low heat flux and large variations of lithospheric thickness in the canadian shield 1 ten new heat flux determinations have been made using measurements in 22 mining exploration boreholes located at latitudes higher than 51 n in the canadian shield they provide data in poorly sampled regions near the core of the north american craton where one expects the lithosphere to be thickest the new heat flux values are all smaller than 34 mw m 2 and are among the lowest recorded so far in the shield for all the new sites there is no relationship between heat flux and heat production in surface rocks in the canadian shield heat flux variations occur at wavelengths 100 km and are mostly of crustal origin local averages in two 250 250 km windows located on archean areas at high latitudes on either side of james bay are 29 mw m 2 and 31 mw m 2 the lowest values found so far at this scale in the canadian shield s wave traveltime delays derived from tomographic models provide the additional constraints needed to resolve differences of deep lithospheric thermal structure there is no significant correlation between average surface heat flux and traveltime delays within the canadian shield confirming that variations of the surface heat flux are mostly of crustal origin traveltime delays cannot be explained by variations in crustal heat production only and require variations of heat supply to the lithosphere and or radiogenic heat production in the lithospheric mantle these variations are associated with changes of lithospheric thickness that may be as large as 80 km the heat flux at the base of the superior lithosphere is constrained to be 11 2 mw m 2

high precision u pb ages for granulite metamorphism and deformation in the archean kapuskasing structural zone ontario implications for structure and development of the lower crust abstract precise u pb isotopic ages for two subhorizontal granulite gneisses in the kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m y after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust mantle delamination volcanism and batholith formation had ceased by 2700 and 2680 ma respectively while granulite formation occurred between 2660 and 2640 ma and ductile deformation continued until 2585 ma emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite widespread tectonic underplating is implied since all superior province granulites have similar young ages anomalous ar ages that post date accretion by about 150 m y and scrambled pb pb and rb sr whole rock systems also suggest that late low temperature alteration accompanied this event temporal relationships and the presence of post volcanic 2670 ma conglomerate cobbles at granulite grade support a model of ductile underplating of a post accretion thermally softened back arc fore arc complex with components welded onto an overlying volcanic plutonic complex at successively deeper levels multistage deformation in active fault zones and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 ma anomalously abundant gold production from archean rocks may indicate that shallow underplating granulitization and devolatization was a widespread phenomenon at this time post accretion ductile underplating must be considered as a primary cause of seismic reflectors in archean terrains

boron and lithium in high grade rocks and minerals from the wawa kapuskasing region ontario there is no preferential partitioning of boron among the principal rock forming minerals in high grade rocks of the kapuskasing structural zone ksz and the wawa domal gneiss region wdg lithium is strongly concentrated in biotite and other ferromagnesian minerals but does not show consistent partitioning between these and the sialic minerals the distribution of b and li within a rock may be studied using an alpha track image which shows that the inconsistencies in partitioning may be largely attributed to disturbance of mineral equilibria by postmetamorphic low grade alteration that deposited b and li boron has similar concentrations in all the rock types studied although it is an incompatible element that elsewhere accumulates in pegmatites lithium concentrations are low in the anorthositic rocks but are otherwise very variable in some but not all rocks higher than usual b and li can be attributed to introduction during alteration boron occurs at low concentrations 2 3 ppm throughout both the k

temperature and rheological properties of the mantle beneath the north american craton from an analysis of heat flux and seismic data 1 we combine heat flux data and seismic velocity models for the north american lithosphere to derive constraints on thermal conditions and deformation mechanisms in the underlying convecting mantle local heat flux averages that are not affected by shallow crustal heat production contrasts allow calculation of reliable lithospheric geotherms and uncertainty ranges for consistency with the seismic data the mantle potential temperature beneath north america must lie within a 1290 c 1450 c range close to that for the oceanic mantle sampled at mid ocean ridges the heat flux at the base of the lithosphere varies laterally from 11 3 mw m 2 beneath the 250 km thick archean core of the superior province to 15 3 mw m 2 beneath the thinner younger appalachians province it is shown that the most likely cause of such rates of heat supply into the north american continent is small scale convection in an unstable boundary layer beneath the rigid mechanical lithosphere this allows useful constraints on the mantle rheological properties we show that the most likely deformation mechanism is dislocation creep in wet mantle rocks ranges for the mantle temperature water content and rheological parameters could be tightened very significantly once strong constraints are obtained on radiogenic heat production in the lithospheric mantle

