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

seasonal activity and morphological changes in martian gullies abstract recent studies of martian dune and non dune gullies have suggested a seasonal control on present day gully activity the timing of current gully activity especially activity involving the formation or modification of channels which commonly have been taken as evidence of fluvial processes has important implications regarding likely gully formation processes and necessary environmental conditions in this study we describe the results of frequent meter scale monitoring of several active gully sites by the high resolution imaging science experiment hirise on the mars reconnaissance orbiter mro the aim is to better assess the scope and nature of current morphological changes and to provide improved constraints on timing of gully activity on both dune and non dune slopes our observations indicate that 1 gully formation on mars is ongoing today and 2 the most significant morphological changes are strongly associated with seasonal frost and defrosting activity observed changes include formation of all major components of typical gully landforms although we have not observed alcove formation in coherent bedrock these results reduce the need to invoke recent climate change or present day groundwater seepage to explain the many martian gullies with pristine appearance

himalayan type escape tectonics along the superior boundary zone in manitoba canada abstract the superior boundary zone separates the neoarchean superior province to the southeast from the paleoproterozoic reindeer zone of the trans hudson orogen to the northwest in northern manitoba late convergence along the zone in the paleoproterozoic has previously been interpreted in terms of a promontory model where the superior province indents the reindeer zone towards the northwest we present a modified promontory model that includes a wnw transport direction of the superior province and lateral escape of a segment of the superior province the split lake block along the northern margin of the promontory the model is based on the kinematics of an array of shear zones along the superior boundary zone that are interpreted as having been active concurrently tectonic escape of the split lake block occurred in a manner similar to the extrusion and rotation of the indochina block along the eastern margin of the himalayas but at a much smaller scale our model explains shear zone kinematics but also the curvilinear nature of some of the shear zones and a high component of along strike stretch along one of the shear zones

precise u pb zircon ages for the molson dyke swarm and the fox river sill constraints for early proterozoic crustal evolution in northeastern manitoba canada precise u pb zircon ages have been obtained for samples from the molson dyke swarm and the fox river sill in ne manitoba canada the ages determined for the cross lake and cuthbert lake dykes are 1 883 7 1 5 1 7and 1 883 2 ma respectively and are in excellent agreement with the 1 882 9 1 4 1 5ma age obtained for the fox river sill these results support the contention that the emplacement of the fox river sill and the molson dyke swarm was contemporaneous and also demonstrate the potential for correlating mafic igneous activity in widely spaced localities the timing of early proterozoic mafic magmatism in the western superior province appears to be synchronous with igneous activity in other parts of the circum superior belt and in the trans hudson orogen to the west the emplacement of the molson dyke swarm at 1 883 ma indicates a 700 ma interval of quiescence between the final igneous activity that is recorded in the archean basement and dyke intrusion the presence of deformed equivalents of molson dykes in the thompson nickel belt indicates that the intense deformation in this belt occurred sometime after 1 883 ma

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

determination of fe mg partitioning between perovskite and magnesiowustite the partition coefficient of fe and mg between perovskite and magnesiowostite k mg fe pv mw has been measured by adopting a b 2 o 3 catalyst technique the present experimental results demonstrate that k mg fe pv mw is a factor of two closer to unity than that obtained in previous studies k mg fe pv mw increases from 0 16 to 0 35 with increasing mg of magnesiowustite from 50 to 86 on the basis of these results the compositions of enstatite and magnesiowustite included in diamond imply that the temperature at the top of the lower mantle is 1870 2040 k the mg 100mg mg fe of perovskite and magnesiowustite at the top of the lower mantle are 93 and 82 83 respectively for a pyrolitic mantle model the upper bound of the lower mantle temperature estimated in this study do not allow a larger fraction of perovskite than the pyrolitic mantle

peak metamorphic temperatures from cation diffusion zoning in garnet a model that relates the characteristic diffusion length and average cooling rate to peak temperature was developed for chemical diffusion in spherical geometries on the basis of geospeedometry principles and diffusion theory the model is quantitatively evaluated for cation diffusion profiles in garnet important model parameters were calibrated empirically using diffusion zoning of ca in garnet from the pikwitonei granulite domain a terrane for which the thermal history has been well characterized the results are used i to empirically test diffusion parameters for mg and fe ii and ii to develop a tool that uses the diffusion zoning of these cations in garnet to constrain peak temperature conditions for garnet bearing rocks the thermometric approach was externally tested by applying it to garnet crystals from various metamorphic terranes worldwide and comparing the results to published peak temperature estimates the results overlap within uncertainties in all cases but result that are based on fe ii and mg chemical diffusion profiles are up to three times more precise than those acquired by conventional methods the remarkable consistency of the results implies that the model is robust and provides a reliable means of estimating peak temperatures for different types of high grade metamorphic rock the tool could be of particular advantage in rocks where critical assemblages for conventional thermometry do not occur or have been replaced during retrogression

a regional perspective of the quetico metasedimentary belt superior province canada alternating greenstone granite and metasedimentary gneiss belts are a first order tectonic feature of the southern superior province the tectonic development of the quetico metasedimentary belt is reviewed with regard to depositional structural and metamorphic plutonic history over its 1200 km length the belt consists of marginal metasedimentary schists of turbiditic origin and interior metasedimentary migmatite and peraluminous leucogranite polyphase deformation has resulted in a steep easterly striking foliation and regional gently east plunging stretching lineation metamorphic grade varies in a low p facies series from greenschist at the belt margins to upper amphibolite and local granulite in the central migmatite intrusive granite zone mineral assemblages in the central zone yield estimates of metamorphic pressure that increase systematically eastward over 800 km from about 250 mpa 2 5 kbar near the canada united states border to 600 mpa 6 kbar in granulites adjacent to the kapuskasi

thermal behavior and ice table depth within the north polar erg of mars abstract we fully resolve a long standing thermal discrepancy concerning the north polar erg of mars several recent studies have shown that the erg s thermal properties are consistent with normal basaltic sand overlying shallow ground ice or ice cemented sand our findings bolster that conclusion by thoroughly characterizing the thermal behavior of the erg demonstrating that other likely forms of physical heterogeneity play only a minor role and obviating the need to invoke exotic materials thermal inertia as calculated from orbital temperature observations of the dunes has previously been found to be more consistent with dust sized materials than with sand since theory and laboratory data show that dunes will only form out of sand sized particles exotic sand sized agglomerations of dust have been invoked to explain the low values of thermal inertia however the polar dunes exhibit the same darker appearance and color as that of dunes found elsewhere on the planet that have thermal inertia consistent with normal sand sized basaltic grains whereas martian dust deposits are generally lighter and redder the alternative explanation for the discrepancy as a thermal effect of a shallow ice table is supported by our analysis of observations from the mars global surveyor thermal emission spectrometer and the mars odyssey thermal emission imaging system and by forward modeling of physical heterogeneity in addition our results exclude a uniform composition of dark dust sized materials and they show that the thermal effects of the dune slopes and bright interdune materials evident in high resolution images cannot account for the erg s thermal behavior

the kapuskasing uplift a geological and geophysical synthesis over the past decade the kapuskasing uplift has been the subject of intense geological and geophysical investigation as lithoprobe s window on the deep crustal structure of the archean superior province enigmatic since its recognition as a positive gravity anomaly in 1950 the structure has been variably interpreted as a suture rift transcurrent shear zone or intracratonic thrust diverse studies including geochronology geothermobarometry and various geophysical probes provide a comprehensive three dimensional image of archean 2 75 2 50 ga crustal evolution and proterozoic 2 5 1 1 ga cooling and uplift the data favour an interpretation of the structure as an intracratonic uplift related to hudsonian collision eastward across the southern kapuskasing uplift erosion levels increase from 10 km in the michipicoten greenstone belt through the wawa gneiss domain 10 20 km into granulites 20 30 km of the kapuskasing structural zone juxtaposed against the low grade swayze greenstone belt al

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

dune slope activity due to frost and wind throughout the north polar erg mars repeat high resolution imaging of dunes within the martian north polar erg have shown that these dune slopes are very active with alcoves forming along the dune brink each mars year in some areas a few hundred cubic metres of downslope sand movement have been observed sometimes moving the dune brink backwards based on morphological and activity timing similarities of these north polar features to southern dune gullies identifying the processes forming these features is likely to have relevance for understanding the general evolution modification of dune gullies to determine alcove formation model constraints we have surveyed seven dune fields each over 1 4 mars winters consistent with earlier reports we found that alcove formation activity occurs during the autumn winter seasons before or while the stable seasonal frost layer is deposited we propose a new model in which alcove formation occurs during the autumn and springtime sublimation activity then enhances the feature summertime winds blow sand into the new alcoves erasing small alcoves over a few mars years based on the observed rate of alcove erasure we estimated the effective aeolian sand transport flux from this we proposed that alcove formation may account for 2 20 of the total sand movement within these dune fields

archean crustal evolution of the northwestern superior craton margin u pb zircon results from the split lake block the split lake block forms a partly retrogressed granulite grade basement segment located at the northwestern margin of the superior province in manitoba unlike other segments along the craton margin the effects of proterozoic tectonism are relatively minor in the split lake block making it amenable to establishing firm temporal constraints for the archean magmatic and metamorphic history of the northwestern superior province margin consequently samples from the main lithological units within the split lake block were selected for precise single grain u pb zircon geochronology heterogeneous zircon populations isolated from representative enderbite tonalite and granodiorite samples reveal a complex growth history with pre 2 8 ga protolith ages e g 284 1 2m a tonalite possibly as old as 3 35 ga as indicated in a granodiorite sample the youngest archean granitic magmatism identified in the eastern split lake block is represented by the 270 8 3m agull lake granite a u pb zircon age of 2695 1 ma obtained for leucosome in mafic granulite is interpreted to reflect the timing of granulite grade metamorphism in the split lake block supported by polyphase zircon growth and or lead loss at ca 2 7 ga in the enderbite sample a younger phase of metamorphic zircon growth at ca 2 62 ga is documented in the tonalite and granodiorite zircon populations the 2 70 2 71 ga crust formation the occurrence of ca 2695 ma high grade metamorphism and broadly contemporaneous paleoproterozoic mafic dykes in both the split lake block and pikwitonei granulite domain imply a common evolution of these high grade terrains along the northwestern superior craton margin since the late archean

