volcano sedimentary processes operating on a marginal continental arc the archean raquette lake formation slave province canada abstract the 200 m thick volcano sedimentary raquette lake formation located in the south central archean slave province represents a remnant arc segment floored by continental crust the formation overlies the gneissic sleepy dragon complex unconformably is laterally interstratified with subaqueous mafic basalts of the cameron river volcanic belt and is considered the proximal equivalent of the turbidite dominated burwash formation a continuum of events associated with volcanism and sedimentation and controlled by extensional tectonics is advocated a complex stratigraphy with three volcanic and three sedimentary lithofacies constitute the volcano sedimentary succession the volcanic lithofacies include 1 a mafic volcanic lithofacies composed of subaqueous pillow pillow breccia and subaerial massive to blocky flows 2 a felsic volcanic lithofacies representing felsic flows that were deposited in a subaerial environment and 3 a felsic volcanic sandstone lithofacies interpreted as shallow water wave and storm reworked pyroclastic debris derived from explosive eruptions the sedimentary lithofacies are represented by 1 a conglomerate sandstone lithofacies consistent with unconfined debris flow hyperconcentrated flood flow and talus scree deposits as well as minor high energy stream flow conglomerates that formed coalescing steep sloped coarse clastic fan deltas 2 a sandstone lithofacies interpreted as hyperconcentrated flood flow deposits that accumulated at the subaerial subaqueous interface and 3 a mudstone lithofacies consistent with suspension sedimentation in a small restricted lagoon type setting the raquette lake formation is interpreted as a fringing continental arc that displays both high energy clastic sedimentation and contemporaneous effusive and explosive mafic and felsic volcanism modern analogues that develop along active plate margins in which continental crust plays a significant role include japan and the baja california peninsula

deposition and imbrication of a 2670 2629 ma supracrustal sequence in the indin lake area southwestern slave province canada new u pb age data from the southwestern slave province demonstrate that units of the indin lake supracrustal belt form an imbricated structural stack the oldest rocks of the belt are undated mafic volcanic flows of the hewitt lake group that are crosscut by a 2670 ma felsic sill itself coeval with mafic through felsic volcanic rocks of the 2668 2671 ma leta arm group the youngest rocks of the belt are 2647 2629 ma turbidites and felsic volcanic rocks of the unconformably overlying chalco lake group tonalite orthogneiss of the adjacent cotterill gneiss complex is 2680 ma suggesting that it does not represent in situ basement to the supracrustal belt intercalation of the older hewitt lake and leta arm groups with the younger chalco lake group is interpreted to result from d1 imbrication and folding between 2629 and 2609 ma the age of a crosscutting tonalite intrusion subsequent d2 folding and regional low pressure metamorphism occurred between 2609 ma and ca 2590 ma d3 normal faulting between the

the origin and evolution of canadian shield brines evaporation or freezing of seawater new lithium isotope and geochemical evidence from the slave craton abstract new chemical and isotopic data for deep seated calcium chloride brine from the miramar con gold mine yellowknife n w t strongly suggest that the brine salinity is of marine origin diagnostic marine properties include uniformly elevated br cl ratios typical of seawater concentrated beyond halite saturation and li br ratios 0 0254 0 0325 and 6 li compositions 32 1 to 36 3 similar to seawater 32 3 the mean 6 li for all mine water samples of 35 1 may reflect minor uptake of li by secondary minerals this interpretation is supported by analyses of altered metabasalt from fault zones which is enriched in li but depleted in 6 li 14 7 to 15 6 relative to the unaltered metabasalt 5 4 the mechanism responsible for concentrating the hypersaline brine end member is not unequivocal as evidence exists to support both evaporative and cryogenic processes on the one hand the devonian sedimentary record in the great slave lake region in conjunction with yellowknife brine isotopic compositions 2 h and 34 s so 4 that are similar to various devonian fluids support an evaporative origin on the other hand the na cl br cl relationship in the brine strongly suggests a cryogenic mechanism regardless of the concentrative mechanism the chemical data indicate that the yellowknife parent brine was concentrated 28 to 30 fold relative to seawater the extreme depletion of mg and enrichment of ca in the brines accompanied by sr ca ratios similar to that of seawater are accounted for by dolomitization of an aragonite rich marine sediment by the brine before infiltration into the crystalline basement rocks subsequent alteration of silicate minerals in the shield added additional ca and sr to the brine as indicated by their radiogenic 87 sr 86 sr ratios up to 0 7147 based on mineral balance calculations the major mineral products of the cryogenic and evaporitic concentration and evolution paths are significantly different the cryogenic evolution results in some 15 mirabilite 60 hydrohalite and 18 dolomite whereas the major minerals formed from the evaporitic evolutionary sequence are 36 halite 8 gypsum 17 dolomite and 30 albite the great similarity between the calcium chloride brine from yellowknife and other such canadian shield brines indicates that they may share a common marine origin

textural transformation of thermally treated anthracites abstract the organic matter of anthracites consists of turbostratic two dimensional crystalline order stacks of three to five aromatic layers called the basic structural units bsu the arrangement of bsu forms the texture of anthracites which corresponds to their chemical and physical properties including optical properties reflectance values thermal treatment leads to the changes in the arrangement of bsu the character of structural and textural transformation of anthracites during heating can be determined based on the changes of reflectance values of anthracites several anthracites of various ranks were thermally treated over the temperature range of 400 673 k 1000 c 1273 k in an inert atmosphere n2 at atmospheric pressure the reflectance indicating surfaces ris were reconstructed and reflectance indicatrix parameters were calculated according to the kilby method specially modified for the study of anthracites it was found that the relative increase of reflectance values rmax rmin was greater for the lower metamorphosed samples than for the higher ones however the higher rank of initial samples the greater their final reflectance values and the anisotropy of the texture attained at 1000 c it indicates that the texture of the higher metamorphosed anthracites meta anthracites is more flexible for rearrangement during heating than the texture of the lower metamorphosed anthracites moreover the difference in the changes of reflectance values versus temperature for the higher and for the lower rank anthracites suggests that meta anthracites have undergone stronger natural metamorphism equivalent to about 200 c in comparison with the other anthracites it could be said that the character of textural transformations of individual anthracites results from the degree of their metamorphism and in a consequence from the texture of parent sediments

comparisons between tethyan anorthosite bearing ophiolites and archean anorthosite bearing layered intrusions implications for archean geodynamic processes

tectonic controls on the endowment of neoarchean cratons in volcanic hosted massive sulfide deposits evidence from lead and neodymium isotopes review and analysis of initial lead isotope ratios from archean volcanic hosted massive sulfide vhms and lode gold deposits and of neodymium isotopes from igneous rocks from the geologic provinces that host these deposits identifies systematic spatial and temporal patterns both within and between the provinces the abitibi wawa subprovince of the superior province is characterized by highly juvenile lead and neodymium most other archean provinces however are characterized by more evolved isotopes although domains within them can be characterized by juvenile isotope ratios the analysis indicates that the endowment measured as the quantity of metal contained in geologic resources per unit surface area of vhms and komatiite associated nickel sulfide deposits is related to the isotopic character and therefore the tectonic history of provinces that host these deposits provinces with extensive juvenile crust have significantly higher endowment of vhms deposits possibly as a consequence of higher heat flow and extension related faults provinces with evolved crust have higher endowment of komatiite associated nickel sulfide deposits possibly because such crust provided either a source of sulfur or a stable substrate for komatiite emplacement in any case initial radiogenic isotope ratios can be useful in predicting the endowment of archean terranes for vhms and komatiite associated nickel sulfide deposits limited data suggest similar relationships may hold in younger terranes lead isotope data may provide constraints on supercraton reconstructions in the archean as lead isotope systematics appear to be highly provincial similar isotope systematics between the superior province and the eastern goldfields superterrane may indicate a linkage during the neoarchean consistent with the similar geologic and metallogenic histories of these provinces

a reconnaissance teleseismic study of the upper mantle and transition zone beneath the archean slave craton in nw canada abstract the objective of this study is to investigate upper mantle structure below the archean slave craton site of the oldest known rocks on earth and numerous diamondiferous kimberlites and thence to gain an understanding of early craton formation and kimberlite genesis to this aim a temporary array consisting of 13 sites equipped with broadband seismometers recorded teleseisms between november 1996 and may 1998 this data set has been augmented with recordings from the yellowknife seismic array our three most robust observations and their interpretation are 1 p wave travel time tomography reveals the oldest part of the craton the central slave basement complex to be underlain by the fastest seismic velocities indicating that this block remains distinct well into mantle depths 2 receiver function analysis requires only the moho as major s wave velocity discontinuity and points to a fairly constant crustal thickness throughout the slave province and 3 sks splitting analysis shows little variation in delay times and fast polarization directions across the array leading us to conclude that the present day plate motion of north america is the primary cause for mantle fabric beneath the entire array furthermore the data let us rule out the possibility that the mackenzie plume had any seismologically detectable effect on the slave lithosphere more speculative results of our investigation namely a possible genetic link between a low seismic velocity anomaly at depth with the overlying lac de gras kimberlite field and a possible archean origin of two shallow low velocity anomalies will require further investigation

evolution of the slave craton greenstone belts in the slave craton represent the episodic accumulation of volcanic and sedimentary rocks over as much as 200 m y in the western craton volcanic and sedimentary rocks were deposited between 2 81 and 2 60 ga on 2 9 3 1 ga sialic basement not tectonically juxtaposed with it deposition of 2 6 ga turbidite and conglomerate sequences across the entire slave craton was synchronous with or followed by the main phase of regional deformation and metamorphism at 2 6 2 58 ga and final stabilization of the craton