the role of granitic intrusions in the evolution of the abitibi belt canada abstract the abitibi greenstone belt canada constitutes a segment of a broad volcanic arc that experienced three distinct periods of volcanic activity peaking at 2730 2725 2718 2714 and 2705 2700 ma during all three volcanic episodes which were generated by subduction related processes mantle plume derived komatiitic volcanic rocks were also added to the composite abitibi arc plume activity resulted in local regional extension and restricted later volcanism within the belt north south shortening produced thrusts and folds followed by se trending dextral strike slip faults between 2700 and 2692 ma the principal belt wide deformation subsequent e w dextral strike slip faulting limited to the major faults continued to 2690 2680 ma destor porcupine fault and even to 2680 2670 ma cadillac larder fault extensional structures resulting from the thickened crust developed around 2660 ma granitic intrusions are associated with all the volcanic episodes and phases of deformation and have been used to constrain the age of many of these events their role in stabilizing and adding to the volcanic crust is perhaps less well appreciated large synvolcanic tonalite dominated intrusions of the tonalite trondhjemite granodiorite ttg suite are associated with the three major periods of volcanic activity of the abitibi greenstone belt and are genetically related to the volcanism these synvolcanic intrusions occur both as high level flat cauldron subsidence complexes and as batholithic masses at the base of the oceanic crust tonalite gneiss dated from 2714 to 2703 ma occurring in structural culminations is considered to be arc underplated material the gneiss crystallized at depth and in contrast to the other tonalites does not appear to have intruded volcanic rocks the gneisses also record the earliest compressional deformation numerous intrusions of island arc affinity both the ttg and monzodiorite mzd suites form stocks and small plutons emplaced during the late phases of major deformation these are more numerous than voluminous and most of the tonalite end members of the suites probably intruded deeper within the sequence retarded by the now thickened crust large tonalite intrusions dominate the uplifted deeper sections adjacent to the abitibi belt in the kapuskasing uplift and grenville parautochthon and presumably make up much of the deep crust of the abitibi belt imaged in lithoprobe seismic profiles small stocks and dyke complexes characterize the late localized strike slip deformational phase these intrusions are porphyritic syenites and related rocks including some alkaline compositions garnetiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2660 ma thus plutonic rocks are a volumetrically major overall component of the abitibi belt and were implicated in every phase of its development from early volcanic arc magmatic activity through the various stages of deformation to final stabilization as continental crust

the nature age and petrogenesis of the cartier batholith northern flank of the sudbury structure ontario canada abstract the cartier batholith exposed about 50 km northwest of sudbury ontario canada is a late archean k rich granitic batholith along with the levack gneiss complex it forms the footwall of the north range of the sudbury igneous complex the cartier batholith dominantly monzogranitic to granodioritic is relatively homogeneous unfoliated and contains k feldspar megacrysts it shows several distinctive chemical features including high concentrations of zr th u and lree and low concentrations of nb ti and hree several discriminants suggest a post orogenic tectonic environment for the batholith this inference is consistent with a u pb age of 2642 1 ma obtained on zircon only slightly younger than the inferred timing of high grade metamorphism of the levack gneiss complex the cartier batholith could have formed as a result of approximately 33 partial melting of the levack gneiss complex

pericontinental crustal growth of the southwestern abitibi subprovince canada u pb hf and nd isotope evidence the abitibi subprovince is widely regarded as a classic example of neoarchean continental crust formed in an ensimatic setting however sialic crust as old as 2 93 ga occurs in the adjacent wawa subprovince and the two subprovinces are thought to have formed a contiguous block since the initiation of abitibi magmatic activity at 2 75 ga a detailed geochemical u pb geochronological and nd hf isotope study of plutonic and volcanic rocks from the southwestern abitibi provides evidence for the involvement of older crust during magma genesis near the abitibi wawa boundary kapuskasing structural zone inherited zircons dated at ca 2 85 ga occur both in 2747 ma tonalite and 2700 ma granodiorite of the rice lake batholith part of the larger kenogamissi plutonic complex igneous zircons from this batholith display large intrasample variations in 176 hf 177 hf which we attribute to mixing of magmas derived from mantle juvenile crust and older crustal sources nd isotope data for felsic volcanic and plutonic rocks in this region indicate a dominantly juvenile source however two older felsic volcanic units 2739 and 2729 ma show nd isotope evidence of significant crustal contamination post tectonic plutonic units that were emplaced at 2676 and 2662 ma also contain inherited zircons but none are older than 2 76 ga in contrast tonalitic units dated at 2744 and 2715 ma from the round lake plutonic complex located 150 km east of the kapuskasing structural zone lack zircon inheritance and display less intrasample hf isotope variability in zircons our data suggest that the influence of older sialic crust during abitibi volcanic and plutonic activity is restricted both in space and time similar evidence for an older crustal influence occurs elsewhere in the southwestern abitibi near the kapuskasing structural zone the data collectively suggest that an approximately 75 km wide belt of abitibi crust adjacent to this structure was underlain by older wawa crust from 2 75 ga onward this crustal geometry is consistent with development of the abitibi sub province at the edge of a wawa protocraton perhaps as the result of rifting of this older continental block

intracrustal extension of an archean orogen revealed using single grain u pb zircon geochronology the metaplutonic wawa gneiss domain in the southern superior province is part of an uplifted block that was exhumed during the proterozoic exposing the lower crustal 30 km levels of the late archean 2720 to 2670 ma abitibi wawa orogen previous structural and geophysical studies of the midcrustal wawa gneiss domain reveal subhorizontal shear zones that formed during orogen parallel extension and correspond to gently dipping seismic reflectors present at depth elsewhere in the orogen in order to bracket the age of this extensional deformation the ages of metre scale igneous bodies that crystallized before and after extension have been determined in four outcrops using single grain u pb zircon dating techniques the results constrain the time of extension to the interval between 2660 and 2645 ma and show that deformation was coeval with pegmatite emplacement partial melting of tonalite gneisses as old as 2925 35 ma and regional circa 2660 ma granulite facies metamorphism of adjacent more deeply eroded crust this extension and associated high grade metamorphism in the wawa gneiss domain occurred 10 to 25 m y after the cessation of regional compression major transcurrent fault movement and sediment deposition in the upper structural levels of the orogen the middle and lower crust was therefore mechanically decoupled from the upper crust during this period of orogen parallel extension this intracrustal style of deformation is interpreted to be the consequence of gravitational forces working on a thermally softened middle crust the detachment of lithosphere from the orogen base is the favored mechanism for generating these forces and for introducing the heat that caused synextension metamorphism cyclic construction and delamination of orogen lithosphere could account for even younger metamorphic strain events evident in nearby exposures of lower crust

