the composite north american craton superior province deep crustal structure and mantle plume model of neoarchaean evolution abstract this paper aims to synthesize geological and geophysical data on superior province and in general archean crust of the composite north america craton relying on experience with deep geological and geophysical studies of composite east european craton by onset of the neoarchean archean provinces of the north american craton were parts of the single stable continent intensely reworked in the neoarchean crustal rocks were combined into a round oval domain wherein distribution of geophysical structural and metamorphic parameters displays concentric zoning the central zone occupies hudson bay basin located immediately above the lithospheric keel the internal zone is mainly composed of granulite facies plutonic volcanic and sedimentary rocks the external zone encompasses southern part of the superior province together with hearne and rae provinces this paper presents 3d crustal models of southern superior province obtained through synthesis of geological maps with a new geological interpretation of seismic sections along lithoprobe geotransects and of the eastern abitibi belt based on higher resolution reflection seismic images neoarchean evolution of the north american craton represents plate tectonic processes initiated by a superplume it is only one of series of similar phenomena recorded 2 75 ga ago in a number of regions the most important features repeated to a certain degree in tectonic units of this type are 1 synchronous formation between 2 79 and 2 58 ga 2 frequently repeated combination of high grade granulite and high temperature amphibolite facies and low or moderate grade greenschist and epidote amphibolite facies metamorphic rocks 3 lower crustal granulite basaltic layer reflectivity zone displayed in seismic images of crust formed or transformed at the final stage of endogenic activity

stratigraphy and radiocarbon chronology of a subaerially exposed holocene coral reef dominican republic a subaerially exposed coral reef records a period of marine conditions following earliest holocene transgression into the technically active subsea level enriquillo valley dominican republic field mapping detailed leveling of reef elevations and i4c dating of fossil corals and mollusks reveal the following chronology of reef development in an unusual geologic and tectonic environment 1 earliest holocene 9760 pm 100 yr b p marine transgression and growth of oysters on an alluvial fan surface approximately 33 m below sea level bsl 2 colonization of reef corals at approximately 34 m bsl on the oyster shell deposits 8990 pm 60 yr b p 3 middle holocene development of a typical caribbean fringing reef in a protected environment 6200 pm 80 to 4760 pm 90 yr b p 4 late holocene sedimentation over the reef with fossil bivalves 5710 pm 90 to 2820 pm 40 yr b p that record a gradual change from marine to brackish water that accompanied growth of shoreline th

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

comparison of esr ages of corals from marine terraces with 14c and 230th 234u ages the ages of corals from marine terraces in the ryukyu islands of japan have been determined by electron spin resonance esr esr ages deduced from the total dosage of natural radiation taking the time dependent dose rate due to 238 u series disequilibrium into account agreed fairly well with those determined by 14 c and 230 th 234 u dating the esr age can be extended well beyond the limits of 14 c dating 50 000 years and the 230 th 234 u method 300 000 years to a million years

evidence for a widespread mafic cover sequence and its implications for continental growth in the northeastern superior province abstract archean greenstone belts of the northeastern superior province nesp were emplaced over a 160 ma period 2 88 2 72 ga spanning a major episode of crustal reworking in which early tonalite trondhjemite plutonism evolved to dominant granite granodiorite and pyroxene bearing felsic plutonism the numerous greenstone belts that crop up across the craton contain lavas belonging to three mafic volcanic suites 1 mg tholeiites that have chemical compositions typical of many archean basalts with 4 10 wt mgo 9 15 wt fe2o3t 0 4 1 2 wt tio2 and la sm ratios between 1 and 3 2 fe tholeiites that have similar mgo contents and la sm ratios but markedly higher fe2o3t 11 20 wt tio2 1 0 2 6 wt and gd yb ratios that may reflect derivation from a distinct garnet bearing mantle source 3 light rare earth element lree enriched rocks that have the chemical characteristics typical of calc alkaline mafic magmas with higher sio2 al2o3 and la sm ratios but lower fe2o3t and tio2 than the tholeiitic suites at a given mgo content the presence of unconformities within single greenstone belts numerous inheritance ages in volcanic rocks replicating those of older volcanic plutonic assemblages and the recognition of widespread geographically separate yet coeval 2 78 ga extrusive assemblages containing the distinctive fe tholeiites do not support a plate tectonic model for a large portion of the nesp the correlations in composition and ages of greenstone belts are more consistent with a model in which they represent the remnants of an extensive autochthonous mafic cover sequence nd isotopic evidence for the presence of evolved crust in some syn volcanic felsic plutons and volcaniclastic rocks imply that this mafic cover sequence was erupted onto an older granitoid crust the amalgamation of two isotopically distinct terranes at ca 2 76 2 74 ga created a proto cratonic mass that consisted of a composite tonalite trondhjemite greenstone crust underplating by mafic mantle melts and widespread insulation after 2 75 ga increased temperatures sufficiently to produce extensive melting of this crust to generate pyroxene bearing felsic rocks and granodiorite to granite partial melts the isotopically enriched character of mantle derived magmas after 2 75 ga may reflect more extensive contamination by a felsic crust affected by widespread partial melting furthermore a decrease of nb th ratios in the lree enriched mafic rocks younger than 2 75 ga reflects a change in the nature of the crustal contaminant from tonalite trondhjemite prior to 2 75 ga to granite granodiorite afterwards

structural analysis and metamorphism of the barlow fault zone chibougamau area neoarchean abitibi subprovince implications for gold mineralization faults and deformation zones are important features of archean terranes because of their significance for structural evolution and the formation of large gold districts in the chibougamau area no

gold mineralization patterns in relation to the lithologic and tectonic evolution of the chibougamau mining district quebec canada the chibougamau mining camp a major gold producer has so far produced a total of 1 050 mt of gold at an average grade of 1 85 g tonne in contrast to a general tendency to group the deposits as vein type with varying compositions this paper demonstrates the strong influence of regional lithologic and tectonic evolution of the area on the gold mineralization patterns a syn volcanic period of mineralization comprises both the volcanogenic massive sulphide and disseminated mineralization and the subsequent epithermal mineralization the latter is also associated with evolving volcanic landforms and syn volcanic intrusions the emplacement of the typical archean lode gold deposits and later cu au deposits of uncertain origin coincides with the syn deformational kenoran period the spatial relationship between the east west trending shear zones and a north east trending fault system is shown to be a possible mechanism for generating added dilatancy during the gold mineralizing phase a late to post kenoran shear system controlled the emplacement of the last major phase of gold mineralization within a stratiform intrusion

open system evolution versus source control in basaltic magmas matachewan hearst dike swarm superior province canada the 2 45 ga matachewan hearst dike swarm was emplaced over 250 000 km2 in diverse granitoid greenstone and metasedimentary terranes of the superior province of canada the fe rich tholeiitic dikes

petrologie des roches volcaniques du sillon de roches vertes archeennes de matagami chibougamau a l ouest de chapais abitibi est quebec 1 le groupe basal de roy in the northeastern part of the abitibi orogenic belt the archean matagami chibougamou greenstone belt 2700 ma includes a basal volcanic sequence named the roy group unconformably overlain by a

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

a review of the superior province of the canadian shield a product of archean accretion abstract the superior province consists of northern and southern high grade gneiss subprovinces and a central region of alternating granite greenstone and metasedimentary belts subprovinces are commonly fault bounded and display contrasting lithological assemblages metamorphic and structural styles geophysical characteristics and ages they are comparable to phanerozoic suspect terranes the middle and late archean rocks of the superior province are of mantle and juvenile crustal origin with little evidence of inheritance of components older than ca 3 1 ga high grade gneiss subprovinces are mainly deeper level equivalents of the supracrustal rich belts but one minnesota river valley is part of an older ca 3 6 ga terrane that was juxtaposed late in the tectonic history granite greenstone terranes are characterized by low grade volcano sedimentary sequences tholeiitic komatiitic submarine lava plain tholeiitic calc alkalic submarine to subaerial central complexes early platform type quartz arenite stromatolitic carbonate late timiskaming type shoshonitic alkalic volcanic fluvial sediment that range in age from ca 3 0 ga to 2 7 ga the volcanics represent oceanic island arc and continental arc volcanism the quartz arenite carbonate sequences deposition on relatively stable early formed ca 3 0 ga crust and the timiskaming type sequences deposition in pull apart basins developed along strike slip faults late archean ca 2 75 2 70 ga greenstone sequences occur throughout the superior province middle archean ca 3 0 2 8 ga sequences are common in northern and central superior province rare or absent in the south volcanism was accompanied by mafic to trondhjemitic plutonism the metasedimentary belts consist of variably metamorphosed volcanogenic turbidites with detrital zircons ranging in age from ca 3 0 ga to 2 7 ga the metasediments were deposited after major volcanism and probably represent accretionary prisms late archean orogenesis accompanied and followed by calc alkalic plutonism youngs southward from about 2 73 2 72 ga to 2 70 2 68 ga it involved large scale north south compression and dextral transpression and displays thin to thick skinned transitions the contemporaneity of events along the lengths of the belts and their southward younging coupled with the evidence of major compression are consistent with subduction driven oblique accretion of oceanic and continental volcanic arcs accretionary sedimentary wedges older microcontinental fragments etc in a convergent margin setting analogous to parts of the pacific rim differences eg abundance of komatiites and tonalitic plutonic suites are attributable to a hotter archean mantle

evaluation and application of the ion microprobe in the strontium isotope geochemistry of carbonates abstract a modified aei im20 ion microprobe has been used to measure 87 sr 86 sr ratios in carbonates a suite of carbonates with varying major elements ca mg fe mn was studied at low and high m m 3000 mass resolution to determine the types and intensities of molecular species isobaric with sr peaks sr data collected at low mass resolution must be corrected for ca 2 and camgo species rb sr ratios are extremely low and correction for 87 rb is not required 87 sr usable sr isotopic data may be obtained from calcite given sr 400ppm and for sr 5000 ppm a precision of 1 0 0007 in 87 sr 86 sr 2 mean can be achieved under optimum conditions the corrections for ca 2 and camgo are smaller than the within run precision in calcite but in dolomite the correction for camgo is 1 5 mass fractionation corrections to 87 sr 86 sr based on 86 sr 88 sr 0 1194 are typically 8 to 10 good agreement between ion probe and solid source mass spectrometer results was found for calcites of known sr isotopic composition st4a 400ppm sr average ion probe 87 sr 86 sr 0 7267 0 0015 solid source 87 sr 86 sr 0 72680 0 00008 14 jcg36 6000ppm sr average ion probe 87 sr 86 sr 0 7056 0 0009 solid source 87 sr 86 sr 0 70588 0 00009 16 jcg44 6000ppm sr average ion probe 87 sr 86 sr 0 7057 0 0006 solid source 87 sr 86 sr 0 70540 0 00008 16 the ability of the ion microprobe to measure 87 sr 86 sr for 10 m spots in calcite was used 1 to measure variation in 87 sr 86 sr of 0 01 on a centimetre scale in a hydrothermally altered basalt from the isle of skye northwestern scotland and 2 to determine the sr isotopic composition of tiny 87 sr 86 sr

235u fission tracks and238u series disequilibria as a means to study recent mobilization of uranium in archaean pyritic conglomerates uranium transport in archaean sediments was investigated by means of two carefully selected suites of variably weathered sediment samples from different localities of the pongola supergroup kaapvaal craton south africa u remobilization on a micron to meter scale was analyzed using u fission track micromapping and spectrometry bulk measurements these studies were supported by ore microscopy neutron activation analysis and spectrometry by combining particle track technique and spectrometry detailed information is obtained on the u transport history of weathered surface samples a simple model of u redistribution processes in geologic systems based on238u series disequilibria is developed and applied to the samples investigated some implications for radiometric field methods are discussed

magmatic affinity of modern and ancient subalkaline volcanic rocks determined from trace element discriminant diagrams when dealing with ancient subalkaline volcanic rocks the alkali total iron magnesium afm diagram is of limited use in assigning a tholeiitic versus calc alkaline affinity because these eleme