mesoarchean ttg magmatism from the northeastern margin of the kaapvaal craton southern africa arguments for an exotic terrane remnant of pilbara craton abstract high grade terranes along archean cratonic margins either represent remobilized equivalents of the adjacent low grade domain or an exotic entity which accreted to the developing craton this study addresses this debate for the southern marginal zone high grade terrane along the northeastern margin of the kaapvaal craton the hout river shear zone separates the high grade terrane from the adjacent low grade pietersburg block of the kaapvaal craton tonalite trondhjemites of the baviaanskloof gneiss unit constitute the dominant rock in the southern marginal zone both pyroxene bearing and pyroxene absent variants occur locally the granitoids are affected by anatectic and metasomatic overprints magmatic textures are better preserved in the massive granitoids u pb zircon analyses of two orthopyroxene bearing granitoids gave mesoarchean crystallization ages 3088 11 ma massive 3121 9 ma banded however overprint ages differ with 2706 50 34 ma from the massive variety and 1925 19 ma 1976 11 ma from the banded variety the syn tectonic 2 71 ga is interpreted as metamorphic overprint age while the post tectonic 2 ga represent age of local overprint available geochemical data on the baviaanskloof granitoids are similar to archean ttgs a possible spatial temporal transition from 3 13 ga low pressure through 3 12 2 98 ga medium pressure to 2 92 ga high pressure ttg is delineated from the northwest towards the southeastern southern marginal zone this trend is compatible with the southward directed polarity of accretion events recognised in the adjacent low grade pietersburg block we argue that the northeastern margin of the kaapvaal craton north of the hout river shear zone is an exotic terrane because 1 the baviaanskloof granitoid event 3 1 2 9 ga with peak at 3 1 ga is older than the accretion episodes 2 97 2 92 ga with peak at 2 95 ga and 2 87 2 84 ga with peak at 2 86 ga in the adjacent pietersburg block 2 the younger 2 86 ga event occurs closer to the hout river shear zone while the older 2 95 ga event was away from it and 3 contemporaneous 3 1 3 0 ga detrital zircons are absent in greenstone belts of the pietersburg block that are proximal to the hout river shear zone possible connection to rocks of similar age from the pilbara craton as part of the vaalbara supercraton is explored

early crustal evolution of the superior craton a u pb hf and o isotope study of zircon from the assean lake complex and a comparison to early crust in other cratons abstract we report u pb hf o isotopic measurements of zircon extracted from meta igneous rocks exposed in the assean lake crustal complex northwestern superior craton canada with crystallization ages of ca 3 22 ga zircon from the assean lake crustal complex are among the oldest reported in the superior craton the hf and o isotope compositions of zircon indicate that these meta igneous rocks derive from a mantle component with a relatively primitive hf isotope composition similar to chondrites that was uncontaminated by elevated 18o material ehf initial 0 and 18ovsmow 5 5 these features are shared by the large majority of the zircon extracted from the oldest meta igneous rocks exposed in archean terranes positive ehf deviations in archean zircons are typically observed in magmas that post date the oldest magmatic activity in a given terrain this temporal variability in magma source regions may reflect a sequence of events comprising i felsic magmatism derived from undepleted mantle components followed by ii re melting of the same mantle component producing the depleted hf signatures in zircon and then iii accretion of the proto craton and reworking of the whole system enabling stabilization

mars south polar spring and summer behavior observed by tes seasonal cap evolution controlled by frost grain size thermal emission spectrometer tes observations of the recession phase of mars south polar cap are used to quantitatively map this recession in both thermal and visual appearance geographically nonuniform behavior interior to the cap is characterized by defining several small regions which exemplify the range of behavior for most of the cap while temperatures remain near the co2 frost point albedos slowly increase with the seasonal rise of the sun then drop rapidly as frost patches disappear over a period of 20 days a cryptic region remains dark and mottled throughout its cold period tes observations are compared with first order theoretical spectra of solid co2 frost with admixtures of dust and h2o the tes spectra indicate that the cryptic region has much larger grained solid co2 than the rest of the cap and that the solid co2 here may be in the form of a slab the mountains of mitchel remain cold and bright well after other areas at comparable latitude apparently as a result of unusually small size of the co2 frost grains we found little evidence for a significant presence of h2o although co2 grain size may be the major difference between these regions incorporated dust is also required to match the observations a self cleaning process carries away the smaller dust grains comparisons with viking observations indicate little difference in the seasonal cycle 12 martian years later the observed radiation balance indicates co2 sublimation budgets of up to 1250 kg m 2 regional atmospheric dust is common localized dust clouds are seen near the edge of the cap prior to the onset of a regional dust storm and interior to the cap during the storm

the pikwitonei granulite domain manitoba a giant neoarchean high grade terrane in the northwest superior provincethis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh n machado t e krogh and w weber are deceased the pikwitonei granulite domain located at the northwestern margin of the superior province is one of the largest neoarchean high grade terranes in the world with well preserved granulite metamorphic assemblages preserved in a variety of lithologies including enderbite opdalite charnockite and mafic granulite u pb geochronology has been attempted to unravel the protolith ages and metamorphic history of numerous lithologies at three main localities natawahunan lake sipiwesk lake and cauchon lake the u pb age results indicate that some of the layered enderbite gneisses are mesoarchean 3 4 3 0 ga and the more massive enderbites are neoarchean the high grade metamorphic history of the pikwitonei granulite domain is complex and multistage with at least four episodes of metamorphic zircon growth identified 1 2716 1 3 8 ma 2 2694 6 0 6 ma 3 2679 6 0 9 ma and 4 2642 5 0 9 ma metamorphic zircon growth during episodes 2 and 3 are interpreted to be regional in extent corresponding

avalanche slope angles in low gravity environments from active martian sand dunes 1 the properties of granular material have an important effect on surface landforms and processes recently it has been suggested that material properties called dynamic and static angle of repose vary with gravitational acceleration which would have a significant effect on many planetary surface processes such as crater collapse and gully formation in order to test that hypothesis we measured lee slopes of active aeolian sand dunes on mars using the high resolution imaging experiment hirise dtms digital terrain model we examined dune fields in nili patera herschel crater and gale crater our measurements showed that the dynamic angles of repose for the sands in these areas are 33 34 in the first region and 30 31 in the other two these results fall within the 30 to 35 window for the dynamic angles of repose for terrestrial dunes with similar flow depths and grain properties and thus show that this angle does not significantly vary with decreasing gravity

late archaean early proterozoic co2 streaming through the lower crust and geochemical segregation considerable evidence exists for large scale migration of co2 rich fluids through the lower continental crust in the late archaean and early proterozoic with transfer of heat producing elements and h2o to higher levels the evidence manifest in the large charnockitic terrains of this age includes deformation related charnockitic alteration of gneisses nearly ubiquitous co2 rich fluid inclusions in minerals depletion of large ion lithophile elements lile particularly rb relative to typical upper crustal rocks similar depletion of 18o in paragneisses and petrographic and geochemical evidence of open system replacement of amphibole gneiss by charnockite a conveyor belt upper mantle source appears to have been necessary for prolonged co2 supply in high grade crustal metamorphism most plausibly by deep subduction of marine carbonate under continental interiors this plate tectonic mode seems consonant with crustal thickening and orogenic patterns inferred for most charnockitic terrains and with co2 transport mechanisms constrained by experimental petrology

nature of assean lake ancient crust manitoba a combined shrimp id tims u pb geochronology and sm nd isotope study abstract the assean lake crustal complex alcc located at the northwest periphery of the superior province in northern manitoba canada hosts an ancient assembly of rocks that record a complex and prolonged crustal history spanning more than two billion years from 3 9 to 1 8 ga a supracrustal assemblage consisting of amphibolite grade greywacke quartz arenite arkose mafic to intermediate volcanics and to a lesser extent iron formation dominates the alcc and likely formed in a shallow marine arc back arc setting the supracrustal rocks are intruded by amphibolite grade calc alkaline tonalite to granite orthogneiss nd model ages for felsic para and orthogneisses range from 3 5 to 4 2 ga and suggest that paleoarchean basement may be preserved at assean lake shrimp u pb zircon ages and id tims conventional u pb zircon and monazite ages show that felsic magmatism occurred at 3 1 3 2 ga whereas a record extending back to at least 3 9 ga is preserved in detritus in paragneiss that may have a depositional age of 3 2 ga orthogneiss was metamorphosed at 2 68 ga whereas local migmatisation of paragneiss occurred at 2 61 ga monazite growth events at 2 6 2 45 and 1 8 ga might represent episodes of accretion of archean crustal segments including the alcc and possibly the sask craton trans hudson basement along the northwest superior province margin at present matching segments of the exotic alcc are unknown but remnants may exist in parts of the sask craton

ion microprobe u pb age determinations on zircon from the late archean granulite facies transition zone of southern india abstract the southern dharwar craton of karnataka and tamil nadu states in south india exposes a depth profile of late archean crust amphibolite facies felsic to intermediate peninsular gneiss of protolith ages 3 4 2 9 ga with subordinate infolded supracrustal relics and the large closepet granite body give way southward to granulite facies orthopyroxene bearing gneisses over a transition zone about 40 km wide with mineralogic pressure indicators increasing from about 4 kbar corresponding to about 15 km paleodepth to about 7 kbar corresponding to near 25 km paleodepth previous whole rock and zircon geochronology have established that amphibolite facies metamorphism migmatization and granite intrusion took place in the latest archean at 2 53 2 51 ga however some workers have considered the foliated granulites as products of an earlier metamorphism distinct from the terminal archean events this work presents whole rock geochemical studies coupled with ion microprobe 207pb 206pb ages and th u

x ray microanalysis of high pressure high temperature phases synthesized from natural olivine in a diamond anvil cell the transformation products of natural mg0 83fe0 17 2sio4 olivine have been prepared at high pressures and high temperature in a laser heated diamond anvil cell dac the study of the quenched high pressure phases has been performed using analytical transmission electron microscopy the iron magnesium partition coefficient between perovskite pv and magnesiowustite mw defined as kmw pv fe mg mw fe mgpv was measured in samples synthesized at pressures from 25 to 75 gpa under various heating conditions in all cases iron goes preferentially into magnesiowustite it was observed however that the partition coefficient decreases strongly between 25 and 40 gpa at pressures higher than 40 gpa it is constant 3 5 within experimental error although there is a systematic increase in k as t decreases the effect of temperature is weak at high pressures from the values of the iron partition coefficient and previously existing thermodynamic values we obtain a consistent thermodynamic data set for the major constituents of the lower mantle the equilibrium compositions of mg fe 2sio4 spinel mg fe sio3 perovskite and mg fe o magnesiowustite phases have been measured and a new high pressure phase of composition mg fe si2o5 is shown to occur in very high temperature regions probably due to high temperature breakdown or non stoichiometry of mg fe sio3 perovskite