pb isotopic composition of late archaean granites and the extent of recycling early archaean crust in the slave province northwest canada initial pb isotopic compositions have been determined for potassium feldspar from ca 2 58 to 2 62 ga plutonic rocks in the southern and central slave province of northwestern canada to evaluate the extent of recycling of ancient crust within the province large differences in initial pb compositions were measured which correlate with geographical areas of the province plutons in the east central part of the province have initial compositions only slightly more radiogenic than estimated mantle values 207pb 204pb 14 8 14 9 and were dominantly deruved from juvenile crustal sources in contrast plutons in the point lake and western contwoyto lake areas of the western slave province have radiogenic compositions 207pb 204pb 15 1 15 2 and indicate significant recycling of pre 3 5 ga crust the pb data support previous interpretations based on nd isotopes for a major isotopic boundary in the central part of the province granites from the southern part of the province near yellowknife have intermediate compositions which indicate 1 the age of the protolith to the granitoids in the yellowknife area is younger than at point lake but older than in the eastern slave or 2 the granitoids in the yellowknife area contain a mixture of an older point lake type component and younger crust the absence of pre 3 2 ga crust in the yellowknife area and lack of evidence for pre 2 8 ga inherited zircons in the yellowknife granitoids favour the former possibility evidence for recycling of ancient crustal sources such as the acasta gneiss is limited to a relatively small area of the west central part of the province suggesting that acasta aged or derived crust is not widespread in the province the marked regionality of isotopic composition may reflect a basement in the western part of the province which is itself a collage of crust of different age being younger ca 3 2 2 9 in the south relative to the point lake region 3 9 3 2 ga

fracture control of late archean pluton emplacement in the northern slave province canada the structural and geophysical characteristics of 2590 2580 ma leucogranites in the northern slave province have been studied to determine the role and conditions of late archean regional deformation during their emplacement the irregularly shaped straight sided c 10 km long ulu pluton is located in the southern part of the high lake greenstone belt northern slave province fabrics in the pluton and its wall rocks indicate that it was emplaced at mid to upper crustal levels during regional e w late archean compression a gravity survey and resulting three dimensional gravity model of the pluton show that it has several linear deep 6 km root zones which feed a relatively thin 2 km tabular body the orientations of the major root zone along the pluton s western side and three subsidiary roots correspond to those of the pluton contacts to within 5 landsat tm data reveal several sets of lineaments interpreted to represent fractures formed during the early assembly of the slave province which cut both the greenstone belt and the younger plutons analysis of these lineaments defines several maxima whose orientations correspond with those of both the feeder zones and the contacts of the ulu pluton the pluton therefore appears to have been emplaced via an intersecting set of vertical fractures magma was arrested by and spread along an unknown horizontal structure subsequent inflation of the pluton was also facilitated by vertical translation on pre existing fractures

correlation between optical chemical and micro structural parameters of high rank coals and graphite abstract in order to identify the parameters that best characterize the chemical and structural evolution of organic matter during coalification the relationships between optical chemical and micro structural parameters in high rank coals and natural graphite were studied the samples include anthracites from penarroya belmez espiel basin spain douro basin portugal and alto chicama basin peru and natural graphite from canada mozambique and austria correlations between the following optical parameters were assessed vitrinite random reflectance rr reflectance indicating surfaces ris axis rmax rint and rmin and ris parameters ram rev and rst as well as bw and ai anisotropy parameters furthermore the chemical parameters used were chosen according to their significant variation in coals namely volatile matter carbon and hydrogen contents calculated in dry ash free basis vmdaf cdaf hdaf as well as the h c atomic ratio structural organization was characterized by micro raman spectroscopy and xrd raman parameters used were the full width at half maximum fwhm and position of g and d1 bands on the first order raman spectrum and the id1 ig intensity area ratio the selected xrd parameters were interlayer spacing d002 and crystallite sizes la and lc results show that i rmax ris axis seems to correlate best with chemical and micro structural parameters ii for the majority of studied samples hdaf and h c atomic ratio are the only chemical parameters with significant correlations with rmax iii the fwhm of the g band of raman spectrum shows good linear correlation with the xrd parameter d002 and iv structural organization of carbon materials as measured by trends in their optical and crystalline parameters is influenced by their hydrogen content daf basis and therefore by the h c atomic ratio

tectonic and thermal history of the anialik river area northwestern slave province canada the anialik river area in the northwestern slave province comprises two geological domains of different age and origin that were tectonically juxtaposed at ca 2650 ma the older domain the kangguyak gneiss belt comprises ca 3300 2700 ma orthogneisses and paragneisses interpreted as the remnants of a mesoarchean continental margin the younger domain the anialik river greenstone belt consists of ca 2680 ma mafic to felsic volcanic rocks interpreted to have formed in an ensimatic island arc setting structural and geochronological evidence suggest collision of the two domains began around 2650 ma in a transpressive regime that involved oblique sinistral subduction of the greenstone belt beneath the kangguyak domain along the tokhokatak shear zone displacement continued until at least ca 2600 ma when late two mica granites intruded along and were deformed in the shear zone following ca 2600 ma rocks in both domains and along the fault cooled rapidly to about 350 c strongly overprinted musco

imaging the lithospheric mantle in northwestern canada with seismic wide angle reflections normal moveout corrected and stacked sections of wide angle reflection data collected in the archean slave province and paleoproterozoic wopmay orogen in northwestern canada image the crust mantle boundary as a subhorizontal discontinuity at about 32 36 km depth along a 450 km lateral extent and for the first time from such data layering within the lithospheric mantle at depths between 75 and 90 km over a lateral distance of 750 km these images correspond to moho and mantle reflections observed on a co located multichannel seismic mcs reflection profile structural features and or the basalt eclogite phase change associated with subduction of oceanic and or delaminated lower crust are possible sources of the deep reflectivity one mantle reflection group lies west of the mcs data and east of the cordilleran deformation front it could have similar sources or result from the mesoproterozoic racklan orogen or meso neoproterozoic rifting of the ancient margin

radiation enhanced fission track annealing revisited and consequences for apatite thermochronometry apatite fission track aft analyses for granitoid and metamorphic bedrock samples from the western superior province ontario the churchill rae province melville peninsula and southampton islan

influence of tectonism and climate on lithofacies distribution and sandstone and conglomerate composition in the archean beaulieu rapids formation northwest territories canada abstract the 0 2 1 km thick archean beaulieu rapids formation is a late orogenic tectonically controlled sedimentary sequence overlying the beaulieu river volcanic belt unconformably on the west and bordering the major n trending beniah lake fault on the east local porphyry stocks characteristic of late orogenic successions were emplaced along strike of the fault four lithofacies comprise the beaulieu rapids formation 1 the 20 320 m thick conglomerate i 2 the 10 305 m thick siltstone sandstone 3 the 20 120 m thick conglomerate ii and 4 the 40 320 m thick quartz rich sandstone lithofacies stacking displays two distinct large scale fining upward sequences represented by the 300 375 m thick conglomerate i and sandstone siltstone sequence a and the 300 400 m thick conglomerate ii and quartz rich sandstone sequence b fining upward sequences a bounding unconformity and the beniah lake fault are compelling indicators of tectonic influence on sedimentation conglomerate i was deposited by debris flows traction currents and sheetfloods on the proximal medial and distal portions of alluvial fans or fan deltas respectively the overlying siltstone sandstone contains two distinct lithological units a sandstone dominated unit formed from fluvial dune migration and overbank deposition in a sandy braidplain setting with scattered ponds whereas sedimentary structures in the siltstone dominated unit are consistent with lacustrine deposition a second phase of tectonic uplift produced sequence b conglomerate ii was deposited in a proximal braided stream setting characterized by abundant longitudinal gravel bars the quartz rich sandstone developed from dune migration on a sandy braidplain mineralogy and textures reflect high sediment influx limited transport distance local sources and climatic influence clast populations in conglomerate indicate a prevalent mafic volcanic source whereas sandstones contain prominent quartz av 76 with subordinate feldspar 13 and lithic fragments 11 the disparity between clast and sandstone composition is a function of chemical weathering typical of a humid climate source areas are restricted to the mafic dominated beaulieu river volcanic belt the 2 8 ga gneisso plutonic sleepy dragon complex and fault related porphyry stocks the humid weathering aggressive archean climate was responsible for destroying labile minerals and lithic fragments in proximal source rocks rich in plagioclase accounting for the high quartz component in the sandstones the beaulieu rapids formation is similar to modern strike slip basins that form along major fault zones in terms of basin geometry faulted and unconformable contacts lithological units and sedimentary facies stacking and organization fault controlled basins of this nature commonly develop during the last stage of terrane accretion on archean cratons

geochemical and nd pb isotopic systematics of late archean granitoids southwestern slave province canada constraints for granitoid origin and crustal isotopic structure new geochemical and nd pb isotopic data for 2 62 2 59 ga granitoids from the southwest slave province are used to determine the source s of granitoid magmas to evaluate the role of pre 2 8 ga basement during this magmatism and to refine the existing nd pb isotopic structure of the western slave province the pb isotopic data require crust older than 3 2 ga as a granitoid protolith whereas the nd isotopic data require input from juvenile crustal material this discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and 2 7 ga juvenile crust in varying proportions specifically granitoids from the southwestern slave province require 10 30 basement whereas granitoids from other parts of the western slave province require 50 incorporation of basement as a protolith may be achieved indirectly by assimilation of basement during juvenile 2 7 ga magmatism or directly during 2 62 2 59 ga magmatism the granitoid isotopic data suggest that indirect basement input was

assessing the age of the late cretaceous danek bonebed with u pb geochronology an ash rich volcaniclastic sandstone immediately underlying dinosaur rich material from the danek bonebed in the horseshoe canyon formation hcf edmonton alberta canada contains accessory zircon which have been dated employing u pb geochronology both laser ablation inductively coupled plasma mass spectrometry la icp ms and chemical abrasion isotope dilution thermal ionization mass spectrometry id tims u pb analyses have been conducted the zircon age distributions are complex with u pb dates ranging from precambrian to cretaceous we consider the youngest id tims 206pb 238u date of 71 923 0 068 ma as the maximum deposition age of the ash rich sandstone placing the overlying danek bonebed in the early maastrichtian this age is compatible with the paleontological assemblage from the danek bonebed and the regional stratigraphy the zircon age distribution also implies that the hcf had a complex provenance of the detritus with some archean and proterozoic zircons a group of mesozoic and a large compliment of cretaceous grains the results highlight the importance of high precision geochronology in constraining the age of important fossil deposits such as the danek bonebed

