle coupled with enriched metasomatic melts derived from the continental lithosphere as rifting progressed the main bulk of the volcanics was generated primarily from the plume mantle with the melting starting at depths of about 120 km and extending to as shallow as the base of the stretched lithosphere at 45 km depth elevated mantle temperatures of 1500 1560 c approximately 150 200 c above normal are inferred from the rare earth element concentrations in the volcanic rocks further constraints on the mantle temperature come f

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

orogen parallel and transverse shearing in the opatica belt quebec implications for the structure of the abitibi subprovince the late archean opatica granitoid gneiss belt is situated within the northern abitibi subprovince along the northern margin of the abitibi greenstone belt approximately 200 km of structural section was mapped along three traverses within the previously unstudied opatica belt the earliest preserved structures are penetrative foliations and stretching and mineral lineations recording regional ductile shearing d1 late d1 deformation was concentrated into kilometre scale ductile fault zones typically with l s tectonite fabrics two families of lineations are associated with d1 indicating shearing both parallel and transverse to the east northeast trend of the belt lineations trending east northeast or northwest southeast tend to be dominant within domains separated by major fault zones in light of the abundant evidence for early north south compression documented throughout southern superior province including the abitibi greenstone belt d1 is interpreted in terms of mid crustal thrusting prob

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

lithospheric flexure and gravity anomalies at proterozoic plate boundaries in the canadian shield abstract proposed proterozoic sutures in the canadian shield show paired positive and negative gravity anomalies previous interpretations ascribe the negative anomaly to an increase in crustal thickness of the older province towards the suture and the positive to increased density and thickness of the younger province here the asymmetrical negative anomaly is interpreted to be the result of flexure of the older lithosphere as it is thrust under the younger more buoyant plate the absence of any visual correlation between topography and observed gravity over suture zones suggests the presence of subsurface loads which are thought to be the primary sources for the related positive gravity anomalies two dimensional modelling of suture anomalies has resulted in the following estimates for elastic plate thicknesses in the canadian shield 60 km for the slave plate and 75 120 125 km for the cape smith labrador and grenville areas of the superior province flexural rigidities at sutures increase with the age of the lithosphere when loaded supporting previous results obtained in oceans and mountain ranges

estimation of the kinetics of geochemical reactions with geophysical models of sedimentary basins and applications abstract the configurational isomerisation and aromatisation reactions which affect the steroid and triterpenoid hydrocarbons present in most organic rich sedimentary rocks are good indicators of the thermal histories of these rocks the extents to which these reactions have occurred can be assessed by gas chromatography mass spectrometry analysis of the organic extracts of the sediment samples we have assumed the reactions can be described as first order and used the model of sedimentary basin formation by extension of the lithosphere to provide thermal histories for the integration of the relevant arrhenius and rate equations governing the reaction rates the activation energies and frequency factors of the reactions were derived by comparing the reaction extents predicted by the model with geochemical observations from basins northern north sea and pannonian basin formed by lithospheric extension which have not experienced major uplift these values were then used to test the thermal consequences of the lithospheric stretching model for nova scotian continental shelf to calculate the amount of uplift and erosion experienced by lower jurassic sediments in the paris and lower saxony basins and the geographical variation in heat flow in a sedimentary basin formed by lithospheric flexure in the foreland basin of alberta in all cases the figures so calculated appear geologically reasonable and receive support from other independent techniques for their estimation the calculated activation energies of all reactions and the frequency factor for aromatisation are chemically sensible the relative values of the frequency factors for the isomerisation reactions could be expected the more sterically hindered c 20 site in a sterane has a lower frequency factor than that for isomerisation at the less hindered c 22 site in hopanes however both values are very low 0 006 and 0 016 s 1 respectively for the back reactions and imply an entropy change of 282 and 271 j mol 1 c 1 respectively the transition states are therefore much more ordered than the reactants this could be the case if catalysis was important no significant variation in catalytic activity can yet be demonstrated in the rocks investigated here