87sr 86sr in kimberlitic carbonates by ion microprobe hydrothermal alteration crustal contamination and relation to carbonatite carbonates in a 30 cm wide zoned kimberlite dyke from the de beers mine kimberley s africa were studied by cathodoluminescence and electron microprobe techniques and their 87sr 86sr ratios were measured using an aei im20 ion microprobe primary carbonates including calcite dendrites rhombohedral calcites in segregation vesicles and mosaic dolomite have high sr 0 69 1 35 wt sro and ba 0 24 0 44 bao and 87sr 86sr ratios in the range 0 7046 to 0 7056 secondary sparry calcite in amygdales and veins is characterised by low ba 0 05 bao and 87sr 86sr near 0 72 rhombohedral calcite 0 5 cm from a contact with 2 900 my old biotite gneiss has minor element chemistry like that of primary carbonate but an elevated 87sr 86sr ratio of 0 7103 possibly indicating crustal contamination in a boundary layer of the kimberlite magma amygdale like segregations of carbonate and or serpentine originated as gas cavities and were not formed by liquid immiscibility they are now filled either by secondary calcite or by minerals precipitated from residual kimberlite liquid however dendritic calcite and primary dolomite and calcite with high sr ba and low 87sr 86sr demonstrate shared chemical characteristics between these carbonates and carbonatite the primary kimberlite magma had initial 87sr 86sr close to 0 7046

the coulon deposit quantifying alteration in volcanogenic massive sulphide systems modified by amphibolite facies metamorphism the coulon deposit is a volcanogenic massive sulphide vms system in the james bay area superior craton quebec that was metamorphosed to amphibolite facies conditions the chemistry and mineralogy of the vms related alteration halo proximal to the mineralized sulphide lenses are investigated using samples collected in the field and 5583 chemical analyses provided by osisko ltd alteration is quantified using mass balance and normative calculations and the application and performance of these methods in an exploration context are investigated in vms systems altered rocks proximal to the ore zones are characterized by multi element metasomatism which is best quantified by mass balance methods that have been successfully applied in the study area however mass balance calculations necessitate the documentation of a precursor which is not always possible in an exploration context therefore an alternative method i e alteration indices was also evaluated in most vms systems proximal alteratio

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

lead isotope zoning in galena an ion microprobe study of a galena crystal from the buick mine southeast missouri ion microprobe techniques have been used to determine the variations of pb isotope ratios within a single octahedral crystal of galena from the buick mine southeast missouri the crystal shows uniform concentric zonation of pb isotope ratios with a total variation of approximately 3 percent in 208 pb 206 pb and approximately 4 percent in 207 pb 206 pb and with the crystal margin being less radiogenic lower 206 204 than the core the range in the observed 208 pb 206 pb ratio is larger than that observed previously for bulk galena analyses from throughout the mine and is almost as large as the total variation reported for the whole southeast missouri ore district the ion probe results are demonstrated to be accurate to 0 1 0 2 percent by comparison with conventional mass spectrometric analyses performed on microsamples of the same crystal the ion probe technique holds promise for establishing a pb isotope chronostratigraphy for the ore bearing solutions which will be useful in delineating temporal and spatial aspects of the fluid circulation patterns

an experimental study of the influence of oxygen fugacity on fe ti oxide stability phase relations and mineral melt equilibria in ferro basaltic systems

conditions and timing of metamorphism in the southern abitibi greenstone belt quebec regional metamorphism ranging in grade from the subgreenschist facies to the greenschist amphibolite facies transition affects all archean supracrustal rocks 2677 ma in the rouyn noranda area contact metamorphic minerals associated with the posttectonic preissac lacorne batholith 2643 ma show no evidence of a regional retrograde event accordingly the age of regional metamorphism can be bracketed between 2677 and 2643 ma three reaction isograds were mapped in subgreenschist facies metabasites dividing the low grade rocks into three metamorphic zones the pumpellyite actinolite zone the prehnite pumpellyite zone and the prehnite epidote zone in addition the pumpellyite actinolite epidote quartz bathograd corresponding to a pressure of approximately 200 mpa occurs on both sides of the porcupine destor fault low pressure regional metamorphism is also indicated both by the occurrence of an actinolite oligoclase zone and the persistence of pre regional metamorphic andalusite the coincidence

the ion microprobe as a tool for obtaining strontium isotopes in magmatic plagioclase a case study at okataina volcanic centre new zealand abstract we investigated the potential of multi collector secondary ion mass spectrometry mc sims as a tool for obtaining sr isotopic compositions in plagioclase a ubiquitous mineral in igneous rocks that serves as a recorder of crystallization history mc sims allows for high spatial resolution analysis 12 m in this study of isotopes and therefore improves the temporal scale at which fluctuations in crystallization conditions can be recognized ultimately improving our understanding of rates of magmatic processes plagioclase crystals from two young rhyolitic deposits from two major eruptive complexes tarawera and haroharo of the okataina volcanic centre in new zealand were analysed results were corrected for matrix effects using linear modelling of mc sims data versus an contents as well as 87sr 86sr ratios acquired via laser ablation inductively coupled plasma mass spectrometry la mc icp ms corrected mc sims sr isotopic ratios had an average 2 uncertainty of 0 0008 per spot and were homogeneous in okataina plagioclase at high spatial resolutions average la mc icp ms 87sr 86sr ratios of plagioclase from both intra caldera volcanic complexes tarawera 87sr 86sr 0 7056 and haroharo 87sr 86sr 0 7054 suggest similar magma sources and similar assimilation and fractional crystallization processes for the two complexes overall homogeneity of plagioclase excluding relict cores indicates no significant changes in contributions i e crustal assimilation mafic influx to the system during the majority of plagioclase crystal growth furthermore lack of 87sr 86sr ratio fluctuations in plagioclase rims suggest interaction between the resident silicic magma and the intruding mafic magma that triggered the eruptions was largely limited to volatile and heat transfer using appropriate standards and analysis this mc sims method can be used to detect short lived open system events in magma reservoirs where differences in 87sr 86sr isotopic ratios are significant

