reworking of sialic crust as represented in late archean age gneisses northern labrador regionally reworked sialic crust has been recognized in the form of 2 800 m y old nebulitic rocks kiyuktok gneisses with high initial 87sr 86sr ratios 0 7081 and pb isotope ratios that suggest a history of the source back to at least 3 500 m y b p the field geochemical and isotopic data are consistent with these rocks having formed by in situ mobilization of granitic components in the 3 600 m y old uivak gneisses by the introduction of aqueous fluids some 2 800 m y ago contemporary but subordinate tonalitic granodioritic orthogneisses derived from discrete intrusive bodies have contrasting sr isotope compositions some display low initial 87sr 86sr ratios 0 7030 that suggest derivation from a source with mantlelike sr isotope composition however most of the late archean age intrusive rocks display higher initial 87sr 86sr ratios 0 7056 0 7059 recognition of these contrasting isotopic patterns suggests that processes of late archean crust formation and stabilization in the north atlantic craton were more complex than previously realized crustal reworking due to interaction of old crust with aqueous fluids must have been accompanied by the introduction of granitic melts derived by melting of ancient deep crustal rocks or contamination of juvenile granitic melts by ancient deep crustal material

nd sr isotopic composition of granulites and constraints on the evolution of the lower continental crust in a statistical study of granulite facies rocks we have analysed 32 samples with ages ranging from 2 900 to 200 myr for sm nd rb and sr and 143nd 144nd and 87sr 86sr ratios the sm nd ratio is found to be constant compared with the highly variable rb sr ratio using the model age formalism we show that in many cases internal differentiation of continental crust follows external differentiation with a large time interval

continental transform tectonics great slave lake shear zone ca 1 9 ga northwest canada great slave lake shear zone gslsz is a northeast trending dextral mylonite zone across which the coeval north trending thelon and taltson magmatic zones on the west margin of the churchill province are offset 300 700 km it is proposed that the gslsz is a continental transform structure related to oblique collisional indentation of the active margin of the churchill province by the microcontinental slave province this accounts for geologic and geophysical differences between the thelon and taltson zones and explains the localization of a tibetan type queen maud uplift north of the gslsz and the abrupt loss of slip on the gslsz east of the uplift it also provides a new model for the athapuscow aulacogen analogous to the modern bay of bengal indo burman ranges sagaing transform andaman sea situation

age constraints on the paleoproterozoic tectonometamorphic history of the committee bay region western churchill province canada evidence from zircon and in situ monazite shrimp geochronology in situ u pb sensitive high resolution ion microprobe shrimp analyses of monazite from upper amphibolite facies paragneiss of the committee bay supracrustal belt central rae domain canada reveal three age populations ca 2350 1850 and 1780 ma the ca 1850 ma age also corresponds to growth of low th u zircon as indicated by u pb shrimp analyses of zircon separates from melanosome and leucosome the contextual advantage of the in situ monazite analysis allows evaluation of the geochronological data in terms of the regional structural and metamorphic evolution the region is dominated by a northeast striking s2 s1 fabric axial planar to tight northwest vergent f2 folds early garnet is enveloped by this biotite sillimanite cordierite s2 fabric garneti hosts ca 1850 ma monazite inclusions with ca 2350 ma cores placing a maximum age on garneti growth and s2 development d2 metamorphic conditions progressed through 3 5 kbar 1 kbar 100 mpa and 600 c to near peak conditions of 5 kbar and 675 c a minimum age for s2 is provided by unstrained ca 1820 ma monzogranite that locally and regionally truncates s2 dominantly ca 1780 ma matrix monazite is interpreted to date post s2 garnetii and cordierite which record 5 kbar and 675 c these data indicate that the committee bay region experienced penetrative d2 tectonometamorphism at ca 1850 1820 ma with a subsequent static overprint the absence of a ca 1 85 ga plutonic suite in the region suggests that low pressure metamorphism was a response to thick skinned crustal thickening initiated at ca 1870 ma the new data highlight the importance of paleoproterozoic reworking of the central rae domain in the hinterland of the trans hudson orogen

influence of relative sea level change and tidal inlet development on barrier spit stratigraphy sandy neck massachusetts abstract sandy neck is a 10 km long sheltered mesotidal barrier spit along the southeastern new england coast bordering cape cod bay its stratigraphy contains evidence of relative sea level rsl rise and tidal inlet development and enables reconstruction of accretion and progradation of this barrier through time core trench and ground penetrating radar gpr data show that the lower boundary of the uppermost eolian facies climbs about 9 m from the proximal to the distal part of the spit away from the mainland anchor point this change is thought to be related primarily to a comparable rise in rsl the same boundary also shows a shallowing trend in a seaward direction indicating net spit progradation through time the absence of inlet channel facies in the stratigraphy of the proxima part of the barrier spit in contrast to the presence of inlet facies in more distal parts of the spit indicates that some time elapsed after barrier initiation before an inlet thalweg developed in its earliest stages this thalweg was poorly defined wide and shallow but its characteristics changed as the barrier spit lengthened in the central part of sandy neck the assemblage of inlet channel facies becomes markedly thicker reflecting thalweg deepening in response to paleotopographic and substrate lithologic factors the stratigraphy of the distal part of the spit shows the effects of continued but more gradual inlet deepening that may be tied directly to increasing barrier length the process controlled stratigraphy presented here differs from current facies models because i accounts for temporal changes in coastal environmental processes and parameters it suggests a high preservation potential for proximal eolian facies in the spit sequence underneath a cover of fine grained and organic back barrier sediment once reliably dated the new stratigraphic data will help to improve the regional rsl curve for the southern cape cod bay area

inversion of the coprod2 magnetotelluric data using a diffusive to propagative mapping dpm diffusive to propagative mapping dpm transforms a diffusive electromagnetic field into a dual wave field to produce electromagnetic reflectivity images similar to non migrated seismic sections an application of dpm to the coprod2 data set gives reflectivity images showing three distinct reflectors attributed to resistivity jumps located at depths ranging from 2 to 50 km the deeper reflector may be related to a major west dipping seismic reflector detected with the cocorp seismic data

archean and early proterozoic events along the snowbird tectonic zone in northern saskatchewan canada abstract the snowbird tectonic zone is a major boundary structure in the canadian shield separating rocks of the rae and hearne lithostructural domains field investigations along portions of the snowbird tectonic zone exposed in northern saskatchewan have revealed a major granulite facies mylonite belt that records deformation and metamorphism at several intervals in the archean and early proterozoic early assemblages are dominated by granulite grade gneisses that were deformed in a major left lateral strike slip shear zone at ca 3200 ma the major episode of deformation along this portion of the snowbird tectonic zone occurred at ca 2600 ma again at granulite facies in a dominantly right lateral strike slip shear zone little evidence of proterozoic deformation is recorded within this region of the snowbird tectonic zone itself but neighboring rocks in the rae province to the west preserve granulite grade right lateral fabrics dated at ca 1850 ma these constraints suggest that the snowbird tectonic zone was a focus of major regional deformation throughout the middle and late archean but that regional deformation was partitioned outward to the surrounding rae and hearne province rocks during the early proterozoic these data suggest that the snowbird tectonic zone was not a major continental suture zone in the early proterozoic