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

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 profiles across the central kapuskasing uplift the central superior province is transected by the intracratonic kapuskasing uplift which contains rocks exhumed from 30 to 35 km paleodepth as part of the lithoprobe kapuskasing transect approximately 52 km of 16 s seismic reflection data were collected in the central segment of the uplift along three profiles that traverse the northern groundhog river block the bounding saganash lake fault and the eastern val rita block the seismic sections have the following characteristics in common i a complexly reflective uppermost portion 1 s limiting correlation of reflective zones and surface features ii numerous subhorizontal east and west dipping reflection zones and iii a significant reduction in reflectivity beyond the refraction defined moho 14 s beneath the groundhog river block a series of straight west dipping 20 reflection zones between 2 and 10 s is underlain by subhorizontal reflections in the lower crust across the saganash lake fault the val rita block is characteriz

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

seismic reflection imaging over a massive sulfide deposit at the matagami mining camp quebec a 2 d seismic reflection profile was shot across the southern flank of the matagami mining camp almost directly above the recently discovered bell allard massive sulfide deposit now estimated at more than 6 million metric tons all orebodies found in the southern part of the mining camp including bell allard are located at the contact between the primarily basaltic wabassee group and the underlying rhyolitic watson lake group seismic reflections were recorded from the basalt rhyolite contacts of the lower wabassee group as well as from gabbro sills that intrude much of the volcanic stratigraphy a strong reflection from the top of the bell allard orebody was also detected but the reflection does not extend over the full width of the deposit as defined by drilling appearing to correlate with the lower pyrite rich zone faulting which can be interpreted from discontinuities in the observed reflections probably controlled the formation of the bell allard deposit if the interpreted gabbro sills are accepted as isotime markers then faulting of the deeper sill complex defines a series of half grabens within the rhyolitic watson lake group the bell allard deposit is found at the intersection of one of these apparently low angle normal faults with the top of the watson lake group indicating that sulfide mineralization may have been associated with fluid flow along the fault which likely penetrates to the underlying mafic intrusion although the precise geometry of subsurface faulting cannot be estimated from a single 2 d seismic profile these results indicate that a full 3 d seismic survey should allow the mapping of many of the subsurface fault systems and the verification of hypotheses of fault controlled deposit formation

a neodymium isotope study of plutons near the grenville front in ontario canada abstract sm nd analysis of meta plutonic rocks near the grenville front in ontario allows their division into two groups granitoid plutons with model ages of 2 ga are intruded into a crustal section formed largely of archean or re worked archean basement plutons with model ages of plutons south of the proposed suture line can be explained by anatexis of penokean age crust alone while plutons north of this line contain mixtures of archean and proterozoic nd these mixed signatures suggest that the nw grenville province may be analogous to northern wisconsin and michigan in this region a very early proterozoic miogeocline supplied by the superior province is overlain by a formation of mixed superior penokean provenance which heralded the collision of a penokean arc with the archean craton despite very strong sm nd evidence for the presence of old crustal components in the plutons u pb analysis did not reveal the presence of inherited zircon hence the absence of such zircons should not be taken in isolation as evidence against crustal re working