oxygen isotopes sea level changes and the temperature history of a coral reef environment in new guinea over the last 105 years abstract coral reef terraces consisting of an upward succession of fringing and barrier reef types are preserved on land along the raising coast of huon peninsula new guinea seven coral reef units i vii from the coast landwards spaced at about 20 kyr interval provide a rather complete record of sea levels oxygen isotopes and temperature of teh surface tropical ocean over the duration of the high sea level events of the last glacial cycle the systematic sampling of the coral reefs was restricted to the massive tridacna gigas species the giant clam which are about 0 7 0 enriched in 18o relative to calcite equilibrium value and the presence of symbionts seems to have no effect on the 18o of their aragonitic skeleton sea levels rose rapidly to 5 6 5 m above present during the last interglacial which on stratigraphic grounds and 18o compositions can be separated into an early phase whose 18o is similar to modern reef viia dated at 138 kyr and a late phase that is 0 6 0 heavier reef viib dated at 118 kyr fringing reefs vi v iv iii and ii record short lived high sea levels stands at 107 85 60 45 to 40 and 31 kyr respectively that stopped short of the present sea level 12 19 28 32 to 42 m the general decline of the high sea level stands corresponds to an average 18o enrichment of 1 7 0 measured between the early interstadials of isotope stage 5 vi and v and the late interstadials of isotope stage 3 iv iiia iiib and ii the maximum 18o enrichment of 1 7 0 relative to present was registered during a regressive phase of reef iiia stratigraphic evidence indicates that sea level lows in the range of 37 to 55 m below present have occurred in between the high sea level stands but those are unlikely to represent the lowest sea levels of the last glacial cycle sea levels and 18o ddta pair offer important clues regarding the partitioning of ice volume and temperature in 18o records the results suggest that the ice volume effect accounts for the observed 18o changes during the early interstadial culminations but only for 29 46 during the late interstadial culminations sea water cooling beyond the threshold limit of coral reef growth 18 c did not occur during the last glacial cycle the temperature of the surface tropical ocean was similar to present during the early interstadials of isotope stage 5 and cooler by 3 c below present during the later interstadials of isotope stage 3

petrogenesis and economic potential of the obatogamau formation chibougamau area abitibi greenstone belt unravelling the petrogenesis and stratigraphy of archean mafic lava flows is essential to our comprehension of the early geodynamic evolution and economic potential of greenstone belts this study

differentiation of ferro basaltic magmas under conditions open and closed to oxygen implications for the skaergaard intrusion and other natural systems because processes such as fractional crystallization and crystallization under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystallization behaviour comparison of experimental data on an iron rich basaltic composition with predictions of the melts free energy minimization algorithm shows that although liquidus temperatures and silicate mineral equilibria are predicted relatively well the saturation of fe ti oxides is not we have used the same experimental data to construct an alternative crystallization model based on known equilibrium phase relations mineral melt partitioning of major elements and mass balance constraints the model is used to explore the consequences of equilibrium and fractional crystallization in systems open and closed to oxygen liquid lines of descent for perfect equilibrium and perfect fractional crystallization are predicted to be very similar in a system open to oxygen the model predicts that magnetite saturation leads to strongly decreasing iron and increasing silica contents of residual liquids whereas systems closed to oxygen crystallize less abundant magnetite leading to a less pronounced iron depletion in the liquid predicted bulk solid compositions and variations offo with falling temperature agree well with those observed or inferred from the cumulates of the skaergaard intrusion but none of the predicted liquid lines of descent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations compositional factors such as water and phosphorus are not thought to be the source of the discrepancy as the cumulates of the basistoppen sill which closely resemble those of skaergaard may be used to calculate a liquid line of descent in agreement with that predicted by the model for fractional crystallization closed to oxygen a comparison of the predicted t io paths and liquid lines of descent with those inferred from natural systems suggests that volcanic centres such as iceland and hawaii evolve under conditions open to oxygen whereas evidence from plutonic environments e g skaergaard and kiglapait layered intrusions suggests that they evolved under conditions more closed to oxygen the compositional evolution of the melt phase in volcanic and plutonic systems may therefore be different although the results of this study suggest that magnetite saturation will limit fe enrichment in all environments to 20wt feo consistent with enrichments reported for volcanic glasses

anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust abstract the sept iles layered intrusion quebec canada is dominated by a basal layered series made up of troctolites and gabbros and by anorthosites occurring 1 at the roof of the magma chamber 100 500 m thick and 2 as cm to m size blocks in gabbros of the layered series anorthosite rocks are made up of plagioclase with minor clinopyroxene olivine and fe ti oxide minerals plagioclase displays a very restricted range of compositions for major elements an 68 an 60 trace elements sr 1023 1071 ppm ba 132 172 ppm and sr isotopic ratios 87 sr 86 sr i 0 70356 0 70379 this compositional range is identical to that observed in troctolites the most primitive cumulates of the layered series whereas plagioclase in layered gabbros is more evolved an 60 an 38 the origin of sept iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts the density of the feo rich tholeiitic basalt parent magma first increased from 2 70 to 2 75 g cm 3 during early fractionation of troctolites and then decreased continuously to 2 16 g cm 3 with fractionation of fe ti oxide bearing gabbros plagioclase an 69 an 60 was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite with further differentiation plagioclase 60 became negatively buoyant and anorthosite stopped forming blocks of anorthosite autoliths even fell downward to the basal cumulate pile the presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and or minor plagioclase nucleation within the main magma body dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid the processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust with gabbro noritic rocks at the base and anorthositic rocks at the top we propose that the lunar crust mainly crystallized bottom up this basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian suite without kreep contamination while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust

plume and arc magmatism in the abitibi subprovince implications for the origin of archean continental lithospheric mantle abstract based on eclogitic mantle xenoliths that appear to be the residue of slab melts it is commonly argued that the formation of distinctive lithospheric keels beneath archean cratons occurred by the tectonic accretion of oceanic crust these models are complicated by evident buoyancy of the keels the varied compositions of the mantle xenolith suite as a whole and a compositional constraint that a large portion of the keels originated in mantle plume melting events resolution of these issues is complicated by the long development history of most cratons 100 s of m y and the multiple cycles of magmatism they represent in contrast the single 50 m y period of arc volcanism in the abitibi subprovince and the immediate onset of orogeny and craton stabilisation avoids many of these uncertainties chemical and isotopic data demonstrate that syn volcanic to syn tectonic adakitic abitibi granitoids were derived via slab melting deep seismic reflection profiles image singular slab like features along the moho these are interpreted as the remnants of oceanic slabs emplaced and torn during orogeny nonetheless their presence directly contradicts subcretion models of keel formation because they occupy regions that should correspond to the top of an earlier formed subcreted keel post adakite shoshonitic granitoids and lamprophyres also attest to the presence of a metasomatised mantle wedge throughout the entire period of abitibi arc magmatism diamond bearing lamprophyres in the abitibi and adjacent wawa subprovince are compositionally equivalent to paleoproterozoic diamondiferous rocks and characteristic of adakite generating shallow subduction rather than evidence of pre existing sub cratonic keels abitibi mantle plume and subduction related volcanism also demonstrates the continuous presence of shallow asthenospheric mantle beneath the belt over a 50 m y period komatiites and then boninites erupted along the central part of the belt halfway through its evolution and rifted arc style volcanism dominated the belt within a few m y of orogeny none of these events are consistent with the development of a progressively thickened continental lithospheric keel the geodynamic evolution of the abitibi belt demonstrates a fact recognised in some xenolith studies the characteristic association of deep keels and archean cratons was not developed gradually during greenstone belt volcanism rather it was initiated by the coupling of shallow imbricated plume and arc related crust with the buoyant residue of mantle plume melting that rose from depths of 80 km to 400 km subduction was active along the southern and eastern margin of the superior province craton before and after ascent of the abitibi plume renewal of subduction in the abitibi as marked by the occurrence of the 2716 ma boninitic volcanic suite delayed the ascent of the deep keel and its coupling to the composite crust final ascent of the plume residue occurred following orogeny and slab loss and may have included entrainment of fragmented slab residue into the rising diapir the coupling of shallow crust and deep keel occurred soon after and the keel s presence beneath the subprovince is recognised by phanerozoic diamond deposits xenolith chemistry and geophysical data

high field strength element ratios in archean basalts a window to evolving sources of mantle plumes abstract in terms of high field strength element ratios nb th zr nb nb y and zr y most basalts from non arc type archean greenstones are similar to oceanic plateau basalts suggestive of mantle plume sources a large number of these basalts have ratios similar to primitive mantle composition perhaps the archean mantle was less fractionated than at present and primitive mantle comprised much of the deep mantle and made a significant contribution to mantle plumes the near absence of archean greenstone basalts similar to nmorb in composition is also consistent with a relatively unfractionated mantle in which a shallow depleted source dm was volumetrically insignificant the element ratios in basalts also indicate the existence of recycled components himu em1 em2 in the mantle by the late archean this suggests that oceanic lithosphere was recycled into the deep mantle and became incorporated in some mantle plumes by the late archean high field strength element ratios also indicate an important contribution of continental crust or and subcontinental lithosphere to some non arc archean greenstone basalts this implies that at least thin continental lithosphere was relatively widespread in the archean

petrogenetic and geodynamic origin of the neoarchean dore lake complex abitibi subprovince superior province canada the neoarchean ca 2728 ma anorthosite bearing dore lake complex in the northeastern abitibi subprovince quebec was emplaced into an association of intra oceanic tholeiitic basalts and gabbros known as the obatogamau formation the obatogamau formation constitutes the lower part of the roy group which is composed of two cycles of tholeiitic to calc alkaline volcanic and volcaniclastic rocks siliciclastic and chemical sedimentary rocks and layered mafic to ultramafic sills in this study we report major and trace element results and nd sr pb and o isotope data for anorthosites leucogabbros gabbros and mafic dykes from the dore lake complex and spatially associated basalts and gabbros of the obatogamau formation to assess their petrogenetic origin and geodynamic setting field and petrographic observations indicate that the dore lake complex and associated volcanic rocks underwent extensive metamorphic alteration under greenschist facies conditions resulting in widespread epidotization 20 40 and chloritization 10 40 of many rock types plagioclase recrystallized mainly to anorthite and albite endmembers erasing intermediate compositions metamorphic alteration also led to the mobilization of many elements e g lile and transition metals and to significant disturbance of the rb sr and u pb isotope systems resulting in 1935 150 and 3326 270 ma errorchron ages respectively the sm nd isotope system was less disturbed yielding an errorchron age of 2624 160 ma on many binary major and trace element diagrams the least altered anorthosites and leucogabbros and the gabbros and mafic dykes of the dore lake complex plot in separate fields signifying the presence of two distinct magma types in the complex the gabbros and mafic dykes in the dore lake complex share the geochemical characteristics of tholeiitic basalts and gabbros in the obatogamau formation suggesting a possible genetic link between the two rock associations initial end 2 6 to 5 0 and 18o 6 1 to 7 9 values for the dore lake complex and gabbros of the obatogamau formation end 2 8 to 4 0 18o 7 3 to 8 0 are consistent with depleted mantle sources all rock types in the dore lake complex and the roy group share the trace element characteristics of modern arc magmas suggesting a suprasubduction zone setting for these two lithological associations on the basis of regional geology and geochemical data we suggest that the dore lake complex and the obatogamau formation represent a dismembered fragment of a suture zone like many phanerozoic ophiolites resulting from closure of a back arc basin between 2703 and 2690 ma

precise timing of the last interglacial period from mass spectrometric determination of thorium 230 in corals the development of mass spectrometric techniques for determination of 230 th abundance has made it possible to reduce analytical errors in 238 u 234 u 230 th dating of corals even with very small samples samples of 6 x 10 8 atoms of 230 th can be measured to an accuracy of 3 percent 2 and 3 x 10 10 atoms of 230 th can be measured to an accuracy of 0 2 percent the time range over which useful age data on corals can be obtained now ranges from about 50 to about 500 000 years for young corals this approach may be preferable to 14 c dating the precision with which the age of a coral can now be determined should make it possible to critically test the milankovitch hypothesis concerning pleistocene climate fluctuations analyses of a number of corals that grew during the last interglacial period yield ages of 122 000 to 130 000 years the ages coincide with or slightly postdate the summer solar insolation high at 65 n latitude which occurred 128 000 years ago this supports the idea that changes in pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth s orbit and rotation axis

mineral deposits of canada a synthesis of major deposit types district metallogeny the evolution of geological provinces and exploration methods wayne d goodfellow editor 1 068 pp geological association of canada mineral deposits division special publication no 5 2007 hardcover isbn 13 978 1 897095 24 9 price members cdn 60 00 non members cdn 80 00 following in the footsteps of other landmark reviews of major ore producing countries such as australia hughes 1990 solomon and groves 1994 and south africa anhaeusser and maske 1986 this substantial 1 000 pages monograph is a great achievement that should be in every geologic research library in the world in many ways it sets a new standard most notably in that all figures and photographs are in color and that the entire volume is available in electronic form both as an included dvd and mostly on the gac website it is common in such compendia to have a spectacular cover photo or frontispiece to catch the reader s attention and this book does not disappoint the frontispiece photo is of a 61 8 kg sample from the dome mine timmins ontario and is stated to contain 20 wt percent gold at today s prices the gold alone would be worth 1 5 million dollars

crustal evolution and the temporality of anorthosites abstract types of rock such as komatiite which formed entirely or dominantly during restricted time periods in solar system history are important indicators of how planetary bodies evolved in deep time in this paper we characterize three different types of temporally restricted anorthosites and discuss their significance to the broad scale evolution of planetary processes primordial anorthosites which constitute the bulk of the lunar crust were sampled during apollo and luna missions and have subsequently been identified in our meteorite collections they are characterized by very calcic plagioclase an93 98 and all have magmatic crystallization ages 4 3 ga suggesting that the earliest planetary crust forming processes on the moon and possibly elsewhere involved substantial if not total melting the magma ocean hypothesis which has endured for nearly 50 years argues that the lunar anorthositic rocks represent global flotation cumulates of plagioclase which crystallized after extensive precipitation and sinking of olivine and pyroxene another possibility revitalized by critical filtering of age data involves serial magmatism whereby the lunar crust was constructed by younger smaller episodic magmatic events on earth two distinct types of temporally restricted anorthosites offer opportunities to understand the evolution of terrestrial geodynamics tectonics and magmatic processes archean megacrystic anorthosites formed only between 3 73 and 2 49 ga as small bodies

iib trioctahedral chlorite from the barberton greenstone belt crystal structure and rock composition constraints with implications to geothermometry iib trioctahedral chlorite in the barberton greenstone belt bgb metavolcanic rocks was formed during pervasive greenschist metamorphism the chem ical composition of the chlorite is highly variable with the fe fe mg ratio ranging from 0 12 to 0 8 among 53 samples the chemical variation of the chlorite results from the chemical diversity of the host rock especially the mgo content of the rock but major details of the variation pattern of the chlorite are due to the crystal structure of the chlorite all major cation abundances in the chlorite are strongly correlated with each other sil icon increases with mg and decreases with fe while aliv and alvi decrease with mg and increase with fe2 a complex exchange vector explains over 90 of the chlorite compositional variation mg4sife2 3alvi 1 aliv 1 which has 3 parts fe mg substitution coupled with one part tschermakite substitution this ratio is required to maintain the charge and site balances and the dimensional fit between the tetrahedral and octahedral sheets the subtle change in al substitution in chlorite implies that alvi is preferentially ordered in the m 4 site and about 84 of the alvi present is in the m 4 sites when they are nearly filled with alvi based on 47 analyzed chlorite bearing rock samples chlorite chl composition is strongly correlated with the mgo content of the host rock calculated correlation coefficients are 0 91 for sio2chl mgorock 0 87 for al2o3chl mgorock 0 89 for mgochl mgorock and 0 85 for feochl mgorock only weak correlations have been found between chlorite oxides and other oxides of rock between same oxides in chlorite and rock sio2 0 67 al2o3 0 59 feo 0 41 however mgochl is saturated at about 36 wt in rocks that have mgo above 22 wt the mgochl is about 5 wt when the host rock approaches 0 wt of mgo this implies that mg substituting into the chlorite is approximately limited to 1 5 9 2 mg atoms per formula unit and 1 0 3 2 aliv chlorite geothermometers can not be applied to all bgb samples however the empirical chlorite geothermometer based on aliv of chlorite may be applicable to chlorites formed under metamorphic conditions because it can predict the chemical composition of the chlorite from basaltic and dacitic samples in this study an estimated temperature of about 320 c for the greenschist metamorphism of the greenstone belt through this geothermometer is consistent with that obtained by other geothermometers

hydrothermal alteration zoning and gold concentration at the kerr addison archean lode gold deposit kirkland lake ontario kerr addison a major archcan lode gold deposit of the abitibi belt with a production of approximately 10 x 10 6 oz au approximately 320 metric tons up to 1983 is localized in a zone of talc carbonate and chlorite carbonate rocks immediately south of a regional structural break the kirkland lake larder lake fault zone relict volcanic textures taken in conjunction with the ratios of diag nostic relatively less mobile elements collectively indicate that most of the wall rocks of the mine were originally mafic and ultramafic rocks along with their clastic derivatives regional greenschist facies metamorphism is locally superposed by a distinct zonation of gold associated hydrothermal alteration characterized by massive introduction of co 2 coupled with variable enrichments of alkali metals and calcium which produced systematic trends of alteration mineral assemblages alteration types of the initial stage contain chlorite which is replaced by muscovite or albite in the advanced stage of alteration with a concomitant increase in the proportion of carbonate minerals numerous generations of crosscutting veins are enveloped by alteration borders in which the sequence of the al bearing minerals namely albite mus covite and chlorite develop from the vein to the wall rocks always in a systematic sequence the same sequence is observed at a larger scale from the ore zone to the periphery of the mine area the intensity of alteration can be monitored by saturation indices co 2 ca fe mg for carbonates and 3k na al 3k al and na al for muscovite albite muscovite and albite respectively molar ratios plots of the moving averages of these indices across the 3 850 ft level of the mine reveal a remarkable correspondence to variations of gold abundance suggesting a genetic relationship of gold enrichment to hydrothermal alteration on a broad scale gold is concentrated to the south of the kirkland lake larder lake fault zone and at a more detailed scale in the transition zone between carbonate muscovite and carbonate albite alteration types during advanced alteration fluid x sub co 2 was approximately 0 1 and na k asymp 10 volcanics of the kinojevis and larder lake groups are 18 o enriched relative to pristine tholeiites and possess distinct mineral delta 18 o values probably due to different conditions of marine water alteration during or shortly after extrusion in domains of advanced alteration these are all shifted down to a common narrow bracket here 18 o 16 o and d h data indicate fairly constant temperatures 270 degrees 300 degrees c and a common isotopic composition of hydro thermal fluids delta 18 o 5ppm delta d 30 to 50ppm across the alteration zoning these results are in common with many other archcan lode gold deposits fe dolomite has a uniform delta 13 c value of 2 8 or 0 42 per mil 1sigma throughout the footwall and ore zones but it is enriched by approximately 1 per mil in the graphite free hanging wall rocks fluid delta 13 c value was 4 or 1 per mil reflecting either a magmatic carbon reservoir or mixtures of marine carbonate carbonaceous and magmatic sources the enrichment of gold at one side of the kirkland lake fault zone is interpreted to be the result of development of a metasomatic zoning induced by gradients in oxidation state the precious metal being concentrated in the reducing side zonation of alteration types south of the kirkland lake larder lake fault zone represents various steps of a continuous process of incremental changes of the rocks to adapt to the composition of an externally generated fluid successive generations of fracture arrays and veins have the same alteration patterns and thus the composition of the incoming hydrothermal fluid seems to have remained essentially unchanged through time gold mineralization at the kerr addison mine and its contiguous alteration halo are interpreted to be the result of repeated cycles of hydraulic fracturing fluid penetration and local reaction with rocks fluid rock interaction produced lateral variations of fluid composition which led to gold concentration probably due to gradients in ph preferentially at the interface between the carbonate muscovite and carbonate albite alteration types which is also the site of pyrite concentration the alteration assemblages and specific site of gold pyrite concentration at kerr addison are common to several other described archcan lode gold deposits

the differentiation of the skaergaard intrusion mcbirney and naslund have missed the two central points that we reiterate for clarity first the composition of the cotectic cumulate extract for the lzc best estimated from the average composition of adcumulate rocks had a substantially lower silica content than any conceivable basaltic liquid so silica enrichment is unavoidable this is true of all the dyke compositions reported by brooks and nielsen even the anomalous rock e with sio2 46 second it is the normal behaviour of tholeiite basaltic magma to produce increasingly iron rich cumulate extracts with little change in silica until feti oxide precipitation occurs when silica enrichment and compositional divergence begins such divergence between equivalent cumulate plutonic and fractionated volcanic rocks hads been widely documented skaergaard magmas do have some uncommon features however similar assemblages exist in volcanic rocks as in the andesitic to rhyolitic glasses of the loch ba ring dyke mull sparks 1988 it is also a basic principle that elements enriched in cumulate rocks often become depleted in the corresponding liquids iron enrichment in the skaergaard cumulates is complementary to iron depletion in liquids

synvolcanic gold mineralization within a deformation zone the chevrier deposit chibougamau abitibi subprovince canada the chevrier gold deposit is located in the northeastern part of the abitibi subprovince within the chevrier volcanic centre cvc this calc alkaline volcanic centre evolved from mafic to felsic in a submarine environment and the chevrier deposit is spatially associated with the uppermost unit composed mostly of massive dacite to rhyolite dated at 2730 ma the chevrier deposit is divided in two zones the north zone is composed of high grade discordant quartz carbonate veins and disseminated pyrite mostly within melanocratic gabbro dykes whereas the south zone is within a low grade concordant pyrite envelope associated with quartz carbonate pyrite veinlets both zones are highly deformed and show strong carbonate sericite and chlorite alterations although these zones share many characteristics with orogenic deposits the crosscutting of the chevrier deposit by structures associated with the major deformation events suggests that mineralization was prior to these events furthermore felsic dykes associated with the formation of the cvc crosscut auriferous veins and zones therefore suggesting a synvolcanic timing for the gold mineralization characteristics of the zones such as similar vein composition and alteration mineral assemblages and location within the volcanic pile suggest that they are part of a single hydrothermal event where the south zone represents the shallower part of the deposit the chevrier deposit is classified as a noncarbonate hosted stockwork and disseminated deposit and shows many similarities with strata bound gold deposits of the andacollo mining district of chile dating of mineralization at the chevrier deposit indicates that the emplacement of gold occurred more than 25 ma before the onset of the orogeny affecting this part of the abitibi subprovince it also illustrates how synvolcanic mineralization can be misinterpreted as an orogenic deposit without the presence of crosscutting relationships with dykes faults or other structures

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

the petrogenesis of plagioclase phyric basalts at mid ocean ridges 1 plagioclase ultraphyric basalts pubs have been sampled along most mid ocean ridges with ultraslow to intermediate spreading rates over the past 40 years the prevalent models for their origin assume positive buoyancy of plagioclase in basaltic liquids resulting in selective concentration of plagioclase phenocrysts by floatation however when the global population of pub lavas is examined this hypothesis becomes less compelling pub host lavas demonstrate a large range of compositions and densities similar to aphyric glasses from the same ridge segments most importantly the majority of pub host liquids are less dense than their phenocryst cargo meaning that plagioclase floatation within a magma chamber cannot be the driving force for phenocryst enrichment furthermore pub lavas have never been sampled on axis at fast spreading centers or from locations with noted contemporaneous axial magma chambers where pubs should be abundant if plagioclase is buoyant in mid ocean ridge basalt morb instead we argue that the high modal abundance of plagioclase results from interaction between magma and preexisting zones of crystal cumulates within the lower crust possibly followed by loss of olivine during magma ascent pubs erupt when the magma maintains an ascent velocity greater than the settling rate of the plagioclase phenocrysts which precludes long crustal residence times for these magmas in addition to being a proxy for lower spreading rates our findings also suggest that pub eruption can also be used as a proxy for the absence of a magma chamber or transport through a conduit system that bypassed an axial chamber

paleogeographic and paleotectonic evolution of a basement controlled archean supracrustal sequence chibougamau caopatina quebec the 7 14 km thick archean volcano sedimentary sequence in the chibougamau caopatina region ccr composed of two mafic to felsic cycles and two thick essentially epiclastic successions is divided into a southern caopatina segment scs and northern chibougamau segment ncs the sedimentary units which evolved after cycle 1 volcanic rocks in the scs and after the combined cycles 1 and 2 in the ncs document a diachronous sedimentary basin evolution this criterion has been employed to distinguish an early evolutionary stage in the south and a late evolutionary stage in the north in each segment the basin episode represents a unique extensional event volcanism preceded the major sedimentary rifting event in both segments composition of sediments and sedimentary facies correlated with the interpreted basin margin faults support the interpretation of two phases of crustal extension volcanic construction and sill intrusion were the prevalent factors governing emergence of volcanic islands in the cc