modest movements spectacular fabrics in an intracontinental deep crustal strike slip fault striding athabasca mylonite zone nw canadian shield geometry and strain partitioning within lower crustal intraplate strike slip shear zones can be extremely complex compared with analogous structural levels of interplate strike slip shear zones sited at plate margins striding athabasca mylonite zone canadian shield is a spectacular ca 500 km long granulite facies continental intraplate shear zone the shear zone is composed of middle archean granulite facies annealed mylonites ca 3 13 ga and late archean ca 2 62 2 60 ga granulite facies ribbon mylonite belts which thread a sinuous course along a chain of crustal scale lozenges cored by relatively stiff rocks of mafic to intermediate composition to the northeast the mylonites form a n s trending 5 10 km thick dextral strike slip belt to the southwest this bifurcates into a pair of conjugate strike slip shear zones overlain by a contemporaneous dip slip shear zone striding athabasca mylonite zone was kinematically inefficient as a strike slip fault and cannot have accommodated large wallrock displacements nevertheless spectacular granulite facies ribbon mylonites were formed throughout the shear zone reflecting the very high temperatures ca 850 1000 c high recrystallization rate strain rate ratios and the transpressive nature of the deformation wk 1 possibly accommodated by significant volume loss by magma migration

geochronological constraints on the legs lake shear zone with implications for regional exhumation of lower continental crust western churchill province canadian shield the legs lake shear zone marks the southeastern boundary of an extensive region 20 000 km2 of high pressure 0 8 1 5 gpa granulite facies rocks in the western churchill province canada the shear zone is one of the largest exhumation related structures in the canadian shield and coincides with the central segment of the 2 800 km long snowbird tectonic zone the movement history of this shear zone is critical for the development of models for the exhumation history of the high pressure region we used electron microprobe u th pb dating of monazite with supplemental id tims u pb geochronology to place constraints on the timing of shear zone activity combining these and other data we suggest that regional exhumation occurred during at least three distinct phases over an 150 million year period the first phase involved high temperature decompression from 1 0 to 0 8 0 7 gpa shortly following 1 9 ga peak metamorphism possibly under an extensional regime the second phase involved rock uplift and decompression of the hanging wall to 0 5 0 4 gpa during east vergent thrusting across the legs lake shear zone at ca 1 85 ga this phase was likely driven by early collision related convergence in the trans hudson orogen the final phase of regional exhumation involving the removal of 15 20 km of overburden from both footwall and hanging wall likely occurred after 1 78 ga and may have been related to regional extensional faulting

acquisition and processing of wide aperture ground penetrating radar data a 40 channel wide aperture ground penetrating radar gpr data set was recorded in a complicated fluvial aeolian environment in eastern canada the data were collected in the multichannel format usually associated with seismic reflection surveys and were input directly into a standard seismic processing sequence filtering static corrections common midpoint gathering velocity analysis normal and dip moveout corrections stacking and depth migration the results show significant improvements over single channel recordings in noise reduction and depth of penetration by stacking and in spatial positioning and reduction of diffraction artifacts by migration these characteristics increase the potential for reliable interpretation of structural and stratigraphic details thus without having to develop any new software gpr data processing technology is brought to the same level of capability flexibility and accessibility that is current in seismic exploration

taltson basement gneissic rocks upb and nd isotopic constraints on the basement to the paleoproterozoic taltson magmatic zone northeastern alberta the taltson magmatic zone is a 300 km long northerly trending belt of archean to paleoproterozoic granitic basement gneiss amphibolite supracrustal gneissic rocks and voluminous paleoproterozoi

crustal history of the rae and hearne provinces southwestern canadian shield saskatchewan constraints from geochronologic and isotopic data abstract the cree lake zone hearne province and western granulite domain rae province in northern saskatchewan are contrasting parts of the variably reworked upper plate western hinterland of the paleoproterozoic trans hudson orogen their junction is defined by the virgin river shear zone a part of the more extensive snowbird tectonic zone subsurface extension of the 2 0 ga thelon orogen lies west of the western granulite domain we present u pb zircon ages sm nd and rb sr isotopic data from the cree lake zone and western granulite domain and from the junction granite which intrudes the virgin river shear zone data from the western granulite domain indicate that prevalent granulite facies metamorphism occurred between 2 3 and 2 0 ga but is imposed on archean protoliths 2 8 3 0 ga old low amphibolite facies retrogression which increases toward the virgin river shear zone probably postdates 2 1 ga data from the cree lake zone also yield archean protolith ages as old as 3 0 ga and some rocks yield interpreted high grade metamorphic overprint ages of 2 3 2 0 ga however two samples yield data indicating hudsonian high grade metamorphic overprint 1 87 to 1 81 ga the junction granite has a u pb zircon age of 1 82 0 30 ga but relict zircons and sm nd data show that it was derived by melting of archean crust high grade mid crustal thermal resetting in the western granulite domain may be coeval with the 1 9 2 0 ga thelon orogen although the data do not preclude an earlier event or events similar 2 3 2 1 ga reworking extends into the cree lake zone but high grade hudsonian 1 87 1 84 ga reworking is also evident in parts of this zone well to the west of the wollaston domain and may extend entirely across the zone to the virgin river shear zone relative importance of the thelon and hudsonian overprints in cree lake zone is unknown data from the junction granite and western granulite domain suggest that all recorded high strain events in the virgin river shear zone postdate 2 3 ga and are thus unrelated to deep crustal archean granulite facies mylonites in the tantato black lake segment of the snowbird tectonic zone north of the athabasca basin

lithology and subsidence in the north sea the north sea sedimentary basin has developed on the northwestern margin of the european tectonic plate and contains an almost continuous record of epeirogenic marine and deltaic sedimentation from carboniferous to recent times the subsidence required to accommodate the pile of sediment deposited which in places exceeds 12 km has been brought about at various times and in various places by differing geodynamical processes as a result the types of sedimentary rocks deposited vary widely both in time and space but the nature of the mechanism is reflected in the sedimentary type deposited the following broad generalizations can be made the late carboniferous was a period of deltaic sedimentation during which eustatic changes in sea level or local variations in subsidence rates are reflected in the typical coal measures swamp deposits late carboniferous early permian times saw the silting up of this basin and in an arid climate aeolian sands were deposited grading laterally to sabkha shales and evaporites the permian culminated in a series of widespread marine incursions during which repetitive evaporites were deposited triassic times were marked by a period of major rifting and the deposition of thick sequences of continental clastics in the north while widespread marine sedimentation persisted in southern areas jurassic times saw the re establishment of marine to deltaic deposition in a series of basins possibly controlled in their distribution by the triassic fault systems late jurassic deposits were laid down in a sea whose bathymetry reflected the structure of the underlying horsts and grabens inherited from triassic times and towards the close of the jurassic the bottom waters at least of this sea become increasingly stagnant sands deposited during the late jurassic were deposited as near shore marine bars beach sands and proximal and distal submarine fans triassic to early cretaceous deposition was concentrated in the areas now occupied by the main grabens of the north sea i e the viking central and moray witch ground grabens subsequent deposition in late cretaceous to tertiary times took place in a more widely subsiding area resulting in progressive onlap onto the surrounding basin margins deposition within this broadly subsiding and relatively unfaulted basin is characterized by chalky limestones in southern areas giving way laterally to shales and minor sands to the north during early tertiary times a large delta was formed in the area beneath the present moray firth and from this delta a supply of sand was fed into submarine fans to the northeast and southeast of the delta front late tertiary deposition is largely represented by a monotonous sequence of marine shales