lithium isotopic systematics of a type granites and their mafic enclaves further constraints on the li isotopic composition of the continental crust article i nfo lithium concentrations and isotopic compositions of 39 a type granites and mafic enclaves from 11 plutons in northeast china and the north china craton are used to constrain their genesis and to characterize further the average composition of the continental crust lithium concentrations 2 8 to 80 ppm in 29 a type granites are slightly higher than those of average i type granites and lower than those of average s type granites reflecting source differences lithium isotopic compositions 7 li 1 8 to 6 9 of a type granites fall within the range of worldwide i type and s type granites by contrast 10 mafic enclaves have significantly higher li concentrations 32 to 179 ppm and less variation in li isotopic compositions 7 li

shallow impact isotopic insights into crustal contributions to the sudbury impact melt sheet abstract the largest known terrestrial impact melt sheet occurs within the 1850 ma sudbury structure ontario in order to evaluate the relative contributions of different target lithologies to the melt sheet we have investigated the pb isotope compositions of feldspar separates from early formed quartz diorite magmas within offset dykes from around the impact structure the samples define a linear array on plots of age corrected 206 pb 204 pb versus 207 pb 204 pb samples from offset dykes hosted by the huronian supergroup south range have a range of 206 pb 204 pb 1850 from 15 424 to 17 255 and 207 pb 204 pb 1850 from 15 390 to 15 801 whilst those hosted by archean gneisses of the superior province north range cluster around 206 pb 204 pb 1850 14 8 and 206 pb 204 pb 1850 15 1 these values can be approximated by binary mixing between the two major groups of target lithologies a mix of 60 70 of superior province gneisses with 30 40 of huronian metasedimentary material closely matches the pb isotope compositions of north range offset dyke samples whereas in the south range the required huronian component is up to ca 80 these mixing proportions are consistent with sr nd and os isotope and trace element constraints a third minor component either locally exposed paleoproterozoic mafic rocks or the lower crust is also required however the isotopic trace element and ni cu platinum group element characteristics of the melt sheet can be accommodated without the involvement of an average lower crustal or meteoritic component a major contribution of huronian supracrustal material which had a pre impact thickness of up to 12 km is required to explain the chemical characteristics of the impact melts which also have a strong upper crustal affinity e g eu sm 0 22 rb sr 0 2 0 35 as such a shallower level of melting is apparent than that predicted by many previous impact models for the sudbury event this can be accommodated by considering approach trajectories for the impactor oblique to the earth s surface in addition the isotopic and trace element variability identified indicates that the melt sheet was heterogeneous at an early stage and may not have been completely homogenised during crater formation our findings have significant implications for the nature of the sudbury impact event the evolution of the melt sheet and the crustal sources of metals contained in sudbury s world class ni cu pge sulphide ores

upper mantle velocity temperature conversion and composition determined from seismic refraction and heat flow 1 we compile upper mantle pn velocities from seismic refraction wide angle reflection surveys in the southern superior province of the canadian shield and compare them with temperatures at the moho deduced from heat flow data calculated moho temperatures and pn velocities correlate well showing that in this area pn depends primarily on temperature the obtained values of v pn t depend weakly on the assumed value of moho heat flow and are on the order of 6 0 10 4 10 km s 1 k 1 within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks comparison between observed pn velocities and predicted values for several mineralogical models at moho temperatures allows constraints on both the moho heat flow and the shallow mantle composition for all moho heat flows undepleted clinopyroxene rich mantle compositions do not allow a good fit to the data for depleted mantle compositions temperatures consistent with the observed pn velocities correspond to values of moho heat flow larger than 12 mw m 2 for our preferred moho heat flow of 15 mw m 2 the best fit mantle composition is slightly less depleted than models for average archean subcontinental lithospheric mantle this may be due to rejuvenation by melt related metasomatism during the keweenawan rifting event the similarity in pn t conversion factors estimated from this empirical large scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies

the composition and petrogenesis of the lower crust a xenolith study granulite facies lower crustal xenoliths from a single basaltic vent hill 32 in the mcbride volcanic province north queensland australia illustrate the extreme lithologic diversity of the deep crust these xenoliths are dominantly mafic but intermediate and felsic granulites represent a significant proportion 20 of the xenolith population all xenoliths have high grade mineralogies and generally well equilibrated textures as well as decompression features which are indicative of derivation from deep crustal levels 0 7 1 0 gpa major and trace element chemistry for 12 xenoliths chosen to span the observed lithologic range are used to constrain the petrogenesis of these rocks in the smaller layered samples metamorphic differentiation may lead to nonrepresentative sampling however in most instances such secondary processes can be identified and the original chemical characteristics delineated the mafic xenoliths formed through a variety of processes including crystallization of mafic magma crystal accumulation from mafic and felsic magmas and partial melting of intermediate compositions to yield a mafic residuum the two analyzed intermediate xenoliths are metasediments based upon high alumina to alkali ratios and rare earth element patterns whereas the two felsic xenoliths have compositions similar to igneous rocks comparisons of noncumulate and or restite xenoliths with unmetamorphosed rock types show that k and rb are variably depleted whereas th and u are strongly depleted in all rock types the large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains suggesting similar processes were operative suggestions of an anomalously ba and sr rich lower crust are not supported by the data by using the observed lithologic proportions of xenoliths at hill 32 and the chemical analyses presented here a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions the weighted mean composition is mafic and formed through a combination of basaltic underplating crystal accumulation tectonic underplating and partial melting no simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in phanerozoic continental margin settings