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

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

radioactive disequilibrium dating of corals by nuclear track detection abstract a simple method of radioactive disequilibrium dating of corals using nuclear track detectors has been attempted the natural alpha activity in a coral sample was monitored by attaching an alpha track plastic detector cr 39 to a sliced sample the u content of sample was measured by the neutron induced fissiontrack density in a muscovite detector the age of a sample can be estimated from the alpha track productivity per u content experimental results for ten corals from three different groups of estimated ages 0 50 000 and 500 000 yr showed a good agreement with a theoretical fit

structural and stratigraphic controls on magmatic volcanogenic and shear zone hosted mineralization in the chapais chibougamau mining camp northeastern abitibi canada the structure and stratigraphy of the chapais chibougamau mining camp in the archean abitibi greenstone belt of the superior craton quebec canada is reevaluated from new aeromagnetic data geochronology and field studies to provide a framework for syngenetic and epigenetic mineralization mineralization in the chapais chibougamau area is synvolcanic volcanogenic massive sulfide vms synmagmatic fe ti v ni cu au ag cu zn pb veins chibougamau type cu au veins and shear zone hosted including opemiska cu au veins subtype vms deposits occur within felsic volcanic rocks of tholeiitic affinity and mafic to felsic volcanic rocks of transitional to calc alkaline affinity at the top of three volcanic cycles of the roy group vms deposits and early polymetallic veins au ag cu zn pb are associated with nnw to nne striking synvolcanic faults fe ti v deposits occur within the layered zone of the lac dore complex especially where it thickens in areas of interpreted synmagmatic faults subeconomic magmatic ni cu deposits occur at the contacts of mafic ultramafic or tonalitic intrusions chibougamau type cu au veins formed through magmatic hydrothermal processes they are cut by dikes that predate regional d2 deformation but are located within and are deformed by nw and ne trending d2 shear zones shear zone hosted au deposits are developed preferentially within regional e w trending deformation corridors and along nne striking sinistral shear zones while opemiska type cu au veins occur within regional overturned anticlines in mafic sills of the cummings complex u pb dating of detrital zircons provides maximum depositional ages for metasedimentary rocks of the blondeau formation of the roy group and the stella formation of the opemisca group which unconformably overlies the roy group the blondeau formation is 2721 3 ma which is coeval to the age of the windfall member in the urban barry belt both show similar exploration potential as they host early polymetallic au ag cu zn pb veins the stella formation is 2704 2 ma and is unaffected by first generation folds thus providing a time constraint for the earliest folding event d1 this new age also indicates that sedimentation in the chapais chibougamau area occurred earlier than for the timiskaming type assemblages in the southern abitibi greenstone belt but the structural setting and presence of alkaline intrusions suggests similar potential for syenite associated au deposits in the stella formation

evolution of the south central part of the archean abitibi belt quebec part i stratigraphy and paleogeographic model the region comprises parts of the southern zone of the abitibi greenstone belt and of the bellecombe gneiss belt and the belleterre angliers greenstone belt the stratigraphic sequence of the abitibi belt consists of 1 a lower ultramafic division 2 a middle tholeiitic division and 3 an upper diverse division these divisions lap onto an older volcanic sequence to the north and west in the bellecombe belt sediments overlie the lower ultramafic division these sediments coarsen toward the abitibi belt and toward the stratigraphic top the volcanic rocks of the belleterre angliers belt overlie the sediments of the bellecombe belt five paleogeographic phases have been recognized during phase i a deep marine ultramafic lava plain covered the south part of the abitibi belt and the bellecombe belt faulting took place at the duparquet destor break during phase ii tholeiitic basalts formed a deep marine lava plain in the west of the abitibi belt whereas flows erupted mainly from central volcanic c

evolution of the southern abitibi greenstone belt based on u pb geochronology autochthonous volcanic construction followed by plutonism regional deformation and sedimentation abstract new mapping and u pb zircon geochronological results in the southern abitibi greenstone belt sagb support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages rather than the collage of allochthonous terranes proposed in recent publications based on lithological and geochronological criteria these supracrustal assemblages also encompass the swayze shining tree and montcalm greenstone belts indicating that despite separation by large granitic intrusions they are in fact part of one extensive greenstone belt in conjunction with the mapping the geochronological results support coherent upward facing stratigraphic sections with about 20 of the samples containing inherited zircons with ages similar to those found in underlying assemblages the seven oldest assemblages represent semi continuous volcanism from 2750 to 2697 ma the volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc alkaline mafic to felsic lavas intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section regional fault control on the distribution of the volcanic assemblages provides evidence for early dip slip movement associated with volcanic extension dating back to at least 2725 ma the two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults the earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 ma and the youngest one of subaerial conglomerates fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 ma the sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons regional folding and reactivated movement on the regional faults probably related to accretion of the abitibi to the superior province craton autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic komatiitic assemblages and two dominantly calc alkaline assemblages as well as the extensive intermingling of these different magma clans in a number of the assemblages appears to be unique to the archean it suggests complex large scale and long lived interaction between mantle plumes and subduction zone magmas the nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with end 2 5 0 5 demonstrating a lack of contamination by ancient enriched components

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

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

tectonic evolution of the northeast portion of the archean abitibi greenstone belt chibougamau area quebec the chibougamau area occupying the northeastern part of the abitibi greenstone belt is a large synclinorium of volcanic and sedimentary rocks enclosed within tonalitic gneisses several east west