pressure and temperature dependence of elastic wave velocity of mgsio3 perovskite and the composition of the lower mantle abstract simultaneous measurements of compressional p and shear s wave velocities and unit cell volume density have been conducted on mgsio 3 perovskite to 9 2 gpa and 873 k using ultrasonic interferometry in conjunction with x ray diffraction finite strain analysis of current data allowed us to determine the elastic moduli and their pressure derivatives independent of the pressure measurement the results are k s 253 2 gpa g 173 1 gpa k s p 4 4 1 and g p 2 0 1 the temperature derivatives of the bulk and shear moduli were tightly constrained from acoustic velocity measurements yielding k s t 0 021 2 gpa k and g t 0 028 2 gpa k p and s wave velocities and density profiles for a pyrolite compositional model consisting of magnesium silicate perovskite and magnesiowustite are calculated at lower mantle depths along a 1600 k adiabatic geotherm the results agree with those of seismic model prem within 0 5 0 5 and 0 3 respectively from 800 to 2600 km the effect of fe and mg partitioning between magnesium silicate perovskite and magnesiowustite on the calculated velocities and density is negligibly small compared to the effects of composition and temperature simultaneous constraints from p and s wave velocities and density support for a homogeneous lower mantle with pyrolitic composition

thermobarometry of garnet clinopyroxene hornblende granulites from the kapuskasing structural zone amphibole mixing properties and standard state properties for the end members tremolite tschermakite pargasite ferrotremolite ferrotschermakite and ferropargasite have been refined through analysis of relevant phase equilibrium experiments coexisting cummingtonite hornblende pairs and natural mineral parageneses when merged with the mineral properties given by r g berman these amphibole properties allow the calculation of numerous equilibria suitable for thermobarometry in a wide range of bulk compositions at amphibolite to granulite metamorphic grades the method was applied to texturally equilibrated garnet pyroxene hornblende gneisses and compared both to results based on anhydrous assemblages and to those for garnet biotite gneisses from the kapuskasing structural zone thermometer resetting is apparent only in garnet biotite gneisses whereas touching garnet pyroxene pairs in mafic gneiss show little evidence of retrograde fe mg exchange computed metamorphic temperatures garnet clinopyrox

element partitioning between magnesium silicate perovskite and ferropericlase new insights into bulk lower mantle geochemistry n this study we investigated iron magnesium exchange and transition metal trace element partitioning between magnesium silicate perovskite mg fe sio3 and ferropericlase mg fe o synthetised under lower mantle conditions up to 115 gpa and 2200 k in a laser heated diamond anvil cell recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy atem and nanometer scale secondary ion mass spectroscopy nanosims iron concentrations in both phases were obtained from x ray energy dispersive spectroscopy measurements and nanosims our results are the first to show that recently reported spin state and phase transitions in the lower mantle directly affect the evolution of fe mg exchange between both phases mg perovskite becomes increasingly iron depleted above 70 80 gpa possibly due to the high spin low spin transition of iron in ferropericlase conversely the perovskite to post perovskite transition is accompanied by a strong iron enrichment of the silicate phase ferropericlase remaining in the fe rich phase though nanoparticles of metallic iron were observed in the perovskite bearing runs suggesting the disproportionation of ferrous iron oxide but were not observed when the post perovskite phase was present implications on the oxidation state of the earth and core segregation will be discussed transition trace element ni mn concentrations determined with the nanosims show similar trends and could thus be used to trace the origin of diamonds generated at depth this study provides new results likely to improve the geochemical and geophysical models of the earth s deep interiors

geochronological constraints on the development of the cross lake greenstone belt northwest superior province manitoba recent 1 20 000 mapping combined with 11 age determinations from metavolcanic metaplutonic and metasedimentary rocks in the cross lake greenstone belt provide a framework for interpreting the stratigraphic structural intrusive and metamorphic evolution of the belt these data in combination with u pb ages from adjacent granitoid domains are used to model a cratonizing event in the northwest superior province the cross lake greenstone belt is made up of three distinct unconformable groups of supracrustal rocks i the pipestone lake group 2760 ma a thick sequence of pillowed and massive tholeiitic basalt flows and related high level gabbros inter preted as a back arc spreading sequence ii the gunpoint group 2730 ma a fining upward sequence of clastic sediments interbedded with rhyodacitic fragmental volcanic rocks and iii the cross lake group 2710 ma a fining upward fluvial marine clastic sedimentary sequence with shoshonitic volcanic rocks near the top deposited in a restricte

age and origin of the jan lake complex a glimpse at the buried archean craton of the trans hudson orogen the largely buried sask craton is a continental fragment or microcontinent that collided with and underthrust the paleoproterozoic flin flon glennie protocontinent along the pelican decollement zone at 1840 1830 ma rocks of the sask craton are exposed in three tectonic windows those of the pelican window have been named the jan lake complex and comprise 2960 ma arc derived leucocratic orthogneisses migmatitic paragneisses and a tholeiitic within plate igneous suite comprising 2488 ma dioritic to gabbroic rocks and 2450 ma enderbitic and charnockitic rocks including the sahli granite ages of ca 2450 ma are also common from the other two tectonic windows to the sask craton suggesting that emplacement of the igneous suite was widespread and perhaps part of the coeval matachewan igneous event the absence of rocks in the 2960 and ca 2450 ma age ranges on both the adjacent superior and rae hearne cratonic margins makes it improbable that the sask craton was derived by simple fragmentation without large scale tectonic transport the overthrust paleoproterozoic rocks represent a high grade northwestern extension of the flin flon volcanic belt and include 1856 ma leucotonalite and 1843 ma quartz monzodiorite an 1830 ma suite of homogeneous calc alkaline enderbitic rocks which intrude burntwood semipelitic migmatites throughout the kisseynew domain has also been emplaced in mylonites of the pelican decollement zone zircon and monazite ages in the 1812 1803 ma range record a high grade metamorphic event that resulted from collision between the sask craton and flin flon glennie complex

postspinel transformations and the width of the 670 km discontinuity a comment on postspinel transformations in the system mg2sio4 fe2sio4 and some geophysical implications by e ito and e takahashi in their recent paper dealing with postspinel transformations in the system mg2sio4 fe2sio4 ito and takahashi 1989 showed that the reaction mg fe 2sio4 mg fe sio3 mg fe o spinel perovskite magnesiowustite occurs over a very narrow pressure interval at 1600 c for values of fe fe mg of between 0 0 and 0 3 it is the purpose of this comment to point out that the argument for a narrow transition interval can be independently confirmed and strengthened using ito and takahashi s data on fe mg partitioning between the phases spinel perovskite and magnesiowustite ito and takahashi s contention that the postspinel transition in mg fe 2sio4 is sharp enough to produce the major or sole contribution to the 670 km discontinuity is reinforced from author

variations in wind velocity and sand transport on the windward flanks of desert sand dunes the magnitudes of increases in wind velocity or speed up factors have been measured on the windward flanks of transverse and linear dunes of varying height on transverse dunes velocity speed up varied with dune shape and height for linear dunes speed up factors varied principally with wind direction relative to the dune with dune shape and dune height the main effect of velocity speed up on the windward flanks of dunes is to increase potential sand transport rates considerably in crestal areas this is greatest for large dunes with winds of moderate velocity blowing at a large angle to the dune changing ratios of base to crest sand transport rates on transverse dunes tend to reduce dune steepness as overall wind velocities increase on linear dunes the tendency for crestal lowering is counteracted by deposition in this area when winds reverse in a bi directional wind regime

giant granulite terranes of northeastern superior province the ashuanipi complex and minto block the ashuanipi complex and minto block of the superior province are large regions that have been classified as high grade gneiss terranes on the basis of the presence of orthopyroxene bearing units like the granite greenstone and metasedimentary subprovinces of the southern superior province the two terranes consist predominantly of intrusive rocks but are distinguished by their primary magmatic orthopyroxene both high grade and gneiss are misnomers because granulite facies gneisses are only sparingly present and the regions consist dominantly of massive unmetamorphosed plutonic rock the ashuanipi complex consists of a deformed metamorphosed package of metasedimentary rocks and primitive early tonalite cut by widespread orthopyroxene garnet granodiorite diatexite as well as plutons of tonalite granite and syenite based on its lithological and chronological similarity and on strike position the complex appears to be the continuation of metasedimentary subprovinces such as the quetico its evolution involved deposition of immature greywacke in an accretionary prism early arc tonalitic magmatism and deformation followed by widespread intracrustal magmatism in the period 2700 2670 ma both metamorphic and igneous rocks record equilibration under granulite facies conditions 700 835 c 0 35 0 65 gpa a 0 3 and indicate exposure levels of 20 km the minto block at the latitude of leaf river consists of several north northwest trending domains of similar scale and diversity to the east trending subprovinces of the southern superior province the central goudalie domain is dominantly amphibolite facies tonalitic rocks including some with ages 3 ga with small belts of volcanic and sedimentary origin lake minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite clinopyroxene orthopyroxene granodiorite orthopyroxene biotite diatexite and granite the hornblende granodiorite suite constitutes most of the utsalik and tikkerutuk domains and is thought to represent continental arc magmatism on the basis of their distinct aeromagnetic and lithological character two additional domains are evident north of the leaf river area the inukjuak domain in the west and the douglas harbour domain in the east the northerly grain of the minto block appears to have been established in situ with respect to the easterly belts of the southern superior province i e no large scale block rotation during the same interval of time 3 0 2 7 ga modification of the tectonic framework for the superior province is required to explain minto arc magmatism in the interval 2730 2690 ma ago a continental magmatic arc built the berens river and bienville subprovinces and minto block on the southern and eastern edges respectively of a northern protocratonic foundation in the same period primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime

the effect of fe spin crossovers on its partitioning behavior and oxidation state in a pyrolitic earth s lower mantle system abstract geophysical interpretations of the earth s interior and its dynamics are significantly influenced by phase transitions of the constituting minerals and their chemical compositions pressure induced fe spin crossovers in the main mineral phases of the earth s lower mantle mg fe silicate perovskite and ferropericlase have been suggested to influence fe partitioning resulting in separate layers with distinct physical properties however previous results remain ambiguous regarding the exact effect of fe spin crossovers and the actual transition pressures we observe here a continuous decrease of the fe 2 mg partition coefficient k d between silicate perovskite and ferropericlase from 25 gpa to 79 gpa in a pyrolitic earth s lower mantle system at about 97 gpa the k d significantly increases with an accompanied decrease of the fe 3 fe ratio in perovskite which therefore leads to an amplified change in the fe 2 k d we conclude that the fe 2 high spin to low spin crossover in ferropericlase and the fe 2 high spin to intermediate spin crossover in perovskite at mid lower mantle pressures 30 80 gpa exert no control on k d but the fe 2 intermediate spin to low spin crossover in silicate perovskite at about 100 gpa preferentially partitions fe into silicate perovskite and reduces its fe 3 content the change in oxidation state and partitioning behavior of fe will increase thermal conductivity and probably could induce a thermal boundary layer at this depth