the teleseismic signature of fossil subduction northwestern canada 1 between june 2003 and september 2005 20 broadband three component seismometers were deployed along the mackenzie liard highway in canada s northwest territories as part of the joint lithoprobe iris canada northwest experiment canoe these stations traverse a paleo proterozoic suture and subduction zone that has been previously documented to mantle depths using seismic reflection profiling teleseismic receiver functions computed from 250 earthquakes clearly reveal the response of the ancient subduction zone on the radial component the suture is evident as a direct conversion from the moho the depth of which increases from 30 km to 50 km over a horizontal distance of 70 km before its signature disappears the structure is still better defined on the transverse component where the moho appears as the upper boundary of a 10 km thick layer of anisotropy that can be traced from 30 km to at least 90 km depth the seismic response of this layer is characterized by a frequency dependence that can be modeled by upper and lower boundaries that are discontinuous in material properties and their gradients respectively anisotropy can be characterized by a 5 variation in shear velocity and hexagonal symmetry with a fast axis that plunges at an oblique angle to the subduction plane the identification of this structure provides an unambiguous connection between fossil subduction and fine scale anisotropic mantle layering previous documentation of similar layering below the adjacent slave province and from a range of precambrian terranes across the globe provides strong support for the thesis that early cratonic blocks were stabilized through processes of shallow subduction

the timing of yellowknife gold mineralization a temporal relationship with crustal anatexis in order to thoroughly evaluate the genesis of orogenic gold mineralization in the context of plate tectonics absolute age data are required to link mineralization with lithological evolution we show that pyrite associated with orogenic gold mineralization from the con deposit in the ca 2700 ma yellowknife greenstone belt has an re os isochron age of 2591 37 ma this is identical to the previously proposed timing of ca 2592 ma for s type pluton related lode gold within ca 2660 ma graywacke mudstone turbidites east of the greenstone belt the re os pyrite data indicate that orogenic gold mineralization was deposited ca 60 to 100 m y after the formation of the host rocks in the yellowknife district and when combined with the regional geologic framework indicate that the gold mineralization formed in a distinct geodynamic setting from that during host rock formation initial os values in the pyrite os i 0 78 0 17 indicate a significant crustal component of os to the ore system compatible with metals being derived from the older host rocks regionally preserved granulites migmatites and charnockites derived in part from the older supracrustal sequences formed at 2595 to 2585 ma corresponding to the timing of gold mineralization s type plutons with ca 2592 ma crystallization ages in the upper crust reflect this high temperature metamorphic event these plutons have turbidite hosted gold deposits associated with their metamorphic aureoles collectively the isotopic and regional geologic data indicate that gold mineralization was related to regional scale crustal anatexis which likely resulted from crustal accretion or collision outside what is now preserved of the craton

evaluation of the structure of bituminous coal from sośnica mine in the upper silesian coal basin poland using reflectance indicating surface ris parameters abstract the effect of a magmatic intrusion on the optical properties reflectance anisotropy and optical character of vitrinite in low rank bituminous coals was examined the study focused on coal samples collected at different distances from an intrusion emplaced into seam no 416 in so nica mine near gliwice in the upper silesian coal basin poland in the immediate contact zone coal transformed into natural coke may be seen for comparison laboratory carbonization of coals weakly changed or apparently unchanged by the intrusion was carried out the results obtained confirmed that during carbonization of the low rank coal heating alone with no external pressure makes possible the reorganization of carbon planes that is responsible for the increase of vitrinite reflectance the heating leads to the destruction of coal structure cross linkages aliphatic groups or heteroatoms and various further transformations of the structural net in addition to the removal of volatile carbonization products external pressures affect the transformation process especially by promoting the rearrangement of small structural units to form further elements of anisotropic structure thus in the case of coal from the immediate contact of the intrusion pressures resulted in values of anisotropy parameters higher than those characterizing coal heated in the laboratory up to 1200 c but in the absence of stress the source of pressure that strongly affected the coal in the mine was probably intrusion related weak tectonism in the region of the so nica mine could explain the fine biaxial optical character of the low rank parent coals more generally pressures associated with thermally evolving gasses conceivably led to the growth of structural anisotropy in the walls of degassing pores finally some structural changes seen in coal adjacent to the intrusion may reflect hydrothermal activity

lithospheric structure in northwestern canada from lithoprobe seismic refraction and related studies a synthesis the snorcle refraction wide angle reflection r war experiment snore 97 included four individual lines along the three transect corridors a combination of snore 97 results with those from earlier studies permits generation of a 2000 km long lithospheric velocity model that extends from the archean slave craton to the present pacific basin using this model and coincident near vertical incidence nvi reflection data and geological information an interpreted cross section that exemplifies 4 ga of lithospheric development is generated the velocity structural models correlate well with the reflection sections and provide additional structural compositional and thermal constraints geological structures and some faults are defined in the upper crust at a larger scale the seismic data identify a variety of orogenic styles ranging from thin to thick skinned accretion in the cordillera and crustal scale tectonic wedging associated with both paleoproterozoic and mesozoic collisions models of poisson

archean magmatism and deformation were not products of plate tectonics abstract the granite and greenstone terrains that dominate upper crust formed from about 3 6 to about 2 6 ga and record magmatic and tectonic processes very different from those of a younger time they indicate heat loss by the archean earth primarily by voluminous magmatism from a mantle much hotter than that of the present plate tectonic processes were not then operating the distinctive array of petrologic structural and stratigraphic features that characterize phanerozoic convergent plate systems ophiolites magmatic arcs accretionary wedges fore arc basins etc have no viable analogues in archean terrains purported archean plate tectonic indicators consist merely of rock types that even superficially resemble actual phanerozoic indicators only when considered in isolation from their association and structure archean ultramafic and mafic volcanic rocks neither resemble ophiolitic rocks in petrology nor occur in ophiolite type successions they often depositionally overlie felsic basement rocks and often overlie and are intercalated with sedimentary and felsic volcanic rocks and they require a mantle about 200 c hotter than now archean graywackes are coherent strata derived from nearby volcanic rocks late in the histories of their regions and they lack the setting and disruption that characterize modern accretionary wedges the lithologic structural and stratigraphic assemblages that typify proterozoic and phanerozoic rifted and reassembled margins also lack archean analogues and no evidence has been found for archean rifting rotation and reassembly of continental plates conversely characteristic archean assemblages lack modern equivalents in any tectonic setting ultramafic lavas erupted at liquidus temperatures are voluminous granite and greenstone terrains have no modern analogues greenstone belts are typically anastomosing networks of upright synforms formed by crowding aside by and sinking between large rising diapiric elliptical composite batholiths the batholiths include both the products of new crustal melts and variably remobilized mid crustal gneisses the greenstone belts are defined by late deformation of regionally semiconcordant volcanic and sedimentary successions and are not relics of successively formed volcanic chains little deformation generally preceded the diapirism and metamorphism was primarily of contact type the regionally uniform areal density and accordant crustal level of the diapiric batholiths their contacts primarily against the oldest strata of the synforms their general age 10 20 million years younger than most of the flanking stratiform rocks and considerations of high archean radiogenic heat productivity all fit the explanation that the batholiths were mobilized by partial melting of hydrous lower crust by radiogenic heating diapirism was accompanied by modest regional orthogonal shortening and extension of the hot upper crust producing the orientations of the batholiths rise of the batholiths greatly increased the petrologic fractionation of the crust and the concentration of radionuclides high in it resulting in cooling of the deeper crust and subjacent mantle and thus cratonization the upper crust containing the granite and greenstone aggregates was decoupled from the gneissic middle crust which underwent flattening and extension subparallel to the elongation of the shallow batholiths this deep deformation may have been driven by flow of dense restites toward delamination loci from which they sank into the mantle the early earth was probably wholly molten the surface of the earth like that of the moon must have been wholly recycled by impacts before 3 9 ga and heavily modified by them until 3 8 ga zircons as old as 4 2 ga have been found as clastic grains in much younger archean quartzites and polycyclic migmatites last partly melted and reconstituted under hydrous conditions only after 3 6 ga contain relict zircons as old as 4 0 ga the lithologies of the early earth protoliths in which these zircons formed have not been established but impact melts and breccias must be represented and magma ocean fractionates may be the nature of the transition in tectonic style into the granite and greenstone mode is unknown plate tectonic rifting and convergence were operating by about 2 0 ga and were in an essentially modern mode by about 0 8 ga the nature of the transition from the granite and greenstone mode at about 2 6 ga to plate mode by about 2 0 ga has yet to be defined the change may have been facilitated by the increasing content of water and carbon dioxide in the mantle as dense but hydrated delaminated archean crust sank into it

products of 2 65 2 58 ga orogenesis in the slave province correlated with slave northern cordillera lithospheric evolution snorcle seismic reflection patterns reflection patterns along lithoprobe slave northern cordillera lithospheric evolution snorcle seismic reflection line 1 in the southwestern slave province are interpreted as products of tectonic wedging during late archean lithospheric convergence the interpretation is aided by application of new seismic imaging techniques by correlation of upper crustal reflection patterns to known geology and by a comparison of lithospheric reflection patterns to similar convergent zones elsewhere in the yellowknife area reflection patterns consist of 1 east dipping reflections at 12 14 s that project into the upper mantle 2 a wedge shaped body in the lower crust with an east dipping reflection fabric that is truncated on the west by 3 a series of west dipping reflections that outline thrust and fold structures in the upper crust the similarity of these reflection patterns to those of the proterozoic fort simpson hottah collision zone 300 km to the west provides support for the interpretation that reflection patterns beneath the slave province are also products of collisional tectonics rocks within the slave province preserve evidence of a 2 65 2 58 ga pan slave orogenic event in which the 2 9 ga central slave basement collided with the 2 7 ga juvenile eastern slave province their suture is interpreted to be a west dipping surface at 4 5 s 12 15 km beneath yellowknife and to project to the surface east of the profile in the lower crust and upper mantle east dipping reflections are interpreted to delineate a coeval subduction zone and accretionary wedge the upper crustal thrust and fold structures are likely linked to gold bearing shear zones at yellowknife these results provide tantalizing evidence that processes similar to those of modern convergent zones were operational at 2 65 2 58 ga