rb sr and u pb ages of volcanism and granite emplacement in the michipicoten belt wawa ontario the michipicoten greenstone belt at wawa ontario is typical of archean volcanic belts in the superior province the supracrustal rocks are divisible into lower middle and upper metavolcanic sequences which are separated by iron formation and clastic metasedimentary rocks these are intruded by granitic stocks and embayed by granitic batholiths this study reports whole rock rb sr and zircon u pb ages for the lower and upper metavolcanics for the granitic rocks that are physically within the greenstone belt internal granites and for the granitic rocks that embay the greenstone belt external granites the apparent rb sr ages for the lower metavolcanics are 2530 90 2285 70 and 2680 490 ma the u pb ages are 2749 2 and 2744 10 ma the internal granites give an rb sr age of 2560 270 ma and a u pb age of 2737 6 ma the external granite at hawk lake indicates an rb sr age of 2550 175 ma and a u pb age of 2747 7 ma it is possible that this unit contains elements older than 2812 ma as

the petrogenesis of saricimen caldiran van quartz monzodiorite implication for initiation of magmatism late medial miocene in the east anatolian collision zone turkey abstract the saricimen porphyry is exposed as a sub volcanic pluton within the upper cretaceous ophiolitic rocks in east anatolian accretionary complex the pluton is quartz monzodioritic in composition consisting of feldspar hornblende and biotite phenocrysts set in a fine grained matrix major element geochemistry indicates the pluton is of high k calc alkaline metaluminous character with a low 0 81 0 90 aluminum saturation index asi trace element and sulfur isotope geochemistry suggests that the saricimen porphyry was mantle derived and contaminated by crustal materials during ascent tectonically this and related volcanic and plutonic rocks in eastern turkey and iran are subduction related and comprise the earliest documented neotectonic igneous activity associated with the final closure of the neo tethys between the arabian and eurasia plates at 14 13 ma

geochronology of wabigoon belt granitoids northwestern ontario rb sr isochrons for seven late tectonic plutons whole rock rb sr isochrons are presented for seven late tectonic granitoid plutons intruded into supracrustal rocks of the wabigoon volcanic plutonic belt age estimates range from 2461 78 ma ottertail lake stock to 2645 158 ma bears passage stock a mean age estimate by weighted pooling of the isochrons gives 2555 41 ma while pooling 39 samples into one regression gives 2568 35 ma these ages concur with estimates for the duration of the kenoran orogeny error limits on our isochrons incorporate the duration of polydiapirism aplitic dyking and deuteric metasomatism available ages indicate that syntectonic and late tectonic granitoid plutonism spanned a geologically short time interval centered at 2565 62 ma that cannot be differentiated by the rb sr technique consistently low initial sr ratios of 0 7008 0 0006 indicate a mantle or low rb sr source

continental crust melting induced by subduction initiation of the south tianshan ocean insight from the latest devonian granitic magmatism in the southern yili block nw china abstract the tianshan belt of the southwestern central asian orogenic belt was generated by paleozoic multi stage subduction and final closure of several extinct oceans including the south tianshan ocean between the kazakhstan yili and tarim blocks however the subduction initiation and polarity of the south tianshan ocean remain issues of highly debated this study presents new zircon u pb ages geochemical compositions and sr nd isotopes as well as zircon hf isotopic data of the latest devonian to early carboniferous granitic rocks in the wusun mountain of the yili paleozoic convergent margin which together with the spatial temporal distributions of regional magmatic rocks are applied to elucidate their petrogenesis and tectonic linkage to the northward subduction initiation of the south tianshan ocean our zircon u pb dating results reveal that these granites were emplaced at the time interval of 362 0 1 2 360 3 1 9 ma suggesting a marked partial melting event of the continental crust in the latest devonian to early carboniferous these granites based on their mineral compositions and textures can be categorized as monzogranites and k feldspar granites geochemically both monzogranites and k feldspar granites have characters of i type granites with high k 2 o contents 4 64 4 83 wt and the k feldspar granites are highly fractionated i type granites while the monzogranites have features of unfractionated i type granites whole rock sr nd isotopic modeling results suggest that ca 20 40 mantle derived magmas may be involved in magma mixing with continental crust partial melts to generate the parental magmas of the granites the mantle derived basaltic magmas was inferred not only to be a major component of magma mixture but also as an important heat source to fuse the continental crust in an extensional setting which is evidenced by the high zircon saturation temperatures 713 727 c and 760 782 c of the studied granites the latest devonian to early carboniferous extensional setting in the wusun mountain region of the yili paleozoic convergent margin is addressed by the subduction initiation of the south tianshan ocean and constituted a late paleozoic nascent arc back arc system in the southwestern caob