diffusion of transition metals in periclase by experiment and first principles with implications for core mantle equilibration during metal percolation diffusion of the divalent transition metals in the deep mantle is important for understanding length scales of mantle heterogeneity transport across the core mantle boundary and the degree of equilibration during core formation here we report the results of experiments and theoretical calculations on ni co fe and mn diffusion in periclase the primary repository of these elements in earth s lower mantle the periclase used in the experiments was doped with 550 ppm al 3 to fix the cation vacancy concentration and the experiments were performed at 2 gpa and temperatures between 1673 and 2073 k the variation in diffusion coefficients among the transition metals was

improved general circulation models of the martian atmosphere from the surface to above 80 km we describe a set of two new generation general circulation models of the martian atmosphere derived from the models we originally developed in the early 1990s the two new models share the same physical parameterizations but use two complementary numerical methods to solve the atmospheric dynamic equations the vertical resolution near the surface has been refined and the vertical domain has been extended to above 80 km these changes are accompanied by the inclusion of state of the art parameterizations to better simulate the dynamical and physical processes near the surface boundary layer scheme subgrid scale topography parameterization etc and at high altitude gravity wave drag in addition radiative transfer calculations and the representation of polar processes have been significantly improved we present some examples of zonal mean fields from simulations using the model at several seasons one relatively novel aspect previously introduced by wilson 1997 is that around northern winter solstice the strong pole to pole diabatic forcing creates a quasi global angular momentum conserving hadley cell which has no terrestrial equivalent within such a cell the coriolis forces accelerate the winter meridional flow toward the pole and induce a strong warming of the middle polar atmosphere down to 25 km this winter polar warming had been observed but not properly modeled until recently in fact thermal inversions are generally predicted above one and often both poles around 60 70 km however the mars middle atmosphere above 40 km is found to be very model sensitive and thus difficult to simulate accurately in the absence of observations

highly varying radiogenic heat production in finland fennoscandian shield abstract radiogenic heat production in finland has been previously studied using airborne gamma ray surveys and glacial till measurements alike for the first time this paper presents a detailed survey on the spatial variation in radiogenic heat production determined using outcrop samples obtained from all important lithologies of the country the dataset of 6465 samples represents mostly mesoarchean about 2 7 ga paleoproterozoic ca 2 2 1 8 ga and mesoproterozoic ca 1 6 1 3 ga rocks nearly all data are from precambrian fennoscandian shield area but heat production appears to be highly variable and above global archean and proterozoic averages spot readings show an arithmetic average of 1 34 1 19 wm 3 and a range from 0 02 to 19 4 wm 3 the interpolated areal average of the whole area is 1 42 1 41 wm 3 the high standard deviation of data is related to the geochemical characteristics of uranium u thorium th and potassium k resulting in a skewed distribution of heat production mesoproterozoic anorogenic rapakivi granites and late paleoproterozoic svecofennian granitoids show the highest heat production values in the range of 3 5 wm 3 the results show no distinct dependencies of heat production with geological age metamorphic grade nor seismic p wave velocity but an increasing trend of heat production with sio2 content and decreasing trends of heat production with fe2o3 content and with rock density are evident surface heat flow 44 borehole data values correlates weakly with heat production r 0 35 the general heterogeneity of heat production calls for supporting information from other geophysical methods for better understanding of the thermal state of the lithosphere in finland and beyond

minto block superior province missing link in deciphering assembly of the craton at 2 7 ga plate tectonic models of the superior province are rooted in the granite greenstone and metasedimentary belts of the southern part of the craton north striking domains of the minto block in the northeastern part of the province evolved at similar times in 3 1 2 8 2 725 and 2 69 ga events requiring expansion of models of late archean assembly to accommodate minto geology western and eastern protocratons 3 1 2 8 ga rifted at 2 79 ga to produce an ocean basin that was mostly consumed by subduction at 2 725 ga the leaf river plutonic suite of cale alkalic hornblende biotite orthopyroxene clinopyroxene granodiorite represents magmatic arcs built on the protocratons whereas the intervening goudalie domain containing fault bounded fragments of rifted continental crust rift volcanics primitive oceanic crust 2724 ma island arc rocks and a 2718 ma back arc assemblage marks the suture terminal collision at 2 7 ga led to thickening and crustally derived granitoid magmatism the southern superior province also experienced vigorous activity between 2 725 and 2 69 ga as island arcs oceanic plateaus continental fragments and accretionary prisms were amalgamated progressively from north to south in a regime of dextral transpression then stitched by granites a northern proto superior craton had continental magmatic arcs built on its eastern and southern flanks in response to west northwest directed subduction orthogonal convergence in the east produced wide plutonic arcs in contrast to terrane accretion tectonics in the more oblique regime along the southern margin

subduction of continental margins and the uplift of high pressure metamorphic rocks abstract the mechanism by which high pressure metamorphosed continental material is emplaced at high structural levels is a major unsolved problem of collisional orogenesis we suggest that the emplacement results from partial subduction of the continental margin which because of its high flexural rigidity produces a rapid change in the trajectory of the descending slab we assume a two fold increase in effective elastic thickness of the lithosphere as the continental margin approaches the subduction zone and calculate the flexural profile of a thin plate for progressive downward migration of the zone of increased rigidity we assess the effect of changes in the flexural profile on the overlying accretionary prism and mantle wedge as the continent approaches by estimating the extra stresses that are imposed on the wedge due to the bending moment exerted by the continental part of the plate the wedges overlying the subduction zones and the subducting slab itself experience substantial extra compressional stress at depths of around 100 km and extensional stress at shallower depths as the continental margin passes through the zone of maximum curvature the magnitudes of such extra stresses are probably adequate to effect significant deformation of the wedge and or the descending plate and are experienced in a time interval of less than 5 m y for typical subduction rates the spatial variation of yield stresses in the region of the wedge and descending slab indicates that much of this deformation may be taken up in the crustal part of the descending slab which is the weakest region in the deeper parts of the subduction zone this may result in rapid upward migration of the crust of the partially subducted continental margin against the flow of subduction high pressure metamorphosed terranes emplaced by the mechanism envisaged in this paper would be bounded by thrust faults below and normal faults above movement on the faults would have been coeval and would have resulted in rapid unroofing of the high pressure terranes synchronous with arrival of the continental margin at the subduction zone and therefore relatively early in the history of a collisional orogen

tectonic and metamorphic events in the westernmost grenville province central ontario new results from high precision u pb zircon geochronology the u pb isotopic ages of metamorphic zircons from deformed amphibolitized metadiabase dykes and associated unstrained anatectic pegmatites have been determined to constrain the times of postdyke ductile flow related recrystallization and deformation samples from the central gneiss belt of the grenville province in the southern georgian bay region provide a means of linking areas formed during common episodes of ductile flow three zircon fractions from one attenuated dyke in the moon river domain gave identical concordant ages of 1062 1065 and 1064 2 ma while those from another gave identical concordant ages of 1052 1051 and 1050 2 ma for the time of metamorphism zircons from a folded dyke from the go home domain gave identical concordant ages of 1049 1047 and 1046 2 ma while a single overgrowth fragment from an unstrained anatectic pegmatite in the fold hinge gave a concordant age of 1047 2 ma as if folding metamorphic zircon growth and postfolding pegmatite are part of the same proc

granulite facies metamorphism in the quetico subprovince north of manitouwadge ontario a zone of granulites defined by an orthopyroxene in isograd and extending more than 100 km in length and about 10 km in width occurs near the southern margin of the quetico subprovince north of manitouwadge ontario mineral assemblages in metasedimentary rocks and associated migmatites consist of quartz plagioclase garnet orthopyroxene biotite cordierite sillimanite k feldspar hercynite magnetite ilmenite and other accessory phases minor mafic gneisses and calc silicate pods or lenses are also present from equilibrium phase relations and thermobarometry the granulites experienced a thermal peak event 4 6 kbar 1 bar 100 kpa 680 770 c a h2o of 0 15 0 25 and fo2 of 1 2 log units above the fmq buffer in association with d2 deformation followed by a retrogression 550 660 c and 3 4 kbar and a later hydrothermal alteration 1 2 kbar and 200 400 c the distribution and calculated peak metamorphic conditions of the granulite zone in the quetico subprovince are similar to those of

garnet orthopyroxene plagioclase quartz barometry refinement and application to the english river subprovince and the minnesota river valley recent experimental theoretical and thermodynamic studies permit better calibration of two reactions for geobarometry grossular pyrope quartz anorthite enstatite grossular almandine quartz anorthite ferrosilite if both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet pyroxene plagioclase quartz assemblage application to a variety of high grade terrains generally yielded excellent results however poor results have been obtained for high fe rocks which can be traced to errors in activity models for garnet and or pyroxene either a two site ideal mixing model for orthopyroxene cf wood and banno 1973 underestimates enstatite activity for high fe orthopyroxenes or the ganguly and saxena 1984 model overestimates pyrope activity in low mg garnets

large scale melt depletion in granulite terranes an example from the archean ashuanipi subprovince of quebec this study uses field petrographic and geochemical methods to estimate how much granitic melt was formed and extracted from a granulite facies terrane and to determine what the grain and outcrop scale melt flow paths were during the melt segregation process the ashuanipi subprovince located in the north eastern superior province of quebec is a large 90 000 km2 metasedimentary terrane in which 85 of the metasediments are of metagreywacke composition that was metamorphosed at mid crustal conditions 820 900 c and 6 7 kbar in a late archean dextral transpressive orogen decrease in modal biotite and quartz as orthopyroxene and plagioclase contents increase together with preserved former melt textures indicate that anatexis was by the biotite dehydration reaction biotite quartz plagioclase melt orthopyroxene oxides using melt orthopyroxene ratios for this reaction derived from experimental studies the modal orthopyroxene contents indicate that the metagreywacke rocks underwent an average of 31 vol partial melting the metagreywackes are enriched in mgo cao and feot and depleted in sio2 k2o rb cs and u have lower rb sr higher rb cs and th u ratios and positive eu anomalies compared to their likely protolith these compositions are modelled by the extraction of between 20 and 40 wt granitic melt from typical archean low grade metagreywackes a simple mass balance indicates that about 640 000 km3 of granitic melt was extracted from the depleted granulites the distribution of relict melt at thin section and outcrop scales indicates that in layers without leucosomes melt extraction occurred by a pervasive grain boundary porous flow from the site of melting across the layers and into bedding planes between adjacent layers in other rocks pervasive grain boundary flow of melt occurred along the layers for a few to tens of centimetres followed by channelled flow of melt in a network of short interconnected and structurally controlled conduits visible as the net like array of leucosomes in some outcrops the leucosomes contain very little residual material 5 biotite orthopyroxene indicating that the melt fraction was well separated from the residuum left in situ as melt depleted granulite only 1 3 vol percentage melt remained in the melt depleted granulites hence the extraction of melt generated by biotite dehydration melting in these granulites was virtually complete under conditions of natural melting and strain rates in a contractional orogen