geochemical and nd isotopic constraints for the origin of late archean turbidites from the yellowknife area northwest territories canada abstract a detailed geochemical and isotopic study of late archean turbidites and volcanic rocks from the yellowknife area slave province was undertaken to constrain the nature of exposed crust at the time of 2 6 to 2 7 ga crustal consolidation the nd t values of the volcanic rocks range from 1 7 to 4 4 this variation can be produced by assimilation of pre 2 8 ga basement by a depleted mantle derived magma possibly followed by fractional crystallization the turbidites are typically metamorphosed to greenschist to amphibolite facies and where metamorphosed to greenschist facies different units of bouma sequence can be observed the different units of bouma sequence were sampled and analyzed separately to evaluate the possible differences in geochemical and isotopic signatures the geochemical data presented here is in accord with the previously proposed model that argues for a mixture of 20 mafic intermediate volcanic rocks 55 felsic volcanic rocks and 25 granitic rocks as a source of these turbidites however our revised calculation with the new data presented here argues for 1 to 2 input from an ultramafic source as well as somewhat higher input from mafic intermediate volcanic sources in the upper shale units compared to the lower sand units the nd t values of the turbidites generally are lower in the upper shale units compared to the lower sand units detailed inspection of trace element data suggest that this is not an artifact of rare earth element rich heavy minerals concentrating in the lower sand units of the turbidites but rather is a result of unmixing of detritus with different nd t values during sediment transportation and deposition the upper shale units of the turbidites are isotopically compatible with a derivation mainly from crustally contaminated volcanic rocks similar to those exposed in the yellowknife area the lower sand units contain a higher proportion of westerly derived plutonic rock detritus characterized by higher nd t suggesting that there are area s west of yellowknife not underlain by older 2 8 4 0 ga basement the trace element characteristics of these turbidites i e cr ni la th sc eu eu and gd n yb n are distinct from those of typical post archean turbidites this observation is consistent with the models that predict that the chemical composition of the upper continental crust was slightly different in the archean compared to post archean time

lower crustal flow kept archean continental flood basalts at sea level large basaltic provinces as much as 15 km thick are common in archean cratons many of these flood basalts erupted through continental crust but remained at sea level although common in the archean record subaqueous continental flood basalts cfbs are rare to absent in the post archean here we show that gravity driven lower crustal flow may have contributed to maintaining archean cfbs close to sea level our numerical experiments reveal that the characteristic time to remove the thickness anomaly associated with a cfb decreases with increasing moho temperature t m from 500 m y for t m approximate to 320 degrees c to 1 m y for t m approximate to 900 degrees c this strong dependency offers the opportunity to assess from the subsidence history of cfbs whether continental geotherms were significantly hotter in the archean in particular we show that the subsidence history of the ca 2 7 ga upper fortescue group in the east pilbara craton western australia requires moho temperatures 700 degrees c applied to eight other unambiguous subaqueous archean cfbs our results indicate moho temperatures 650 degrees c at the time of eruption we suggest that the decrease in the relative abundance of subaqueous cfbs over earth s history could reflect the secular cooling of the continental lithosphere due to the decrease in radiogenic heat production

field relations u pb geochronology and sm nd isotope geochemistry of the point lake greenstone belt and adjacent gneisses central slave craton n w t canada data from the point lake area central slave craton suggest an intimate tectonic and paleogeographic association between volcano sedimentary supracrustal rocks and adjacent gneisses granite plutons and orthogneisses yield u pb zircon crystallization ages ranging from ca 3230 to 2818 ma numerous mafic dykes cut the gneisses and two have been dated by u pb zircon geochronometry at 2673 3 and 2690 3 ma ages similar to those of volcanic rocks in the point lake greenstone belt although high strain zones form the greenstone gneiss in most places a structural repetition of granite about 4 km east of keskarrah bay is cut by numerous mafic dykes and apparently overlain depositionally by pillow basalt mafic volcanic and plutonic rocks from point lake have initial 2 7 ga end values ranging from about 2 2 to 6 3 significantly lower than the depleted mantle at that time the nd data suggest either derivation from a more isotopically evolved reservoir or assimilation of crust similar to the granit

evolution of an archean basement complex and its autochthonous cover southern slave province canada abstract the sleepy dragon complex sdc is one of several antiformal exposures of a 4 0 2 85 ga basement block underlying the west central region of the archean slave province northwestern canada this basement block the central slave basement complex csbc is overlain by an autochthonous to locally parautochthonous dominantly quartz rich clastic sequence the 2 85 2 80 ga central slave cover group together these units comprised a fundamental building block during neoarchean growth and assembly of the slave craton the well exposed patterson lake morose lake area represents a type locality for study of the csbc central slave cover group and overlying rocks of the yellowknife supergroup field mapping and u pb geochronology employing both thermal ionisation mass spectrometry tims and sensitive high resolution ion microprobe shrimp document a prolonged crustal history oldest intact basement units consist of foliated to gneissic tonalite dated in two places at 2955 12 ma and 2944 9 ma indirect evidence for a ca 3150 ma basement component is obtained both from xenocrystic zircon in tonalite and detrital zircons from the overlying central slave cover group which here consists of the patterson lake formation a granite boulder from this formation is dated at 2934 3 ma and may be locally derived a sheared mafic volcanic unit along the basement cover contact contains 2942 3 ma metamorphic titanite which provides a minimum deposition age this volcanic unit is therefore also a part of the basement complex younger events in the sdc mainly reflect tonalite trondhjemite granodiorite plutonism during construction of the overlying cameron river and beaulieu river greenstone belts of the yellowknife supergroup a unit of k feldspar megacrystic granodiorite is dated at 2726 3 ma and tonalite and granodiorite bodies located toward the centre of the sdc have primary crystallization ages of 2683 3 2 ma 2677 2 ma and 2672 7 6 ma the core of the southern sdc is occupied by a late tectonic to post tectonic granite pluton previously dated at 2586 2 ma an intravolcanic unconformity along the southern margin of the sdc developed in part on 2 7 ga granodiorite is constrained by existing u pb data to have formed sometime between ca 2685 2660 ma an abundance of synvolcanic plutons of similar age and the presence of a 3 km thick mafic volcanic sequence overlying a less dense magmatically active felsic crust suggest that gravitational instability and magmatic diapirism may have played important roles in doming and uplift of the sdc prior to regional d 1 deformation the younger toward centre zonation of plutonic rocks in the sdc is documented in other domal granite gneiss complexes of archean age e g pilbara craton western australia where it is attributed to upward and outward displacement of older crust during repeated magma channeling into a growing mid crustal dome in the west central slave craton such a process might account for the regional presence of local unconformities of apparently similar age the domal basement plutonic complexes would have been further amplified after 2660 ma during the development of regional f 1 and f 2 fold belts

the central slave basement complex part i its structural topology and autochthonous cover new field and geochronological data are used to define the distribution of mesoarchean basement rocks in the south central slave province this distribution reflects a single contiguous basement terrane that we propose to call the central slave basement complex it shows a structural topology that is internally consistent and compatible with known regional folding and faulting events a sample of a proposed basement gneiss below the courageous lake greenstone belt central slave province has been dated by u pb methods and yields an age of 3325 8 ma consistent with the new basement distribution this sample also contains 2723 3 ma metamorphic zircon and ca 2680 ma titanite the central slave basement complex is overlain by a thin discontinuous but distinctive cover sequence that includes minor volcanic rocks clastic sedimentary rocks and banded iron formation all previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the

recognition of vitrinite with non uniaxial negative reflectance characteristics non uniaxial negative reflectance characteristics are being reported for coals from around the world with increasing frequency these data have traditionally been collected from oriented sections cut from a block of coal to determine the magnitudes of the principal reflectance axes to make data collection less time consuming a method of determining the principal reflectance axes from a standard crushed coal sample has been developed the method requires measurement of the maximum and minimum apparent reflectances from a series of randomly oriented vitrinite particles the reflectance crossplot displays these two reflectance values against their difference or bireflectance the paper describes the method of interpreting the resultant pattern to determine the values of the three principal reflectance axes analysis of coals from the british columbia rocky mountains and foothills revealed biaxial and uniaxial positive as well as uniaxial negative reflectance characteristics a new measure of maturation rev is proposed which is based purely on the lengths of the three principal reflectance axes rm approximates this value but is influence by any eccentricity of the reflectance indicating surface ris vitrinite with a non uniaxial reflectance characteristics cannot be adequately described by a single measure such as rmaxorrm but requires reference to both the size and shape of the ris a reflectance determination such as rev or rm provides rank astm information whereas the new measures rst and ram describe a reflectance indicating surface shape which is the result of maturation and tectonic histories vitrinites with non uniaxial negative reflectance characteristics are abundant in the coalfields of the rocky mountains and foothills of british columbia rmax does not fully describe the reflectance character of these coals and does not describe the potentially useful ris shape

stratigraphy sedimentology and physical volcanology of the henik group central ennadai rankin greenstone belt northwest territories canada late archean paleogeography of the hearne province and tectonic implications abstract extensive areas of the hearne and rae provinces are underlain by archean metavolcanic and metasedimentary rocks in the rae province supracrustal sequences with prominent continental to shallow water quartz arenites have been interpreted by others as rift and passive margin deposits in the hearne province to the southeast the ennadai rankin greenstone belt is exposed in an area ca 700 200 km forming the second largest greenstone belt in canada it comprises mainly greenschist facies grade mafic and felsic volcanic siliciclastic chemogenic and volcaniclastic rocks stratigraphic sedimentologic and physical volcanologic data based on 1 50 000 scale mapping in the central part of the belt indicate deposition in a subaqueous mafic volcanic plateau slope basin system that was well removed from a continental influence water depth was below storm wave base as indicated by preservation of delicate lamination in pelitic and iron formation units absence of features suggestive of subaerial exposure oscillatory flow or tidal currents and lack of interfingering rocks that could represent laterally adjacent shelf coastal or continental environments reconnaissance level regional geologic and geochronologic data indicate an overall pattern of supracrustal deposition in the hearne province consisting of a large mafic volcanic plain isolated felsic volcanic centres with lenses and belts of deep water sedimentary rocks at least two speculative yet testable tectonic models are consistent with the inferred paleogeography for both the rae province is considered to be the western continental hinterland to the ensimatic hearne province the rae and hearne provinces may record late archean continental extension sea floor spreading oceanic volcanic arc growth diachronous back arc spreading and collapse and arc continent collision with thrusting of oceanic rocks on to attenuated rae crust alternatively the hearne province may represent northwestward accretion of multiple volcanic arc trench pairs

the archean deep marine environment turbidite architecture of the burwash formation slave province northwest territories the slave province is an archean craton that contains an unusually large proportion of preserved sedimentary rock including thick turbidite successions the burwash formation is exposed in the southern slave province between the anton and sleepy dragon basement massifs at the base of the succession volcanics and clastic metasedimentary rocks of the raquette lake formation record initiation of the basin in a rifted arc environment these are overlain by thin black slates representing a transgression followed by well over 5 km of burwash formation metamorphosed turbiditic sandstones and slates interspersed with thin felsic tuff layers lateral correlation within the formation is possible using airphotos and recognizable tuff units burwash formation sandstones include thinly bedded units displaying bouma sequences and thicker bedded units with scour and fill structures and stratification bands characteristic of dense sediment gravity flows the sedimentary rocks are organized in architectural elements that include channel fill sandstones and conglomerates muddy levees interchannel sandstones resembling high amplitude reflection packages harps described from modern fans and possible depositional lobes the overall sedimentary architecture was probably controlled by events in the tectonically active source area or areas the archean turbidites resemble their phanerozoic and modern analogues although they show less voluminous levees and are generally less organized than large modern passive margin fans which probably have no equivalents in the archean

mantle heterogeneities and their significance results from lithoprobe seismic reflection and refraction wide angle reflection studiesthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent lithoprobe contribution 1486 within lithoprobe s 10 transects data from more than 20 000 km of multichannel seismic mcs reflection profiling and 12 refraction wide angle reflection r war surveys were acquired while the

oxidatively and thermally altered high volatile bituminous coals in high temperature coal fire zone no 8 of the wuda coalfield north china abstract coal fires have received increasing attention due to their environmental economic and social impacts their significant influence on coal properties is widely documented by geophysical and geochemical methods the present paper demonstrates by means of coal petrological methods significant changes of microstructure optical appearance and optical properties of medium rank bituminous coals affected by an underground coal fire the studied changes are suggested to result from distinct oxidative and thermal alterations induced by a high temperature regime temperatures over 800 c encountered in the coal fire zone no 8 wuda coalfield the investigated coal seam no 9 represents a high volatile a bituminous coal and is dominated by vitrinite followed by inertinite with minor amounts of liptinite the microstructure of the coal fire affected coals facilitate a broad spectrum of characteristic features the microscopic analyses revealed development of 1 non tectonic fissures and non tectonic fractures 2 distinct micropores 3 dark oxidations rims and 4 bright spots interpreted as incipient mesophases optical properties were characterised for megasporinite and vitrinite in the examined megasporinite a noticeable anisotropy and distinct granular appearance was observed in the investigated coals both a dominant increase as well as a minor decrease in both random and maximum vitrinite reflectances were documented for 1 coal grains and 2 dark rimmed vitrinite at non tectonic fissures and non tectonic fractures in addition also a characteristic bimodal vitrinite population was measured the increase in bireflectance was clearly documented for two sub zones within the coal fire zone no 8 the applied kilby s cross plots suggested a large overlapping of the data sets of r max and r min indicating marked changes in vitrinite structure the applied coal petrological approach revealed distinct changes to the coal s microstructure appearance as well as to optical properties induced by the underground coal fire the presented coal petrographic approach may help determine the extent of coal fires in coal mines and the direction of propagation of coal fire front

static stress drop in the mw 9 tohoku oki earthquake heterogeneous distribution and low average value static stress drop distribution and its average value over the rupture area contain important information on the mechanics of large earthquakes here we derive static stress drop distributions from 40 published rupture models for the 2011 mw 9 tohoku oki earthquake that are based on various multidisciplinary observations average stress drop value over the fault area encompassed by the 5 m coseismic slip contour is not unusually large for each rupture model the mean for the 40 models is 2 3 1 3 mpa assuming a uniform rigidity 40 gpa the value for the entire rupture zone and with a more realistic rigidity structure will be even lower in the majority of the models local stress drop in parts of the rupture zone well exceeds 20 mpa the heterogeneous stress change distribution with large stress drop being accompanied by large stress increase leads to the small average for the earthquake

origin and evolution of mid to late archean crust in the hanikahimajuk lake area slave province canada evidence from u pb geochronological geochemical and nd pb isotopic data abstract a geochronological major trace element geochemical and nd pb isotopic study of early to mid archean basement granitoids gneisses 2 68 ga greenstone belt and 2 58 2 62 ga late archean granitoids from the hanikahimajuk lake area of the northern slave province was undertaken to constrain the tectonomagmatic evolution of the north central area of slave province northwest territories canada the basement tonalites are precisely dated at 3378 1 ma by u pb zircon methods and are characterized by steeply fractionated ree patterns with no negative eu anomalies they were likely produced through partial melting of mafic crust at 0 8 gpa the endt of these rocks range from 0 3 to 2 1 implying involvement of even older crustal material in their genesis two different types of 2 68 ga mafic to intermediate volcanic rock are recognized in the hanikahimajuk lake area tholeiitic rocks with no ta nb depletions and calc alkaline rocks with slight negative ta nb depletions all of these rocks show clear evidence for contamination by the 3 4 ga basement tonalite thus precluding the possibility that some of these tholeiitic mafic rocks were produced in a mid oceanic ridge setting from the data presented here we conclude that the tholeiitic rocks were produced in a back arc setting likely similar to that of the japanese arc the calc alkaline rocks may either be a remnant of the volcanic arc or part of the same back arc system however the close proximity with the tholeiitic rocks in the field favours the latter possibility mafic to intermediate rocks with similar geochemical and isotopic characteristics are also found from the yellowknife area of the southwestern slave province and from the indin lake area of the westernmost slave province south of grenville lake area this observation may indicate that the mafic dominated volcanic belts of the western slave province may be a remnant of a single now collapsed and shortened back arc basin the existence of syn volcanic calc alkaline plutonic rocks with similar geochemical and isotopic features as the calc alkaline volcanic rocks supports this possibility the geochemistry of late archean granitoids suggests that they are produced mainly through partial melting of mafic igneous protoliths and clastic sedimentary rocks and possibly tonalitic rocks similarity in the end 2 6 values between these granitoids and the supracrustal rocks in the area is in accord with this model such protoliths are also consistent with the tectonic setting at which the 2 68 ga volcanic rocks were generated because mafic igneous rocks and clastic sedimentary rocks with lesser felsic igneous rocks are the major constituents of a back arc basin

on the optically biaxial character and heterogeneity of anthracites abstract the results of the study of optical properties of 13 anthracites from different parts of the world are presented in this paper measurements of reflectance values were made on non oriented vitrinite grains for a minimum of 300 points per sample the reconstruction of reflectance indicating surfaces ris were made by kilby s method kilby w e 1988 recognition of vitrinite with non uniaxial negative reflectance characteristics int j coal geol 9 267 285 kilby w e 1991 vitrinite reflectance measurement some technique enhancements and relationships int j coal geol 19 201 218 it was found that the use of kilby s method for strongly anisotropic materials like anthracites did not give unambiguous results some improvement in kilby s method consisting of the division of the cumulative cross plot into several elemental components is suggested each elemental cross plot corresponds to a textural class of anthracite which is characterized by the values of ris main axes rmax k rint k and rmin k k 1 2 n n number of classes the global texture of anthracite is characterized as a ris with main axes calculated as the weighted means of r max r int and r min for each class of this anthracite the division of cumulative kilby s cross plot on elemental components makes possible the calculation of new coefficients ht and h10 characterizing the heterogeneity of the structure and texture of anthracites the results of our study show that all anthracites have biaxial negative textures but their heterogeneity varies in a wide range of ht and h10 coefficients depending upon the individual coal basin

lithospheric structure beneath the archaean slave province and proterozoic wopmay orogen northwestern canada from a lithoprobe refraction wide angle reflection survey the slave geological province a relatively small craton in nw canada that includes the oldest rocks in the world is one of five cratons that form the archaean continental core of north america the palaeoproterozoic wopmay orogen lying between the craton and the phanerozoic northern canadian cordillera represents crustal growth through addition of a series of north south oriented magmatic arcs and accreted terranes as part of lithoprobe s multidisciplinary studies line 11 of the snore 97 seismic refraction wide angle reflection survey investigated the structure of the lithosphere in this region interpretation of crustal and mantle arrivals from 13 record sections followed an iterative procedure of traveltime inversion and forward modelling results show that crustal thickness at 32 35 km remains relatively constant from the archaean regions in the east to proterozoic domains in the west contrary to average values of crustal thicknesses from global compilations that suggest thicker proterozoic crust at the westernmost end of the line where crustal extension and basin formation are inferred the crust thins to 30 km upper crustal velocities to about 12 km depth lie in the range of 5 8 6 2 km s 1 the middle crust of thickness 8 10 km has velocities between 6 2 and 6 6 km s 1 average velocities for the lower crust differentiated as a 10 km thick layer range between 6 6 and 6 9 km s 1 with a higher velocity region 7 1 km s 1 in the transition zone between the fort simpson magmatic arc and hottah accreted terrane in the wopmay orogen where a palaeoproterozoic subduction zone has been inferred from coincident multichannel reflection data in the same area anomalously low upper mantle velocities 7 5 km s 1 have been found indicating that the effects of collision subduction and accompanying physical changes in the rocks comprising the mantle lithospheric wedge above the underthrusting structure may have been retained for 1800 ma data from most record sections with offset distances greater than 250 km show continuous wide angle reflections from upper mantle depths of about 75 km and almost 100 km although poorly constrained velocities in this depth range vary from 8 4 to 8 8 km s 1 interpretation of these phases and mantle reflections from equivalent depths in coincident near vertical incidence multichannel data relates them to subduction and underthrusting of oceanic crust and subsequent phase changes that occurred during the assembly of the craton and the evolution of the palaeoproterozoic orogen

thermal regime of the lithosphere in the canadian shieldthis article is one of a series of papers published in this special issue on the theme lithoprobe parameters processes and the evolution of a continent geothermal studies were conducted within the framework of lithoprobe to systematically document variations of heat flow and surface heat production in the major geological provinces of the canadian shield one of the main conclusions is that in the shield the variations in surface heat flow are dominated by the crustal heat generation horizontal variations in mantle heat flow are too small to be resolved by heat flow measurements different methods constrain the mantle heat flow to be in the range of 12 18 mw m 2 most of the heat flow anomalies high and low are due to variations in crustal composition and structure the vertical distribution of radioelements is characterized by a differentiation index di that measures the ratio of the surface to the average crustal heat generation in a province determination of mantle temperatures requires the knowledge of both the surface heat flow and di mantle temperatures increase with an increase in surface heat flow but decrease with an increase in di stabi

climatic and tectonic influences on fan deltas and wave to tide controlled shoreface deposits evidence from the archaean keskarrah formation slave province canada the 2 6 ga keskarrah formation located in the central slave province northwest territories canada is a late orogenic tectonically controlled sedimentary sequence that developed under unusual climatic and depositional conditions the formation is adjacent to the crustal scale north trending beniah lake fault and overlies the 3 15 ga augustus granite the 2 69 2 7 ga mafic volcanic peltier formation and the turbiditic contwoyto formation unconformably principal lithofacies in the keskarrah formation include conglomerate sandstone and siltstone sandstone the conglomerate lithofacies represents coalescing gravelly streamflow dominated fan deltas adjacent to topographic highs up section quartz rich arenites and quartz arenites of the sandstone lithofacies are interpreted to be shallow water shoreface deposits influenced by wave action and tides the overlying feldspathic litharenites of the siltstone sandstone lithofacies are consistent with a lower shoreface to proximal offshore environment dominated by wave and tide interaction tidal influence in both sandstone dominated lithofacies is inferred from the presence of mudstone laminae between bedforms and on foresets of cross beds as well as from abundant reactivation surfaces with local mudstone drapes intense chemical weathering during the archaean resulting from elevated atmospheric co2 levels higher temperatures and moist climatic conditions played an important role in the development of quartz rich arenites that appear to be first cycle deposits few lithic fragments and feldspar grains are preserved due to in situ host rock weathering chemical weathering during transport and wave and tide action hydraulic sorting and abrasion in the shoreface environment contributed to the continued breakdown and transport of labile minerals increased proportions of lithic fragments in sandstone beds of the conglomerate lithofacies are the result of shorter transport distances from source areas to the depositional environment abundant conglomerate with up to 4 m large granitic boulders derived from the adjacent augustus granite and mafic clasts from the peltier formation indicate high relief and fault related uplift and subsidence the intimate association of fan deltas and wave and tide influenced shallow marine deposits in association with quartz rich sandstones forming in a high relief area make the keskarrah formation remarkable in the rock record

integrated nd isotopic and u pb detrital zircon systematics of clastic sedimentary rocks from the slave province canada evidence for extensive crustal reworking in the early to mid archean abstract a combined u pb geochronology and nd isotopic analysis of clastic metasedimentary rocks from the archean slave province northwest territories canada is presented a series of clastic sedimentary rocks with deposition age of 3 13 ga to 2 58 ga was collected from the west central slave province these samples include conglomerates as well as finer sedimentary rocks such as greywacke arkose and quartzite although it is generally agreed that the nd model age t dm of clastic sedimentary rocks represents the average sedimentary provenance age t dm of samples studied here is generally older than the average u pb age of detrital zircons extracted from the same sample rather there seems to be a better 1 1 relationship between the t dm and u pb age of the oldest zircon from each sample a simple model relationship between u pb geochronology and t dm suggests that such correlation is a result of archean crustal evolution of the slave province being dominated by crustal reworking i e intracrustal processes rather than addition of juvenile materials from the mantle one exception is the period between 3 1 and 3 2 ga during which period a significant portion of detrital zircons have u pb ages exceeding the t dm indicating a large input of juvenile material and or decreasing level of crustal reworking an alternative explanation for the u pb vs t dm trend may be a mixing of older mafic juvenile crust containing no zircon and younger felsic juvenile crust however the general lack of extensive pre 2 8 ga mafic crust in the slave province makes this scenario unlikely the results of this study cannot constrain the change in the volume of continental crust at any given period of time since the addition of juvenile crust may always be counterbalanced by recycling of crustal material back into the mantle however a dominance of crustal reworking during the early to mid archean strongly implies that a large part of ancient crust was lost during the younger tectonomagmatic events since the majority of zircon may be lost during crustal reworking processes unless they are preserved as an inherited core absence of a large quantity of ancient detrital zircon cannot be used as evidence for the lack of large sialic crust existing on the early earth

geology alteration and lithogeochemistry of the hood volcanogenic massive sulfide vms deposits nunavut canada the hood volcanogenic massive sulfide vms deposits are hosted by the 2 68 ga amooga booga volcanic belt abvb in the northwestern archaen slave craton and consist of three deposits hood 10 41 and 41a and three occurrences 46 461 and 462 the mineralized zones consist of massive to semi massive pyrrhotite pyrite chalcopyrite sphalerite and galena hosted predominantly by felsic volcanic flows within the predominantly mafic abvb the mineralized lenses occur at different stratigraphic levels and have textural alteration and stratigraphic features consistent with formation via subseafloor replacement the felsic volcanic rocks in the hood deposits can be subdivided into groups based on immobile trace element geochemistry the main felsic types a and b are petrographically indistinguishable type a has higher high field strength element hsfe and rare earth element ree contents than type b suggesting a higher temperature of formation type a rocks also have higher nb ta values indicative of a greater mantle input in their genesis compared to type b rocks mineralization is more closely associated with type a than type b rocks the two mafic volcanic rock types previously identified in the abvb type i and type ii both occur within the hood deposits the type ii mafic group is interpreted to be the result of variable crustal contamination of type i magma the volcanic rocks of the abvb are interpreted to have formed in a continental margin arc back arc setting the genesis of these magmatic suites involved magmatic underplating and emplacement through pre existing sialic basement that resulted in crustal melting mantle crust mixing and contamination leading to the aforementioned geochemical features in both mafic and felsic suites this type of extensional tectonic environment was likely associated with high heat flow and is similar to global vms environments proximal to extending continental margins e g sturgeon lake bathurst and finlayson lake the similarities of the abvb to other slave craton greenstone belts further highlights the overall potential for greenstone hosted vms mineralization in the slave craton chlorite sericite quartz is the dominant hydrothermal alteration assemblage in the hood deposits and is typical of vms style mineralization mass change calculations illustrate that elemental changes are typical of vms environments with gains in fe2o3 mgo and base metals associated with chlorite alteration near mineralized zones k2o gains associated with sericite alteration and losses of na2o in both alteration types

the nature of mesoarchaean seawater and continental weathering in 2 85 ga banded iron formation slave craton nw canada abstract banded iron formations bif have been extensively used as proxies to infer the chemical composition of ancient bulk seawater however their proximity to ancient crust suggests that they might also be used to reveal the composition of emergent continental landmass at the time of their deposition here we use the combination of geochemistry and sm nd isotopes on a layer by layer basis to interpret the relative contributions of hydrothermal hydrogenous and terrestrial input to one of the oldest documented superior type bif in the world the 2 85 ga central slave cover group bif is deposited within a rift basin related to a continental margin and is found associated with basement gneisses as well as shoreline and shallow shelf type facies such as fuchsitic quartzite and pebble to cobble conglomerate that confirm a near shore depositional setting for the bif the bif ranges from a pure chemical oxide magnetite silicate grunerite actinolite sediment with low al 2 o 3 2 o 3 10 wt and the occurrence of ferro hornblende biotite and garnet the silica bands have low trace metal content e g ni low ree average of 6 ppm and a shale normalized rare earth and yttrium rey pattern that is hree to lree enriched with pr yb sn values reaching sn of 0 5 during active rifting of the basement excess of eu 2 impacted the basin yielding seawater with eu anomalies eu eu sn as high as 3 85 average 2 75 larger than similarly aged bif high resolution geochemistry shows that there is more silica 19 4 wt sio 2 in the iron bands than iron 8 7 wt fe 2 o 3 in the silica bands implying that dissolved fe 2 came to the bif site in pulses and that silica likely represents background deposition consistently radiogenic e nd t values for the iron bands average 1 7 show that the dissolved rey in the source water during ferric iron precipitation was provided by submarine hydrothermal fluids with relatively uniform 143 nd 144 nd the silica bands by contrast reveal high variation in seawater 143 nd 144 nd as evident from the bimodal e nd t distribution with one segment exhibiting negative e nd t values averaging 1 1 and another with positive e nd t values averaging 2 5 this suggests input of dissolved rey into the upper seawater from weathering of isotopically different crustal components in the source region collectively we speculate that the low rey in the upper seawater and the overall low ni content implies a highly weathered crustal surface that was unable to contribute a significant dissolved load to the shelf environment

the bell lake group and anton complex a basement cover sequence beneath the archean yellowknife greenstone belt revealed and implicated in greenstone belt formation u pb zircon and monazite geochronology indicates the presence of a 2 93 ga basement anton complex and 2 8 ga cover sequence bell lake group beneath the 2 70 to 2 72 ga kam group of the yellowknife greenstone belt in the southern slave province the bell lake group comprises remnants of a quartzite rhyolite banded iron formation succession the ages of detrital zircons from the quartzite unit constrain its deposition to be younger than 2 92 and range up to 3 7 ga u pb ages for gneisses beneath the bell lake group are in excess of 2 93 ga and they are locally overlain by the quartzite the contact between the tholeiitic to calc alkaline kam group and the bell lake group is poorly exposed and equivocal approximately 6 km higher in the section the ranney chert a felsic volcaniclastic layer separates 2 70 2 72 ga tholeiitic to calc alkaline upper kam group rocks from a lower tholeiitic section containing sheeted dike complexes zircons from the ranney chert yield pb pb ages ranging from 2 72

archaean sedimentary sequences of the slave craton abstract the slave craton is composed of five archaean terranes that accreted during the neoarchean the central slave superterrane is the only tectonic block containing evidence of four major sedimentary successions including 1 a quartz arenite dominated succession deposited on an extensive tide influenced coastline along a rifted continental margin 2 a turbidite dominated succession that developed as submarine fans in a fore arc or intra arc basin 3 a second turbidite succession representing deep water deposition in a back arc basin and 4 late conglomerate dominated successions representing fluvial alluvial fan and shallow water depositional settings in strike slip basins deposition of the sedimentary rocks took place between ca 3 1 ga and 2 6 ga indicating a complex tectonic history an unconformity between the gneissic basement and the overlying quartz arenite succession marks a marine flooding surface consistent with a first order sea level change a major hiatus lies between the quartz arenite successions and arc backarc related volcanic deposits with interstratified and conformably overlying turbidite strata late conglomerate successions are exposed along north trending lineaments and overlie all sedimentary and volcanic rock units unconformably this depositional hiatus represents a fundamental change from compression and uplift to transcurrent motion during the final stages of craton stabilization between ca 2630 and 2580 ma the slave province experienced considerable pluton emplacement which disaggregated much of the once continuous stratigraphy a wheeler diagram illustrates the major tectonic events that occurred during formation of the central slave superterrane however remaining uncertainties concerning the evolution of the entire slave craton reflect the need for additional detailed field investigations

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

evidence of large scale repeating slip during the 2011 tohoku oki earthquake 1 the repetition of slip during rupture process of earthquake is a debate issue which had never been confirmed clearly in the past big events due to the lack of dense near field observations and limited resolution in time of source model the 2011 m9 0 tohoku oki earthquake generated a wealth seismic records which provided us an unprecedented opportunity to study the rupture evolution of giant earthquake at a high spatio temporal resolution here we use teleseismic local strong motion and near field coseismic geodetic data to investigate the source rupture process of this event based on the parallel inversion technique the results reveal a broad slip zone with remarkable large scale repeating slip during the earthquake the inverted source model shows several time periods of energy release with three main peaks these energy bursts and temporal rupture snapshots suggest repetition of a large scale slip on the biggest asperity this rupture behavior resulted in 50 m slips on the slip zone and prolonged the entire rupture process for a long duration of 160 seconds the proposed source model is in a good agreement with the aftershock distribution and can interpret the characteristics of local strong motions further investigations of repeating slip during this event are crucial which will deeply transform earthquake science from dynamic point of view

geochemical and metallogenic relations in volcanic rocks of the southern slave province implications for late neoarchean tectonics late neoarchean volcanic belts in the southern slave province include 1 in the east the cameron river beaulieu river belts which are characterized by stratigraphically thin flow rich classic calc alkaline arc type sequences with accompanying syngenetic volcanogenic massive sulphide deposits and 2 in the west the yellowknife belt which is characterized by stratigraphically thick structurally complex pyroclastic rich adakitic back arc basin type sequences with accompanying epigenetic lode gold deposits the volcanic belt association bears persuasive chemical evidence of subduction initiated magma generation however the greenstone belts together with coeval matching patterned belts in superior province of the southern canadian shield bear equally persuasive evidence of prevailing autochthonous parautochthonous relations with respect to component stratigraphic parts and to older gneissic basement the eastern and western volcanic belts in question are petrogenetically ascribed to a westerly inclined present geography subduction zone s that produced shallower east to deeper west slab initiated mantle wedge generated parent magmas this early stage microplate tectonic process involved modest mantle subduction depths small tectonic plates and small sialic cratons in the larger context of earth s progressively cooling hence subduction deepening mantle this late neoarchean greenstone belt development 2 73 2 66 ga merged with the massive end archean tonalite trondhjemite granodiorite granite ttgg bloom 2 65 2 55 ga resulting in greatly enhanced craton stability successive subduction deepening plate craton enlarging stages with appropriate metallotectonic response across succeeding proterozoic time and beyond led to modern mode plate tectonics

recognizing distinct portions of seamounts using volcanic facies analysis examples from the archean slave province nwt canada abstract archean volcanic rocks in the mafic dominated ca 2 66 2 69 ga point lake and beaulieu river belts slave province northwest territories are significant in demonstrating the facies that characterize specific portions of pillow volcanoes or seamounts irrespective of tectonic setting three distinct localities mapped in detail display facies consistent with 1 proximal deep water 2 medial to distal deep water and 3 medial shallow water seamount settings the proximal facies in the point lake belt include a 55 m thick non vesicular pillowed sequence cut by numerous mafic dykes and sills dykes contain multiple chilled margins indicating successive magma pulses which contributed to edifice construction abundant feeder conduits in addition to the absence of fragmental facies and vesicles are typical of the central deep water portion of seamounts where growth is initiated the medial to distal deep water facies in the point lake belt are represented by a 30 80 m thick assemblage of disorganized pillow breccia and pillowed and massive flows with 5 27 vesicularity massive non vesicular hyaloclastite intermingled with sedimentary material fluidal peperite in addition to thin shale units interstratified with pillow breccia and hyaloclastite indicate that sedimentation and volcanism were contemporaneous an increase in fragmental units and vesicularity relative to the proximal deep water facies is suggestive of the medial to distal part of a seamount in shallower water bedded tuffs laterally along strike with massive flows are the results of turbidity current deposition immediately following localized subaqueous eruptions a medial shallow water seamount setting is represented in the beaulieu river belt by a 5 85 m thick sequence of vesicular lobate pillowed and massive flows stratified pillow breccia and hyaloclastite and mafic dykes vesicularity ranges from 21 49 in pillowed flows 5 40 in massive flows and 20 35 in pillow breccia and hyaloclastite stratified pillow breccia developed along steep flow fronts in shallow water whereas bedded hyaloclastite formed during reworking and redeposition of autoclastic hyaloclastite on seamount flanks in shallow water the volcanic facies associations in the study areas are analogous to those of modern seamounts associated with the mid atlantic ridge and east pacific rise as well as mesozoic cenozoic seamounts in the canary islands fiji southwest japan the sea of japan and cyprus volcanological studies in the point lake and beaulieu river volcanic belts and subsequent comparisons with phanerozoic analogues demonstrate the manner in which distinct portions of ancient seamounts can be recognized in similar archean terranes

constraints on the composition of the crust and uppermost mantle in northwestern canada vp vs variations along lithoprobe s snorcle transect shear wave seismic data recorded along four profiles during the snore 97 1997 slave northern cordillera refraction experiment refraction wide angle reflection experiment in northwestern canada are analyzed to provide s wave velocity vs models these are combined with previous p wave velocity vp models to produce cross sections of the ratio vp vs for the crust and upper mantle the vp vs values are related to rock types through comparisons with published laboratory data the slave craton has low vp vs values of 1 68 1 72 indicating a predominantly silicic crustal composition higher values 1 78 for the great bear and eastern hottah domains of the wopmay orogen imply a more mafic than average crustal composition in the western hottah and fort simpson arc values of vp vs drop to 1 69 these low values continue westward for 700 km into the foreland and omineca belts of the cordillera providing support for the interpretation from coincident seismic reflection studies that much of the crust fro

the 1991 1996 natmap slave province project introduction the 11 papers contributed to this issue on the geology of the slave province are summarized and where appropriate links between data from the various contributions are emphasized in the discussion we provide a synthesis of the present knowledge base on pre yellowknife supergroup basement rocks throughout the slave province all available age data on mesoarchean and paleoarchean basement rocks are compiled and in a first attempt to seek regional correlations 10 events of crustal growth and or reworking are recognized spanning well over 1 billion years of earth s history the spatial distributions of these events across the craton are discussed finally we pose a series of important questions that remain and that may help guide future research

geology and u pb geochronology of rocks of the eokuk uplift a pre 2 8 ga basement inlier in the northwestern slave province nunavut canada rocks of the eokuk uplift have been mapped in detail along the coast of coronation gulf and 10 key units have been dated by u pb analysis of zircon monazite and titanite the combined data indicate that this inlier of the slave province has a 3 2 ga crustal component evidence of a granulite grade orogenic event predating 2 8 ga and a lack of evidence for any significant orogenic activity corresponding to the 2 7 2 6 ga events common in the rest of the slave province the oldest rocks in the study area are a succession of granitoid and supracrustal gneisses that have been metamorphosed to amphibolite to granulite facies from field relationships the oldest rock is a granodiorite to tonalite orthogneiss with a zircon crystallization age of 3254 13 6 ma a granite gneiss which may be a small felsic intrusion or an anatectic melt of the tonalite gneiss yields a zircon age of 3216 14 13 ma a k feldspar megacrystic monzogranite gneiss contains old discordant possibly inherited zircons with 207pb 206p

modelling u pb discordance in the acasta gneiss implications for fluid rock interaction in earth s oldest dated crust abstract the u pb isotopic system in zircon is the tool of choice to interrogate high temperature geological processes yet this system has potential to investigate lower temperature fluid rock interaction as well in some cases removal of radiogenic pb is incomplete potentially allowing regression of discordant u pb data on a concordia diagram to determine both the age of crystallization and the timing of fluid driven isotopic disturbance however in rocks preserving more complex histories simple regression is not effective at resolving multiple pb loss events here we use a concordant discordant comparison cdc test to establish the times of u pb disturbance in the acasta gneiss complex agc canada agc c 4 03 to c 3 40 ga orthogneisses experienced a long and complex post crystallization history for which u pb zircon data reflects not only the heterogeneous nature of the rock but also the varying degrees and duration of crustal reworking that inevitably involved open system processes the cdc test calculates the similarity between the concordant age structure and a modelled age structure the latter inferred from discordant analyses over a wide range of potential disturbance times our analysis reveals concordant zircon components implying new growth and or recrystallization at 3992 5 3501 6 3442 5 and 3126 6 ma in addition we establish episodes of radiogenic pb loss driven by fluid rock interaction at 3150 50 ma and probably at 2875 50 ma and c 2590 ma these pb loss episodes correlate with previously recognised events recording growth of zircon rims during metamorphism granite emplacement and unroofing pb loss within the agc shows an antithetic relationship in different samples that are in close geographic proximity we suggest that zircon alteration and associated new growth effectively rendered those grains that underwent pb loss at a particular time less susceptible to alteration during the next episode of fluid interaction

lithosphere development in the slave craton a linked crustal and mantle perspective the late tectonic evolution of the slave craton involves extensive magmatism deformation and high temperature low pressure ht lp metamorphism we argue that the nature of these tectonic events is difficult to reconcile with early pre 2 7 ga development and preservation of a thick tectosphere and suggest that crust mantle coupling and stabilization occurred only late in the orogenic development of the craton the extent and repetitiveness of the tectonic reworking documented within the mesoarchean basement complex of the western slave together with the development of large volume extensional mafic magmatism at 2 7 ga within the basement complex argue against preservation of a widespread thick cool mesoarchean tectosphere beneath the western slave craton prior to neoarchean tectonism broad scale geological and geophysical features of the slave craton including orientation of an early f1 fold belt distribution of ca 2 63 2 62 ga plutonic rocks and the distribution of geochemical petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an ne sw structural grain to the craton these trends are oblique to the earlier ca 2 7 ga north south trending boundary between mesoarchean and neoarchean crustal domains and are interpreted to represent a younger structural feature imposed during northwest or southeast vergent tectonism at ca 2 64 2 61 ga extensive plutonism in part mantle derived crustal melting and associated ht lp metamorphism argue for widespread mantle heat input to the crust a feature most consistent with thin 100 km lithosphere at that time we propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the d1 structural grain producing the early 2 63 2 62 ga arc like plutonic rocks subsequent collision external to the present craton boundaries possibly accompanied by partial delamination of some of the underthrust lithosphere produced widespread deformation d2 and granite plutonism throughout the province at 2 6 2 58 ga an implication of this model is that diamond formation in the slave should be neoarchean in age d 2003 elsevier b v all rights reserved

stacked uppermost mantle layers within the slave craton of nw canada as defined by anisotropic seismic discontinuities 1 a 20 station seismic array in nw canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the mohorovicic and lehman discontinuities following wavefield decomposition groups of seismograms were source normalized through simultaneous deconvolution to estimate the near receiver impulse response and thus detect major discontinuities beneath each seismic station stations within the lac de gras kimberlite field display an unusually strong negative impulse on the radial component within the nw quadrant and two moderate impulses on the transverse component forward modeling of these impulses suggests a mantle layer dipping at 22 to the southeast with a mildly anisotropic 2 upper discontinuity at 120 135 km depth and another mildly anisotropic 2 discontinuity at about 170 km depth superimposed on these layers is another stronger anisotropic 4 layer between 110 and 180 km depths that dips to the west stations outside of the lac de gras field but within the southeastern slave craton display more numerous but weaker impulses the most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed farther north the prominent negative impulse observed on the radial component is interpreted to arise from structural preferred orientation in the form of a stockwork of wehrlite dykes beneath the lac de gras field interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000 2900 ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 ma the layer beneath the se slave craton has a similar but distinct tectonic history of nw verging underthrusting associated with the 2635 2615 ma defeat suite of plutonism taken together these interpretations indicate that the slave craton was assembled from at least four lithospheric blocks prior to its cratonization about 2580 ma each block is 90 120 km thick and all four blocks abut across a steep suture boundary beneath mackay lake in the central slave craton significant amounts of carbon and eclogites could have become incorporated into the central slave mantle within the diamond stability field during the proposed underthrusting of lithosphere to explain their relatively common occurrence in kimberlite eruptions within the slave craton

evolution of optical properties of vitrinite sporinite and semifusinite in response to heating under inert conditions abstract the objective of the study was to characterize changes of reflectance reflectance anisotropy and reflectance indicating surface ris shape of vitrinite sporinite and semifusinite subjected to thermal treatment under inert conditions examination was performed on vitrinite liptinite and inertinite concentrates prepared from channel samples of steam coal r r 0 70 and coking coal r r 1 25 collected from seam 405 of the upper silesian coal basin the concentrates were heated at temperatures of 400 1200 c for 1 h time in an argon atmosphere all components examined in this study vitrinite sporinite and semifusinite as well as matrix of vitrinite and liptinite cokes despite of rank of their parent coal show in general the most important changes of reflectance value and optical anisotropy when heated at 500 c 800 c with the exception of bireflectance value of sporinite and 1200 c after heating the steam coal at 1200 c the vitrinite and the semifusinite reveal similar reflectances whereas the latter a slightly stronger anisotropy sporinite and matrix of liptinite coke have lower reflectances but anisotropy r bi and r am values similar to those observed for vitrinite and semifusinite however at 1000 c sporinite and matrix of liptinite coke have the highest reflectivity of the studied components the ris at 1200 c is the same for all components the optical properties of the three macerals in the coking coal become similar after heating at 1000 c coke obtained at 1200 c did not contain distinguishable vitrinite grains at 1200 c semifusinite and vitrinite coke matrix have highest r r values among the examined components maximum reflectance r max reach similar values for vitrinite and sporinite slightly lower for semifusinite matrix of liptinite coke and matrix of vitrinite coke have considerably stronger anisotropy r bi and r am values than other components ris at 1200 c is also similar for all components

stress in western canada from regional moment tensor analysis more than 180 regional moment tensor rmt solutions for moderate sized earthquakes m 4 are used to examine the contemporary stress regime of western canada and provide valuable information relating to earthquake hazard analysis the overall regional stress pattern shows mainly ne sw oriented p axes for most of western canada with local variations in the northern cordillera the maximum compressive stress direction 1 varies from east west to north south to ne sw from south to north the stress direction 1 is consistent with the p axis direction for the largest earthquakes except in the central and northern mackenzie mountains where there is a 16 difference the yakutat collision zone shows a steady change in 1 from east west in the east to north south in the west in the canada united states border region rmt solutions suggest a north south compressional regime may extend through southern british columbia and northern washington to the eastern cordillera in the vancouver island puget sou

geology and geochemistry of an archean mafic dike complex in the chan formation basis for a revised plate tectonic model of the yellowknife greenstone belt an archean mafic dike complex in the chan formation at the base of the mafic volcanic section of the yellowknife greenstone belt consists of multiple metagabbro dikes and sills separated by screens of pillowed mafic volcanic rocks which are cut by a younger postvolcanic metadiabase dike swarm field relationships and geochemical characteristics are compatible with a comagmatic origin for the metagabbro and metavolcanic rocks that were fed through and deposited on an older rift related supracrustal sequence of the dwyer group the synvolcanic metagabbro dikes have extended the strike length of the volcanic section by at least 100 the mafic rocks of the chan formation are geochemically similar to mid ocean ridge basalt possibly with a minor subduction zone component preliminary end values for metagabbroic rocks are consistent with the derivation of magmas predominantly from a normal depleted mantle source the chan formation is interpreted to have formed in a marginal basin like setting adjacent to

timing of plutonism deformation and metamorphism in the yellowknife domain slave province canada u pb zircon titanite and monazite ages for plutonic rocks are used to constrain the timing of post 2660 ma deformational and metamorphic events in the yellowknife domain southern slave province diorite and tonalite plutons of the defeat suite yield ages of 2628 3 and 2624 3 ma older than previous less precise estimates for this suite of ca 2618 2620 ma the defeat suite plutons crosscut and postdate upright f1 folds in turbidites of the ca 2680 2660 ma burwash formation d1 deformation is therefore bracketed between 2660 and 2630 ma the d1 event significantly predates deposition of polymict conglomerates of the jackson lake formation a second folding event f2 represented by northwest to north trending steeply plunging folds of bedding and d1 structures affected the area after intrusion of the hidden lake pluton at 2606 2 ma and synchronously with intrusion of the 2596 2 ma sparrow lake pluton a member of the two mica granite prosperous suite the time of initiation and the duratio

palaeoproterozoic thermal events recorded in the 4 0 ga acasta gneiss canada evidence from shrimp u pb dating of apatite and zircon u pb isotopes of apatites and zircons extracted from a sample of the acasta gneisses that occurs along the western margin of the slave craton in northwest territories of canada have been measured using a shrimp ii ion microprobe eleven apatite analyses 30 m spot give 238u 204pb 206pb 204pb and 204pb 206pb 207pb 206pb isochron ages of 1905 86 ma 2 and 1936 28 ma 2 respectively analyses 20 m spot on the partly recrystallised central portions of fourteen zircons mostly define a mixing array on a 238u 206pb 207pb 206pb concordia plot with concordia intercepts at 4014 25 ma 2 and 1967 93 ma 2 the former agrees with 3 96 4 02 ga protolith ages obtained on the oldest components of the acasta gneisses by other workers the lower concordia intercept for the zircon data is consistent with the u pb age of apatites and may be related to a 500 c thermal event perhaps early in the palaeoproterozoic wopmay orogeny