retrograde fluids in the archean shawmere anorthosite kapuskasing structural zone ontario canada the archean shawmere anorthosite lies within the granulite facies portion of the kapuskasing structural zone ksz ontario and is crosscut by numerous linear alteration veins containing calcite quartz dolomite zoisite clinozoisite margarite paragonite chlorite these veins roughly parallel the trend of the ivanhoe lake cataclastic zone equilibria involving clinozoisite margarite quartz calcite plagioclase show that the vein minerals were stable at t 0 9 thus vein formation while clearly retrograde spanned a range of temperatures and fluid compositions evolved from h2o rich to co2 rich the calcite in the retrograde veins has 18o values that range from 8 4 to 11 2 average 9 7 0 9 and 13c values that range from 3 9 to 1 6 average 3 1 0 6 these values indicate that the fluids from which calcite precipitated underwent extensive exchange with the anorthosite and other crustal lithologies the fluids may have been initially derived either from devolatilization of metamorphic rocks or crystallization of igneous rocks in the adjacent abitibi subprovince vein quartz contains co2 rich fluid inclusions final melting t 57 0 to 58 7 c that range in size from 5 to 17 m measured homogenization temperatures t h range from 44 0 to 14 5 c however for most inclusions 46 of s1 t h 44 0 to 21 1 c co2 1 13 to 1 05 g cm3 at 400 to 600 c these densities correspond to pressures of 3 5 to 7 kbar which is the best estimate of pressures of vein formation it has been argued that some high density co2 rich fluid inclusions found in the ksz were formed during peak metamorphism and thus document the presence of a co2 rich fluid during peak granulite facies metamorphism rudnick et al 1984 the association of high density co2 rich fluid inclusions with clearly retrograde veins documents the formation of similar composition and density inclusions after the peak of metamorphism thus the coincidence of entrapment pressures calculated from fluid inclusion density measurements with peak metamorphic pressures alone should not be considered strong evidence for peak metamorphic inclusion entrapment all fluid inclusion results are consistent with an initially semi isobaric retrograde p t path

petroleum secondary migration and accumulation in the central junggar basin northwest china insights from basin modeling the timing of petroleum generation migration and accumulation in the central junggar basin has been evaluated using basin modeling technology to decipher the processes of petroleum secondary migration and accumulation in the jurassic carrier beds during the yanshan and himalayan orogenies during the early yanshan orogeny j2 j3 crude oils sourced mainly from the permian source rocks migrated laterally from depocenters to adjacent structural high points during the late yanshan orogeny cretaceous eocene the hydrocarbon secondary migration pattern changed significantly specifically in the cretaceous a large scale regional second phase of generation and migration occurred in the western part of the central junggar basin in the eastern part of the central junggar basin crude oils emplaced in the depressions migrated mainly to the eastern uplifts in the early tertiary oils in the source kitchens generally began migrating to the north and mixed with the jurassic generated hydrocarbons during the himalayan orogeny neogene quaternary this migration was strengthened continually the modeled results agree with migration associated geochemical information of the jurassic accumulations integration of the regional geological setting and some petroleum geology data suggest that the jurassic tectonic high points and facies change and unconformity associated subtle traps are of crucial importance for current oil exploration both overpressured bodies in the triassic and permian systems and volcanic rock reservoirs of the carboniferous basement in the uplifts have important implications for future natural oil and gas exploration this work should help reduce exploration risk in this region

deep crustal oxygen isotope variations the wawa kapuskasing crustal transect ontario we have studied the oxygen isotopic composition of rocks from a 100 km transect through the central superior province of ontario representing progressively the shallower terrains of the kapuskasing structural zone ksz the wawa gneiss terrane wgt and the michipicoten greenstone belt mgb these rocks range in age from 2 76 to 2 60 ga and correspond to a section through approximately 20 km of crustal thickness equivalent lithologic types have similar range of 18o values at each crustal level tonalitic to granodioritic rocks 6 4 to 9 5 dioritic and anorthositic rocks 5 5 to 7 6 mafic gneisses group 1 majority 5 7 to 7 1 group 2 8 1 to 9 5 18o values exhibit a remarkable correlation with sio2 values similar to that observed in unaltered plutonic rocks of equivalent composition paragneisses have significantly higher 18o values 9 3 to 12 2 low grade metavolcanic and metasedimentary rocks of the mgb are 18o enriched compared to their high grade equivalents in the ksz and wgt 7 4 to 13 3 for mafic to felsic metavolcanic rocks 11 4 to 14 7 for clastic metasediments coexisting minerals exhibit 18o fractionation consistent with equilibrium but corresponding to uniform isotopic temperatures about 553 to 584 c across the entire transect lower than the inferred metamorphic temperatures in the highest grade ksz terrane the lack of distinctive isotopic differences between equivalent rock types in the ksz wgt and mgb suggests that there is no significant gradient in 18o with depth in the crust the majority of mafic gneisses group 1 appear to have been emplaced either as subaerial extrusives intrusive sills or less likely as submarine extrusives that were hydrothermally altered at high temperatures the less abundant group 2 mafic rocks have the 18o values typical of greenstones that were altered at low temperature by seawater and isotopically resemble low grade rocks in the michipicoten and abitibi belts in general no major changes in whole rock isotopic composition appear to have occurred during granulite facies metamorphism implying limited flux of water or co2 the continuous linear gradient in 18o versus sio2 in the high grade rocks cannot be due to differentiation of a mafic source magma a model involving an association between mantle derived mafic magma and 18o enriched crustal materials is more consistent with the oxygen isotopic and the ree data