paleomagnetism anisotropy of magnetic susceptibility and argon argon geochronology of the clearwater anorthosite saskatchewan canada abstract the clearwater anorthosite in saskatchewan is a major gabbro anorthosite layered intrusion lying within the archean rae province along the northwestern hinterland of the paleoproterozoic trans hudson orogen tho the intrusion which could be at least 2 4 ga old carries a well defined remanent magnetization with a direction of declination 112 and inclination 38 k 316 95 3 1 n 8 the corresponding paleomagnetic pole plat 6 5 n plong 48 2 w dp 2 2 dm 3 7 is the first one obtained from the archean western granulite domain of the rae province field observations supplemented by anisotropy of magnetic susceptibility data show that the magnetization post dates ductile deformation of the anorthosite argon argon plateau dates of 1917 7 ma for hornblende and 1839 5 ma for biotite are interpreted as cooling ages with the hornblende age approximating the time of remanence the pole position and others of comparable age from the northwestern margin of the tho form a loose cluster the mean of which is about 90 of arc away from that of a broadly contemporaneous group of poles from the superior province this relation supports previous interpretations of paleomagnetic data that prior to the tho at about 1 9 ga ago a major ocean the manikewan ocean at least 5000 km across separated the superior and rae hearne provinces

thermal state in the north viking graben north sea determined from oil exploration well data we can deduce thermal conductivities and thermal gradients from extensively available oil exploration data geophysical well logs cores cuttings formation thicknesses and temperatures thermal conductivity is predicted at three significant scales first it is computed at the scale of well log electrofacies thicknesses from 1 to 10 m using a geometric mean model calibrated on laboratory measurements made on the main sedimentary rocks the electrofacies conductivity is calculated as a function of the mineralogy the porosity and the saturating fluids second it is estimated at formation scale at each well site thicknesses from 100 to a few thousand meters using a serial model that accounts for the anisotropy due to sediment stacking and for temperature effects finally for each formation thicknesses on the order of 1 km the average conductivity field is mapped at basin scale extent on the order of 100 km using a geostatistical treatment accounting for lateral facies and or porosity changes fo

3d pseudo seismic imaging of transient electromagnetic data a feasibility study we investigate a pseudo seismic approach based on the so called inverse q transform as an alternative way of processing transient electromagnetic tem data this technique transforms the diffusive tem response into that of propagating waves obeying the standard wave equation these transformed data can be input into standard seismic migration schemes with the potential of giving higher resolution subsurface images such images contain geometrical and qualitative information about the medium but no quantitative results are obtained as in model based inversion techniques these reconstructed images can be used directly for geological interpretation or in further constraining possible inversions we extend the original q transform based on an electrical source formulation to the case of a large loop tem source moreover an efficient discrete version of the inverse of this modified q transform is presented using a regularization method application of this inverse transform to the measured tem responses gives the corresponding pseudo seismic data which are input into a 3d migration scheme we then use a 3d boundary element type of kirchhoff migration to ensure high computational efficiency this proposed method was applied to both synthetic data as well as field measurements taken from an engineering geology survey the results indicate that the resolution of the tem data is significantly improved when compared with standard apparent resistivity plots demonstrating that higher resolution 3d transient electromagnetic imaging is feasible using this method

focused synthetic aperture radar processing of ice sounder data collected over the greenland ice sheet the authors developed a synthetic aperture radar sar processing algorithm for airborne spaceborne ice sounding radar systems and applied it to data collected in greenland by using focused sar phase corrected coherent averaging they improved along track resolution by a factor of four and provided a 6 db processing gain over unfocused sar coherent averaging without phase correction based on a point target analysis for a greenland ice sounding data set also they demonstrated that the focused sar processing reduced clutter and enabled them to identify bedrock interface returns buried in clutter using focused sar technique they processed data collected over a key 360 km long portion of the 2000 m contour line of southwest greenland to the best of their knowledge these are the first high quality radar ice thickness measurements over this key location moreover these ice thickness measurements have been used for improving mass balance estimates of the greenland ice sheet

a basement interacted fluid in the n81 deposit pine point pb zn district canada sr isotopic analyses of single dolomite crystals the mississippi valley type pb zn deposits of the pine point district northwest territories canada are located close to the eastern edge of the present day western canadian sedimentary basin the deposits are thought to have formed as the result of basin wide fluid flow in the presqu ile barrier the host to the ore deposits a laser ablation multi collector inductively coupled plasma mass spectrometric study of 87sr 86sr ratios of ore related dolomites from the n81 deposit at pine point indicates that at least two sources of sr were present in the mineralizing system one fluid has a range in sr isotopic values from 0 07073 to 0 71200 and is interpreted to be derived from middle devonian seawater that interacted with clastic units in the basin the second fluid has higher sr isotopic values up to 0 71520 similar to those found in some canadian shield brines and is interpreted to represent an evaporated seawater derived brine which has interacted with crystalline basement rocks reactivation of old structures in the basement may have provided a pathway for cross formation fluid flow to the site of mineralization the data suggest that the stratigraphic location of the pine point district near the interface between the western canadian sedimentary basin and its basement may have exerted a fundamental control on the formation of these deposits

crustal evolution and age of thermotectonic reworking in the western hinterland of the trans hudson orogen northern saskatchewan abstract the hearne province in northern saskatchewan forms the western hinterland of the trans hudson orogen it is a fault bounded terrane lying structurally and geographically between the rae province to its west and the wathaman batholith and orogenic internides to its east the hearne province is comprised primarily of high grade metamorphic archean rocks structurally overlain by archean or proterozoic metasedimentary rocks and intruded by paleoproterozoic granites whereas the rae terrane on its west boundary is comprised primarily of still higher grade archean rocks the calc alkaline wathaman batholith was intruded at ca 1 855 ga and the orogenic internides to the east of it include mostly high grade mafic and intermediate volcanic rocks crystallized between 1 92 and 1 86 ga terminal collision and orogenesis in the internides have been constrained to be later than 1 82 ga and earlier than 1 78 ga mineral u pb data presented here unequivocally demonstrate that high grade metamorphism and large volume anatexis across the southern hearne province were approximately synchronous with terminal collision and high grade metamorphism across the trans hudson orogen zircon data for most samples form complex arrays in concordia space but demonstrate a high grade metamorphic overprint at 1810 1800 ma zircons typically have inherited components of archean age suggesting the presence of extremely old material in the hearne and a potential archean connection with the rae province sm nd data indicating ca 3500 ma model ages for similar parageneses in both terranes further support the proposed archean connection between hearne and rae province and the great antiquity of hearne crust sm nd data also elucidate the similarity of the ca 1 80 ga anatectic granites of the hearne to the 1 82 ga junction granite which intruded the virgin river shear zone and constrains the timing of final motion on that structure monazite 207 pb 206 pb ages are not older than 1812 ma or younger than 1769 ma and rutile 207 pb 206 pb ages and u pb ages are tightly constrained at about 1750 ma these data in combination with results of standard thermobarometric analysis permit determination of post orogenic cooling and denudation rates high cooling 7 c ma and denudation 240 m ma rates suggest fault accomodated uplift during post orogenic collapse