u pb geochronologic constraints on the crustal evolution of the long range inlier newfoundland the long range mountains of newfoundland expose the northeastern most basement inlier of the appalachian orogen u pb results for two samples of basement gneiss reveal crustal formation ages of 1466 10 ma western brook pond charnockite and 1530 8 ma cat arm road gneiss indicating an affinity with pinwarian magmatism 1510 1450 ma in labrador a third sample of basement gneiss from the cat arm road dam site yielded a minimum formation age of 1631 ma these basement gneisses were intruded by granite plutons during two periods of grenvillian magmatism group i at 1032 1022 ma and group ii at 993 985 ma group i intrusions include the 1032 0 1 5 ma lomond river granite and the 1022 0 2 0 ma lake michel igneous suite group ii intrusions include the 999 4 ma potato hill charnockite 993 7 ma horse chops granite 984 9 1 6 ma cloud river granite and may include the apsy granite the metamorphic history of the long range inlier is complex and three discrete proterozoic metamorphic events can

constraints on the u pb systematics of metamorphic rutile from in situ la icp ms analysis abstract in situ laser ablation icp ms u pb dating of metamorphic rutile from granulite facies metapelitic rocks of the archaean pikwitonei granulite domain manitoba canada provides constraints on pb diffusion and characterizes the closure behavior of rutile the analysis of transects of 35 m spots across 15 rutile grains having a size of 120 to 280 m yielded concordant ages with core ages of ca 2450 ma and core to rim younging towards 2280 ma age profiles indicate that volume diffusion of pb occurs in rutile implying that the ages represent cooling ages to investigate the closure behavior of pb in rutile closure temperature profiles t c x were constructed based on different models combined with experimentally determined diffusion parameters the classical t c x model of dodson 1986 mat sci forum 7 145 154 indicates a rapid decrease of t c in the rims of grains providing unrealistic estimates for the cooling rate when combined with u pb ages a new t c x model was constructed based on the analyzed age profiles that are described by an error function this model shows a more steady decrease in t c throughout the grain from ca 640 c in the core depending on grain size to a rim intercept t c rim of 490 c 7 2 which is interpreted to be the extrapolated theoretical absolute temperature of insignificant pb diffusion in rutile the new model provides a better description of the relation between age and t c for the analyzed grains however both t c x models demonstrate that even in small grains the variations of t c can be significant making it impossible to derive one representative t c for pb in rutile the error function based t c x model allows the determination of cooling rates which show a decrease over time from ca 2 2 to 0 4 c ma agreeing well with previous estimates for the pikwitonei granulite domain this consistency supports the validity of our model and indicates that cooling rates can be estimated from single grains by la icp ms u pb dating of rutile providing constraints on the cooling history of a metamorphic terrane the slow cooling rates imply that exhumation was slow

u pb geochronology of the opatica tonalite gneiss belt and its relationship to the abitibi greenstone belt superior province quebec u pb ages for zircon monazite and titanite from samples collected from two transects across the opatica tonalite gneiss belt in the superior province of quebec indicate that the belt contains rocks that are significantly older than those in the adjacent low grade northern abitibi belt tonalite and tonalite gneiss which make up most of the belt formed over an interval of 100 ma from pre 2800 to 2702 ma five samples have ages of ca 2807 13 2773 23 and 2702 3 ma zircon growth at 2718 2721 ma in the two oldest samples may record an early period of high grade metamorphism in the belt hornblende diorite monzodiorite and tonalite plutons were intruded at 2693 2696 ma particularly along the southern boundary with the abitibi belt these include the canet pluton at ma the lac ouescapis pluton at 2693 2 ma and the barlow pluton at 2696 3 ma pink granite plutons and dykes are the youngest intrusive rocks in the belt three samples have yielded zircon ages of 2690 2 and 2686 4 ma

thermo tectonic evolution of an archean accretionary complex u pb geochronological constraints on granulites from the quetico subprovince ontario canada abstract u pb geochronological data have been obtained for a granulite zone in the quetico subprovince an archean accretionary complex of the superior province north of manitouwadge ontario canada this granulite zone represents the culmination of a low pressure high temperature regional metamorphism in the quetico subprovince and an apparent continuation of a northward increase in metamorphic grade in the wawa subprovince and across the quetico wawa boundary a u pb zircon age of 2666 1 ma from a concordant magnetite bearing granitic pegmatite in the quetico granulite zone is consistent with the inferred timing of regional metamorphism in amphibolite facies zones 2671 2665 ma and indicates that the low pressure high temperature metamorphism in the quetico subprovince commenced during the main compressional deformation titanite in a calc silicate rock from low leucosome fraction layered migmatites that might represent incipient melting gave a 207pb 206pb age of 2658 1 ma the timing of the granulite facies metamorphism is constrained by u pb zircon ages of 2650 2 and 2651 3 ma from mafic granulite and garnet bearing tonalitic leucosome respectively that are similar to reported u pb ages for mafic granulites in the upper levels of the kapuskasing structural zone the present u pb data suggest a diachronous development of peak metamorphic conditions of the regional metamorphism in the quetico and wawa subprovinces this diachronous development and the zonal distribution of the regional metamorphism are probably related to oblique collision at a convergent plate margin and differential rates of erosional uplift away from the quetico wawa boundary

trace element systematics in granulite facies rutile implications for zr geothermometry and provenance studies metamorphic rutile from granulite facies metapelitic rocks of the archean pikwitonei granulite domain pgd manitoba canada provides constraints on the systematics of trace elements in rutile during high temperature conditions and subsequent slow cooling compositional profiles and maps of the zr concentrations in rutile grains 120 600 m from three metapelitic gneisses were acquired by electron probe micro analysis using a spatial resolution of down to 2 m simultaneously profiles were analysed for nb cr and v which have significantly different diffusion characteristics in rutile the profiles of all elements show relatively homogeneous concentrations within most grains but significant inter grain differences even within a single thin section some rutile grains display a slight concentration decrease from a neighbouring garnet towards the matrix for all measured elements the lack of diffusion profiles for all analysed elements shows that these are highly immobile in rutile and that distributions of these elements are primary and preserve prograde information the nb and cr concentrations overlap with ranges that are ascribed to different provenances indicating that source discrimination based on these elements is not possible in all cases high retentiveness for zr implies that the zr in rutile geothermometer is highly robust to diffusive re equilibration even during very slow cooling 2 c ma 1 from granulite facies conditions most grains have high zr contents 3000 4600 ppm differences between high zr contents suggest that during growth under vapour absent conditions there may not be saturation of zr in rutile even if zircon is present therefore several rutile grains need to be analysed in a sample to obtain a useful minimum peak temperature the highest zr concentrations correspond to 900 c this is significantly higher than previous peak temperature estimates of 820 c based on two feldspar thermometry on a regional scale this implies that part of the pgd was affected by ultra high temperature uht metamorphism it also implies that rutile is able to preserve primary compositions even to uht conditions this study shows that if combined with textural information zr in rutile has the potential to be a very useful tool for estimating rutile crystallization temperatures and peak metamorphic conditions for granulite facies rocks zr in rutile yields more reliable peak metamorphic temperatures than most other exchange geothermometers which tend to partially re equilibrate by diffusion during cooling

timing of high pressure metamorphism in the yukon tanana terrane canadian cordillera constraints from u pb zircon dating of eclogite from the teslin tectonic zone zircon from eclogite near last peak in the teslin tectonic zone yielded a u pb isotopic age of 269 2 ma 2 the first precise age for such a rock in the yukon tanana terrane of the canadian cordillera both the morphology and geochemistry of the eclogitic zircons indicate a metamorphic origin and the u pb age therefore constrains the timing of peak high pressure metamorphism in this rock the u pb age demonstrates for the first time that an early permian high pressure metamorphic event occurred in rocks now making up the teslin tectonic zone and possibly elsewhere in the yukon tanana terrane this u pb age provides a new geochronologic pin in the evolution of the yukon tanana terrane prior to its mesozoic accretion to the north american continental margin and combined with recent 40ar 39ar muscovite data indicates that high pressure metamorphism at this time was a relatively short lived event

evolution of an archean metamorphic belt a conventional and shrimp u pb study of accessory minerals from the jimperding metamorphic belt yilgarn craton west australia u pb tims analyses of zircon monazite titanite and apatite combined with shrimp u th pb analyses of zircons from magmatic and metasedimentary rocks of the jimperding metamorphic belt in the southwestern part of the yilgarn craton western australia provide reliable information on the evolution of this belt shrimp analyses on single detrital zircon grains suggest the sediments originated from archean sources similar in age to the narryer gneiss complex deposition of the clastic metasediments is bracketed by the youngest concordant grain 3055 pm 12 ma and the minimum age of a possible amphibolite dike crosscutting the sediments 2711 pm 7 ma shrimp and tims analyses on zircons from a porphyritic granite and an orthogneiss indicate that magmatism occurred during the prograde stage of a regional metamorphic event 2660 2670 ma metamorphic monazites from pelitic schists define an age of 2661 pm 1 ma that relates to mineral growth also during the prograde stage of metamorphism peak

formation and evolution of granite magmas during crustal reworking the significance of diatexites taylor mclennan 1985 the physical processes ina continuous section through reworked archaean crust records the volved particularly how granite magma is formed remain generation of granitic magma and its subsequent development in the controversial miller et al 1988 white chappell opatica subprovince in the canadian shield there the transition 1990 a partially melted source can form a granitic from palaeosome to granite was a closed system process through magma only if large volumes of melt are separated from intermediate stages of patch migmatite and diatexite the average most of their residuum understanding this process is degree of partial melting was less than 30 but the melt crucial to determining how crustal recycling and differfraction was redistributed within individual diatexite layers during entiation occurs for example melt residuum separation deformation regions that lost melt became residual diatexites could be a nearly perfect single stage process at the enriched in tio2 feot mgo cao sc cr co sr rare earth site of partial melting as described for leucosomes in elements ree and high field strength elements hsfe melt metatexite migmatites wickham 1987a sawyer 1991 accumulated to create diatexite magmas enriched in large ion 1994 brown et al 1995 alternatively the separation lithophile elements lile but contaminated with residuum macould be an imperfect multi stage process that yields a terial such diatexite magmas are parental to granites found at magma with a large residual component as is observed higher crustal levels in the terrane flow of the diatexite magma in in diatexite migmatites bea 1991 greenfield et al response to deformation separated some of its residuum into schlieren 1996 sawyer 1996 the metatexite model necessarily parautochthonous plutons were created where ascending granitic produces leucocratic residuum free granites such as magma locally ponded below impermeable layers and structures those described by le fort 1981 and montel et al magma left the anatectic region in dykes and lost its remaining residuum as it crystallized consequently the allochthonous granite 1991 and maximizes the geochemical signature of magmas that rose through 20 km of crust to feed the highest level crustal differentiation in contrast the diatexite model plutons in this region are highly fractionated and essentially free of can produce residuum rich granites and so reduce the residuum geochemical effect of crustal differentiation as chappell 1996 noted how anatectic magmas formed in the deeper crust evolve to the granitic magmas emplaced in the upper crust is also poorly known because few crustal sections