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

tectonic setting of late archean bimodal volcanism in the michipicoten wawa greenstone belt ontario the tectono stratigraphic relationships depositional environments rock associations and major and trace element compositions of the late archean 2744 2696 ma bimodal basalt rhyolite volcanic rocks of the michipicoten wawa greenstone belt ontario are compatible with an origin along a convergent plate margin that varied laterally from an immature island arc built on oceanic crust to a more mature arc underlain by continental crust this environment is similar to that of the cenozoic taupo kermadec tonga volcanic zone michipicoten basaltic rocks most of which are proximal deposits compositionally similar la yb n 0 63 1 18 to modern oceanic island arc tholeiites are interpreted as having formed along the largely submerged island arc voluminous michipicoten rhyolitic pyroclastic rocks la yb n 4 3 18 7 ybn 5 7 15 9 probably erupted subaerially from the continental arc with distal facies deposited subaqueously on the adjacent oceanic island arc and proximal facies deposited in subaeri

rb sr biotite and whole rock data from the kapuskasing uplift and their bearing on the cooling and exhumation history rb sr isotopic ratios were measured on biotite and whole rocks from a suite of 21 granitoid rocks from the southern kapuskasing uplift results of 18 of the whole rock analyses fall on an isochron with an age of 2 677 0 057 ga and sri 0 70080 biotite model ages range from 2 50 to 1 93 ga in a general spatial relationship of decreasing age with depth in the structural section as calibrated with the igneous hornblende barometer for tonalitic rocks and garnet pyroxene barometers for granulites near the michipicoten belt in the west biotite ages of 2 ga reflect disturbances also recorded by paleomagnetic and whole rock rb sr systems to the east in a 70 km long northwest southeast transect toward the ivanhoe lake fault zone ages decrease from 2 50 to 1 95 ga in a southern transect ages are in the range 2 30 2 38 ga to within 15 km of the fault reflecting consistent erosion levels corresponding to 0 53 mpa but decrease to 1 93 ga to the east in concert with paleopressures in the 0 7 1 0 gpa ra

zircon u pb ages from the wabigoon manitou lakes region wabigoon subprovince northwest ontario three distinct stratigraphic units have been recognized in the crow lake savant lake belt these are a basal unit consisting of high magnesium tholeiitic flows a middle unit consisting mostly of tholeiitic to calc alkaline pyroclastic rocks and flows and an upper unit of high iron tholeiitic flows a zircon u pb study has been carried out on seven rocks from the wabigoon manitou lakes region of this belt in the manitou lakes area a minimum age of 2755 ma on a porphyry pluton intruded into the wapageisi volcanics gives a minimum age for the high magnesium tholeiitic sequence a trondhjemite phase of the atikwa batholith near wabigoon lake that is dated at 2732 2 2 9 ma has the same age within experimental error as a rhyolite flow from the lower wabigoon volcanics in the same area that is dated at however a trondhjemite phase of the atikwa batholith at eagle lake that is dated at is distinctly younger than a dacite flow collected nearby from the lower wabigoon volcanics that is dated at both thes

high grade metamorphism in the chapleau foleyet area ontario archean rocks superior province kapuskasing structural zone in migmatitic mafic gneiss paragneiss dioritic and tonalitic rocks a lower grade garnet clinopyroxene plagioclase gt cpx pl zone and patchy higher grade orthopyroxene zones are distinguished based on mineral melt equilibria minimum conditions for the gt cpx pl zone are 750 degrees c 6 kbar a h 2 o 0 5 0 7 and for the orthopyroxene zone 770 degrees c 6 kbar a h 2 o 800 degrees c pressure values are 5 4 8 4 kbar and define a nne trending area of relatively high p in the kapuskasing zone supporting the interpretation of a tilted crustal section modified journal abstract

yield estimation from spectral amplitudes of direct p and p coda recorded by the wake island deep ocean hydrophone array abstract spectral amplitudes of direct p and p coda between 0 5 and 6 hz have been measured for 14 eastern kazakh test explosions using 8 hydrophones of the wake island array and a new model for these data has been developed to estimate relative yields of those explosions each log spectral amplitude is considered to be the sum of four parameters and an error term three parameters are frequency dependent and represent 1 the average eastern kazakh spectrum at the wake island array 2 the spectral shape characteristics unique to each event and 3 the station or hydrophone corrections the fourth parameter represents relative yield a total of 346 parameters including the 14 relative yields were needed to model all 901 spectral amplitudes that were measured and first order linear regression techniques were used to invert the data for these parameters standard deviations of the computed relative yields are very small averaging only 0 015 m b units deviations between the relative yields and their corresponding relative neis body wave magnitudes however are larger averaging 0 14 m b units this difference is interpreted to be an indicator of the level of inhomogeneity in the worldwide pattern of energy radiated from eastern kazakh tests there is evidence that this pattern can be correlated with the precise epicentral location of each explosion within the test site and is due to focusing and defocusing near the source it is proposed that the yield of a large explosion might be disguised by siting the explosion at a location that selectively defocuses energy towards continents where most seismic stations are located the precision with which a relative yield is determined from the data of any single hydrophone averages 0 035 m b units thus the model appears to provide a very stable measure of yield errors associated with frequencies greater than 2 hz are only slightly greater than errors associated with frequencies less than 2 hz verifying the utility of using the higher frequencies observed in direct p and p coda to estimate yield