analysis of bottom morphology of the david glacier drygalski ice tongue east antarctica data from radio sounding measurements have been analysed to determine the ice thickness and the bottom morphology of drygalski ice tongue antarctica the morphology and the structure of the bottom surface has been studied through an electromagnetic interpretation a function that includes the gain loss due to the geometrical shape of the reflecting surfaces has been calculated such a function has been evaluated assuming some physical electromagnetic quantities the temperature of the glacier the complex dielectric permittivity of ice sea ice and sea water the ice water interface shows both concave and convex faces toward the sounding system producing a focusing or deocusing effect detected as absolute or relative amplitude variation in the echo signal it is shown that the calculated function follows quite well the observed bottom rippled surface of the glacier tongue estimated from the time arrival measurements of the echo signal

palaeozoic mesozoic and contemporaneous megaspores from the tertiary of southern england indicators of sedimentary provenance and ancient vegetation megaspores are recorded from several localities in the thanet and reading beds upper palaeocene of southern england evidence from their taxonomic affinity colour compression and state of preservation indicates three separate groups two reworked from lower westphalian coal measures and lower cretaceous sequences respectively and one of contemporaneous forms mesozoic megaspores redeposited in the british tertiary have not previously been recorded isolated carboniferous forms have been found before but their possible value in provenance studies has not been fully explored the new material includes a diverse assemblage of numerous westphalian and lower cretaceous forms from a single horizon in the thanet beds of possible source areas it seems most likely that these spores were reworked from the moray firth basin and elsewhere near the present eastern coast of scotland by means of longshore drift following a roughly n s palaeocoastline evidence from other studies of heavy mineral assemblages and palaeogeography also indicates derivation of thanet beds sediments by this means the cenozoic megaspores include an azolla water fern and two distinct lycopsid forms one probably of selaginellalean and one of isoetalean affinity these latter provide the first clear evidence of lycopsids in british tertiary floras and expand our knowledge of freshwater macrophytes of that time the assemblages of minerisporites show morphological trends which may prove to be of biostratigraphic value

development of rift basins illustrated by the structural evolution of the oseberg feature block 30 6 offshore norway the mesozoic tilted fault block in norwegian north sea block 30 6 containing the giant oseberg oil and gas field is used to illustrate basin development during the jurassic and cretaceous there is seismic evidence for an early rifting phase in the northern viking graben on which the jurassic cretaceous rifting phase early bathonian to berriasian is overprinted the oseberg structure is representative of other fault blocks bordering the northern viking graben axis planar normal faults and listric faults which were restricted to the rifting phase dominate both fault types exhibit syn fault sedimentation listric faulting is recognized on seismic sections by the differential tilting reverse drag antithetic faulting and a wedge shaped syn rift reflector package the initiation of rifting is dated by baselap onto pre rift reflectors and cessation is dated by an unconformity beneath the generally onlapping post rift sediments steeply dipping planar normal faults often active along previously established fault zones played a significant part in the post rift development of the basin the last movement on planar normal faults in the oseberg fault scarp becomes younger away from the rift axis and this produced the complex fault scarp visible on many seismic lines the topography defined by the post rift unconformity is a composite surface which formed in response to subsidence processes it is not the result of orogenic events such as the kimmerian 9orogeny9 footwall uplift associated with planar normal faults caused local unconformities in complex fault scarps and on fault block crests

the age and pb loss behaviour of zircons from the isua supracrustal belt as determined by ion microprobe ion microprobe uthpb analyses of zircons from a conglomeratic metavolcanic unit in the isua supracrustal belt of west greenland show that their magmatic age is3807 1ma and failed to detect any older zircon cores or xenocrysts low u areas within zircons at a 30 m scale lost only small amounts of radiogenic pb and only in the recent past whereas high u areas within grains lost much greater proportions of their pb both recently and as early as the archaean some areas in the zircons have concordant upb and thpb ages the ion probe results agree with earlier determinations for the isua supracrustals and clarify minor discrepancies between previous multigrain and single crystal analyses

sm nd pb pb and rb sr geochronology of the manfred complex mount narryer western australia the sm nd pb pb and rb sr systems in manfred complex samples ranging from metaleucogabbro to metapyroxenite indicate formation of the complex 3700 ma ago and multiple metamorphic disturbances over the succeeding 2000 ma the isotopic responses to metamorphism were largely determined by the accessory mineral contents of the rocks and by country rock influences whole rock data give a five point sm nd isochron date of 3680 70 ma and the six samples with least radiogenic pb give a pb pb isochron date of 3689 146 ma whole rock rb sr data reflect the widespread yilgarn event at 2700 ma but some samples show additional rb sr disturbance 2000 ma ago data for various mineral fractions show substantial internal re equilibration of sm nd during granulite facies metamorphism at 3320 ma and further disturbance at least as recently as 3100 ma about half of the samples examined show evidence of partial pb isotopic re equilibration or u gain at 2900 ma possibly a time of retrograde amphibolite facies metamorphism a high apparent source 10 2 0 4 from the pb pb whole rock isochron could indicate assimilation of significant quantities of older 4000 ma crustal material the measured initial nd 0 2 0 7 is consistent with this but is not definitive the various dates correspond well to those determined by kinny et al kinny p d williams i s froude d o ireland t r and compston w 1988 precambrian res 38 325 341 and wijbrans wijbrans j r 1985 ph d thesis australian national university and complement the geological framework presented by myers myers j s 1988 precambrian res 38 309 323

migration of ground penetrating radar data with a finite element method that considers attenuation and dispersion we use a numerical model to study the effects of attenuation and dispersion upon the migration of ground penetrating radar gpr profiles a finite element method fem is developed that incorporates attenuation and is used to generate synthetic gpr profiles with random noise these profiles are then migrated with and without the attenuation term using our fem codes the misfit between the position of interfaces in the model and the position of corresponding interfaces in the migrated profile is greatly decreased when the attenuation term is considered in the migration process the improvement in resolution results from the use of the group velocity rather than any phase velocity consequently for dispersive media the attenuation term of high frequency gpr waves cannot be ignored in migration

smnd upb and rbsr geochronology and lithostructural relationships in the southwestern rae province constraints on crustal assembly in the western canadian shield the rae province is an area of archean crust bounded by the early proterozoic thelon orogen to the northwest and mylonites of the snowbird tectonic zone to the southeast the southern part of the province in saskatchewan is a highgrade complex of ultramafic mafic and felsic orthogneisses with interlayered supracrustal units the oldest rocks in the area were derived from layered mafic plutonic complexes and supracrustal units the earliest deformation d1 resulted in development of a layer parallel foliation preserved in small lensoid pods of anorthosite gabbro and pyroxenite a pyroxenite yields a sm nd tdm age of 3 5 ga and is isotopically similar to a mafic granulite metabasalt associated with early supracrustal units these rocks were intruded by a sequence of mafic and tonalitic orthogneisses that yield a sm nd isochron age of 3 32 2 87 and tdm ages of 3 2 3 1 ga the mafic tonalitic gneisses were subsequently migmatized producing anatectic net veins of tonalitic to granitic pegmatite these were deformed during d2 deformation the terrane was then intruded by minor granodioritic to tonalitic plutonic sheets and a swarm of mafic dikes that were deformed by the last major period of inhomogeneous deformation d3 which ended at 2 0 ga the clearwater complex a large 24 8 km long anorthosite massif intruded the gneiss complex following d3 and yields a sm nd mineral wholerock isochron age of 1 85 ga sm nd rb sr and u pb shrimp data record multiple thermotectonic events and provide constraints on crustal evolution of the rae province u pb shrimp data for metamorphic zircons separated from an early anorthosite associated with units that give pre 3 2 ga nd model ages show considerable variation in th u ratio zircons yield virtually concordant data that record archean 2 9 ga and several early proterozoic between 2 3 and 1 85 thermotectonic events sm nd data are generally poorly correlated possibly as a result either of slight remobilization of ree s during granulite facies metamorphism or that rocks with distinct origins were compared rb sr whole rock and mineral data show substantial re equilibration in mafic tonalitic gneisses at 2 8 ga and in supracrustal units at 2 3 ga also a discordant granitic dike and biotite separated from a metapelite yield rb sr tdm ages of 1 9 ga these metamorphic ages are interpreted to reflect thermotectonism associated with formation and assembly of juvenile early proterozoic crust in the talston thelon plutonic zone to the west and with terminal collision in the trans hudson orogen to the east

neoarchean continental crust formation and paleoproterozoic deformation of the central rae craton committee bay belt nunavut integrated mapping structural analysis and u pb geochronology of the committee bay area nunavut establish a record of neoarchean crustal growth followed by penetrative paleoproterozoic deformation supracrustal rocks include a lower ca 2 73 ga mafic ultramafic volcanic dominated sequence a middle economically significant 2 71 ga intermediate volcanic bearing sequence with intercalated sulphidized gold bearing iron formation and an upper 2 69 ga clastic komatiite quartzite sequence following an 80 million year hiatus this succession was intruded by voluminous ca 2 61 2 57 ga granodiorite tonalite granite diorite which do not appear to have thermally or tectonically affected the supracrustal belt instead three generations of structures record polyphase paleoproterozoic deformation of the region d1 structures are consistent with a doubly vergent structural fan developed at ca 2 35 ga in response to the arrowsmith orogeny that affected the western rae margin penetrative d2 structures do

examples of reverse time migration of single channel ground penetrating radar profiles a single channel ground penetrating radar gpr profile portrays a distorted unfocused image of subsurface structure due to apparent position shifts associated with dipping reflectors and to diffractions from corners and edges a focused image may be produced from such data by using any of the migration algorithms previously developed for seismic data we use reverse time migration based on the scalar wave equation field work was performed over a simple stratigraphic soil sequence and a complicated fluvial environment in the migrated images reflector continuity is enhanced and the level of detail available for high resolution interpretation is significantly increased

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

sm nd systematics of a tonalitic augen gneiss and its constituent minerals from northern michigan abstract the sm nd isotopic system of a tonalitic augen gneiss and its constituent minerals from northern michigan was disturbed during metamorphism sm nd zircon ages are lower than the wholerock sm nd model age however closely associated pairs of minerals for example sphene and biotite or apatite and plagioclase retain their apparent metamorphic ages the sm nd model age for the tonalitic augen gneiss of 3919 30 myr appears to reflect open system behavior during metamorphism a mineralogically different gneiss from the same location has a sm nd model age of 3520 70 myr the two whole rocks differ in their sm nd and rb sr systematics and in their chondrite normalized rare earth element ree patterns the whole rock normalized mineral ree patterns show the contribution of the major and trace minerals to the ree content of the whole rock the trace minerals contain a significant amount of the total ree

accretion history of the trans hudson orogen in manitoba and saskatchewan from paleomagnetism lithoprobe s sponsorship has led to the acquisition of paleomagnetic data from 20 units throughout the paleoproterozoic trans hudson orogen tho of saskatchewan and manitoba essentially the first such data for the region discussed summarily in this paper they provide an initial framework for the tho s tectonic evolution they show that the archean hearne and superior cratons were at subtropical and subpolar paleolatitudes respectively at 1875 ma with the lynn lake laronge arc midway between them in the manikewan ocean by 1855 ma this ocean was still 5500 700 km wide and its seafloor was subducting northwestward under the hearne craton and pericratonic peter lake domain margin with the coeval development of an andean type continental magmatic arc the wathaman chipewyan batholith between 1855 and 1810 ma coalescing apparent polar wander path apwp segments record closing of the ocean at a rate of 12 cm a trapping and accreting the several separate intervening terranes flin flon hanson lake lynn lake laronge rottenstone and presumably also other tho terranes from 1815 to 1775 ma the assembled terranes drifted as a coherent craton yielding a stillstand and hairpin in the apwp

a cordilleran type batholitic belt in the churchill province in northern saskatchewan abstract the rottenstone and western la ronge domains of the south central churchill province in saskatchewan consist mostly of hudsonian batholithic complexes spatially and structurally related to an aphebian hudsonian volcanic arc the batholithic belt is of comparable dimensions and compositional variation to cordilleran type phanerozoic batholiths and appears to be emplaced in close proximity to a prior late archean early aphebian continental crustal margin a quartz diorite line separating dominant quartz diorite tonalite granodiorite plutonic rocks from dominant granodiorite quartz monzonite granite intrusives can be defined these general characteristics as well as more detailed features of aphebian deposition and hudsonian thermotectonism within and on either side of the rottenstone domain suggest a continental margin rather than an intracontinental location for evolution of the hudsonian orogenic belt of the south central churchill province and provide some insights into the possible development of this part of the canadian shield as part of a hudsonian wilson cycle