metaconglomerate preserves evidence for kimberlite diamondiferous root and medium grade terrane of a pre 2 7ga southern superior protocraton article i nfo we studied heavy minerals extracted from a diamondiferous metaconglomerate that formed 2697 2701 ma in a successor basin within the michipicoten greenstone belt mgb of the wawa abitibi terrane southern superior craton the conglomerate is metamorphosed in the greenschist facies and contains mainly locally derived igneous mafic to felsic detritus but also very minor components of medium grade metamorphic minerals diamonds and paragenetic diamond indicator minerals comparison of the size distribution resorb tion and n aggregation of diamonds in nearby wawa lamprophyres and the metaconglomerate diamonds confirms that the latter were not derived from the proximal lamprophyric source the heavy minerals in the metaconglomerate include diopside olivine corundum chromite almandine pyrope with kelyphitic rims picroilmenite amphibole and anorthite low abundances of the heavy minerals several grains per 4 70 tons of the metaconglomerate are in part explained by their complete or partial replacement by the greenschist mineral assemblage detrital almandine and amphibole are inferred to originate in amphibolite facies rocks cr diopside olivine chromite and anorthite were sourced from mafic ultramafic anorthosite and chromitite bearing layered complexes mapped in the mgb the presence of pyrope with more than 6 wt cr2o3 suggests derivation from a cratonic root picroilmenite has compositions typical of kimberlite and unlike that of ultramafic lamprophyres and other unconventional diamondiferous volcanics the wawa metaconglomerate therefore should be considered analogous to the witwatersrand successor basin conglomerate in recording indirect evidence for archean kimberlites the tight localization of the diamondif erous conglomerate in time and space was controlled by a quick 3 ma erosion of the source kimberlite body the location of the kimberlite bearing n2 7 ga superior protocraton was inferred from the provenance of the metaconglomerate detrital material the clasts could have originated as close as the northern wawa abitibi terrane or as distant as the opatica terrane the pre 2 7 ga diamonondiferous cratonic root below the southern superior was removed in the neoarchean proterozoic the existence of archean kimberlites and deep diamondiferous roots below smaller pre 2 7 ga protocratons emphasizes the similarity of neoarchean and phanerozoic mantle processes

sand dune movement in the victoria valley antarctica we use vertical aerial photographs and lidar topographic survey data to estimate dune migration rates in the victoria valley dunefield antarctica between 1961 and 2001 results confirm that the dunes migrated an average of 1 5 m year these values are consistent with other estimates of dune migration from cold climate deserts and are significantly lower than estimates from warm deserts dune migration rates are retarded by the presence of entrained ice soil moisture and a reversing wind regime dune absorption merging and limb extension are apparent from the time series images and account for significant changes in dune form and the field scale dune pattern dune field pattern analysis shows an overall increase in dune field organization with an increase in mean dune spacing and a reduction in total crest length and defect density these data suggest that dunes in other cold desert environments on earth mars or titan that may also have inter bedded frozen laminae still have the potential to migrate and organize albeit at lower rates than dunes in warm deserts

timing and kinematics of crustal movement in the northern superior superterrane insights from the gull rapids area of the split lake block manitoba abstract the northern superior superterrane in the northwestern superior province is characterized by an ancestry back to ca 3 8 ga and contains one of the largest and best preserved high grade terranes on earth the pikwitonei granulite domain and its northeast extension the split lake block this high grade terrane occurs in the superior boundary zone a complex transitional zone that bounds the northwestern margin of the archean superior province and separates it from the paleoproterozoic crust of the trans hudson orogen detailed structural and geochronological data from the exceptionally well exposed gull rapids area at the eastern margin of the split lake block shows evidence for both neoarchean kenoran and paleoproterozoic trans hudson deformation and metamorphism more importantly it indicates that the uplift of the split lake block was achieved by shear zone movement in the neoarchean not in the palaeoproterozoic synchronous with peak metamorphism and dyke emplacement at ca 2686 ma this indicates that uplift of the high grade domains in the northern superior superterrane was neither related to its collision with the north caribou superterrane at ca 2 72 ga nor to the ca 1 8 ga trans hudson orogeny instead this uplift might have occurred during a major period of superior wide active tectonism toward the end of the neoarchean kenoran orogenesis

southern high latitude dune fields on mars morphology aeolian inactivity and climate change in a study area spanning the martian surface poleward of 50 s 1190 dune fields have been identified mapped and categorized based on dune field morphology dune fields in the study area span 116 400 km2 leading to a global dune field coverage estimate of 904 000 km2 far less than that found on earth based on distinct morphological features the dune fields were grouped into six different classes that vary in interpreted aeolian activity level from potentially active to relatively inactive and eroding the six dune field classes occur in specific latitude zones with a sequence of reduced activity and degradation progressing poleward in particular the first signs of stabilization appear at 60 s which broadly corresponds to the edge of high concentrations of water equivalent hydrogen content observed by the neutron spectrometer that have been interpreted as ground ice this near surface ground ice likely acts to reduce sand availability in the present climate state on mars stabilizing high latitude dunes and allowing erosional processes to change their morphology as a result climatic changes in the content of near surface ground ice are likely to influence the level of dune activity spatial variation of dune field classes with longitude is significant suggesting that local conditions play a major role in determining dune field activity level dune fields on the south polar layered terrain for example appear either potentially active or inactive indicating that at least two generations of dune building have occurred on this surface many dune fields show signs of degradation mixed with crisp brinked dunes also suggesting that more than one generation of dune building has occurred since they originally formed dune fields superposed on early and late amazonian surfaces provide potential upper age limits of 100 my on the south polar layered deposits and 3 ga elsewhere at high latitudes no craters are present on any identifiable dune fields which can provide a lower age limit through crater counting assuming all relatively stabilized dune fields represent a single noncontiguous surface of uniform age their estimated crater retention age is 10 000 years an average sized uncratered dune field 94 km2 has a crater retention age 8 my this apparent youth suggests that present day climate conditions are responsible for the observed degradation and reduced level of aeolian activity a lack of observed transport pathways and the absence of large dune fields in the largest basins hellas and argyre planitiae are consistent with the previously proposed idea that dune sands are not typically transported far from their source regions on mars

origin and evolution of granulites in normal and thickened crusts rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies they can be exposed at the earth9s surface within this one erogenic cycle for example the granulites exposed in british columbia have concordant u pb zircon ages of 65 85 ma in contrast rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger lower grade rocks at the cessation of orogeny such rocks will undergo near isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness these granulites require a second orogeny to uplift and expose them at the surface as an example the archean granulites of enderby land antarctica were metamorphosed at 3070 ma at 1000 c at 8 10 kbar pressure they then followed at least a 400 c isobaric cooling path they also had a prolonged residence time of 2000 m y near the base of the crust before uplift no doubt a continuum exists between these extremes but it is proposed that many intermediate and high pressure granulites form in the lower parts of thickened crust and therefore require a two stage cycle of tectonism to be uplifted and exposed at the earth9s surface although most granulites probably form in thickened crust at active plate margins underplating of normal 30 40 km thickness crust by mantle melts during extension can also produce granulites these granulites would also undergo isobaric cooling and require a later orogeny to expose them an example is the paleozoic lachlan fold belt of eastern australia which has undergone only minor uplift since the major ultrametamorphism at about 400 ma however isotopic petrologic and geochemical differences are to be expected between these extremes of granulite formation

an extended episode of early mesoproterozoic metamorphic fluid flow in the reynolds range central australia the products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the upper calcsilicate unit in the granulite portion of the late palaeoproterozoic reynolds range group northern arunta block central australia the zones of retrogression characterized by minerals such as wollastonite grossular and clinohumite local resetting of oxygen isotopic compositions and local major element metasomatism were channelways for water rich fluids derived from granulite facies metapelites u th pb isotopic ages measured by the shrimp ion microprobe on zircon and monazite from a granulite facies semipelite an early semiconcordant aluminous quartz rich fluid flow segregation and a late discordant quartz rich segregation record some of the extended thermal history of the area zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 11 ma old itself derived from a source containing zircon as old as 2 2 ga low th u overgrowths on the zircon grew during granulite facies metamorphism at 1594 6 ma monazite cooled to its blocking temperature at 1576 8 ma zircon cores from the semiconcordant segregation are dominantly 2 3 ga old indicating that the source of the fluids was not the particular metamorphosed semipelite studied two generations of low th u overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 8 and 1582 8 ma respectively the monazite age is the same 1576 12 ma zircon from the late discordant segregation gave 1568 4 ma fluid flow occurred for at least 18 3 ma and ended 26 3 ma after the peak of metamorphism suggesting a very slow cooling rate of 3 c ma 1 the last regional high grade metamorphism in the reynolds range occurred at 1 6 ga not 1 78 ga as previously thought the high grade event at 1 78 ga is a separate event that affected only the basement to the reynolds range group

elasticity composition and temperature of the earth s lower mantle a reappraisal the interpretation of seismological models for the earth s lower mantle in terms of chemical composition and temperature is sensitive both to the details of the methodology employed and also to uncertainties in key thermoelastic parameters especially for the dominant mg fe sio3 perovskite phase here the alternative approaches adiabatic decompression of the lower mantle for comparison at zero pressure with laboratory data and the projection of laboratory data to lower mantle p t conditions for direct comparison with seismological observations are assessed along with the equations of state on which they are based it is argued that adiabatic decompression of the lower mantle is best effected by a strategy in which consistent third order finite strain expressions are required simultaneously to fit the strain dependence of both the seismic parameter and the density this procedure accords due weight to the most robust seismological observations namely the wave speeds and reduces to an acceptable level the otherwise very strong covariance among the fitted coefficients it is demonstrated that this approach can readily be adapted to include the effects of relaxation from the hill average to the reuss lower bound of the aggregate bulk modulus which governs the radial variation of density for the projection of laboratory thermoelastic data to lower mantle p t conditions the preferred equation of state is of the mie gruneisen type involving the addition at constant volume of the pressure along a finite strain principal 300 k isotherm and the thermal pressure calculated from the debye approximation to the lattice vibrational energy a high degree of consistency is demonstrated between these alternative equations of state possible departures of the lower mantle from conditions of adiabaticity and large scale homogeneity are assessed these are negligible for the prem model bullen parameter b 0 99 0 01 but significant for ak135 b 0 94 0 02 reviving the possibility of a substantially superadiabatic temperature gradient experimentally determined thermoelastic properties of the major mg fe sio3 perovskite and mg fe o magnesiowustite phases are used to constrain the equation of state parameters employed in the analysis of seismological information although the scarcity of information concerning the pressure and temperature dependence of the elastic moduli for the perovskite phase remains a serious impediment nevertheless it is demonstrated that the combination of a pyrolite composition simplified to the three component system sio2 mgo feo with molar xpv 0 67 and xmg 0 89 and a lower mantle adiabat with a potential temperature of 1600 k consistent with the preferred geotherm for the upper mantle and transition zone is compatible with the prem seismological model within the residual uncertainties of the thermoelastic parameters for the perovskite phase in particular such consistency requires values for the perovskite phase of ks ks p t and q ln ln v t of approximately 3 8 and 2 and g t near 0 022 gpa k 1 for the lower mantle assemblage more silicic models which have sometimes been advocated can also be reconciled with the seismological data but require lower mantle temperatures which are much higher by about 700 k for pyroxene stoichiometry however the absence of seismological and rheological evidence for the pair of thermal boundary layers separating convection above and below which is implied by such models remains a formidable difficulty under these circumstances the simplest possible model that of grossly uniform chemical composition throughout the mantle is preferred

partitioning of iron between magnesian silicate perovskite and magnesiowustite at about 1 mbar abstract the disproportionation products of olivine fo 90 in diamond anvil cell experiments at about 1 mbar are magnesian silicate perovskite mg 94 and a series of magnesiowustites mg 85 90 yielding a recommended value of 0 4 0 1 for the distribution coefficient that defines the exchange of mg and fe 2 between the two phases k d the distinctive compositional trends which would signify that ferric iron had been stabilised during experiments are lacking pressure does not exert any significant effect on distribution behaviour despite thermodynamic predictions of major reallocation of fe 2 from magnesiowustite into and in favour of perovskite towards the base of the lower mantle nor does the presence of al 3 in magnesian silicate perovskite in amounts appropriate for the lower mantle substantially modify distribution behaviour and hence phase chemistry provided the oxygen content of the system remains essentially constant wood s earth planet sci lett 174 2000 341 multi anvil study has been used to link k d with temperature and to calculate that magnesian silicate perovskite mg 93 0 0 6 would coexist with magnesiowustite mg 82 6 0 9 just beyond the 660 km discontinuity 1800 k whilst at the core mantle boundary 2650 k the corresponding values would be 91 9 0 4 and 84 9 0 8

zirconium abundance in granulite facies minerals with implications for zircon geochronology in high grade rocks zircon growth in high grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium zr we have measured the concentration of zr in the major minerals in granulite facies rocks of differing bulk composition both garnet and hornblende contain tens of parts per million zr and no other major phase contains significant zr simple calculations show that reaction of either garnet or hornblende to form non zr bearing phases will release sufficient zr to account for at least some new zircon growth u pb ages from new zircon grown as a result of either hornblende or garnet breakdown are not expected to record the time of peak metamorphism but rather will record the time of particular metamorphic reactions allowing direct correlation of zircon ages with petrologically derived pressure temperature time paths this approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible

eoarchean to neoarchean evolution of the nuvvuagittuq supracrustal belt new insights from u pb zircon geochronology the nuvvuagittuq supracrustal belt in northern quebec is a rare 9 km2 fragment of the earth9s early crust the belt contains a metamorphosed volcano sedimentary sequence that is at least eoarchean in age including amphibolites of the ujaraaluk unit that may have protoliths as ancient as 4400 million years old ma o9neil and others 2012 upper amphibolite facies metamorphism and high strain deformation have obscured many primary field relationships and disturbed whole rock isotope systematics leading to debate over the interpretation of previous geochronological data we report new shrimp u pb isotopic analyses of zircons from key meta igneous and newly identified metasedimentary units of the belt the analyzed samples fall into four categories a felsic gneisses including sheets interlayered with the supracrustal assemblage and a tonalitic orthogneiss at the margin of the belt b a newly identified metasedimentary unit c fuchsite bearing quartz rich layers d mafic gneisses including a garnet biotite amphibolite from the ujaraaluk unit and a meta gabbro the felsic lithologies place constraints on the minimum age of the supracrustal assemblage oscillatory zoned zircons separated from a felsic orthogneiss sheet form a discordant array with an upper intercept age of 3774 32 ma and the tonalitic orthogneiss yielded oscillatory zoned zircons with an upper intercept age of 3781 11 ma a metasedimentary unit dominated by quartz albite and clinozoisite has been identified within a lower strain domain interfolded with amphibolites of the ujaraaluk unit a sample of this unit yielded zircons with highly variable cl structures and near concordant ages of between 3390 and 3780 ma zircons from a fuchsite bearing quartz layer have oscillatory zonation similar to the orthogneiss grains and define an upper intercept age of 3794 16 ma petrologic observations and geochemical data suggest that this lithology is a metasomatically altered felsic orthogneiss band rather than a meta sedimentary rock belonging to the supracrustal package electron microscopy of polished thin sections was used to locate zircons within 6 samples of the ujaraaluk unit amphibolites and two samples of meta gabbro large 500 m zircons separated from a garnet biotite amphibolite together with grains from a meta gabbro are sector zoned and yield ages of 2700 to 2640 ma recording the timing of pervasive high temperature metamorphism in the region the new zircon u pb data add important new detail to the geological history of the nuvvuagittuq belt the volcano sedimentary sequence and mafic ultramafic intrusive rocks were formed prior to 3800 million years ago subsequent geological evolution included extensive and protracted early archean ttg magmatism and crustal re melting paleo to mesoarchean uplift and sedimentation and neoarchean high grade metamorphism and regional magmatism during consolidation of the superior craton

sapphirine bearing granulites from the sipiwesk lake area of the late archean pikwitonei granulite terrain manitoba canada sapphirine occurs in the orthopyroxene cordierite and feldspar sillimanite granulites in the sipiwesk lake area of the pikwitonei granulite terrain manitoba 97 40 w 55 05 n the orthopyroxene cordierite granulites have extremely high al2o3 24 5 wt and mgo 24 6 wt contents and contain sapphirine up to 69 2 wt al2o3 aluminous orthopyroxene up to 8 93 wt al2o3 cordierite spinel phlogopite and corundum sapphirine forms coronas mantling spinel and corundum corona sapphirine is zoned and its composition varies through the substitution mg fe mn si 2 al as a function of the phases with which it is in contact textural and chemical relationships of sapphirine with coexisting phases indicate that spinel cordierite reacted to form orthopyroxene sapphirine under conditions of increasing pressure moreover decreasing core to rim variation of al2o3 in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation on the basis of experimentally determined p t stability of the assemblage enstatite sapphirine cordierite and the al content of hypothetical fe2 free orthopyroxene associated with sapphirine and cordierite metamorphic temperatures and pressures are estimated to be 860 890 c and 3 0 11 2 kbar

high precision u pb geochronology in the minnesota river valley subprovince and its bearing on the neoarchean to paleoproterozoic evolution of the southern superior province high precision u pb ages have been obtained for high grade gneisses late kinematic to postkinematic granitic plutons and a crosscutting mafic dike of the archean minnesota river valley tectonic subprovince at the southern ramparts of the superior craton of north america the antiquity of the minnesota river valley terranes is confirmed by a high precision u pb zircon age of 3422 2 ma for a tonalitic phase of the morton gneiss voluminous late kinematic monzogranites of the benson ortonville granite and morton sacred heart granite blocks yield identical crystallization ages of 2603 1 ma illustrating the synchrony and rapidity of deep crustal melting and plutonism throughout the minnesota river valley terranes postkinematic 2591 2 ma syenogranites and aplitic dikes in both blocks effectively constrain the final penetrative deformation of the minnesota river valley subprovince monazite growth from 2609 to 2595 ma in granulitic paragneisses of the benson and montevideo blocks is interpreted to record prograde to peak granulite facies metamorphic conditions associated with crustal thickening and magmatism neoarchean metamorphism and plutonism are interpreted to record the timing of collisional accretion and terminal suturing of the mesoarchean continental minnesota river valley terranes to the southern margin of the superior province along the western great lakes tectonic zone subsequent paleoproterozoic rifting of this margin is recorded by voluminous basaltic dike intrusion expressed in the minnesota river valley by major wnw trending tholeiitic diabase dikes dated at 2067 1 ma only slightly younger than the structurally and geochemically similar 2077 4 ma fort frances kenora kabetogama dike swarm of northern minnesota and adjoining canada

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

exposed cross sections through the continental crust implications for crustal structure petrology and evolution abstract the continental crust is exposed in cross section at numerous sites on the earth s surface these exposures which appear to have formed by obduction along great faults during continental collision may be recognized by exposures of deep crustal rocks exhibiting asymmetric patterns of metamorphic grade and age across the faults and by distinctive bouguer anomaly patterns reflecting dipping basement structure and an anomalously deep mantle from an examination of five complexes which meet these criteria it is concluded that the most prominent layering in the crust is not compositional but metamorphic the lower crust consists of granulite facies rocks of mafic to intermediate composition while the intermediate and shallow levels consist predominantly of amphibolite facies gneisses and greenschist facies supracrustal rocks respectively post metamorphic granitic intrusions are common at intermediate to shallow levels position of discontinuities in refraction velocity where present commonly correspond to changes in composition or metamorphic grade with depth the continental crust is characterized by lateral and vertical heterogeneities of varying scale which are the apparent cause of the complex seismic reflections recorded by cocorp field observations coupled with geochemical data indicate a complex evolution of the lower crust which can include anatexis multiple deformation polymetamorphism and reworking of older crustal material the complexity of the crust is thus the result of continuous evolution by recycling and metamorphism through time in a variety of tectonic environments

radiogenic heat production in the continental crust abstract the thermal structure and evolution of continents depend strongly on the amount and distribution of radioactive heat sources in the crust determining the contribution of crustal rocks beneath a superficial layer is a major challenge because heat production depends weakly on major element composition and physical properties such as seismic wavespeed and density enriched granitic intrusives that lie at the current erosion level have a large impact on the surface heat flux but little influence on temperatures in the deep crust many lower crustal rocks that are poor in heat producing elements are restites from ancient orogenic events implying that enrichment of the upper crust was achieved at the expense of deeper crustal levels for the same total heat production concentrating heat sources in an upper layer acts to reduce temperatures in the lower crust thereby allowing stabilization of the crust the present day structure of the crust is a consequence of orogeny and should not be adopted for thermal models of the orogenic event itself this review summarizes information extracted from large data sets on heat flow and heat production and provides estimates of crustal stratification and heat production in several geological provinces analysis of global and regional data sets reveals the absence of a positive correlation between surface heat flow and crustal thickness showing that the average crustal heat production is not constant differences of heat flow between geological provinces are due in large part to changes of crustal structure and bulk composition collating values of the bulk crustal heat production in a few age intervals reveals a clear trend of decrease with increasing age this trend can be accounted for by radioactive decay indicating that thermal conditions at the time of crustal stabilization have not changed significantly for the average crustal thickness of 40 km moho temperatures are near solidus values at the time of stabilization suggesting an intrinsic thermal control on crustal thickness and heat production distribution crustal thickening by more than about 10 km above this mean value induces changes of gravitational potential energy that exceed the strength of the lithosphere for several provinces where strong constraints on heat production are available it is shown that prior to intracrustal fractionation only modest amounts of thickening were needed to generate the conditions of ultra high temperature metamorphism the tell tale signature of crustal heat production is anatectic and metamorphic events that lag the cessation of orogenic activity by several tens of million years crustal heat production is decreasing with time because it is due to the radioactive decay of uranium thorium and potassium its rundown is responsible for the secular cooling of lithospheric roots at a typical rate of about 100 k gy 1 implying complex thermal interactions with a convecting mantle that is not cooling at the same rate

seasonality of present day martian dune gully activity martian slope gullies are argued to be evidence for recent liquid water flow on the surface of mars to explain the source of water a wide range of environmental conditions and processes has been invoked however a lack of information about the environmental context or timing of gully activity makes it difficult to evaluate the theories here we present new observations of extensive gully modification over the past 6 mars years within dune gullies with slope gully morphology observed activity within 18 gullies in 7 dune fields constrains timing to winter which is consistent with observed slope gully activity these observations show that fluvial processes are unlikely to cause present day martian dune gully activity and imply that co 2 frost accumulation may play the dominant role