the petrology and geochemistry of granitic gneisses from the east arunta inlier central australia implications for proterozoic crustal development the entia gneiss complex is an early to middle proterozoic inlier composed of upper amphibolite facies orthogneisses and paragneisses in the eastern end of the arunta block in central australia it is separated from overlying structurally distinct compositionally different and probably younger gneisses of the harts range complex by a subhorizontal detachment zone this paper discusses the petrology of four major granitic gneiss bodies one of these the bruna gneiss has intruded this detachment zone the other three are components of the structurally underlying entia gneiss complex the entia gneiss complex has a distinctive subhorizontally layered character with recumbent folds and flat lying schistosity the granitic gneisses that are components of this complex appear to have intruded an actively deforming terrain and are folded sheet like bodies the orthogneisses of the entia complex range from gabbro to true granite in composition and the granites are i type on both geochemical and petrographic criteria they are significantly different from the relatively alkali rich granites of equivalent age which occur in some of the other northern australian proterozoic blocks and are most like the granites of the modern cordilleran belts particularly in the association of diorite and gabbro with granodiorite it is suggested they are formed in narrow collisional belts formed as a result of closure of intracratonic rift basins the high surface area to volume ratio of these granites which is a consequence of the deformational environment into which they were intruded has resulted in considerable wall rock exchange in the more differentiated end members of the suites this has led to contamination by a net gain of k2o rb and ba and loss of relatively compatible elements including the ree y nb and tio2 compared with the syn tectonic entia granites the bruna granite gneiss is post tectonic and shows geochemical affinities with a type or anorogenic palaeozoic granites most of this granite is confined to the detachment zone but the very latest phases also intrude the harts range cover sequence an important conclusion of this study is that detachment zones of the type described from this area of the eastern arunta block exercise strong control on plutonism into the early to middle proterozoic crust as long as the very mobile style of early proterozoic tectonics continues the intracrustal dislocation zone confines syn tectonic granitic magmatism to deeper levels

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

age structure and orogenic significance of the berens river composite batholiths western superior province the berens river area of northwestern ontario is underlain mainly by archean felsic plutonic rocks which enclose minor supracrustal and gneissic enclaves and merge with the greenstone belt rich uchi subprovince to the south u pb geochronology using zircon and monazite shows that the batholiths evolved mainly between 2750 and 2690 ma by sequential and essentially continuous intrusive activity into an older substratum composed of 3000 2800 ma volcanic and tonalitic crust there is a broad but not strict compositional transition from early biotite tonalite and hornblende tonalite progressing with time towards a greater abundance of hornblende granodiorite to granite and finally to late biotite granite rare peraluminous granites and sanukitoid dioritic monzodioritic to granitic plutons the tonalite suites were predominantly synvolcanic the late granitic intrusions postdated volcanism but were largely synchronous with the main compressional events that caused widespread sedimentation deformation

coronitic metagabbro and eclogite from the grenville province of western quebec interpretation of u pb geochronology and metamorphism we present new u pb and metamorphic data on high pressure coronitic metagabbros from three distinct structural settings in the parautochthonous belt of the grenville province in western quebec intrusive ages are i for metagabbro close to the grenville front correlative with the sudbury dykes defined in ontario ii for an eclogitized lens at the base of the highest structural level sl4 a new age for mafic magmatism in the western grenville and iii for metagabbro from sl4 interpreted as correlative with metagabbros from the algonquin and shawanaga domains in ontario metamorphism in all cases is grenvillian with the best constrained age of 1069 3 ma for the metagabbro of sl4 metamorphic grade increases from the grenville front to the south the mafic rocks preserve relict igneous textures overprinted by garnet clinopyroxene that developed as coronas and or pseudomorphs after igneous phases the highest grade metagabbros contain omphacite and some lack primary plagioclase therefore b

40ar 39ar thermochronology along a western quebec transect of the grenville province canada ar ar thermochronology on 24 hornblendes 3 biotites 2 muscovites and 2 k feldspars collected along a 400 km long nw se geotraverse through the grenville province in western quebec is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the grenville orogen in the grenville front zone the c 1000 ma time of exhumation previously established from thermobarometric and isotopic studies is supported by the hornblende age data presented here from 60 km to 160 km se of the front reworked archaean migmatites of the parautochthonous reservoir dozois terrane rdt 1004 ma old metamorphic monazites contain hornblendes with 972 950 ma cooling ages assuming metamorphic geotherms between 25 and 30 c km 1 calculated cooling and unroofing rates are about 6 c ma 1 and 0 33 km ma 1 in the p t range 725 c 800 mpa and 450 c 400 mpa hornblendes from monocyclic rocks of the mont laurier and morin terranes mlt and mt monazite ages c 1165 ma give ages of about 1040 and 1010 ma respectively calculation of cooling unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated grenville collision which took place approximately between 1060 and 1020 ma cooling ages of c 900 ma for muscovite and biotite and 860 810 ma for k feldspar show that cooling rates decreased to around 1 5 c ma 1 under retrograde greenschist facies conditions in the mlt on a time vs distance diagram the hornblende data define several distinct age ranges suggesting that each terrane had a characteristic thermal history thus cooling was diachronous and probably non homogeneous throughout this segment of the grenville orogen the time lag between the cooling history of the parautochthon 972 950 ma and the allochthons 1040 1010 ma is compatible with an earlier pre 1040 ma peak of metamorphism in the allochthons the reservoir cabonga allochthon was transported toward the nnw from its probable root zone in the mlt during the 1060 1020 ma grenvillian collision as a partially cooled slab the remobilization of the archaean parautochthon is attributed to this collision in the grenville front zone slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid tectonic exhumation shortly after collision the minor delay 20 30 ma in unroofing of the mt compared to the adjacent mlt is most likely related to post 1040 ma extensional displacement along the labelle shear zone in terranes like those described above where metamorphism is diachronous determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure however identification of contrasting cooling histories contributes to unravelling the independent movement of terranes

precise u pb ages for grenvillian and pre grenvillian thrusting of proterozoic and archean metamorphic assemblages in the grenville front tectonic zone canada while the position of the grenville front is placed at a structural feature associated with the onset of high grade rocks when approached from the north the time of first metamorphism in these rocks is in general pre grenvillian and highly variable at killarney amphibolite to granulite grade rocks were metamorphosed lineated and foliated to their present front parallel configuration mainly by 1453 6 ma north of north bay amphibolite grade migmatites have been subjected to a pre 2650 ma metamorphism granulite grade gneisses south of the grenville front near val d or record a 2640 ma metamorphic event south of chibougamau mafic enclaves in tonalite south of the front were converted to migmatitic amphibolite at about 2680 to 2690 ma in central labrador a 1650 ma foliated granite is present south of the grenville front and at least some of the banding is this old at the atlantic coast granulite facies metamorphism in front parallel gneisses predates a 1790 1662 ma deformation in contrast each of these areas shares a near common age of second metamorphism that is variable in degree and accompanies nw directed thrusting or crustal loading near killarney on georgian bay titanite records an episode of 977 2 ma partial resetting the degree of which increases with distance into the grenville province here grenvillian deformation where present is manifest as feldspar porphyroclastic fabric with strong se plunging lineations near north bay archean pegmatites in the grenville front zone were similarly deformed at 988 10 ma and strongly rodded ductile amphibolite underwent late melting to produce postlineation tonalite in which new zircon tips grew at 987 2 ma a 991 4 ma cooling age for titanite in this melt is analytically identical to the zircon growth age hence rapid cooling occurred south of chibougamau new zircons grew in high pressure coronite in response to tectonic loading at 995 6 ma and in central labrador melt pods that developed in 1650 granites in response to thrusting record a zircon new growth age of 995 2 ma titanite in the granitic host rock has an analytically identical age of 992 4 ma again indicating rapid cooling this second event is recorded by 28 titanite and zircon analyses for samples up to 225 km apart that record a 1651 5 ma to 988 5 ma two age mixing line grenville resetting at about 1037 19 ma in the grenville front zone on the labrador coast is indicated but major thrusting is pre 1662 ma u pb isotopic results indicate that the grenville front zone is characterized along its length by partially reset u pb systems high pressure assemblages local melting and rapid cooling crustal loading and shearing with lead loss related to mechanically induced recrystallization and local heat that was rapidly terminated by rebound and cooling are required

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