determination of the ice dielectric permittivity using the data of the test in antarctica of the ground penetrating radar for mars 98 mission a ground penetrating radar will be integrated inside the guiderope ballast of the mars 98 balloon mission a prototype of this impulse system working in the 5 15 mhz band was tested close to dumont d urville the french antarctic station in february 1993 the authors obtained a large set of profiles in cold ice with a glacier thickness from 200 to about 900 m for each depth the measured echoes are very clear with a good signal to noise ratio snr in this paper the authors firstly describe the performances of the radar and they study the electromagnetic propagation in the antarctica glacier this knowledge allows them to use the frequency wavenumber migration to focus the signal to increase the snr and to rebuild an image of the subsurface the migration process gives a good estimation of the real part of the permittivity i e the velocity the fact that the signal comes from different depths is used to measure the imaginary part of the permittivity which defines the ice attenuation the spectral analysis gives its frequency dependence

phase field imaging the electromagnetic equivalent of seismic migration an alternative approach to the interpretation of electromagnetic em data is to downward continue the observations to construct a two dimensional resistivity image of the medium that produced them the downward continuation can be done in a straightforward manner by drawing a parallel between the em and acoustic wave equations with appropriate approximations and identification of specific terms that are equivalent and with transformation from an initial value problem to a boundary value problem em data can be imaged using any of the methods originally developed for migration of seismic data the method we chose here is claerbout s one way finite difference wave equation migration the algorithm is implemented in the frequency domain independent images of relative reflectivity are obtained by processing e and h pseudosections separately the best estimate of the actual reflector position is the region of coincidence of the two images the same approach can be used in reverse for modeling the em respo

provenance of the proterozoic thelon basin nunavut canada from detrital zircon geochronology and detrital quartz oxygen isotopes abstract the paleoproterozoic thelon basin in nunavat canada is one of several large quartz dominated sedimentary basins developed on paleoproterozoic and archean rocks of the canadian shield coarse grained quartz to sub feldspathic sandstones and conglomerates of the thelon formation comprise the basin fill detrital zircons from the thelon formation have lam icp ms 207 pb 206 pb ages ranging from 3 94 to 1 78 ga but most are neoarchean 2 70 2 50 ga the 18 o values of quartz range from 7 7 to 13 9 and are typically 10 12 ages u zr ratios and oxygen isotopic compositions of detrital quartz are consistent with a predominantly neoarchean metasedimentary source such as surrounds the eastern thelon basin and comprises the bulk of basement rocks within the rae domain the three stratigraphic sequences of the thelon formation have 18 o ranges for detrital quartz and zircon ages that further constrain the source areas and support previous sequence stratigraphic interpretations the lower sequence contains reworked siltstone and sandstone clasts and 2 50 1 85 ga zircons indicating a contribution from paleoproterozoic terranes detrital quartz 18 o values range from 9 8 to 13 9 indicating a metamorphic source also which is consistent with a proximal source for the lower sequence the middle sequence contains zircons exhibiting the widest range of ages 3 94 1 78 ga a 3 4 ga paleoarchean zircon population cannot be correlated to the immediately surrounding basement and therefore indicates a distal source possibly in the order of 1000 2000 km away the upper sequence contains relatively few zircons due to extensive alteration but ages are predominantly early paleoproterozoic and neoarchean the upper sequence strata contain the lowest 18 o values of quartz indicating an influx of granitic material late in the depositional history of the basin the geochronology and geochemistry of detrital grains within the thelon formation indicate that the source areas evolved from predominantly proximal to distal and back to proximal as the basin filled

radiogenic isotope chemostratigraphy reveals marine and nonmarine depositional environments in the late mesoproterozoic borden basin arctic canada

sr and nd isotope systematics of polymetamorphic archean gneisses from southern west greenland and northern labrador sr and nd isotopic data for middle to late archean polymetamorphic felsic gneisses from localities in the nuuk area west greenland are compared and contrasted with new isotopic results for early archean amitsoq gneisses and with data for isotopically reworked kiyuktok gneisses from the saglek area labrador sr isotopic data for individual suites of felsic gneisses record the time integrated effect of variable rb sr fractionation during prograde and retrograde events as well as the effect of source inhomogeneity contrasting petrologic and sr nd isotopic characteristics are the result of differences in level of exposure caused partially by juxtaposition of terranes of different metamorphic character by movement on ductile shear zones and post shearing folding deformation sm nd systematics of felsic gneisses from nordafar ikerasakitsup akornga tinissaq and kangimut sammisoq qasigianguit define a geologically meaningless ca 3280 ma nd isochron which is the result of mixing of samples from unrela

neoarchean convergent margin ni cu mineralization axis lake and nickel king ni cu deposits in the south rae craton of the canadian shield abstract the southern portion of the rae craton in the canadian shield is host to multiple ca 2 63 ga ni cu rich metamorphosed mafic intrusions with noritic noritic anorthositic and pyroxenitic compositions including those hosting the undeveloped axis lake and nickel king deposits found in northern saskatchewan and southeast northwest territories in order to understand the setting of these intrusions and their concomitant mineralization we investigated their geochemical and isotopic characteristics mineralized intrusions at axis lake and nickel king are metamorphosed up to granulite facies with local partial melting the intrusions preserve whole rock major element geochemistry consistent with a sub alkaline basaltic composition e g sio2 46 54 wt tio2 the nd isotope values of the axis lake intrusion ndt2630 0 3 to 2 1 are more juvenile and closer to the depleted mantle value relative to those of the nickel king deposit ndt2630 2 4 to 0 4 this agrees with their also distinctive th yb and nb yb distributions suggesting oceanic and continental arc like settings respectively the axis lake and nickel king nd isotope compositions are indistinguishable from other arc like 2 61 to 2 58 ga mafic to felsic intrusions and volcanic rocks recognized in the rae craton and the neighboring chesterfield block the age spatial distribution and geochemistry of the studied intrusions coincide with the initial stages of a recently defined 2 63 2 60 ga continental arc the roots of which are found in the south rae craton our data indicate that neoarchean intrusions with an arc like geochemistry may be a significant exploration target in the rae craton this broadens possible host suites beyond the established paleoproterozoic rift related rocks most commonly considered as the prime target for ni cu pge mineralization

evidence for isotopic equilibration of smnd whole rock systems in early archaean crust of enderby land antarctica smnd isotopic data indicate that differential ree mobility occurred on a whole rock scale during transitional amphibolite to granulite facies regional metamorphism 700 c 7 kbar in early archaean rocks 3930 ma of the napier complex of enderby land antarctica the degree of mobility is independent of metamorphic grade but correlates directly with development of tectonic fabric whole rock samples with d3 m3 internal fabrics lie along an array corresponding to an age of 2410 100ma whereas samples preserving only earlier fabrics preserve an older albeit imprecisely defined isochron age in contrast to a widely held belief such changes did not require the presence of a large hydrous fluid flux if the mechanism responsible for smnd resetting at this locality where tchur ages range from 1990 ma to 6090 ma is more widespread than is currently recognised isolated smnd model ages particularly in complex terrains should be treated with caution

the upper jurassic boulder beds and related deposits a fault controlled submarine slope ne scotland during the late jurassic kimmeridgian rifting in the north sea large scale normal faults developed to form relatively deep marine basins the north western margin of one of these basins defined by the helmsdale fault is described to show the nature of sedimentation associated with a submarine fault scarp subsidence to the se of the helmsdale fault led to a fault scarp down which various sediment mass flows moved including debris flow rockfall high and low concentration turbidity currents and possible storm surge wash over deposits redeposition by soft sediment sliding was important as a slope accretion process deep channels were incised into the slope and infilled by coarse grained elastics contemporaneous small scale submarine fans located farther off in the present north sea may have developed as a result of slope bypassing sediment flows down such channels the succession is characterized by extreme sediment heterogeneity compared to many deep water environments and it may be part of an ancient fan delta the main influences upon slope accretion were the synsedimentary movements on the helmsdale fault and the nature of the available sediment input such that the succession represents structurally controlled and sedimentologically modified slope development

smnd isotopic systematics of enderby land granulites and evidence for the redistribution of sm and nd during metamorphism measurements of143nd 144nd and147sm 144nd are reported for whole rocks and mineral separates from granulites of the napier complex at fyfe hills charnockites leuconorites and gabbros yield a whole rock smnd isochron age of3060 160m y and an initial143nd 144nd ratio of0 50776 10 nd 3060m y 2 0 1 8 the negative nd value and the presence of geologically induced dispersion in the data suggest that the isochron age does not represent the time of primary crystallization of the complex but instead indicates a time of later redistribution of sm and nd and partial re equilibration of143nd 144nd ratios this probably occurred during the upper granulite facies metamorphism which has also been dated at 3100m y by rbsr and upb zircon studies 1 coexisting clinopyroxene apatite and total rock fractions in two adjacent samples define an approximately linear array corresponding to an age of 2300 300 m y this array indicates that redistribution of sm and nd and re equilibration of143nd 144nd ratios occurred on an intermineral scale during the upper amphibolite to lower granulite facies metamorphism at 2450m y due to the resetting of the smnd system on both whole rock and mineral scales the primary crystallization age of the igneous protolith is not well constrained by the present data although it is clearly3100m y if it is assumed that the complex was derived initially from a depleted mantle reservoir nd t 2 evolution of the negative nd value of 2 0 with the observed sm nd ratios requires a prehistory of 380m y this implies a primary age of 3480m y however substantially older primary ages can be inferred if the source reservoirs had nd t 2 and or substantial reductions in the sm nd ratio occurred in whole rocks during the granulite facies metamorphism at 3100 m y such an inferred reduction in the sm nd ratio may have been the result of preferential loss of sm relative to nd or introduction of a low sm nd fluid with nd 0 during granulite facies metamorphism

nd evidence for extensive archean basement in the western churchill province canada to determine the extent of reworked archean crust in the western churchill province we have examined sm nd crustal residence ages of basement cores in southern alberta and southwestern saskatchewan along with crustal residence ages calculated for other sm nd data available from the churchill province the deep drill hole samples from the interior platform give sm nd crustal residence ages that average 2 8 ga granulites from northeastern alberta and composite gneisses from northern saskatchewan also provide archean crustal residence ages these data demonstrate that the presence of reworked archean crust in the western churchill province is more widespread than has been confirmed previously we suggest that sm nd crustal residence ages are valuable estimates of crust formation times especially in orogenic areas where other isotope systems have been disturbed

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

pseudo seismic wavelet transformation of transient electromagnetic response in engineering geology exploration 1 this paper presents some new theoretical analysis and numerical simulations of that transient electromagnetic diffusion field response is transformed into a pseudo seismic wavelet in engineering geology exploration it can clearly reveal the electric interface under ground to simplify the integral equation used in the transformation the integral range is separated into seven windows and each window is compiled into a group of integral coefficients then the accuracy of the coefficients is tested and the calculated coefficients are used to derive the pseudo seismic wavelet by optimization method finally several geo electric models are designed so that model responses are transformed into the pseudo seismic wavelet the transformed imaginary wave shows that some reflection and refraction phenomena appear when the wave meets the electric interface this result supports the introduction of the seismic interpretation in data processing of transient electromagnetic method

proterozoic collisional tectonism in the trans hudson orogen saskatchewan isotopic and structural data from the juvenile reindeer zone of the trans hudson orogen northern saskatchewan indicate a pre 1 85 ga thermotectonic event possibly reflecting arc continent collision followed by a more extensive nappe forming ca 1 83 1 80 ga thermotectonism during terminal continent continent collision preliminary data from the adjacent ensialic cree lake zone suggest high grade reworking of archean crust by the pre 1 85 ga event in the rae province to the west high grade metamorphism and reworking of archean crust occurred about 2 0 ga and may be related to the formation of the coeval taltson magmatic zone

thermochronological history of the northern canadian shield abstract the northern canadian shield is comprised of multiple archean cratons that were sutured by the late paleoproterozoic to form the canadian component of supercontinent nuna more than 2000 combined k ar and 40ar 39ar cooling ages from across the region reveal a stark contrast in upper and lower plate thermal responses to nuna forming events with the churchill province in particular revealing near complete thermal reworking during the late paleoproterozoic we review the detailed cooling history for five regions that span the churchill province and trans hudson orogen tho thelon tectonic zone south rae reindeer zone south hall peninsula and the cape smith belt the cooling patterns across churchill province are revealed in two 1500 km transects at the plate scale churchill s cooling history is dominated by tho accretionary and collisional events during which it formed the upper plate cooling ages generally young from west to east across both southern and central churchill and latest cooling in the tho is 50 myr older in southernmost churchill reindeer zone compared to eastern churchill hall peninsula indicating diachronous thermal equilibration across 2000 km strike length of the tho churchill exhibits relatively high post terminal tho cooling rates of 4 c myr which support other geological evidence for widespread rapid exhumation of the tho upper plate following terminal collision potentially in response to lithospheric delamination

four zircon ages from one rock the history of a 3930 ma old granulite from mount sones enderby land antarctica ion microprobe u th pb analyses of zircons from a granulite grade orthogneiss from mount sones enderby land antarctica record the ages of four principal events in the history of the gneiss three of which already have been recognized through previous isotopic dating of other samples the structure of the zircons indicates at least four different stages of growth the several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed stratigraphic succession of growth and thereby defining separate u pb discordance patterns for each stage the stratigraphically oldest zircon rare discrete cores is indistinguishable in age from the most common euhedrally zoned zircon both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927 10 ma ago making the orthogneiss currently the oldest known terrestrial rock the outer parts of most grains and some whole grains recrystallized at 2948 31 17 ma during or immediately after possibly 100 ma of high granulite grade metamorphism the recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the napier complex at 1000 ma in the intervening time at 2479 23 ma the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic pb which left most of the crystals with radiogenic pb deficiencies but produced local radiogenic pb excesses in others a new generation of zircon characterized by very high th u and low u grew at that time that event deformation and possibly a minor rise in temperature produced widespread perturbations of other isotopic systems throughout the napier complex

very early archean crustal accretion complexes preserved in the north atlantic craton the north atlantic craton contains very early archean supracrustal rocks orthogneisses and massive ultramafic rocks most units of supracrustal rocks are dominated by mafic volcanic rocks layered gabbros and banded iron formations bust some also contain abundant felsic volcanic sedimentary rocks quartzites and marbles some quartzites contain detrital zircons derived from rocks identical in age to felsic volcanic sedimentary rocks in these sequences ca 3800 ma and also from older ca 3850 ma sources the presence of the ca 3850 ma detrital zircons suggests that the supracrustal units containing them were deposited on or close to ca 3850 ma sialic crust the massive ultramafic rocks have chemical affinities to upper mantle rocks the voluminous suites of tonalitic gneisses are dominated by 3700 3730 ma bodies that intrude the supracrustal sequences but they also locally contain components with ages between 3820 and 3920 ma the diverse supracrustal units upper mantle rocks and ge 3820 ma components in the gneisses were tectonically interleaved in very early archean convergent plate boundaries giving rise to accretion complexes in the period 3700 3730 ma voluminous tonalitic magmas produced by partial melting of predominantly mafic rocks in the base of the accretion complexes were emplaced at higher levels forming juvenilemore continental crust and leaving behind a refractory lower crustal to upper mantle substrate less

the arrowsmith orogeny geochronological and thermobarometric constraints on its extent and tectonic setting in the rae craton with implications for pre nuna supercontinent reconstruction abstract geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the rae craton canada provide important temporal and spatial links between previously investigated areas in the northern and southern rae craton that had been interpreted to have been reworked during the ca 2 5 2 3 ga arrowsmith orogeny on southern boothia peninsula upper amphibolite to granulite facies metamorphism dated by in situ shrimp u pb analysis of monazite occurred at ca 2 64 2 60 2 54 2 50 2 49 2 44 and 2 37 2 34 ga with retrograde monazite growth at 2 27 ga shrimp u pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca 2 54 and 2 50 ga which also corresponds to the age determined for a k feldspar augen granite textural relationships indicate that the regional foliation formed between ca 2 54 ga and 2 50 2 49 ga peak metamorphic conditions of 700 c and 5 6 kbar are interpreted to have been reached during the 2 37 2 34 ga event in the mccann mcarthur lake region of the southwestern rae craton upper amphibolite to granulite facies metamorphism is dated by in situ shrimp monazite ages of ca 2 53 2 47 and 2 37 2 28 ga textural relationships indicate deformation initiated by ca 2 37 ga and terminated by ca 2 30 ga metamorphic conditions of 5 kbar and 700 735 c were quenched at ca 2 3 ga following peak conditions of 7 kbar and 800 c the evidence for regional deformation at 2 54 2 49 ga on southern boothia peninsula supports an andean type accretionary margin setting for the arrowsmith orogeny a collisional event may have terminated high grade metamorphism in the northern region by ca 2 34 ga while metamorphism associated with continental arc magmatism continued in the south until at least ca 2 28 ga these combined data demonstrate the continuity and broad extent of the arrowsmith orogen along the western flank of the rae craton extending 1500 km from northern saskatchewan to northern baffin island the broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper the inferred convergent tectonic activity between ca 2 54 and 2 28 ga is interpreted to have occurred on the flank of a growing supercontinent a setting that could account for an apparent slowdown but not shutdown in global tectonics

age and provenance of the paleoproterozoic thluicho lake group based on detrital zircon u pb shrimp geochronology new insights into the protracted tectonic evolution of the southwestern rae province canadian shield abstract the middle paleoproterozoic thluicho lake group comprises coarse grained continental clastic rocks deposited in fault controlled alluvial basin s in the southwestern rae province of the canadian shield the group nonconformably overlies crystalline basement that records late taltson high grade metamorphism ca 1 93 ga and is folded and metamorphosed to greenschist facies providing an important window of insight into tectonic activity in this region this paper reports 100 detrital zircon ages for two samples from the lower and upper parts of the thuicho lake group four analyses of one of the youngest detrital zircons from the stratigraphically higher member define a weighted mean 207pb 206pb age of 1922 6 ma establishing a maximum depositional age for the group this result accords with regional geological relationships indicating that the group is intermediate in age between development of two major tectonic fronts an older ca 1 93 ga east southeast trending front that formed at the height of thelon taltson orogeny and a younger ca 1 91 1 90 ga northeast striking front expressed by the black bay fault and related northeast striking structures affiliated with the snowbird tectonic zone the younger age limit of the group may therefore be older 1 91 1 90 ga than the previously reported minimum age constrained by cross cutting dykes dated at ca 1 82 ga in terms of isotopic provenance the bulk of analyses of the combined dataset lie within two principal age ranges 2750 2500 ma and 2475 2250 ma the older of these is dominated by ca 2600 ma zircons whereas the younger contains two statistically distinct subgroups of 2410 and 2310 ma this signature is consistent with derivation almost exclusively from immediately surrounding basement rocks of the nolan taltson zemlak ena and beaverlodge domains abundant 2475 2250 ma zircons provide indirect evidence for the 2 45 2 3 ga arrowsmith orogeny despite proximity to the taltson magmatic zone 1990 1960 ma zircons are nearly completely absent suggesting that thluicho lake basins formed in the hinterland of the orogen possibly separated from the foreland taltson arc by a continental divide alternatively taltson arc plutons may not have been unroofed at the time of deposition of thluicho lake strata the data presented herein support the previously proposed broad correlation between the thuicho lake and nonacho groups however there are apparent differences in detail between respective basin settings and controls suggesting that this correlation requires further investigation

u pb geochronology in the trans hudson orogen northern saskatchewan canada we have obtained u pb ages on zircons from volcanic and plutonic units in several lithotectonic domains of the southern trans hudson orogen in northern saskatchewan these data constrain the timing of early proterozoic orogenic events in the region and enhance our understanding of both the relationships among local domains and the relationship of the trans hudson orogen to other early proterozoic orogens in north america with one exception all units studied so far yield zircon ages of 1890 1835 ma most of which are systematically earlier than previously reported rb sr isochron ages on the same or similar units suggesting open system behavior in the rb sr systems five metarhyolites from volcanic sequences in the la ronge domain glennie domain and hanson lake block give ages ranging from 1888 to 1876 ma most of the plutons we dated ranging from gneissic syntectonic tonalites and granodiorites to less deformed late intrusions such as the wathaman batholith and other smaller bodies yield ages of 18

evidence for neoarchean ni cu bearing mafic intrusions along a major lithospheric structure a case study from the south rae craton canada abstract deformed and metamorphosed mafic to ultramafic intrusive rocks are common along and adjacent to the snowbird tectonic zone a major paleoproterozoic structure separating the rae and hearne cratons on the rae side of this major lithospheric structure we investigated a series of mafic bodies that intrude high grade paragneiss and are associated with ni cu pge mineralization by linking field observations geochronology and metamorphic petrology our results indicate archean 2 63 ga crystallization ages for these mafic intrusions over a minimum along strike extent of 500 km and a later high temperature 820 c and 7 5 kbar paleoproterozoic metamorphic overprint at t