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

timing of late archaean terrane assembly crustal thickening and granite emplacement in the nuuk region southern west greenland abstract a breakthrough in understanding the structural and metamorphic evolution of archaean gneiss complexes occurred with the recognition that the nuuk region of southern west greenland comprised separate terranes assembled in the late archaean from northwest to southeast these are the akia 3220 2970 ma akulleq 3870 3600 and 2820 ma and tasiusarsuaq 2920 2860 ma terranes the minimum time of assembly is recorded by the first event common to all component terranes using shrimp u pb zircon geochronology the oldest events common to all terranes including emplacement of crustally derived granites contemporaneous metamorphism and anatexis have been dated at 2710 2725 ma in the akulleq terrane areas where in situ diatexite formed abundant granitoid sheets were intruded and common growth of metamorphic zircon mostly low th u in most lithologies occurred in the akia and tasiusarsuaq terranes there was only intrusion of a lesser number of 2720 ma granitoid sheets because metamorphic zircons of that age have not been found the 2720 ma event is interpreted as marking or shortly following terrane assembly when the diverse components of the akulleq terrane were tectonically juxtaposed with the other two this new documentation of metamorphic and associated igneous events within an accreting cratonic region is an illustration that the stabilisation of extensive areas of archaean gneisses can be due to accretionary tectonics long after the individual components were first formed

norite from the mahalapye granitoid complex northern edge of kaapvaal craton implications for the extent of the paleoproterozoic bushveld lip abstract the paleoproterozoic centred at 2 054 ga bushveld lip is one of the largest intracratonic lips in the world its components are primarily distributed within the archean kaapvaal craton this study reports dyke like and sill like norite intrusions from lose quarry within the mahalapye complex a paleoproterozoic granitoid migmatitic gneiss terrane along the northern edge of kaapvaal craton field relations indicate the norite postdate the 2 061 ga diorite gneiss and predate the 2 039 ga granodiorite significantly it exhibit structures indicative of comingling with 2 053 ga leucosome leucogranite a product of partial melting of diorite gneiss the cross cutting relations provide unequivocal evidence that the norite and the leucogranite were liquid at the same time and should be of similar age fe rich orthopyroxene xmg 0 43 0 46 constitutes the dominant mafic mineral in the norite geochemical characteristics of the mahalapye complex norites are similar to those of marginal facies norites from the bushveld complex b1 sills and molopo farms complex layered intrusions two of the prominent constituents of the bushveld lip this is the first solid evidence that the original extent of the bushveld lip covered an area larger than the current known limits of the archean kaapvaal craton into the youngest and westernmost terrane of the high grade limpopo complex the potential of high grade terranes along archean cratonic margins as locales to look for lip fragments is explored

ultrahigh resolution topographic mapping of mars with mro hirise stereo images meter scale slopes of candidate phoenix landing sites 1 the objectives of this paper are twofold first to report our estimates of the meter to decameter scale topography and slopes of candidate landing sites for the phoenix mission based on analysis of mars global surveyor mgs mars orbiter camera moc images with a typical pixel scale of 3 m and mars reconnaissance orbiter mro high resolution imaging science experiment hirise images at 0 3 m pixel 1 and second to document in detail the geometric calibration software and procedures on which the photogrammetric analysis of hirise data is based a combination of optical design modeling laboratory observations star images and mars images form the basis for software in the u s geological survey integrated software for imagers and spectrometers isis 3 system that corrects the images for a variety of distortions with single pixel or subpixel accuracy corrected images are analyzed in the commercial photogrammetric software socet set bae systems yielding digital topographic models dtms with a grid spacing of 1 m 3 4 pixels that require minimal interactive editing photoclinometry yields dtms with single pixel grid spacing slopes from moc and hirise are comparable throughout the latitude zone of interest and compare favorably with those where past missions have landed successfully only the mars exploration rover mer b site in meridiani planum is smoother moc results at multiple locations have root mean square rms bidirectional slopes of 0 8 4 5 at baselines of 3 10 m hirise stereopairs one per final candidate site and one in the former site yield 1 8 2 8 slopes at 1 m baseline slopes at 1 m from photoclinometry are also in the range 2 3 after correction for image blur slopes exceeding the 16 phoenix safety limit are extremely rare

u pb geoehronology and evolution of the english river subprovince an archean low p high t metasedimentary belt in the superior province u pb isotopic data for zircon monazite and titanite are used to assess the evolution of the late archean english river subprovince ers and adjacent domains of the winnipeg river subprovince in the northern superior province ontario the ers consists of turbiditic metasedimentary rocks deposited during the final stages of magmatic and tectonic accretion within the greenstone granite uchi subprovince to the north at around 2720 to 2710 ma the sedimentary rocks were intruded by a suite of calc alkalic plutons at 2698 ma major regional deformation amphibolite to granulite facies metamorphism anatexis and emplacement of an extensive peraluminous granitic suite culminated at 2691 ma late episodes of metamorphism metasomatism and emplacement of pegmatites occurred locally at circa 2680 and 2669 ma in contrast the winnipeg river subprovince to the south consists of widespread tonalitic suites formed mainly between 3200 and 2800 ma and granodioritic to granitic intrusions formed between 2710 and 2690 ma metamorphism occurred at around 2710 ma in southern domains and a distinct granulite facies event at around 2680 ma affected central and northern domains of the winnipeg river subprovince the rapid sequence of events recorded in the ers supports the notion that metamorphism was caused by thermal perturbations related to injection of granitic magmas these melts were generated through partial melting of the lower crust by a thermal anomaly presumably caused by mantle upwelling related to collapse of the inactivated subducting plate the short lived nature of the metamorphic events and their cyclicity follow the behaviour predicted by recent models linking the evolution of strain rates to episodes of segregation and upward magma transfer in the course of prolonged orogenic events

new and recent gully activity on mars as seen by hirise 1 fresh appearing gully deposits are found at tens of sites in the southern hemisphere of mars these deposits have latitudinal and azimuthal dependences similar to the overall preferences of southern hemisphere gullies suggesting that most gullies can undergo such events definite changes are seen at ten sites including two previously reported these include visible modification of gully channels and aprons those formation intervals constrained to better than one mars year tend to include winter and exclude summer suggesting seasonal activity this seasonal activity is consistent with proposed models for gully formation driven by co2 frost although at least some of the new deposits are probably due to dry granular flow with no volatile involvement as these deposits are capable of effecting distinct topographic changes to gully aprons and channels they represent a significant component of recent gully evolution

mars global digital dune database mgd3 north polar region mc 1 distribution applications and volume estimates the mars global digital dune database mgd3 now extends from 90 n to 65 s the recently released north polar portion mc 1 of mgd3 adds 844 000 km2 of moderate to large size dark dunes to the previously released equatorial portion mc 2 to mc 29 of the database the database available in gis and tabular format in usgs open file reports makes it possible to examine global dune distribution patterns and to compare dunes with other global data sets e g atmospheric models mgd3 can also be used by researchers to identify areas suitable for more focused studies the utility of mgd3 is demonstrated through three example applications first the uneven geographic distribution of the dunes is discussed and described second dune derived wind direction and its role as ground truth for atmospheric models is reviewed comparisons between dune derived winds and global and mesoscale atmospheric models suggest that local topography may have an important influence on dune forming winds third the methods used here to estimate north polar dune volume are presented and these methods and estimates 1130 km3 to 3250 km3 are compared with those of previous researchers 1158 km3 to 15 000 km3 in the near future mgd3 will be extended to include the south polar region published in 2011 this article is a us government work and is in the public domain in the usa

u pb geochronology and trace element composition of zircon from the horseshoe lake greenstone belt superior province canada implications for the tectonic and metamorphic history1 depth profile techniques for u pb geochronology and rare earth element ree geochemistry were conducted on unpolished archean zircon with metasomatic rims from metasedimentary rocks within the horseshoe lake greenstone belt western superior province canada these zircon crystals are shown to have isotopically distinct rims typically 100 million years younger than the cores the 207pb 206pb rim ages can be temporally correlated with regional magmatism and with a later potentially au bearing hydrothermal event synchronous with greenschist facies metamorphism and regional deformation notably the zircons do not record evidence of local ca 2741 2715 ma magmatism manifested by the emplacement of a quartz feldspar porphyry dike swarm laser ablation inductively coupled plasma mass

the paleomagnetic significance of new u pb age data from the molson dyke swarm cauchon lake area manitoba u pb geochronology paleomagnetism and petrography indicate that the molson dyke swarm along the western margin of the superior craton is a composite of at least two ages of intrusion the more extensive younger dyke set the molson swarm sensu stricto generally has a 030 trend is mainly pyroxenitic to noritic with subordinate diabase and has been related to rifting in a back arc environment during closure of the manikewan ocean at about 1920 1800 ma which culminated in the trans hudson orogen a u pb zircon age from one of these dykes located at cauchon lake indicates emplacement at 1877 7 150 4 similar to two previous u pb age determinations on molson dykes another dyke from cauchon lake yields a baddeleyite zircon u pb date of 2091 2 ma and appears to be part of an older mainly diabasic suite of east northeast trending dykes that may represent a continental rifting episode that preceded the opening of the manikewan ocean the new u pb age data require a revision to the interpretation of