quantification of lower crustal synmetamorphic fluid fluxes in the kapuskasing structural zone based on oxygen isotope profiles across two paragneiss mafic gneiss contacts detailed oxygen isotope profiles were measured across two very similar amphibolite grade paragneiss mafic gneiss contacts located 2 km apart within the kapuskasing structural zone the first profile is asymmetrical about die contact 18 o values for whole rocks and minerals are homogeneous in the paragneiss whole rock 18 o 10 4 to 11 4 smoothly decrease by about 2 over 2 5 m moving into the mafic gneiss and are homogeneous in the remainder of the mafic gneiss whole rock 18 o 7 0 to 7 4 the second profile is flat with homogeneous whole rock and mineral 18 o values in bom lithologies and mafic gneiss values whole rock 18 o 8 7 to 9 8 that are higher by about 3 relative to typical mafic gneiss values mineral mineral isotopic fractioations are fairly constant in all samples and are typical of high grade metamorphic rocks despite their differences both profiles can be explained by the advection of a mixed h 2 o co 2 fluid in equilibrium with the paragneiss across the contact into the mafic gneiss coupled with diffusion of oxygen in the fluid modeling constrains the net fluid flux required to form me first profile to be 1 3 m 3 m 2 and for me second profile to be 10 m 3 m 2 thus the net fluid flux during metamorphism varied probably by at least a factor of five over 2 km these fluxes could have been generated locally within the kapuskasing structural zone during metamorphism either by metamorphic devolatilization reactions or by the release of volatiles from crystallizing magmas

crustal geochemistry in the wawa foleyet region ontario fifty three rock samples from the kapuskasing structural zone ksz and 56 from the wawa domal gneiss terrane wgt ontario have been analysed for major elements 32 trace elements and 18o d was measured in a few samples average chemical compositions for the ksz and wgt regions have been calculated from map unit averages weighted by regional abundance compared with estimates of the composition of the upper continental crust ucc the ksz is enriched in al fe mg ca p transition elements sc and sr depleted in si b most rare earth elements zr hf nb ta li na k u th ba and rb but similar in composition to estimates of the lower continental crust the wgt is closer in composition to the ucc these data support the interpretation on geophysical and petrological grounds that the crust here is layered and has been uplifted the wgt and ksz regions representing progressively deeper levels igneous rock and orthogneiss 18o values in the ksz and wgt show good correlation with the weigh

u pb zircon and monazite ages from the kapuskasing uplift age constraints on deformation within the ivanhoe lake fault zone a decade of u pb dating of zircon and monazite from high grade metamorphic rocks in the kapuskasing uplift has identified a series of magmatic and metamorphic events between 2700 and 2585 ma and indicates that the onset of regional granulite metamorphism took place at mid crustal levels of the southern superior craton ca 2660 ma new u pb ages for zircon and monazite have been used to constrain the age of ductile deformation fabrics at two sites in the ivanhoe lake fault zone the structure along which the granulite facies kapuskasing structural zone was uplifted these results suggest that the fault zone was probably active in the late archean as young as 2630 ma and again at approximately 2500 ma

u pb zircon geochronology of the kapuskasing structural zone and vicinity in the chapleau foleyet area ontario geological development of the archean abitibi and wawa greenstone granite subprovinces of the superior province occurred in three main stages 1 a volcanic sedimentary depositional stage 2750 2696 ma ago 2 a metamorphic tectonic event 2700 2685 ma ago and 3 a late plutonic event at 2685 2668 ma the area between wawa in the west and ivanhoe lake in the east represents an oblique section through some 20 km of the crust where geochronological relationships can be examined at different structural levels much of the eastern wawa subprovince is characterized by tonalite granodiorite gneiss containing abundant mafic xenoliths and represents a crustal level intermediate between high level metavolcanic rocks near lake superior and the deep seated gneisses of the kapuskasing structural zone a tonalitic phase of the complex gneiss of the wawa subprovince has a minimum u pb zircon date of 2707 ma but the fine zircons are partly reset possibly in response to lit par lit injections of gneissic to foliat

u pb zircon geochronology of the selbaie cu zn ag au mine abitibi subprovince canada the selbaie mine is a large tonnage low grade cu zn ag au deposit with both stratiform and epithermal characteristics mineralization comprises lower grade disseminated zn ag ore and pyritic siliceous sinter of the a1 zone at the surface and higher grade cu zn ag au quartz sulfide lenses of the a2 zone below and transitional with the a1 zone and of the b zone 1 km to the west the copper rich quartz sulfide lenses comprise networks of subvertical quartz sulfide veins vein arrays and breccia they have irregular locally gradational boundaries and variable attitudes with dips from 25 o to 90os the mineralization is within a volcanic stratigraphic succession of heterolithic breccias and felsic and intermediate tuffs and flows which are cut by mafic and felsic intrusions and felsite dikes the felsic volcanic rocks and a tonalitic marginal phase from the adjacent brouillan pluton are calc alkalic and have geochemical signatures that support a cogenetic relationship the mineralization and mine stratigraphic succession are truncated by the brouillan faults a series of subparallel west directed predominantly brittle thrusts with the brouillan pluton on the upper plate above the mine stratigraphic succession galenas from stratiform and vein ore have uniform lead isotope compositions of 2o6pb 2o4pb 13 184 to 13 211 2o7pb 2o4pb 14s351 to 14 369 and 2ospb 2o4pb 33s039 to 33 100 with model values of 8 1 8 2 and model k values of 4 1 to 4 2 n 5 among the most primitive of abitibi volcanic associated massive sulfide depositss they have model ages that range from 2 71 to 2 68 ga a u pb zircon age of 2729 ma for a sericite altered calc alkalic quartz porphyry flow 150 m up section from stratiform mineralization closely approximates the age for this mineralization type a u pb zircon age for a felsite dike that cuts postore east west foliation is 2726 3 ma slightly younger but overlapping at the 20 level with the age for the quartz feldspar porphyry flow indicating that the copper rich quartz sulfide lenses are essentially coeval with the lower grade stratiform mineralization a sample of sheared chloritic granodiorite taken from one of the brouillan faults near the margin of the brouillan pluton has a complex zircon opulation six concordant anal ses of abraded multigrain fractions individual grains or grain tips 20 p 206 s

rare earth element patterns in archean high grade metasediments and their tectonic significance abstract metasediments from two contrasting types of archean high grade terrains are interpreted as being derived from distinct tectonic settings the kapuskasing structural zone canada represents the deep roots of a typical greenstone belt whereas the limpopo province southern africa and western gneiss terrain australia mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism upper amphibolite to granulite fades paragneisses from the kapuskasing structural zone have ree patterns similar to those of greenstone belt sediments except where partial melting has occurred forming restites with eu enrichment and melts with eu depletion except in this latter instance metamorphism has not affected the primary ree patterns ree patterns in archean upper amphibolite to granulite facies metasediments from the central limpopo province and western gneiss terrain show wide differences ranging from patterns which resemble those in post archean terrigenous sediments to typical archean sedimentary rock patterns the diversity in ree patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances samples with post archean patterns displaying eu depletion are interpreted as being derived from k rich granitic plutons which were portions of small stable early archean terrains precursors of the widespread late archean proterozoic episode of major cratonic development

heat production in an archean crustal profile and implications for heat flow and mobilization of heat producing elements we have measured concentrations of heat producing elements th u and k in 58 samples representative of the main lithologies in a 100 km transect of the superior province of the canadian shield from the michipicoten wawa greenstone belt near wawa ontario through a domal gneiss terrane of amphibolite grade to the granulite belt of the kapuskasing structural zone near foleyet this transect has been interpreted as an oblique cross section through some 25 km of crust uplifted along a major thrust fault and thus provides an opportunity to examine in detail a continuous profile into deep continental crust of archean age mean heat production values for these terranes based on aereal distribution of major rock types and calculated from their th u and k concentrations are michipicoten greenstone belt 0 72 w m 3 wawa domal gneiss terrane amphibolite grade 1 37 w m 3 kapuskasing granulites 0 44 w m 3 among the silicic plutonic rocks tonalites granites and their derivative gneisses the relatively large variation in heat production correlates with modal abundances of accessory minerals including allanite sphene zircon and apatite we interpret these variations as primary pre metamorphic the relatively high weighted mean heat production of the domal gneiss terrane can be accounted for by the larger proportion there of late stage th u and k rich granitoid plutons these may have been derived from the underlying kapuskasing granulite terrane leaving it slightly depleted in heat producing elements transport of th u and k therefore could have taken place in silicate melts rather than in aqueous or carbonic metamorphic fluids this conclusion is supported by the lack of a statistically significant difference in heat production between tonalites tonalite gneisses and mafic rocks of amphibolite versus granulite grade the pre metamorphic radioactivity profile for this crustal section is likely to have been uniformly low with a mean heat production value less than 1 w m 3 this result is distinctly different from measured profiles in more silicic terranes which show decreasing heat production with depth this implies fundamental differences in crustal radioactivity distributions between granitic and more mafic terranes and may be an important factor in selective reactivation of lithologically different terranes possibly resulting in preferential stabilization of basic terranes in the geological record our results indicate that a previously determined apparently linear heat flow heat production relationship for the kapuskasing area does not relate to the distribution of heat production with depth low but significant heat production 0 4 0 5 w m 3 continues to lower crustal depths with no correlation to the depth parameter from the linear relationship this low heat production may be a minimum average granulite heat production and suggests that in general heat flow through the moho is 8 10 mw m 2 lower than the reduced heat flow calculated from the heat flow heat production regression

age petrogenesis and tectonic setting of the thessalon volcanic rocks huronian supergroup canada abstract the 2 5 ga huronian supergroup volcanic rocks in northern ontario represent one of the better preserved paleoproterozoic continental flood basalt sequences on earth the youngest age of huronian supergroup volcanism is refined to 2452 5 6 2 ma based on new u pb zircon analyses of the copper cliff rhyolite recent mapping and geochemical analyses of thessalon formation volcanic rocks define a well developed chemostratigraphy a lower series consisting of four geochemically distinct mafic lava sequences and an upper basalt andesite volcanic series these two mafic volcanic series are separated by felsic volcanic rocks rhyolite and dacite the mafic lavas are low ti tholeiites dominated by basalt and andesite and are characterized by enriched light rare earth element profiles and high th contents up to 12 ppm most basalt andesite sequences show some internal geochemical variation reflecting a combination of fractional crystallization and in some cases limited crustal contamination but each sequence represents a discrete magma batch derived by different degrees of partial melting in the subcontinental lithospheric mantle all magma batches display prominent negative nb ta and ti anomalies on primitive mantle normalized multi element diagrams interpreted to reflect melting of a heterogeneous and previously metasomatized garnet lherzolite that acquired some chemical characteristics of a source typical of subduction zone magmatism at least three distinct mantle source compositions are required to explain the range in the observed lava compositions the most unusual of these reservoirs has a number of geochemical features akin to upper continental crust and is characterized by high th yb 10 and low zr nb 11 this reservoir is interpreted to be peridotite modified through metasomatism and entrapment of arc magmas during northward neoarchean subduction along the southern superior craton margin

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas